Evaluation of MWT Materials Accusorb MRI Shield. Dr. E. Kanal, Department of Radiology, UPMC. Wednesday, April 21, 2010

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1 Evaluation of MWT Materials Accusorb MRI Shield Dr. E. Kanal, Department of Radiology, UPMC Wednesday, April 21, 2010 Background: In almost all x-ray and ultrasonographic imaging examinations, in order to diagnostically examine patient anatomy and/or physiology, external energies are transmitted into patients in a manner that is restricted to the anatomy being examined. Receivers then detect modifications of this transmitted energy induced by the varying anatomic structures the energy passes through. These modifications are then analyzed and utilized to provide anatomic and/or physiologic information regarding the anatomy that had been probed by these transmitted energies. Due to the resonant manner in which magnetic resonance imaging (MRI) is executed, large volumes of patient anatomy are irradiated with radiofrequency (RF) energies even if the anatomy being studied is restricted to a very small percentage thereof. Spatially localized anatomic and physiologic information is then gleaned, via resonance phenomenology, from what is typically only a small fraction of the tissue that had initially undergone RF irradiation and its related power deposition. Thus, the risks associated with the potential thermal sequelae of RF irradiation are experienced by ALL the tissue of the patient that was irradiated, even if the volume for which diagnostic information was acquired was actually only much smaller than that which had been irradiated for the study. This holds true for all aspects and types of MRI examinations, including but not limited to magnetic resonance angiography, magnetic resonance spectroscopy, functional MRI, diffusion weighted MRI, perfusion weighted MRI, etc. It also applies regardless of the MR imaging sequence utilized, be it gradient echo or spin echo or inversion recovery variants or fast/turbo spin echo or echo planar sequences, etc. Practically speaking, since RF power deposition generally falls off very rapidly as one leaves the volume contained by the RF transmitter coil(s), in MRI we are essentially RF irradiating the entire volume contained within the RF transmitter coils even if we are only studying a very localized anatomic region of interest contained within it. The rationale for this is relatively straightforward: in ultrasound, the ultrasonographic beam energy can be directed and/or focused specifically to the anatomic region to be examined. In CT the X-ray beam can be restricted or collimated to the precise anatomy undergoing diagnostic evaluation. An X-ray beam can be nominally narrowed to 1 mm thickness to produce diagnostic information regarding a 1 mm thick anatomic slice in the human being studied. Such is not the case with MR-related imaging, where resonance phenomena within the volume that had been irradiated produce diagnostic information about the slice or sub-volume to be examined. This is because there have been no effective RF collimators available to restrict the transmitted RF energies to the specific anatomy to be evaluated and no more. Therefore, since the total amount of RF power absorbed by the patient is itself proportional to the total amount of tissue exposed to these

2 RF energies, the patients find themselves in the unusual situation of absorbing far more RF exposure than is actually necessary for the requested diagnostic evaluation. Especially in this day in which the Image Gently campaign is so strongly encouraged among all radiologists for patients of all ages, this seems to be quite inefficient and wasteful, exposing all MR patients to more or far more RF energies and their related (especially thermal) risks than is actually needed for their requested study. In order to successfully restrict transmitted RF energies to just the anatomic region to be examined, the equivalent of a safe RF collimator needs to be developed. This material would need to markedly and efficiently restrict RF energies from passing through them (such as by reflection and/or absorption) while at the same time successfully dissipating any absorbed RF energies in a manner that does not in and of itself pose any threat to the patient or RF transmitting (or MRI-related) hardware. Early in the development of the MRI industry there had been several early attempts at developing such RF shields, but their efficacy and/or safety was not felt to be sufficient or successful and they are not used in clinical or research MR environments today. To this end, we researched material that was initially designed to be used to help absorb radar energies to assist the military in enabling objects to produce a much attenuated radar signature so-called stealth technology. This technology works primarily by absorbing, attenuating, and safely dissipating microwave (and RF) energies to which it is exposed. After discussions with the manufacturer of this material confirmed that these characteristics should work with RF energies of the frequencies utilized in 1.5 Tesla MR scanning environments, we elected to pursue evaluation of custom designed RF shields for human application to be used in MR imaging environments to attempt to restrict RF power deposition to the anatomic regions of interest diagnostically desired - and no more. This report summarizes our experience with thermal testing of the RF Shield material provided by MWT. In vitro testing: Heating measurements were obtained and recorded using a Luxtron m3300 fluoroptic thermography unit with True Temp software. Testing was performed utilizing an acrylic head/torso phantom filled with polyacrylic acid mixture, 2.7 grams per liter of distilled filtered water. The test device used for heating testing was as per the FDA design. This is a insulated 0.5 mm thick wire with a 1 cm length of insulation stripped from each end. This implant was suspended within the gel-filled torso phantom oriented parallel to the long axis of the phantom and RF bore and situated at isocenter but far left laterally within the torso portion of the phantom. Fiber optic probes were placed at the tip of the device, in the middle of the exposed metal at one end of the wire/device, and at the juncture of the exposed wire/insulation at one end of the device. The fourth probe was placed within the gel itself at a position remote from the device but within the RF irradiated volume of the transmitting body RF coil (Figure 1).

