Resonance Frequency Shift MR Thermometry Research Articles

Field drift correction of proton resonance frequency shift temperature mapping with multichannel fast alternating nonselective free induction decay readouts.

PurposeTo demonstrate that proton resonance frequency shift MR thermometry (PRFS‐MRT) acquisition with nonselective free induction decay (FID), combined with coil sensitivity profiles, allows spatially resolved B 0 drift‐corrected thermometry.MethodsPhantom experiments were performed at 1.5T and 3T. Acquisition of PRFS‐MRT and FID were performed during MR‐guided high‐intensity focused ultrasound heating. The phase of the FIDs was used to estimate the change in angular frequency δωdrift per coil element. Two correction methods were investigated: (1) using the average δωdrift over all coil elements (0th‐order) and (2) using coil sensitivity profiles for spatially resolved correction. Optical probes were used for independent temperature verification. In‐vivo feasibility of the methods was evaluated in the leg of 1 healthy volunteer at 1.5T.ResultsIn 30 minutes, B 0 drift led to an apparent temperature change of up to –18°C and –98°C at 1.5T and 3T, respectively. In the sonicated area, both corrections had a median error of 0.19°C at 1.5T and –0.54°C at 3T. At 1.5T, the measured median error with respect to the optical probe was –1.28°C with the 0th‐order correction and improved to 0.43°C with the spatially resolved correction. In vivo, without correction the spatiotemporal median of the apparent temperature was at –4.3°C and interquartile range (IQR) of 9.31°C. The 0th‐order correction had a median of 0.75°C and IQR of 0.96°C. The spatially resolved method had the lowest median at 0.33°C and IQR of 0.80°C.ConclusionFID phase information from individual receive coil elements allows spatially resolved B 0 drift correction in PRFS‐based MRT.

Multiple-point magnetic resonance acoustic radiation force imaging.

Purpose:To implement and evaluate an efficient multiple-point MR acoustic radiation force imaging pulse sequence that can volumetrically measure tissue displacement and evaluate tissue stiffness using focused ultrasound (FUS) radiation force.Methods:Bipolar motion-encoding gradients were added to a gradient-recalled echo segmented EPI pulse sequence with both 2D and 3D acquisition modes. Multiple FUS-ON images (FUS power > 0 W) were interleaved with a single FUS-OFF image (FUS power = 0 W) on the TR level, enabling simultaneous measurements of volumetric tissue displacement (by complex subtraction of the FUS-OFF image from the FUS-ON images) and proton resonance frequency shift MR thermometry (from the OFF image). Efficiency improvements included partial Fourier acquisition, parallel imaging, and encoding up to 4 different displacement positions into a single image. Experiments were performed in homogenous and dual-stiffness phantoms, and in ex vivo porcine brain.Results:In phantoms, 16-point multiple-point magnetic resonance acoustic radiation force imaging maps could be acquired in 5 s to 10 s for a 2D slice, and 60 s for a 3D volume, using parallel imaging and encoding 2 displacement positions/image. In ex vivo porcine brain, 16-point multiple-point magnetic resonance acoustic radiation force imaging maps could be acquired in 20 s for a 3D volume, using partial Fourier and parallel imaging and encoding 4 displacement positions/image. In 1 experiment it was observed that tissue displacement in ex vivo brain decreased by approximately 22% following FUS ablation.Conclusion:With the described efficiency improvements it is possible to acquire volumetric multiple-point magnetic resonance acoustic radiation force imaging maps, with simultaneous proton resonance frequency shift MR thermometry maps, in clinically acceptable times.

First clinical experience with a dedicated MRI-guided high-intensity focused ultrasound system for breast cancer ablation.

ObjectivesTo assess the safety and feasibility of MRI-guided high-intensity focused ultrasound (MR-HIFU) ablation in breast cancer patients using a dedicated breast platform.MethodsPatients with early-stage invasive breast cancer underwent partial tumour ablation prior to surgical resection. MR-HIFU ablation was performed using proton resonance frequency shift MR thermometry and an MR-HIFU system specifically designed for breast tumour ablation. The presence and extent of tumour necrosis was assessed by histopathological analysis of the surgical specimen. Pearson correlation coefficients were calculated to assess the relationship between sonication parameters, temperature increase and size of tumour necrosis at histopathology.ResultsTen female patients underwent MR-HIFU treatment. No skin redness or burns were observed in any of the patients. No correlation was found between the applied energy and the temperature increase. In six patients, tumour necrosis was observed with a maximum diameter of 3–11 mm. In these patients, the number of targeted locations was equal to the number of areas with tumour necrosis. A good correlation was found between the applied energy and the size of tumour necrosis at histopathology (Pearson = 0.76, p = 0.002).ConclusionsOur results show that MR-HIFU ablation with the dedicated breast system is safe and results in histopathologically proven tumour necrosis.Key Points• MR-HIFU ablation with the dedicated breast system is safe and feasible • In none of the patients was skin redness or burns observed • No correlation was found between the applied energy and the temperature increase • The correlation between applied energy and size of tumour necrosis was good

