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Artifacts can arise in reconstructed SPECT images using CT-based attenuation correction (CTAC) due to patient respiratory motion. We investigate the extent of these artifacts using different CT scanners ranging from single-slice to state-of-the-art multi-slice units. The 4D NCAT phantom was used to realistically model different patient respiratory patterns (breathhold, shallow, normal, and deep breathing). In-111 ProstaScint® and Tc-99m Sestamibi SPECT emission projection data including the effects of attenuation, collimator-detector response and scatter were simulated from the phantoms. CT images were generated using different CT scanners with varying rotation speeds (0.5 to 14 sec/rotation). The CT data were converted into attenuation maps and used to reconstruct the emission projections with attenuation correction (AC). In each case, the CT-based AC SPECT images (with and without artifacts) were compared to assess the effect of the respiratory motion. CT respiratory artifacts were found to increase with slower rotation speeds and to affect the SPECT reconstructions using CTAC. Though less susceptible to respiratory motion, the fastest CT scanner was still found to result in artifacts in the SPECT images due to the mismatch between the CT (∼breathhold) and SPECT (average motion) data. In both cases (CT motion and CT-SPECT mismatch), the artifacts were reduced using a shallow breathing pattern. We conclude that respiratory motion is an important consideration in SPECT-CT imaging when using CT-based AC. Careful work must be done to design protocols to reduce CT artifacts while minimizing the mismatch between the CT and SPECT data.