3 Figure 1 After acquiring 3-plane scout scans, an RF aggressive pulse sequence was prescribed resulting in estimated specific absorption rates (SAR) of 2 W/kg average total body weight with peak SAR values of 4 W/kg. Specific scan parameters utilized were as follows: Scan parameters were as follows: Axial, fast spin echo TE=14ms, TR=900ms, ETL=64 FOV=48cmx48cm 36 slices, 10mm with a 2.5mm gap NEX=20 Matrix=128x128 Scan Time=24:11 EST SAR=1.9986, Peak SAR= The first test consisted of scanning the device with the RF shield completely surrounding the torso section of the phantom containing the embedded device, as illustrated in Figure 2:

4 Figure 2 This was compared to the temperature changes observed during imaging of the same phantom and device with the identical scan parameters and scan time duration with no RF shield present. With no RF shield present significant, roughly 17 degree Celsius increases in surface temperatures were observed during MR imaging at the tip of the implanted device as seen in Figure 3:

5 Figure 3 (We attribute the abrupt modification of the temperature reading for the tip probe to variation in probe tip positioning that initially occurred during scanning) Compare this to the results observed with the RF shield completely surrounding the device within the scanned phantom, where no significant temperature modification was seen in any probe/lead, as shown in Figure 4:

6 Figure 4 The RF shield was then superiorly displaced so that it only covered half the superoinferior extent of the scanned device/phantom, as shown in figure 5: Figure 5 Scanning at this time demonstrated device heating to as high as 14 degrees Celsius, which was between that which was seen with no RF shield present versus that which was seen when the device was fully RF shielded, as seen in Figure 6:

7 Figure 6 We then studied the effectiveness of the RF shield when only draped over the entire phantom and device, not circumferentially entirely surrounding it, as demonstrated in Figure 7:

8 Figure 7 Device heating was far more restricted here (figure 8), with maximal tip heating observed to only increase by roughly 6 degrees Celsius. Figure 8 We then tested the imaging impact of the presence of the RF shield. Recognizing that the MR imaging process requires precise magnitudes of RF energies to reach the tissue to be examined, we examined the impact on the images acquired of the tissue phantom with versus without the presence of the surrounding RF shield when positioned superoinferiorly half way down the SI length of the torso phantom. These results are illustrated in images 9 and 10, where the RF attenuating properties of the RF shield are clearly evident:

9 Figure 9 No RF Shield present

10 Figure 10 Same MR imaging sequence, RF shield present surrounding the superior half of the torso phantom. As a result of these in vitro testing results we elected to proceed with testing thermal and imaging effects of the utilization of the RF shield on human imaging. Since one of the the targeted theoretical objectives of such RF shielding material might be to shield cardiac pacemakers and pacemaker leads from transmitted RF energies to diminish arhythmogenic and thermal possible risks of such exposures, it was elected to perform our first human imaging tests on MR imaging of the cervical spine region. We wanted to ensure that diagnostic MR images of the cervical spine could still be obtained while severely attenuating RF energies that would reach the thoracic cage. We therefore designed our next research protocol to focus on targeted MR imaging of the human

