Modeling of Active Shimming of Metallic Needles for Interventional MRI
Saikat Sengupta1

1Department of Radiology, Vanderbilt University Medical Center, Nashville, TN, United States


Needle artifacts caused by large magnetic susceptibility differences between metallic needles and stylets and the surrounding tissue are a persistent problem in interventional MRI. In this abstract, we present the concept, design and modeling results of a active shim system for needles inspired from degaussing coils used in naval vessels. Field disturbance induced by a Titanium needle at 3 Tesla is modeled and an active orthogonal shim coil insert design is presented to demonstrate shimming of the field variation around the needle. This work lays the foundation for designing full generalized active shim systems for Interventional MRI probes.


Needle artifacts have long presented a challenge in Interventional MRI (IMRI)1-3. The large difference in magnetic susceptibility between needles and the surrounding tissue induces significant field perturbations that result in a variety of artifacts around the needle. These artifacts obscure targets for example in biopsies4 and prevent accurate image based monitoring in therapeutic applications5. The purpose of this work is to therefore introduce a solution for this problem by developing active shim coils to correct the field disturbance caused by the needle. In this abstract, we present conceptual details of the method and initial results of the design and modeling of the shim coils.


The proposed solution is inspired from degaussing coil technology used in ships and submarines for defense against magnetic field sensitive sea mines as well as recent work in local shim coils for brain MRI6-9. Similar to a ship’s degaussing coil structure, a shim insert with DC coils is proposed that will produce the necessary corrective field outside the needle.

Needle Design and Field Modeling : We modeled the shimming of a 10 Gauge (3.4/2.7 mm OD/ID) Titanium needle ( Volume Susceptibility : 𝟀 = 182*10-6 ) in a surrounding medium of Water (𝟀 = -9.05*10-6 )10. A 100 mm hollow needle was designed in SolidWorks (Dassault Systemes, MA, USA) with a 30o single sided bevel at the tip. The design was exported as an .STL file into MATLAB ( Mathworks, MA, USA) and voxelized to yield a 3D grid of points defining the needle in space (Figure 1). The voxel resolution was set to 0.1 mm. A sub-grid of 40 mm3 ( 4003 voxels) was used for modeling the field from the top 3 cm of the needle. The field was modeled for two orientations with needle orientated perpendicular to the B0 field, along the magnet’s up-down (X) axis. The field distortion was computed using Fourier-analysis based field modeling at 3 Tesla11,12.

Shim Coil Design and Modeling : Two shim coils with 26G (0.4 mm OD) wire were designed in the needle’s coordinate system ( XN : Along needle length, YN , ZN : along radial directions) with normals along ZN (CN0) and YN (CN90). CN0 was designed with an angled split loop at its tip (with half the current in each arc) that followed the needle tip bevel. Coil geometry was transformed into the scanner’s coordinate frame assuming a (0,0,0) degree orientation along the magnet’s X axis. (Figure 3). The field from each coil in the 4003 target grid was then estimated by using an implementation of Biot-Savart’s field induction formula7-9, with a current path resolution element of 0.4 mm. A current of 1 Amp was used to calculate the unit Bz(r) field for both shim coils.

Field Shimming : The field produced by the two shim coils were used to shim the needle induced field by multilinear least squares fitting in Matlab with a mask defined to exclude voxels within the needle. The currents were constrained to 600 mAmperes in each coil.


Figure 2 shows the modeled ΔB0 of the needle for two orientations. The familiar dipolar pattern is observed that varies with needle orientation. Figure 4 shows the fields produced by the two coils for 1 Amp. The field variation is similar to that induced by the needle, which indicates the feasibility of shimming. Note that while the CN0 field matches the needle ΔB0, the CN90 field has a phase offset. As the needle is rotated about the X axis, the two coils therefore perform in a phased manner. Figure 5 shows the results of shimming. There is good correction of the ΔB0 induced by the needle in both orientations. The standard deviation of the field outside the needle was reduced from 334.6 Hz to 178.7 Hz for orientation 1 with currents of 510 mA and 2 mA and from 335 Hz to 192 Hz for orientation 2 with currents of 0 mA and 494 mA for CN0 and CN90 respectively. A small uncompensated field remained at the tip of the needle where the coils did not reach.


The results demonstrate the feasibility of shimming a needle with active shim coils. Generalized coil paths can be designed to provide robust shimming for all needle orientations, including ones designed using stream function methods13. The wire sizes and current constraints will clearly scale with needle size. For the 26G wire used here, the current capacity is ~505 mA assuming a coefficient of 500 circular-mils/Amp14. Our future work will include all these considerations in the design and eventual fabrication of such needles.


This work was supported by NIBIB 1R21EB025258. I would like to thank Dr Megan Poorman and Dr William A Grissom for advice and discussions on field modeling.


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Figure 1 : Structure of 10 G needle (a) Needle body designed in Solidworks. One tip had a 30o bevel (b) Voxelized needle grid in the scanner’s coordinate system. Midline sections in three orientations are shown. The needle is oriented in the base ΘX = 0oY = 0o and ΘZ = 0o orientation, where the angles represent needle rotations about the magnet’s X (up-Down) , Y ( left-Right) and Z (foot-head) axes. White voxels representing the needle were assigned the magnetic susceptibility of Titanium and black voxels were Water.

Figure 2 : Estimated needle Induced field at 3 Tesla for the two orientations. Midline sections in three orientations are shown. Note that the dipolar pattern observed in the midline coronal slice does not change with orientation due to needle symmetry at this X position. The resolution is 0.1 mm/voxel isotropic.

Figure 3 : Design of the shim coils (a) Solidworks sketch of the shim coils in the needle’s coordinate frame. All dimensions are in mm. CN0 and CN90 were designed orthogonally with the angle representing deviation from the Needle's Y axis (blue arrow). Only one angle is used to define the coils since all coils will be designed orthogonal to the needle’s radial plane. The X and O represent direction of current flow into and out of the plane (b) CN0 with split loop at the tip and (c) CN90 in the needle and magnet coordinate frames.

Figure 4 : Unit 1 Ampere fields induced by the shim coils in orientation 1 (a) and orientation 2 (b). Strong field variation is seen in both cases. Coronal slices was shown at two levels indicated in the top left axial frame by white dotted lines. Note in Coronal slice 1 that CN0 produces a field in phase with the needle’s induced field (Figure 2 Coronal slice). However, as the needle and the shim coils rotate about the X axis, CN0 field no longer matches the needles field. CN90 is now required to produce the corrective field.

Figure 5 : Results of simulated needle shimming for orientation1 (a & b) and orientation 2 (c & d). The grey voxels represent the mask used to exclude the voxels within the needle’s OD. Good compensation of the field inhomogeneity is predicted around the needle for both orientations, at all sections of the needle. For orientations at which ΘY and ΘZ ≠ 0o, a third coil may be required to shim the tip well while concurrently not damaging the shim more proximally at the body of the needle. Also the required currents are expected to be halved for 1.5 Tesla.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)