A method for improving the efficiency of myocardial perfusion imaging using conventional SPECT and SPECT/CT imaging systems

Siemens has developed a new IQ-SPECT™ product to improve the efficiency of myocardial perfusion imaging (MPI) using conventional large-field-of-view SPECT and SPECT/CT systems. In this article we present the key technology components that enable this product to perform MPI in less than 5 minutes or at an equivalently lower dose. The enabling hardware is a specially designed variable-focus collimator for cardiac imaging. Images are acquired with the collimator mounted on a Symbia SPECT/CT system rotating about the patient in a cardio-centric orbit at a fixed radius of 28 cm. The acquired data are reconstructed using an iterative reconstruction technique employing the conjugate-gradient method with the Mighell chi-square objective function accounting for Poisson statistics. Each collimator is characterized by measuring the orientations of its holes to account for the deviations from design specifications that are introduced in the casting process. In addition, the 3D point-response function (PRF) is modeled from the autocorrelation of the hexagonal shapes of the collimator holes at the entrance and exit sides. This PRF is no longer just an approximate Gaussian but is more conical at the distances of interest. The system matrix accounts for the deflection of the heads as they rotate about the patient. The deflections were measured for a number of systems using an Optotrak™ optical fixture to obtain accurate 3D orbit information for each head. The reconstruction engine applies the flood-field uniformity corrections (instead of being applied to the raw data) and also estimates patient motion vectors from the distortion-corrected projection images. Attenuation compensation is applied using a patient-specific CT-derived mu map, and an energy-window-based estimate is used to correct for patient-induced scatter. Phantom and patient studies are presented to demonstrate the diagnostic quality of the images acquired using fast or low-dose protocols.