In vivo evaluation of multi‐echo hybrid PRF/T1 approach for temperature monitoring during breast MR‐guided focused ultrasound surgery treatments

Abstract

Purpose To evaluate the precision of in vivo temperature measurements in adipose and glandular breast tissue using a multi-echo hybrid PRF/T1 pulse sequence. Methods A high bandwidth, multi-echo hybrid PRF/T1 sequence is developed for monitoring temperature changes simultaneously in fat- and water-based tissues. The multiple echoes are combined with the optimal weightings for magnitude and phase images, allowing for precise measurement of both T1 and the proton resonance frequency (PRF) shift. The sequence was tested during in vivo imaging of 10 healthy volunteers in a breast specific MR-guided focused ultrasound (MRgFUS) system and also during FUS heating of excised breast adipose tissue. Results The in vivo results indicate that the sequence can measure PRF temperatures with 1.25×1.25×3.5 mm resolution, 1.9 second temporal resolution and 1.0 °C temperature precision, and can measure T1 values with 3.75×3.75×3.5 mm resolution, 3.8 second temporal resolution and 2.5 to 4.8 % precision. The excised tissue heating experiments demonstrate the sequence’s ability to monitor temperature changes simultaneously in water- and fat-base tissues. Conclusion The addition of a high bandwidth, multi-echo readout to the hybrid PRF/T1 sequence improves the precision of each measurement, providing a sequence that will be beneficial to several MR-guided thermal therapies.

Magnetic resonance thermometry: Methodology, pitfalls and practical solutions

“…This includes but is not limited to proton density (32), T1 relaxation (33,34), T2 relaxation (35,36), apparent water diffusion (37,38) and magnetization transfer (39). The merits and limitations of MR thermometry based upon these MR parameters are surveyed in an excellent review (40).…”

“…To mitigate this error, hybrid methods combining the PRF shift and other temperature dependent MR parameters like apparent diffusion coefficient (64) or T1 (34,65,66) are investigated. Multi gradient echoes with variable flip angles allow using the magnitude information for T1 mapping and temperature related T1 changes in lipids and the phase information for PRF thermometry in aqueous tissue (65,66).…”

“…In breast cancer treatment, using the PRF method alone evoked difficulties in the localization of the thermal hotspot due to fat regions (131). Here hybrid methods using additional T1 contrast for temperature readings in fat reduced the PRF error significantly (34). Treating liver cancer with MRgFUS is hindered by the ribcage interfering with an efficient focusing of the acoustic energy and respiratory motion.…”

Thermometry and ablation monitoring with ultrasound

“…In addition, it was observed that fat did not exhibit the same temperature dependent frequency shift, and the technique could not measure temperature in this tissue. As a result, applying PRF shift methods in fat-rich organs such as breast has been challenging and typically requires alternative methods based on T1 or T2 relaxation (20, 21). Furthermore, PRF shift thermometry is challenging to apply in the abdomen due to the proximity of the lungs and subsequent motion of organs due to respiration.…”

Spatiotemporal filtering of MR‐temperature artifacts arising from bowel motion during transurethral MR‐HIFU

“…This is necessary because a reduced SNR is typically observed in the direction of ultrasound heating caused by signal loss due to the temperature dependence of T1 relaxation. 24 The spatially dependent SNR mask is derived using an approximate analytic expression for the ultrasonic heating beam Ψ simu , described in Eq. (2), and built-up empirically from previous heating experiments in a tissue-mimicking gel phantom 1 and the knowledge of the transducer position at each acquisition…”