Computerized diagnosis of emphysema

A picture is worth a thousand words. This familiar saying has formed the basis of radiology for more than a century, with the radiologist being the sorcerer given the exclusive power to interpret images. A definite trend in modern medicine is quantification. That which can be expressed in numerical form gives the impression of being more reliable, truer. The reason for this development is not only due to computers, but their magic plays a significant role. Computers provide accurate, reproducible measurements, working relentlessly around the clock without evidence of fatigue. Luckily, modern radiological equipment is based on computers that have made the explosive development of our discipline possible. More and more, we use our computer-based machines for measurements: tumor size, lengths, angles, and densities are measured daily in support of our judgments, as reflected in reports. To date, however, computer power has mainly been used to assist the radiologist’s professional skills. Computer-aided diagnosis (CAD) can be used to diagnose cancer in the breast (5) and lungs (7), and its field of use is expanding, for example, to the diagnosis of polyps on computed tomography (CT) colonography (10). Although it might take some of the magic out of our profession, computer-aided diagnosis of diseases is desirable, provided it is accurate. Herein lies the greatest challenge. Accurate diagnosis implies a computer algorithm with high sensitivity and specificity. The human eye and brain in combination provide a relatively good means for this purpose, but translation of this knowledge into a computer algorithm is still a great problem. In this issue of Acta Radiologica, STAVNGAARD et al. (8) report their experience with computer diagnosis of pulmonary emphysema. Their method is not new. They utilize an algorithm described already in 1988 by MÜLLER et al. (6), known as a ‘‘density mask. ’’ The algorithm is based on the fact that normal alveoli are too small to be demonstrated by CT. When the emphysematous process destroys the alveolar walls, larger air-containing spaces are formed which, when they reach a certain size, may be seen in CT images as areas of lower density than the surrounding lung tissue. The amount of tissue destruction necessary for visual demonstration is critical and depends on factors such as image resolution, slice thickness, reconstruction algorithm, and window settings. The influence of window width and level may be obviated by having the computer recognize and count the number of pixels with Hounsfield values (HU) consistent with emphysema. This method has been calibrated against pathologic specimens, with cutoff thresholds of 2910 HU for slices with a thickness of 10 mm (6) and 2950 HU for HRCT (2). The density-mask method has been used for many years for the quantification of visually demonstrable pulmonary emphysema. Imaging techniques continue to develop, however, and the quantification method has been continuously adapted. Thus, GEVENOIS et al. (4) recently re-adapted their technique to multidetector-row CT by recommending the use of a cut-off threshold as low as 2960 to 2970 HU in images with a slice thickness of 1 mm. When presenting scientific results, standardized and calibrated methods must be used, unless other forms of validation can be provided. In their paper, STAVNGAARD et al. are at odds with previously validated methods. In their quantification, they use a slice thickness of 5 mm with reconstruction of overlapping slices and a cut-off threshold of 2910 HU. Moreover, they use a low-dose technique with 40 mA. This set of image parameters has not been used previously, and thus proper validation is missing. The authors then compare the computercalculated results with those obtained by visual inspection of three HRCT slices, and claim that the computer better describes the distribution of emphysema. Although this is an unfair comparison, it may be reasonable to believe that it is true, even if it cannot be proven. The interesting thing in this paper, however, is that the authors take one step further. They suggest that patients without visible emphysema may still have emphysema based on a high number of pixels v2910 HU. This means that the authors try to introduce computer diagnosis of emphysema. Personally, I think this is incorrect, even if it may be true. There is, however, no method available for validation: there is no physiologic method allowing diagnosis of this small degree of emphysema, and biopsy cannot be considered ethically correct. Perhaps new advances in functional imaging with polarized He may make evaluation possible (1), but there are other facts