Original Article
Design and implementation of magnetically maneuverable capsule endoscope system with direction reference for image navigation
Proc IMechE Part H: J Engineering in Medicine 2014, Vol. 228(7) 652–664 Ó IMechE 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0954411914540876 pih.sagepub.com
Zhen-Jun Sun1,2, Bo Ye1, Yi Sun2, Hong-Hai Zhang1,2 and Sheng Liu1,2
Abstract This article describes a novel magnetically maneuverable capsule endoscope system with direction reference for image navigation. This direction reference was employed by utilizing a specific magnet configuration between a pair of external permanent magnets and a magnetic shell coated on the external capsule endoscope surface. A pair of customized Cartesian robots, each with only 4 degrees of freedom, was built to hold the external permanent magnets as their endeffectors. These robots, together with their external permanent magnets, were placed on two opposite sides of a ‘‘patient bed.’’ Because of the optimized configuration based on magnetic analysis between the external permanent magnets and the magnetic shell, a simplified control strategy was proposed, and only two parameters, yaw step angle and moving step, were necessary for the employed robotic system. Step-by-step experiments demonstrated that the proposed system is capable of magnetically maneuvering the capsule endoscope while providing direction reference for image navigation.
Keywords Capsule endoscope, configuration optimization, direction reference, image navigation, magnetic maneuvering
Date received: 11 June 2013; accepted: 29 May 2014
Introduction As a less invasive and more comfortable method, compared with traditional endoscopy, capsule endoscopy has been gradually used to explore the gastrointestinal (GI) tract over the past decade.1,2 Current designs of capsule endoscope (CE) integrate function modules such as illumination, image capture, wireless data transceiver, and power supply modules. Thus, it can automatically acquire images from the inspected GI tract and wirelessly send data to an external memory when moving by means of visceral peristalsis after being ingested. The images can be viewed in a real-time mode or off-line mode depending on whether the working external memory for image storage is in a workstation or portable recorder. Although CE has gained wide acceptance, it suffers from certain limitations.3 Among these limitations, the walking ability, including locomotion and navigation, is a basic issue and may be solved first. For locomotion, various methods have been employed, including the spiral-type capsule,4 motor-legged capsule robot,5–7
worm-like capsule robot,8,9 and magnetic maneuvering capsule.10–14 Compared with other methods, magnetic maneuvering seems to be a more promising method because of its wireless operation capability, without the need for an extra power supply. Keller et al.10 used a handheld external permanent magnet (EPM) to steer a magnetic capsule endoscope (MCE) in the exploration of human stomach. Carpi and colleagues11,15 employed a robotic magnetic navigation system to maneuver an MCE coated with a magnetic shell. A digital fluoroscope with a threedimensional (3D) localization error of 1 mm was 1
School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan, China 2 National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, China Corresponding author: Sheng Liu, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, F101, Wuhan 430074, Hubei, China. Email: [email protected]
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employed to provide feedback for the active maneuvering of the MCE. Ciuti et al.12 employed another commercially available robot to hold an EPM. The robot had 6 degrees of freedom (DOFs). The EPM had a cylindrical shape and was magnetized perpendicularly to its main axis, whereas four small cylindrical and symmetrically arranged permanent magnets were imbedded in the swallowed CE and magnetized axially. Both vision feedback and inertial feedback were used to provide useful information during the maneuvering. For navigation, it is natural to employ images from CE because this makes possible the full use of what the CE provides. Although image-navigated magnetic maneuvering has been mentioned by Keller et al.10 and Carpi et al.,15 no details have been provided. For image navigation, direction reference is necessary and important as it is important in some other work related to image processing16,17 in which the lack of direction reference increased the difficulty of the algorithm. This study simply employed a reasonable magnet configuration between two EPMs and the magnetic shell of the MCE, without any embedded sensor, for direction reference. To the best of our knowledge, this is the first time that a method for providing direction reference through a reasonable magnet configuration has been explicitly proposed in the CE field. This specific magnet configuration allows an image-navigated control strategy to be proposed. An extensive magnetic analysis was carried out qualitatively and quantitatively to support the control strategy and optimize the control parameters and the structure of external robotic arms. Step-by-step, in-vitro experiments validated the effectiveness of the image-navigated magnetic maneuvering system.
System design System overview The proposed system, schematically represented in Figure 1, is mainly composed of a capsule device and Cartesian robotic subsystem. The two Cartesian robotic arms of the robotic subsystem are placed across each other on two lateral sides of the ‘‘patient bed.’’ Each robotic arm clamps a cylindrical EPM as its end-effector. A CE coated with a magnetic shell/ingested permanent magnet (IPM) on the intermediate part of its external surface is called a MCE. The horizontal layout of two axially magnetized EPMs with the diametrically magnetized IPM between them provides the MCE direction reference for image navigation. This layout also makes it feasible to magnetically maneuver the MCE in 3D space using the 4DOF Cartesian robotic arms. With the direction reference, images taken by the MCE can be intuitively and conveniently used for direction recognition. According to these images, the operator sets control parameters through a human–machine interface (HMI) to maneuver the EPMs with the robotic arms, and finally drives the MCE to implement an inspection of the GI tract.
EPMs and IPM. The EPMs and IPM are all made of Nd–Fe–B and are cylindrical, with a hole in the center. Each EPM is mounted on a robotic arm as its endeffector, whereas the IPM is coated on the external surface of the CE. Each EPM is magnetized axially and has an external diameter of 100 mm, an internal diameter of 30 mm, a height of 55 mm, a weight of 4.1 kg, and a magnetization of 820 kA/m, whereas the IPM is magnetized diametrically and has an external diameter of 16.5 mm, an internal diameter of 13.5 mm, a height of 16.5 mm, a weight of 8.9 g, and a magnetization of 850 kA/m. Capsule device. The capsule device (Hangzhou Hitron Technologies Co., Ltd, China) consists of CEs, an antenna array, a data receiver, and a workstation with dedicated software. A CE has a diameter of 13.4 mm, length of 28 mm, and weight of 4.3 g. An off-line mode and real-time mode can both be accessed at a high image resolution of 480 dpi 3 480 dpi or a low resolution of 240 dpi 3 240 dpi. A CE can last more than 8 h while operating at the low resolution and at 2 FPS. This device also takes advantage of the ability to wirelessly turn a CE on or off.18 Cartesian robotic subsystem. The Cartesian robotic subsystem consists of two Cartesian robotic arms and their servo control routine. Figure 2 shows the defined coordinate system (CS). Here, (XM, YM, ZM) is fixed on the EPM, and (XC, YC, ZC) is fixed on the MCE. The corresponding axes of these two CSs are parallel at their initial state. (XW, YW, ZW) is the world CS, which is the same as the initial state of (XM, YM, ZM). Initially, YM is parallel to the length of the stationary patient bed, ZM is along the vertical line, and the origin OM is the geometric center of the EPM. Each robotic arm has 4 DOFs: three translational movements along XW, YW, and ZW; and one rotation about ZW. The strokes in the XW, YW, and ZW directions are approximately 220 mm, 490 mm, and 185 mm, respectively.
Control strategy based on image navigation A control strategy based on image navigation will be more reliable with the participation of the operator who inputs the commands after reading the images during the maneuvering. However, the images from a common CE are not suitable for operator’s direction recognition because of the rolling of the CE and the lack of a direction reference. The employment of a localization system11 or an embedded inertial sensor12 to provide absolute orientation and position feedback is an effective way to provide direction reference. However, because the control of the MCE is actually the control of its position relative to the inspected intestinal wall, absolute feedback methods can just act as a complement to a control method based on image navigation. While images from a CE provide this kind
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Figure 1. Schematic of the proposed system. USB: universal serial bus; DOF: degree of freedom; EPM: external permanent magnet; MCE: magnetic capsule endoscope; LED: light-emitting diode; IPM: ingested permanent magnet.
of relative position feedback, a direction reference for direction recognition can be acquired by exploiting the EPM and IPM layout, which has the advantage of not requiring an extra embedded electronic device for navigation.
Direction reference. In Figure 2, EPMs are placed with their main axes XM in the horizontal plane. When the diametrically magnetized IPM is placed between these two EPMs, its pole direction will also be in the horizontal plane. If the EPMs rotate about their individual ZM axes simultaneously, the IPM will rotate accordingly, while its pole direction remains in the horizontal plane. Thus, the IPM will not roll if the EPM pole directions are maintained in the horizontal plane, and the IPM
pole direction can be used as the direction reference. When the external surface of a CE is coated with an IPM, the horizontal line (XI in Figure 3(b)) of images from CE shall accord with the IPM pole direction. This CE and IPM assembly (i.e. MCE) provides CE’s camera with a direction reference for the image-navigated control strategy.
Principle of image-navigated control strategy. A magnetically maneuvered CE should always be adjusted to align with the surrounding tissues and then advance a small distance, as depicted in Figure 3. In a typical GI image captured by a CE, there will normally be features (e.g. dark area or bubbles) indicating where the exit of the intestinal lumen is located. Adjusting the MCE to point
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Figure 2. Definition of coordinate systems for EPM and MCE and diametrically magnetized direction (adapted from Sun et al.19). EPM: external permanent magnet; MCE: magnetic capsule endoscope.
