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The aim of the current study was to improve temperature‐monitoring precision using multiecho proton resonance frequency shift‐based thermometry with view‐sharing acceleration for MR‐guided laser interstitial thermal therapy (MRgLITT) on a 0.5‐T low‐field MR system. Both precision and speed of the temperature measurement for clinical MRgLITT treatments suffer at low field, due to reduced image signal‐to‐noise ratio (SNR), decreased temperature‐induced phase changes, and limited RF receiver channels. In this work, a bipolar multiecho gradient‐recalled echo sequence with a temperature‐to‐noise ratio optimal weighted echo combination is applied to improve the temperature precision. A view‐sharing–based approach is utilized to accelerate signal acquisitions while preserving image SNRs. The method was evaluated using ex vivo (pork and pig brain) LITT heating experiments and in vivo (human brain) nonheating experiments on a high‐performance 0.5‐T scanner. In terms of results, (1) after echo combination, multiecho thermometry (i.e., ~7.5–40.5 ms, 7 TEs) provides ~1.5–1.9 times higher temperature precision than the no echo combination case (i.e., TE7 = 40.5 ms) within the same readout bandwidth. Additionally, echo registration is necessary for the bipolar multiecho sequence; (2) for a threefold acceleration, the view‐sharing approach with variable‐density subsampling shows around 1.8 times lower temperature errors than the GRAPPA method. Particularly for view‐sharing, variable‐density subsampling performs better than Interleave subsampling; and (3) ex vivo heating and in vivo nonheating experiments demonstrated that the temperature accuracy was less than 0.5 °C and that the temperature precision was less than 0.6 °C using the proposed 0.5‐T thermometry. It was concluded that view‐sharing accelerated multiecho thermometry is a practical temperature measurement approach for MRgLITT at 0.5 T. [ABSTRACT FROM AUTHOR]

Aims: This study aimed to explore the accuracy of non-invasive temperature measurement based on proton resonance frequency (PRF) phase subtraction in microwave ablation (MWA).Methods and Material: The signal change of the agar phantom during the ablation process was monitored by the gradient echo sequence under 1.5 T superconducting magnetic resonance imagining (MRI), and the temperature change was converted by the phase subtraction method of the PRF, which was compared with the temperature measured using an optical fiber.Statistical Analysis Used: SPSS software version 22.0 was used for data processing, and the independent sample t-test was used for comparative analysis. P < 0.05 indicated statistical significance.Results: The maximum error between the MRI temperature measurement and the standard value was 3.61°C, whereas the minimum and average errors were 0.01°C and 1.19°C ± 0.78°C, respectively.Conclusions: The temperature measurement technology, which is based on the PRF phase method, has good accuracy in MRI-guided MWA. [ABSTRACT FROM AUTHOR]

Objective: Artifacts limit the application of proton resonance frequency (PRF) thermometry for on-site, individualized heating evaluations of implantable medical devices such as deep brain stimulation (DBS) for use in magnetic resonance imaging (MRI). Its properties are unclear and the research on how to choose an unaffected measurement region is insufficient. Methods: The properties of PRF signals around the metallic DBS electrode were investigated through simulations and phantom experiments considering electromagnetic interferences from material susceptibility and the radio frequency (RF) interactions. A threshold method on phase difference Δϕ was used to define a measurement area to estimate heating at the electrode surface. Its performance was compared to that of the Bayesian magnitude method and probe measurements. Results: The B0 magnetic field inhomogeneity due to the electrode susceptibility was the main influencing factor on PRF compared to the RF artifact. Δϕ around the electrode followed normal distribution but was distorted. Underestimation occurred at places with high temperature rises. The noise was increased and could be well estimated from magnitude images using a modified NEMA method. The Δϕ-threshold method based on this knowledge outperformed the Bayesian magnitude method by more than 42% in estimation error of the electrode heating. Conclusion: The findings favor the use of PRF with the proposed approach as a reliable method for electrode heating estimation. Significance: This study clarified the influence of device artifacts and could improve the performance of PRF thermometry for individualized heating assessments of patients with implants under MRI. [ABSTRACT FROM AUTHOR]

MRS thermometry has been utilized to measure temperature changes in the brain, which may aid in the diagnosis of brain trauma and tumours. However, the temperature calibration of the technique has been shown to be sensitive to non-temperature-based factors, which may provide unique information on the tissue microenvironment if the mechanisms can be further understood. The focus of this study was to investigate the effects of varied protein content on the calibration of MRS thermometry at 3 T, which has not been thoroughly explored in the literature. The effects of ionic concentration and magnetic field strength were also considered. Temperature reference materials were controlled by water circulation and freezing organic fixed-point compounds (diphenyl ether and ethylene carbonate) stable to within 0.2 °C. The temperature was measured throughout the scan time with a fluoro-optic probe, with an uncertainty of 0.16 °C. The probe was calibrated at the National Physical Laboratory (NPL) with traceability to the International Temperature Scale 1990 (ITS-90). MRS thermometry measures were based on single-voxel spectroscopy chemical shift differences between water and N-acetylaspartate (NAA), Δ(H20-NAA), using a Philips Achieva 3 T scanner. Six different phantom solutions with varying protein or ionic concentration, simulating potential tissue differences, were investigated within a temperature range of 21-42 °C. Results were compared with a similar study performed at 1.5 T to observe the effect of field strengths. Temperature calibration curves were plotted to convert Δ(H20-NAA) to apparent temperature. The apparent temperature changed by −0.2 °C/% of bovine serum albumin (BSA) and a trend of 0.5 °C/50 mM ionic concentration was observed. Differences in the calibration coefficients for the 10% BSA solution were seen in this study at 3 T compared with a study at 1.5 T. MRS thermometry may be utilized to measure temperature and the tissue microenvironment, which could provide unique unexplored information for brain abnormalities and other pathologies. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

