The aim of this study was to evaluate the assessment of the left ventricular ejection fraction (LVEF) in patients by gated IQ-SPECT. Methods: Twenty-eight patients were examined using gated 99m Tc-sestamibi IQ-SPECT. Two different reconstruction datasets were created using the same projection data. The number of iterations, subsets, and gaussian filtering were based on 2 different recommendations from the manufacturer. For each dataset, end diastolic volume, end systolic volume, and LVEF were calculated using 4DMSPECT. A multigated planar equilibrium radionuclide ventriculography (MUGA) study with 99m Tc-labeled red blood cells was used as a reference for the LVEF. Results: The values of the different datasets were tested using the Bland-Altman analysis method. The calculated mean and 95% limits of agreement for the LVEF when dataset 1 and 2 were compared were −1.1% and ±15% points; when dataset 1 was compared with MUGA, the mean was calculated to −3.1% points and ±17% points for the 95% limits of agreement. When dataset 2 and MUGA were compared, the mean was −4.2% and 95% limits of agreement of ±18% points. Conclusion: Neither of the gated reconstructed datasets analyzed with 4DMSPECT was comparable to LVEF estimated by MUGA, with a tendency to overestimate LVEF. However, large random variations of the end diastolic volume, end systolic volume, and LVEF between the 2 gated reconstructed datasets were found. The reconstructed datasets were not interchangeable. Thus, these values should only be used with great caution when evaluating the functional state of the heart. Key Words: ejection fraction; IQ-SPECT; gated blood pool; myocardial perfusion imaging J Nucl Med Technol 2015; 43:193-200 DOI: 10.2967/jnmt.115.155382 As a diagnostic technique in coronary artery disease, electrocardiography (ECG)-gated SPECT (gSPECT) data acquisition is an established method. Reconstruction and analysis of the data provide information of left ventricular perfusion, wall motion, wall thickness, and quantification of parameters as left ventricular end diastolic and systolic volumes (EDV and ESV, respectively) and ejection fraction (LVEF). The study is normally performed using dual-or triple-head g-camera systems with low-energy all-purpose or low-energy high-resolution (LEHR) collimators installed and with a 90°or L detector configuration (1). To generate adequate imaging statistics, the overall acquisition time can be 20-30 min. Developments in hardware and software continue improving gSPECT image quality (2,3). In December 2008, Siemens Healthcare announced the Food and Drug Administration clearance of their newly developed hardware and software package called IQ-SPECT, with the purpose of shortening the acquisition time considerably. The Siemens IQ-SPECT system consists of SMARTZOOM collimators (magnifying collimators with a complex design), gantry movement control, and special reconstruction software. SMARTZOOM collimators center on the heart, collecting up to 4 times more counts than LEHR collimators. These collimators magn...