Temperature mapping considerations in the breast with line scan echo planar spectroscopic imaging

Abstract

A line-scan echo planar spectroscopic imaging (LSEPSI) sequence was used to serially acquire spectra from 4096 voxels every 6.4 s throughout the breasts of nine female subjects in vivo. Data from the serial acquisitions were analyzed to determine the potential of the technique to characterize temperature changes using either the water frequency alone or the watermethylene frequency difference. Fluctuations of the apparent temperature change under these conditions of no heating were smallest using the water-methylene frequency difference, most probably due to a substantial reduction of motion effects both within and without the imaged plane. The approach offers considerable advantages over other methods for temperature change monitoring in the breast with magnetic resonance but suffers from some limitations, including the unavailability of lipid and water resonances in some voxels as well as a surprisingly large distribution of water-methylene frequency differences, which may preclude absolute temperature measurement. Most of the magnetic resonance (MR) parameters that contribute to the signal intensity of MR images are temperature sensitive and so of potential utility for guiding thermal therapies (1-5). Of the available MR parameters, there is a growing consensus that the temperature sensitivity of the water proton resonant frequency (PRF) is most suited for guiding thermal therapies. Utilizing the water PRF is advantageous because it appears to be tissue type independent, at least in nonfatty tissues (6). Furthermore, the PRF appears to be minimally affected by heat-induced tissue changes and can be accessed using standard MR methods like gradient echo imaging sequences (7-10). Overall, the PRF appears to be more sensitive than other endogenous parameters like water relaxation times or water diffusion (4). The PRF method has been shown to be useful in numerous animal studies for online guidance of thermal ablation (10 -13) and its use has been demonstrated in several clinical studies (14 -19). The temperature sensitivity of the water PRF arises from changes in the degree of electron screening induced by temperature-induced variations in the hydrogen bonding network (20).Typically, changes in water PRF are estimated using phase-difference images acquired with gradient echo sequences (7). Difference images are generally employed to remove the phase variations caused by spatial inhomogeneities in the magnetic field. Because of the use of image subtractions, the method is sensitive to non-temperaturerelated effects that alter the local magnetic field between scans. These changes can include drift in the static field (21), patient motion outside of the imaging plane (22), and local or global changes in magnetic susceptibility, which can be altered by heating itself (23,24). The sensitivity of the phase-difference MR thermometry to motion sensitivity is particularly problematic. For example, when motion occurs between scans, phase maps acquired during heating are no longer aligned with the baseline scans used in the...

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