MAGNETIC RESONANCE IN MEDICINE 16,476-480 ( 1990)

Human Brainstem Auditory Evoked Potentials ( BAEP) before and after MR Examinations *

s. MULLER AND M. HOTZ

MR Center and Department of ORL, University Hospital, CH-4031 Basel, Switzerland

Received July 26, 1990; revised September 17, 1990

Recently significant changes of human brainstem auditory evoked potentials ( BAEP) after exposure to static magnetic fields were reported (L. von Klitzing, Clin. Physiol. Meus. 7(2), 157 ( 1986); L. Stojan, D. Sperber, and K. Dransfeld, Naturwissenschajen 75,622 ( 1988)). We recorded BAEPs of 1 1 subjects before and after a routine MRI examination at 1.5 T. In addition the BAEP of a healthy volunteer was measured in five different static magnetic fields (0-2.0 T). Our results indicate that routine MRI investigations do not significantly alter the interpeak latencies of the BAEPs. o 1990 Academic Press, Inc.

INTRODUCTION

The measurement of brainstem auditory evoked potentials (BAEP) is a well-estab- lished diagnostic tool in audiology. Electrical events generated within the brainstem auditory pathways by acoustic signals are recorded from the scalp of man by using computer averaging techniques. The technique is a reliable and objective test for the identification of retrocochlear lesions in the auditory system of the brainstem.

Recently changes in amplitude and latency intervals of the human BAEP signal after exposure to static magnetic fields were reported (1-3). It was found that, e.g., the characteristic time interval between wave I and V was significantly increased by about 0.2 and 0.4 ms while the natural variation within one subject is less than 0.1 ms ( 3 ) . No causal interpretation of these findings could be given. The goal of this study was to examine the extent of changes of the BAEPs under clinical magnetic resonance imaging conditions.

METHODS AND MATERIALS

Eleven patients (mean age, 47 years; SD, 17; range, 27-78) were examined in a routine MR imaging protocol (six neuro-, three spine, and two extremity imaging). None of the patients were suspected to have brainstem lesions nor was one subsequently located. For the MR examination a rampable Siemens whole-body Magnetom oper- ating at a magnetic field strength of 1.5 T was used. Standard radiofrequency coils for head, spine, and extremities were used, and conventional spin-echo and gradient-echo imaging sequences were applied. The mean time spent in the magnetic field was 50 min (range, 14-67 min).

* A preliminary account of this work was made at the 9th Annual Meeting of the Society of Magnetic Resonance in Medicine, 1990, New York, abstract, p. 605.

0740-3 194/90 $3.00 Copynght 0 1990 by Academic Press, Inc. All rights ofreproduction in any form reserved

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The BAEPs of each patient were recorded 15 min before and 2-4 min after the MR examination in an adjacent room with a residual magnetic field of about 0.1 mT. Left and right ear were measured for most patients. For the BAEP recording a standard Nicolet Compact Four System was used; 100-ps clicks with a sound pressure level of 1 15 dB acted as a sweep trigger; 1000 BAEP acquisitions were averaged during 4 min. The whole BAEP measurement lasted 10- 15 min.

In a second experiment of the BAEPs of one healthy male volunteer were acquired in the magnetic field at different field strengths (0,0.5, 1,1.5,2.0 T). For this experiment the subject was positioned in supine position with his head in the homogeneous region of the magnet. No BO gradients or rf exposure was used during this experiment. In the time gaps between the BAEP measurements the magnetic field was changed to the next value. The whole experiment time was 3 h.

RESULTS

In Fig. 1 the result of a typical BAEP recording is shown. The position of the waves I to V is indicated. The characteristic peak positions and interpeak latencies are used for the evaluations. In Table 1 the results obtained from the routine MRI examination are summarized. The BAEP waves were recorded immediately before and after the MR investigation as described under Methods and Materials. The first and second columns in the table give the number and the mean position for the peaks in the BAEP curve. For the value in column 3, which describes the relative change of the peak positions due to the exposure to the magnetic field, the natural patient to patient variability was taken into account. Therefore the relative changes of the peak positions before ( poshfore) and after ( posafte,) the MR examination were calculated for each patient according to

[I1 ( Posafter) - ( Posbefore

0.5 ' ((Posafter) -k (POSbefore)) '

This value would reflect a magnetic field influence on the BAEP. Its mean value (averaged over all patients) is shown for each BAEP peak in column 3 . In addition the patient to patient variability was taken into account by calculating the standard deviation of the values in [ 11. These standard deviations are given in column 4 for each peak. Table 1 demonstrates that no statistically significant alterations of the BAEP latency intervals occurred.

The BAEP waves of the healthy volunteer acquired in five different static magnetic field intensities are shown in Fig. 2. The positions of peaks I and V were determined using the software package integrated in the Nicolet recording system and are indicated in the figure. Taking a normal 0.1-ms variability of the peak positions into account it can be noted that no significant changes in peak latencies of the acoustic potentials are observed.

