Radiology
Florian M. Vogt, MD Mark E. Ladd, PhD Peter Hunold, MD Serban Mateiescu, RT Franz X. Hebrank, PhD Al Zhang, PhD Jo ¨ rg F. Debatin, MD, MBA Susanne C. Go ¨ hde, MD
Index terms: Magnetic resonance (MR), biological effects Magnetic resonance (MR), safety Magnetic resonance (MR), threedimensional Nerves, MR Published online before print 10.1148/radiol.2332030428 Radiology 2004; 233:548 –554 Abbreviations: FDA ⫽ Food and Drug Administration IEC ⫽ International Electrotechnical Commission PNS ⫽ peripheral nerve stimulation
Increased Time Rate of Change of Gradient Fields: Effect on Peripheral Nerve Stimulation at Clinical MR Imaging1 PURPOSE: To increase gradient stimulation from 100% to a fixed 120% level and to assess patient acceptance of the degree of peripheral nerve stimulation (PNS) at magnetic resonance (MR) imaging. MATERIALS AND METHODS: Two hundred ten patients underwent MR imaging of various body regions according to clinical indications. An additional threedimensional fast low-angle shot sequence with the 120% stimulation level was performed. A patient questionnaire was distributed after MR imaging to document the presence, degree, and location of PNS. Degree was measured with an 11-point scale (score range, 0 –10). Age was analyzed between the sexes for significant statistical differences. Furthermore, correlation between location of examination and location and degree of stimulation was performed. To determine stimulation discomfort relative to other factors typically present at MR imaging, the degree of discomfort due to room temperature, size of magnet bore, acoustic noise, examination time, and heating sensation was determined for comparison, as well.
1
From the Department of Diagnostic and Interventional Radiology, University Hospital Essen, Hufelandstrasse 55, 45122 Essen, Germany (F.M.V., M.E.L., P.H., S.M., J.F.D., S.C.G.); Siemens Medical Solutions, Erlangen, Germany (F.X.H.); and Siemens Medical Solutions, Chicago, Ill (A.Z.). Received March 31, 2003; revision requested June 18; final revision received February 10, 2004; accepted February 27. Address correspondence to F.M.V. (email:
[email protected]).
RESULTS: Thirty-five (16.7%) patients reported a stimulation sensation during imaging in one or more locations, while six (2.9%) felt very uncomfortable local stimulation (score of 8 –10). No significant difference between male and female patients regarding age, sex, and appearance or degree of stimulation sensation could be detected. No significant correlation between location of examination and location and degree of stimulation was recorded. Compared with other side effects, PNS was considered relatively unimportant. CONCLUSION: The 120% gradient stimulation level seems acceptable for routine clinical imaging with this gradient system, since only 2.9% of patients experienced very uncomfortable local stimulation. ©
Author contributions: Guarantors of integrity of entire study, M.E.L., J.F.D.; study concepts, A.Z., F.X.H., J.F.D.; study design, A.Z., F.X.H., M.E.L.; literature research, F.M.V., M.E.L.; clinical studies, S.M., F.M.V., P.H., S.C.G.; data acquisition, S.M., F.M.V., P.H., S.C.G.; data analysis/interpretation, M.E.L., P.H., F.M.V., J.F.D.; statistical analysis, S.C.G., M.E.L., F.M.V.; manuscript preparation, F.M.V., P.H., M.E.L., S.C.G.; manuscript definition of intellectual content, F.M.V., J.F.D., F.X.H., M.E.L.; manuscript editing, F.M.V., A.Z., M.E.L., S.C.G.; manuscript revision/review, F.M.V., M.E.L., P.H., S.C.G., F.X.H., A.Z.; manuscript final version approval, all authors ©
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Magnetic resonance (MR) imaging has been used with great success for many years in the clinical routine for various indications and anatomic regions. Because of the emerging demand for faster sequences with higher spatial and temporal resolutions, the development of modern high-performance gradient systems has led to an increase of gradient strength and slew rate by a factor of nearly 10 in the past 20 years. The further increase in gradient output might not only reduce imaging time but also reduce artifacts, allow shorter breath-hold times, provide higher spatial resolution or increased coverage while keeping imaging times equal, and reduce susceptibility and flow-related artifacts. It is important to set appropriate limits for these gradient systems, however, to limit the manifestation of uncomfortable side effects and maintain MR imaging as a safe and comfortable imaging modality. Fast-switching gradients induce a time-varying electric field that may cause undesirable peripheral nerve stimulation (PNS) at sufficiently high amplitudes and therefore present a potential limitation for safe use in clinical MR imaging (1–3). Since the late 1970s, safety
Radiology
