The effect of scatter and attenuation on aerosol deposition as determined by gamma scintigraphy

Gamma scintigraphy is often used to quantify deposition patterns from aeros ol inhalers. The errors caused by scatter and tissue attenuation in planar Tc-99m gamma scintigraphy were investigated based on the data collected fro m four subjects in this study. Several error correction methods were tested . The results from two scatter correction methods, Jaszczak's method and fa ctor analysis of dynamic sequences (FADS), were similar. Scatter accounted for 20% of raw data in the whole lung, 20% in the oropharynx, and 43% in th e central airways and esophagus. Three attenuation correction methods were investigated and compared. These were: uniform attenuation correction (UAC) , a known method used for inhalation drug imaging work, the broad-beam atte nuation correction used for organ imaging in nuclear medicine; and a narrow -beam inhomogeneous tissue attenuation correction proposed in this study. T he three methods differed significantly (p < 0.05), but all indicated that attenuation is a severe quantification problem. The narrow beam attenuation correction with scatter correction, showed that raw data underestimated tr acer deposition by 44% in the lung, 137% in the oropharynx, and 153% in the trachea/esophageal region. To quantify aerosol lung deposition using plana r scintigraphy even in relative terms, corrections are necessary. Much of t he literature concerning quantified aerosol dose distributions measured by gamma scintigraphy needs to be interpreted carefully.