Morphometry of Human Nasal Airways In Vivo Using Magnetic Resonance Imaging

Abstract

The epithelium of the nasal airways is recognized as being at risk of developing cancer from inhalation exposure to toxic aerosols. Because groups that are revising or developing respiratory tract dosimetry models have pointed out the lack of information on the deposition of aerosols in human nasal airways, particularly as a function of the size and shape of these airways, we utilized magnetic resonance imaging to obtain invivo measurements of the nasal airways of a human subject. A series of 40 contiguous 3–mm–thick coronal sections were obtained using a 1.5 Tesla proton imaging unit. These images were then digitized and perimeters and cross–sectional areas of the left and right airways were obtained. The results indicated that there were marked differences in areas between the left and right sides of this subject at any given time, and that the size of the airways were significantly smaller than areas that have been reported previously based on measurements obtained from cadavers. A separate set of images were also obtained after the subject had been administered a nasal decongestant. These airways dimensions were quantitatively more similar to the cadaver data. These preliminary data indicate that nasal airway dimensions are likely to be smaller than those previously reported. These differences may play a role in determining the patterns of aerosol deposition to be expected in nasal airways. It has become increasingly evident that the epithelium of the nasal airways is at risk of developing cancer from inhalation exposure to a variety of environmentally derived gases and particles (Bross etal., 1978; Roush 1979; Gerhardsson etal., 1985; Brinton etal., 1985; Olson and Asnaes 1986; Benjamin etal., 1979; Boecker etal., 1986). To establish appropriate exposure-dose-response relationships, particularly among different species, it is important to understand the factors that affect the deposition, retention and translocation of inhaled toxicants in the nasal airways. Several deposition studies of different sized aerosols in the human nasal airways have been reported, both in terms of total airway deposition (Heyder and Rudolf 1960; Hounam etal., 1971) and also deposition in gross subdivisions of the nose (Fry and Black, 1973). Those data, although useful, do not provide the degree of experimental detail needed to improve our current understanding of the regional deposition and retention of aerosols of widely differing sizes in accurately defined anatomical regions. The studies noted above have also not addressed the potential influence of size and shape of the nasal airways on aerosol deposition. To supplement the invivo data, several experimenters have used physical casts of human nasal airways (Patra etal., 1986; Proetz 1951; Itoh etal., 1985; Proctor and Swift, 1970; Girardin etal., 1983). These casts were obtained from cadavers using rubber or plastic injection techniques. Although these techniques are regarded as accurate in their rendering of the size, shape and configuration of the nasal airways at the time that the casts were made, it is doubtful that the casts provide realistic models of the state of the nasal airways in a normal living individual. There are several reasons for this. First, the mucosa that lines the entire nasal cavity, except the vestibule, is endowed with a rich blood supply that allows for the dilation and constriction of the venous cavernous tissue in the mucosa of the conchae and septum. Both unilateral and bilateral changes in nasal airway patency occur with time such that in normal individuals, air flow is neither constant nor equally distributed between the two nasal airways. The size and shape of each nasal airway is not only likely to be different but temporally variable, and the size of the airway is probably significantly smaller than those of human cadavers because the latter represents a nonphysiological state. Second, shrinkage of tissue as a result of fixation would also tend to increase the size of the nasal airways beyond the range of the normal physiological state. The purpose of this study was to obtain data on the dimensions of human nasal airways invivo using magnetic resonance imaging (MRI), a technique that images the air–mucosa interface directly, and to compare the quantitative measurements of airway cross–sectional areas and perimeters to those that have previously been published and that were derived from cadavers. This comparison provides an indication of the realism of the physical models that have been used previously, and the need for additional studies.