Original Research

BOLD-fMRI Response to Single-Pulse TranscranialMagnetic Stimulation (TMS)

Daryl E. Bohning, PhD,1* Ananda Shastri, PhD,1 Eric M. Wassermann, MD,5

Ulf Ziemann, MD,6 Jeffrey P. Lorberbaum, MD,2 Ziad Nahas, MD,2

Mikhail P. Lomarev, MD, PhD,1,7 and Mark S. George, MD1–4

Five healthy volunteers were studied using interleavedtranscranial magnetic stimulation/functional magneticresonance imaging (TMS/fMRI) and an averaged singletrial (AST) protocol. Blood oxygenation level-dependent(BOLD)-fMRI response to single TMS pulses over the motorcortex was detectable in both the ipsilateral motor cortexunder the TMS coil and the contralateral motor cortex, aswell as bilaterally in the auditory cortex. The associatedBOLD signal increase showed the typical fMRI hemody-namic response time course. The brain’s response to asingle TMS pulse over the motor cortex at 120% of thelevel required to induce thumb movement (1.0%–1.5% sig-nal increase) was comparable in both level and duration tothe auditory cortex response to the sound accompanyingthe TMS pulse (1.5%–2.0% signal increase). J. Magn. Re-son. Imaging 2000;11:569–574. © 2000 Wiley-Liss, Inc.

Index terms: single-pulse transcranial magnetic stimulation(TMS); single-event fMRI; motor cortex; imaging

TRANSCRANIAL MAGNETIC STIMULATION (TMS) is atechnique in which a pulsed magnetic field from a smallcoil is used to create localized neuron-depolarizing cur-rents in the cerebral cortex (1). It has been used toinvestigate brain function for over a decade and showsclinical promise in the treatment of depression (2). Re-cently, the use of TMS in combination with single-pho-

ton emission computed tomography (SPECT) (3),positron emission tomography (PET) (4–6), and MRI(7–10) has also generated interest in using TMS to stim-ulate or inhibit local response for neuroimaging inves-tigations of functional connectivity.

Typically applied in trains, a combined TMS/PETstudy (5) reported 5% increases in cerebrospinal fluid(CBF) in primary motor/somatosensory cortex underthe coil with as few as five 5-pulse trains of TMS at 10Hz (25 pulses). In another TMS/PET study but using 1Hz TMS, Fox et al (6) reported focal increases of 12%–20% at the site after stimulation at 120% of the motorthreshold (MT) for 30 minutes. However, because oftheir low time resolution, PET images necessarily rep-resent an integrated blood flow response rather thanthe time course of blood flow increase or decrease as-sociated with a single event. In a combined TMS/fMRIstudy (8), 18-second trains of 110% MT TMS pulses at1 Hz (18 pulses) induced 3%–4% increases in the bloodoxygenation level-dependent (BOLD)-fMRI signal in thearea of stimulation. Although that study had an imag-ing time resolution of 3 seconds rather than the approx-imately 60 seconds in the PET studies, it used a blockdesign stimulation pattern, so the hemodynamic re-sponse could not be directly compared with the hemo-dynamic response observed in cognitive task single-event studies. Niehaus et al (11) used transcranialDoppler sonography to measure blood flow in the mid-dle and posterior cerebral arteries after applications ofTMS in 5-pulse trains at 10 Hz and measured a 5%increase in hemodynamic response curves not unlikethose seen in large vessels in response to cognitivetasks. However, a local cortex response could not beobserved with this technique, and, again, the TMS wasapplied in multiple pulse trains.

There is no doubt that a single TMS pulse applied tothe motor cortex is capable of causing a neuronal re-sponse, since its consequences can be clearly seen inthe form of an overt movement of the contralateral ex-tremity. However, to date, the only functional neuroim-aging technique in which the response to single-pulseTMS has been observed is electroencephalography(EEG) (12). With its superior temporal resolution, EEGwas able to confirm a direct association between theTMS stimulation and the neuronal response (12), but

