Institute of Physics and Engineering in Medicine Phys. Med. Biol. 60 (2015) 4355–4370
Physics in Medicine & Biology doi:10.1088/0031-9155/60/11/4355
An optical setup for electric field measurements in MRI with high spatial resolution Simon Reiss1, Andreas Bitzer2, and Michael Bock1 1
Radiology—Medical Physics, University Medical Center Freiburg, 79106 Freiburg, Germany 2 Biolab Technology AG, 8008 Zürich, Switzerland E-mail: [email protected] Received 31 October 2014, revised 23 February 2015 Accepted for publication 14 April 2015 Published 18 May 2015 Abstract
Electric field measurements in the magnetic resonance (MR) imaging environment are important to assess potentially dangerous radio-frequency (RF) heating in the vicinity of metallic structures such as coils, implants or catheters. So far, E-field measurements have been performed with dipole antennas that lag of limited spatial resolution and which are difficult to use in the magnet bore as they interfere with the RF transmit field of the MRI system. In this work an electro-optic sensor is presented that utilizes the Pockels effect to measure the E-field in a clinical MR system with high spatial resolution. This sensor consists of dielectric materials only and thus, it only minimally influences the measured E-field distribution. A 10 m long flexible optical fiber connects the small sensor head to a remote processing unit where the optical signal is transformed into an electrical output signal. Spatially resolved qualitative E-field measurements were performed in a 1.5 T clinical MR system in the vicinity of metallic samples and an active tracking catheter with a resolution of up to 1 mm. The near-field pattern of a resonant U-shaped metallic sample was clearly identified and compared with numerical simulations. A more complex field behavior was found for the tracking catheter where strong E-field enhancements were observed at the distal tip and at its proximal part outside the phantom solution. Due to its sub-mm spatial resolution the optical sensor approach provides detailed insight into the complex and difficult to access field distributions close to implants and metallic structures and has turned out to be promising tool for MRI field and safety inspections.
0031-9155/15/114355+16$33.00 © 2015 Institute of Physics and Engineering in Medicine Printed in the UK
4355
S Reiss et al
Phys. Med. Biol. 60 (2015) 4355
Keywords: electric field measurement, magnetic resonance safety, optical sensor, interventional MRI, implant safety, Pockels effect (Some figures may appear in colour only in the online journal) Introduction During a magnetic resonance imaging (MRI) study the human body is exposed to radio-frequency (RF) fields at the Larmor frequency. The magnetic component of the RF field, B1, is required to excite the magnetization so that a measurable induction signal is generated in a receiver antenna (RF coil). Unfortunately, the magnetic components of the RF fields are accompanied by unwanted electric RF fields which are not required for the MR measurement, but which can deposit substantial amounts of energy in the tissue (Shellock 2000). To avoid potential safety hazards for the patient, an upper regulatory limit is defined for the absorbed RF energy: the specific absorption rate (SAR). SAR limits are given in energy per gram of tissue, and they are often defined so that the tissue temperature does not rise by more than 1 °C. Using RF power measurements at the input ports of the RF transmit coils, and models for the mass distribution and the dielectric properties of the human body, SAR values can be estimated reliably at clinical field strengths (B0 ≤ 3 T) already before the start of an MRI exam. With increasing B0 the wavelength λ of the RF field in tissue decreases leading to standing wave patterns in the patient and, thus, to an inhomogeneous energy deposition. These inhomogeneities can cause so-called hot spots which are already visible at 3 T. Hot spots can also occur when the patient has electrically conducting structures such as catheters, pacemaker leads or metallic prothesteses in the body. Depending on their geometrical and constructional details, conducting structures can act as resonant antennae, which lead to high RF power deposition at locations in the vicinity of these structures. At the end of a wire, for example, electrical field strengths increase be several orders of magnitude when the wire length approaches the resonance length in tissue. These high local electric fields cause displacement currents in the adjacent tissue, which in turn heat up or coagulate the tissue. Even though the heating might be very localized, these hot spots must be avoided at any time, because they can lead to tissue damage in vital structures or functional impairment of the implanted devices (e.g. near pacemaker leads). To enable imaging of patients with implanted devices the use of a so-called safety index has been proposed (Yeung et al 2002). The safety index is a scaling factor for the SAR value such that the energy deposition stays below the regulatory limits everywhere in the patient including the potential hot spots near the implants, if the down-scaled SAR value is used as a new power limit for the MRI experiment. Safety indices can be calculated from RF simulations, but they need to be verified in experiments since simulations often use coarse calculation grids due to limitations in computing power and memory space, and thus they do not provide locally accurate field strength values. Currently, the measurement of electric fields with a spatial resolution of better than 2 mm is challenging. The standard sensor for electric field measurements is the dipole antenna (Taylor et al 1997, Nordbeck et al 2008); if the dipole length L is smaller than the RF wavelength (L
