Coherence factor of speckle from a multi-row probe

The coherence factor provides a quantitative measure of image quality. It is defined as the ratio of the coherent sum across array elements to the incoherent sum and measures the distribution of ultrasonic energy between the main beam and side lobes of a radiation pattern. Values range from 0 to 1. For low values most of the energy is outside of the main beam, and for high values it is in the main beam. The authors have applied the van Cittert-Zernike theorem to determine analytic solutions of the coherence factor for single and multi-row arrays. The solution depends only on the number of rows and columns in a transducer array. With a multi-row probe, the authors imaged a uniform tissue-mimicking phantom and saved coherent signals. Images of the phantom were produced based on coherent and incoherent summations of array elements. They then combined the two images to produce a coherence factor image. Within the focal region, average coherence was 0.50 for the phantom which compares favorably to a value of 0.53 from the analytic solution. Next, phase distortions of pi/2 and pi radians were electronically introduced at specific elements, and the phantom was imaged again. Phase distortion greatly effects energy distribution for coherent summations but has a minimal effect on incoherent summations. An introduced distortion of pi/2 decreased the average coherence factor to 0.33. A distortion of pi further decreased it to 0.11. Results of human studies showed decreased average coherence factors compared to undistorted phantom images. These results suggest that the coherence factor provides a quantitative measure of beam quality for in vivo imaging.