1Functional Neuroimaging Research Division, Department of Radiol-ogy, Medical University of South Carolina, Charleston, South Carolina29425.2Functional Neuroimaging Research Division, Department of Psychia-try, Medical University of South Carolina, Charleston, South Carolina29425.3Functional Neuroimaging Research Division, Department of Neurol-ogy, Medical University of South Carolina, Charleston, South Carolina29425.4The Ralph H. Johnson Veterans Hospital, Charleston, South Carolina29425.5Intramural Research Program, NINDS, Bethesda, Maryland 20892.6Department of Neurology, J.W.Goethe-University, D-60580 Frankfurt,Germany.7Institute of the Human Brain, St. Petersburg, 197376 Russia.Contract grant sponsors: National Alliance for Research in Schizophre-nia and Depression (NARSAD) and the Ted and Vada Stanley Founda-tion; Contract grant sponsor: NIMH; Contract grant number: R21.*Address reprint requests to: D.E.B., Radiology Department, MedicalUniversity of South Carolina, 171 Ashley Avenue, Charleston, SC29425. E-mail: bohninde@musc.eduReceived December 10, 1999; Accepted February 24, 2000.

JOURNAL OF MAGNETIC RESONANCE IMAGING 11:569–574 (2000)

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with a spatial resolution inferior to both PET and fMRI,it was unable to give good localization for that response.

The main objective of this study was to determinewhether interleaved TMS and fMRI (7) could be usedwith an averaged single trial (AST) (13,14) protocol todetect BOLD response to neuronal activation inducedby a single TMS pulse, and, further, to measure its timecourse.

The technique is important because it will allow thecomparison of different TMS events using their associ-ated BOLD responses. For example, with a single-eventtechnique it might be possible to compare different TMSintensities, or coil orientations, or single versus pairedstimulation (through one coil, or possibly through twodifferent coils, one conditioning coil, and one test coil).Such studies could provide a bridge between electro-physiology (variation of motor evoked potential ampli-tudes) and fMRI (variation of BOLD response).

MATERIALS AND METHODS

General Experimental Design

Five healthy male volunteers (mean age 34.6 6 4.0 SDyears) participated in this study under a protocol ap-proved by the Medical University of South Carolina’sInstitutional Review Board for Human Research. SingleTMS pulses were applied to the area of the left primarymotor cortex associated with thumb movement at 120%of MT on alternate 12-second epochs of a REST epoch-TMS epoch stimulation cycle. It was decided to stimu-late over the motor cortex area for the thumb becausethis allowed independent verification in the form ofthumb movement of proper functional location acrossand within subjects. During each epoch, 20 5-slice vol-umes of BOLD-EPI images were acquired at the rate of0.6 seconds per volume. The REST-TMS cycle was re-peated 15 times. Since previous work (8,10) has shownthat the sound of the TMS coil provides a good cognitivereference activation in the auditory cortex, an addi-tional epoch with a reference volitional movement wasdropped to allow a larger number of averages.

A 1.5-T clinical MR scanner (EDGE, Rel.9.4, PickerInternational, Highland Heights, OH) was used for themultislice single-shot gradient-echo echoplanar (EPI)-fMRI acquisitions (68 3 66 matrix reconstructed to a128 3 128 image, field of view 270 mm, a 40°, TE 40.0msec, slice thickness 5 mm, gap 1.5 mm, with fat sat-uration). The in-magnet stimulation was performed us-ing a Dantec MagPro (Dantec Medical A/S, Skovlunde,Denmark) with a special nonferromagnetic TMS coilwith a figure-8 design. The details of the method havebeen published elsewhere (7–9).

TMS Coil Placement

The TMS coil was rigidly mounted in the MR head coilwith vitamin E capsules placed at the ends and centerof the TMS coil to help locate its position in the struc-tural images. Subjects wore earplugs, and vision wasunconstrained. While lying on the gantry outside thescanner bore, the subjects inserted their heads into theMR head coil. Then, while the TMS coil was intermit-tently pulsed at high intensity (90% machine output

when MT was unknown, lower for subjects whose ap-proximate MT was known from previous studies), theyadjusted their head position until visible movement ofthe contralateral (right) hand abductor pollicis brevis(thumb) was optimally induced. Formal electromyo-graphic (EMG) determination of the MT was not per-formed because of the MR scanner’s high magneticfield, although with this stimulator a close concordancehas been found between EMG-determined MT and MTdetermined visually (15).

