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 magnetic resonance 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 T 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. Magn Reson Med 78:2299-2306, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Keywords: ablation; interventional magnetic resonance imaging; magnetic resonance thermometry; microwave; z-shimming.

Figure 1

The proposed dual echo z-shimmed PRF-shift thermometry pulse sequence.

Figure 2

(a) First echo images in an agar gel phantom with different values of p show different levels and distribution of signal recovery near the probe. Zoomed near-probe regions are shown in the lower left insets. The green highlighted region in the zoomed p = 100% image is the near-probe ROI for measuring signal recovery. The ROI was defined as the set of voxels around the probe where the second echo image had less than 70% of the mean signal amplitude in the phantom. (b) Mean voxel magnitudes in the near-probe ROI in sum-of-squares combined dual-echo images, normalized by the mean intensity in the same ROI in the second echo (100%) image. ROI signal is maximized at p = 140%. (c) Magnitude images of the first echo (p = 140%), the second echo and the combined two-echo images. The region near the probe is zoomed in for each case. A spin echo image is also shown to illustrate how close the recovery comes to filling in the hole around the probe.

Figure 3

Image signal recovery and temperature map precision in agar gel phantoms with different probe and slice orientations and no heating. The value of p used is reported for each case. Magnitude images of the first echo and second echo images are shown in the first two columns, and their combined magnitude is shown in the third column. For the combined images, mean signal in the low-signal ROI around the wire is reported as a percentage of the mean ROI signal in the second echo images. The positions of the perpendicular slices are indicated by the yellow lines in the magnitude images in the parallel slice images. Maps of temperature standard deviation (σ) across time are shown in the fourth and fifth columns, for the fully-refocused echo alone and for the dual-echo scan. The number of voxels with σ > 1 °C is reported on an inset zoomed-in σ map for each case; smaller numbers are better. Figure S1 plots the temperature curve over time for the recovered voxel indicated by the white arrow in the parallel probe/perpendicular slice σ map, to illustrate the range of temperature errors with single- and dual-echo scans.

Figure 4

Temperature maps in the agar gel phantoms after five minutes of heating in each needle and slice orientation using single echo baseline subtraction and the proposed dual echo method. The positions of the perpendicular slices are indicated by the yellow lines in the magnitude images in the parallel slice images. To facilitate comparison, the single echo maps were masked to zero where the dual echo maps were zero. Zoomed near-probe regions are shown in the lower left insets. In all cases the dual-echo maps are smoother near the probe.

Figure 5

Porcine muscle heating experiment. (a) Mean image signal recovery in the near-probe ROI as a function of p. The signal is maximized at p = 50%. (b) First echo (p = 50%) and second echo images from the dual-echo sequence. (c) Temperature standard deviation across 100 dynamics before heating. The number of voxels with σ > 1 °C is reported; smaller numbers are better. (d) Temperature maps at low heat (< 24.5°C, before temperature wrapping would occur in the single echo map), and high heat, as well as maximum temperature across the 180 second ablation at 25 Watts, using single echo baseline subtraction and the proposed dual echo method. Zoomed near-probe regions are shown in the lower left insets in (b), (c), and (d). To facilitate comparison, the single echo maps were masked to zero where the dual echo maps were zero.