Modeling Deposition of Inhaled Particles - Aerosols: Measurement, Dosimetry, and Health Effects - page 94

FIGURE 5.5 Morphological model of the human nasal passages derived from MRI imaging data. Note the
complex cross section of the nasal passage.
directly from in vivo medical images or from the imaging of casts, provide a physiologically real-
istic foundation for studying the inluence of local airway features on the motion and deposition
of particles. Spencer et al. 104 have derived a morphological model of the lung airways using data-
driven surface modeling techniques. In this model, anatomical data were used to deine airway
lengths, diameters, and orientation angles. The surfaces of the resulting airway network were then
realized using advanced graphics rendering techniques. Speciically, nonuniform rational B-splines
(NURBS) were used to model smooth airway connections and realistic lung surface features.
Models derived from imaging or cast data are especially useful in developing representations of
the extrathoracic airways, as these passages are not easily modeled as idealized tubes. Figure 5.5
depicts a geometric model of the morphology of the human nasal passages derived from MRI images
of an adult male. The irregular, tortuous shape of the nasal passages (as seen in the cross sections) will
result in a distinct particle deposition pattern that cannot be predicted using simpliied geometries.
Morphological models have been developed for the nose 105 and the oral cavity. 106 Recently, Rosati
et al. 107 have presented a combined extrathoracic-lung model for use in particle deposition studies.
This model uses a nose and mouth constructed from imaging data from the U.S. National Library
of Medicine Visible Human Project. 108 The extrathoracic model is combined with a ive-lobe lung
airway model constructed using the morphological data of Yeh and Schum. 109 The combined system
is depicted in Figure 5.6, which shows modeled deposition in a typical lobar path within the lung.
5.4.3.5 Surface Features
There is much evidence to suggest that surface features in the lungs should be considered when
modeling particle deposition. Both the cartilaginous rings (which are a pronounced anatomical fea-
ture of the TB airways), and the carinal ridges (which are situated at airway bifurcations) have been
studied. Bronchoscopy images depicting these surface features are shown in Figure 5.7.
Cartilaginous rings affect airlow patterns in the large airways. 12,105 Using CFD modeling,
Musante and Martonen 110 demonstrated that small eddies, produced between the rings, may increase
localized particle deposition. They also predicted that low instabilities produced by the rings could
affect deposition in locations downstream from the rings themselves. In addition, errors in large
airway deposition of up to 35% were possible if the rings were ignored. 111