The subject’s head was then stabilized with foam-padded inflatable restraints. MT was determined bygradually decreasing stimulation intensity until move-ment (slight twitch of right thumb) was observed ap-proximately 50% of the time. The stimulator was thenset to 120% of the subject’s MT. After scanner tuningand acquisition of T1-weighted reference images, andbefore the TMS/fMRI acquisition was started, the TMScoil to head position was rechecked with one or moreindividual manual TMS pulses. Figure 1a shows a sam-ple EPI-BOLD fMRI image, a coronal section approxi-mately perpendicular to the TMS coil and passingthrough the intersection of the figure-8; the overdrawnfigure-8 indicates the relative position of the TMS coil.

Data Processing

Following the study, the images were transferred to aSun workstation (Sun Microsystems, Mountain View,CA) and converted to ANALYZE (CNSoftware, West Sus-sex, UK) format.

Motion Check

A check to be certain that subject movement was lessthan 3 mm along all the three major axes (x, y, z) wasperformed on each image set using MEDx 3.0 (SensorSystems, Sterling, VA). None of the data sets failed thischeck. The images were then co-registered to a commonmean image using the Automated Image Registration(AIR) software (16).

t-Test for Areas of Activation

Pixel-by pixel paired t-tests compared acquisitions6–15 (3.6–9 seconds after the TMS pulse, to allow forhemodynamic lag) of the TMS epochs with the corre-ponding acquisitions in the preceding REST epoch. Theresulting t-maps, images representing the pixel-by-pixel t-values for the comparision between the two con-ditions, TMS minus REST, describe regional brain ac-tivation.

BOLD Activity Time Course

Using the positions of the three vitamin E capsulemarkers mounted on the TMS coil, a point 2 cm fromthe face of the coil along a line through its center andperpendicular to it was computed and located in thevolume defined by the t-map slice set. The cluster ofpixels with the highest level of activation (highest t-values) in the immediate vicinity of this point was takenas the TMS-induced activation of the ipsilateral motorcortex. In addition, clusters of pixels distinguished by

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relatively high local levels of activation that consistentlyappeared in approximately the same location in all fivesubjects were identified in the t-maps.

Finally, in the slice in which ipsilateral motor re-sponse under the TMS coil was found, two separateclusters of four pixels each were selected, one from aCSF-filled area and one from an area in the white mat-ter in the center of the contralateral hemisphere. Theseareas were used to check for any nonspecific responseto the TMS pulse and to provide a measure of the levelof noise in the data. For each cluster, the BOLD signaltime series was obtained by extracting the mean inten-sity of the voxels in the cluster from the functionalimages, acquisition by acquisition (ie, time point bytime point). These time series were then high pass fil-tered to remove signal changes with periods longer than24 seconds and averaged over the 15 cycles. The cycle-averaged time courses for all five subjects were sepa-rately normalized by the mean signal during the RESTepoch and then averaged point by point across corre-sponding clusters in the five subjects to obtain a sub-ject and cycle averaged time course. The error bars weretaken as the standard deviation (SD) of the subjectaverage.

RESULTS

The t-map for the section in which pixel clusters for themotor cortex were identified for a subject with relativelystrong responses is shown in Fig. 2a. A small cluster ofpixels can be seen in the ipsilateral motor cortex, alongwith larger clusters in the medial ipsilateral motor cor-tex, contralateral motor cortex, and contralateral audi-tory cortex. The cycle averaged time courses from theindicated pixel clusters (arrows), corresponding to themedial ipsilateral motor, ipsilateral motor under theTMS coil, contralateral auditory, and contralateral mo-tor areas are shown in Fig. 2e, respectively. Ipsilateralauditory activation was in a more posterior section inthe data for this volunteer. The mean t-values for theclusters (t-mean) have been included for comparison of

relative statistical significance and to convey a sense ofthe possible influence of any TMS coil-induced suscep-tibility artifact. The prominent cluster near the vertex isthought to be in the medial motor cortex (medial pri-mary motor cortex, eg, leg area) or possibly the supple-mentary motor cortex (SMA). It shows a time coursesimilar to those seen bilaterally, in both more lateralmotor cortex and auditory cortex, but since it was iden-tified in only two of the five subjects, it was not viewedas a consistent TMS-induced activation.