Fat‐referenced MR thermometry in the breast and prostate using IDEAL

To demonstrate a three-echo fat-referenced MR thermometry technique that estimates and corrects for time-varying phase disturbances in heterogeneous tissues. Fat protons do not exhibit a temperature-dependent frequency shift. Fat-referenced thermometry methods exploit this insensitivity and use the signal from fat to measure and correct for magnetic field disturbances. In this study, we present a fat-referenced method that uses interpolation of the fat signal to correct for phase disturbances in fat free regions. Phantom and ex vivo tissue cool-down experiments were performed to evaluate the accuracy of this method in the absence of motion. Non-heated in vivo imaging of the breast and prostate was performed to demonstrate measurement robustness in the presence of systemic and motion-induced field disturbances. Measurement accuracy of the method was compared to conventional proton resonance frequency shift MR thermometry. In the ex vivo porcine tissue experiment, maximum measurement error of the fat-referenced method was reduced 42% from 3.3 to 1.9°C when compared to conventional MR thermometry. In the breasts, measurement errors were reduced by up to 70% from 6.4 to 1.9°C. Ex vivo and in vivo results show that the proposed method reduces measurement errors in the heterogeneous tissue experiments when compared to conventional MR thermometry.

ARFI‐prepared MRgHIFU in liver: Simultaneous mapping of ARFI‐displacement and temperature elevation, using a fast GRE‐EPI sequence

MR acoustic radiation force imaging (ARFI) is an elegant adjunct to MR-guided high intensity focused ultrasound for treatment planning and optimization, permitting in situ assessment of the focusing and targeting quality. The thermal effect of high intensity focused ultrasound pulses associated with ARFI measurements is recommended to be monitored on line, in particular when the beam crosses highly absorbent structures or interfaces (e.g., bones or air-filled cavities). A dedicated MR sequence is proposed here, derived from a segmented gradient echo-echo planar imaging kernel by adding a bipolar motion encoding gradient with interleaved alternating polarities. Temporal resolution was reduced to 2.1 s, with in-plane spatial resolution of 1 mm. MR-ARFI measurements were executed during controlled animal breathing, with trans-costal successively steered foci, to investigate the spatial modulation of the focus intensity and the targeting offset. ARFI-induced tissue displacement measurements enabled the accurate localization, in vivo, of the high intensity focused ultrasound focal point in sheep liver, with simultaneous monitoring of the temperature elevation. ARFI-based precalibration of the focal point position was immediately followed by trans-costal MR-guided high intensity focused ultrasound ablation, monitored with a conventional proton resonance frequency shift MR thermometry sequence. The latter MR thermometry sequence had spatial resolution and geometrical distortion identical with the ARFI maps, hence no coregistration was required.

Quantifying tissue damage due to focused ultrasound heating observed by MRI.

Focused ultrasound heating of ex vivo bovine kidney and liver was monitored using magnetic resonance imaging (MRI) to investigate the quantitative relationship between time-dependent temperature elevations and altered contrast in MR images due to thermal coagulation. Proton resonance frequency shift MR thermometry was performed during heating at 10 sec intervals (single-slice fast spoiled GRASS [FSPGR], theta/TE/TR 30 degrees/11/39 msec, field of view 8 cm, 256 x 256, 3 mm slice thickness, 1 NEX); post-heating MR images were T1-weighted (3D-FSPGR, theta/TE/TR 60 degrees/25/200 msec, 1 mm slice thickness, 3 NEX). Analysis of the resulting temperature versus time data using the Arrhenius relationship and a simple binary discrimination model showed that thermal coagulation occurred with heating at approximately 54 degrees C for 10 sec in both tissues and could be predicted with approximately 625 microm spatial resolution. These results suggest that quantitative MR guidance of thermal coagulation therapy is feasible, and they provide information useful for designing future investigations in vivo.