Driving circuitry for focused ultrasound noninvasive surgery and drug delivery applications

Abstract

Recent works on focused ultrasound (FUS) have shown great promise for cancer therapy. Researchers are continuously trying to improve system performance, which is resulting in an increased complexity that is more apparent when using multi-element phased array systems. This has led to significant efforts to reduce system size and cost by relying on system integration. Although ideas from other fields such as microwave antenna phased arrays can be adopted in FUS, the application requirements differ significantly since the frequency range used in FUS is much lower. In this paper, we review recent efforts to design efficient power monitoring, phase shifting and output driving techniques used specifically for high intensity focused ultrasound (HIFU).

Keywords: beam-steering; focused ultrasound; high-intensity focused ultrasound; hyperthermia; integrated circuits; noninvasive surgery; phase shifter; phased arrays; phased-locked loop; power amplifier; pulser; tissue ablation.

Figure 1.

(a) A block-diagram of a typical single-element HIFU system. If used in an n-element system, the area enclosed in the dashed line should be repeated n times. (b) A more practical multi-element implementation that employs phase shifters.

Figure 2.

The output power applied to a 50 Ω load with and without feedback, reproduced from [15].

Figure 3.

(a) FPGA based power meter block diagram. (b) An example of the sampled forward voltage waveform of a 2 MHz signal applied in a burst of 3 pulses with a burst period of 3 μs.

Figure 4.

A simple phase shifter using switchable delay lines.

Figure 5.

Isolation transformer used to create 180° phase shifts [17].

Figure 6.

Digital phase shifter using programmable counters. (a) Schematic diagram, and functional operation examples of (b) a 45° phase shift and (c) and 360° phase shift.

Figure 7.

Inverter based digital delay lines. (a) Rise time control, reproduced from [18], and both rise and fall time control, reproduced from [22].

Figure 8.

AND gate delay element, reproduced from [19].

Figure 9.

Block diagram of a linearized PLL.

Figure 10.

The impact that the number of phases used in a HIFU phase array has on the peak pressure amplitude square (P²), simulation reproduced from [20].

Figure 11.

Basic schematic of a class-D power amplifier.

Figure 12.

(a) Basic schematic of a class-E power amplifier. (b) The drain voltage waveform of an ideal class-E amplifier. (c) The effects of adjusting the output network components of a class-E amplifier [27].

Figure 13.

Schematic of a direct-modulation transmitter.

Figure 14.

Differential class-E power amplifier with mode-locking, reproduced from [40].

Figure 15.

Basic schematic of the lock-mode power amplifier [27].