Temperature monitoring is used during MR-guided High-Intensity Focused Ultrasound (MR-HIFU) induced hyperthermia to ensure controlled thermal uptake. Temperature change in non-adipose tissue can be calculated from the water-proton resonance frequency shift (PRFS), while the absolute temperature in adipose tissue can be calculated from an apparent T2 map¹. Interleaved scanning of gradient-echo and dual fast spin-echo sequences has been previously demonstrated in order to simultaneously acquire PRFS and apparent T2 maps during sonication². The temporal resolution of temperature measurements at the ultrasound focus should be high during rapid heating, but can be lowered while maintaining hyperthermia or during the cooling phase. The objective of this study is to demonstrate the feasibility of dynamically controlling the temporal resolution of both interleaved scans during different stages of a MR-HIFU hyperthermia application and in response to actual temperature changes.

Experiments were performed using a Sonalleve V2 MR-HIFU system (Philips Healthcare, Best, The Netherlands) with a 5-channel reception coil (Window and Pelvis coils). A Python script acquired images and controlled the interleaved scanning using a modified version of the MatMRI³ toolbox that allows for dynamic control of MR parameters. This modified version of MatMRI utilized a version of the Philips eXTernal Control⁴ (XTC) scanner interface adapted to dynamically update the rhythm/duty-cycle of each interleaved scan in real-time. The HIFU system operation was controlled using MatHIFU³. As shown in Figure 1, an ex vivo porcine leg muscle with a subcutaneous fat layer was used as the MR-HIFU hyperthermia target. The tissue was previously degassed at a negative pressure of -27 inHg for one hour and vacuum-sealed in a 16.5×14.9 cm Ziploc bag for MR-HIFU hyperthermia.

An interleaved scanning protocol was configured with single-slice gradient-echo and dual fast spin-echo dynamic sequences for PRFS and T2-mapping MR-thermometry methods, respectively, as described in Figure 2. A 200 ms delay was added before each apparent T2-mapping acquisition in order to reduce signal influences from the previous dynamically acquired PRFS acquisitions². The position of the respective slices are shown in Figure 1. The interleaved scanning protocol consisted of three PRFS dynamics interleaved with one T2 segment (kernel). Maximum and average temperatures were calculated within a 16 mm diameter region for apparent T2 temperature maps, and a 10 mm diameter region for PRFS temperature maps due to the smaller diameter of heating at the ultrasound focal point. A 2^nd order phase-drift correction algorithm was used for the PRFS thermometry.

The MR-HIFU hyperthermia protocol began with a 4 mm radial volumetric sonication trajectory at an acoustic power of 40 Watts for rapid heating until reaching a target temperature of 8 degrees above the baseline temperature. The power was then reduced to 12 W for 180 seconds to sustain the target temperature. To demonstrate the on-demand feature, apparent T2 acquisition was disabled during the rapid heating phase to accelerate PRFS acquisition, then re-enabled while maintaining hyperthermia and during the subsequent cooling.

The temporal resolutions of the interleaved scans were successfully modified in real-time during MR-HIFU induced hyperthermia. As can be seen in Figure 3, the apparent T2 scan was disabled and the temporal resolution of the PRFS scan increased when the initial rapid heating began. After obtaining the target temperature, the T2 scan was re-enabled resulting in decreased temporal resolution of the PRFS scan. Considering that the temporal resolution is limited by the largest time delay between temperature measurements, the temporal resolution was approximately 3.2 seconds/measurement for the interleaved PRFS scan, and approximately 0.2 s/measurement for the non-interleaved PRFS scan (or when the T2 scan was disabled). When activated, the temporal resolution of the interleaved apparent T2 scan was approximately 14.4 s/measurement, only slightly longer than the 12.0 s/measurement for a non-interleaved apparent T2 scan.

To determine the potential mutual signal influences of the interleaved scans, the same MR-HIFU induced hyperthermia protocol was used in an additional two experiments. The first experiment used only the PRFS scan, and the second used only the T2 scan (both without interleaved scanning). As shown in Figures 4 and 5, there was no noticeable difference between the interleaved and non-interleaved PRFS temperature results. The observed greater heating in the subcutaneous fat than at the ultrasound focus may be due to the closeness of the ultrasound focus to the fat.

In this study, we demonstrated the feasibility of dynamically modifying the temporal resolutions of interleaved scans on-demand. This new approach is very promising for achieving better control of near field heating in adipose tissue for different MR-HIFU applications and for general MRI interventional applications.