Image Processing and Interpretation

Myocardial perfusion imaging (Mpi) is the most commonly performed noninvasive test for the diagnosis and risk stratification for patients with an intermediate or high probability of coronary artery disease (cAD). 1 however, image acquisition, processing, and interpretation protocols can vary significantly between different nuclear labs. The following is a general overview of processing and interpretation of single-photon emission computed tomography (specT) images for the assessment of perfusion. proTocol VAriAbiliTy inter-lab differences exist not only in image acquisition procedures but also in processing and interpretation. 2 For example, some labs use attenuation correction, while others do not. even among those that do use attenuation correction, differences exist in the data used for correction (moving line sources versus cT correction). 2,3 some labs rely heavily on quantitative analyses, 4 while others rely solely on visual interpretation. 2 Approaches to quantitation also vary depending on the algorithm used, 5 and visual interpretation requires an experienced reader and is associated with interobserver variability. 6 processiNG pArAMeTers Key specT image processing parameters include the following 7 : Filtering—Filtering helps improve image clarity by removing noise and blur before and after backprojection of raw specT data. 7 The butterworth filter is the standard filter for specT, but the optimal order and cutoff varies by vendor. 7 Reconstruction—reconstruction reorients specT data to the 3 axes of the patient's heart, creating horizontal long axis, vertical long axis, and short axis slices 7 (Figure 1). image reconstruction has traditionally been performed using filtered backprojection. Motion Correction—inspection of cine display of planar projection data (ie, rotating images) is the recommended method of detecting motion because it demonstrates both vertical and horizontal motion. 7,8 A static sinogram is also sometimes used but does not show horizontal motion. 8 Figure 1. specT display showing conventional slice alignment of perfusion images (rows) and rotating, or cine, images (bottom right). Attenuation Correction—The cine display of planar projection images is also recommended for identifying the sources of attenuation. 8 if automated attenuation correction is used, the data should be processed first without attenuation correction and then reprocessed with attenuation correction. 7 Quantitation—The degree of ischemia versus infarction can be assessed by quantitation of the stress images compared with the rest images. 8 Quantitative analysis is usually displayed as a bull's-eye or polar plot. 8 A number of software packages are available for quantitation of myocardial slices. 9 image courtesy of Mark c. hyun, cNMT.