Capacitive micromachined ultrasonic transducer (CMUT) arrays for endoscopic ultrasound [electronic resource]
Keywords: Electrical Engineering.

As healthcare technology advances, we are seeing a growing demand for multi-modal medical systems capable of generating high quality 3D images in real time. Ultrasound has become more prevalent as a medical tool to fit this need, and establishing techniques to build robust ultrasound systems has become a priority. Capacitive micromachined ultrasonic transducers, or CMUTs, have several specific advantages over traditional piezoelectric devices in the medical imaging domain: they have a wide bandwidth, which is ideal for imaging, and their use of IC fabrication techniques not only enable fabrication of high frequency devices with arbitrary geometries but also allow for easy integration with electronics. Ultrasound imaging has gained popularity as a medical tool for several reasons. Unlike x-ray imaging, ultrasound imaging is considered safe for both patient and practitioner, as it does not emit ionizing radiation. It is a minimally invasive tool, as often times imaging and therapy can be done externally to the organ of interest. Because highly complex hardware is not required, ultrasound tools are much cheaper for clinicians than MRI or PET scanners, which can cost millions of dollars. Importantly, processing of volumetric ultrasound data can be done in real time, allowing clinicians to make quick decisions and provide fast feedback for their patients. There are several areas of medical research that can greatly benefit from the use of ultrasound-based devices. One such area is in the staging and diagnosis of bladder cancer. Current diagnosis methods with urine tests are insufficient and often are followed by transurethral surgery to obtain a biopsy. The risks associated with a biopsy include bladder trauma and perforation, which may in return require more surgery. In order to minimize the number of required procedures, and as a result, minimize the risk of trauma for the patient, a minimally-invasive, high resolution imaging device is desired to improve diagnostic sensitivity. Ultrasound provides high resolution of soft tissues, where x-rays typically do not perform well. Specifically in this application, a forward-facing device is desirable, and when combined with a 2D transducer array for 3D imaging, can allow the practitioner to visualize the entire volume of the tumor during the procedure. Additionally, enhancing ultrasound imaging with photoacoustic imaging or ablation techniques could vastly improve diagnosis and treatment of this disease. I have developed a forward-facing ultrasound-based endoscope that leverages CMUT technology to generate high resolution, real-time 3D images. This dissertation first presents the design and fabrication of a novel QuadRing CMUT array. The CMUT array is composed of four independent, concentric 128-element ring arrays. The center frequency of each array was chosen such that their natural foci align. Following transducer fabrication, a new custom charge amplifier IC is introduced and characterized. This new IC is integrated with the CMUT to improve the signal-to-noise ratio (SNR) of the received ultrasound signal. One of the challenges in building a robust 3D medical imaging system is in the integration and packaging of the device. This dissertation addresses many of these associated challenges, including integration with flexible printed circuit boards and the use of 3D-printed components to realize an endoscope form factor that can easily be used for imaging inside the body. These techniques are leveraged to build a forward-facing CMUT-based endoscope. The pulse-echo response of this endoscope is characterized, and high resolution, real-time, volumetric images are obtained using these imaging devices. Additionally, a novel CMUT biasing scheme that enables complete ground shielding of the array is introduced and experimentally validated, a key step towards clinical use of these devices. The ring structure of these arrays allows the introduction of other therapeutic devices, such as fibers for photoacoustic imaging or high intensity focused ultrasound (HIFU) arrays for tissue ablation, through the center of the endoscope without increasing the overall package size. By combining these therapeutic devices with a forward-facing imaging device, imaging and therapy are automatically co-registered and can be performed simultaneously.