Usefulness and pitfalls of planar γ-scintigraphy for measuring aerosol deposition in the lungs:: A Monte Carlo investigation

Planar gamma -scintigraphy is often used to quantify pulmonary deposition patterns from aerosol inhalers. The results are quite different from those obtained using 5-dimensional PET and SPECT. The purpose of this study was to characterize the effects of scatter and tissue attenuation on the distribution of radiolabeled aerosol as measured by planar scintigraphy using Monte Carlo simulations. This study also investigated the applicability of a few correction methods used in inhalation studies. Methods: Body density maps were derived from CT scans. Regions of interest-lungs, major airways, and esophagus-were defined from the same CT volume. Two radioactivity source distribution patterns in the lung, uniform and nonuniform, were used. A Monte Carlo program, SIMIND, was used to generate anterior and posterior gamma -images of the composed inhalation distributions for 2 energy windows, photopeak (127-153 keV) and scatter (92-125 keV). The effects of scatter and attenuation were estimated on the basis of the imaging components separated from the simulation. A scatter correction method and 2 attenuation correction methods, all applied to inhalation scintigraphy, were evaluated using the simulated images. Results: The amount of scatter ranges from 24% to approximately 29% in the lungs and from 29% to approximately 35% in the central (airway or esophagus) region on the planar images. Significant differences were found among regions and between source distributions (P < 0.05), The fraction k used for dual-energy- based scatter correction also varied and was found to be less than the commonly used k = 0.5, The simplified narrow-beam attenuation correction and the effective (broad-beam) correction methods were found to either under- or overcorrect the regional activities. Conclusion: The amount of scatter and tissue attenuation in the thorax region depends on source distribution and body attenuation. In applying planar scintigraphy for aerosol inhalation studies, it is difficult to obtain precise quantitative measurements because of the uncertainties associated with scatter and attenuation corrections. Accurate corrections require knowledge of both source and density distributions.