DISCUSSION

Several effects of electromagnetic fields on man are known to exist. They can be subdivided into thermic and nonthermic effects ( 4 ) . For a thermic effect the specific

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0 1 2 3 4 5 6 7 8 ms

FIG. 1. Typical BAEP signal recorded with a Nicolet Compact Four System ( 1000 averages). The acoustic trigger pulse is visible as an artifact at -0 ms. BAEP waves are labeled I-V.

absorption rate (SAR) in the body is the most important parameter. In MRI the SAR values are limited to 0.4 W / kg in whole-body exposure and 2 W /kg in local exposure. This implies a maximum body temperature increase of 1 "C. In our examinations the SAR was below 0.1 W / kg; thus a thermal effect on the BAEP recording could be ruled out. In addition a rise in body temperature would result in a proportional decrease of BAEP latencies ( 5 ) , in contrast to the observed increase in BAEP latencies as described in (1-3).

Nonthermal effects are also described in the literature. They are difficult to assess quantitatively. They seem to depend on frequency and have complex field strength relationships. At a cellular level changes in membrane potentials and kinetics have

TABLE 1

BAEPs of 1 1 Subjects before and after MRI at 1 .S T

Relative difference before Peak Mean position and after MRI Standard deviation

I I1

111 IV V

1.59 ms 2.68 ms 3.80 ms 5.15 ms 5.74 ms

-0.01 1 -0.022 +0.009 -0.006 -0.004

0.063 0.026 0.064 0.040 0.020

Note. The BAEP values were recorded before and after a routine MRI examination and the mean relative time shift of each wave (averaged over all subjects) was calculated according to Eq. [ 11 (column 3). The patient to patient variability is shown in column 4, where the standard deviations of the values in column 3 are given. No statistically significant alterations of the BAEP latency intervals were measured.

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1.60rns . 6.08 ms

6.08 ms 1.5$ ms

1.52 ms

i 6.08 rns

0.0 T

0.5 T

1.0 T

1.5 T

2.0 T

ms 0 1 2 3 4 5 6 1

FIG. 2. BAEP waves acquired from a normal volunteer in five different static magnetic fields. The subject was in supine position with his head in the homogeneous region of the magnet. No rf or gradient switching was applied. It can be noted that the positions of BAEP signals I and V, for which changes of 0.2-1.8 rns were observed in (3 , 7 ) , show no systematic field dependence.

been postulated (6). In man disturbances to the circadian rythm have also been de- scribed ( 4 ) .

Our experiments provided no supportive evidence for the latency and amplitude changes described in ( 1-3). It must be emphasized that our imaging conditions such

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as field strength and exposure time were different from those of the quoted authors. The most probable explanation for the changes observed by these authors appears to be technical interactions between the recording and shielding systems. For example, von Klitzing and co-workers (2) mention that the magnetic field alters the capacitive resistance of the electrodes. It is also possible, although unlikely since a population of at least five test subjects was examined, that some changes could be explained by the normal test-retest variability of BAEP recording.

In our work the influence of routine MR imaging examinations at 1.5 T on the human BAEPs was investigated. Standard hardware and software were used and pa- tients suffering from different diseases were investigated. The results summarized in Table 1 show that the alterations observed in (1-3) cannot be reproduced in our experiments. While a natural variation of the peak positions is present in each case, a systematic prolongation of a few percent as described, e.g., in ( 3 ) was not measured, and the mean change for all peaks is far below 0.1 ms.

In ( 7) strong influences of the magnetic field strength on the latency intervals were observed. For example, for wave V a 1.8-ms delay was measured at 2.0 T. We could not find a field dependence of this order. In contrast our results in Fig. 2 indicate that the characteristic time intervals of the BAEPs are not altered in magnetic fields ranging from 0.0 to 2.0 T.

In conclusion our results show that routine MRI examinations do not produce pathological changes in auditory evoked potentials and presumably not in brainstem structures which generate these potentials.

ACKNOWLEDGMENTS

The authors thank J. H. J. Allum, P. Abt, and J. Seelig for useful discussions and help. This work was supported by the Swiss National Science Foundation under Grant 4.889.85.18.

REFERENCES

1. L. VON KLITZING, E. IHNEN, AND B. TERWEY, Naturwissenschaften 71, 538 ( 1984). 2. L. VON KLITZING, Clin. Physiol. Meas. 7(2) , 157 (1986). 3. L. STOJAN, D. SPERBER, AND K. DRANSFELD, Naturwissenschaften 75,622 ( 1988). 4. BASLER AND HOFFMANN, in “Der Einfluss nichtionisierender elektromagnetischer Strahlung auf die

5. K. H. CHIAPPA, in “Evoked Potentials in Clinical Medicine,” p. 227, Raven Press, New York, 1983. 6. A. A. BRAYMAN AND M. W. MILLER, Radiat. Environ. Biophys. 25, 329 (1986). 7. L. VON KLITZING, in “NMR in Medicine” (F. Niisslin and H. Wendhauser, Eds.), p. 199, Urban &

Umwelt,” Schriftenreihe Umweltschutz No. 98, p. 10, Bern, 1988.

Schwamenberg, Miinchen, 1987.