Saikat Sengupta1 and Xinqiang Yan1
1Department of Radiology, Vanderbilt University Institute of Imaging Science, Nashville, TN, United States
Synopsis
Artifacts caused by large magnetic susceptibility differences
between metallic probes and the surrounding tissue are a persistent problem in
interventional MRI. In 2019, we presented the concept, design and modeling of
active shims for metallic probes inspired from naval degaussing systems. Field
disturbance induced by a metallic probe at 3 Tesla was modeled and an active
shim insert design was presented to shim the field variation around the probe. In
this abstract we present the first experimental results showing effective
recovery of metallic probe induced signal loss at 3 Tesla.
INTRODUCTION
Metal probe artifacts, primarily signal loss stemming from the large difference in susceptibility between metal and the surrounding tissue have long presented a challenge in Interventional MRI (IMRI)1-3. These artifacts obscure targets in biopsies4 and prevent accurate image-based monitoring in therapeutic applications5. The goal of this work is to introduce a solution to this problem by developing active shim coils to correct the field disturbance caused by the probe. In this abstract we present the first experimental results showing effective recovery of probe induced signal loss at 3 Tesla.METHODS
Our technique
is inspired from degaussing coil technology used in ships and submarines for
defense against magnetic field sensing sea mines6, as well as recent
work in local shim coils for brain MRI7-9. In our earlier work, we
demonstrated effective correction of the ΔB0 outside a titanium
probe using a two-coil shim insert in simulations10. Here, we
modeled and fabricated such a two-coil system for in scanner experiments.
Simulations:
We modeled a
6.35/4.57 mm OD/ID thick wall Titanium rod (Volume Susceptibility: 𝟀 = 182*10-6 ) in a
surrounding medium of Water (𝟀
= -9.05*10-6 )11. The
rod was defined within a 40 mm3 (4003 voxels,
voxel resolution of 0.1 mm) grid of points in Matlab (Figure 1a, b). The induced
ΔB0 was estimated
with the rod orientated vertically, perpendicular to B0 , using
Fourier-analysis based field calculation at 3 Tesla12-14. Next, a
two coil shim insert was modeled using 26 gauge (0.4 mm) wire to fit inside the
rod. The coils labeled CN0 and CN90 were designed
orthogonal to each other with simple rounded tip geometries (Figure 1c). The
numbers indicated the angle in the Needle’s radial direction. 1 Ampere Bz
fields generated by the coils were computed with a Matlab implementation of
Biot Savart’s law15. Finally, shimming was simulated by fitting the rod induced ΔB0 with the
coil fields in an unconstrained least squared fit.
Experiments: A 12-inch CP Grade 2 hollow Titanium rod
(Titanium Processing Center, MI, USA) with the same OD/ID as above was used for
the experiments. For the shim insert, a 12 cm long 4 mm cylindrical former with
four 0.8 mm diameter grooves for shim wire placement was 3D printed. 26-gauge
enameled copper wire was placed into the slots to form the two coils. The coils
were insulated with a single layer of insulation tape. The shim set was
connected to a constant current power supply (KeySight E3631A, Keysight
Technologies, CA, USA) using 20 feet long twisted cables. Four inhouse made
broadband Toroidal chokes (~25-30 dB S21 isolation @ 127 MHz) were placed in
line with each shim coil to suppress RF and gradient induced voltages.
All experiments were performed on a Philips Elition 3 Tesla scanner
with a 2 channel transmit/receive body coil. The shim set was inserted into the
titanium rod and the setup placed vertically in a water bottle phantom to match
the simulation condition. 3D GRE imaging with field mapping (1 mm3
voxels, 240 x 96 x 96 mm3 FOV, TR/TE/ΔTE = 9.5/4.7/0.5ms, FA = 8o,
Tacq = 100 s) was then performed in
three conditions, phantom without any rod, with rod and no shimming, with rod
and active shimming.RESULTS
Figure 1 shows
the setup and results of the simulations. Figures 1d and e show the Titanium
rod induced ΔB0 and the 1 Ampere field produced by the CN0 coil. A
similar dipolar pattern is observed in both, indicating the feasibility of
shimming. Figure 1f shows the coronal slices across the shimmed field with a
current of -1.5 Amps in CN0 and 0 Amps in CN90. Excellent
compensation of the field is observed, indicating the potential of signal
recovery around the rod. The central rod is masked in all fieldmaps.
Figure 2a
shows the Titanium rod, the shim insert with the two shim coils and a zoomed in
view of the same (inset). The base plate of the shim insert holds the chokes.
Figure 2b shows the insert placed inside the rod. This setup is placed
vertically in the scanner, perpendicular to B0.
Figure 3
shows magnitude images in three planes of the bottle without the rod, with the
rod but no shims and with -1.5 Amp CN0 shim. The 6.35 mm diameter
Titanium rod produces a conspicuous signal void of greater than 20 mm in
diameter. With shimming, the signal around the rod is recovered to a large
extent in the upper section of the bottle where the insert reaches. The signal
void shrinks to ~13 mm in diameter, i.e. a recovery of almost 58% of lost signal around the rod.
Figure 4 shows the fieldmaps in the same planes. Without shimming the bipolar ΔB0 pattern is observed around the signal void,
which is minimized after shimming. DISCUSSION
The results
demonstrate that it is possible to shim and recover signal losses around metallic
probes with active shim coils. Generalized coil paths can be designed to
provide robust shimming for all probe orientations16. Shims can also
be designed for probes with specific geometries, tip shapes and materials.
Artifact reduction can be potentially achieved in a variety of qualitative and
quantitative IMRI applications. Acknowledgements
This work was supported by NIBIB 1R21EB025258. I would like to
thank Dr William A Grissom and Dr Megan Poorman for advice on field modeling.References
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