standards for MR imaging have been declared by regulatory agencies, such as the Food and Drug Administration (FDA) and International Electrotechnical Commission (IEC), to protect patients from PNS and associated potential risks (4). Fulfillment of these regulations is mandatory for the commercialization and implementation of medical devices by manufacturers. However, the introduction of new technologic developments (ie, new gradient hardware and pulse sequences, such as echo-planar imaging) in the past few years has made it possible to exceed the older recommended limits in routine clinical practice. Current FDA guidance classifies timevarying gradient fields as a substantial risk when the “time rate of change of gradient fields (dB/dt) is sufficient to produce severe discomfort or painful nerve stimulation” (5,6). Recently, the FDA has been involved in the formulation of new IEC regulations. Any new FDA guidelines will therefore probably be consistent with these newly determined IEC limitations on gradient output. The newly proposed IEC regulations (7) divide operation into three modes— normal operating mode, first-level controlled operating mode, and second-level controlled operating mode—although only the first two modes are of relevance for routine clinical imaging. The gradient output limits for the first two modes can be determined by using one of two methods— default numeric values or directly determined values— on the basis of results of experimental study in human subjects for the gradient system in question. The default values for the normal and first-level modes for a whole-body gradient system at a stimulus duration of 0.2 msec are a time rate of change of gradient fields of 44 T/sec and 56 T/sec, respectively. For experimentally determined values, the gradient output in the normal operating mode should not exceed 80% of the directly determined mean threshold PNS. In the first-level operating mode, the gradient output is limited to 100% of this level to minimize the occurrence of painful PNS. The mean threshold PNS level is therefore hereafter referred to as the 100% level. The mean stimulation threshold (100% level) of each pulse sequence depends on the duration , the shape and polarity of the gradient pulses, the number of gradient pulses in a pulse train, and the orientation of the gradients (8,9). Therefore, the IEC requires a control Volume 233
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mechanism, adapted to each imaging system, to ensure that the limits of the normal and first-level operating mode cannot be exceeded by using different sequence types. The 100% level must be adapted for each sequence to account for the varying imaging parameters by using a validated model. Threshold PNS is defined in the IEC guidelines as the onset of sensation, painful PNS as the level that is tolerable to the patient when properly informed and motivated, and intolerable PNS as the level at which the patient requests that the imaging procedure be terminated immediately. The mean threshold PNS level was chosen by the IEC as the 100% level based in part on data from Bourland et al (10). These data were re-presented in figure 9 of Schaefer et al (6) and indicate that less than 5% (ie, 2%–5%) of a healthy population with normal nerve conduction systems would experience “uncomfortable” (ie, painful) stimulation at this gradient output, thereby restricting the risk of PNS to an acceptable level. Intolerable stimulation would be expected in less than 1% of a healthy population. The results of an internal study performed in 30 healthy volunteers by Siemens Medical Solutions, Erlangen, Germany (F. Hebrank, written communication, 2001), showed that 3% felt uncomfortable stimulation at the 100% limit. The 100% level of our imager is set to the mean threshold PNS, as specified by the IEC guidelines. In our own experience in 2 years of routine clinical imaging, however, only one patient who reported painful PNS could be recalled by our personnel, indicating that the gradient output might be increased above the 100% level without causing the occurrence of painful PNS in a significant number of patients (ie, less than 5% of patients should experience painful PNS). Most of the literature regarding stimulation deals with studies in healthy volunteers to determine mean threshold PNS and its dependence on gradient axis, wave shape, duration and number of applied pulses, and patient position (10 – 16). These studies are performed by varying the stimulation level from subthreshold values to superthreshold values and requesting the volunteer to report any stimulation sensations at each level. Thus, the purpose of our study was to increase the stimulation level from 100% to a fixed 120% level and assess patient acceptance regarding the degree of PNS.
