Nowadays Nuclear Medicine has become more and more important in diagnosing diseases (e.g. tumour). SPECT (Single Photon Emission Computed Tomography) is a widely used nuclear imaging technique in Nuclear Medicine. During the SPECT imaging process a radiolabeled substance is injected into the body that plays a role in specific biological activity (e.g. the radiopharmaceutical will be located in the myocardium). Through the radioactivity distribution in the body it is possible to gain functional information about the examined organ.
In SPECT imaging gamma photon emitted radioactive tracer is used. The emitted gamma photons are detected with a gamma camera that is placed in different angular positions around the body. That way the two-dimensional projections of the three-dimensional activity distribution are collected. At the end, the three-dimensional activity distribution can be reconstructed from projections.
A gamma camera consists of a scintillation crystal, a collimator and of PMTs (Photomultiplier Tubes). The collimator is a thick sheet of lead with holes in it that is placed in front of the crystal and has the role to detect only gamma photons reaching the detector perpendicularly. The geometrical properties of the collimator (hole and septa size) have basically influence on sensitivity and spatial resolution.
From the collected projection images the three-dimensional activity distribution can be reconstructed using an appropriate reconstruction algorithm. Maybe the most widely used reconstruction algorithm is the iterative MLEM (Maximum Likelihood Expectation Maximization) algorithm. The main advantage of the iterative MLEM algorithm is that the distance dependent detector blurring can be involved in the forward projection step resulting in a better image quality.
The EM algorithms are able to reconstruct isotopes with single energy range. So far simulations with the GATE simulation toolkit have been achieved using SPECT isotopes that have multiple energy value. The reconstruction software is based on the EM algorithm. In addition, it reconstructs a 67Ga radioisotope that has multiple energy value. The ML-EM algorithm was completed in a way that it is able to reconstruct isotopes that have multiple energy range. The two-dimensional reconstruction pictures, which were created by the new ML-EM algorithm, were compared with the pictures that were created by the original algorithm.