Figure 3. Image-navigated control strategy: (a) top-level control strategy and (b) schematic of control based on image navigation.
at this exit will align it with the surrounding tissues and facilitate the subsequent movement. Repeating these procedures for adjusting the direction and moving forward will allow the MCE to explore the entire intestinal tract if it does not become stuck. Control parameters and DOFs of robotic arm of proposed system. While the MCE is under control, the (XC, YC, ZC) coordinates fixed on the MCE continuously change. Because the operator stands in the world CS, the images captured in the CS of the CE should be transformed to the world CS to correctly recognize the direction with the help of a localization device. While the MCE is under control, the operating commands should be transformed back to the CS of the CE because what should be adjusted is the position and orientation of the CE relative to the inspected intestinal wall. These make the robotic control relatively complex. The employment of a diametrically magnetized IPM, placed between two axially magnetized EPMs, prevents
the MCE from rotating about its main axis and provides its images with direction reference. Three advantages come consequently. First, because of the direction reference, the moving direction of the MCE’s next step can be directly observed from GI images. Next, the transformation between CSs will be easier, which simplifies the control algorithm for the external robot(s). Third, this specific layout loses control of the MCE rotation about XC and releases the MCE constraint in the vertical plane. That is, the pitch angle Du is no longer under control, and movement in the YCOCZC plane is free. This makes the MCE more compliant and less invasive in the GI tract and reduces its possibility of getting stuck. This also allows the MCE to be navigated in 3D space using the two-dimensional (2D) images of the proposed MCE. Thus, the control parameters are the yaw step angle Dc and moving step d. Because the EPM pitch must be fixed to acquire the direction reference, and the EPM roll has no influence on the MCE, the necessary DOFs for one robotic arm shrink to four, which are one
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rotation about ZM and three translations along XW, YW, and ZW.
Magnetic maneuvering of MCE When the two EPMs with different magnetic poles face each other, the main axis of the diametrically magnetized IPM/MCE located between the EPMs will be aligned parallel to the EPM radial direction. This means the MCE can freely pitch in the vertical plane, and the MCE will finally align itself with the surrounding intestinal lumen in the vertical plane. In this case, the movement of the EPMs along the length direction of the patient bed can drag the MCE forward (Figure 4(a)). As the EPMs rotate counterclockwise, the MCE will rotate clockwise (Figure 4(b)). After the direction adjustment, the movement of the EPMs in the direction parallel to the main axis of the MCE can drag the MCE to move along its main axis, which is actually not a wise strategy because of the need for a large stroke in the XW direction and the possibility of interference between the EPMs and the inspected patient. An improved control strategy fixes EPMs in the nearest XW direction to the MCE and requires the EPM movements only in the YW and ZW directions (Figure 4(c)). An exception occurs in a situation where the MCE axis nearly points toward the EPMs (70° \ CMCE 4 110° or 250° \ CMCE 4 290°). In these cases, if only the magnetic force exists, the MCE is out of control and moves toward the closer EPM. When the EPM on the side of the desired movement direction is further from the MCE, the other EPM should move further from the MCE to drive it in the right direction (Figure 4(d)). By inputting the two parameters through the HMI according to the images from the MCE, the operator can continuously control the robotic system to change the MCE position and orientation and finish the diagnosis. During maneuvering, the directions of the two control parameters are directly set according to the images from the MCE, whereas the magnitudes can be approximately set in advance according to the forces and torques computed in the following section. Sometimes, disabling or changing the step values of rotation Dc or forward movement d is necessary, which is mainly dependent on the specific situation.
Magnetic link between EPMs and MCE Analytical background Suppose the magnetizations of the IPM/MCE and two EPMs are all rigid and uniform. According to Kelvin’s ! formula, the incremental magnetic force d F exerted by the EPMs on an infinitesimal element (dV) of the IPM can be given by ! ! ! dF = m0 r MIPM HEPM
ð1Þ
!
where MIPM is the magnetization of the IPM, and * HEPM is the magnetic field strength generated by the EPMs. The total force can be calculated as follows Z Z ! ! ! ! F ¼ dFdV ¼ m0 ðMIPM HEPM ÞdV ð2Þ V
Using CE’s CS (XC, YC, ZC), we have !
ð3Þ
MIPM = MIPM eYC !
HEPM ¼ HEPMXC eXC þ HEPMYC eYC þ HEPMZC eZC
ð4Þ
Suppose the IPM always aligns its magnetization direction with the magnetic field generated by the EPMs. Then, equation (4) can be written as !
ð5Þ
HEPM = HEPMYc eYc
and equation (2) can be re-written as !
ð
F=
m0 MIPM HEPMYC dV’
V
m0 VIPM MIPM rHEPMYC
ð6Þ
When the IPM aligns with the magnetic field of the EPMs, there is no torque between the EPMs and IPM. When the EPMs rotate by a small angle DCEPM (Figure 3), a misalignment angle DCIPM between the IPM and the changed magnetic field is consequently ! generated. Suppose HEPM 0 is the new magnetic field. The torque on the IPM exerted by the EPMs can be approximately calculated as follows !
!
0 T = m0 VIPM MIPM 3HEPM = m0 VIPM ðMIPM eYC Þ
3ðHEPM 0 eYC 0 Þ = m0 VIPM MIPM HEPM 0 sinðDCIPM Þ
ð7Þ
According to section ‘‘Control parameters and DOFs of robotic arm of proposed system,’’ we can simply treat the control of the proposed system as an equivalent control problem in the horizontal plane. The results of the experiment in section ‘‘Magnetic maneuvering of MCE based on given model with dedicated codes’’ confirmed the feasibility of this simplification. In the following section, the computation will simply be conducted in the horizontal plane.
Magnetic parameters for both EPM and IPM The magnetic flux densities (B) of the EPMs and IPM were all measured with a Gauss meter. COMSOL was used to conduct a finite element analysis of B to match the measured data (Figure 5). The magnetizations of the EPMs and IPM are 820 and 850 kA/m, respectively.
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Figure 4. Strategy of magnetic maneuvering: (a) supposed initial position, (b) rotation control, (c) translation control (0 \ CEPM 4 80°) and (d) translation control (80° \ CEPM 4 90°). EPM: external permanent magnet; MCE: magnetic capsule endoscope.
the symmetry of its solved magnetic field. Thus, the two symmetric points, OCL0 and OCR0 , have the same magnetic fields, both in amplitude and direction. Then, the magnetic field where the MCE is located is equivalent to twice the magnetic field on point OCL0 or point OCR0 , which can be conveniently solved by a 2D symmetric model in COMSOL.
Relationship between rotation angle of EPMs and corresponding angle of MCE Figure 5. Magnetic flux densities (B) of both EPM and IPM. The EPM curves are the B values along its axial direction. The IPM curves show the B values in the radial direction through the geometric center of IPM, where the curves in dotted lines are B values along the pole direction and the other two are B values on the N–S interface. EPM: external permanent magnet; IPM: ingested permanent magnet.
Simplified computation of magnetic field generated by two EPMs Suppose Figure 4(a) is the initial position of the MCE and two EPMs, and the MCE is always located in the middle of the EPMs. When the EPMs rotate at an angle of CEPM clockwise, the relative positions of MCE to the left and right EPMs are, respectively, on point OCL0 and point OCR0 in Figure 6, if the two rotating EPMs are treated as one stationary EPM. Thus, the magnetic field generated by the EPMs at the MCE position is equivalent to the sum of the magnetic fields on point OCL0 and point OCR0 . These two points are symmetrical about the geometric center of the stationary EPM. When the EPMs rotate clockwise by CEPM in a real situation, line OCL0 OCR0 will rotate counterclockwise by CEPM. The geometric symmetry of EPM shall lead to
Suppose the MCE is in the middle of the EPMs. The nearest distance (dEPM-MCE in Figure 6) between one EPM and the MCE is set to be 150 mm. Figure 7 depicts the corresponding relationship between the rotation angle of the IPM/MCE (CMCE) and the rotation angle of the EPMs (CEPM is in the range of 0°–90°) when dEPM-MCE is 150, 180, or 200 mm. When CEPM is in the range of 290° to 0°, the relationship between CMCE and CEPM is symmetrical about point (0°, 0°), with the curves shown in Figure 7. When CEPM is in the range of 90°–180°, the relationship between CMCE and CEPM is symmetrical about point (90°, 90°), with the curves shown in Figure 7. Suppose the MCE is initially in the middle of the EPMs, which have an angle of CEPM and begin to move along the length direction of the patient bed to drag the MCE. Because of the insufficient force at the beginning, the MCE will probably not be able to move and will just rotate about ZC. In these cases, the relationship between CMCE and the moving distance is depicted in Figure 7(b) when the EPMs maintain an angle of CEPM.
Magnetic force According to the proposed control strategy, when CEPM is in the range of 0°–80°, the EPMs only move
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Figure 6. Principle of simplified computation of magnetic field HEPM generated by two EPMs. EPM: external permanent magnet; MCE: magnetic capsule endoscope.
along the length direction of the patient bed to drag the MCE. When dEPM-MCE is 150 mm and the MCE cannot move but can just rotate about ZC, the approximate force acting on the MCE by the movement of the EPMs maintaining a certain rotation angle of CEPM is calculated using equation (6) and depicted in Figure 8. When CEPM is in the range of 80°–90°, the EPM whose position is opposite to the moving direction of the MCE shall move away from the patient bed in the transverse direction to correctly drive the MCE. In this case, as an example, the force acting on the MCE/IPM when CEPM is 90° is plotted in Figure 8 with an unmarked line.
