1. Imaging apparatus, comprising: a first device which obtains a first image, by a first modality, said image being an ionizing radiation image, wherein said first image is registered to a system of coordinates; a second device which obtains a second, structural image, by a second modality, said structural image being an ultrasonic image; and a computerized system, which comprises: a registrator for co-registering said second, structural image to said system of coordinates, and an attenuation-instruction generator configured to compute a set of attenuation instructions for said first image, based at least on non-uniformities in said second, structural image.