Number of Body Regions Investigated (Region Placed in the Magnet Isocenter) Examination Region
No. of Patients
Head Neck Heart Spine Abdomen Colon Pelvis Vessels Extremities
29 4 9 13 85 30 7 18 15
Total
210
MATERIALS AND METHODS Patients and Sequences Between August 2001 and March 2002, 210 patients were consecutively enrolled in the study without any particular selection criteria. The inclusion of this number of patients was based on statistical considerations in view of the expected rate of patients who would report uncomfortable PNS (⬍5%). Patients underwent MR imaging of various body regions according to clinical indications (Table). The study was approved by the local institutional review board, and written informed consent was obtained from all study participants. Two hundred ten patients (mean age, 50 years; range, 17– 83 years), including 113 male patients (mean age, 49 years; range, 17– 81 years) and 97 female patients (mean age, 50 years; range, 18 – 83 years), participated in this study. Mean body weight was 74 kg (range, 43–135 kg), and mean height was 173 cm (range, 149 –197 cm). All MR imaging was performed with a 1.5-T whole-body imager (Magnetom Sonata; Siemens, Erlangen, Germany) equipped with high-amplitude gradients (40 mT/m maximum amplitude) and a corresponding high slew rate (200 mT/ m/msec) and low rise time (200 sec). All examinations were conducted by using the whole-body gradient coil—that is, no head-coil gradient insert was available. The maximum field of view of the gradient coil was 40 cm. Depending on the region of interest, a head coil, extremity coil, body array coil, and/or spine array coil was used for signal reception. The patients were placed in the imager in either the supine or prone (in the case of colonic examination) position, with either head or feet (only in certain extremity examinations) first. The patients’
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Figure 1. Pulse diagram of three-dimensional fast low-angle shot MR sequence characterized by increased stimulation level of 120%.
hands were unclasped, with arms flat on the table at the patients’ sides. Each patient was provided with an emergency squeezable bulb and instructed to activate it if discomfort became intolerable. Furthermore, all patients were informed that interruption due to intolerable stimulations during the examination would not influence the diagnostic outcome of the study. This was guaranteed by performing the sequence with 120% stimulation level as the final sequence in the examination. Patients were not aware of the start time of the additional sequence. The body region being investigated was placed at the isocenter of the magnet. Prior to examination, all patients were asked to take notice of the occurrence of PNS and to note its degree and location. In addition to the normal clinical protocol, an additional three-dimensional fast low-angle shot spoiled gradient-echo sequence (repetition time msec/ 550
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echo time msec, 1.66/0.62; flip angle, 14°) with trapezoidal bipolar Gx, Gy, and Gz gradient waveforms and an in-plane data acquisition matrix of 154 ⫻ 256 was used with a field of view of 263 ⫻ 350 mm, which rendered a pixel size of 1.7 ⫻ 1.4 mm. The section thickness was 2.4 mm. The sequence was performed in the coronal plane with the in-plane phaseencoding direction along the left-right axis. This sequence was characterized by an increased stimulation level of 120%, which is comparable to a time rate of change of gradient fields of 93 T/sec at a stimulus duration of 0.2 msec. At the 100% stimulation level, the repetition time of this protocol is limited to 1.78 msec, leading to a 7% increase in imaging time. A pulse diagram of this sequence is shown in Figure 1. The three-dimensional fast low-angle shot sequence with the increased 120% stimulation level had an acquisition time of 7 seconds. To pro-
vide imaging duration long enough to allow recognition of potential “intolerable” PNS, the sequence was repeated four times without interruption. Thus, the entire duration amounted to 28 seconds, long enough to allow patients to interrupt the examination because of intolerable stimulations. All other sequences were limited to the 100% level, although the gradient output of most sequences did not exceed the 80% level and were thus applied in the normal operating mode. The control mechanism of our imager, which determines the 100% level for various imaging sequences and parameters, uses the SAFE (Stimulation Approximation by Filtering and Evaluation) model (17), which was developed by using healthy volunteers and has been accepted by the FDA. It is able to predict the sequence-specific stimulation threshold for arbitrary gradient waveforms and arbitrary section orientations and is thereVogt et al
Radiology
fore suitable to be implemented as a gradient time rate of change of gradient fields watchdog. Hence, the model allowed calculation of a 120% stimulation level for arbitrary gradient waveforms and all orientations.
