Multi-echo gradient echo pulse sequences: which is best for PRFS MR thermometry guided hyperthermia?

PURPOSE: MR thermometry (MRT) enables noninvasive temperature monitoring during hyperthermia treatments. MRT is already clinically applied for hyperthermia treatments in the abdomen and extremities, and devices for the head are under development. In order to optimally exploit MRT in all anatomical regions, the best sequence setup and post-processing must be selected, and the accuracy needs to be demonstrated. METHODS: MRT performance of the traditionally used double-echo gradient-echo sequence (DE-GRE, 2 echoes, 2D) was compared to multi-echo sequences: a 2D fast gradient-echo (ME-FGRE, 11 echoes) and a 3D fast gradient-echo sequence (3D-ME-FGRE, 11 echoes). The different methods were assessed on a 1.5 T MR scanner (GE Healthcare) using a phantom cooling down from 59 °C to 34 °C and unheated brains of 10 volunteers. In-plane motion of volunteers was compensated by rigid body image registration. For the ME sequences, the off-resonance frequency was calculated using a multi-peak fitting tool. To correct for B0 drift, the internal body fat was selected automatically using water/fat density maps. RESULTS: The accuracy of the best performing 3D-ME-FGRE sequence was 0.20 °C in phantom (in the clinical temperature range) and 0.75 °C in volunteers, compared to DE-GRE values of 0.37 °C and 1.96 °C, respectively. CONCLUSION: For hyperthermia applications, where accuracy is more important than resolution or scan-time, the 3D-ME-FGRE sequence is deemed the most promising candidate. Beyond its convincing MRT performance, the ME nature enables automatic selection of internal body fat for B0 drift correction, an important feature for clinical application.

An MR-compatible antenna and application in a murine superficial hyperthermia applicator.

In this work, a novel magnetic resonance (MR)-compatible microwave antenna was designed and validated in a small animal superficial hyperthermia applicator. The antenna operates at 2.45 GHz and matching is made robust against production and setup inaccuracies. To validate our theoretical concept, a prototype of the applicator was manufactured and tested for its properties concerning input reflection, sensitivity for setup inaccuracies, environment temperature stability and MR-compatibility. The experiments show that the applicator indeed fulfils the requirements for MR-guided hyperthermia investigation in small animals: it creates a small heating focus (<1 cm3), has a stable and reliable performance (S11< -15 dB) for all working conditions and is MR-compatible.