Role of Magnetic Resonance Thermometry in Magnetic Resonance Image-guided Focused UltrasonographyTreatment: Advantages and Pitfalls

Purpose or Learning Objective: To review the importance of magnetic resonance thermometry, its technical advances but also possible pitfalls regarding technical obstacles pertaining to temperature measurement accuracy in noninvasive thermal ablation treatments such as magnetic resonance image-guided focused ultrasonography (MRgFUS).

To present a retrospective study of our experience operating MRgFUS in bone lesions, particularly bone metastasis and osteoid osteomas, to critically analyze our decisions while performing this technique, and to share our data, concerns, and improvement suggestions for this technique.

Methods or Background: Persistent pain is a common consequence of bone lesions, such as advanced cancer bone metastasis and osteoid osteomas. MRgFUS is an emerging and promising alternative for the noninvasive thermal ablation of benign bone tumors and bone metastases.

Although high-intensity focused ultrasonography (HIFU) can be performed as an US-guided technique, magnetic resonance imaging (MRI) guidance is preferred for bone treatments. MRI thermometry provides a near real-time assessment of temperature and a thermal-dose distribution on surrounding tissues. This feature allows monitoring of thermal damage on the treated lesions and nearby healthy tissues, and it is essential in modulating the energy to apply for ablation.

The thermal dose can be correlated with the clinical outcomes. For that reason, accurate thermometry in thermal ablation treatments is critical. One of the most used methods is proton resonance frequency (PRF) thermometry.

Understanding temperature-related biophysical and physiologic processes helps further MR thermometry hardware and software developments, needed to improve absolute temperature measurements and therefore the efficiency of thermal therapy.

Results or Findings: PRF thermometry is an essential tool to adjust sonication power, to monitor thermal doses, and to guarantee coagulation at the target. Nevertheless, certain intrinsic factors of tissues influence the temperature readings, such as temperature-dependent chemical shifts, temperature-dependent changes in the electromagnetic properties of different tissues, and regional perfusion-based changes.

Minimal intra-scan motions caused by respiratory, cardiac, peristaltic, or bulk motion can also lead to PRF temperature misregistration with standard deviations > ± 10°C even without any application of heat. Additionally, the presence of non-contraindicated electrically conductive implants (e.g., stents, osteosynthesis components, clips, sleeves), susceptibility artifacts, and signal voids also compromise precise temperature determinations.

Conclusion: The reliability of temperature information has major implications on temperature-related biological processes research and on clinical settings, contributing not only to the efficiency of thermal therapies but also to their safety.

Publication History

Article published online:
26 May 2023