Ultrasound
Acoustical Holography: Physical Parameters and Potential Clinical Applications 1 Alexander Flllmonoy, Ph.D., Joseph J. Gerdes, M.D., and Richard L. Clark, M.D. Acoustical holography achieves real-time imaging of bodily structures through ultrasound. The fundamentals of acoustical holography and a description of a prototype unit undergoing trials at the authors' institution are presented. Physical parameters and means of calibrating the acoustic beam are discussed, and results of preliminary experimental and clinical studies reviewed. Acoustical holography has the potential for providing complementary diagnostic information which, after further technical developments, may furnish clinically useful information. INDEX TERMS:
Holography, acoustical • Ultrasound, apparatus and equipment
Radiology 114:687-691, March 1975
of acoustical (or ultrasonic) holography have been known for some time (1, 2) but, until recently, the potential usefulness of this diagnostic modality in clinical medicine has been largely speculative (3). Recent work by Holbrooke and associates (4) and by Anderson and Curtin (5) demonstrated impressive theoretical resolution capabilities of acoustical holography in homogeneous phantoms, and fair visualization of anatomical detail in excised biological specimens. The results of these investigations and our own initial unpublished observations suggested that with the prototype units being currently evaluated in the field, in vivo applications would be limited. Nevertheless, because acoustical holography permits real-time imaging without ionizing radiation, its potential as a medical diagnostic tool should not be ignored. Therefore, our ener-
gies were· initially directed toward studying the physical parameters of the acoustic beam and analyzing tissue energy deposition at currently employed diagnostic energy levels. In addition, a series of in vivo laboratory investigations in rabbits was conducted which suggest a potential clinical application of acoustical holography in the diagnosis and management of tendon injury and other soft-tissue extremity lesions.
T
HE PRINOIPLES
MACHINE DESCRIPTION AND PHYSICAL PARAMETERS
The essential elements of the acoustical holography unit2 undergoing trials at the University of North Carolina Medical School are presented in Figure 1. The unit may be operated at ultrasound frequencies of 1, 3, and 5 megahertz, which are produced when a 10 X 10 cm 2
TV CAMERA POLAROID CAMERA
OBJECT TRANSDUCER OPTIC LENS COUPLING SURFACE
HOLOGRAPHIC SURFACE
SOUND REFLECTOR
REFERENCE TRANSDUCER SOUND REFLECTOR
COUPL ING MODULE \ Fig. 1.
HOLOGRAPHIC MODULE
Schematic representation of acoustical holography unit.
1 From the Department of Radiology, University of North Carolina School of Medicine, Chapel Hill, N. C. Presented in part at the Work in Progress session of the Fifty-ninth Scientific Assembly and Annual Meeting of the Radiological Society of North America, Chicago, 111., Nov. 25-30, 1973. shan 2 Model 300, Prototype, 1973, Holosonics, Inc., Richland, Wash.
687
A. FILIMONOV, J. J. GERDES AND R. L. CLARK
688
Table I:
March 1975
Reported Insonification Safety Levels
Parameter
Curtis (15)
Taylor & Pond (16)
Weiss & Holyoke (17)
Frequency Pulse length Duty factor Peak intensity Average intensity
1 MHzt 106 Its** 0.1 18 wjcm 2 1.8 wjcm 2
0.5,1,2,6 MHz 104 itS 0.1 56 wjcm 2 5.6 wjcm 2
9 MHz Continuous 1.0 25 wjcm 2 2.5 wjcm 2
Acoustical Holography* 3 MHz 380 itS 0.02 7.5 wjcm 2 0.15 wjcm 2
* These intensities represent the maximum capability of the unit. Actual average imaging intensities are on the order of 10 mWatts/cm 2• t Megahertz ** Microseconds quartz crystal is set into vibration by periodic voltage pulses. The ultrasound beam is pulsed every 16.67 milliseconds with pulse widths ranging from 38 to 380 microseconds. The ultrasound beam does not propagate in air, thus requiring a coupling medium, usually water, between the source of ultrasound and the object to be viewed. This coupling is achieved directly by immersion of the object and transducer in a water bath, or indirectly by placing the object between two water-filled rubber bags coated with an acoustic coupling agent. The realtime image is formed by transmission of the ultrasound beam through the object. The transmission beam is modified in amplitude and phase, reflected, focused by two liquid lenses, and reflected again toward the surface of the imaging tank. At this point, the object beam interferes with an ultrasonic beam coming from a reference source, which is in phase with the object beam transducer. This interaction produces a characteristic ripple pattern or hologram which is read by a 10-milliwatt argon ion laser. The reflected laser light, containing the image information, is passed through a spatial filter which selects the first-order diffracted light coming from the surface of the liquid. Finally,oa beam splitter divides the light output between a TV camera and a groundglass screen or Polaroid camera. The acoustical beam power may be measured by a precision balance, since ultrasound exerts a tangible force on a given surface (6-9). Calorimetric techniques (10-12) may also be used since a certain portion of the beam energy is absorbed and produces a detectable temperature rise. Although various other methods, such as those depending on optical, chemical, and piezoelectric effects can also be used to measure ultrasonic power levels, the radiation force method, in our experience, appears to be the most easily adapted for use under clinical conditions. This method has the advantage of not requiring sophisticated equipment or special calibrations and, in addition, has acceptable accuracies (± 10%). The geometric distribution of the beam, as well as the intensities produced by the large transducer used with the holography unit are treated in a separate paper (13). However, the measurements do show that imaging of thick body parts at 3 megahertz may require intensity levels close to 100 milliwatts per cm 2 • It is appropriate to compare the characteristics of this acoustical holography unit with the tentative safety
