Laleh Golestanirad1, Boris Keil1, Maria Ida Iacono2, Giorgio Bonmassar1, Leonardo M Angelone2, Cristen LaPierre1, and Lawrence L Wald1
1Radiology, Massachusetts General Hospital, Charlestown, MA, United States, 2Division of Biomedical Physics, Office of Science and Engineering Laboratories, Center for Devices and Radiological Health, US Food and Drug Administration, Silver Spring, MD, United States
Synopsis
Recently we presented the feasibility study of using a reconfigurable DBS-friendly head coil, composed of a patient-adjustable
rotating birdcage transmitter, and an integrated 32-channel receiver array to reduce SAR during imaging of patients with deep brain stimulation
implants. Here we introduce the first prototype of such coil system, and present results of finite element simulations on
patient-derived numerical models of realistic DBS lead trajectories, which characterize its SAR reduction
performance.Introduction
Each year approximately 300,000 patients
with medical implants including deep brain stimulation (DBS) devices
are denied magnetic resonance imaging (MRI) examination due to safety concerns
1. Recently
we presented results of a feasibility study for using reconfigurable DBS-friendly
transmit/receive head coils, composed of a patient-adjustable rotating birdcage
transmitter, and an integrated 32-channel receiver array at 1.5T
2. Here we present the
first prototype of such coil system, along with the results of measurements and
comprehensive numerical simulations that characterize its image quality and specific
absorption rate (SAR) profile.
Materials and Methods
Hardware
development and safety tests
The
coil system was composed of a linearly-polarized rotating birdcage transmitter,
and an anthropomorphic 32-channel receive array at 1.5 T (see Fig.1). The transmitter
has a slab-like region of low electric field that is systematically adjusted for
each patient to encompass the DBS implant. This technique reduces the local SAR
at the implant tip by 500 times below that of a circularly polarized (CP)
birdcage. An array of 32-channel surface receiver coils was designed for
maximum reception. Details of array design and construction can be found in 3. The ensemble of
transmit-receive array went through a comprehensive battery of safety tests to
assess the quality of active detuning and SNR maps, as well as evaluating
temperature increases (both in the coil’s circuitry due to eddy-currents, and
in a head phantom due to RF absorption) during long continuous scans (~15 min).
Numerical models
The coil’s SAR reduction performance was assessed for both generic DBS
lead trajectories and patient-derived realistic lead models. Generic DBS leads models
were integrated into a 1×1×1mm3 numerical head model with 15
different tissue classes. Details of segmentation and construction of the head
model are given in 4. The location of
the sub-thalamic nucleus (STN), —one of anatomical structures targeted for
DBS, was estimated from the MRI data upon which the head model was built. After
the STN was located, a series of entry points were selected on the skull based
on typical angles of approach for the STN DBS (~60˚ in the anterior-posterior
direction and 15˚–30˚ from the sagittal plane 5). The rest of the
lead was placed in the subcutaneous structure along anterior-posterior
position. To evaluate the SAR reduction performance in practical cases, leads
with the extra-cranial portion looped around the surgical burr hole were also
modeled.
Patient-specific simulations were performed on four realistic DBS lead
trajectories, extracted from post-operative CT images of patients operated on at
Massachusetts General Hospital. The lead trajectories were manually segmented from
CT images and a polyline representing lead trajectory was constructed and
exported to the electromagnetic solver (HFSS, ANSYS Inc). Next, a DBS lead model including
4 cylindrical contacts (“electrodes”) connected through a solid core surrounded
by hollow insulation was constructed in HFSS. Finally, homogeneous head models
were built from the CT data, and were assigned the electric properties of grey
matter at 64 MHz.
For both generic and realistic cases, FEM simulations were performed
for the full range of coil rotation angles and 1g-averaged SAR was calculated using
HFSS built-in SAR module.
Results and Future Work
The
ensemble of transmit/receive coil showed less than 3˚C temperature rise in coil
components during test scans. Images showed increased SNR up to 5 fold in
cortical structures, and up to 2 fold at the level of deep brain nuclei with
respect to a circularly polarized (CP) birdcage (see Fig. 1). In all simulations including (1) generic
DBS lead trajectories with wire segments that were 15˚–30˚ out of
plane (Fig. 2c), (2) leads with looped segments (Fig. 2d), and (3) patient-derived
realistic lead trajectories (Figs. 3 and 4), an optimum coil angle was
found that reduced the maximum local SAR at electrode tip below the maximum SAR
value in the head. The location of this optimum position was however dependent
on the individual lead trajectory, indicating the need to have a
patient-adjustable transmitter.
In
the last step, the sensitivity of the results to electrical conductivity of the
anatomical structure surrounding the DBS (labeled as “grey matter” in the numerical
model) was evaluated. Simulations were performed with a range of conductivities
from 0.01-3 S/m, and with different coil rotation angles. We found that when
the coil was in the optimum position, the maximum SAR values were not sensitive
to electrical conductivity of the structure surrounding the electrode (see Fig.
5).
We
are now in the process of evaluating a cohort of 20 DBS patients to perform
uncertainly budget calculations assessing the safety of this new technology for
post-operative imaging of DBS patients.
Acknowledgements
This work was partially supported by NIH grant P41EB015896.References
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