Thermal dose optimization via temporal switching in ultrasound surgery

Abstract

Temporal switching has been simulated and implemented in vivo experiments as a method to optimize thermal dose in ultrasound surgery. By optimizing the thermal dose over a tissue volume, the peak temperature is decreased, less average power is expended, and overall treatment time is shortened. To test this hypothesis, a 16 element, spherically sectioned array has been constructed for application in ultrasound surgery guided by magnetic resonance imaging. A simulation study for the array was performed to determine an optimal treatment from a set of multiple focus fields. These fields were generated using the mode scanning technique with power levels determined numerically using a direct weighted gradient search in the attempt to create an optimally uniform thermal dose over a 0.6x0.6x1.0 cm(3) tissue volume. Comparisons of the switched fields and a static multiple focus field indicate that the switching technique can lower power requirements and decrease treatment time by 20%. More importantly, the peak temperature of the sonication was lowered 13 degrees C, thus decreasing the possibility of cavitation. The simulated results of the 16 element array were then experimentally tested using MRI to noninvasively monitor temperature elevations and predict lesion size in rabbit thigh muscle in vivo. In addition, the results show that the switching technique can be less sensitive to tissue inhomogeneities than static field sonication while creating contiguous necrosis regions at equal average powers.

Real‐time monitoring of radiofrequency ablation of rabbit liver by respiratory‐gated quantitative temperature MRI

“…The temperature and TD maps (evaluated with the use of Sapareto's equation (7,8)), were superimposed on magnitude images to visualize the spatial extension of the thermal treatment during the ablation.…”

“…However, the limited precision of such images necessitates the acquisition of dynamic gadolinium (Gd)-enhanced T1-weighted (T1w) images at the end of the procedure (6). MRI may also provide temperature imaging for quantitative monitoring based on the thermal dose (TD) concept (7,8). Thus, several MR thermometry methods have been developed (9).…”

The effect of electronically steering a phased array ultrasound transducer on near‐field tissue heating

“…Indeed, previous studies have indicated that MRgHIFU treatment times can be reduced through the use of phased array transducers. [1][2][3] For effective, accurate and fast thermal treatments, the design objectives for a phased array transducer include a small focal spot size, suppressing grating lobes and side lobes, maximizing the range of electronic steering in 3D and achieving maximum pressure at the focal zone. 4 These objectives are achieved by manipulating the frequency of operation, radius of curvature, number of elements, interelement spacing, element size, and aperture diameter of the transducer.…”

Optimal patterns for sequentially multiple focusing in high intensity focused ultrasound and their application to thermal dose

“…Therefore, optimal multiple-focus patterns which produce a clean ultrasound field can serve as the basis for temporal switching. The previous studies [1,5,6,11] mainly used the predefined multiple focus patterns where the spacing between adjacent focal points was close. Besides, a single pattern with four foci or double patterns with four and five foci were repeatedly scanned in a raster fashion like a single-focus scan.…”

“…Temporal switching technique [1,4,5] is to rapidly switch between patterns for sequentially multiple focusing to increase the effective focal volume. By the way, the larger the number of focal points used, the more likely it is that the ultrasound field will have unexpected hot spots.…”