In these time course plots, the values of the three timepoints immediately after the TMS pulse were set to zeroto eliminate an off-scale excursion at 12 seconds due toan artifact. This artifact was caused by residual currentin the TMS coil because of the short (600-msec) intervalbetween acquisitions.

Figure 3 shows the BOLD signal time courses aver-aged over all five subjects for the six regions consis-tently identified in all subjects: a) noise, b) CSF, c)ipsilateral auditory, d) contralateral auditory, e) ipsilat-eral motor under the TMS coil, and f) contralateral mo-tor. The time course of the BOLD signal shows thetypical hemodynamic response associated with neuro-nal activation bilaterally in both the motor and auditorycortex. The response in the auditory cortex due to thesound accompanying the TMS pulse (1.5%–2.0% signalincrease) appears to be slightly larger than that causedby a single TMS pulse over the motor cortex sufficient toinduce thumb movement (1.0%–1.5% signal increase).The averaged responses appear significantly smallerin the averaged curves compared with the corre-sponding curves for the individual in Fig. 2, partlydue to the strong responses of that individual, butalso because these data have not yet been normalizedto give a mean of zero during the REST epoch like thedata used for the subject averaged means in Fig. 3.For example, in Fig. 2d, one can see that the “re-sponse” peak would be shifted downward by about1% by this normalization.

Figure 1. Sample EPI-BOLD images from fMRI acquisition. a) Normal windowing and leveling. b) Windowed and leveled toaccentuate TMS coil susceptibility artifact. The position of the TMS coil is indicated by the figure-8. Some signal loss and twoindentations at the edge of the brain can be seen, the latter corresponding to the two loops of the coil.

BOLD-fMRI Response to Single-Pulse TMS 571

Figure 2. a) t-map indicating clusters of pixels identified with the BOLD response to single-pulse TMS in the : medial ipsilateralmotor, ipsilateral motor under the TMS coil, contralateral auditory and contralateral motor areas. The cycle-averaged timecurves associated with the clusters are as follows: b) medial ipsilateral motor, c) ipsilateral motor under the TMS coil, d)contralateral auditory, and e) contralateral motor.

Figure 3. Time courses of BOLD-fMRI response to single-pulse transcranial magnetic stimulation (TMS) from pixel clustersidentified with a) baseline noise, b) CSF, c) ipsilateral auditory, d) contralateral auditory, e) ipsilateral motor, and f) contralateralmotor areas.

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DISCUSSION

This is the first combined TMS/fMRI averaged singletrial study and provides the first measurements of thetime course of the BOLD response to single TMS pulses.It demonstrates that the hemodynamic time course ofthis response, in both the motor cortex under the coil,as well as the contralateral motor cortex, is comparablein both shape and magnitude to that seen in the audi-tory response to the short “snap” of the TMS coil whenit is pulsed. The response in the auditory cortex due tothe sound accompanying the TMS pulse (subject aver-aged 1.5%–2.0% signal increase) appears to be slightlylarger than that induced in the motor cortex by the TMSpulse itself (subject averaged 1.0%–1.5% signal in-crease). Since this is seen even on the side of the braincontralateral to the TMS coil where coil artifact is not anissue, it might be due to transcallosal attenuation, or itmight just be the result of suboptimal placement of theTMS coil over motor cortex.

The present study and previous studies with TMS/fMRI (7–10) are consistent in showing a TMS-inducedincrease in BOLD signal, presumably correlated withincreased blood flow. In a TMS/PET study (5), stimula-tion over the left primary sensorimotor cortex at sub-motor threshold intensity also increased local CBF forthe smallest number of pulses administered, 25. How-ever, overall, local CBF correlated negatively with thenumber of 5-pulse (at 10 Hz) trains, falling almost lin-early from a value of 15% at 5 trains (25 pulses) to 23%at 20 trains where it stayed out to 30 trains.

