The project describes the developement of a multimodality detector dedicated to breast imaging. It combines aring of highly segmented PET detectors able to obtain high spatial resolution (1 mm or better) and sensitivity,with an array of ultrasonic transducers, which allows to acquire tomographic data. Ultrasound reflection time,transmission time and attenuation can be measured at all angles around the breast. Alogwith the scannerdetectors, the will be developed acquisition electronics, data processing software and tomographic imagereconstruction software, as well as fusion and navigating the image codes, and algorithms for the identificationof the tubular structures in the breast tissue. This toolkit has the potential of advantageously replacing X­raymammography as preventive screening for breast cancer. The system will allow the use of the ultrasonicmodality to acquire full breast volumes in few minutes without administration of ionizing radiation and, in thecases where the ultrasound image hints to the presence of any significant abnormality, to acquire highresolution PET images which can be displayed fused and fully registered to the ultrasonic image modalities.