Many recent studies have demonstrated the efficacy of interstitial ablative approaches for the treatment of hepatic tumors. Despite these promising results, current systems remain highly dependent on operator skill, and cannot treat many tumors because there is little control of the size and shape of the zone of necrosis, and no control over ablator trajectory within tissue once insertion has taken place. Additionally, tissue deformation and target motion make it extremely difficult to place the ablator device precisely into the target. Irregularly shaped target volumes typically require multiple insertions and several overlapping (thermal) lesions, which are even more challenging to accomplish in a precise, predictable, and timely manner without causing excessive damage to surrounding normal tissues.
In answer to these problems, we have developed a steerable acoustic ablator called the ACUSITT with the ability of directional energy delivery to precisely shape the applied thermal dose . In this paper, we address image guidance for this device, proposing an innovative method for accurate tracking and tool registration with spatially-registered intra-operative three-dimensional US volumes, without relying on an external tracking device. This method is applied to guid-ance of the flexible, snake-like, lightweight, and inexpensive ACUSITT to facilitate precise placement of its ablator tip within the liver, with ablation monitoring via strain imaging. Recent advancements in interstitial high-power ultrasound applicators enable controllable and penetrating heating patterns which can be dynamically altered. This paper summarizes the design and development of the first synergistic system that integrates a novel steerable interstitial acoustic ablation device with a novel trackerless 3DUS guidance strategy.
Steerability in percutaneous medical devices is highly desirable, enabling a needle or needle-like instrument to avoid
sensitive structures (e.g. nerves or blood vessels), access obstructed anatomical targets, and compensate for the
inevitable errors induced by registration accuracy thresholds and tissue deformation during insertion. Thus, mechanisms
for needle steering have been of great interest in the engineering community in the past few years, and several have been
proposed. While many interventional applications have been hypothesized for steerable needles (essentially anything
deliverable via a regular needle), none have yet been demonstrated as far as the authors are aware. Instead, prior studies
have focused on model validation, control, and accuracy assessment. In this paper, we present the first integrated
steerable needle-interventional device. The ACUSITT integrates a multi-tube steerable Active Cannula (AC) with an
Ultrasonic Interstitial Thermal Therapy ablator (USITT) to create a steerable percutaneous device that can deliver a
spatially and temporally controllable (both mechanically and electronically) thermal dose profile. We present our initial
experiments toward applying the ACUSITT to treat large liver tumors through a single entry point. This involves
repositioning the ablator tip to several different locations, without withdrawing it from the liver capsule, under 3D
Ultrasound image guidance. In our experiments, the ACUSITT was deployed to three positions, each 2cm apart in a conical pattern to demonstrate the feasibility of ablating large liver tumors 7cm in diameter without multiple parenchyma punctures.
Three dimensional heat-induced echo-strain imaging is a potentially useful tool for monitoring the formation of thermal
lesions during ablative therapy. Heat-induced echo-strain, known as thermal strain, is due to the changes in the speed of
propagating ultrasound signals and to tissue expansion during heat deposition. This paper presents a complete system for
targeting and intraoperative monitoring of thermal ablation by high intensity focused acoustic applicators. A special
software interface has been developed to enable motor motion control of 3D mechanical probes and rapid acquisition of
3D-RF data (ultrasound raw data after the beam-forming unit). Ex-vivo phantom and tissue studies were performed in a
controlled laboratory environment. While B-mode ultrasound does not clearly identify the development of either necrotic
lesions or the deposited thermal dose, the proposed 3D echo-strain imaging can visualize these changes, demonstrating
agreement with temperature sensor readings and gross-pathology. Current results also demonstrate feasibility for realtime
computation through a parallelized implementation for the algorithm used. Typically, 125 frames per volume can
be processed in less than a second. We also demonstrate motion compensation that can account for shift within frames
due to either tissue movement or positional error in the US 3D imaging probe.