levels reported in the literature (TABLE I). The acoustical intensities required for imaging by the unit under study are well below currently accepted maximum dosages. However, it is evident from a study of the literature that the precise interactions of tissue and ultrasound are not completely understood, and we are thus unable to imply with absolute certainty that currently accepted safety levels are truly "safe." The resolution capabilities of acoustical holography have been recently reviewed by Anderson and Curtin (5) and have an important bearing on the selection of the appropriate imaging frequency. Considering the practical requirements of resolution and penetration, one can conclude that the 3 megahertz frequency provides the best compromise while still operating well within currently accepted safety margins. At 1 megahertz, although transmission is excellent, resolution is unacceptable. At 5 megahertz, the reverse holds, and maximum machine power is required for viewing small body parts. EXPERIMENTAL AND PRELIMINARY CLINICAL APPLICATIONS
Several modifications of the Holosonics Model 300 prototype acoustical imager were made for the sake of better imaging and ease of examination. The unit ordinarily provides acoustical coupling of the subject to the machine by placing the part to be studied between two water-filled neoprene rubber bags. Water, mineral oil, or a commercially available acoustic coupling agent such as Aquasonic 1003 , diluted with water, is used to eliminate minute air gaps. However, rotation of the subject between the water bags tends to disturb the acoustical coupling and degrade the image. For this reason, the lower water bag was replaced with a Plexiglas water bath. Two sizes of tanks were used. The smaller unit, measuring 45 X 60 X 15 cm was useful in imaging rabbits and is also suitable for infants. A larger tank, me.?suring 70 X 125 X 17 cm provided a comfortable way to examine small children. A standard mixing valve and drain were added to the system providing temperatureregulated continuously circulating water. Recently, a modification of the upper coupling device, consisting of the attachment of a water-filled 20-cm extension cylinder between the transducer and the neoprene bag, has 3
Parker Laboratories, Irvington, N. J. 07111.
689
ACOUSTICAL HOLOGRAPHY
Vol. 114
Ultrasound
=
Fig. 2. A. Acoustical holographic image of the distal portion of the hind leg of a normal rabbit: large solid arrow anterior muscle bundle ; small solid arrow tibia; curved open arrow gastrocnemius muscle ; large open arrow Achilles tendon. Band C. Two images of a partial Achilles tendon injury in a rabbit. Note the slight irregularity of the tendon indicated by the arrow.
=
=
allowed us greater versatility in examining the larger irregular body contours of adults without the need for immersion . An in vivo study of tendon and muscle injuries in rabbits was conducted to assess the potential clinical application of acoustical holography in the diagnosis of softtissue injuries . An example of a typical acoustical hologram of a normal shaved rabbit leg is shown in Figure 2, A. Two types of injuries were studied: Achilles tendon divisions and chemical myositis. Two animals had partial tendon divisions and six had complete tendon divisions. Two of the latter group underwent bilateral Achilles tendon sectioning . Approximately 50 % of the fibers were severed in those tendons with partial lacerations. When viewed in real time, and with previous knowledge of the exact site of injury, an abnormality consisting of a small notch at the point of tendon injury was noted. In the static Polaroid images (Fig. 2, B and C) without the benefit of movement and manipulation , such a lesion is somewhat difficult to perceive. An example of a com-
=
plete tendon laceration is shown in Figure 3, A . With the foot dorsiflexed, the gap between the tendon ends widens (Fig. 3, B). After one week, considerable retraction of the tendon ends occurs (Fig. 3, C). Several attempts at surgical tendon repair were unsuccessful because of the animals persistent ability to remove or damage leg casts. The chemical injuries were introduced in six animals by intramuscular injection of varying concentrations of either hydrochloric acid or trichloracetic acid into the anterior compartment of the rabbit's hind leg: 0.1 ml of 0.1 N hydrochloric acid produced clinically evident local inflammation. Similar changes were produced with 0 .1 ml of 12 % or 25 % trichloracetic acid . With one exception, acoustical holography failed to disclose any abnormality, apart from increase in size (due to edema) of the anterior compartment, when compared with the uninjected hind leg. In one rabbit, injected with 0.1 ml of 25 % trichloracetic acid, an area of induration was noted in addition to the inflammatory edema . The
Fig. 3. Complete Achilles tendon laceration . A. With plantar flexion, the severed tendon ends (arrows) approximate each other. B. With dorsiflexion the severed tendon ends separate. C. After one week, there is considerable retraction of the tendon ends.
