IEEE Trans Ultrason Ferroelectr Freq Control. 2020 Jul;67(7):1366-1376. doi: 10.1109/TUFFC.2020.2973678. Epub 2020 Feb 13.
Treatment of tumors in organs obscured by the ribs is a challenge for high-intensity focused ultrasound (HIFU) array. The ribs absorb the ultrasound beam's energy causing the temperature of the ribs to rise, the beam to be distorted, and the focal heat deposition to be limited. The challenges presented by the ribs motivated the development of the limited power deposition (LPD) refocusing algorithm, which is capable of limiting the power deposition over the ribs while maximizing the energy deposition at the focus, thus achieving an efficient and safe ablation. In this article, a new iterative sparse LPD (ISLPD) approach that provides similar focal heating to previously developed methods while reducing the number of transducers excited during the HIFU treatment is discussed. It will be possible to conduct processes, such as imaging and motion tracking in parallel with the HIFU treatment by using the elements no longer used by the refocusing technique. The approach removes transducer elements by using an iteratively reweighted penalty matrix to achieve a more sparse solution over the course of multiple rounds of sparsity induction by harshly punishing the use of elements that previously had low excitation magnitudes. The semidefinite relaxation (SDR) method is used as a means of relaxing the nonconvex constraints into convex form and induces sparsity using the one-norm squared as a convex surrogate for the zero-norm. A 1-MHz spherical phased-array focused on a target in an inhomogeneous medium is simulated to test the algorithms' efficacy. The propagation of the waves as they travel from the array toward the target was modeled using a finite-difference time-domain propagation model. Temperature simulations that utilized the inhomogeneous bioheat transfer equation (BHTE) were used to determine the temperature rise profile within the region of interest (ROI). These simulation results illustrate the benefits of the optimization-based approach proposed in this article over the ray-tracing (shadowing) method for element selection.
肋骨遮挡下的器官肿瘤的治疗是高强度聚焦超声（HIFU）阵列面临的挑战。肋骨会吸收超声波束的能量，导致肋骨温度升高、波束变形以及焦点热沉积受限。肋骨带来的挑战促使人们开发了有限功率沉积（LPD）重聚焦算法，该算法能够限制肋骨上方的功率沉积，同时最大限度地增加焦点处的能量沉积，从而实现高效、安全的消融。本文讨论了一种新的迭代稀疏 LPD（ISLPD）方法，该方法在提供与先前开发的方法类似的焦点加热的同时，减少了在 HIFU 治疗过程中激励的换能器数量。通过使用不再用于重聚焦技术的元件，可以与 HIFU 治疗并行进行成像和运动跟踪等过程。该方法通过迭代重新加权惩罚矩阵来移除换能器元件，在多次稀疏性诱导过程中通过严厉惩罚先前激励幅度较低的元件，实现更稀疏的解。半定松弛（SDR）方法被用作将非凸约束松弛为凸形式的手段，并使用一范数的平方作为零范数的凸替代物来诱导稀疏性。模拟了一个 1MHz 的球形相控阵，聚焦于不均匀介质中的一个目标，以测试算法的效果。使用有限差分时域传播模型模拟了波从阵列传播到目标的过程。利用非均匀生物传热方程（BHTE）进行的温度模拟用于确定感兴趣区域（ROI）内的温升分布。这些模拟结果说明了本文提出的基于优化的方法相对于射线追踪（阴影）方法在元件选择方面的优势。
