RESOLUTION AND QUANTITATIVE ACCURACY IMPROVEMENTS IN ULTRASOUND TRANSMISSION IMAGING (COMPUTED TOMOGRAPHY)

The type of ultrasound transmission imaging, referred to as ultrasonic computed tomography (UCT), reconstructs distributions of tissue speed of sound and sound attenuation properties from measurements of acoustic pulse time of flight (TOF) and energy received through tissue. Although clinical studies with experimental UCT scanners have demonstrated UCT is sensitive to certain tissue pathologies not easily detected with conventional ultrasound imaging, they have also shown UCT to suffer from artifacts due to physical differences between the acoustic beam and its ray model implicit in image reconstruction algorithms. Artifacts are expressed as large quantitative errors in attenuation images, and poor spatial resolution and size distortion (exaggerated size of high speed of sound regions) in speed of sound images. In this work, methods are introduced and investigated which alleviate these problems in UCT imaging by providing improved measurements of pulse TOF and energy.;Reduction of refractive errors in attenuation images using an array receiver is investigated and compared to conventional single element receiver techniques. An approach is introduced which uses computer synthetic focussing of the array to arbitrary depths within the object, in combination with incoherent signal averaging. Also a purely phase insensitive application of the array is demonstrated to be resistant to refraction related errors. As with the speed of sound imaging tests, single projections and full image reconstructions of a refractive test object are used to demonstrate accuracy improvements in attenuation imaging afforded by the array receiver methods.;Traditionally pulse TOF is measured using leading edge detection techniques which define pulse TOF as that of the earliest arriving wave. The alternative TOF measure proposed here utilizes the cross correlation of received signals with a reference signal of known transit time. Superiority in resolution and reduced image distortion of cross correlation over leading edge detection methods is demonstrated by single projectional views and full image reconstructions of simple test objects. Also a comparison of speed of sound images of soft tissue structures, scanned in vivo, is presented to illustrate image quality improvements which can be realized on existing UCT clinical scanners via implementation of cross correlation.