Torque exerted on MCE by two EPMs When placed a certain distance away from each other, with the MCE/IPM located in their middle, the EPMs rotate to turn the MCE around. At the beginning of this control, the torque will probably be insufficient to overcome the applied force of the surrounding GI tract. When the EPMs keep a certain angle of CEPM and the rotation of the MCE lags by an angle of 3°, 6°, or 9°, the approximate torque acting on the MCE can be calculated using equation (7) and is shown in Figure 9(a). Figure 9(b) depicts the approximate torque exerted on the MCE/IPM when the EPMs with a distance of 300 mm move to drag the MCE which remains stationary.
Experiments and result Magnetic maneuvering of MCE based on given model with dedicated codes An experiment was first conducted in transparent polyvinyl chloride (PVC) tubes with both simple and complex shapes (Figure 10(a) and (b)). The capsule device was not involved. Instead, an IPM-coated CE shell without electronic circuits was used. A dedicated
routine for the corresponding model was employed to verify the MCE locomotion capability when maneuvered by the proposed system (Figure 10(c)). The simple model was placed horizontally and employed to help validate the translation and rotation in the horizontal plane, whereas it was placed vertically to validate the flexibility in the vertical plane. Its centerline is a planar line. And it had a tube diameter of 25 mm, total length of 440 mm, and minimum circle radius of 70 mm. It was fixed on a wooden base and placed in the middle of the EPMs, with its length direction parallel to the length direction of the patient bed. Two EPMs were set 300 mm apart from each other and had the same center height as the model. For the horizontally placed model, the dedicated codes considered the model geometry. Once the robotic system began to run, it automatically drove the IPM to the end of its journey in the model. It took the IPM approximately 230 s to travel in this model, with a running speed of approximately 2 mm/s. A similar experiment was also performed with this model inside a porcine intestine and showed similar results. For the vertically placed model, the IPM can be driven by translation of the EPMs only in the YW direction, with little effect from the model geometry, which indicated the MCE flexibility in the vertical plane. Then, a complex model (its centerline is a spatial line) with a length of 260 mm, width of 120 mm, maximum height of 80 mm, and minimum circle radius of 30 mm was employed. It took approximately 7 min to move through this complex model at a speed of approximately 2 mm/s.
Magnetic maneuvering of MCE with direction reference for image navigation In this experiment, the simple model was wrapped in a light-tight plastic bag, and a real MCE was used. Every
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Figure 7. Angle of MCE. (a) CEPM versus CMCE. The three curves are for different dEPM-MCE (150, 180, and 200 mm). (b) dlMCE (depicted in Figure 6) versus CMCE when EPMs maintain an angle of CEPM (0°, 15°, 30°, 45°, 60°, 75°, 90°).
Figure 8. Distance versus force acting on MCE/IPM when EPMs with rotation angle of CEPM (0°, 15°, 30°, 45°, 60°, 75°, 90°) move distance of dlMCE (depicted in Figure 6). EPM: external permanent magnet; IPM: ingested permanent magnet.
step of the maneuvering was manually controlled by an operator using a mouse and dedicated HMI according to the images from the MCE. The amplitudes of the two main control parameters of the EPMs, rotation step Dc and moving step d, were set in advance at 3° and 5 mm, respectively. Sometimes, the rotation or forward movement step was disabled or changed according to the specific situation. The time required for this experiment varied from 400 to 600 s at a speed of approximately 2 mm/s. An ex-vivo experiment was also performed inside an inflated porcine intestine (Figure 11) which we think is a better choice to mimic the intestine in a live body, compared to a collapsed intestine. The porcine intestine was cleaned with the mixture of saline water and vinegar and then washed by running water. The other simple model was used to accommodate part of the porcine intestine to make it light-tight. It was produced by a 3D
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Figure 9. Torque on MCE/IPM. (a) CEPM (CMCE lags 3°, 6°, and 9°) versus torque acting on MCE/IPM. The curves marked with the same symbol in a decreasing sequence of values correspond to a distance sequence of 150, 180, and 200 mm between the EPM and MCE and (b) dlMCE versus torque acting on MCE/IPM when EPMs keep a rotation angle of CEPM (0°, 15°, 30°, 45°, 60°, 75°, 90°). EPM: external permanent magnet; IPM: ingested permanent magnet.
printer and had a cuboid profile (140 3 120 3 90 mm3) with a curved hole across it. The section of the porcine intestine with less grease inside was selected to insert in the model hole whose diameter is approximately 36 mm. The porcine intestine was inflated and then checked to be smooth and not winding after the MCE was inserted. Two-end orifices of the intestine were folded to keep the gas inside. The MCE can be imagenavigated from one side of the model to the other side.
Discussion Analyses of computational curves for magnetic link between IPM and EPM The curves for the angle relationship, force, and torque between the IPM and EPMs provide theoretical support for the proposed control strategy. They are also
useful for maneuvering the MCE. Some data (e.g. an EPM partition angle of 80° for the two different control methods) were written into the control routine, whereas other data (e.g. 60–80 mm as the effective dragging distances) were directly used in magnetic maneuvering through the HMI. The following are some necessary analyses of the curves in Figures 7–9. From Figure 8, the curve with a 75° angle for the EPMs obviously has a lower force than the others (except the 90° curve). The curves for the larger EPM angles have rapidly decreasing forces, which indicate that the control method should be changed. From Figure 7(a), when the EPM rotation angle is 80°, the corresponding MCE angle is approximately 70°, and the MCE nearly points to the EPMs. Thus, 80° is suggested as the partition angle of the two different control methods. From Figure 8, when CEPM is in the range of 0°–80°, at a distance of approximately 60 mm, the forces are
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Figure 10. Simulated intestinal model and proposed system: (a) simple model, (b) complex model, and (c) proposed system. CE: capsule endoscope; EPM: external permanent magnet; MCE: magnetic capsule endoscope; HMI: human–machine interface.
almost at their maximums. From Figure 9(b), when CEPM is 60° or 75°, and the EPM moving distance is 60 mm, the torque exerted on the MCE is approximately 1.15 mN m or 1.07 mN m, respectively, which would probably be large enough to prevent the MCE from maintaining its original orientation. However, from Figure 7(b), the corresponding variation in the MCE’s orientation is approximately 4.8° or 6° when the EPMs move from 0 to 60 mm. This indicates that the EPM movement has little influence on the MCE orientation, and the MCE orientation adjustment can be provided by the EPM rotation. When the EPMs continue to move another 20 mm, to a total distance of 80 mm, the forces exerted on the MCE remain at a high level. However, the MCE orientation will have a large variation of 16.5° when CEPM is 75°, and larger direction adjustment will be needed for the subsequent maneuvers. When CEPM is around 90°, if the
force is not large enough, the MCE will be allowed to move naturally with peristalsis. If the force at a moving distance of 60 mm is not large enough to drive the MCE, a larger moving distance will still not have an effect. Thus, the minimum among all the maximum forces with different values of CEPM (except 90°) in Figure 8 can be used as the final active force to resist the total frictional force exerted on the MCE. That is, the effective active force is approximately 255 mN at an angle of 75° and an EPM moving distance of 60 mm.
Possible combinations of IPM and EPM Although the proposed system provides a feasible control mechanism for the active maneuvering of an IPM/ MCE using two EPMs, several other IPM and EPM combinations are also available. Three indices can be
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0.48
Axial Axial Diac Dia Axial Axial Dia Dia Dia Dia Dia Dia 1 2 3 4 5 6 7 8 9 10 11 12
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IPM: ingested permanent magnet; DOF: degree of freedom; EPM: external permanent magnet. a Torque is calculated when there is a distance of 150 mm and an angle misalignment of 3° between the IPM and EPM (cEPM = 0). b Force is calculated when there is a lateral distance of 150 mm between the IPM (cIPM = 0) and EPM (cEPM = 0) and an axial distance varying from 0 to 80 mm; the column ‘‘Dist.,’’ together with the column ‘‘Max.,’’ indicates that at that distance the axial force reaches that maximum value; the column ‘‘0 mm’’ lists the lateral force when the moving distance of EPM is 0 mm; and the column ‘‘80 mm’’ lists the lateral force when the moving distance of EPM is 80 mm. c ‘‘Dia’’ is the short form for diametrical, which means the described magnet is magnetized diametrically. d ‘‘5(1)’’ indicates that there are five useful DOF for the IPM, and one DOF is induced by the rotation of EPM’s main axis (roll). e Roll of IPM is not a useful DOF.
