Estimation of spatial resolution for high-frequency imaging systems using a novel anechoic-sphere phantom

A new method has been developed to estimate the spatial resolution of high-frequency ultrasound (HFU, >20 MHz) imaging systems in 3-D using a tissue-mimicking phantom (TMP). The TMP is divided into eight adjacent compartments containing anechoic spheres (6 vol %) with diameters increasing from 0.1 to 1.09 mm, suspended in an echogenic background consisting of 3.5-μm-diameter glass beads. The ability of an HFU imaging system to detect these spheres against a speckle background provides a realistic estimation of its minimum 3-D resolution. Here, the TMP was scanned using a 40-MHz single-element transducer and a linear array (VisualSonics™ Vevo 770 and 2100, respectively), and a custom HFU annular-array-based system with a 40-MHz nominal center frequency. Depth-of-field and penetration depth of the custom system were increased using synthetic focusing and chirp-coded excitation. Resulting B-mode images indicated that the custom system could resolve spheres down to 0.2 mm, while the commercial systems resolved spheres down to 0.3 mm. In addition, the annular array provided a larger depth-of-field and higher contrast-to-noise ratios at all depths versus the commercial transducers. [Work supported by NIH Grant EB008606.]