64-element intraluminal ultrasound cylindrical phased array for transesophageal thermal ablation under fast MR temperature mapping: an ex vivo study

Abstract

This work was undertaken to investigate the feasibility of using a cylindrical phased array for transoesophaeal thermal ablation under magnetic resonance (MR) imaging guidance. Sixty-four transducers (0.45 mm wide by 15 mm tall), operating at 4.6 MHz, were spread around the periphery of a 10.6-mm-diam cylinder. The head of the applicator was covered with a 65-microm thick latex balloon attached using watertight seals. This envelope was inflated with degassed water to provide acoustic coupling between the transducer and the tissues. The underlying operating principle of this applicator is to rotate a plane ultrasound beam electronically. For this purpose, eight adjacent transducers were excited with appropriate delay times so as to generate a plane wave. The exposure direction was changed by exciting a different set of eight elements. Ex vivo experiments conducted on 47 samples of pig liver under MR temperature monitoring demonstrated the ability of this applicator to generate cylindrical or sector-based coagulation necroses at depths up to 19 mm with excellent angular precision by applying 20 W/cm2. MR thermometry was performed in "real-time" with segmented echo-planar imaging gradient echo sequences. The temporal resolution was approximately 3 s/ image. The average value for the temperature baseline in liver tissue close to the applicator was 0.3 degrees C (+/- 0.6 degrees C). The thermal dose delivered in tissues was computed on-line during temperature imaging. Excellent MR compatibility was demonstrated, all MR acquisitions were performed without susceptibility artifacts or radio-frequency interferences with the ultrasound device. Thermal lesions identified on post-treatment follow up showed good correlation with online MR thermometry data. The individual differences between measurements performed visually and using MRI thermal dose maps were about 11% of volume. This study demonstrated the feasibility of thermal ablation using a phased array intraluminal ultrasound applicator and on-line MR monitoring.

Figure 1

Head of the applicator. (1) Transducers. (2) Hole for the water cooling circuit. (3) Place for watertight seals.

Figure 2

Efficiency of the transducers.

Figure 3

Experimental system and electrical equipment for in vitro experiments.

Figure 4

Applicator embedded in Agar gel block viewed with turbo spin echo sequences. TR=800 ms; TE=20 ms; flip angle=90°; slice thickness=6 mm; field of view (FOV) 128×128 mm² (a) Longitudinal plane. (b) Axial plane.

Figure 5

Applicator embedded in Agar gel block viewed in transverse plane with fast gradient echo sequences. TR=500 ms; TE=21 ms; flip angle=35°; slice thickness=3mm; field of view 128×128 mm². (a) Magnitude. (b) Phase. The applicator is in the centre of the figure.

Figure 6

(a) T2w image of the ultrasound device (longitudinal plane) inserted in a sample of pig liver. (b) T1-IR image of the ultrasound device (longitudinal plane). The balloon is inflated around the applicator.

Figure 7

Temperature monitoring in a transverse plane during two ultrasound exposures (sequence F). Colour map: red above 52°C, green 47 to 52 °C, blue 42 to 47°C.

Figure 8

Temperature time course for two successive sonications along the same direction (sequence G). Three locations along the acoustic axis were considered.

Figure 9

(a to d) Temperature and (A to D) equivalent time at 43°C (thermal dose) monitoring in a transverse plane during sequence D. (a to d) colour map: red above 52°C, green 47 to 52°C, blue 42 to 47°C. (A to D) colour map: red above 20 hours equivalent time at 43°C, green 2 hours equivalent time at 43°C, blue 1 hour equivalent time at 43°C.

Figure 10

Liver tissue treated ex vivo (section perpendicular to the axis of the applicator). (a) using sequence C. (b) using sequence H.

Figure 11

Mean, minimal and maximal values of the treatment depth for circular lesions in function of the acoustical intensity (sequences A, B, C, E).

Figure 12

Lesion obtained by exciting eight transducers with the same phase (20 W/cm² - 90 seconds).