Usefulness and pitfalls of planar gamma-scintigraphy for measuring aerosoldeposition in the lungs: A Monte Carlo investigation

Planar gamma -scintigraphy is often used to quantify pulmonary deposition p atterns from aerosol inhalers. The results are quite different from those o btained using 5-dimensional PET and SPECT. The purpose of this study was to characterize the effects of scatter and tissue attenuation on the distribu tion 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 e sophagus-were defined from the same CT volume. Two radioactivity source dis tribution patterns in the lung, uniform and nonuniform, were used. A Monte Carlo program, SIMIND, was used to generate anterior and posterior gamma -i mages of the composed inhalation distributions for 2 energy windows, photop eak (127-153 keV) and scatter (92-125 keV). The effects of scatter and atte nuation were estimated on the basis of the imaging components separated fro m the simulation. A scatter correction method and 2 attenuation correction methods, all applied to inhalation scintigraphy, were evaluated using the s imulated images. Results: The amount of scatter ranges from 24% to approxim ately 29% in the lungs and from 29% to approximately 35% in the central (ai rway or esophagus) region on the planar images. Significant differences wer e found among regions and between source distributions (P < 0.05), The frac tion k used for dual-energy- based scatter correction also varied and was f ound to be less than the commonly used k = 0.5, The simplified narrow-beam attenuation correction and the effective (broad-beam) correction methods we re found to either under- or overcorrect the regional activities. Conclusio n: The amount of scatter and tissue attenuation in the thorax region depend s on source distribution and body attenuation. In applying planar scintigra phy for aerosol inhalation studies, it is difficult to obtain precise quant itative measurements because of the uncertainties associated with scatter a nd attenuation corrections. Accurate corrections require knowledge of both source and density distributions.