TISSUE ATTENUATION CORRECTIONS IN GAMMA-SCINTIGRAPHY

Gamma scintigraphy may be used to quantify deposition patterns from ae rosol inhalers provided that appropriate corrections are made for the attenuation of gamma rays by different body tissues. Tissue attenuatio n correction factors (ACFs) were determined in a group of 10 healthy v olunteers, using four different techniques: measurement of body thickn ess, transmission scans, a sealed radioactive source, and a perfusion scan. Different combinations of these ACFs were then used to calculate deposition values from data obtained in the same volunteers in a prev ious study in which the deposition pattern of a drug inhaled from two different asthma inhalers was investigated. In addition, deposition pa tterns were determined from data that had not been corrected for tissu e attenuation. Lung and stomach ACFs were relatively independent of th e correction method (means, 1.84-2.16 for the lungs and 3.42-3.81 for the stomach), although ACFs for the oropharynx were more variable (mea ns, 1.41-2.29). There were significant differences (P < 0.05) between the deposition patterns calculated using five different combinations o f ACFs, although the actual magnitudes of the differences were small. Lung deposition was significantly underestimated (P < 0.05) by not cor recting for tissue attenuation. It is therefore suggested that correct ions for tissue attenuation must be made when quantifying aerosol depo sition and that a simple approach based on the measurement of body thi ckness offers a convenient means of doing this without exposing subjec ts to an additional radiation dose.