89
2 156
150 70 mm
5 214 89 110 71 96 153 198 299 212 403 192
Roll of IPM
50
217
140 50 mm The same as 1 100 60 mm 125 60 mm 140 60 mm 59 60 mm 187 60 mm 57 70 mm The same as 4 104 60 mm 0.49 0.51 0.41 0.49 0.87 0.50 1.07 0.48 0.46 0.54 Yaw of IPM (cIPM) Yaw of IPM Yaw of IPM Yaw of IPM Yaw of IPM Yaw of IPM Yaw of IPM Yaw of IPM 3 3
5 3 Pitch of IPM (u) 5 3 Pitch of IPM d 5(1) Pitch of IPM 3 5(1) Pitch of IPM 3 4 3 Roll of IPMe 4 3 Roll of IPMe 4 Roll of IPMe 3 4 Roll of IPMe 3 5(1) Yaw of IPM Pitch of IPM 5(1) Yaw of IPM Pitch of IPM This one is not a valid combination 4(1) Yaw of IPM 3 Left–right Up–down Left–right Up–down Left–right Up–down Left–right Up–down Left–right Up–down Left–right Up–down
0 mmb Max.
Dist. Yaw Pitch Roll
Rotation of EPM Useful DOF Relative bearing Magnetized direction of IPM Magnetized direction of EPM No.
Table 1. EPM and IPM arrangements and combinations.
used to compare these combinations. The first is the magnetic force acting on the MCE. This force should have a relatively large axial component to drive the MCE and a relatively small lateral component acting on GI tissue. The second is the magnetic torque acting on the MCE. A slight EPM rotation angle should induce a torque with less amplitude fluctuation and a relatively consistent IPM rotation angle. The third is the EPM installation. Two rotations always require a more complex mechanism than one rotation. Rotation about the main axis requires a much simpler mechanism than rotation about either of the other two. If the EPM(s) and IPM have the same dimensions and magnetizations as those used in the proposed system, there are 6 arrangements and combinations of one EPM and one IPM for a dexterous mechanism (Figure 12), and 12 arrangements and combinations of one EPM and one IPM for both vertically placed and horizontally placed mechanisms (Table 1). If treating as vertically placed combination, Figure 12(a)–(f) corresponds to the even groups in Table 1. Groups 1–8 are the basic EPM and IPM arrangements and combinations. Groups 9–12 are the variants of Groups 3, 4, 7, and 8, respectively. The variation of groups 9–12 involves achieving the IPM yaw by the rolling of EPM, which will probably simplify the clamping mechanism. Groups with pitch and yaw control capability for an IPM (i.e. Groups 1–4 and 9 and 10) are suitable for accurate orientation control of an IPM, whereas the other groups are suitable for the control strategy proposed in this article. Note that the torque and force of the even groups with the EPM and IPM placed horizontally vary during the EPM rotation, and the torques of these groups in Table 1 are almost at their upper limits. To achieve accurate IPM orientation control, the combinations with a diametrically magnetized EPM and axially magnetized IPM (Groups 3, 4, 9, and 10) may be better choices for the compromise between the
Axial Axial Axial Axial Dia Dia Dia Dia Axial Axial Dia Dia
Lateral
b
b
Axial
Forceb (mN) Torquea (Yaw, mN m)
Figure 11. Inflated porcine intestine across light-tight model for ex-vivo experiment.
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Figure 12. Arrangements and combinations of one EPM and one IPM for dexterous mechanism. (a)–(f) correspond to Groups 2, 4, 6, 8, 10, and 12 in Table 1, respectively. OI and OE are depicted in Figure 4. EPM: external permanent magnet; MCE: magnetic capsule endoscope.
torque, axial force, lateral force, and simplification of mechanism. This was the combination employed by Ciuti et al.12 For the control strategy proposed in this article, Group 5 or 7 may be better choices. Compared with maneuvering using one EPM, maneuvering with two EPMs can generate a much larger torque, more effective axial force, and smaller lateral force on the intestinal wall during most of the maneuvering time.
Conclusion
selection basis of the proposed control strategy. Simulated intestine experiments demonstrated the feasibility of this magnetic maneuvering system using a direction reference for image navigation. An examination of different configurations for the EPM(s) and magnetic shell suggested the use of magnets for providing a direction reference for magnetic maneuvering. Future work will be devoted to making better use of the advantage of providing this direction reference with a specific magnet configuration to improve our MCE. Acknowledgements
A direction reference technique for image navigation was proposed using a specific configuration of an IPM/ MCE and two EPMs. An image-navigated control strategy was analyzed, and only two parameters were found to be necessary for magnetically maneuvering an MCE with the 4-DOF external robots. The computations of the angle, force, and torque relationship between the magnetic shell and two EPMs were conducted as the theoretical background and parameter
The authors thank Wes Jones for his kind provision of the code package ‘‘CDirectoryChangeWatcher’’ on CodeProject. The authors also thank the reviewers for their valuable comments and suggestions on the improvement of this article. Declaration of conflicting interests The authors declare that there is no conflict of interest.
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Funding This work was supported by the National High Technology Research and Development Program of China (863) (no. 2008AA04Z313). References 1. Iddan G, Meron G, Glukhovsky A, et al. Wireless capsule endoscopy. Nature 2000; 405: 417. 2. Van Gossum A, Munoz-Navas M, Fernandez-Urien I, et al. Capsule endoscopy versus colonoscopy for the detection of polyps and cancer. N Engl J Med 2009; 361(3): 264–270. 3. Fisher R and Hasler L.New vision in video capsule endoscopy: current status and future directions. Nat Rev Gastroenterol Hepatol 2012; 9(7): 392–405. 4. Sendoh M, Ishiyama K and Arai I. Fabrication of magnetic actuator for use in a capsule endoscope. IEEE T Magn 2003; 39: 3232–3234. 5. Quirini M, Menciassi A, Scapellato S, et al. Design and fabrication of a motor legged capsule for the active exploration of the gastrointestinal tract. IEEE: ASME T Mech 2008; 13(2): 169–179. 6. Valdastri P, Webster J, Quaglia C, et al. A new mechanism for mesoscale legged locomotion in compliant tubular environments. IEEE T Robot 2009; 25(5): 1047–1057. 7. Park H, Park S, Yoon E, et al. Paddling based microrobot for capsule endoscopes. In: Proceedings of the IEEE international conference on robotics and automation, Roma, 10–14 April 2007, pp.3377–3382. New York: IEEE. 8. Dario P, Ciarletta P, Menciassi A, et al. Modeling and experimental validation of the locomotion of endoscopic robots in the colon. Int J Robot Res 2004; 23: 549–556. 9. Wang X and Meng Q-H. An inchworm-like locomotion mechanism based on magnetic actuator for active capsule endoscope. In: Proceedings of the IEEE/RSJ international conference on intelligent robots and systems, Beijing, China, 9–15 October 2006, pp.1267–1272. New York: IEEE. 10. Keller J, Fibbe C, Volke F, et al. Inspection of the human stomach using remote-controlled capsule endoscopy: a feasibility study in healthy volunteers (with videos). Gastrointest Endosc 2011; 73(1): 22–28. 11. Carpi F and Pappone C. Magnetic maneuvering of endoscopic capsules by means of a robotic navigation system. IEEE Trans Biomed Eng 2009; 56(5): 1482–1490. 12. Ciuti G, Valdastri P, Menciassi A, et al. Robotic magnetic steering and locomotion of capsule endoscope for diagnostic and surgical endoluminal procedures. Robotica 2010; 28: 199–207. 13. Gao M, Hu C, Chen Z, et al. Design and fabrication of a magnetic propulsion system for self-propelled capsule endoscope. IEEE Trans Biomed Eng 2010; 57(12): 2891– 2902.
14. Lien G-S, Liu C-W, Jiang J-A, et al. Magnetic control system targeted for capsule endoscopic operations in the stomach—design, fabrication, and in vitro and ex vivo evaluations. IEEE Trans Biomed Eng 2012; 59(7): 2068– 2079. 15. Carpi F, Kastelein N, Talcott M, et al. Magnetically controllable gastrointestinal steering of video capsules. IEEE Trans Biomed Eng 2011; 58(2): 231–234. 16. Ojala T, PietikaE`inen M and MaE`enpaE`a T. Multiresolution gray-scale and rotation invariant texture classification with local binary patterns. IEEE T Pattern Anal 2002; 24(7): 971–987. 17. Vilarin˜o F, Spyridonos P, De Iorio F, et al. Intestinal motility assessment with video capsule endoscopy: automatic annotation of phasic intestinal contractions. IEEE Trans Med Imaging 2010; 29(2): 246–259. 18. Jiang H, Zhang L, Zhang C, et al. Wireless switch for implantable medical devices based on passive RF receiver. Electron lett 2008; 44(17): 1006–1007. 19. Sun Z, Ye B, Qiu Y, et al. Preliminary study of a legged capsule robot actuated wirelessly by magnetic torque. IEEE T Magn, in press.