690
A. FILIMONOV, J. J. GERDES AND R. L. CLARK
Fig. 4. Muscle injury. The arrow indicates a trichloracetic acid-induced walled-off necrotic area in the anterior muscle compartment of the lower portion of a rabbit's hind leg.
acoustical hologram disclosed an apparent cavity just under the skin, at the site of the palpable induration (Fig. 4). Surgical exploration confirmed the presence of a cavity containing a small amount of liquefied necrotic material. To gain experience with holographic images of normal human anatomy and to assess the ability of the available acoustical holography unit to display soft-tissue structures in various parts of the body, examinations were performed on normal infants, children, and adults . The details of these studies will form the basis of a subsequent report, but in general, anatomical detail with this prototype was considered acceptable only in upper and lower extremity studies. DISCUSSION It now appears likely that some form of real-time ultrasonic imaging will eventually become a clinical reality. Liquid surface acoustical holography as utilized in the system we are evaluating is but one of several emerging techniques of through-transmission ultrasonic imaging. A review of these alternative methods is beyond the scope of this paper, but the reader is referred to an excellent review of ultrasonic image detection systems by Berger (17). Other acoustical holographic techniques are discussed in detail by Hildebrand and Brenden (18). The major problems which currently prevent the widespread application of liquid surface acoustical ho-
March 1975
lography to medical diagnosis are both technical and theoretical in nature. The current prototype equipment is awkward to use in a clinical setting. It is difficult to ensure adequate coupling of the patient to the transducer with the present neoprene water bag system. In addition, not all patients can be appropriately examined in an immersion water bath. Our experience with abdominal and thoracic examinations has been uniformly disappointing. Nevertheless. in vivo gallbladder imaging has been recently demonstrated (19) utilizing a newer, technically updated version of the Model 300. It is evident that further equipment development and patient handling research is necessary before full determination of abdominal and thoracic imaging capability can be evaluated. It should be emphasized , however, that the ability to visualize a structure by acoustical holography does not always imply clinical usefulness, particularly when other more convenient and equally safe diagnostic modalities are available. When used in industrial applications for nondestructive testing, flaws as small as 0.5 mm can be detected in homogeneous materials . This impressive capability cannot even be approached in man, because of the marked inhomogeneity of living tissue. Every muscle bundle, facial plane, vascular structure and tendon in a relatively thin structure such as the forearm contribute to beam modulation by the edge phenomena of diffraction and reflection, rather than by simple attenuation (such as in radiography). The resulting images have an incredible amount of excess information which must be analyzed. Even with the ability to " focus" on a O.5-cm tissue plane, distorted information from in front of, or behind the plane of interest continues to degrade the image. We are hopeful. however, that some of these technical problems will be improved, resulting in an improved signal-to-noise ratio. In certain clinical situations, we feel that the potential of acoustical holography as a useful diagnostic tool should be explored. As we have demonstrated in animal work, there may well be an application for acoustical holography in the evaluation of tendon, muscle, and other soft-tissue injuries, particularly in extremities . The close proximity of many potential lesions to bone will provide some additional reflection problems, but the capability of real-time imaging in various planes and projections may provide practical clinical information in the near future. Our present clinical investigations are directed towards the evaluation of tendon injury repair and tendon graft healing in the distal portions of the extremities. Initially, it was felt that congenital hip dysplasia might be evaluated by acoustical holography, since cartilage is relatively sonolucent, and repeated radiographic exposure to infants is undesirable. Our preliminary experience has been disappointing, but further work is indicated in this area, as well as with other pediatric orthopedic problems. Another area of possible usefulness might be the
Vol. 114
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ACOUSTICAL HOLOGRAPHY
