Evaluation of Ultrasonic Beam Refraction in HIFU Therapy and Focal Depth Optimization in Ultrasonic Transducer to Increase the Treatment Accuracy

Background and purpose: Nowadays High-intensity focused ultrasound (HIFU) as nonionizing radiation is used for cancer treatment. Ultrasound beams when crossing the border of two environments are refracted causing decrease in the beam focusing within a tumor. Also, it may induce damage to normal tissues. In this study, we evaluated the focal depth shift induced by ultrasonic beam refraction. Materials and methods: At first, the level of shift caused by the difference in two specific tissues were calculated for one element of transducer. Then it extended to all of the transducer elements and total shift was calculated. Designed algorithms were implemented in MATLAB. Also, HIFU simulator was used according to the US Food and Drug Administration (FDA) regulations to simulate HIFU therapy. Results: The results showed that beam refraction caused by different velocity shifted the focal point. Therefore, an algorithm was proposed for controlling the focal depth and was tested on four models with two layers of tissue. Results showed that using the proposed algorithm corrected much of the focal point shift. Also, it was noted that a part of this shift was caused by absorption of the beam in tissues. Conclusion: Not controlling the focal point shift can induce damage to normal tissues. Therefore, using computational algorithms significantly corrected the focal point shift and lead to increasing safety and having adequate control in HIFU therapy.