The use of combined single photon emission computed tomography and X-ray computed tomography to assess the fate of inhaled aerosol

Abstract

Background: Gamma camera imaging is widely used to assess pulmonary aerosol deposition. Conventional planar imaging provides limited information on its regional distribution. In this study, single photon emission computed tomography (SPECT) was used to describe deposition in three dimensions (3D) and combined with X-ray computed tomography (CT) to relate this to lung anatomy. Its performance was compared to planar imaging.

Methods: Ten SPECT/CT studies were performed on five healthy subjects following carefully controlled inhalation of radioaerosol from a nebulizer, using a variety of inhalation regimes. The 3D spatial distribution was assessed using a central-to-peripheral ratio (C/P) normalized to lung volume and for the right lung was compared to planar C/P analysis. The deposition by airway generation was calculated for each lung and the conducting airways deposition fraction compared to 24-h clearance.

Results: The 3D normalized C/P ratio correlated more closely with 24-h clearance than the 2D ratio for the right lung [coefficient of variation (COV), 9% compared to 15% p < 0.05]. Analysis of regional distribution was possible for both lungs in 3D but not in 2D due to overlap of the stomach on the left lung. The mean conducting airways deposition fraction from SPECT for both lungs was not significantly different from 24-h clearance (COV 18%). Both spatial and generational measures of central deposition were significantly higher for the left than for the right lung.

Conclusions: Combined SPECT/CT enabled improved analysis of aerosol deposition from gamma camera imaging compared to planar imaging. 3D radionuclide imaging combined with anatomical information from CT and computer analysis is a useful approach for applications requiring regional information on deposition.

FIG. 1.

Example images demonstrating the inclusion of activity in the bronchi and part of the trachea in the 2D regions representing the lung. The left image shows an anterior planar gamma camera view of the lungs with the lung regions overlaid. The right image depicts a transverse section of the SPECT dataset with lung regions overlaid. Activity in the trachea is clearly separate from the lung regions.

FIG. 2.

The total deposition in each lung determined from planar and SPECT imaging.

FIG. 3.

The total activity leaving the nebulizer assessed from (1) the difference in activity put into the nebulizer and that left after inhalation and (2) the sum of the activity accounted for in the body and exhalation filter.

FIG. 4.

The variation of 3D normalized central-to-peripheral ratio with the corresponding 2D ratio. The line is the line of identity.

FIG. 5.

The variation of 24-h clearance for the right lung with (a) 2D normalized central to peripheral ratio (▴) and (b) 3D normalized central-to-peripheral ratio (□). The lines are the lines of least squares linear regression.

FIG. 6.

(a) The variation of 24-h clearance with the estimated fractional deposition in generations 2–15. The line is the line of identity. (b) The variation of 24-h clearance with conducting airways deposition fraction assuming the definition of the conducting airways in the Weibel Model. The line is the line of identity.

FIG. 7.

Comparison of total deposition in the right and left lungs. The line is the line of identity.

FIG. 8.

Comparison of the 3D normalized central-to-peripheral ratio in the right and left lungs. The line is the line of identity.

FIG. 9.

Comparison of the bronchial airways deposition fraction in the left and right lungs.