evaluation of deep venous structures in the extremities. With current equipment, superficial veins can be visualized, and it has been demonstrated in vitro that clotted and non-clotted blood give different images (20). Careful study of the extremities and hand reveals that pulsating structures can be visualized. To date however, it has been difficult to precisely localize arteries because the pulsations are transmitted to numerous adjacent structures. The possibility of utilizing acoustical holography to visualize the developing fetus in utero has been well recognized but until recently because of certain technical problems, in vivo studies have been disappointing. However, one is able in selective cases to observe fetal outlines and motion in the third trimester of pregnancy (19). Further work is indicated, and a study to evaluate fetal activity, in particular, respiratory movement, is underway. We are optimistic that certain new engineering and electronic modifications may substantially improve the resolution capabilities of the present unit. However, it should be emphasized that conventional pulse echo techniques currently provide a more reliable means of assessing fetal size, overall shape, and number. It has been shown (19) that the use of frequencies swept over a range of 1-5 megahertz has improved the images by partially averaging out the speckle formed by the coherent sound waves. The use of grids in the imaging tank will improve the response of the imaging system at low spatial frequencies. In addition, the elimination of a reference transducer by the grid will remove some of the restrictions on the placement of the lenses and object due to the necessity of maintaining a correct angular displacement between object and reference source. Energy determinations have convinced us that when current equipment is operated at 3 megahertz on thin body parts such as the extremities, the maximum energy deposition falls far below currently accepted permissible dosages. Nevertheless, as we have emphasized earlier, the ultimate safety of ultrasound in general and acoustical holography in particular is an area of continued debate. For this reason, the widespread application of acoustical holography to thicker body parts, which require greater acoustic energy levels for adequate transmission, seems undesirable at this time. Finally, it is evident that being able to evaluate acoustical energy conveniently in any clinical setting is highly advisable. The methods mentioned in this paper [described in' greater detail elsewhere (13)] have proved convenient and accurate and thus deserve further application. ACKNOWLEDGMENTS: We wish to thank Frank Miller, R. T., Terry Walser and B. G. Thompson for their excellent technical
Ultrasound
and engineering help. The capable secretarial assistance of De Lois Popp and the photographic capabilities of Robert Strain are also appreciated. The continued support of Dr. James Scatliff, Chairman of the Department of Radiology, is greatly appreciated. Finally, we wish to thank Holosonics, Inc., Richland, Wash. for their advice and continuing interest in our work.
Richard L. Clark, M.D. Department of Radiology University of North Carolina School of Medicine Chap~H~N.C.
27514
REFERENCES 1. Brendon BB: A comparison of acoustical holographic methods. [In] Acoustical Holography. New York, Plenum Press. Vol 1, 1969, pp 57-71 2. Metherell AF: Acoustical holography. Sci Amer 221:36-44, Oct 1969 3. Sikov MR, Pimmel RL, Reich FR, et al: Visualization of soft tissue by ultrasonic holography. Radiology 102:191-192, Jan 1972 4. Holbrooke DR, McCurry EM, Richards V, et al: Acoustical holography for surgical diagnosis. Ann Surg 178:547-558, Oct 1973 5. Anderson RE, Curtin HR: Ultrasonic holography: a promising medical imaging tool. Radiology 109:417-421, Nov 1973 6. Huetter TF, Bolt RH: Sonics. New York, John Wiley, 1955, pp 43-48 7. Fry WJ, Dunn F: Ultrasound: analysis and experimental methods in biological research. [In] Physical Techniques in Biological Research, Mastuk WL, ed. New York, Academic Press, 1962, Vol 4, pp 348-355 8. Kossoff G: Calibration of ultrasonic therapeutic equipment. Acustica 12.2:84-92, 1962 9. Hill CR: Calibration of ultrasonic beams for bio-medical applications. Phys Med Bioi 15:241-248, Apr 1970 10. Fry WJ, Dunn F: Physical Techniques in Biological Research, Mastuk WL, ed. New York, Academic Press, 1962, Vol 4, pp
261-394 11. Fry WJ, Fry RB: Determination of absolute sound levels and acoustical absorption coefficients by thermocouple probes-s-theory. J Acoust Soc Amer 26:294-310, 1954 12. Sokollu A: On the measurement of ultrasound intensity in tissue: reservations on radiation force and the case of calorimetry. [In] Interaction of Ultrasound and Biological Tissues. Workshop Proceedings, U. S. Dept. of HEW, 1972, pp 155-170 13. Filimonov AB: Characteristics and dosimetry of an acoustical holography unit. Med Phys (to be published) 14. Curtis JC: Action of intense ultrasound on intact liver. [In] Ultrasonic Energy, Kelly E, ed. Illinois, Urbana, 1965, pp 85-116 15. Taylor KJW, Pond J: The effects of ultrasound of varying frequencies on rat liver. J Path 100:287-293, Apr 1970 16. Weiss L, Holyoke ED: Detection of tumors in soft tissues by ultrasonic holography. Surg Gynec Obstet 128:953-962, May 1969 17. Berger H: A survey of ultrasonic image detection methods. [In] Acoustical Holography, Metherell AF, ed. New York, Plenum Press, 1969, Vol 1, pp 27-48 18. Hildebrand BP, Brenden BS: An Introduction to Acoustical Holography. New York, Plenum Press, 1972 19. Deichman J [Holosonics, Inc., Richland, Wash]: Personal communication 20. Anderson RE: Potential medical applications for ultrasonic holography. [In] Acoustical Holography, Green PS, ed. New York, Plenum Press, 1974, Vol 5, pp 505-513
