Interstitial Dual-Mode Ultrasound With a 3-mm MR-Compatible Catheter for Image-Guided HIFU and Directional In Vitro Tissue Ablations

Current interstitial techniques of tumor ablation face challenges that ultrasound (US) technologies could meet. The ablation radius and directionality of the US beam could improve the efficiency and precision. Here, a nine-gauge magnetic resonance (MR)-compatible dual-mode US catheter prototype was experimentally evaluated for ultrasound image-guided high-intensity focused ultrasound (USgHIFU) conformal ablations. The prototype consisted of 64 piezocomposite linear-array elements and was driven by an open research programmable dual-mode US platform. After verifying the US image guidance capabilities of the prototype, the high-intensity focused US (HIFU) output performances (dynamic focusing and HIFU intensities) were quantitatively characterized, together with the associated 3-D HIFU-induced thermal heating in tissue phantoms [using MR thermometry (MRT)]. Finally, the ability to produce robustly HIFU-induced thermal ablations in in vitro liver was studied experimentally and compared to numerical modeling. Investigations of several HIFU dynamic focusing allowed overcoming the challenges of miniaturizing the device: monofocal focusing maximized deep energy deposition, while multifocal strategies eliminated grating lobes. The linear-array design of the prototype made it possible to produce interstitial US images of tissue and tumor mimics in situ. Multifocal pressure fields were generated without grating lobes and transducer surface intensities reached up to ${I}_{\text {sapa}} = 14~\text {W}\cdot \text { cm}^{-{2}}$ . Seventeen elementary thermal ablations were performed in vitro. Rotation of the catheter proved the directionality of ablation, sparing nontargeted tissue. This experimental proof of concept demonstrates the feasibility of treating volumes comparable to those of primary solid tumors with a miniaturized USgHIFU catheter whose dimensions are close to those of tools traditionally used in interventional radiology while offering new functionalities.