The Effect of Scatter and Attenuation on Aerosol Deposition as Determined by Gamma Scintigraphy

Abstract

Gamma scintigraphy is often used to quantify deposition patterns from aerosol inhalers. The errors caused by scatter and tissue attenuation in planar Tc-99m gamma scintigraphy were investigated based on the data collected from four subjects in this study. Several error correction methods were tested. The results from two scatter correction methods, Jaszczak's method and factor 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 the 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 attenuation correction used for organ imaging in nuclear medicine; and a narrow-beam inhomogeneous tissue attenuation correction proposed in this study. The 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 tracer deposition by 44% in the lung, 137% in the oropharynx, and 153% in the trachea/esophageal region. To quantify aerosol lung deposition using planar scintigraphy even in relative terms, corrections are necessary. Much of the literature concerning quantified aerosol dose distributions measured by gamma scintigraphy needs to be interpreted carefully.

Regional Distribution and Kinetics of Inhaled Pharmaceuticals

“…Borgstrom [72] found that planar imaging overestimated by 27% the amount of lung deposition of drug from a dry powder inhaler. This is precisely the predicted effect due to over-emphasis of peripheral deposition at the expense of central airway deposition [64] [61,62,69]. The same effect was most likely the cause of observed variability in deposition by Newman [4] , who observed anomalously high lung penetration of large particles in a study of the effect of particle size on distribution.…”

“…For example, the absorption rates of Tc-99m pertechnetate used in a common fashion as a tracer for triamcinolone acetonide drug particles in the human lung were measured, and then the absorption of [ 11 C]triamcinolone acetonide was determined using the same drug preparation in the same human volunteers. The absorption rate constant of Tc was approximately 0.07 min -1 , while that of triamcinolone acetonide was 0.03 min -1 [60][61][62].…”

“…Also, attenuation and scatter of radiation by the body have important effects on quantification which are difficult to correct because of the variability in body size and mass distribution. Effective scatter and attenuation corrections are also important in qualitative imaging because of the large effect they have on perceived tracer distribution within the body, but they are rarely done using sufficiently rigorous methods [61,62,[65][66][67][68][69].…”

“…Improvement of relative planar quantification is possible if correction methods for scatter and attenuation are applied. Even so, our direct comparison between standard planar imaging techniques and PET to measure a drug that distributed in decreasing fashion from the mouth to the peripheral lung showed that planar quantification of such a distribution is, at best, relative [61,62,69]. Quantitatively, many studies have shown strong variability in planar imaging of doses from inhalers.…”

The Use of Combined Single Photon Emission Computed Tomography and X-ray Computed Tomography to Assess the Fate of Inhaled Aerosol

“…The ability of planar imaging to measure total deposition in the lung has come under particular scrutiny in the literature. (16,17) It was therefore felt important to include a clear validation of the planar analysis of this study. This was done in two ways: (1) using the dose accountability experiment in which the total activity leaving the nebulizer measured using an isotope calibrator was compared to that obtained from gamma camera imaging, and (2) by comparing total lung activity measurements from planar imaging with those from SPECT.…”