Key Points
- Physicians use thermometry, the measure of temperature, to monitor the brain and the skull during focused ultrasound treatments.
- This article reviews the most clinically applicable technical parameters of thermometry and should be of interest to clinical teams.
MR Thermometry during Transcranial MR Imaging–Guided Focused Ultrasound Procedures: A Review
Focused ultrasound brain treatments for tissue ablation use heat to destroy tissue at the target location (e.g., thalamotomy for essential tremor). Precise – less than 1 mm – targeting is imperative to decrease side effects such as gait imbalance, hemiplegia, sensory abnormalities, paresthesia, and even spontaneous pain.
To monitor the rise and fall of the temperature in the brain and at the skull during these procedures in real time, treating physicians use thermometry, which is the measure of temperature. Because focused ultrasound brain procedures are performed with MRI guidance, proton resonance frequency (PRF) MRI thermometry (MRT) is currently the primary method of thermometry used to monitor current clinical treatments.
In general, MRT represents a group of thermal-sensitive MR imaging parameters, such as PRF shift, T1 and T2 relaxation times, diffusion, proton density, and magnetization transfer. PRF MRT is used for MR-guided focused ultrasound treatments due to fast acquisition times and high temperature sensitivity within aqueous tissues such as the brain. The factors that influence PRF MRT include heterogeneous heat deposition at the focal zone, tissue properties, tissue inhomogeneity, the presence of increased susceptibility, timing of the acquisition, and motion artifacts.
Consideration of each of these factors is critical to achieve successful transcranial MR-guided focused ultrasound treatments, and a deep understanding of MRT sequences and their pitfalls and limitations helps clinical teams avoid complications during procedures.
This article, which was written by a collaborative team of expert clinicians brought together by Kazim H. Narsinh, MD, a neurointerventional radiologist at the University of California San Francisco, reviews each of the most clinically applicable technical parameters of PRF MRT. It should be of interest to every clinical team that is using focused ultrasound for brain treatments.
“Interest in performing transcranial MR imaging–guided focused ultrasound procedures continues to grow,” said Dr. Narsinh. “This collaborative article provides an updated review on the technical underpinnings and considerations of the most common clinically used MRT sequences.”
“Understanding and improving MRT is essential in all ablation techniques, especially in the brain, to ensure successful ablation of the target and minimize off-target heating and side effects,” said Suzanne LeBlang, MD, the Foundation’s director of clinical relationships. “In addition to performing a clinical examination during the procedure, analysis of real-time MRT images to determine when the procedure is complete was discussed in depth at the recent Focused Ultrasound Foundation essential tremor targeting workshop (held in September 2023). The white paper summarizing this workshop is now available on our website.
“MRI monitoring of focused ultrasound therapies is an active area of research,” said Eugene Ozhinsky, PhD, assistant professor of radiology and biomedical imaging at the University of California San Francisco and co-corresponding author of the article. “With a wider availability of advanced pulse sequences and MRI coils compatible with focused ultrasound transducers, we will soon be able to dramatically improve the speed, quality, and coverage volume of MR thermometry imaging.”
See the American Journal of Neuroradiology (Open Source)