1H MRS thermometry has been investigated for brain trauma and hypothermia monitoring applications but has not been explored in brain tumours. The proton resonance frequency (PRF) of water is dependent on temperature but is also influenced by microenvironment factors, such as fast proton exchange with macromolecules, ionic concentration and magnetic susceptibility. 1H MRS has been utilized for brain tumour diagnostic and prognostic purposes in children; however, the water PRF measure may provide complementary information to further improve characterization. Water PRF values were investigated from a repository of MRS data acquired from childhood brain tumours and children with apparently normal brains. The cohort consisted of histologically proven glioma (22), medulloblastoma (19) and control groups (28, MRS in both the basal ganglia and parietal white matter regions). All data were acquired at 1.5 T using a short TE (30 ms) single voxel spectroscopy (PRESS) protocol. Water PRF values were calculated using methyl creatine and total choline. Spectral peak amplitude weighted averaging was used to improve the accuracy of the measurements. Mean PRF values were significantly larger for medulloblastoma compared with glioma, with a difference in the means of 0.0147 ppm ( p < 0.05), while the mean PRF for glioma was significantly lower than for the healthy cohort, with a difference in the means of 0.0061 ppm ( p < 0.05). This would suggest the apparent temperature of the glioma group was ~1.5 °C higher than the medulloblastomas and ~0.7 °C higher than a healthy brain. However, the PRF shift may not reflect a change in temperature, given that alterations in protein content, microstructure and ionic concentration contribute to PRF shifts. Measurement of these effects could also be used as a supplementary biomarker, and further investigation is required. This study has shown that the water PRF value has the potential to be used for characterizing childhood brain tumours, which has not been reported previously. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

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Thermal therapy procedures being carried out under MR guidance would be safer if temperature changes could be accurately monitored in both water-based and fat-based tissues. To this end, we present a hybrid proton resonance frequency (PRF)/ T1 approach for simultaneously measuring PRF shift temperatures in water-based tissues and T1 changes in fat-based tissues. The hybrid PRF/ T1 sequence is a standard radiofrequency spoiled gradient echo sequence executed in a dynamic mode with two flip angles alternating every time frame. The PRF information is extracted every time frame using the image phase in the standard approach, and the T1 information is extracted every two time frames using a variable flip angle approach. Simulation studies, ex vivo high intensity focused ultrasound heating experiments, and in vivo stability experiments were performed to test the feasibility of the approach. The results indicate that the hybrid PRF/ T1 approach provides PRF temperature maps of the same quality as those obtained by traditional PRF methods while simultaneously being able to track T1 changes in fat-based tissues. Although several potential error sources exist for the T1 measurements, the approach is a promising start toward realizing quantitative temperature measurements in both water-based and fat-based tissues. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Abstract: The purpose of this study is to develop a fast and accurate temperature mapping method capable of both fat suppression and reduced field-of-view (rFOV) imaging, using a two-dimensional spatially-selective RF (2DRF) pulse. Temperature measurement errors caused by fat signals were assessed, through simulations. An 11×1140μs echo-planar 2DRF pulse was developed and incorporated into a gradient-echo sequence. Temperature measurements were obtained during focused ultrasound (FUS) heating of a fat–water phantom. Experiments both with and without the use of a 2DRF pulse were performed at 3T, and the accuracy of the resulting temperature measurements were compared over a range of TE values. Significant inconsistencies in terms of measured temperature values were observed when using a regular slice-selective RF excitation pulse. In contrast, the proposed 2DRF excitation pulse suppressed fat signals by more than 90%, allowing good temperature consistency regardless of TE settings. Temporal resolution was also improved, from 12 frames per minute (fpm) with the regular pulse to 28 frames per minute with the rFOV excitation. This technique appears promising toward the MR monitoring of temperature in moving adipose organs, during thermal therapies. [Copyright &y& Elsevier]

Purpose: The purpose of this study was to evaluate different methods of magnetic resonance thermometry (MRTh) for the monitoring of intradiscal laser ablation therapy in an open 1.0 Tesla magnetic resonance (MR) scanner. Material and methods: MRTh methods based on the two endogenous MR temperature indicators of spin-lattice relaxation time T1 and water proton resonance frequency (PRF) shift were optimised and compared in vitro. For the latter, we measured the effective spin-spin relaxation times T2* in intervertebral discs of volunteers. Then we compared four gradient echo-based imaging techniques to monitor laser ablations in human disc specimens. Criteria of assessment were outline of anatomic detail, immunity against needle artefacts, signal-to-noise ratio (SNR) and accuracy of the calculated temperature. Results: T2* decreased in an inverse and almost linear manner with the patients’ age ( r = 0.9) from 70 to 30 ms (mean of 49 ms). The optimum image quality (anatomic details, needle artefacts, SNR) and temperature accuracy (±1.09°C for T1-based and ±1.11°C for PRF-based MRTh) was achieved with a non-spoiled gradient-echo sequence with an echo time of TE = 10 ms. Conclusion: Combination of anatomic and thermometric non-invasive monitoring of laser ablations in the lumbar spine is feasible. The temperature accuracy of the investigated T1- and PRF-based MRTh methods in vitro is high enough and promises to be reliable in vivo as well. [ABSTRACT FROM AUTHOR]