Interleaved water and fat MR thermometry for monitoring high intensity focused ultrasound ablation of bone lesions

Abstract

Purpose To demonstrate that interleaved MR thermometry can monitor temperature in water and fat with adequate temporal resolution. This is relevant for high intensity focused uUltrasounds (HIFU) treatment of bone lesions, which are often found near aqueous tissues, as muscle, or embedded in adipose tissues, as subcutaneous fat and bone marrow. Methods Proton resonance frequency shift (PRFS)‐based thermometry scans and T1‐based 2D variable flip angle (2D‐VFA) thermometry scans were acquired alternatingly over time. Temperature in water was monitored using PRFS thermometry, and in fat by 2D‐VFA thermometry with slice profile effect correction. The feasibility of interleaved water/fat temperature monitoring was studied ex vivo in porcine bone during MR‐HIFU sonication. Precision and stability of measurements in vivo were evaluated in a healthy volunteer under non‐heating conditions. Results The method allowed observing temperature change over time in muscle and fat, including bone marrow, during MR‐HIFU sonication, with a temporal resolution of 6.1 s. In vivo, the apparent temperature change was stable on the time scale of the experiment: In 7 min the systematic drift was <0.042°C/min in muscle (PRFS after drift correction) and <0.096°C/min in bone marrow (2D‐VFA). The SD of the temperature change averaged over time was 0.98°C (PRFS) and 2.7°C (2D‐VFA). Conclusions Interleaved MR thermometry allows temperature measurements in water and fat with a temporal resolution high enough for monitoring HIFU ablation. Specifically, combined fat and water thermometry provides uninterrupted information on temperature changes in tissue close to the bone cortex.

Simultaneous proton resonance frequency T1 ‐ MR shear wave elastography for MR‐guided focused ultrasound multiparametric treatment monitoring

“…As mentioned previously, the Ernst angle—which maximizes the SNR for gradient‐recalled echo–type pulse sequences—is temperature‐dependent (due to the temperature dependence of T 1 ) and therefore will change throughout the experiment. As showed by Lena et al, the best T 1 precision is achieved when the Ernst angle for the middle of the expected temperature range is used 90 . This study was performed for a single tissue type, so the relationship will be more complicated when imaging, for example, both muscle and fat.…”

“…In this work, the two flip angles were selected as 71% of the Ernst angle at room temperature, as estimated over a region of interest (ROI) by a vendor‐supplied T 1 mapping sequence. It can be noted that the Ernst angle depends on T 1 , which in turn depends on temperature, so the Ernst angle will also be changing with temperature, and the most accurate approach is to select the Ernst angle that corresponds to the middle of the temperature range of interest 89,90 …”