1. Imaging apparatus, comprising: a first device, for obtaining a first image, by a first modality, selected from the group consisting of SPECT, PET, CT, an extracorporeal gamma scan, an extracorporeal beta scan, x-rays, an intracorporeal gamma scan, an intracorporeal beta scan, an intravascular gamma scan, an intravascular beta scan, and a combination thereof, wherein said first image is registered to a system of coordinates; a second device, for obtaining a second, structural image, by a second modality, selected from the group consisting of a three-dimensional ultrasound, an MRI operative by an internal magnetic field, an extracorporeal ultrasound, an extracorporeal MRI operative by an external magnetic field, an intracorporeal ultrasound, an intracorporeal MRI operative by an external magnetic field, an intravascular ultrasound, and a combination thereof; 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 on said second, structural image.