1. In apparatus for obtaining images of biological tissue by passing X-ray radiation through a biological tissue to form an image in a receptor, the apparatus comprising an X-ray source for emitting X-ray radiation, first and second compression surfaces adapted for immobilizing the biological tissue therebetween, and a receptor disposed adjacent the second compression surface, the X-ray source disposed adjacent the first compression surface so that X-ray radiation emitted from the source passes through the biological tissue and is received by the receptor, the improvement comprising: a compression plate that is radiolucent and sonolucent, the compression plate having first and second surfaces, the second surface forming the first compression surface; an ultrasonic transducer disposed adjacent to the first surface of the compression plate; and drive means for moving the ultrasonic transducer across the first surface of the compression plate while the biological tissue remains immobilized between the first and second compression surfaces.