Multidetector CT Guided Histotripsy – A Targeting Solution for Closed Bore Imaging Systems

Purpose: Using focused ultrasound, histotripsy provides a noninvasive, nonthermal, and nonionizing local tumor treatment based on acoustic cavitation. Currently, the histotripsy transducer is manually aligned to the target using real-time diagnostic ultrasound. Alternative targeting approaches using x-ray C-arms are being developed to overcome ultrasound limitations with anatomic obstructions and deep targets. In clinical practice, however, tumors are primarily diagnosed, monitored, and intervened on using closed bore imaging systems such as multidetector CT (MDCT). Compared to the open C-arm, a closed bore requires another new targeting workflow to accommodate histotripsy hardware. This study demonstrates the feasibility of performing histotripsy in a conventional closed bore MDCT.   

Materials and Methods: Imaging was performed using an MDCT system (Discovery 750HD, GE), which was registered to a histotripsy transducer with robotic positioner (UR5e, Universal Robots). The registration was performed by attaching a helical calibration phantom (instead of the transducer) to the robotic arm and acquiring a CT for a known robotic arm and patient table position. Histotripsy treatments (2 cm diameter) were performed in 5 agarose-based multimodality phantoms (5x5x5 cm). For each, a pre-treatment axial CT scan was acquired, and a target zone defined. The phantom was then moved 50 cm out of the CT bore using table translation to allow space for the histotripsy transducer. The robot arm was automatically positioned based on the established CT-to-histotripsy calibration and known table translation. After treatment, CT was acquired to visualize the treatment zone. Targeting accuracy was determined by manually segmenting the actual treatment zone on the post-treatment CT and calculating the distance and overlap of the zone on the pre-treatment CT plan.

Results: All treatments were successfully delivered and visualized on CT. The average (STD) Euclidean distance between the planned and actual treatment zone centers was 0.8 ± 0.7 mm. The Dice coefficient (0 = no overlap, 1 = perfect overlap) between planned and actual treatments was 0.94 ± 0.05. The range of actual treatment diameters was 1.9-2.1 cm.

Conclusion: Histotripsy targeting can be performed accurately with a closed bore conventional MDCT system by including table position in the robotic arm calibration. This could enable integration of gold standard imaging techniques for tumor localization into histotripsy treatment planning and delivery.