Acoustical Holography: Physical Parameters and Potential Clinical Applications 1 Alexander Flllmonoy, Ph.D., Joseph J. Gerdes, M.D., and Richard L. Clark, M.D. Acoustical holography achieves real-time imaging of bodily structures through ultrasound. The fundamentals of acoustical holography and a description of a prototype unit undergoing trials at the authors' institution are presented. Physical parameters and means of calibrating the acoustic beam are discussed, and results of preliminary experimental and clinical studies reviewed. Acoustical holography has the potential for providing complementary diagnostic information which, after further technical developments, may furnish clinically useful information. INDEX TERMS:
Holography, acoustical • Ultrasound, apparatus and equipment
Radiology 114:687-691, March 1975
of acoustical (or ultrasonic) holography have been known for some time (1, 2) but, until recently, the potential usefulness of this diagnostic modality in clinical medicine has been largely speculative (3). Recent work by Holbrooke and associates (4) and by Anderson and Curtin (5) demonstrated impressive theoretical resolution capabilities of acoustical holography in homogeneous phantoms, and fair visualization of anatomical detail in excised biological specimens. The results of these investigations and our own initial unpublished observations suggested that with the prototype units being currently evaluated in the field, in vivo applications would be limited. Nevertheless, because acoustical holography permits real-time imaging without ionizing radiation, its potential as a medical diagnostic tool should not be ignored. Therefore, our ener-
gies were· initially directed toward studying the physical parameters of the acoustic beam and analyzing tissue energy deposition at currently employed diagnostic energy levels. In addition, a series of in vivo laboratory investigations in rabbits was conducted which suggest a potential clinical application of acoustical holography in the diagnosis and management of tendon injury and other soft-tissue extremity lesions.
T
HE PRINOIPLES
MACHINE DESCRIPTION AND PHYSICAL PARAMETERS
The essential elements of the acoustical holography unit2 undergoing trials at the University of North Carolina Medical School are presented in Figure 1. The unit may be operated at ultrasound frequencies of 1, 3, and 5 megahertz, which are produced when a 10 X 10 cm 2
TV CAMERA POLAROID CAMERA
OBJECT TRANSDUCER OPTIC LENS COUPLING SURFACE
HOLOGRAPHIC SURFACE
SOUND REFLECTOR
REFERENCE TRANSDUCER SOUND REFLECTOR
COUPL ING MODULE \ Fig. 1.
HOLOGRAPHIC MODULE
Schematic representation of acoustical holography unit.
1 From the Department of Radiology, University of North Carolina School of Medicine, Chapel Hill, N. C. Presented in part at the Work in Progress session of the Fifty-ninth Scientific Assembly and Annual Meeting of the Radiological Society of North America, Chicago, 111., Nov. 25-30, 1973. shan 2 Model 300, Prototype, 1973, Holosonics, Inc., Richland, Wash.
687
A. FILIMONOV, J. J. GERDES AND R. L. CLARK
688
Table I:
March 1975
Reported Insonification Safety Levels
Parameter
Curtis (15)
Taylor & Pond (16)
Weiss & Holyoke (17)
Frequency Pulse length Duty factor Peak intensity Average intensity
1 MHzt 106 Its** 0.1 18 wjcm 2 1.8 wjcm 2
0.5,1,2,6 MHz 104 itS 0.1 56 wjcm 2 5.6 wjcm 2
9 MHz Continuous 1.0 25 wjcm 2 2.5 wjcm 2
Acoustical Holography* 3 MHz 380 itS 0.02 7.5 wjcm 2 0.15 wjcm 2
* These intensities represent the maximum capability of the unit. Actual average imaging intensities are on the order of 10 mWatts/cm 2• t Megahertz ** Microseconds quartz crystal is set into vibration by periodic voltage pulses. The ultrasound beam is pulsed every 16.67 milliseconds with pulse widths ranging from 38 to 380 microseconds. The ultrasound beam does not propagate in air, thus requiring a coupling medium, usually water, between the source of ultrasound and the object to be viewed. This coupling is achieved directly by immersion of the object and transducer in a water bath, or indirectly by placing the object between two water-filled rubber bags coated with an acoustic coupling agent. The realtime image is formed by transmission of the ultrasound beam through the object. The transmission beam is modified in amplitude and phase, reflected, focused by two liquid lenses, and reflected again toward the surface of the imaging tank. At this point, the object beam interferes with an ultrasonic beam coming from a reference source, which is in phase with the object beam transducer. This interaction produces a characteristic ripple pattern or hologram which is read by a 10-milliwatt argon ion laser. The reflected laser light, containing the image information, is passed through a spatial filter which selects the first-order diffracted light coming from the surface of the liquid. Finally,oa beam splitter divides the light output between a TV camera and a groundglass screen or Polaroid camera. The acoustical beam power may be measured by a precision balance, since ultrasound exerts a tangible force on a given surface (6-9). Calorimetric techniques (10-12) may also be used since a certain portion of the beam energy is absorbed and produces a detectable temperature rise. Although various other methods, such as those depending on optical, chemical, and piezoelectric effects can also be used to measure ultrasonic power levels, the radiation force method, in our experience, appears to be the most easily adapted for use under clinical conditions. This method has the advantage of not requiring sophisticated equipment or special calibrations and, in addition, has acceptable accuracies (± 10%). The geometric distribution of the beam, as well as the intensities produced by the large transducer used with the holography unit are treated in a separate paper (13). However, the measurements do show that imaging of thick body parts at 3 megahertz may require intensity levels close to 100 milliwatts per cm 2 • It is appropriate to compare the characteristics of this acoustical holography unit with the tentative safety
