1. An apparatus for ultrasonically scanning a chestwardly compressed breast of a patient, comprising: a support arm; and a compression/scanning assembly supported by the support arm and configured to move under operator control between a non-scanning position in which the assembly is out of contact with the patient's breast to a scanning position in which the assembly is in contact with the breast and compresses the breast chestwardly, wherein said compression/scanning assembly comprises: (i) a housing member supported by the support arm, said housing member having an opening; (ii) a compressive membrane secured to said assembly across at least a part of said opening of said housing member and positioned to contact the breast and compress the breast in a chestward direction as the compression/scanning assembly is moving into said scanning position, and to maintain the breast compressed when in said scanning position, wherein said compressive membrane is porous with respect to an acoustic coupling liquid; (iii) an ultrasound transducer disposed within the housing member; and (iv) a transducer translation mechanism disposed within the housing member and coupled to the ultrasound transducer, the transducer translation mechanism being configured to mechanically control the ultrasound transducer when said compression/scanning assembly is in said scanning position to maintain contact with said compressive membrane while scanning the breast through said compressive membrane.