THE APPLICATION OF AXICON TRANSDUCERS TO MEDICAL ULTRASOUND IMAGING.

摘要

Two factors which have limited the diagnostic potential of current B-mode ultrasound systems are their limited spatial resolution and the presence of a distracting speckle pattern characteristic of this coherent imaging technique. This thesis investigates the improvements that can be made in both areas by the use of line focus, or axicon transducers.; The chief advantage of an axicon is the maintenance of a narrow beamwidth over a large depth of field. The properties of one type of axicon, the cone, are shown to be well suited to medical applications and are examined in detail. Acoustic fields of conical transducers are described for continuous wave and pulsed excitation. The relatively high off-axis sensitivity of the cone precludes its use as both a transmitter and receiver. However, a suitable point spread function can be achieved by a hybrid system which combines a spherically focused annular array transmitter with a conical receiver. A practical hybrid was constructed in which the conical focusing was obtained by two aluminum mirrors and a piezoelectric ceramic disk. This system operates at 4 MHz and attains lateral resolution of 0.33 mm and axial resolution of 0.45 mm over a 7 cm depth with excellent signal to noise ratio.; Further improvements in image quality were obtained by dividing the receiving disk into eight sectors and processing the signals from each sector. Two methods, multiplicative processing and phase insensitive addition, reduced speckle fluctuations. This effect was more marked with the latter technique but the former reduced artifacts characteristic of the conical geometry.; To quantitatively assess image improvement, a new quantity, called contrast to speckle ratio (CSR) is proposed. The CSR is a measure of the image contrast of cylindrical voids in a random scattering medium relative to contrast fluctuations caused by speckle. Measurement of CSR for various cylinder diameters showed the superiority of the signal processing methods and agreed well with subjective ranking of image quality.; Finally, future developments to improve sensitivity, CSR, and image acquisition speed are suggested. The feasibility of detecting microcalcifications in the breast is also discussed.