CMOS Integrated Circuits to Interface with a 2D Array of Ultrasound Transducers for Ultrasound Brain Stimulation

Abstract

Neurological disorders profoundly affect quality of life, impairing essential functions like memory, cognition, and movement. Traditional treatments are often limited due to the complexity of the brain, the invasiveness of procedures, or the poor precision of the treatment. Ultrasound (US) neuromodulation offers a promising and minimally invasive alternative, using ultrasound waves to modulate neural activity in deep brain regions with high spatial resolution. Despite its potential, a technological gap limits preclinical ultrasound neuromodulation studies, as current ultrasound devices are too bulky and imprecise for flexible and continuous neuromodulation in freely moving rodents. Moreover, precise targeting is essential for effective neuromodulation, necessitating fine control over the focal spot location. This thesis aims to discover new system-level and circuit-level approaches to designing a miniaturized, power-efficient, and high-spatial-resolution 2D phased array transmitter application-specific integrated circuit (ASIC). These approaches are guided by the vision of implementing a miniaturized ultrasound transducer device for precise ultrasound neuromodulation in freely moving rodents, ultimately laying the foundation for future clinical applications in neurological disorder treatment.
The thesis begins with a literature review to determine the acoustic parameters required for effective ultrasound neuromodulation. Then, a comprehensive system-level study using the k-Wave Matlab toolbox is conducted, examining factors critical to the design of a 2D phased array transducer. This study highlights how parameters such as driving voltage, phase quantization resolution, frequency, and array length significantly impact the performance of the 2D phased array ultrasound transducer by influencing key acoustic and electrical attributes. Optimizing these parameters is essential to achieving high focal gain and spatial resolution while minimizing power consumption. The study finds that a three-bit phase resolution is optimal for accurate ultrasound beam steering. Additionally, while increasing array length improves spatial resolution and phased array gain, it also increases the overall size of the ultrasound transducer, constrained by the small brain sizes of rodents in preclinical studies. This study demonstrates that higher frequencies enhance spatial resolution and phased array gain. However, due to kerf limitations in the dicing manufacturing process of piezoelectric transducers, increasing frequency reduces the effective area of the transducers. Moreover, the pitch size should be reduced with increasing frequency in order to avoid the appearance of the grating lobes in the beam profile, forming a tradeoff between the frequency and the available area for the beamforming channels. Since higher frequencies are desired, particularly for applications in which higher spatial resolution is needed, high-voltage beamforming channels have been developed to investigate the highest frequency that allows for implementing a high-voltage beamforming channel. In this regard, two beamforming channels have been implemented and validated in TSMC 180-nm BCD technology, allowing for driving the ultrasound transducers with 12-MHz 36-V pulses. On the other hand, the power consumption of the beamforming channels has a crucial impact on the overall power efficiency of the ASIC. Therefore, a power-efficient HV pulser is developed, leading to 40.9% power consumption reduction with respect to the conventional HV pulser.
Utilizing the proposed beamforming channels, a prototype 2D phased array transmitter ASIC is developed to validate the findings of the initial system-level study. The transmitter ASIC includes a 12*12 array of beamforming channels that generate 12-MHz pulses with 20-V amplitude. The phasing circuitry in the ASIC generates the required delay for each beamforming channel, hence allowing the ASIC to steer the ultrasound beam. The measurement results proved the functionality of the phased-array transmitter ASIC in terms of phase quantization, showing a maximum DNL of 0.35 LSB. Furthermore, the ASIC allows for controlling the driving voltage and the burst duration, as needed for US neuromodulation applications. Based on the prototype 2D phased array transmitter ASIC, a US-guided US transmitter ASIC is developed for preclinical US neuromodulation experiments in freely moving rats. Considering the size of the rat's brain, the size of the ASIC is limited to 5*5 mm2. Given the prior discussion, the frequency was set to 12 MHz to achieve higher spatial resolution and phased array gain. Taking into account the proposed 20-V beamforming channel, the size of the 2D array was set to 66*66. According to the author's knowledge, this is the first transmitter ASIC that allows for preclinical US neuromodulation experiments on freely moving animals. Moreover, a temperature sensor was added to the ASIC to monitor the temperature rise in the ASIC. In order to monitor the location of the focal spot during the US neuromodulation, synthetic aperture US imaging capabilities are added to the ASIC to track the tissue displacement caused by the US neuromodulation push. In this regard, the ASIC includes eight groups of receiver channels that will be connected to 96 piezoelectric transducers using an analog multiplexer. To prevent side lobes in the beam profile, a system-level study was conducted to determine the optimal placement of receiver channels within the 2D array. The study revealed that a random distribution of receiver elements among the 2D transmitter elements is the most effective arrangement for minimizing side lobe appearance. To the best of the author's knowledge, this is the first ultrasound neuromodulation ASIC with integrated imaging capabilities. The measurement results proved the functionality of the ASIC in generating 20-V 12-MHz pulses with a programmable burst duration. The simulation results have shown that the receiver channels are capable of recording the echoes required for US imaging to monitor the location of the focal spot.
In summary, this thesis describes innovations in system-level and circuit-level integrated circuit design for developing an ultrasound ASIC that can be integrated with a 2D array of piezoelectric transducers, forming a miniaturized phased array US transducer that allows 3D US beam steering and focusing with the highest combination of spatial resolution and penetration depth in the context of pre-clinical studies. The ASIC includes the receiver channels that can be used for US imaging, allowing for monitoring real-time monitoring of the location of the focal spot during the US neuromodulation. This US ASIC leads to the emergence of miniaturized US transducers compatible with preclinical behavioral experiments in rodents, paving the way for the development of therapies to treat neurological disorders in humans.