Appendix 1 Notation Variables ! F
! H EPM ! M IPM T Dc DcIPM, MCE
cEPM, cMCE
magnetic force acting on ingested permanent magnet (IPM) by external permanent magnets (EPMs) (N) magnetic field strength generated by two EPMs (A/m) magnetization of IPM (A/m) magnetic torque acting on IPM by EPMs (N m) yaw step angle (°) ! misalignment angle between M IPM and the changed magnetic field ! H EPM 0 (°) rotation angle of EPM, MCE (°)
Definitions of coordinate systems (XC, YC, ZC) (XI, YI, ZI) (XM, YM, ZM) (XW, YW, ZW)
CE/MCE/IPM’s coordinate system image coordinate system EPM’s coordinate system world coordinate system
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Design and implementation of magnetically maneuverable capsule endoscope system with direction reference for image navigation
Proc IMechE Part H: J Engineering in Medicine 2014, Vol. 228(7) 652–664 Ó IMechE 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0954411914540876 pih.sagepub.com
Zhen-Jun Sun1,2, Bo Ye1, Yi Sun2, Hong-Hai Zhang1,2 and Sheng Liu1,2
Abstract This article describes a novel magnetically maneuverable capsule endoscope system with direction reference for image navigation. This direction reference was employed by utilizing a specific magnet configuration between a pair of external permanent magnets and a magnetic shell coated on the external capsule endoscope surface. A pair of customized Cartesian robots, each with only 4 degrees of freedom, was built to hold the external permanent magnets as their endeffectors. These robots, together with their external permanent magnets, were placed on two opposite sides of a ‘‘patient bed.’’ Because of the optimized configuration based on magnetic analysis between the external permanent magnets and the magnetic shell, a simplified control strategy was proposed, and only two parameters, yaw step angle and moving step, were necessary for the employed robotic system. Step-by-step experiments demonstrated that the proposed system is capable of magnetically maneuvering the capsule endoscope while providing direction reference for image navigation.
Keywords Capsule endoscope, configuration optimization, direction reference, image navigation, magnetic maneuvering
Date received: 11 June 2013; accepted: 29 May 2014
Introduction As a less invasive and more comfortable method, compared with traditional endoscopy, capsule endoscopy has been gradually used to explore the gastrointestinal (GI) tract over the past decade.1,2 Current designs of capsule endoscope (CE) integrate function modules such as illumination, image capture, wireless data transceiver, and power supply modules. Thus, it can automatically acquire images from the inspected GI tract and wirelessly send data to an external memory when moving by means of visceral peristalsis after being ingested. The images can be viewed in a real-time mode or off-line mode depending on whether the working external memory for image storage is in a workstation or portable recorder. Although CE has gained wide acceptance, it suffers from certain limitations.3 Among these limitations, the walking ability, including locomotion and navigation, is a basic issue and may be solved first. For locomotion, various methods have been employed, including the spiral-type capsule,4 motor-legged capsule robot,5–7
worm-like capsule robot,8,9 and magnetic maneuvering capsule.10–14 Compared with other methods, magnetic maneuvering seems to be a more promising method because of its wireless operation capability, without the need for an extra power supply. Keller et al.10 used a handheld external permanent magnet (EPM) to steer a magnetic capsule endoscope (MCE) in the exploration of human stomach. Carpi and colleagues11,15 employed a robotic magnetic navigation system to maneuver an MCE coated with a magnetic shell. A digital fluoroscope with a threedimensional (3D) localization error of 1 mm was 1
School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan, China 2 National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, China Corresponding author: Sheng Liu, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, F101, Wuhan 430074, Hubei, China. Email: [email protected]
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employed to provide feedback for the active maneuvering of the MCE. Ciuti et al.12 employed another commercially available robot to hold an EPM. The robot had 6 degrees of freedom (DOFs). The EPM had a cylindrical shape and was magnetized perpendicularly to its main axis, whereas four small cylindrical and symmetrically arranged permanent magnets were imbedded in the swallowed CE and magnetized axially. Both vision feedback and inertial feedback were used to provide useful information during the maneuvering. For navigation, it is natural to employ images from CE because this makes possible the full use of what the CE provides. Although image-navigated magnetic maneuvering has been mentioned by Keller et al.10 and Carpi et al.,15 no details have been provided. For image navigation, direction reference is necessary and important as it is important in some other work related to image processing16,17 in which the lack of direction reference increased the difficulty of the algorithm. This study simply employed a reasonable magnet configuration between two EPMs and the magnetic shell of the MCE, without any embedded sensor, for direction reference. To the best of our knowledge, this is the first time that a method for providing direction reference through a reasonable magnet configuration has been explicitly proposed in the CE field. This specific magnet configuration allows an image-navigated control strategy to be proposed. An extensive magnetic analysis was carried out qualitatively and quantitatively to support the control strategy and optimize the control parameters and the structure of external robotic arms. Step-by-step, in-vitro experiments validated the effectiveness of the image-navigated magnetic maneuvering system.
System design System overview The proposed system, schematically represented in Figure 1, is mainly composed of a capsule device and Cartesian robotic subsystem. The two Cartesian robotic arms of the robotic subsystem are placed across each other on two lateral sides of the ‘‘patient bed.’’ Each robotic arm clamps a cylindrical EPM as its end-effector. A CE coated with a magnetic shell/ingested permanent magnet (IPM) on the intermediate part of its external surface is called a MCE. The horizontal layout of two axially magnetized EPMs with the diametrically magnetized IPM between them provides the MCE direction reference for image navigation. This layout also makes it feasible to magnetically maneuver the MCE in 3D space using the 4DOF Cartesian robotic arms. With the direction reference, images taken by the MCE can be intuitively and conveniently used for direction recognition. According to these images, the operator sets control parameters through a human–machine interface (HMI) to maneuver the EPMs with the robotic arms, and finally drives the MCE to implement an inspection of the GI tract.
EPMs and IPM. The EPMs and IPM are all made of Nd–Fe–B and are cylindrical, with a hole in the center. Each EPM is mounted on a robotic arm as its endeffector, whereas the IPM is coated on the external surface of the CE. Each EPM is magnetized axially and has an external diameter of 100 mm, an internal diameter of 30 mm, a height of 55 mm, a weight of 4.1 kg, and a magnetization of 820 kA/m, whereas the IPM is magnetized diametrically and has an external diameter of 16.5 mm, an internal diameter of 13.5 mm, a height of 16.5 mm, a weight of 8.9 g, and a magnetization of 850 kA/m. Capsule device. The capsule device (Hangzhou Hitron Technologies Co., Ltd, China) consists of CEs, an antenna array, a data receiver, and a workstation with dedicated software. A CE has a diameter of 13.4 mm, length of 28 mm, and weight of 4.3 g. An off-line mode and real-time mode can both be accessed at a high image resolution of 480 dpi 3 480 dpi or a low resolution of 240 dpi 3 240 dpi. A CE can last more than 8 h while operating at the low resolution and at 2 FPS. This device also takes advantage of the ability to wirelessly turn a CE on or off.18 Cartesian robotic subsystem. The Cartesian robotic subsystem consists of two Cartesian robotic arms and their servo control routine. Figure 2 shows the defined coordinate system (CS). Here, (XM, YM, ZM) is fixed on the EPM, and (XC, YC, ZC) is fixed on the MCE. The corresponding axes of these two CSs are parallel at their initial state. (XW, YW, ZW) is the world CS, which is the same as the initial state of (XM, YM, ZM). Initially, YM is parallel to the length of the stationary patient bed, ZM is along the vertical line, and the origin OM is the geometric center of the EPM. Each robotic arm has 4 DOFs: three translational movements along XW, YW, and ZW; and one rotation about ZW. The strokes in the XW, YW, and ZW directions are approximately 220 mm, 490 mm, and 185 mm, respectively.
Control strategy based on image navigation A control strategy based on image navigation will be more reliable with the participation of the operator who inputs the commands after reading the images during the maneuvering. However, the images from a common CE are not suitable for operator’s direction recognition because of the rolling of the CE and the lack of a direction reference. The employment of a localization system11 or an embedded inertial sensor12 to provide absolute orientation and position feedback is an effective way to provide direction reference. However, because the control of the MCE is actually the control of its position relative to the inspected intestinal wall, absolute feedback methods can just act as a complement to a control method based on image navigation. While images from a CE provide this kind
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Figure 1. Schematic of the proposed system. USB: universal serial bus; DOF: degree of freedom; EPM: external permanent magnet; MCE: magnetic capsule endoscope; LED: light-emitting diode; IPM: ingested permanent magnet.
of relative position feedback, a direction reference for direction recognition can be acquired by exploiting the EPM and IPM layout, which has the advantage of not requiring an extra embedded electronic device for navigation.
Direction reference. In Figure 2, EPMs are placed with their main axes XM in the horizontal plane. When the diametrically magnetized IPM is placed between these two EPMs, its pole direction will also be in the horizontal plane. If the EPMs rotate about their individual ZM axes simultaneously, the IPM will rotate accordingly, while its pole direction remains in the horizontal plane. Thus, the IPM will not roll if the EPM pole directions are maintained in the horizontal plane, and the IPM
pole direction can be used as the direction reference. When the external surface of a CE is coated with an IPM, the horizontal line (XI in Figure 3(b)) of images from CE shall accord with the IPM pole direction. This CE and IPM assembly (i.e. MCE) provides CE’s camera with a direction reference for the image-navigated control strategy.
Principle of image-navigated control strategy. A magnetically maneuvered CE should always be adjusted to align with the surrounding tissues and then advance a small distance, as depicted in Figure 3. In a typical GI image captured by a CE, there will normally be features (e.g. dark area or bubbles) indicating where the exit of the intestinal lumen is located. Adjusting the MCE to point
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Figure 2. Definition of coordinate systems for EPM and MCE and diametrically magnetized direction (adapted from Sun et al.19). EPM: external permanent magnet; MCE: magnetic capsule endoscope.
