Review Article Table 1. A comparision of Tc99m and 201Tl isotopes. Half-life T1/2 [h] γ photon energy [keV] Given activity of a substance [mCi] Method of acquisition of the isotope Technet-99m 6.02 140 (98%) 10–30 Generator Tal-201 73 167 (8%), 135 (2%) 2 Cyclotron Mother radionuclide Molybdenum-99 Thallium-203 [10,11]. As g photons fall onto the crystal, they react with it and are transformed into light photons. After an appropri- ate amplification (using secondary emission) and shaping, light photons are converted into a digital image. Collected images are processed by a computer unit (Figure 1). Detector Rotation The first investigations on the range of rotation of SPECT detector have been attempted in the 80s. There was a ques- tion of which rotation range (180° or 360°) is more beneficial to studying myocardial perfusion [12–14]. Due to a substan- tial improvement in image contrast and shortening of study time, image acquisition with a 180° rotation of the detector became a standard in this kind of myocardial diagnostics [15]. In case of a rotation of 180°, the study begins with a detec- tor placed in LPO-45° (left posterior oblique) and ends in RAO-45° (right anterior oblique) position. This allows for obtaining a clearer spatial distribution of left ventricular regions than in the case of the rotation of 360°. During the investigation, 32 acquisitions are obtained with the 64×64 matrix [16], but study protocols with 128×128 matrix can also be used (Figure 2). Topographic Image Reconstruction Back-projection method In single photon emission computer-aided tomography, reconstruction of the image with the back-projection meth- od focuses on filtering of intended projections for a given acquisition angle. Images obtained with SPECT method represent the distribution of activity of an administered radiopharmaceutical in the diagnosed object. In these kinds of reconstructions, mathematical equations are applied, such as the Radon transform (1) and Fourier transform (2) [17,18], which allow to obtain a two- or three-dimensional activity distribution on the basis of one- dimensional measurements:  g(s, )   f (s cos  u sin , s sin  u cos )du (1)  where: (s,q) – detector coordinates (s – position, position angle); 50 Computer © Pol J Radiol, 2011; 76(3): 49-52 Positioning system System of photomultipliers NaI Crystal Collimator Figure 1. Construction scheme of scintillation gamma camera. Almost only gamma rays parallel to the openings in the collimator pass through the collimator. A RAO-45° LPO-45° Figure 2. Positioning of SPECT detection system. A - patient positioned perpendicularly to this article, with his head from the outside. u – variable describing the location of the summed points, F (u)  e(uu0 )2 / 2 2 ; u0 (2) During image reconstruction, in order to eliminate noise in acquired projections, filters are used to suppress i.a. high frequencies in places where signal is not detected and where noise dominates in measurements. An addi- tional function of the applied filters is to smooth the acquired image in order to improve its quality and clarity (Figure 3). Common use of filters results from the fact that they can be easily implemented, which is associated with intuitive expectations of a user of software applied in scintigraphic image processing. Iterative methods Iterative methods, next to the back-projection method, find application in SPECT image reconstruction. In single pho- ton emission tomography, beside a filtered back-projection method, various algorithms such as MLEM (Maximum Likelihood Expectation Maximization) and MAPEM (Maximum A Posterior Expectation Maximization) are used