Suitability of a tumour-mimicking material for the evaluation of high-intensity focused ultrasound ablation under magnetic resonance guidance

Abstract

This study tests the suitability of a tumour-mimic for targeting magnetic resonance (MR)-guided high-intensity focused ultrasound (HIFU). An agarose-based tumour-mimic was injected as a warm solution that polymerized in tissue. Thermal characteristics and acoustic absorption of the mimic were observed within the values reported for tissues. The relaxation times at 3T were 1679 ± 15 ms for T1 and 41 ± 1 ms for T2. The mimic was clearly visible on in vivo images. With lower contrast the tumour-mimic was visible on T2-weighted images, where it was possible to detect the ablated tissue surrounding the mimic after sonications. HIFU sonications were performed to induce thermal ablation on and around the mimic using a Sonalleve system (Philips). MR thermometry maps were performed during HIFU. The average temperature when the sonication was done at the tumour-mimic was 67.6 ± 8.0 °C in vitro and 67.6 ± 5.0 °C in vivo. The average temperature for sonications at tissues was 68.4 ± 8.7 °C in vitro (liver) and 66.0 ± 2.6 °C in vivo (muscle), with no significant difference between tissue and tumour-mimic (p > 0.05). The tumour-mimic behaviour when using MR-guided HIFU was similar to tissues, showing that this mimic can be used as an alternative to tumour models for validating MR-guided HIFU devices targeting.

An experimental model to investigate the targeting accuracy of MR-guided focused ultrasound ablation in liver

“…The presented method demonstrated no risk of tissue contamination with air bubbles and no irregular heating pattern was observed on PRFS temperature maps during HIFU sonication, as opposite to [35]. Importantly, the needle and the syringe were mounted together and filled with liquid solution prior to the incision.…”

“…Tissue effraction is minimal and direct assessment of the ballistic accuracy of millimeter-range (i.e. elementary) HIFU lesions aiming to overlap the marker location is meaningful due to the small size of the marker itself, unlike the previous model [32,35]. …”

“…Accurate targeting in abdominal organs during breathing is a challenging problem. Unlike previous reports dealing with static conditions during the sonication [ 32 , 35 ], our sonications in pig liver were performed under continuous breathing and gating. Internal organ drift between targeting and sonication steps [ 48 ], acoustic aberration of the HIFU beam versus the theoretical beam forming in homogeneous tissue and geometric distortion of MR images sequence-specific are inherent limits to the achievable accuracy for centering the lesions on the T1-contrast marker.…”

“…That model was a large unperfused 1 cm hyperechoic lesion on sonograms (US reflecting) and the polymerized solution had attenuation coefficient of 0.39 dB.cm −1 at 1 MHz, as compared to 0.68 dB cm -1 for liver parenchyma [34]. Recently, Pichardo et al [35] proposed using the same agarose-based solution for MR targeting and thermal monitoring during HIFU sonication on living static tissue (rabbit muscle). The model applied to static muscular tissue in rabbits induced significant edema probably due to intrinsic temperature of the injected solution, while the animals were sacrificed immediately post-treatment.…”

Tissue-mimicking thermochromic phantom for characterization of HIFU devices and applications

“…Several HIFU TMP formulations based on agar, gelatin, or polyacrylamide have been reported that often include additives to adjust TMP thermal and acoustic properties to be comparable to human soft tissues [10, 11, 12, 13, 14, 15]. For example, the TMP acoustic attenuation and speed of sound may be adjusted via the addition of silicon dioxide particles, condensed milk, bovine serum albumin, corn syrup, glass beads, intralipid, graphite, or propanol.…”