Figure 3. Image-navigated control strategy: (a) top-level control strategy and (b) schematic of control based on image navigation.
at this exit will align it with the surrounding tissues and facilitate the subsequent movement. Repeating these procedures for adjusting the direction and moving forward will allow the MCE to explore the entire intestinal tract if it does not become stuck. Control parameters and DOFs of robotic arm of proposed system. While the MCE is under control, the (XC, YC, ZC) coordinates fixed on the MCE continuously change. Because the operator stands in the world CS, the images captured in the CS of the CE should be transformed to the world CS to correctly recognize the direction with the help of a localization device. While the MCE is under control, the operating commands should be transformed back to the CS of the CE because what should be adjusted is the position and orientation of the CE relative to the inspected intestinal wall. These make the robotic control relatively complex. The employment of a diametrically magnetized IPM, placed between two axially magnetized EPMs, prevents
the MCE from rotating about its main axis and provides its images with direction reference. Three advantages come consequently. First, because of the direction reference, the moving direction of the MCE’s next step can be directly observed from GI images. Next, the transformation between CSs will be easier, which simplifies the control algorithm for the external robot(s). Third, this specific layout loses control of the MCE rotation about XC and releases the MCE constraint in the vertical plane. That is, the pitch angle Du is no longer under control, and movement in the YCOCZC plane is free. This makes the MCE more compliant and less invasive in the GI tract and reduces its possibility of getting stuck. This also allows the MCE to be navigated in 3D space using the two-dimensional (2D) images of the proposed MCE. Thus, the control parameters are the yaw step angle Dc and moving step d. Because the EPM pitch must be fixed to acquire the direction reference, and the EPM roll has no influence on the MCE, the necessary DOFs for one robotic arm shrink to four, which are one
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rotation about ZM and three translations along XW, YW, and ZW.
Magnetic maneuvering of MCE When the two EPMs with different magnetic poles face each other, the main axis of the diametrically magnetized IPM/MCE located between the EPMs will be aligned parallel to the EPM radial direction. This means the MCE can freely pitch in the vertical plane, and the MCE will finally align itself with the surrounding intestinal lumen in the vertical plane. In this case, the movement of the EPMs along the length direction of the patient bed can drag the MCE forward (Figure 4(a)). As the EPMs rotate counterclockwise, the MCE will rotate clockwise (Figure 4(b)). After the direction adjustment, the movement of the EPMs in the direction parallel to the main axis of the MCE can drag the MCE to move along its main axis, which is actually not a wise strategy because of the need for a large stroke in the XW direction and the possibility of interference between the EPMs and the inspected patient. An improved control strategy fixes EPMs in the nearest XW direction to the MCE and requires the EPM movements only in the YW and ZW directions (Figure 4(c)). An exception occurs in a situation where the MCE axis nearly points toward the EPMs (70° \ CMCE 4 110° or 250° \ CMCE 4 290°). In these cases, if only the magnetic force exists, the MCE is out of control and moves toward the closer EPM. When the EPM on the side of the desired movement direction is further from the MCE, the other EPM should move further from the MCE to drive it in the right direction (Figure 4(d)). By inputting the two parameters through the HMI according to the images from the MCE, the operator can continuously control the robotic system to change the MCE position and orientation and finish the diagnosis. During maneuvering, the directions of the two control parameters are directly set according to the images from the MCE, whereas the magnitudes can be approximately set in advance according to the forces and torques computed in the following section. Sometimes, disabling or changing the step values of rotation Dc or forward movement d is necessary, which is mainly dependent on the specific situation.
Magnetic link between EPMs and MCE Analytical background Suppose the magnetizations of the IPM/MCE and two EPMs are all rigid and uniform. According to Kelvin’s ! formula, the incremental magnetic force d F exerted by the EPMs on an infinitesimal element (dV) of the IPM can be given by ! ! ! dF = m0 r MIPM HEPM
ð1Þ
!
where MIPM is the magnetization of the IPM, and * HEPM is the magnetic field strength generated by the EPMs. The total force can be calculated as follows Z Z ! ! ! ! F ¼ dFdV ¼ m0 ðMIPM HEPM ÞdV ð2Þ V
Using CE’s CS (XC, YC, ZC), we have !
ð3Þ
MIPM = MIPM eYC !
HEPM ¼ HEPMXC eXC þ HEPMYC eYC þ HEPMZC eZC
ð4Þ
Suppose the IPM always aligns its magnetization direction with the magnetic field generated by the EPMs. Then, equation (4) can be written as !
ð5Þ
HEPM = HEPMYc eYc
and equation (2) can be re-written as !
ð
F=
m0 MIPM HEPMYC dV’
V
m0 VIPM MIPM rHEPMYC
ð6Þ
When the IPM aligns with the magnetic field of the EPMs, there is no torque between the EPMs and IPM. When the EPMs rotate by a small angle DCEPM (Figure 3), a misalignment angle DCIPM between the IPM and the changed magnetic field is consequently ! generated. Suppose HEPM 0 is the new magnetic field. The torque on the IPM exerted by the EPMs can be approximately calculated as follows !
!
0 T = m0 VIPM MIPM 3HEPM = m0 VIPM ðMIPM eYC Þ
3ðHEPM 0 eYC 0 Þ = m0 VIPM MIPM HEPM 0 sinðDCIPM Þ
ð7Þ
According to section ‘‘Control parameters and DOFs of robotic arm of proposed system,’’ we can simply treat the control of the proposed system as an equivalent control problem in the horizontal plane. The results of the experiment in section ‘‘Magnetic maneuvering of MCE based on given model with dedicated codes’’ confirmed the feasibility of this simplification. In the following section, the computation will simply be conducted in the horizontal plane.
Magnetic parameters for both EPM and IPM The magnetic flux densities (B) of the EPMs and IPM were all measured with a Gauss meter. COMSOL was used to conduct a finite element analysis of B to match the measured data (Figure 5). The magnetizations of the EPMs and IPM are 820 and 850 kA/m, respectively.
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Figure 4. Strategy of magnetic maneuvering: (a) supposed initial position, (b) rotation control, (c) translation control (0 \ CEPM 4 80°) and (d) translation control (80° \ CEPM 4 90°). EPM: external permanent magnet; MCE: magnetic capsule endoscope.
the symmetry of its solved magnetic field. Thus, the two symmetric points, OCL0 and OCR0 , have the same magnetic fields, both in amplitude and direction. Then, the magnetic field where the MCE is located is equivalent to twice the magnetic field on point OCL0 or point OCR0 , which can be conveniently solved by a 2D symmetric model in COMSOL.
Relationship between rotation angle of EPMs and corresponding angle of MCE Figure 5. Magnetic flux densities (B) of both EPM and IPM. The EPM curves are the B values along its axial direction. The IPM curves show the B values in the radial direction through the geometric center of IPM, where the curves in dotted lines are B values along the pole direction and the other two are B values on the N–S interface. EPM: external permanent magnet; IPM: ingested permanent magnet.
Simplified computation of magnetic field generated by two EPMs Suppose Figure 4(a) is the initial position of the MCE and two EPMs, and the MCE is always located in the middle of the EPMs. When the EPMs rotate at an angle of CEPM clockwise, the relative positions of MCE to the left and right EPMs are, respectively, on point OCL0 and point OCR0 in Figure 6, if the two rotating EPMs are treated as one stationary EPM. Thus, the magnetic field generated by the EPMs at the MCE position is equivalent to the sum of the magnetic fields on point OCL0 and point OCR0 . These two points are symmetrical about the geometric center of the stationary EPM. When the EPMs rotate clockwise by CEPM in a real situation, line OCL0 OCR0 will rotate counterclockwise by CEPM. The geometric symmetry of EPM shall lead to
Suppose the MCE is in the middle of the EPMs. The nearest distance (dEPM-MCE in Figure 6) between one EPM and the MCE is set to be 150 mm. Figure 7 depicts the corresponding relationship between the rotation angle of the IPM/MCE (CMCE) and the rotation angle of the EPMs (CEPM is in the range of 0°–90°) when dEPM-MCE is 150, 180, or 200 mm. When CEPM is in the range of 290° to 0°, the relationship between CMCE and CEPM is symmetrical about point (0°, 0°), with the curves shown in Figure 7. When CEPM is in the range of 90°–180°, the relationship between CMCE and CEPM is symmetrical about point (90°, 90°), with the curves shown in Figure 7. Suppose the MCE is initially in the middle of the EPMs, which have an angle of CEPM and begin to move along the length direction of the patient bed to drag the MCE. Because of the insufficient force at the beginning, the MCE will probably not be able to move and will just rotate about ZC. In these cases, the relationship between CMCE and the moving distance is depicted in Figure 7(b) when the EPMs maintain an angle of CEPM.
Magnetic force According to the proposed control strategy, when CEPM is in the range of 0°–80°, the EPMs only move
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Figure 6. Principle of simplified computation of magnetic field HEPM generated by two EPMs. EPM: external permanent magnet; MCE: magnetic capsule endoscope.
along the length direction of the patient bed to drag the MCE. When dEPM-MCE is 150 mm and the MCE cannot move but can just rotate about ZC, the approximate force acting on the MCE by the movement of the EPMs maintaining a certain rotation angle of CEPM is calculated using equation (6) and depicted in Figure 8. When CEPM is in the range of 80°–90°, the EPM whose position is opposite to the moving direction of the MCE shall move away from the patient bed in the transverse direction to correctly drive the MCE. In this case, as an example, the force acting on the MCE/IPM when CEPM is 90° is plotted in Figure 8 with an unmarked line.
