Improvement and Quantitative Assessment of B-mode Images Produced by Annular Array/Cone Hybrids

Abstract

We have developed a hybrid ultrasound imaging system consisting of a spherically focused annular array transmitter and a conical receiver. The high resolution (0.3 mm at 4 MHz) of this system has provided superior images of the female breast. Recently, we have applied two signal processing techniques that offer further improvement in image quality. In the first, phase insensitive sector addition (PISA), the cone is divided into eight sectors and images are obtained with each sector. The final image, made by adding these independent images, has much smoother speckle at the expense of some loss of resolution. In the second method, multiplicative processing (MP), the B-scan is formed from the produce of the eight rf signals. This technique, which also provides smoother speckle with some loss of resolution, reduces the sensitivity of the hybrid to scatterers more than one millimetre off axis and thus suppresses some artifacts characteristic of axicon focusing. To quantitatively assess these signal processing methods, and indeed any B-mode imaging system, we have devised a measure that we call the contrast to speckle ratio (CSR). Images are made of a scattering phantom containing anechoic cylindrical holes. The CSR is a measure of the contrast of these objects in the image relative to the contrast fluctuations caused by speckle. Plots of CSR vs. hole size show the superiority of PISA and MP over conventional coherent processing. This result correlates with subjective impressions of images of the test object and freshly excised breast tissue. It appears that some of the resolution of the hybrid system can be “traded off” for smoother and less distracting speckle to achieve even better breast images.