The utility of sparse 2D fully electronically steerable focused ultrasound phased arrays for thermal surgery: a simulation study

Abstract

Sparse arrays are widely used in diagnostic ultrasound for their strong performance and relative technical simplicity. This simulation study assessed the efficacy of phased arrays of varied sparseness for thermal surgery, especially with regard to power consumption and near-field heating. It employs a linear ultrasound propagation model and a semi-analytical solution to the Pennes' bioheat transfer equation. The basic design had 4912 cylindrical transducers (500 kHz) arranged on a flat 12 cm disk (1.5 mm spacing). This array was compared to randomly-thinned sparse arrays with 75%, 50% and 25% populations. Temperature elevations of 60 and 70 °C were induced in sonication times of 5-20 s, at foci spanning depths of 50-150 mm and radii of 0-60 mm. The sparse arrays produced nearly indistinguishable focal patterns but, averaged across the foci, required 132%, 200% and 393% of the power of the full array, respectively, applied through fewer transducer elements. Comparable results were found at 1 MHz from equivalent arrays. Simulated lesions were formed (thermal dose ⩾ 240 equivalent minutes at 43 °C (T(43))) and 'transition' and 'unsafe' regions (both defined as 5 min < T(43) < 240 min) were identified, the former immediately surrounding the lesion and the latter anywhere else. At a depth of 100 mm, sparse arrays were found to produce comparable lesions to the full array at the focus, but 'unsafe', over-heated near-field regions after some ablated lesion volume: about 12 mL for the 25% array, around 100 mL for the 50% array, while the 75% and full arrays produced 150 mL lesions safely.

Image-guided ultrasound phased arrays are a disruptive technology for non-invasive therapy

“…With random and simulation-optimized phased arrays, this energy is incoherently distributed amongst a large volume, thereby minimizing the formation of local hot spots in tissue. The reduction in focal intensity is proportional to the transducer surface area that is not emitting ultrasound [96]. Therefore, more total energy is required from a sparse array to achieve the same acoustic intensity or temperature elevation at the focus than a fully populated array, all else equal.…”

“…Therefore, more total energy is required from a sparse array to achieve the same acoustic intensity or temperature elevation at the focus than a fully populated array, all else equal. This is a significant drawback for thermal surgery treatments since the extra energy is transmitted through the superficial tissues, causing increased near-field heating that limits the rate of ablation (Figure 5) [96, 97]. However, sparse arrays may be useful for cavitation-based therapy [98], where the required time-averaged powers are much lower, meaning that superficial tissue heating is no longer a limiting factor.…”

“…Performance of flat, 2D random sparse arrays for FUS therapy [96]. (a) Lateral pressure profiles of random sparse arrays are shown with a different number of active elements relative to the full array (500 kHz, 4912 elements, element spacing = λ/2), which demonstrates good focal quality even with 253 of active elements.…”

“…(a) Example of a sonication raster pattern used to generate extended lesions [96]. Each of the five sonication cells contain 25 foci, shown in detail in the middle cell.…”

Optimal design of a concave annular array transducer to generate high intensity focused ultrasound

“…[8] 이 이론을 토대로 구동주파수를 단일 연속파에서 Chirp 주파수로 변경 시켜 그레이팅 로브를 최소화시킨 연구가 수행되었 고, [9] 주어진 트랜스듀서의 기하학적 구조에 대한 최 적의 주파수를 찾는 연구도 수행되었다. [10] 기하학 적 구조에 관하여도, 채널과 채널 사이의 간격을 반 파장으로 설정하거나, [6] 환상형 채널을 갖는 트랜스 듀서의 각 채널을 여러 개의 아치형으로 분리시키고 분리된 채널에 소용돌이형 위상차를 주어서 그레이 팅 로브의 영향을 줄이고자 하는 연구가 수행되었 다. [11] 그리고 그레이팅 로브를 억제시키면서 빔 조 향이 가능하도록 하는 방법으로 더 적은 개수의 채 널을 사용하는 Sparse 배열 트랜스듀서에 대한 연구 도 수행되었으며, [5,12] 그 목적을 극대화시키기 위해 최적화 기법을 이용한 트랜스듀서 구조 설계에 대한 연구가 수행되었다.…”