A dual-mode ultrasound system for imaging and High Intensity Focused Ultrasound (HIFU) with a single 2-D Capacitive Micromachined Ultrasonic Transducer (CMUT) array
Keywords: Electrical Engineering.

Ultrasound imaging technology has many applications for the medical field and for the public. Thanks to ultrasound imaging, parents can meet their precious child even before the baby is born. In clinical applications, ultrasound is inexpensive, portable and reveals the structure and movement of organs in real time, allowing physicians to monitor the growth and physical development of a fetus. Because there is no ionizing radiation exposure to the patient, it is a very safe technology. In addition to diagnostic applications, ultrasound has been used for therapeutic treatment. High intensity focused ultrasound (HIFU) has been widely used to treat different types of tumors, including those of prostate, liver, breast, kidney, bone and pancreas because of its non-invasive and precise approach for tissue ablation. The basic concept of using HIFU is to focus continuous ultrasound at the focal point and a temperature increase beyond a certain point creates a lesion without damaging the surrounding tissue. For successful HIFU operation, it is important to have a reliable method for guidance and monitoring of the treatment such as ultrasound imaging. Most ultrasound image-guided HIFU systems need separate imaging and HIFU transducers, and require a cooling system due to properties of piezoelectric transducers such as narrow fractional bandwidth and self-heating. As an alternative, capacitive micromachined ultrasonic transducers (CMUTs) have a distinctive advantage over piezoelectric transducers in respect to self-heating and a wide fractional bandwidth. Thus, CMUTs are especially beneficial in dual-mode operations where a single transducer is used for both imaging and therapy. By taking advantage of this CMUT technology, I developed a compact dual-mode ultrasound system that can perform both ultrasound imaging and HIFU with a single 2-D CMUT array. A dual-mode ultrasound probe is equipped with a dual-mode application-specific integrated circuit (ASIC) and a 2-D 32x32-element CMUT array. The dual-mode ASIC consists of pulsers, transmit beamforming circuitry, and low-noise amplifiers for imaging mode and high voltage (HV) switches for HIFU mode. By turning HV switches on and off, the system can alternately operate imaging mode and HIFU mode on demand. A 2-D 32x32-element CMUT array was fabricated to have a center frequency of 5 MHz in immersion. Both ASIC and CMUT array were flip-chip bonded to a custom-designed flexible printed circuit board (flex PCB). After polydimethylsiloxane (PDMS) encapsulation, the acoustic performance of the probe was evaluated. I successfully demonstrated the imaging mode of the dual-mode probe using nylon wire phantom. Using HIFU mode, I measured 7.4 MPa peak-to-peak pressure at 8 mm focal depth. To get higher pressure for the ablation, high AC and DC voltage were used, and CMUT arrays got shorted due to the insulator breakdown. With this probe, obtaining high pressure levels needed for tissue ablation was problematic with CMUTs due to device failure at high voltages. Therefore, I re-optimized a CMUT design that can produce higher output pressure without breakdown or device failure. With CMUT simulation software, the design parameters of CMUT element were optimized with a gap height of 0.13 um and a top plate thickness of 1 um. After it was fabricated and integrated, the dual-mode probe was tested again in an acoustic setup. Compared with previous results, the device shows improved performance without device failure. The focused pressure at F-1 (8 mm) was measured to 16 MPa peak-to-peak. More importantly, most of the device can produce high pressure levels reliably without device failure. Using HIFU simulation software, the specification for HIFU ablation was explored if the dual-mode probe can ablate the tissue. It shows that even with 10 MPa peak-to-peak the dual-mode probe can create the lesion. An ablation test was successfully performed on HIFU phantom gel and ex-vivo tissue using HIFU mode of the dual-mode probe. Another important evaluation as a HIFU probe was the heating of the device. While CMUT array has very low self-heating, because of the power dissipation on HV switches of dual-mode ASIC, the ASIC was heated during HIFU mode. To reduce the heating of dual-mode ASIC, the copper heat sink rod, the chiller, and the water circulation heat sink were added to the system and it significantly reduced the heating. With the thermal management system, the probe was thermally stable around the body temperature during HIFU mode and imaging mode. Lastly, I successfully demonstrated ultrasound image-guided HIFU on HIFU phantom gel with guide wires by switching between imaging mode and HIFU mode using dual-mode ultrasound system. Our studies established a dual-mode HIFU system that will improve the non invasive ablation of tissue. This work of the dual mode system certainly shows the possibility of the new treatment application that was impossible to achieve using the conventional image-guided HIFU system.