Anechoic sphere phantoms for estimating 3-D resolution of very-high-frequency ultrasound scanners.

Two phantoms have been constructed for assessing performance of high-frequency ultrasound imagers. They also allow for periodic quality assurance tests and training technicians in the use of higher-frequency scanners. The phantoms contain eight blocks of tissue-mimicking material; each block contains a spatially random distribution of suitably small anechoic spheres having a small distribution of diameters. The eight mean sphere diameters are distributed from 0.10 to 1.09 mm. The two phantoms differ primarily in terms of the frequency dependence of the backscatter coefficient of the background material. Because spheres have no preferred orientation, all three (spatial) dimensions of resolution contribute to sphere detection on an equal basis; thus, the resolution is termed 3-D. Two high-frequency scanners are compared. One employs single-element (fixed focus) transducers (25 and 55 MHz), and the other employs variable focus linear arrays (20, 30, and 40 MHz). The depth range for detection of spheres of each size is determined corresponding to determination of 3-D resolution as a function of depth. As expected, the single-element transducers are severely limited in useful imaging depth ranges compared with the linear arrays. In this preliminary report, only one human observer analyzed images.