Questionnaire A questionnaire distributed immediately after the MR examination documented the patients’ impressions regarding the degree of stimulation by using an 11-point scale (0 –3 ⫽ no or nonobjectionable stimulation, 4 –7 ⫽ tolerable stimulation, 8 –10 ⫽ very uncomfortable stimulation). Scores for both global PNS and the degree of stimulation at individual locations were recorded. The location of stimulation could be reported as eyes, nose, arms, chest, abdomen, upper spine, lumbar spine, buttocks, or legs, or the patient could specify a location not listed explicitly on the questionnaire. Multiple answers were possible. Furthermore, degrees of discomfort due to room temperature, size of the magnet bore, acoustic noise, needle placement, contrast agent injection, examination time, heating sensation, and enema, if performed, were obtained with the 11point scale, as well. Mean scores of the different categories were compared.
Statistical Analysis The SPSS (version 10.0 for Windows; SPSS, Chicago, Ill) statistics package was used for subsequent statistical analysis. Age was analyzed for significant statistical differences between the sexes by using a paired t test. The statistical significance of the difference between sexes for the median stimulation degree was performed by using the Mann-Whitney U test. P values lower than .05 were considered to indicate a significant difference. The degree of discomfort from the stimulation sensation was compared against the degree of discomfort of other origin (eg, needle puncture). For all patients, a first test was performed by using the Kruskal-Wallis H test for nonparametric data (group A) to compare all sources of discomfort. This unpaired test was chosen because needle puncture, contrast agent injection, and enema were not applicable to all patients. The Friedman test was then applied to the subgroup of patients who underwent all sources of discomfort, including contrast agent injection and enema (group B). In case a statistically sigVolume 233
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Figure 2. Bar graph shows the number of reported global stimulations for each degree of stimulation at MR imaging.
nificant difference was found between the various sources of discomfort by using these tests, all possible pair combinations were tested with the Wilcoxon signed rank test for group A and the Mann-Whitney U test for group B. For these pairwise tests, P ⬍ (.05 divided by number of pairs) denotes the border of significance by taking into account the Bonferoni correction, where the number of pairs denotes the respective number of possible pairwise comparisons for the variables. Correlation of the degree of discomfort between the different sources was tested with the Spearman rank correlation coefficient. Difference in stimulation degrees among the different body locations was tested in pairwise fashion with the Wilcoxon signed rank test, taking into account the Bonferoni correction for multiple comparisons. Differences between the stimulation degree for the different examination locations (landmark) or for the different stimulation sites (arms, legs, etc) were investigated with one-way analysis of variance by using post hoc tests for multiple comparisons.
