Abstract

The overlaying rib cage is a major hindrance in treating liver tumors with high intensity focused ultrasound (HIFU). The problems caused are overheating of the ribs due to its high ultrasonic absorption capability and degradation of the ultrasound intensity distribution in the target plane. In this work, a correction method based on binarized apodization and geometric ray tracing approach was employed to avoid heating the ribs. A detailed calculation of the intensity distribution in the focus plane was undertaken to quantify and avoid the effect on HIFU beam generated by a 1-MHz 256-element random phased array after the ultrasonic beam passes through the rib cage. Focusing through the ribs was simulated for 18 different idealized ribs-array configurations and 10 anatomically correct ribs-array configurations, to show the effect of width of the ribs, intercostal spacing and the relative position of ribs and array on the quality of focus, and to identify the positions that are more effective for HIFU applications in the presence of ribs. Acoustic simulations showed that for a single focus without beam steering and for the same total acoustic power, the peak intensity at the target varies from a minimum of 211 W/cm2 to a maximum of 293 W/cm2 for a nominal acoustic input power of 15 W, whereas the side lobe level varies from 0.07 Ipeak to 0.28 Ipeak and the separation between the main lobe and side lobes varies from 2.5 mm to 6.3 mm, depending on the relative positioning of the array and ribs and the beam alignment. An increase in the side lobe level was observed by increasing the distance between the array and the ribs. The parameters of focus splitting and the deterioration of focus quality caused by the ultrasonic propagation through the ribs were quantified in various possible different clinical scenarios. In addition to idealized rib topology, anatomical realistic ribs were used to determine the focus quality of the HIFU beam when the beam is steered both in axial and transverse directions and when the transducer is positioned at different depths from the rib cage.

Highlights

  • Hepatocellular Carcinoma (HCC) is the most common form of liver cancer and one of the leading causes of cancer-related deaths [1]

  • In addition to idealized rib topology, anatomical realistic ribs were used to determine the focus quality of the High intensity focused ultrasound (HIFU) beam when the beam is steered both in axial and transverse directions and when the transducer is positioned at different depths from the rib cage

  • We have demonstrated the dual nature of imaging and therapy of the spherical random phased array, providing a platform to image the strong aberrating structures such as rib cage in real time and tailor the acoustic field to achieve high peak focal intensity while avoiding heating of the ribs [21]

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Summary

Introduction

Hepatocellular Carcinoma (HCC) is the most common form of liver cancer and one of the leading causes of cancer-related deaths [1]. Treatment options include liver transplantation and surgery; only about 25% of patients are suitable for surgical resection, depending on the size and location of the tumor. High intensity focused ultrasound (HIFU) is a non-invasive medical procedure that uses high-amplitude waves to heat and ablate the tumor. The feasibility of HIFU for treatment of a range of tumors including liver cancer has been demonstrated in several studies [2,3,4]. One of the major limitations is the presence of intervening structures such as rib cage that obstruct the acoustic beam path [5,6]. Three problems are associated with the presence of rib cage in the HIFU beam path. Absorption of acoustic energy by the ribs causing local heating and skin burns [5]. Reduction in the intensity at the focal spot due to the shadowing of elements of the phased array by the ribs [7], and, focus deterioration due to the periodic structure of the ribs [8]

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