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.

Mingming Wu,Axel Haase,Gerard C. van Rhoon,Marion I. Menzel,Paul Baron,Hendrik T. Mulder,Eduardo Coello

doi:10.1002/mrm.28302

Abstract

PurposeThe linear change of the water proton resonance frequency shift (PRFS) with temperature is used to monitor temperature change based on the temporal difference of image phase. Here, the effect of motion‐induced susceptibility artifacts on the phase difference was studied in the context of mild radio frequency hyperthermia in the pelvis.MethodsFirst, the respiratory‐induced field variations were disentangled from digestive gas motion in the pelvis. The projection onto dipole fields (PDF) as well as the Laplacian boundary value (LBV) algorithm were applied on the phase difference data to eliminate motion‐induced susceptibility artifacts. Both background field removal (BFR) algorithms were studied using simulations of susceptibility artifacts, a phantom heating experiment, and volunteer and patient heating data.ResultsRespiratory‐induced field variations were negligible in the presence of the filled water bolus. Even though LBV and PDF showed comparable results for most data, LBV seemed more robust in our data sets. Some data sets suggested that PDF tends to overestimate the background field, thus removing phase attributed to temperature. The BFR methods even corrected for susceptibility variations induced by a subvoxel displacement of the phantom. The method yielded successful artifact correction in 2 out of 4 patient treatment data sets during the entire treatment duration of mild RF heating of cervical cancer. The heating pattern corresponded well with temperature probe data.ConclusionThe application of background field removal methods in PRFS‐based MR thermometry has great potential in various heating applications and body regions to reduce motion‐induced susceptibility artifacts that originate outside the region of interest, while conserving temperature‐induced PRFS. In addition, BFR automatically removes up to a first‐order spatial B0 drift.

Highlights

Mild hyperthermia (HT) treatment (40-44°C) of different types of cancer is applied to sensitize tumor cells to radiotherapy and chemotherapy.[1]
Because common MR thermometry methods rely on a reference scan, the methods are very sensitive to any interscan motion
Displacement of tissue outside the region of interest (ROI) originating from breathing or digestive motion alters the underlying macroscopic magnetic field with time, which would be misinterpreted as temperature change

Summary

Introduction

Mild hyperthermia (HT) treatment (40-44°C) of different types of cancer is applied to sensitize tumor cells to radiotherapy and chemotherapy.[1]. For mild HT of the pelvis, radio frequency (RF) hyperthermia (RF-HT) provides both deep penetration depth and a large enough tissue coverage during heating. Double Echo Gradient Echo (DEGRE) is used for monitoring mild HT of tumors in the pelvis[2,5] and sarcomas.[6]. If the region of interest (ROI) is displaced, the reference method cannot be applied any longer. Displacement of tissue outside the ROI originating from breathing or digestive motion alters the underlying macroscopic magnetic field with time, which would be misinterpreted as temperature change. The alteration of the magnetic susceptibility distribution induces a field change equivalent to the susceptibility difference convoluted with a dipole. Because the dipole propagates into neighboring tissue, large errors in temperature maps are induced.[5,9] We first investigated the impact of breathing on the magnetic field change within the pelvic region inside and outside the RF applicator in the presented work

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