RESULTS Of the 210 patients examined (group A), 34 formed subgroup B, since they underwent contrast agent injection and enema in addition to all other sources of discomfort. No significant difference between male and female patients regarding age and the appearance or degree of stimulation sensation could be detected for patients in either group A or group B. Fifty-seven of the 210 (27.1%) patients reported a global stimulation sensation
during the examination (grade 1–10). Only five (2.4%) felt uncomfortable global stimulation, as defined by a score of 8 –10 (Fig 2). Only five patients reported global stimulation degrees higher than any of the other sources of discomfort; four of these reported a global stimulation of 8 –10. No patient activated the emergency squeeze bulb or requested that the examination be terminated as a result of PNS. Compared with other sources of discomfort during MR imaging, stimulation sensations were considered less objectionable (statistically significant) than acoustic noise, examination time, and size of the magnet bore when averaged over all participants (group A; Fig 3). Furthermore, the average discomfort level of stimulation was less than half that of needle placement and also less than that of room temperature. The stimulation sensation was less objectionable than enema within the contrast agent injection and enema subgroup (group B). Any source of discomfort of grade 8 –10 was reported by 43 of 210 (20.5%) patients; grade 8 –10 was more frequent for all other sources of discomfort than for stimulation, except for contrast agent injection (Fig 4). The degree of discomfort from stimulation correlated with the degree of discomfort from all other sources (P ⬍ .01)—that is, those who felt more discomfort from stimulation also registered more discomfort from each of the other sources. The only exception was for enema, where statistical significance could not be shown because of the few enemas performed. The most common anatomic locations
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Figure 3. Bar graph shows that stimulation sensations at MR imaging (black bar) were considered least important compared with other sources of discomfort (gray bars) when averaged over all patients. ⴱ ⫽ statistically significant difference with respect to global stimulation sensation.
for PNS were the arms, abdomen, and upper spine, followed by the legs and chest (Fig 5). No stimulation in any individual location (eg, arms, nose) was recorded by 97 male patients and 78 female patients. Only 35 (16.7%) patients reported a stimulation of more than 0 in one or more individual locations, of which six (2.9%) had an uncomfortable grade (grade 8 –10), which is slightly inconsistent with the five patients who registered uncomfortable stimulation in the global stimulation question (Fig 2). Although abdominal and colonic (50%) examinations were the most frequent investigations and, consequently, most stimulations occurred when imaging the abdomen, no significant correlation between the location of examination (landmark) and location and degree of stimulation could be found. Significantly different degrees of stimulation for individual locations were seen only between the arms (with the highest degree of stimulation) and the eyes (with the lowest degree of stimulation) and between the arms and the nose (with the second lowest degree of stimulation) (Fig 6).
DISCUSSION Even though PNS per se is not permanently harmful, excessive gradient-induced patient discomfort should be avoided, while at the same time maximizing the gradient operating zone for high-performance imaging. The three points we believe to be important from this study on the effect of increased time rate of change of gradient fields on peripheral nerve stimulation in clinical use can be summarized as the 552
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Figure 4. Bar graph shows that the percentage of patients that reported high degrees of discomfort at MR imaging (n ⫽ 8 –10) is higher for almost all other sources of discomfort (gray bars) if compared with global stimulation (black bar) (not statistically significant because of low case numbers).
