Introduction: Attenuation has a strong influence on the data and consequently on the quality of the image. Attenuation of photons is the decrease in the number of counts coming from the body. Attenuation correction corrects the weakening of the rays in various depths. The non-diagnostic, low-dosage CT is used for the attenuation correction when the images are taken with the SPECT/CT. Purpose: The goal of this master’s thesis is to find out how the attenuation correction in the case of the SPECT/CT influences the quality of the image. Methods: The Phantom NEMA IEC BodyPhantom was filled with the isotope technetium-99m. Eight images were captured, each with a different ratio of the specific activity between the phantom background and the spheres. The images were reconstructed in the program called Oasis by three different reconstructions: the filtered back projection, the non-corrected iterative reconstruction, and with the attenuation correction using the CT. The number of counts in the background and in all six spheres was measured. This was followed by the comparison of the contrast in images that were reconstructed using different methods. A descriptive statistic and repeated ANOVA measure were conducted. The Wilcoxon signed-rank test was carried out in Matlab. Results: On the images that were processed by the filtered back projection or the iterative reconstruction, the background is not homogeneous. On the images that were corrected using the CT, the number of counts is evenly spread across the entire background of the phantom, thus making the background homogeneous. The statistical analysis showed that p < 0.001, meaning that, statistically, contrast is typically divergent among the different methods of reconstruction. Discussion and conclusion: It was discovered that the increase in the number of counts, and consequently the image contrast, is proportional to the size of the sphere, and to the increased ratio of activity between the background and the sphere with all three types of reconstruction. The CT-AC images have the best contrast; images with iterative reconstruction are second best; images processed by the filtered back projection are third. The images reconstructed with the CT-AC have the least negative contrast, while the negative contrast in the images with iterative reconstruction and FBP images is far more prominent. Because the CT-AC images are of better quality than the non-corrected images, it is recommended that the CT-AC be used for all tests; however due to the removal of artefacts caused by attenuation correction, it is also important to examine the non-corrected images every time.