Radiological and Thermal Dose Correlations in Pallidothalamic Tractotomy With MRgFUS

Abstract

Background: MR-guided focused ultrasound (MRgFUS) offers the possibility of safe and accurate lesioning inside the brain. Until now, most MRgFUS thermal applications have been based on temperature or energy protocols. Experimental studies support however an approach centered on thermal dose control. Objective: To show the technical feasibility and lesion size predictability of a thermal dose approach during MRgFUS pallidothalamic tractotomy (PTT) against chronic therapy-resistant Parkinson's disease (PD). Methods: MR and thermal dose data were analyzed in 31 MRgFUS interventions between January and December 2017 in patients suffering from chronic therapy-resistant Parkinson's disease (PD) using a standardized PTT target covered by 5 to 7 target lesion sub-units. Results: Good correlations were found between (1) the mean axial T2 lesion diameter intraoperatively and the mean 240 cumulative equivalent min at 43°C (240 CEM) thermal dose diameter (r = 0.52), (2) the mean axial T2 diameter 48 h post-treatment and the mean 18 CEM thermal dose diameter (r = 0.62), and (3) the mean axial T2 diameter intraoperatively and 48 h post-treatment (r = 0.62). Conclusion: Our current approach using a thermal dose steering for multiple target lesion sub-units could be reproduced in 31 interventions with a good lesion size predictability.

Keywords: MRgFUS (magnetic resonance-guided focused ultrasound surgery); Parkinson's disease; functional neurosurgery; imaging; incisionless; pallidothalamic tractotomy; radiology; thermal dose.

Figure 1

(A) Applied target sub-units 1,3 and 4 at plan DV0, 2 and 5 at V1 and 7 at D1. (B) 240 CEM thermal dose surfaces reached (dark blue) in sub-target 1 (2.6 × 2.3 mm), 2 (2.6 × 2.2), 3 (2.7 × 2.9), 4 (4.1 × 2.8), 5 (1.8 × 1.7) and 7 (2.1 × 2.2). 18 CEM thermal dose surfaces (light blue) reached in sub-target 1 (4.7 × 4), 2 (4.5 × 4.1), 3 (4.2 × 4.2), 4 (4.1 × 2.8), 5 (3.6 × 3.6), 7 (1.8 × 1.2 and 4 × 3.9). (C) shows the projection of the axial T2 MR lesions measured intraoperatively (green, corresponding to D) and at 48 h post-treatment (orange, corresponding to E), displayed at high magnification.

Figure 2

(A) shows 31 cases with measurements of their mean lesion diameter in axial T2 intra- and postoperatively. (B) shows intraoperative mean axial T2 measurements of the lesions as well as their 240 and 18 CEM thermal dose mediolateral and anteroposterior diameters. (C) shows the postoperative T2 lesion diameters at 2 days with their thermal doses.

Figure 3

Thirty one PTT targets are displayed for correlations between mean axial T2 lesion diameter intraoperatively and the mean 240 CEM thermal dose diameter in (A) (r = 0.52). In (B) mean T2 axial diameter 48 h post treatment was correlated with the mean 18 CEM thermal dose diameter (r = 0.62). In (C), mean axial T2 lesion diameter intraoperatively was correlated with mean T2 axial diameter 48 h post-treatment (r = 0.62).

Figure 4

Example of insufficient target coverage of a patient treated before this study series which led to initially good but later partial therapeutic effect on the symptoms. (A) Target was chosen at (L7.5, MCL, DV0) and repeated sonications (n = 4) with temperatures ranging from 55 to 57°C and of 13 s duration were applied on the same spot. The 240 CEM thermal dose outline (dark blue circle) was projected in (B) on the axial T2 scan intraoperatively (4 × 3.8 mm) and both 18 CEM (light blue, 6.3 × 4.9 mm) and 240 CEM thermal dose outlines are shown in (C) on the T2 axial scan performed 2 days after the treatment.

Figure 5

Two patients in whom longer sonication durations were applied treated before this series. Spatial coverage was increased as shown in the thermal dose values for 240 CEM but as shown in the axial T2 scan 48 h after the treatment, the hyperintense signal was difficult to associate with any thermal dose equivalent. In (A) temperatures reached in final sonications were 56, 56, and 54°C (sonication time: 28, 28, and 20 s, power: 450, 500, and 600 W) with final 18 CEM (light blue) and 240 CEM (dark blue) surfaces in axial projections of 6.1 × 6.8 and 3.5 × 3.9 mm, respectively. In B: temperature reached was 57°C in 24 s (450 W) for final 18 and 240 CEM thermal dose surfaces of 4.7 × 5.4 and 3.2 × 3.6 mm, respectively. In (A,B), the PTT single target coordinates were L7.5, MCL, and DV0.

Figure 6

Two treatments performed in the same patient within 2 years (A–D and later E–H) because of insufficient target coverage and partial symptom recurrence. (A,B) show the target placed at L7.5, MCL, and V0.5. Temperatures reached in final sonications were 55, 56, 58, 57, and 55°C (sonication time: 13 s for all, power: 350, 450, and 500 W) with final 18 CEM (light blue) and 240 CEM (dark blue) thermal dose surfaces on axial projections of 6.6 × 6.1 mm and 3.9 × 4.2 mm, respectively. (C,D) show the lesions in axial T2 scan performed intraoperatively (C) and 2 days after it (D) with projections of their 18 and 240 CEM thermal dose outlines. In a second session, a complement of PTT was planned (E) in 3 target sub-units: 1 (L7.5, MCL-2.7, V1), 2 (L5.5, MCL-2.7, V1), and 3 (L6.5, MCL-1.7, DV0). Temperatures reached in final sonications were 55, 56, and 55°C (sonication time: 10, 10, and 16 s, power: 550, 550, and 650 W). Final 18 and 240 CEM thermal dose surfaces were shown in (F). (G,H) show the 240 and 18 CEM thermal dose outlines projected on axial T2 scans performed intraoperatively (G) and 2 days after it (H).