Ultrasound Imaging with Flexible Array Transducer for Pancreatic Cancer Radiation Therapy

Simple Summary Ultrasound (US) imaging has been widely used for tumor tracking in image-guided radiotherapy. The quality of US images using conventional probes highly depends on user proficiency and anatomical changes, which has severely hindered the use of US-based organ motion monitoring for pancreatic cancer. The flexible array transducer is a novel and promising solution to address the limitation of conventional US probes. At the same time, its strength and flexible geometry also makes the image reconstruction and delay-and-sum (DAS) beamforming very challenging. Inaccuracy in delay calculation results in defocused and distorted US images. In this study, we proposed a novel shape estimation for flexible array transducer to enhance abdomen motion monitoring. Pancreatic cancer with less than 10% 3-year survival rate is one of deadliest cancer types and greatly benefits from enhanced radiotherapy. Organ motion monitoring helps spare the normal tissue from high radiation and, in turn, enables the dose escalation to the target that has been shown to improve the effectiveness of RT by doubling and tripling post-RT survival rate. The flexible array transducer is a novel and promising solution to address the limitation of conventional US probes. We proposed a novel shape estimation for flexible array transducer using two sequential algorithms: (i) an optical tracking-based system that uses the optical markers coordinates attached to the probe at specific positions to estimate the array shape in real-time and (ii) a fully automatic shape optimization algorithm that automatically searches for the optimal array shape that results in the highest quality reconstructed image. We conducted phantom and in vivo experiments to evaluate the estimated array shapes and the accuracy of reconstructed US images. The proposed method reconstructed US images with low full-width-at-half-maximum (FWHM) of the point scatters, correct aspect ratio of the cyst, and high-matching score with the ground truth. Our results demonstrated that the proposed methods reconstruct high-quality ultrasound images with significantly less defocusing and distortion compared with those without any correction. Specifically, the automatic optimization method reduced the array shape estimation error to less than half-wavelength of transmitted wave, resulting in a high-quality reconstructed image.