Magnetic resonance thermometry: Methodology, pitfalls and practical solutions

Abstract

Clinically established thermal therapies like thermo ablative approaches or adjuvant hyperthermia treatment rely on accurate thermal dose information for the evaluation and adaptation of the thermal therapy. Intratumoral temperature measurements have been correlated successfully with clinical endpoints. Magnetic resonance imaging is the most suitable technique for non-invasive thermometry avoiding complications related to invasive temperature measurements. Since the advent of MR thermometry two decades ago, numerous MR thermometry techniques have been developed continuously increasing accuracy and robustness for in vivo applications. While this progress was primarily focused on relative temperature mapping, current and future efforts will likely close the gap towards quantitative temperature readings. These efforts are essential to benchmark thermal therapy efficiency, understand temperature related biophysical and physiological processes and to use these insights to set new landmarks for diagnostic and therapeutic applications. With that in mind, this review summarizes and discusses advances in MR thermometry providing practical considerations, pitfalls and technical obstacles constraining temperature measurement accuracy, spatial and temporal resolution in vivo. Established approaches and current trends in thermal therapy hardware are surveyed with respect to potential benefits for MR thermometry.3

Experimental measurement of microwave ablation heating pattern and comparison to computer simulations

“…Such a simulation approach would also be computationally less demanding as the electromagnetic problem (Equation 5) does not need to be solved. While we employed infrared thermometry in this study, the proposed technique to estimate SAR changes during heating may also be employed with magnetic resonance thermometry [35] to facilitate 3D measurements in vivo .…”

Spin-crossover and high-spin iron(ii) complexes as chemical shift19F magnetic resonance thermometers

“…1,4,6 Currently, methods based on water PRF shift are the most widely used for imaging temperature in clinical studies due to their high-resolution and independence on tissue type. 7 However, these techniques suffer from a low temperature sensitivity of ca. –0.01 ppm per °C, and their ability to accurately determine absolute temperature is limited.…”

High‐resolution intravascular MRI‐guided perivascular ultrasound ablation

“…Although not used here, the localized sensitivity profile of each projection may also provide a means of motion‐correction . Like conventional thermometry, high‐resolution IV‐MRI thermometry is subject to drift in the reference phase during procedures . Acquiring a reference phase image before commencing each ablation, limits the potential drift period to the duration of each ablation.…”

“…Postablation, thermal dose maps, measured in cumulative equivalent minutes at 43°C ("CEM 43 "), were derived by temporally integrating the dynamic real-time temperature maps 32,33 after de-noising and co-registering them with anatomical images. A CEM 43 of 340 has been assumed as a threshold for producing necrosis, 24,34,35 so the MRI-based lesion-dose-threshold CEM 43 = 340 contours were calculated, smoothed to generate continuous loops, and overlaid on co-registered photographs of lesions. The areas of the continuous CEM 43 = 340 loops were measured as a potential IV-MRI proxy for lesion size.…”