following: (a) Only 2.9% of all patients felt uncomfortable stimulation. (b) In comparison to other sources of discomfort associated with the examination, stimulation sensations were considered less disturbing. (c) Setting the stimulation limit for the first controlled mode to 120% of the mean threshold PNS level seems to be acceptable for routine clinical imaging with this gradient system. Because we were not interested in individual threshold values, the experiment was not conducted by increasing the gradient performance parameters and stopping as soon as the personal stimulation limit was reached. Rather, the 120% limit was exploited in each patient, and the impressions regarding the degree of stimulation at this level were studied. Early regulatory standards sought to set limitations on time rate of change of gradient fields to 20 T/sec to avoid both cardiac and peripheral nerve stimulation. The regulatory standards were set to a factor of three below PNS means to provide adequate protection from cardiac stimulation at all gradient ramp times (18,19), since the difference between the cardiac stimulation threshold and the mean PNS threshold becomes smaller for longer ramp times. For approximately the past 10 years, commercial systems have been capable of exceeding these limits (20), and it became recognized that the older limits were overly conservative and hindered the introduction of faster higher-resolution imaging sequences. Given the problems of how and where to measure time rate of change of gradient fields and given the fact that the time integral of time rate of change of gradient fields (ie, the end amplitude of the gradient magnetic field) is more indicative of
the stimulation potential of an imaging sequence (21), new regulations have been introduced to avoid fixed numeric limits and to allow PNS to be experienced by some patients while seeking to minimize the chance of painful PNS (7) in the population as a whole. On the basis of results of the study of Bourland et al in 1999 (10), a 2%–5% probability of uncomfortable PNS seems to be considered by the FDA and IEC as an acceptable compromise between gradient performance and patient comfort when imaging at the maximum allowable performance level, which corresponds to the first-level controlled operating mode of IEC standards. The study of Bourland et al (10) was conducted in 84 healthy subjects with well-controlled laboratory conditions— that is, with no magnetic field and therefore no acoustic noise from gradient switching and with none of the distractions of a typical MR examination. The goal of the present study was to select a fixed stimulation level of 120% and determine what percentage of a clinical population would register uncomfortable PNS during routine examinations. It is difficult to determine each individual’s perception of what is “uncomfortable” or “intolerable.” Even in patients who reported very uncomfortable stimulation, none of them asked for termination of the examination, although all were equipped with an emergency-call squeezable bulb and were instructed that they could terminate the examination at any time without diagnostic consequence. Therefore, according to the IEC definitions, no patient experienced “intolerable PNS.” Furthermore, compared with other Vogt et al
Radiology
Figure 5. Bar graph shows the number of reported stimulations in each anatomic location at MR imaging.
side effects of MR imaging, PNS was considered relatively unimportant on average. Among the most disturbing factors were acoustic noise, size of the magnet bore, length of the examination, and, if performed, application of an enema. Since the stimulation threshold is known to be dependent on the design of the sequence used (1,13,22), the maximum gradient amplitudes and slew rates have to be adapted dynamically to the particulars of the sequence parameters to take advantage of the full imager capability. As indicated in the Materials and Methods section, our system performs a calculation that allows sequence-dependent determination of the mean PNS threshold, with parameters determined by an empirical model that was introduced by Hebrank and Gebhardt (17). By assuming that these model parameters are accurate, the data collected in the present study for the fast low-angle shot sequence should hold for any arbitrary sequence and section orientation in the imager—that is, the stimulation limit of each sequence can be increased to 120%. Budinger et al (12) and Bourland et al (10,23), who studied the gradient-induced pain threshold, determined that most persons found stimulation painful at a level about 50%–70% above their personal nerve stimulation threshold, depending on the gradient axis. Only 2%–5% of subjects are expected to feel uncomfortable at the level of time rate of change of gradient fields that represents the mean nerve stimulation threshold (100% level) for the population (Gy gradient axis at a ramp duration of 300 sec) (6). Although only 2.9% of all 210 patients in the present study felt uncomfortable PNS despite the higher stimulation limit, this rate is consistent with the data of Schaefer et al (6) (Fig 9) because of the residual uncertainty of their data. Volume 233
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Figure 6. Bar graph shows the average reported degrees of stimulations in each anatomic location (gray bars) at MR imaging. Statistically significant differences (ⴱ) were seen only between “arms” (black bar) and “nose” and between “arms” and “eyes.”
