The US detector has a center frequency of 6.25 MHz and a bandwidth of over 80% (Refs. 14 and 15) with a measured minimum detectable pressure of 18 Pa. The array has a radius of curvature of 40 mm and covers 85 deg of a circle. The elevation focus is roughly 1 mm at a distance of 48 mm, and the in-plane resolution for 12 views around the object is . A 32-channel pulser/receiver (Lecoeur-Electronique, Chuelles, France) sampling at is used for data acquisition. Illumination uses an Nd:YAG laser (Quanta-Ray pro 250, Spectra Physics, Mountain View, California) pumping an optical parametric oscillator (VersaScan- L532, GWU, Erftstadt, Germany) at 10 Hz. The radial and angular positions of the fiber bundles were adjusted to achieve roughly equal sized contiguous spots of 11-mm diameter at the skin surface. The pulse energy per fiber bundle was 6.5 mJ, giving an average fluence of at the finger, well below the maximum permissible exposure stated in IEC 60825- 1. US images were made with a commercial US machine (Siemens Acuson S2000TM, Siemens AG, Erlangen, Germany).