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

The spatial resolution of high-frequency ultrasound (HFU, >;20 MHz) imaging systems is usually determined using wires perpendicular to the beam. Recently, a tissue-mimicking phantom (TMP) was developed to estimate resolution in three dimensions (3D). The TMP consisted of eight slabs of anechoic spheres with diameters increasing from 0.1 to 1.09 mm. The ability of a HFU system to detect these spheres against a speckle background provides a realistic estimate of its 3D resolution. In the present study, the TMP was used with HFU systems using single-element transducers, linear arrays and annular arrays. The TMP was immersed in water and each slab was scanned using a VisualSonics^™ Vevo 770 and Vevo 2100, and a custom HFU system based on a 5-element annular array with a nominal center frequency of 40 MHz, a focal length of 12 mm, and a total aperture of 6 mm. A synthetic focusing algorithm and a chirp excitation were used to increase the depth-of-field and penetration depth of the custom system, respectively. System performances were compared in terms of sphere detection and contrast-to-noise ratio (CNR). Resulting B-mode images indicated that all three HFU systems were able of detecting the 0.3 mm anechoic spheres, but only the annular-array system showed a sufficient CNR and depth-of-field to clearly resolve the 0.2 mm spheres deep into the phantom.