mm-resolution focal depth controller for focused ultrasound

Abstract

Cerebrovascular diseases account for the second cause of death of Korean. And also, aftereffects of cerebrovascular diseases like paralysis, cognitive disorder increase the death rate too. Although advance in neurosurgery’s surgical technique has made survival rate of cerebrovascular diseases raised, still about half of survivors suffer from aftereffects that rehabilitation is such an important treatment for them. One of the previous studies represents that if brain neuromodulation is implemented with rehabilitation simultaneously, behavioral function can be improved. Therefore, neuromodulation is considered as promising treatment method for its non-invasiveness. Neuromodulation requires two big factors

small size focused ultrasound and reconfigurable focal depth according to patient’s symptoms and locations of lesions. To make small size focused ultrasound, large size of transducer which converts electrical signals into acoustic energy is needed. Also, arrayed structure of transducer is required to make focal point change. Arrayed structure transducers which make controllable focal point are driven by beamforming principle. Constructive interference at focal point is achieved by addition of different time delay to compensate the path differences between the transducer elements. To make deeper focal depth, smaller time difference between delayed signals is required. As a result, fine control of time delay between paths to the transducer can make small size focused ultrasound move along the z-axis(depth) minutely. In this thesis, mm-resolution focal depth controller for focused ultrasound is achieved in 0.18um CMOS technology. This controller makes 0.5 MHz pulsed signals with different time delays needed to make transducer focus the acoustic energy to the target location of brain with mm-resolution. Time delays applied to the electrical signals are determined by bit number of DAC which is used to make reference voltages of comparators. Addition of resistor and capacitor to the oscillator makes output wave closer to the sawtooth shape than sinusoidal. The transition time of output pulse is determined by meeting point of oscillator’s output wave and two outer reference voltages from DAC

high and low. The designed 16-channel focal depth controller consumes 6.2 mW of low power and is fully integrated in a small chip occupying $1.66 mm^{2}$ includ-ing pads.