Tuija Kangasmaa Evaluation and Optimization of Novel Reconstruction Methods for Myocardial Perfusion SPECT

Myocardial perfusion imaging (MPI) is one of the most common types of SPECT studies, being highly valued for its diagnostic accuracy. However, the quality of MPI is seriously impaired by image noise, photon attenuation, Compton scatter, collimatordetector response (CDR) and patient motion. These image degrading factors have been investigated for more than three decades now and the modern reconstruction-based compensation methods can greatly improve the image quality. Recently CDR compensation has attracted considerable interest, because it allows diagnostically satisfying images to be acquired in half of the acquisition time currently in use. Shorter scan times both reduce artifacts related to patient motion and increase the patient throughput. The utilization of modern compensation methods is not always straightforward. Their application can be time consuming and they can generate new image artifacts. The aim of this thesis was to validate and optimize novel SPECT reconstruction and compensation techniques for use with MPI. The specific focus was on scan time reduction and on CDR compensation. In the first part of the thesis, half acquisition time imaging was studied in combination with Monte Carlo (MC)based scatter compensation, which is a statistical technique dependent on the count level of the acquisition. The MC-based scatter compensation was not observed to hinder half acquisition time imaging. The image quality obtained with half acquisition time using CDR and MC-based scatter compensation was better than with full acquisition time and conventional reconstruction, but it did not achieve the image quality obtained with full acquisition time combined with CDR and MC-based scatter correction. In the second part of the thesis, the MC-simulator used in the first part was developed further for simultaneous 201Tl/99mTc cardiac imaging. Simultaneous 201Tl/99mTc could also reduce the total acquisition time by half, because it allows the stress and rest scans to be performed in one single session. One problem encountered in simultaneous 201Tl/99mTc is the down-scatter from 99mTc to the 201Tl energy window. The MC-simulator was optimized for 201Tl/99mTc imaging. This was found to greatly improve the image quality and produce an image in less than 3 minutes. Even with reduced scan times, some patients cannot stay still during the acquisition, which leads to artifacts that can be falsely interpreted as perfusion deficits. The third part of this thesis focused on reconstruction-based motion correction in MPI-imaging. Different motion correction methods were examined and optimized in terms of their performance and speed so that efficient motion correction could be achieved in a few minutes. In the fourth and final part of the thesis, novel methods to reduce CDR related artefacts were developed. The CDR artefacts could be greatly reduced if CDR was used with Bayesian reconstruction methods. It was found that the best performance could be obtained with Bayesian reconstruction with an anatomical prior. National Library of Medicine Classification: WN 440, WN 206, WG 280,