IMPLICATIONS OF DUAL-ENERGY-WINDOW (DEW) SCATTER CORRECTION INACCURACIES FOR IN-111 QUANTITATIVE GEOMETRIC MEAN IMAGING

There is increasing clinical interest in the use of quantitative imagi ng for radiopharmaceuticals labelled with In-111. Dual-energy-window ( DEW) scatter correction is a frequently used component of planar geome tric mean quantitative imaging, but it is known that the scatter multi plier k suffers from significant dependence on the characteristics of the scatter medium. Phantom studies with a variety of source geometrie s were carried out to determine the clinical impact of this dependence on the quantitative accuracy of tumour imaging carried out in conjunc tion with attenuation correction. Spheres of various sizes (5-20 ml vo lumes) containing approximately 3.7 MBq (100 mu Ci) In-111 were imaged at a variety of depths (4.8-10.5 cm) within an elliptical water-fille d phantom, as well as in air. Geometric mean emission images were acqu ired using a 20% photopeak window at 247 keV and a 10% scatter window at 205 keV. These emission images were corrected for attenuation using measured Tc-99(m) transmission data that were scaled to In-111 photon energies. Scatter correction was performed in two ways: (1) using the standard DEW method and (2) using a modified DEW method that takes in to account benign scatter in the detector crystal. Errors in the activ ity estimates ranged from -4% to +3% for method 1 in water, and -5% to +3% for method 2 in water. In air, method 1 ranged from -13% to -5%, and method 2 ranged from -10% to -1%. Method 1 was found to yield an a ccuracy equivalent to that of method 2, except in conditions of very l ow patient scatter, when the modified method behaved significantly bet ter. We conclude that in a variety of realistic geometries, variations in scatter fraction as determined by the DEW scatter correction metho d combined with appropriate attenuation correction need not inhibit ac curate absolute quantitation of spherical 'tumours' labelled with In-1 11 when using planar imaging.