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Temeprature imaging during focused ultrasound procedure overlaid on anatomical images MR unique ability to provides temperature images of internal organs makes it the prefered imaging modality to guide thermal ablation procedures.



Besides being one of the best and most versatile diagnostic imaging modality, MR is also unique in allowing to monitor with temperature images a thermal ablation procedure in real time. There are several MR measurable quantities that are temperature dependent: water molecules Apparent Diffusion Coefficient (ADC), T1 relaxation time, Proton Resonance Frequency (PRF),etc... It is possible to exploit these temperature dependent measurable quantities to produce temperature images in-vivo as D. Parker first demonstrated in a 1983 seminal paper.

Quantity Tissue dependent Reliable measurement
ADC yes difficult in real life (motion sensitivity, susceptibility effects...)
T1 yes difficult compromise between acquisition speed and accuracy
PRF no easy and fast measurement but motion sensitive.

Proton resonance frequency appears as a very interesting way to measure temperature with MR images as it is both easy to measure and tissue independent which is essential for the reliability of the method. With the PRF method, MR temperature images can be acquired in real time during a thermal ablation procedure enabling a proper control of the treatment progress.

temperature and thermal dose maps overlaid on anatomic images during an RF ablation procedure
Temperature (left) and thermal dose (right) maps overlaid on anatomic images

Temperature history is essential to calculate a quantity called the thermal dose. Thermal dose has been shown to be the best indicator of the final, heat induced, tissue destruction and this regardless of tissue types. So temperature monitoring is most important to ensure that healthy tissues around the tumor are spared and thermal dose reliably shows in real time how treatment progress and indicates when the therapy endpoint is reached.


Because of volumetric acquisitions, excellent contrast and above all temperature imaging, MRI shows major advantages in term of monitoring focused ultrasound ablation. However MR is extremely sensitive to radiofrequency interferences (hence the Faraday cage around it) and magnetic susceptibility variations. This means that to profit from all the advantages MR has to offer in terms of patient safety and treatment effectiveness, the ablation device has to be specifically designed for active compatibility with the MR environment.

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