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.

Keywords: MR thermometry; PRFS; VFA; bone; high-intensity focused ultrasound; interleaved MRI.

FIGURE 1

A, Fat and water images acquired with the interleaved framework (magnitude images at two FAs for 2D‐VFA MRT and phase images for PRFS MRT). B, 2D‐VFA T₁ mapping. C, 2D‐VFA‐based and PRFS‐based MRT calculation. D, Fusion of MRT images

FIGURE 2

Experimental setup for the ex vivo experiment. The ultrasound transducer is outlined in blue and the ultrasound beam path in red. The imaging slice was positioned to match the focal location (in yellow)

FIGURE 3

Combined water and fat thermometry during a HIFU sonication experiment. A, the asterisks indicate the location of the voxels considered. B, 2D‐VFA‐based temperature changes (°C) vs time estimated in fat voxels. C, Temperature changes vs time estimated using 2D‐VFA‐based MRT in fat and using PRFS‐based MRT in muscle

FIGURE 4

Temperature change maps at the temperature peak. A, From PRFS in water voxels. B, From 2D‐VFA T1 in fat voxels. C, Overlaying the two maps, a temperature change map from the interleaved thermometry is provided

FIGURE 5

A, Temporal SD map of VFA‐based temperature change in fat voxels of a transverse slice, located on the lower leg of a volunteer. B, Temperature change estimates, after spatial averaging with a gaussian filter, in the volunteer study without heating: 2D‐VFA‐based temperature changes in three voxels in the bone marrow and PRFS‐based temperature change in three voxels in the muscle