MR-guided focused ultrasound (MRgFUS) brain treatments are currently guided by one thermometry sequence: single-slice 2DFT MR thermometry. In this work, we divided treatment monitoring into two tasks, with different thermometry design goals for each, and developed sequences optimized for these goals: “Focal Spot Localization” and “Monitoring”. These sequences achieve greater imaging performance by utilizing multi-echo spiral thermometry¹, region-specific update rates, and MASTER² slice interleaving.

Currently, focal spot targeting confirmation requires several low-power sonications to obtain high-resolution measurements in three dimensions. We developed a Focal Spot Localization sequence that obtains high-resolution measurements in-plane, at improved temperature precision compared to 2DFT, while also providing multiple-slices for through-plane characterization. This would reduce the number of sonications required, improving treatment time. Additionally, even lower-power sonications could be detected, improving patient safety.

During ablative treatment sonications, single-slice monitoring cannot detect through-plane shifts of the ultrasound focus, or unexpected out-of-plane heating. We developed a multi-rate thermometry Monitoring sequence that interleaves different sequences at different update rates to simultaneously achieve fast and precise focal monitoring, 3-dimensional focal spot measurement, and full brain monitoring. Fast and precise multi-slice monitoring of the focus ensures accurate thermal dose estimates for treatment feedback, while (slower) full-brain monitoring ensures patient safety

All sequences were implemented using RTHawk (HeartVista, Menlo Park, CA) on a GE 3T 750 Signa scanner (GE Healthcare, Milwaukee, WI) equipped with the Insightec Exablate Neuro (Insightec, Haifa, Israel). All imaging was performed with the body coil, as is normally used with the Exablate system. Conventional 2DFT was implemented as a gold standard for comparison. The new sequences all use a 36 cm FOV to ensure that the transducer and water bath do not alias. Volunteer imaging was done with informed consent under IRB approval. Multi-frequency reconstruction¹ was performed on all data spiral data. Temperature uncertainty was measured as the per-voxel temporal standard deviation of temperature measurements, and averaged within manually segmented ROIs. Performance was also tested inside the transducer, using a gel phantom. Sequence parameters and MR parameters are compiled in Tables 1-2.

The Focal Spot Localization sequence is a 5-slice 3-echo thermometry sequence, with doubled in-plane resolution as compared to conventional 2DFT (1.1x1.1 mm vs 1.09x2.18 mm), and acquisition time of 7 s. The Monitoring sequence interleaves 3 distinct sequences at different rates to monitor 29 total slices. The “Focus” is monitored using 3-slice 3-echo spiral imaging, for high-speed high-precision measurement of focal heating. Two adjacent slices, the “Boundary”, are monitored at half the temporal rate (also using 3-echo spiral) to fully characterize the focal spot. The remaining 24 slices, “Background”, were acquired using 8 blocks of 3-slice MASTER², with spiral readouts. Use of MASTER improves temperature uncertainty, compared to traditional slice interleaving, by increasing echo time. Limiting each MASTER block to 3 slices reduces inherent diffusion and motion-encoding artifacts. The three component sequences were interleaved such that Focus utilized 45% (15% per-slice) of the timeline, Boundary used 15% (7.5% per-slice), and Background the remaining 40% (1.7% per-slice).

Figure 1 compares temperature uncertainty in vivo between 2DFT and Focal Spot Localization while Figure 2 compares 2DFT with Monitoring. Large images compare the center slice, while stacks of images at the right show additional slices monitored by the new sequences. Figure 3 compares uncertainty within the transducer. The new sequences obtained better uncertainty than 2DFT, with average uncertainty compiled in Table 1. Relative “Efficiency” is also listed in Table 1, which accounts for differences in speed and voxel volume. Each multi-echo spiral sequence is more than 150% as efficient as 2DFT. Background is 69% as efficient (but collects 3 slices per TR, for an effective 120% efficiency.)In this work, we have shown that multi-slice multi-echo spiral thermometry is an effective imaging approach for volumetric treatment monitoring. Improved imaging performance was successfully used to achieve imaging objectives for different aspects of ablative treatments. Focal spots may be localized faster and with reduced heating using the higher-resolution higher-precision Focal Spot Localization sequence. The mixed update rate Monitoring sequence successfully delivers high-speed high-precision monitoring of the targeted focus, while improving safety by simultaneously monitoring the full brain at a lower update rate. These sequences have also been validated within the transducer, to help ensure they will work in the clinical setting.