Both motor and auditory increases seem to besmaller than the '3% increase reported in the earlierTMS/fMRI studies (7–10) using trains of either 18 or 21TMS pulses (at 1 Hz). They are also smaller than the 5%increase observed in the TMS/PET study (5) for five400-msec-long trains of five pulses (effectively 10 Hz)1600 msec apart, or 25 pulses delivered over a totalperiod of about 8400 msec. This may indicate that thehemodynamic response builds with additional TMSpulses, although sublinearly. The TMS/PET data fur-ther imply a relatively rapid depletion and, eventualinhibition of response to the TMS. The level of BOLDresponse evoked in the auditory cortex by the sound ofthe single TMS pulse observed in the present study iscomparable to that observed in another fMRI studyusing averaged single trials in response to 100-msectone bursts (17)

To increase the certainty that the activated areaschosen in the different subjects were, in fact, homolo-gous, spatial normalization of the data into Telairachspace would have been desirable. Unfortunately, due tothe small number of relatively thin slices and the factthat they were not acquired perpendicular to the AC-PCline, the spatial normalization performed by MEDx wasnot reliable. Consequently, since the exact Telairachcoordinates of the ipsilateral and the contralateral mo-tor cortex activations are not known, it cannot be cer-tain that this is an example of interhemispheric activa-tion of homologous motor cortical areas.

The prominent clusters observed in two subjects nearthe vertex are thought to be in the medial motor cortex(medial primary motor cortex, eg, leg area, or possibly

the SMA). This area was also co-activated in a studyusing 18FDG-PET that reported 8% increases in theregional cerebral metabolic rate of glucose in responseto long trains (15 trains, each 60 seconds long) of 2 HzTMS of the left sensorimotor cortex (18). Interestingly,they also reported that the magnitude and extent of thesensorimotor cortex was significantly greater with avoluntary imitation of the TMS-induced movements.Although not directly comparable because of the differ-ences in technique and train lengh, the continued pos-itive response in that study and the Fox et al study (6),is hard to reconcile with the Paus et al study (5) unlessfrequency is taken into account.

The reduced significance of the ipsilateral motor cor-tex response under the TMS coil may be due to thesusceptibility artifact caused by the coil, but it is notsevere. Figure 1 shows a sample EPI-BOLD fMRI image,windowed and leveled normally (Fig. 1a), comparedwith the same fMRI image that has been windowed andleveled to accentuate the TMS coil-induced susceptibil-ity artifact (Fig. 1b). This coronal section is approxi-mately perpendicular to the TMS coil and passesthrough the intersection of the figure-8. The suscepti-bility artifact is noticeable, and another group has im-plemented a mechanism for moving the TMS coil awayfrom the head during the actual acquisition of the EPIimages (19). However, it is not clear whether such aprecaution is necessary, since there is less than a 15%reduction in SNR immediately under the coil (9), andsuch a maneuver does constrain the possible TMSstimulation patterns somewhat.

CONCLUSIONS

Single TMS pulses applied over the motor cortex withsufficient intensity to induce thumb movement pro-duced BOLD-fMRI responses detectable in both the ip-silateral motor cortex under the TMS coil and the con-tralateral motor cortex, as well as bilaterally in theauditory cortex. The signal increase induced in the mo-tor cortex by the TMS pulse is comparable to the signalincrease in the auditory cortex caused by the sound ofthe TMS pulse and shows the standard fMRI hemody-namic response pattern, in both magnitude and timecourse. Thus, interleaved TMS/fMRI can be used inaveraged single-pulse trials, and BOLD response to sin-gle-pulse TMS can be detected under the TMS coil de-spite the coil-induced susceptibility artifact. Furtherwork is needed to remove this artifact (19), or, alterna-tively, to calculate its effect on the signal and the asso-ciated limitations on activation detection sensitivity.With further refinement, this combination of single-pulse TMS and averaged single trial fMRI will probablybe of considerable interest for in vivo neurophysiology.The temporal resolution of electrophysiology (based onthe amplitude of motor evoked potentials) will comple-ment the spatial resolution of functional neuroimaging(based on the local variation of BOLD response).

ACKNOWLEDGMENTS

Partial equipment and salary support was receivedfrom the National Alliance for Research in Schizophre-

BOLD-fMRI Response to Single-Pulse TMS 573

nia and Depression (NARSAD; Young Investigator andIndependent Investigator Award; M.S.G.), the Ted andVada Stanley Foundation (M.S.G. and D.E.B.), and theNIMH (grant R21 to D.E.B.).

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