There seems to be no clear difference regarding the presence of uncomfortable stimulation in healthy volunteers and in patients with various underlying diseases, as in our study population. Contrary to Ehrhardt et al (15), who found that the typical location of stimulation was close to the region of maximum time rate of change of gradient fields, our data correspond to those of Ham et al (14). Depending on the clinical question being investigated, our patients were placed in the gradient coil at one of nine different locations. Stimulation symptoms were reported in various anatomic regions, and we could not find any correlation between the known position of the maximum value of time rate of change of gradient fields of the gradient coil and the location of reported peripheral nerve stimulation. The high rate of stimulation sensations in the arms might result from vibrations of the imager that were interpreted as stimulations, which would reduce the number of patients with true PNS. Since our sequence used all three gradient axes together, we were not able to differentiate the influence of single energized gradient coils on the anatomic site of stimulation, as reported by Schaefer et al (24). Mean body weight, height, and the distribution of patient sex in our group were similar to those in other studies with healthy volunteers, but our population showed a higher standard deviation for weight and height. Even with regard to the higher mean age in the present study, obviously resulting from the fact that we included only patients with underlying diseases, we noted a lack of dependence of stimulation threshold on body weight, height, and patient sex, which is consistent with findings in previous studies of Abart et al (11) and Chronik and Ramachandran (25). Other investigators have
revealed that subjects with greater diameter tended to have higher thresholds (26). In contrast to certain previously published studies in healthy volunteers, the acquisition of our data sets was performed with real clinical conditions. Hence, we used a sequence typically applied in clinical routine with implementation of all three gradient units and, furthermore, an applied radiofrequency pulse and the presence of a stationary magnetic field (10,17). Although no significant differences have been observed with or without a 1.5-T static magnetic field present along the long axis in dogs (10,27,28), the stationary magnetic field leads to high acoustic noise typically associated with MR imaging gradients and to vibrations of the magnet and patient bed during gradient switching. The patients in the present study were not aware of the time point of the start of the 120% fast gradient sequence within the series of routine clinical sequences. Therefore, possible vibrations during turbo spin-echo or other sequences could have been misinterpreted as stimulation. On the other hand, distraction due to accompanying noise and intermittent verbal commands by the staff have to be considered as reasons for possible higher personal stimulation thresholds, resulting in a decrease of reported stimulations. Even when considering these circumstances, however, probably only an increase or decrease of reported minor or tolerable stimulations, not uncomfortable stimulations, should be expected. As discussed earlier, a low percentage of subjects (⬍5%) is expected to experience uncomfortable stimulation for values of time rate of change of gradient fields at the 100% limit, indicating that not only the 120% fast low-angle shot sequence but any sequence used in the
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routine clinical protocol could potentially lead to PNS (17). Therefore, it is impossible to differentiate the amount of reported PNS between the 100% and the 120% limits in this study. The goal of this study, however, was not the evaluation or specification of the increase in intolerable stimulation between the 100% and 120% limits but rather to answer the question of what percentage of the clinical population would identify PNS as uncomfortable at 120%. Hence, the study was performed without including a control group at 100%. It is also to be expected that even with sequences that remain well below the 100% limit, a low percentage of patients would report stimulation due to sensitization by the inclusion in the study protocol and the signing of the consent form. Again, to answer this question, data for a separate control group undergoing identical examinations except for the reduced time rate of change of gradient fields would have to be collected. This study was conducted for only one particular gradient system (with a 40-cm field of view). Thus, whether or not other gradient systems would show a similar rate of uncomfortable PNS at the 120% level would have to be verified independently in future studies. In this relatively large collection of patients with various underlying diseases, it was observed that most findings were consistent with previous results in the literature that were determined in healthy volunteers. If the goal of the IEC and FDA is to limit the occurrence of uncomfortable PNS to less than 5% of the population, the results of the present study indicate that raising the level of the first controlled mode to 120% of the mean threshold PNS from the current 100% may be warranted, since only about 3% of the population will experience uncomfortable stimulation. References 1. Reilly JP. Peripheral nerve stimulation by induced electric currents: exposure to time-varying magnetic fields. Med Biol Eng Comput 1989; 27:101–110. 2. Brand M, Kimmlingen R, Heid O, Haase A. Peripheral nerve stimulation: head gradient coils vs. switch coils (abstr). In: Proceedings of the Tenth Meeting of the International Society for Magnetic Resonance in Medicine. Berkeley, Calif: International Society for Magnetic Resonance in Medicine, 2002; 707. 3. Bowtell R, Bowley RM. Analytic calcula-
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