The effects of different corrections for background activity in the estimation of
low organ uptake of radiopharmaceuticals have been examined using myocardial perfusion
imaging agents. Estimates of myocardial uptake of 99mTc-labelled MIBI and tetrofosmin
were made both at rest and after exercise. Patients were given one or other of the
agents (12 MIBI; 17 tetrofosmin) and the measurements at rest and after exercise were
made within a week of each other using a planar gamma camera method incorporating
an attenuation-corrected, geometric mean technique. Myocardial uptakes were estimated
using two different background corrections and also with no background subtraction.
Mean values were in the range 1.3 to 3.0% and showed that, for both agents, uptakes
estimated with and without background correction could differ by a factor of two.
Although the study was not designed to compare myocardial uptakes of the two agents,
a background correction which accounted separately for activity in tissue over- and
under-lying the heart resulted in similar mean values for tetrofosmin (1.7% both at
rest and after exercise) and for MIBI (1.8% rest; 1.9% exercise). For both agents,
no significant difference was observed between myocardial uptakes at rest and after
exercise measured at about two hours post-injection.