The basis of quantitative pulmonary computer tomography

The present paper deals with the basis for regional and global ventilation studies of the lungs using computer tomography. For this purpose the usual CT values have been replaced by so-called scaled computer tomographic quotients (sCTQ values). Using a suitable scaling factor, these correspond accurately with absolute density in mg/cm3 or g/cm3 as far as the lungs are concerned. From the sCTQ values of pulmonary regions and their changes caused by respiration, it is possible to calculate the friction of air-free tissues and of respiratory air contained within them, and to obtain the changes in air volume in the pulmonary region under investigation. Moreover, absolute or relative distribution of pulmonary ventilation can be determined quantitatively by means of an absolute and relative distribution function (PHI-or-PSI-function). Examples of local changes in respiratory parameters were obtained using four thoracic computer tomograms (regional transverse sections). These were obtained from a normal subject after maximal inspiration, after normal deep expiration and after maximal expiration with the subject supine, and after normal deep expiration with the subject in the prone position. These parameters, like the CT and sCTQ values, can be used for a pictorial demonstration of the lung. In addition, mean values and percentage deviations of the pulmonary sCTQ values are given for each of the four conditions. For the calculations it was necessary to isolate the pulmonary tissues from the rest of the thoracic structures. For this purpose 3 masks have been developed. For pulmonary ventilation analysis a relatively coarse picture matrix proved appropriate. Coarsing of the matrix should be effected only after the pulmonary tissue has been defined. (orig.)