levels reported in the literature (TABLE I). The acoustical intensities required for imaging by the unit under study are well below currently accepted maximum dosages. However, it is evident from a study of the literature that the precise interactions of tissue and ultrasound are not completely understood, and we are thus unable to imply with absolute certainty that currently accepted safety levels are truly "safe." The resolution capabilities of acoustical holography have been recently reviewed by Anderson and Curtin (5) and have an important bearing on the selection of the appropriate imaging frequency. Considering the practical requirements of resolution and penetration, one can conclude that the 3 megahertz frequency provides the best compromise while still operating well within currently accepted safety margins. At 1 megahertz, although transmission is excellent, resolution is unacceptable. At 5 megahertz, the reverse holds, and maximum machine power is required for viewing small body parts. EXPERIMENTAL AND PRELIMINARY CLINICAL APPLICATIONS
Several modifications of the Holosonics Model 300 prototype acoustical imager were made for the sake of better imaging and ease of examination. The unit ordinarily provides acoustical coupling of the subject to the machine by placing the part to be studied between two water-filled neoprene rubber bags. Water, mineral oil, or a commercially available acoustic coupling agent such as Aquasonic 1003 , diluted with water, is used to eliminate minute air gaps. However, rotation of the subject between the water bags tends to disturb the acoustical coupling and degrade the image. For this reason, the lower water bag was replaced with a Plexiglas water bath. Two sizes of tanks were used. The smaller unit, measuring 45 X 60 X 15 cm was useful in imaging rabbits and is also suitable for infants. A larger tank, me.?suring 70 X 125 X 17 cm provided a comfortable way to examine small children. A standard mixing valve and drain were added to the system providing temperatureregulated continuously circulating water. Recently, a modification of the upper coupling device, consisting of the attachment of a water-filled 20-cm extension cylinder between the transducer and the neoprene bag, has 3
Parker Laboratories, Irvington, N. J. 07111.
689
ACOUSTICAL HOLOGRAPHY
Vol. 114
Ultrasound
=
Fig. 2. A. Acoustical holographic image of the distal portion of the hind leg of a normal rabbit: large solid arrow anterior muscle bundle ; small solid arrow tibia; curved open arrow gastrocnemius muscle ; large open arrow Achilles tendon. Band C. Two images of a partial Achilles tendon injury in a rabbit. Note the slight irregularity of the tendon indicated by the arrow.
=
=
allowed us greater versatility in examining the larger irregular body contours of adults without the need for immersion . An in vivo study of tendon and muscle injuries in rabbits was conducted to assess the potential clinical application of acoustical holography in the diagnosis of softtissue injuries . An example of a typical acoustical hologram of a normal shaved rabbit leg is shown in Figure 2, A. Two types of injuries were studied: Achilles tendon divisions and chemical myositis. Two animals had partial tendon divisions and six had complete tendon divisions. Two of the latter group underwent bilateral Achilles tendon sectioning . Approximately 50 % of the fibers were severed in those tendons with partial lacerations. When viewed in real time, and with previous knowledge of the exact site of injury, an abnormality consisting of a small notch at the point of tendon injury was noted. In the static Polaroid images (Fig. 2, B and C) without the benefit of movement and manipulation , such a lesion is somewhat difficult to perceive. An example of a com-
=
plete tendon laceration is shown in Figure 3, A . With the foot dorsiflexed, the gap between the tendon ends widens (Fig. 3, B). After one week, considerable retraction of the tendon ends occurs (Fig. 3, C). Several attempts at surgical tendon repair were unsuccessful because of the animals persistent ability to remove or damage leg casts. The chemical injuries were introduced in six animals by intramuscular injection of varying concentrations of either hydrochloric acid or trichloracetic acid into the anterior compartment of the rabbit's hind leg: 0.1 ml of 0.1 N hydrochloric acid produced clinically evident local inflammation. Similar changes were produced with 0 .1 ml of 12 % or 25 % trichloracetic acid . With one exception, acoustical holography failed to disclose any abnormality, apart from increase in size (due to edema) of the anterior compartment, when compared with the uninjected hind leg. In one rabbit, injected with 0.1 ml of 25 % trichloracetic acid, an area of induration was noted in addition to the inflammatory edema . The
Fig. 3. Complete Achilles tendon laceration . A. With plantar flexion, the severed tendon ends (arrows) approximate each other. B. With dorsiflexion the severed tendon ends separate. C. After one week, there is considerable retraction of the tendon ends.