Physics in Medicine & Biology doi:10.1088/0031-9155/60/11/4355
An optical setup for electric field measurements in MRI with high spatial resolution Simon Reiss1, Andreas Bitzer2, and Michael Bock1 1
Radiology—Medical Physics, University Medical Center Freiburg, 79106 Freiburg, Germany 2 Biolab Technology AG, 8008 Zürich, Switzerland E-mail: [email protected] Received 31 October 2014, revised 23 February 2015 Accepted for publication 14 April 2015 Published 18 May 2015 Abstract
Electric field measurements in the magnetic resonance (MR) imaging environment are important to assess potentially dangerous radio-frequency (RF) heating in the vicinity of metallic structures such as coils, implants or catheters. So far, E-field measurements have been performed with dipole antennas that lag of limited spatial resolution and which are difficult to use in the magnet bore as they interfere with the RF transmit field of the MRI system. In this work an electro-optic sensor is presented that utilizes the Pockels effect to measure the E-field in a clinical MR system with high spatial resolution. This sensor consists of dielectric materials only and thus, it only minimally influences the measured E-field distribution. A 10 m long flexible optical fiber connects the small sensor head to a remote processing unit where the optical signal is transformed into an electrical output signal. Spatially resolved qualitative E-field measurements were performed in a 1.5 T clinical MR system in the vicinity of metallic samples and an active tracking catheter with a resolution of up to 1 mm. The near-field pattern of a resonant U-shaped metallic sample was clearly identified and compared with numerical simulations. A more complex field behavior was found for the tracking catheter where strong E-field enhancements were observed at the distal tip and at its proximal part outside the phantom solution. Due to its sub-mm spatial resolution the optical sensor approach provides detailed insight into the complex and difficult to access field distributions close to implants and metallic structures and has turned out to be promising tool for MRI field and safety inspections.
0031-9155/15/114355+16$33.00 © 2015 Institute of Physics and Engineering in Medicine Printed in the UK
4355
S Reiss et al
Phys. Med. Biol. 60 (2015) 4355
Keywords: electric field measurement, magnetic resonance safety, optical sensor, interventional MRI, implant safety, Pockels effect (Some figures may appear in colour only in the online journal) Introduction During a magnetic resonance imaging (MRI) study the human body is exposed to radio-frequency (RF) fields at the Larmor frequency. The magnetic component of the RF field, B1, is required to excite the magnetization so that a measurable induction signal is generated in a receiver antenna (RF coil). Unfortunately, the magnetic components of the RF fields are accompanied by unwanted electric RF fields which are not required for the MR measurement, but which can deposit substantial amounts of energy in the tissue (Shellock 2000). To avoid potential safety hazards for the patient, an upper regulatory limit is defined for the absorbed RF energy: the specific absorption rate (SAR). SAR limits are given in energy per gram of tissue, and they are often defined so that the tissue temperature does not rise by more than 1 °C. Using RF power measurements at the input ports of the RF transmit coils, and models for the mass distribution and the dielectric properties of the human body, SAR values can be estimated reliably at clinical field strengths (B0 ≤ 3 T) already before the start of an MRI exam. With increasing B0 the wavelength λ of the RF field in tissue decreases leading to standing wave patterns in the patient and, thus, to an inhomogeneous energy deposition. These inhomogeneities can cause so-called hot spots which are already visible at 3 T. Hot spots can also occur when the patient has electrically conducting structures such as catheters, pacemaker leads or metallic prothesteses in the body. Depending on their geometrical and constructional details, conducting structures can act as resonant antennae, which lead to high RF power deposition at locations in the vicinity of these structures. At the end of a wire, for example, electrical field strengths increase be several orders of magnitude when the wire length approaches the resonance length in tissue. These high local electric fields cause displacement currents in the adjacent tissue, which in turn heat up or coagulate the tissue. Even though the heating might be very localized, these hot spots must be avoided at any time, because they can lead to tissue damage in vital structures or functional impairment of the implanted devices (e.g. near pacemaker leads). To enable imaging of patients with implanted devices the use of a so-called safety index has been proposed (Yeung et al 2002). The safety index is a scaling factor for the SAR value such that the energy deposition stays below the regulatory limits everywhere in the patient including the potential hot spots near the implants, if the down-scaled SAR value is used as a new power limit for the MRI experiment. Safety indices can be calculated from RF simulations, but they need to be verified in experiments since simulations often use coarse calculation grids due to limitations in computing power and memory space, and thus they do not provide locally accurate field strength values. Currently, the measurement of electric fields with a spatial resolution of better than 2 mm is challenging. The standard sensor for electric field measurements is the dipole antenna (Taylor et al 1997, Nordbeck et al 2008); if the dipole length L is smaller than the RF wavelength (L
