Correlation of left ventricular count rate with patient weight in Tc-99m myocardial perfusion imaging

The aim of this retrospective investigation was to devise a protocol for obtaining consistent myocardial counts for patients attending myocardial perfusion imaging.

A total of 229 patients were included in the study sample. For each rest single-photon emission computed tomography data set, a three-dimensional ellipse was manually defined to isolate the left ventricle. The count rate per voxel was measured from a threshold region of interest covering the normally perfused myocardium (70-100% of maximum counts). Data were normalized for net administered activity and plotted against patient weight. The correlation between the variables was used to derive a protocol for adjusting injected activity or scan time to obtain consistent counts from patient to patient.

There was a significant correlation between counts per second per voxel per MBq and patient weight. The best fit was achieved with an inverse model. A correlation equation was derived for patients weighing more than 60 kg (combined R=0.79): [CPS/voxel/MBq=(0.0163/weight(kg)]-3.68×10+A in which A, a sex-dependent constant, was 0 for men and -2.920×10 for women.

There was a significant inverse correlation between patient weight and CPS per voxel per MBq.