690
A. FILIMONOV, J. J. GERDES AND R. L. CLARK
Fig. 4. Muscle injury. The arrow indicates a trichloracetic acid-induced walled-off necrotic area in the anterior muscle compartment of the lower portion of a rabbit's hind leg.
acoustical hologram disclosed an apparent cavity just under the skin, at the site of the palpable induration (Fig. 4). Surgical exploration confirmed the presence of a cavity containing a small amount of liquefied necrotic material. To gain experience with holographic images of normal human anatomy and to assess the ability of the available acoustical holography unit to display soft-tissue structures in various parts of the body, examinations were performed on normal infants, children, and adults . The details of these studies will form the basis of a subsequent report, but in general, anatomical detail with this prototype was considered acceptable only in upper and lower extremity studies. DISCUSSION It now appears likely that some form of real-time ultrasonic imaging will eventually become a clinical reality. Liquid surface acoustical holography as utilized in the system we are evaluating is but one of several emerging techniques of through-transmission ultrasonic imaging. A review of these alternative methods is beyond the scope of this paper, but the reader is referred to an excellent review of ultrasonic image detection systems by Berger (17). Other acoustical holographic techniques are discussed in detail by Hildebrand and Brenden (18). The major problems which currently prevent the widespread application of liquid surface acoustical ho-
March 1975
lography to medical diagnosis are both technical and theoretical in nature. The current prototype equipment is awkward to use in a clinical setting. It is difficult to ensure adequate coupling of the patient to the transducer with the present neoprene water bag system. In addition, not all patients can be appropriately examined in an immersion water bath. Our experience with abdominal and thoracic examinations has been uniformly disappointing. Nevertheless. in vivo gallbladder imaging has been recently demonstrated (19) utilizing a newer, technically updated version of the Model 300. It is evident that further equipment development and patient handling research is necessary before full determination of abdominal and thoracic imaging capability can be evaluated. It should be emphasized , however, that the ability to visualize a structure by acoustical holography does not always imply clinical usefulness, particularly when other more convenient and equally safe diagnostic modalities are available. When used in industrial applications for nondestructive testing, flaws as small as 0.5 mm can be detected in homogeneous materials . This impressive capability cannot even be approached in man, because of the marked inhomogeneity of living tissue. Every muscle bundle, facial plane, vascular structure and tendon in a relatively thin structure such as the forearm contribute to beam modulation by the edge phenomena of diffraction and reflection, rather than by simple attenuation (such as in radiography). The resulting images have an incredible amount of excess information which must be analyzed. Even with the ability to " focus" on a O.5-cm tissue plane, distorted information from in front of, or behind the plane of interest continues to degrade the image. We are hopeful. however, that some of these technical problems will be improved, resulting in an improved signal-to-noise ratio. In certain clinical situations, we feel that the potential of acoustical holography as a useful diagnostic tool should be explored. As we have demonstrated in animal work, there may well be an application for acoustical holography in the evaluation of tendon, muscle, and other soft-tissue injuries, particularly in extremities . The close proximity of many potential lesions to bone will provide some additional reflection problems, but the capability of real-time imaging in various planes and projections may provide practical clinical information in the near future. Our present clinical investigations are directed towards the evaluation of tendon injury repair and tendon graft healing in the distal portions of the extremities. Initially, it was felt that congenital hip dysplasia might be evaluated by acoustical holography, since cartilage is relatively sonolucent, and repeated radiographic exposure to infants is undesirable. Our preliminary experience has been disappointing, but further work is indicated in this area, as well as with other pediatric orthopedic problems. Another area of possible usefulness might be the
Vol. 114
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ACOUSTICAL HOLOGRAPHY