Torque exerted on MCE by two EPMs When placed a certain distance away from each other, with the MCE/IPM located in their middle, the EPMs rotate to turn the MCE around. At the beginning of this control, the torque will probably be insufficient to overcome the applied force of the surrounding GI tract. When the EPMs keep a certain angle of CEPM and the rotation of the MCE lags by an angle of 3°, 6°, or 9°, the approximate torque acting on the MCE can be calculated using equation (7) and is shown in Figure 9(a). Figure 9(b) depicts the approximate torque exerted on the MCE/IPM when the EPMs with a distance of 300 mm move to drag the MCE which remains stationary.
Experiments and result Magnetic maneuvering of MCE based on given model with dedicated codes An experiment was first conducted in transparent polyvinyl chloride (PVC) tubes with both simple and complex shapes (Figure 10(a) and (b)). The capsule device was not involved. Instead, an IPM-coated CE shell without electronic circuits was used. A dedicated
routine for the corresponding model was employed to verify the MCE locomotion capability when maneuvered by the proposed system (Figure 10(c)). The simple model was placed horizontally and employed to help validate the translation and rotation in the horizontal plane, whereas it was placed vertically to validate the flexibility in the vertical plane. Its centerline is a planar line. And it had a tube diameter of 25 mm, total length of 440 mm, and minimum circle radius of 70 mm. It was fixed on a wooden base and placed in the middle of the EPMs, with its length direction parallel to the length direction of the patient bed. Two EPMs were set 300 mm apart from each other and had the same center height as the model. For the horizontally placed model, the dedicated codes considered the model geometry. Once the robotic system began to run, it automatically drove the IPM to the end of its journey in the model. It took the IPM approximately 230 s to travel in this model, with a running speed of approximately 2 mm/s. A similar experiment was also performed with this model inside a porcine intestine and showed similar results. For the vertically placed model, the IPM can be driven by translation of the EPMs only in the YW direction, with little effect from the model geometry, which indicated the MCE flexibility in the vertical plane. Then, a complex model (its centerline is a spatial line) with a length of 260 mm, width of 120 mm, maximum height of 80 mm, and minimum circle radius of 30 mm was employed. It took approximately 7 min to move through this complex model at a speed of approximately 2 mm/s.
Magnetic maneuvering of MCE with direction reference for image navigation In this experiment, the simple model was wrapped in a light-tight plastic bag, and a real MCE was used. Every
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Figure 7. Angle of MCE. (a) CEPM versus CMCE. The three curves are for different dEPM-MCE (150, 180, and 200 mm). (b) dlMCE (depicted in Figure 6) versus CMCE when EPMs maintain an angle of CEPM (0°, 15°, 30°, 45°, 60°, 75°, 90°).
Figure 8. Distance versus force acting on MCE/IPM when EPMs with rotation angle of CEPM (0°, 15°, 30°, 45°, 60°, 75°, 90°) move distance of dlMCE (depicted in Figure 6). EPM: external permanent magnet; IPM: ingested permanent magnet.
step of the maneuvering was manually controlled by an operator using a mouse and dedicated HMI according to the images from the MCE. The amplitudes of the two main control parameters of the EPMs, rotation step Dc and moving step d, were set in advance at 3° and 5 mm, respectively. Sometimes, the rotation or forward movement step was disabled or changed according to the specific situation. The time required for this experiment varied from 400 to 600 s at a speed of approximately 2 mm/s. An ex-vivo experiment was also performed inside an inflated porcine intestine (Figure 11) which we think is a better choice to mimic the intestine in a live body, compared to a collapsed intestine. The porcine intestine was cleaned with the mixture of saline water and vinegar and then washed by running water. The other simple model was used to accommodate part of the porcine intestine to make it light-tight. It was produced by a 3D
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Figure 9. Torque on MCE/IPM. (a) CEPM (CMCE lags 3°, 6°, and 9°) versus torque acting on MCE/IPM. The curves marked with the same symbol in a decreasing sequence of values correspond to a distance sequence of 150, 180, and 200 mm between the EPM and MCE and (b) dlMCE versus torque acting on MCE/IPM when EPMs keep a rotation angle of CEPM (0°, 15°, 30°, 45°, 60°, 75°, 90°). EPM: external permanent magnet; IPM: ingested permanent magnet.
printer and had a cuboid profile (140 3 120 3 90 mm3) with a curved hole across it. The section of the porcine intestine with less grease inside was selected to insert in the model hole whose diameter is approximately 36 mm. The porcine intestine was inflated and then checked to be smooth and not winding after the MCE was inserted. Two-end orifices of the intestine were folded to keep the gas inside. The MCE can be imagenavigated from one side of the model to the other side.
Discussion Analyses of computational curves for magnetic link between IPM and EPM The curves for the angle relationship, force, and torque between the IPM and EPMs provide theoretical support for the proposed control strategy. They are also
useful for maneuvering the MCE. Some data (e.g. an EPM partition angle of 80° for the two different control methods) were written into the control routine, whereas other data (e.g. 60–80 mm as the effective dragging distances) were directly used in magnetic maneuvering through the HMI. The following are some necessary analyses of the curves in Figures 7–9. From Figure 8, the curve with a 75° angle for the EPMs obviously has a lower force than the others (except the 90° curve). The curves for the larger EPM angles have rapidly decreasing forces, which indicate that the control method should be changed. From Figure 7(a), when the EPM rotation angle is 80°, the corresponding MCE angle is approximately 70°, and the MCE nearly points to the EPMs. Thus, 80° is suggested as the partition angle of the two different control methods. From Figure 8, when CEPM is in the range of 0°–80°, at a distance of approximately 60 mm, the forces are
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Figure 10. Simulated intestinal model and proposed system: (a) simple model, (b) complex model, and (c) proposed system. CE: capsule endoscope; EPM: external permanent magnet; MCE: magnetic capsule endoscope; HMI: human–machine interface.
almost at their maximums. From Figure 9(b), when CEPM is 60° or 75°, and the EPM moving distance is 60 mm, the torque exerted on the MCE is approximately 1.15 mN m or 1.07 mN m, respectively, which would probably be large enough to prevent the MCE from maintaining its original orientation. However, from Figure 7(b), the corresponding variation in the MCE’s orientation is approximately 4.8° or 6° when the EPMs move from 0 to 60 mm. This indicates that the EPM movement has little influence on the MCE orientation, and the MCE orientation adjustment can be provided by the EPM rotation. When the EPMs continue to move another 20 mm, to a total distance of 80 mm, the forces exerted on the MCE remain at a high level. However, the MCE orientation will have a large variation of 16.5° when CEPM is 75°, and larger direction adjustment will be needed for the subsequent maneuvers. When CEPM is around 90°, if the
force is not large enough, the MCE will be allowed to move naturally with peristalsis. If the force at a moving distance of 60 mm is not large enough to drive the MCE, a larger moving distance will still not have an effect. Thus, the minimum among all the maximum forces with different values of CEPM (except 90°) in Figure 8 can be used as the final active force to resist the total frictional force exerted on the MCE. That is, the effective active force is approximately 255 mN at an angle of 75° and an EPM moving distance of 60 mm.
Possible combinations of IPM and EPM Although the proposed system provides a feasible control mechanism for the active maneuvering of an IPM/ MCE using two EPMs, several other IPM and EPM combinations are also available. Three indices can be
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0.48
Axial Axial Diac Dia Axial Axial Dia Dia Dia Dia Dia Dia 1 2 3 4 5 6 7 8 9 10 11 12
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IPM: ingested permanent magnet; DOF: degree of freedom; EPM: external permanent magnet. a Torque is calculated when there is a distance of 150 mm and an angle misalignment of 3° between the IPM and EPM (cEPM = 0). b Force is calculated when there is a lateral distance of 150 mm between the IPM (cIPM = 0) and EPM (cEPM = 0) and an axial distance varying from 0 to 80 mm; the column ‘‘Dist.,’’ together with the column ‘‘Max.,’’ indicates that at that distance the axial force reaches that maximum value; the column ‘‘0 mm’’ lists the lateral force when the moving distance of EPM is 0 mm; and the column ‘‘80 mm’’ lists the lateral force when the moving distance of EPM is 80 mm. c ‘‘Dia’’ is the short form for diametrical, which means the described magnet is magnetized diametrically. d ‘‘5(1)’’ indicates that there are five useful DOF for the IPM, and one DOF is induced by the rotation of EPM’s main axis (roll). e Roll of IPM is not a useful DOF.
