Real-time volumetric ultrasound imaging with capacitive micromachined ultrasonic transducer (CMUT) probes [electronic resource]
Keywords: Electrical Engineering.

In ultrasound imaging, an array of ultrasonic transducers is used to generate ultrasound beams and obtain the echo data reflected by the imaging targets. The echo data are sent to a back-end imaging system and processed for image reconstruction. Currently, most of the commercial ultrasound probes in the market are made of piezoelectric transducer arrays. Capacitive micromachined ultrasonic transducers (CMUTs) are another type of transducers which emerged as an alternative to piezoelectric transducers. Being fabricated using standard micromachining processes, they have advantages in fabricating transducer arrays with arbitrary geometry or many transducer elements, as well as in integrating them with a supporting front-end electronics. 2-D transducer arrays enable volumetric imaging without mechanically scanning the probe. However, a 2-D transducer array consists of a large number of transducer elements, which increases the imaging system complexity and the data processing time. To eliminate these issues, at the expense of degraded image quality, a sparse 2-D array with fewer transducer elements can be used in volumetric imaging. The first part of this dissertation describes an optimization method to find a sparse array configuration that provides optimal image quality with limited number of transducer elements. Among the various array shapes we can implement using CMUT fabrication technologies, the ring geometry is particularly attractive in many applications including intracardiac and intravascular applications. A ring transducer array, which is a type of 2-D sparse array, enables volumetric imaging with much fewer transducer elements compared to a fully populated 2-D array. To find the optimal imaging scheme for real-time imaging with a ring array, various imaging options were investigated and compared in both simulations and experiments. Commercial ultrasound imaging systems are mainly designed for probes with standard geometry and conventional imaging techniques. Therefore, they are not readily accessible for probes with non-standard geometry, such as a ring array. In addition, it is difficult to use them with non-conventional imaging schemes that may be ideal for non-standard array geometries. For real-time volumetric imaging with various types of CMUT arrays, a flexible imaging system that works with arbitrary probe geometry and various imaging schemes including non-conventional imaging techniques was designed and implemented. The raw data obtained by the custom imaging system are transferred to a host PC, and then processed for real-time image reconstruction by custom imaging software. The custom imaging software was first developed on a multi-core CPU platform, and then on a graphics processing unit (GPU) platform for better real-time imaging performance and more functionalities, such as real-time volume rendering and dual-mode imaging with both photoacoustic and ultrasound images. Using the custom imaging system and software, real-time imaging was demonstrated for various types of CMUT probes and imaging schemes. The imaging results presented in this dissertation show successful demonstration of real-time imaging for 1-D, rectangular, and annular CMUT arrays with various imaging phantoms.