Absolute MR thermometry using time‐domain analysis of multi‐gradient‐echo magnitude images

MRI allows for absolute temperature measurements in substances containing two spectral resonances of which the frequency difference Δf(T) is related to absolute temperature. This frequency difference can be extracted from spectroscopic data. An image‐based MR technique that allows for the acquisition of spectroscopic data at high temporal and spatial resolution is the multi‐gradient‐echo sequence. In this work, the application of the multi‐gradient‐echo sequence for MR thermometry purposes was further developed. We investigated the possibility of postprocessing the multi‐gradient‐echo data into absolute temperature maps, using time‐domain analysis of the magnitude of the multi‐gradient‐echo signals. In this approach, instead of an indirect computation of Δf(T) from separately found frequencies, Δf(T) is a direct output parameter. In vitro experiments were performed to provide proof of concept for retrieving absolute temperature maps from the time‐domain analysis of multi‐gradient‐echo magnitude images. It is shown that this technique is insensitive to both field drift and local field disturbances. Furthermore, ex vivo bone marrow experiments were performed, using the fat resonance as a reference for absolute temperature mapping. It is shown that the postprocessing based on the magnitude signal in the time domain allows for the determination of Δf(T) in bone marrow. Magn Reson Med 64:239–248, 2010. © 2010 Wiley‐Liss, Inc.     

Noninvasive temperature mapping with MRI using chemical shift water‐fat separation

Tissues containing both water and lipids, e.g., breast, confound standard MR proton reference frequency‐shift methods for mapping temperatures due to the lack of temperature‐induced frequency shift in lipid protons. Generalized Dixon chemical shift–based water‐fat separation methods, such as GE's iterative decomposition of water and fat with echo asymmetry and least‐squares estimation method, can result in complex water and fat images. Once separated, the phase change over time of the water signal can be used to map temperature. Phase change of the lipid signal can be used to correct for non‐temperature‐dependent phase changes, such as amplitude of static field drift. In this work, an image acquisition and postprocessing method, called water and fat thermal MRI, is demonstrated in phantoms containing 30:70, 50:50, and 70:30 water‐to‐fat by volume. Noninvasive heating was applied in an Off1‐On‐Off2 pattern over 50 min, using a miniannular phased radiofrequency array. Temperature changes were referenced to the first image acquisition. Four fiber optic temperature probes were placed inside the phantoms for temperature comparison. Region of interest (ROI) temperature values colocated with the probes showed excellent agreement (global mean ± standard deviation: ?0.09 ± 0.34°C) despite significant amplitude of static field drift during the experiments. Magn Reson Med 63:1238–1246, 2010. © 2010 Wiley‐Liss, Inc.     

Real‐time MR thermometry for monitoring HIFU ablations of the liver

A high‐resolution and high‐speed pulse sequence is presented for monitoring high‐intensity focused ultrasound ablations in the liver in the presence of motion. The sequence utilizes polynomial‐order phase saturation bands to perform outer volume suppression, followed by spatial‐spectral excitation and three readout segmented echo‐planar imaging interleaves. Images are processed with referenceless thermometry to create temperature‐rise images every frame. The sequence and reconstruction were implemented in RTHawk and used to image stationary and moving sonications in a polyacrylamide gel phantom (62.4 acoustic W, 50 sec, 550 kHz). Temperature‐rise images were compared between moving and stationary experiments. Heating spots and corresponding temperature‐rise plots matched very well. The stationary sonication had a temperature standard deviation of 0.15° C compared to values of 0.28° C and 0.43° C measured for two manually moved sonications at different velocities. Moving the phantom (while not heating) with respect to the transducer did not cause false temperature rises, despite susceptibility changes. The system was tested on nonheated livers of five normal volunteers. The mean temperature rise was ? 0.05° C, with a standard deviation of 1.48° C. This standard deviation is acceptable for monitoring high‐intensity focused ultrasound ablations, suggesting real‐time imaging of moving high‐intensity focused ultrasound sonications can be clinically possible. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

MR safety: Fast T1 thermometry of the RF‐induced heating of medical devices

Determining the MR compatibility of medical implants and devices is becoming increasingly relevant. In most cases, the heating of conductive implants due to radiefrequency (RF) excitation pulses is measured by fluoroptic temperature sensors in relevant tests for approval. Another common method to determine these heating effects is MR thermometry using the proton resonance frequency. This method gives good results in homogeneous phantoms. However in many cases, technical shortcomings such as susceptibility artifacts prohibit exact proton resonance frequency thermometry near medical implants. Therefore, this work aimed at developing a fast T₁‐based method which allows controlled MR‐related heating of a medical implant while simultaneously quantifying the spatial and temporal temperature distribution. To this end, an inversion recovery snapshot Fast Low‐Angle Shot (FLASH) sequence was modified with additional off‐resonant heating pulses. With an accelerated imaging method and a sliding‐window technique, every 7.6 s a new temperature map could be generated with a spatial in‐plane resolution of 2 mm. The temperature deviation from calculated temperature values to reference fluoroptic probe was found to be smaller than 1 K. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.