Dual‐echo Z‐shimmed proton resonance frequency‐shift magnetic resonance thermometry near metallic ablation probes: Technique and temperature precision

Abstract

Purpose To improve the precision of proton resonance frequency-shift MR thermometry near ablation probes, by recovering near-probe image signals that are typically lost due to magnetic susceptibility-induced field distortions. Methods A dual-echo gradient-recalled echo sequence was implemented, in which the first echo was under- or over-refocused in the slice dimension to recover image signal and temperature precision near a probe, and the second echo was fully-refocused to obtain image signal everywhere else in the slice. A penalized maximum likelihood algorithm was implemented to estimate a single temperature map from both echoes. Agar phantom and ex vivo experiments with and without microwave heating at 3 Tesla evaluated how much temperature precision was improved near a microwave ablator compared to a conventional single-echo scan as a function of slice and needle orientation in the magnet. Results The number of near-probe voxels with temperature standard deviation σ > 1 °C was decreased by 51% in the phantom experiment, averaged across orientations, and by 31% in the pork. Temperature maps near the probe were more smoother and more complete in all orientations. Conclusion Dual echo z-shimmed temperature imaging can recover image signal for more precise temperature mapping near metallic ablation probes.

Modeling of active shimming of metallic needles for interventional MRI

“…Lower artifact levels can reduce procedure times and error rates, especially when multiple probes are used in the same procedure. Furthermore, active shimming should be compatible and synergistic with most imaging‐based approaches for minimizing needle artifacts 26‐31,53 . Therefore, a combination of techniques could be used in materials such as stainless steel that have large susceptibility differences with water.…”

Experimental assessment of microwave ablation computational modeling with MR thermometry

“…). While prior studies 35 noted considerable impact of artifacts in proximity to metallic ablation probes, limited artifact may have been observed in this study since the water‐cooled ablation applicator does not have any metallic components in direct contact with tissue; rather, the antenna is surrounded by circulating water that is enclosed by plastic tubes.…”

“…[32][33][34] In these studies, MR thermometry images were acquired with different imaging sequences such as spoiled gradient-recalled echo (SPGR) and fast-SPGR in a single plane with an update time of~13-26 s. Prior studies have also employed gradient eco sequences (GRE) with Z-shimming for minimizing artifacts in the proximity of metallic microwave ablation applicator. 35 With advances in MR thermometry sequence development, multi-slice thermometry with update times of under 10 s are now available, offering a potential avenue for three-dimensional (3D) measurement of transient temperature profiles during ablation procedures. 23 In our earlier study, 36 we presented a preliminary assessment of 3D computational models of microwave thermal therapy (over the temperature range 20°C-45°C) in an agar phantom using a 14.1 T ultra-high field small-animal MR scanner.…”

“…Some prior studies have investigated the use of 1.5 T MRI thermometry for monitoring microwave heating of ex vivo tissues such as brain, muscle, liver, kidney, in vivo rabbit brain, as well as ablation of prostate cancer in humans 32–34 . In these studies, MR thermometry images were acquired with different imaging sequences such as spoiled gradient‐recalled echo (SPGR) and fast‐SPGR in a single plane with an update time of ~13–26 s. Prior studies have also employed gradient eco sequences (GRE) with Z ‐shimming for minimizing artifacts in the proximity of metallic microwave ablation applicator 35 . With advances in MR thermometry sequence development, multi‐slice thermometry with update times of under 10 s are now available, offering a potential avenue for three‐dimensional (3D) measurement of transient temperature profiles during ablation procedures 23 …”

MRI monitoring of focused ultrasound sonications near metallic hardware

“…Recently presented enhancements of conventionally used proton‐resonance‐frequency‐shift (PRFS) thermometry have the potential to additionally provide noninvasive temperature measurement in the presence of moderate off‐resonance of a few hundred Hertz . However, there exists no technique for monitoring focused ultrasound sonications near larger metallic hardware such as hip arthroplasty, which typically induce off‐resonance in the range of many kHz.…”