Modeling of active shimming of metallic needles for interventional MRI

Abstract

Purpose Artifacts caused by large magnetic susceptibility differences between metallic needles and tissue are a persistent problem in many interventional MRI applications. The signal void caused by the needle can hide procedure targets and prevent accurate image‐based monitoring. In this paper, a solution to this problem is presented in the form of an active shim insert inspired from degaussing coils used in naval vessels, that is designed to correct the field disturbance (ΔB0) caused by the needle. Methods The ΔB0 induced by a 10 gauge hollow single‐beveled titanium needle at 3T is modeled in different orientations. A set of 63 orthogonal coil pairs with unique tip paths are evaluated for shimming performance, and an optimal coil pair is chosen. Shimming performance and current demands are evaluated over a range of needle orientations. Results Robust correction of the titanium needle induced ΔB0 is predicted using a flat no‐loop coil combined with an orthogonal 1½ turn loop coil angled at the bevel angle for most orientations, with currents well below 1 amp per coil. Reductions in ΔB0 standard deviations with shimming ranged from ~49% to ~10% depending on needle orientation, with performance worsening as the needle is aligned more along B0. Conclusion Simulations predict that it is possible to minimize metallic probe induced ΔB0 and signal losses using externally supplied direct current shim coil inserts in arbitrary orientations for potential benefits in many interventional MRI applications.

Minimal artifact actively shimmed metallic needles in MRI

“…When the needle was parallel to B 0 (0, −90, 0 degrees), shimming did not have noticeable impact on the signal void. This was expected from simulations based on the chosen coil structure and has been discussed in detail in our earlier simulation work 41 . The phase precision maps did not show significant differences between the shimmed and unshimmed cases, apart from the much larger uncorrected void in the latter.…”

“…This was expected from simulations based on the chosen coil structure and has been discussed in detail in our earlier simulation work. 41 The phase precision maps did not show significant differences between the shimmed and unshimmed cases, apart from the much larger uncorrected void in the latter. However, a much steeper phase gradient around the void was noted in the unshimmed needle.…”

“…Without any rotations (0, 0, 0‐degree), the needle was oriented vertically with the beveled tip pointing down along ‐X M (Figure 1A). We have reported shimming simulations in detail in our earlier work, 41 and a similar procedure was followed here. Shim coils were simulated for a 4/3 mm outside diameter/inside diameter (OD/ID), hollow Titanium needle with a 30° single‐sided bevel at the tip (volume susceptibility: χ = 182 × 10 −6 ) 42 with water inside and around it (χ = −9.05 × 10 −6 ) 42 at 3T.…”

“…The goal of this work is to explore a solution to this problem in the form of a multicoil active shim insert that is designed to fit inside the metallic needle and produce a field that corrects the ΔB 0 outside of it. In our earlier work, we presented modeling studies to demonstrate the correction of the ΔB 0 around a 10‐gauge titanium needle 41 . In this paper, we present the fabrication of a shim insert and correction of the signal void artifact around a titanium needle at 3T.…”

“…In our earlier work, we presented modeling studies to demonstrate the correction of the ΔB 0 around a 10-gauge titanium needle. 41 In this paper, we present the fabrication of a shim insert and correction of the signal void artifact around a titanium needle at 3T. Signal recovery and correction of the underlying ΔB 0 across different imaging sequences and needle orientations is demonstrated.…”