Lucia Navarro de Lara1,2, Mohammad Daneshzand1,2, Anthony Mascarenas3, Douglas Paulson3, Sergey Makarov1,4, Jason Stockmann1,2, Larry Wald1,2,5, and Aapo Nummenmaa1,2
1Radiology, A.A Martinos Center for Biomedical Imaging/MGH, Charlestown, MA, United States, 2Harvard Medical School, Boston, MA, United States, 3Tristan Technologies, San Diego, CA, United States, 4Department of Electrical and Computer Engineering, A.A Martinos Center for Biomedical Imaging/MGH, Worcester, MA, United States, 5HST/MIT, Cambridge, MA, United States
Synopsis
We investigate the feasibility of using
small-diameter three-axis TMS coil as a basis for constructing a simultaneous
stimulation and imaging array. We present an MR-compatible 3-axis TMS coil
prototype comprising of three orthogonal coil X/Y/Z units. We assess the
influence of the TMS coil element on the MRI images and measure the sound
pressure levels with systematically varying the current amplitudes and coil
orientation with respect to the magnetic field. Supported by simulations, we conclude
that construction of a large-scale multichannel; system using such a 3-axis TMS
elements as a basis appears feasible but the acoustic properties should be
improved
Introduction
Multichannel Transcranial
Magnetic Stimulation (mTMS) is an emerging technology for non-invasive stimulation
of the human brain. The use of multiple TMS coils in an array configuration
enables shifting the location TMS ‘hot spot’ electronically without any mechanical
movement. This is achieved by computationally determining the current
amplitudes to be passed to each of the coil elements to synthesize a desired
target field pattern1. The mTMS technique would be particularly
powerful when used in conjunction with functional MRI (fMRI), since maneuvering
multiple TMS coils inside the scanner environment either manually or
robotically becomes rather cumbersome.
While
the combination of TMS and fMRI has been demonstrated2 and
subsequently optimized in terms of TMS-compatible RF receive coil arrays3,
the main challenge is that for maximizing the degrees of freedom for the mTMS
targeting approach, multiple coil elements need to be employed that are smaller
than the conventional MR-compatible coils, resulting in significantly increased
current densities and magnetic fields. Here, our goal was to fabricate a small
diameter ‘three-axis’ TMS coil (See Fig.1B) that is proposed to be used in a
large-scale integrated mTMS/MRI system (Fig.1A) that is currently under
development. Subsequently, we evaluated the compatibility with 3T MR environment.
Finally, we assessed whether a 16x3=48 channel TMS array could be used to
synthesize electric field patterns suitable for stimulating the motor cortex. Methods
An MRI
compatible 3-axis TMS coil was fabricated (see a 3D-CAD model in Fig1.C) employing
vibration damping material (blue material in the figure) to mechanically
decouple the X/Y and Z coil elements while allowing cooling air to flow in/out
through the coil assembly. The coil embedded in the damping material was then
encapsulated in an epoxy-fiberglass chassis to harden the structure. The final
prototype is shown in Fig.1D.
The 3-axis TMS
coil and the commercial MR compatible TMS (MRi-B91, MagVenture, Denmark) (see
Fig2.A) were evaluated in terms of B0 effects acquiring GRE field map sequence
(FOV 220mm, 75 slices, FA=75, TR=1200ms, TE1=10ms, TE2=12.64ms, SL=2mm, 1.7mm
in-plane resolution, axial). To assess the possible drop-outs/distortions on
the functional images, EPI images (FOV=200mm, SL=5mm, TR=1710ms, TE=31ms,
FA,90, 24 slices , 2.3mmx2.3mm in-plane resolution) were acquired of a 19 cm
standard spherical phantom placing the TMS coils on the top of the phantom. B0
maps were calculated using MATLAB and displayed in Hz.
To perform an
assessment of the acoustic noise produced by the new developed coil, sound
pressure levels were acquired using an MR compatible microphone and the
corresponding sound meter (2238 Mediator, Bruel & Kjaer, Denmark). See
measurement setup in Fig2.B. Sound pressure waveforms were acquired using the
NI 6341 (National Instruments, USA) and evaluated as Peak dB (20log(Pin/Pref)
dB) in MATLAB. First, sound increase depending on stimulation intensity was
investigated for the Z coil at fringe and at isocenter. Then, at 25% of maximal
stimulator output (MSO) (100% = 1800 V stimulator charging voltage), the effect
of the angle between the Z coil normal and the B0 in the sound was evaluated
for Z and Y coils, for both cases, at fringe and isocenter. Finally, we
simulated a 16x3 = 48 channel mTMS system using an anatomically realistic model
of a human head. We used the minimum-norm approach1 with and without
constrained optimization to determine the current amplitudes to synthesize the
electric fields corresponding to a commercially available TMS coil. Results
B0 maps and EPI images are
shown in Fig3. Standard deviation for the 3-axis TMS coil was 1.27 Hz and for
the MagVenture coil 0.67Hz.
Results of the sound
evaluation are summarized in Fig4. using the absolute sound pressure level peak
as the measure. For comparison, the
maximal values reported by the sound meter (time windowing of 10ms and
frequency weighting) were 113.9 dB for the Z coil in the fringe with 90° at 50
%. For the MagVenture coil, the maximal sound pressure level recorded was 104.8
dB for 100% and 90°.
The capability of the multichannel
system to ‘emulate’ the commercial MRI-compatible TMS coil shown in Fig 5. The
E-fields of the MRi-B91 coil are shown for current rate-of-change (dI/dt) of 94
A/µs that corresponds to 50%MSO (Fig.5 A-B). The dI/dt values required to synthesize
the corresponding field using the 16x3 array (Fig.5C) were calculated either
with no constraint (maximal achievable dI/dt =120A/µs) (Fig.5D-E) or
constrained minimum-norm optimization (dI/dt < 60 A/µs = 50% MSO) (Fig.5G-H). In each of the cases, the desired E-field could be synthesized but in the
constrained case more channels with reduced amplitudes were used.Discussion
We demonstrated the
feasibility of constructing an MR-compatible three-axis TMS coil that can be
utilized as a basic element in a large-scale hybrid mTMS/MRI system. The B0
artefacts imply that the materials selected are well suited for concurrent
TMS/MRI. The acoustic noise from the current three-coil prototype is reaching
the limits of EPI gradient switching noises4 and substantially
larger than the large-diameter MRi-B91. However, our current coil prototype
does not yet incorporate any materials for muffling the sound from the coils,
which is expected to reduce the sound levels significantly. In conclusion, our
results support the feasibility of constructing an MR-compatible multichannel
system using the three-axis coil as a basic element. Acknowledgements
This work was funded by NIH
R00EB015445, R01MH111829, NIH R00EB021349.References
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(3) Navarro de Lara et al., MRM, 74(5), p.1492(10), 2015
(4) Ravicz et al., J Acoust Soc Am. 108(4): 1683–1696;
October