Respiratory average CT for attenuation correction in myocardial perfusion SPECT/CT.

OBJECTIVE: Cine average CT (CACT) and interpolated average CT (IACT) have been proposed to improve attenuation correction (AC) for PET/CT in oncologic and cardiac studies. This study aims to evaluate their effectiveness on myocardial perfusion SPECT/CT using computer simulation and physical phantom experiments.

METHODS: We first simulated normal male with 99m Tc-sestamibi distribution using digital XCAT phantom with respiratory motion amplitudes of 2, 3, and 4 cm. Average activity and attenuation maps represented static SPECT and CACT, while the attenuation maps of end-inspiration and end-expiration represented two helical CTs (HCTs), respectively. Sixty noise-free and noisy projections were simulated over 180° using an analytical parallel-hole projector. We then filled 673 MBq 99m Tc into an anthropomorphic torso phantom with normal heart or heart with a defect which placed on a programmable respiratory platform to model various respiratory amplitudes. Sixty projections were acquired over 180° using a clinical SPECT/CT scanner. The CACT, standard HCT, and 2 HCTs at extreme phases were acquired. Interpolated CT phases were generated between them using affine plus b-spline registration, and IACT was obtained by averaging the interpolated phases and the 2 original extreme phases for both simulation and phantom experiments. Projections were reconstructed with AC using CACT, IACT, and HCTs, respectively. Polar and 17-segment plots were analyzed by relative difference (RD) of the uptake. Two regions-of-interest (ROI) were drawn on the defect and background area to obtain the intensity ratio (IR).

RESULTS: No substantial difference was observed on the polar plots generated from different AC methods, while the quantitative RD measurements showed that SPECTCACT were most similar to the original phantom, followed by SPECTIACT , with RDmax  <8 and <10% in the simulation study. The RD of SPECTHCTs deviated from the original phantom and SPECTCACT in various segments, with RDmax of 19.76 and 16.68% in the simulation and phantom experiment, respectively. The IR of SPECTHCTs fluctuated more from the truth for higher motion amplitude.

CONCLUSIONS: Both CACT-AC and IACT-AC reduced respiratory artifacts and improved quantitation in myocardial perfusion SPECT as compared to HCT-AC. The use of IACT further reduced the radiation dose.