89
2 156
150 70 mm
5 214 89 110 71 96 153 198 299 212 403 192
Roll of IPM
50
217
140 50 mm The same as 1 100 60 mm 125 60 mm 140 60 mm 59 60 mm 187 60 mm 57 70 mm The same as 4 104 60 mm 0.49 0.51 0.41 0.49 0.87 0.50 1.07 0.48 0.46 0.54 Yaw of IPM (cIPM) Yaw of IPM Yaw of IPM Yaw of IPM Yaw of IPM Yaw of IPM Yaw of IPM Yaw of IPM 3 3
5 3 Pitch of IPM (u) 5 3 Pitch of IPM d 5(1) Pitch of IPM 3 5(1) Pitch of IPM 3 4 3 Roll of IPMe 4 3 Roll of IPMe 4 Roll of IPMe 3 4 Roll of IPMe 3 5(1) Yaw of IPM Pitch of IPM 5(1) Yaw of IPM Pitch of IPM This one is not a valid combination 4(1) Yaw of IPM 3 Left–right Up–down Left–right Up–down Left–right Up–down Left–right Up–down Left–right Up–down Left–right Up–down
0 mmb Max.
Dist. Yaw Pitch Roll
Rotation of EPM Useful DOF Relative bearing Magnetized direction of IPM Magnetized direction of EPM No.
Table 1. EPM and IPM arrangements and combinations.
used to compare these combinations. The first is the magnetic force acting on the MCE. This force should have a relatively large axial component to drive the MCE and a relatively small lateral component acting on GI tissue. The second is the magnetic torque acting on the MCE. A slight EPM rotation angle should induce a torque with less amplitude fluctuation and a relatively consistent IPM rotation angle. The third is the EPM installation. Two rotations always require a more complex mechanism than one rotation. Rotation about the main axis requires a much simpler mechanism than rotation about either of the other two. If the EPM(s) and IPM have the same dimensions and magnetizations as those used in the proposed system, there are 6 arrangements and combinations of one EPM and one IPM for a dexterous mechanism (Figure 12), and 12 arrangements and combinations of one EPM and one IPM for both vertically placed and horizontally placed mechanisms (Table 1). If treating as vertically placed combination, Figure 12(a)–(f) corresponds to the even groups in Table 1. Groups 1–8 are the basic EPM and IPM arrangements and combinations. Groups 9–12 are the variants of Groups 3, 4, 7, and 8, respectively. The variation of groups 9–12 involves achieving the IPM yaw by the rolling of EPM, which will probably simplify the clamping mechanism. Groups with pitch and yaw control capability for an IPM (i.e. Groups 1–4 and 9 and 10) are suitable for accurate orientation control of an IPM, whereas the other groups are suitable for the control strategy proposed in this article. Note that the torque and force of the even groups with the EPM and IPM placed horizontally vary during the EPM rotation, and the torques of these groups in Table 1 are almost at their upper limits. To achieve accurate IPM orientation control, the combinations with a diametrically magnetized EPM and axially magnetized IPM (Groups 3, 4, 9, and 10) may be better choices for the compromise between the
Axial Axial Axial Axial Dia Dia Dia Dia Axial Axial Dia Dia
Lateral
b
b
Axial
Forceb (mN) Torquea (Yaw, mN m)
Figure 11. Inflated porcine intestine across light-tight model for ex-vivo experiment.
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Figure 12. Arrangements and combinations of one EPM and one IPM for dexterous mechanism. (a)–(f) correspond to Groups 2, 4, 6, 8, 10, and 12 in Table 1, respectively. OI and OE are depicted in Figure 4. EPM: external permanent magnet; MCE: magnetic capsule endoscope.
torque, axial force, lateral force, and simplification of mechanism. This was the combination employed by Ciuti et al.12 For the control strategy proposed in this article, Group 5 or 7 may be better choices. Compared with maneuvering using one EPM, maneuvering with two EPMs can generate a much larger torque, more effective axial force, and smaller lateral force on the intestinal wall during most of the maneuvering time.
Conclusion
selection basis of the proposed control strategy. Simulated intestine experiments demonstrated the feasibility of this magnetic maneuvering system using a direction reference for image navigation. An examination of different configurations for the EPM(s) and magnetic shell suggested the use of magnets for providing a direction reference for magnetic maneuvering. Future work will be devoted to making better use of the advantage of providing this direction reference with a specific magnet configuration to improve our MCE. Acknowledgements
A direction reference technique for image navigation was proposed using a specific configuration of an IPM/ MCE and two EPMs. An image-navigated control strategy was analyzed, and only two parameters were found to be necessary for magnetically maneuvering an MCE with the 4-DOF external robots. The computations of the angle, force, and torque relationship between the magnetic shell and two EPMs were conducted as the theoretical background and parameter
The authors thank Wes Jones for his kind provision of the code package ‘‘CDirectoryChangeWatcher’’ on CodeProject. The authors also thank the reviewers for their valuable comments and suggestions on the improvement of this article. Declaration of conflicting interests The authors declare that there is no conflict of interest.
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Funding This work was supported by the National High Technology Research and Development Program of China (863) (no. 2008AA04Z313). References 1. Iddan G, Meron G, Glukhovsky A, et al. Wireless capsule endoscopy. Nature 2000; 405: 417. 2. Van Gossum A, Munoz-Navas M, Fernandez-Urien I, et al. Capsule endoscopy versus colonoscopy for the detection of polyps and cancer. N Engl J Med 2009; 361(3): 264–270. 3. Fisher R and Hasler L.New vision in video capsule endoscopy: current status and future directions. Nat Rev Gastroenterol Hepatol 2012; 9(7): 392–405. 4. Sendoh M, Ishiyama K and Arai I. Fabrication of magnetic actuator for use in a capsule endoscope. IEEE T Magn 2003; 39: 3232–3234. 5. Quirini M, Menciassi A, Scapellato S, et al. Design and fabrication of a motor legged capsule for the active exploration of the gastrointestinal tract. IEEE: ASME T Mech 2008; 13(2): 169–179. 6. Valdastri P, Webster J, Quaglia C, et al. A new mechanism for mesoscale legged locomotion in compliant tubular environments. IEEE T Robot 2009; 25(5): 1047–1057. 7. Park H, Park S, Yoon E, et al. Paddling based microrobot for capsule endoscopes. In: Proceedings of the IEEE international conference on robotics and automation, Roma, 10–14 April 2007, pp.3377–3382. New York: IEEE. 8. Dario P, Ciarletta P, Menciassi A, et al. Modeling and experimental validation of the locomotion of endoscopic robots in the colon. Int J Robot Res 2004; 23: 549–556. 9. Wang X and Meng Q-H. An inchworm-like locomotion mechanism based on magnetic actuator for active capsule endoscope. In: Proceedings of the IEEE/RSJ international conference on intelligent robots and systems, Beijing, China, 9–15 October 2006, pp.1267–1272. New York: IEEE. 10. Keller J, Fibbe C, Volke F, et al. Inspection of the human stomach using remote-controlled capsule endoscopy: a feasibility study in healthy volunteers (with videos). Gastrointest Endosc 2011; 73(1): 22–28. 11. Carpi F and Pappone C. Magnetic maneuvering of endoscopic capsules by means of a robotic navigation system. IEEE Trans Biomed Eng 2009; 56(5): 1482–1490. 12. Ciuti G, Valdastri P, Menciassi A, et al. Robotic magnetic steering and locomotion of capsule endoscope for diagnostic and surgical endoluminal procedures. Robotica 2010; 28: 199–207. 13. Gao M, Hu C, Chen Z, et al. Design and fabrication of a magnetic propulsion system for self-propelled capsule endoscope. IEEE Trans Biomed Eng 2010; 57(12): 2891– 2902.
14. Lien G-S, Liu C-W, Jiang J-A, et al. Magnetic control system targeted for capsule endoscopic operations in the stomach—design, fabrication, and in vitro and ex vivo evaluations. IEEE Trans Biomed Eng 2012; 59(7): 2068– 2079. 15. Carpi F, Kastelein N, Talcott M, et al. Magnetically controllable gastrointestinal steering of video capsules. IEEE Trans Biomed Eng 2011; 58(2): 231–234. 16. Ojala T, PietikaE`inen M and MaE`enpaE`a T. Multiresolution gray-scale and rotation invariant texture classification with local binary patterns. IEEE T Pattern Anal 2002; 24(7): 971–987. 17. Vilarin˜o F, Spyridonos P, De Iorio F, et al. Intestinal motility assessment with video capsule endoscopy: automatic annotation of phasic intestinal contractions. IEEE Trans Med Imaging 2010; 29(2): 246–259. 18. Jiang H, Zhang L, Zhang C, et al. Wireless switch for implantable medical devices based on passive RF receiver. Electron lett 2008; 44(17): 1006–1007. 19. Sun Z, Ye B, Qiu Y, et al. Preliminary study of a legged capsule robot actuated wirelessly by magnetic torque. IEEE T Magn, in press.
Appendix 1 Notation Variables ! F
! H EPM ! M IPM T Dc DcIPM, MCE
cEPM, cMCE
magnetic force acting on ingested permanent magnet (IPM) by external permanent magnets (EPMs) (N) magnetic field strength generated by two EPMs (A/m) magnetization of IPM (A/m) magnetic torque acting on IPM by EPMs (N m) yaw step angle (°) ! misalignment angle between M IPM and the changed magnetic field ! H EPM 0 (°) rotation angle of EPM, MCE (°)
Definitions of coordinate systems (XC, YC, ZC) (XI, YI, ZI) (XM, YM, ZM) (XW, YW, ZW)
CE/MCE/IPM’s coordinate system image coordinate system EPM’s coordinate system world coordinate system
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