where: MTF – function modulating data transmission; x – coefficient determining the range. Another filter used in SPECT image reconstruction is the Weiner filter, a two-dimensional filter of the Fourier area [23,25]: W ( f )  MTF 1  MTF 2 /(MTF 2  N / O) where: W(f) – filter value as a function of spatial frequency; MTF – function modulating data transmission; N – noise power spectrum; O – object energy spectrum; Weiner filter (4) configured in such a manner serves the fol- lowing functions: improves resolution as the reverse MTF and reduces noise as a low-pass filter. Another filter applied in single photon emission tomog- raphy is the Butterworth filter, which is an example of a smoothing filter [26]. Similar to the Metz filter, Butterworth is defined in respect to frequency B(n): B( )  1    n 1/2 1     c   © Pol J Radiol, 2011; 76(3): 49-52 Pleśniak J – Single Photon Emission Computed Tomography as a diagnostic… (4) (5) 51 Figure 3. The principle of measurement projection in single photon emission computed tomography method (SPECT). Detector estimates activity of radiopharmaceutical passed along one closely named direct. for reconstruction of topographic images registered by SPECT gamma camera. Iterative methods of image reconstruction are used in emission tomography due to large errors in acquired data. They are aimed at most accurate estimation of algorithm coefficients that are contained in a linear equation g=A×f, with g being a vector of a value in the sinogram, A – fixed matrix, f – unknown vector of a pixel value in the recon- structed image [17,19]. Another advancement in computer technology used, among others, in nuclear medicine is the application of a new OSEM algorithm (Ordered Subset Expectation Maximization), which is a modification of the MLEM algo- rithm [20,21]. Iterative OSEM method, in combination with Fourier filtering, reduces noise better than FBP, and the reconstructed topographic images are improved in regard to quality and clarity [22]. Filters: Metz, Weiner and Butterworth An integral part of the image reconstructed with the back- projection method is the choice and implementation of an appropriate filter for obtaining the most accurate reflection of the acquired image of the diagnosed organ. The first filter used for image reconstruction with SPECT technique was the Metz filter [23,24], defined in terms of frequency M(n) as: M ( )  1 [1 MTF 2 ( )]x MTF ( ) (3) where: n – frequency; nc – cut-off frequency determining the point where func- tion reaches 0. Conlusions In the context of contemporary imaging modalities (CT, MRI), radioisotope diagnostic examinations of heart mus- cle, due to their non-invasiveness, are to a great extent irreplaceable in predicting cardiovascular events. The advantage of SPECT over other diagnostic methods com- prises the possibility of assessing regional blood flow dis- tribution in the myocardium and determining left ven- tricular functional parameters in gated SPECT of the heart (GSPECT) such as: ejection fraction, index of systolic wall thickening. In single photon emission tomography there is a con- stant search for new solutions that would lead to quality improvement of organ imaging and shortening of the study time. An example of such solution was the introduction of a two-headed rotating camera into radioisotope diagnos- tics of myocardial perfusion, which significantly shortened the testing time for any single patient, as well as GSPECT, which beside assessing perfusion deficits in myocardial segments allows for obtaining additional clinical informa- tion regarding volume and ejection fraction of the left ven- tricle, myocardial contractility and systolic wall thickening index.