The extent and severity are two important parameters when describing a regional defect in myocardial single-photon emission computed tomography (SPECT) perfusion imaging. Intuitively, these two parameters should be independent of each other, but we have shown in a previous study that there is an interdependence. This interdependence has been investigated in two commercially available analysis programs (AutoQUANT and 4D-MSPECT) using Monte Carlo-simulated SPECT data.