11 cervical spine with and without the presence of the RF shielding material surrounding the thorax of these research subjects. Purpose: To assess the RF shielding capabilities of the MWT RF shield while assessing thermal impact on the patient and shielding material. We also assessed the impact on the diagnostic content of the MR images of the cervical spine as well as the rapidity of signal attenuation once reaching the (thoracic) volume encircled by the RF shield. Methods: A total of seven research subjects were studied with IRB approval. For each research subject studied the patient underwent the typical, clinically requested MR imaging examination of the cervical spine in one of our 1.5 Tesla General Electric MR scanners as per the routine clinical imaging protocol. This included a positioning scout sequence followed by sagittal spin echo T1 weighted imaging, sagittal fast spin echo T2 weighted imaging, axial spoiled gradient echo imaging, and three dimensional gradient echo axial imaging. For each research subject, four Luxtron fiber optic temperature probes were utilized for all temperature measurements, with measurements taken at each probe tip at a frequency of 1 Hz. During the portion of the exam where the patient/research subject was not wearing the RF shield, probes 1 and 2 were placed directly on patient s skin, with probe 1 over the manubrium and probe 2 positioned on the left side of neck just inferior to angle of the mandible. Probes 3 and 4 were placed on patient s gown with lead 3 at the level of xyphoid and lead 4 inferior to lead 3 at a location midway between the xyphoid and the umbilicus. The times correlating to the beginning (of the pre-scan phase) and end of each MR imaging sequence were recorded. Upon completion of the clinical portion of the examination the probes were removed and the Accusorb RF shield was placed around the patient s chest in such a way as to ensure that the so anterior and posterior edge of the vest was positioned as superiorly as possible without covering/obscuring the anatomic structures of the cervical spine. The subject was then re-positioned in a manner matching as precisely as possible that of the prior, clinically indicated study just completed. The fiber optic probes were then repositioned on the subject with probes 1 and 2 placed in the identical locations as before and again contacting the patient s skin directly. Probe 1 was again placed at level of the manubrium, ensuring that the superiormost extent of the RF shield covered the probe tip. Probe 2 was again positioned on left side of the neck in such a way that the probe tip was NOT covered by the RF shield even in part. Probe 3 was positioned directly on the anterosuperiormost edge of the shield, located slightly superior to probe tip 1 s position. Probe 4 was placed directly on the left anterior collar surface of the RF shield, somewhat inferior to probe tip 2. The MR examination (and all associated imaging parameters) executed with the presence of the RF shield was in every way identical to that performed without the presence of the RF shield in place. Scan start/stop time data for each sequence was again recorded in same manner as it had been for the clinical scan portion of the examination.

12 Thermal data were analyzed to compare temperatures measured at the probe tips with the RF shield in place relative to that experienced without the presence of the RF shield. The RF shield serves as another layer of clothing, as it wrapped around the chest as would be a vest, and thus some minimal elevation of temperature underneath the shield was expected relative to sampling without the shield present. The study was performed with the vest off and with the vest on in all instances (the order of which was decided by for patient, technologist, and study convenience and efficiency). This research project was performed with the approval of the Institutional Review Board of the University of Pittsburgh. Results: A total of seven patients agreed to serve as research subjects for this study. Of these, there was inconsistent probe tip positioning/dislodgement during the study in one patient (the first studied in this protocol) in whom there was also inconsistent positioning of the study before and after RF shield placement. As such we discarded all data from this research subject. Thus, there is complete data available for six research subjects. Of all the studies performed on each research subject, the greatest degree of temperature elevation observed in the RF shield material itself was in a single research subject (# ) where the temperature at the RF shield edge increased by 3.21 degrees Celsius over the course of a 5 minutes and six second study. In this sequence, 3 degrees Celsius of temperature elevation had already been reached by 3 minutes and fifteen seconds into the study after which the measured temperature essentially leveled off despite continued RF power deposition at the same levels and rates throughout the entire study. This suggested that the RF shield material was able to absorb and dissipate this level of RF imaging power without undue heating of the surfaces or edges of the RF shield. In all other cases the temperature elevations observed on the patient s skin was considered trivial and consistent with physiologic temperature changes that one might observe when lying quietly while wearing a sweater or similar such outer garment. No patient/research subject thermal complaints were reported and no focal hot spots were palpated or observed on the skin or the RF shield.

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18 Imaging of the cervical spine demonstrated very sharp cut-off of signal from the superior edge of the RF shield when this was included in the displayed field of view (100% of all cases in the sagittal plane). This provided for diagnostic MR imaging in the anatomic region of interest not covered by the RF shield while rendering the anatomy uninterpretable in the regions (intentionally) enveloped by the RF shielding material. Signal drop off to half of initial signal intensities were within one to two centimeters in all cases (superoinferiorly).

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22 Conclusions: The data have confirmed that the RF thermal loads produced by routine clinical MR imaging of the cervical spine were capably absorbed and dissipated by the RF shielding material tested in a manner that posed no safety threat to the patient. Further, imaging signal intensities were tightly restricted to the volume not enveloped by the RF shield material, thus providing further evidence of the successful RF shielding capabilities of this material. It should therefore be possible to significantly decrease total patient RF energy deposition from routine MR imaging studies by encasing the anatomy which the diagnostician does NOT want to examine (yet might still be located be within the RF excitation/transmission coil) in such RF shielding material, thus effectively collimating the RF energies to the anatomic regions of interest to be examined. Further, in the case of implanted leads, wires, or electrically conductive devices, it might well be possible to significantly reduce thermal concerns and the potential for focusing of RF energies via an antenna effect at the tips of these electrically conductive materials by shielding these implants with such RF shielding material if they are located in or near anatomy that is not designated for study yet might still otherwise be exposed to significant RF excitation energies in the course or routine MR imaging.