Modeling Deposition of Inhaled Particles - Aerosols: Measurement, Dosimetry, and Health Effects - page 107

5.5.2.2 Deposition Patterns Deduced from Clearance Studies
Experimental methods have been developed in which the regional deposition of particles is deduced
from measurement of the time-course of clearance of particles from the thorax. 133 Speciically,
radiolabeled particles are inhaled and a whole body counter is used to measure the amount of
radioactive activity in the stomach, chest, and extrathoracic regions. Since the removal of particles
(by mucociliary clearance) in the TB region occurs at a faster rate than removal (by macrophagic
clearance) in the pulmonary region, deposition in the two regions can be deduced from the two-part
slope of a normalized retention curve for the thorax.
5.5.2.3 Light-Scattering Methods
Traditionally, total deposition of particles in the respiratory tract has been quantiied using light-
scattering photometry to compare the concentration of particles in the inhaled and exhaled
air. 133,250,262,263 When a monodisperse aerosol is used, and ventilation is simultaneously measured,
the deposition fraction in the respiratory system can be calculated. However, photometry cannot
distinguish between differences in inspiratory and expiratory aerosol concentration and changes in
aerosol size distribution; therefore, these methods are inappropriate for polydisperse or hygroscopic
aerosols. 250 Rosati et al. 264 have developed a light-scattering, particle-sizing system that may be the
best option for determining total deposition of polydisperse aerosols in the respiratory tract. This
system also has the potential to be applied to hygroscopic aerosols as it can determine particle sizes
of inhaled and exhaled aerosols, and works well for varying-sized polydisperse aerosols. 264,265
5.5.2.4 Imaging Studies
Radionuclide imaging has been widely used to measure both the concentration and spatial distribu-
tion of inhaled aerosols. In these studies, particles are tagged with a radioisotope (such as 99m Tc) and
then inhaled. 2D (planar) or 3D imaging modalities can then be used to measure the radioisotope
emissions from speciic locations within the body.
Planar gamma cameras can be used to obtain projections of the spatial distribution of inhaled
radiolabeled aerosols. These images may be useful in predicting total deposition within the lung or
extrathoracic passages, but the 2D nature of the images may obscure important deposition patterns. 266
Planar imaging studies have been performed for a variety of inhaled aerosols. 132,267,268 To assist in
the interpretation of planar gamma camera data, Martonen et al. 267,268 have developed methods to
associate regions of images (Figure 5.10A) with computer models of the human lung (Figure 5.10B).
The computer model serves as a template to be superimposed on actual images, thus permitting the
generational airway composition within the central (C), intermediate (I), and peripheral (P) zones
of planar images to be predicted (Figure 5.10C). Tossici-Bolt et al. 269,270 have recently described
methods of constructing 3D representations of aerosol deposition from planar scintigraphic images.
Recently, 3D tomographic imaging modalities have been applied to the study of particle deposition
patterns. Both 3D single photon emission computed tomography (SPECT) 271-273 and positron emission
tomography (PET) 274-276 have been employed in particle deposition studies. 3D methods provide a
powerful means of associating particle deposition with distinct local regions within the respiratory
system. In a series of papers, Martonen et al. 101,272,277-280 have recently presented computational
methods for correlating the individual airways of a lung morphology model with the voxels of 3D
SPECT images. These methods provide a means of validating 3D CFPD and deposition models
using SPECT data while simultaneously providing a framework for predictive laboratory studies of
targeted aerosol delivery. In practice, the computer model may be superimposed on the voxels of a
SPECT image to allow a quantiication of particle deposition. Figure 5.11 shows an example SPECT
image and a detailed view of the airway composition of an associated voxel.
5.5.2.5 Microdosimetry
In the experimental methods described earlier, particle deposition may be measured by region (e.g.,
by clearance studies) or airway-by-airway (e.g., in casts). However, in the study of the health risks