Simultaneous reconstruction of activity and attenuation in emission tomography

Acquisition of high quality functional images in SPECT and PET relies on an accurate estimation and compensation for photon scatter processes in the patient. Since stand-alone SPECT and novel hybrid PET/MR devices do not explicitly measure the necessary individual photon attenuation distribution, alternative approaches are needed to access this information. In this thesis work, novel methods were developed to recover attenuation maps without using additional transmission measurements. In contrast to other approaches in hybrid PET/MR imaging, the new method combines the information of both modalities in order to extract the individual anatomical shapes and its corresponding attenuation coefficients of the patient. Furthermore, cutting-edge estimation techniques for accidental coincidences have been embedded in iterative 3D reconstruction to improve quantitative accuracy of PET. Aside from that, a clinically feasible method for attenuation correction in myocardial perfusion SPECT imaging was developed, where the combination of measured emission data and patient unspecific a priori data provide sufficient information for accurate 3D image reconstruction in the human thorax. These novel methods provide high quality functional imaging for both modalities, stand-alone SPECT and PET/MR, without applying additional dose to the patient or loss of image quality.