Pulse lengths, beam widths, and sizes of the resolution cell at different central frequencies ranging from 2 to 3.5 MHz.

Pulse lengths were measured from two-way pulse-echo testing of the transducer. Beam widths were evaluated from the widths of the lateral distributions (measured at 20% of the maximum) of the B-mode intensity corresponding to the point targets (diameters: 0.1 mm) in a commercial phantom (Model 054GS, CIRS, Norfolk, Virginia, USA). For each central frequency, five measurements were carried out at the focal lengths of 1 cm and 4 cm, respectively (left: data measured at 1-cm focus; right: data measured at 4-cm focus).