1. In apparatus for obtaining radiological 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, an upper compression surface and a lower compression surface adapted for immobilizing the biological tissue therebetween, and a receptor disposed beneath the lower compression surface, the X-ray source disposed above the upper 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 an upper surface and a lower surface, the lower surface forming the upper compression surface; an ultrasonic transducer disposed above the compression plate; drive means for moving the ultrasonic transducer through a predetermined path across the upper surface of the compression plate while the biological tissue remains immobilized between the upper and lower compression surfaces, so that the ultrasonic transducer generates a plurality of ultrasound images of the biological tissue that are in geometric registration with the image formed in the receptor.