evaluation of deep venous structures in the extremities. With current equipment, superficial veins can be visualized, and it has been demonstrated in vitro that clotted and non-clotted blood give different images (20). Careful study of the extremities and hand reveals that pulsating structures can be visualized. To date however, it has been difficult to precisely localize arteries because the pulsations are transmitted to numerous adjacent structures. The possibility of utilizing acoustical holography to visualize the developing fetus in utero has been well recognized but until recently because of certain technical problems, in vivo studies have been disappointing. However, one is able in selective cases to observe fetal outlines and motion in the third trimester of pregnancy (19). Further work is indicated, and a study to evaluate fetal activity, in particular, respiratory movement, is underway. We are optimistic that certain new engineering and electronic modifications may substantially improve the resolution capabilities of the present unit. However, it should be emphasized that conventional pulse echo techniques currently provide a more reliable means of assessing fetal size, overall shape, and number. It has been shown (19) that the use of frequencies swept over a range of 1-5 megahertz has improved the images by partially averaging out the speckle formed by the coherent sound waves. The use of grids in the imaging tank will improve the response of the imaging system at low spatial frequencies. In addition, the elimination of a reference transducer by the grid will remove some of the restrictions on the placement of the lenses and object due to the necessity of maintaining a correct angular displacement between object and reference source. Energy determinations have convinced us that when current equipment is operated at 3 megahertz on thin body parts such as the extremities, the maximum energy deposition falls far below currently accepted permissible dosages. Nevertheless, as we have emphasized earlier, the ultimate safety of ultrasound in general and acoustical holography in particular is an area of continued debate. For this reason, the widespread application of acoustical holography to thicker body parts, which require greater acoustic energy levels for adequate transmission, seems undesirable at this time. Finally, it is evident that being able to evaluate acoustical energy conveniently in any clinical setting is highly advisable. The methods mentioned in this paper [described in' greater detail elsewhere (13)] have proved convenient and accurate and thus deserve further application. ACKNOWLEDGMENTS: We wish to thank Frank Miller, R. T., Terry Walser and B. G. Thompson for their excellent technical
Ultrasound
and engineering help. The capable secretarial assistance of De Lois Popp and the photographic capabilities of Robert Strain are also appreciated. The continued support of Dr. James Scatliff, Chairman of the Department of Radiology, is greatly appreciated. Finally, we wish to thank Holosonics, Inc., Richland, Wash. for their advice and continuing interest in our work.
Richard L. Clark, M.D. Department of Radiology University of North Carolina School of Medicine Chap~H~N.C.
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REFERENCES 1. Brendon BB: A comparison of acoustical holographic methods. [In] Acoustical Holography. New York, Plenum Press. Vol 1, 1969, pp 57-71 2. Metherell AF: Acoustical holography. Sci Amer 221:36-44, Oct 1969 3. Sikov MR, Pimmel RL, Reich FR, et al: Visualization of soft tissue by ultrasonic holography. Radiology 102:191-192, Jan 1972 4. Holbrooke DR, McCurry EM, Richards V, et al: Acoustical holography for surgical diagnosis. Ann Surg 178:547-558, Oct 1973 5. Anderson RE, Curtin HR: Ultrasonic holography: a promising medical imaging tool. Radiology 109:417-421, Nov 1973 6. Huetter TF, Bolt RH: Sonics. New York, John Wiley, 1955, pp 43-48 7. Fry WJ, Dunn F: Ultrasound: analysis and experimental methods in biological research. [In] Physical Techniques in Biological Research, Mastuk WL, ed. New York, Academic Press, 1962, Vol 4, pp 348-355 8. Kossoff G: Calibration of ultrasonic therapeutic equipment. Acustica 12.2:84-92, 1962 9. Hill CR: Calibration of ultrasonic beams for bio-medical applications. Phys Med Bioi 15:241-248, Apr 1970 10. Fry WJ, Dunn F: Physical Techniques in Biological Research, Mastuk WL, ed. New York, Academic Press, 1962, Vol 4, pp
261-394 11. Fry WJ, Fry RB: Determination of absolute sound levels and acoustical absorption coefficients by thermocouple probes-s-theory. J Acoust Soc Amer 26:294-310, 1954 12. Sokollu A: On the measurement of ultrasound intensity in tissue: reservations on radiation force and the case of calorimetry. [In] Interaction of Ultrasound and Biological Tissues. Workshop Proceedings, U. S. Dept. of HEW, 1972, pp 155-170 13. Filimonov AB: Characteristics and dosimetry of an acoustical holography unit. Med Phys (to be published) 14. Curtis JC: Action of intense ultrasound on intact liver. [In] Ultrasonic Energy, Kelly E, ed. Illinois, Urbana, 1965, pp 85-116 15. Taylor KJW, Pond J: The effects of ultrasound of varying frequencies on rat liver. J Path 100:287-293, Apr 1970 16. Weiss L, Holyoke ED: Detection of tumors in soft tissues by ultrasonic holography. Surg Gynec Obstet 128:953-962, May 1969 17. Berger H: A survey of ultrasonic image detection methods. [In] Acoustical Holography, Metherell AF, ed. New York, Plenum Press, 1969, Vol 1, pp 27-48 18. Hildebrand BP, Brenden BS: An Introduction to Acoustical Holography. New York, Plenum Press, 1972 19. Deichman J [Holosonics, Inc., Richland, Wash]: Personal communication 20. Anderson RE: Potential medical applications for ultrasonic holography. [In] Acoustical Holography, Green PS, ed. New York, Plenum Press, 1974, Vol 5, pp 505-513
