ABSTRACT
Poor sensitivity and poor signal to noise ratio because of low injected thallium dose and presence of scattered photons are the main problems in using thallium in scintigraphic imaging of the heart. Scattered photons are the main cause of degrading the contrast and resolution in SPECT imaging that result in error in quantification. Thallium decay is very complicated and photons are emitted in a wide range of energies of 68-82 keV. It seems possible to achieve better primary to scattered radiation ratio and better image sensitivity simultaneously if the energy window setting is carefully selected. This investigation was performed in three steps: Monte Carlo simulation, phantom experiment and clinical study. In simulation step, the new 4D digital NCAT phantom was used to simulate the distribution of activity [201Tl] in patient torso organs. The same phantom was used to simulate the attenuation coefficient of different organs of the typical patient's body. Two small defects on different parts of left ventricle also were generated for further quantitative and qualitative analysis. The simulations were performed using the SimSET simulator to generate images of such patient. The emissions arising from Tl-201 decay were simulated in four steps using the energies and relative abundances. Energy spectra for primary and scatter photons were calculated. Changing the center and width of energy windows, optimum energy window characteristics were determined. In next step jaszczak phantom was prepared and used for SPECT imaging in different energy windows. In last step SPECT images of 7 patients who had angiographic data were acquired in different energy windows. All of these images were compared qualitatively by four nuclear medicine physicians independently. The optimum energy window was determined as a wider asymmetric window [77keV?30%] that its center is not placed on photo-peak of energy spectrum. This window increased the primary counts rate and PTSR considerably as compared with the conventional symmetric energy window [67keV%]. In a comparison which performed between clinical images acquired in suggested 77-30% window with conventional 67-20% window, a considerable increase was found in myocardial to defect contrast [1.541 +/- 0.368] and myocardial to cavity contrast [1.171 +/- 0.099]. A negligible increase was also found in total counts of images using this window. We found that conventional symmetric energy window [67keV +/- 10%] couldn't be a suitable choice for thallium heart imaging; furthermore three energy windows, 73keV-30%, 75keV-30% and 77keV-30%, were determined as optimum window options. For further analysis the images from such windows were compared in each three steps of this investigation. In all steps conventional symmetric energy window [67keV-20%] was introduced as the worst case and the asymmetric 77keV-30% was determined as the most suitable
ABSTRACT
There are several technical features that make PET an ideal device for the noninvasive evaluation of cardiac physiology. Organ motion due to respiration is a major challenge in diagnostic imaging, especially in cardiac PET imaging. These motions reduce image quality by spreading the radiotracer activity over an increased volume, distorting apparent lesion size and shape and reducing both signal and signal-to-noise ratio levels 4D average male torso [2 cm diaphragmatic motion] produced by NCAT phantom was used for simulations. Emission sinograms generated by Eidolon PET simulator were reconstructed using iterative algorithm using STIR. The respiratory motion correction [RMC] applied to data sets using an automatic algorithm. Cross section views, activity profiles, contrast-to-noise ratios and left ventricle myocardium widths of corrected and non-corrected images were compared to investigate the effect of applied correction. Comparison of respiratory motion corrected and non corrected images showed that the algorithm properly restores the left ventricle myocardium width, activity profile and improves contrast-to-noise ratios in all cases. Comparing the contrast recovery coefficient []shows that the applied correction effected phases of number 7,8 and 9 of cardiac cycle more than the other 13 phases and the maximum value being 1.43 +/- 0.07 for phase number 8. The maximum value of ratio of the left ventricle myocardium width for non-RMC and RMC images along the line profile passing the apicobasal direction and along the line profile passing from the middle of the lateral wall of the left ventricle were 1.38 +/- 0.07 for phase number 9 and 1.12 +/- 0.03 for phases of number 8 and 9 respectively. Blurring and ghosting of each image depends on the speed of diaphragm during that respiratory phase. This simulation study demonstrates that respiratory motion correction has good overall effect on PET cardiac images and can reduces errors originating from diaphragmatic motion and deformation. Effect of such a correction varies from one cardiac phase to another and this depends on the blurring and ghosting of all respiratory phases used to form this cardiac phase. Using an automatic algorithm capable of correcting respiratory motion using full signal may be very useful to prevent lengthening of the overall scan time to obtain same motionless lesion signal levels