Improving conformal tumour heating by adaptively removing control points from waveform diversity beamforming calculations: a simulation study.

Waveform diversity is a phased array beamforming strategy that determines an optimal sequence of excitation signals to maximise power at specified tumour control points while simultaneously minimising power delivered to sensitive normal tissues. Waveform diversity is combined with mode scanning, a deterministic excitation signal synthesis algorithm, and an adaptive control point removal algorithm in an effort to achieve higher, more uniform tumour temperatures. Simulations were evaluated for a 1444 element spherical section ultrasound phased array that delivers therapeutic heat to a 3 cm spherical tumour model located 12 cm from the array. By selectively deleting tumour control points, the tumour volume heated above 42°C increased from 2.28 cm3 to 11.22 cm3. At the expense of a slight increase in the normal tissue volume heated above the target temperature of 42°C, the size of the tumour volume heated above 42°C after tumour points were deleted was almost five times larger than the size of the original heated tumour volume. Several other configurations were also simulated, and the largest heated tumour volumes, subject to a 43°C peak temperature constraint, were achieved when the tumour control points were located along the back edge of the tumour and laterally around the tumour periphery. The simulated power depositions obtained from the results of the adaptive control point removal algorithm, when optimised for waveform diversity combined with mode scanning, consistently increased the penetration depth and the size of the heated tumour volume while increasing the heated normal tissue volume by a small amount.