Ultrasound beam distortion and pressure reduction in transcostal focused ultrasound surgery

Although it has been long expected to extend the use of image-guided focused ultrasound to transcostal treatment of hepatic, renal and pancreatic tumours, the presence of the ribcage remains a significant hindrance. Major side effects described in the literature include pain, bone injuries and insufficient energy delivered to the target organs. Localized hot spots may also exist at the interfaces between the ribs and soft tissue and in the highly absorptive regions such as the skin and connective tissue. This study investigates the factors affecting the acoustic beam patterns with propagation of focused ultrasound through the ribcage. A realistic ribcage model and a simplified absorbing ribcage model with adequate rib thickness were used in the experimental and numerical explorations. The acoustic pressure at the focus (target) was determined as functions of the dimensions of the ribcage, the position of the intended target relative to the ribcage, and the F-number and driving frequency of the ultrasonic transducer. Results show that the resulting ultrasonic beam distortion behind the ribcage depends on all these factors mentioned above. The phenomena of beam splitting, bending and shifting result in a significant reduction in the acoustic pressure or intensity at the target and hence will decrease the ablation efficiency there. This qualitative and quantitative analysis of focusing ultrasound through the ribcage will provide useful information on optimizing sonication protocols i.e. delivering sufficient ultrasound energy into the tumour site and avoiding potential damage to surrounding normal tissue and structures in clinical applications.