A phase-cycled temperature-sensitive fast spin echo sequence with conductivity bias correction for monitoring of mild RF hyperthermia with PRFS

Abstract

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Correction of motion‐induced susceptibility artifacts and B0 drift during proton resonance frequency shift‐based MR thermometry in the pelvis with background field removal methods

“…The latter solution may raise concerns about patient comfort due to acoustic noise. A more time‐efficient acquisition scheme, for instance using a fast spin echo sequence 26 or a slightly reduced FOV would increase the likelihood of scanning during a time period without intrascan motion. Although applying BFR worked robustly on the volunteer data sets, interscan bulk motion due to patient discomfort is more likely to occur during mild RF‐HT and was the reason of the dropout of 1 examined patient treatment data set.…”

“…The low spatial resolution limits the correct representation of the susceptibility‐induced field change and, therefore, the power of the background field removal step. Using a 3D gradient echo acquisition scheme, a fast spin echo sequence, 26 or switching to 3 T, would all potentially lead to higher SNR. Enough image coverage is needed to detect potential hotspots and is advantageous for reliably solving the background field problem but requires longer scan time.…”

“…The cylindrical phantom mimicked the electrical properties of human tissue. It was made of 89.76% demineralized water, 10% of a hydrophilic organic polymer solidifying powder, called “super stuff” (TX‐151), 0.205% NaCl, and 0.04% NaN3 (percentage values of the total weight are given), and was used in previous validation experiments 26 . The phantom was 30 cm in diameter and 40 cm in depth.…”

“…One common approach for extracting and correcting for B 0 field drift uses references outside the heated area and extrapolates a first‐order spatial field drift over the acquired image. External reference tubes were filled with oil or silicon 25,26 . Problems associated with external reference tubes include an increased imaging volume, low signal‐to‐noise‐ratio (SNR) of the reference signal, and thus low robustness of the correction method.…”

“…In the context of deep mild RF‐HT, the reference tubes are placed in the periphery of the field of view, where it suffers from increased field inhomogeneity and thus a shorter and lower SNR. Alternatively, the patient’s adipose tissue signal was used as a reference 26 . As discussed before, 11,12 this will only work if the fat tissue is not heated itself.…”