Special Requirements for Specific Disease Processes

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Pneumothorax

On both screen-film and digital chest radiographs, pneumothoraces may be difficult to detect. Detection of a pneumothorax depends on the detection of two findings: the first is the radiolucency of the lung outside of the edge of the lung; the second is the detection of the lung edge. Because digital image processing can change the density of regions of the image, density equalization programs may make the radiolucency harder to detect. The pleural edge can be very thin and can be superimposed on the ribs. Detection of the edge of the lung depends somewhat on the thickness of that edge, which varies among individuals. Using a slight degree of image processing to enhance edges increases the conspicuity of the lung edge and makes it easier to see the pneumothorax (Fig. 4). When the edge of the lung is superimposed on a rib, a low-kilovolt-peak chest X-ray technique may make it harder to see the lung edge because the relative radiodensity of the lung is increased, therefore a high-kilovolt-peak technique is preferred. The outer 2 cm of the lungs are more radiolucent than the more central portion since there is less lung, fewer blood vessels, and therefore less tissue density. If the image is too dark, it may become quite difficult to see detail in this region. If you are using a workstation to look at the image, making the image lighter can enhance the conspicuity of a pneumothorax. In carefully controlled settings, the detection of pneumothora-ces on screen-film and digital images has been shown to be equivalent [4,5,6].

Emphysema

The edges of emphysematous bullae and blebs are often very fine curved lines. Such lines can be blurred by the penumbra resulting from the X-ray tube focal spot. Small degrees of edge enhancement can enhance the conspicuity of these structures. For this reason, it can be easier to detect small amounts of emphysema on digital images (Fig. 4).

Interstitial Disease

Edge enhancement will increase the conspicuity of normal lung structures and interstitial lung disease. With edge enhancement, several reports document no difference in detection of interstitial lung disease [4,7,8]. Without edge enhancement, one study [9] showed that interstitial disease is less conspicuous (Figs. 5a-5c).

Interstitial disease is composed of fine and coarse lines and nodules. The conspicuity of these can be increased by a small amount of edge enhancement. The interstitial disease seen on digital chest radiographs, when minimal, may be quite difficult to see on conventional screen-film chest radiographs.

Figure 4 Pneumothorax detection in a 22-year-old male with a spontaneous pneumothorax. The pneumothorax is visible laterally. A few very small blebs are minimally visible at the apex of the lung.

Figure 5 Effect of edge enhancement algorithm on detection of interstitial lung disease in patient with methotrexate induced interstitial lung disease. Interstitial disease shown with no edge enhancement (a), standard edge enhancement (b), and exaggerated edge enhancement (c). Increasing enhancement accentuates the fine curved fibrotic lines. However, the small nodules are better seen in the less enhanced images. A small degree of edge enhancement (b) shows the pattern of interstitial disease best.

Figure 5 Effect of edge enhancement algorithm on detection of interstitial lung disease in patient with methotrexate induced interstitial lung disease. Interstitial disease shown with no edge enhancement (a), standard edge enhancement (b), and exaggerated edge enhancement (c). Increasing enhancement accentuates the fine curved fibrotic lines. However, the small nodules are better seen in the less enhanced images. A small degree of edge enhancement (b) shows the pattern of interstitial disease best.

Thus, one is likely to detect minimal interstitial disease more often on the digital images. At the same time, the vessels may be more conspicuous on digital chest radiographs, so it is important to carefully assess visible lines. In general, if the lines branch, they are vessels; if they fail to branch, they are likely interstitial lines. On underexposed images, the noise can become visible and may simulate very small, miliarylike nodules. Figure 3b shows the nodular appearance of noise on a severely underexposed film. Conversely, the use of a blurring algorithm to decrease the visibility of noise may also blur fine interstitial lines and nodules decreasing their conspicuity, particularly if they are in lighter areas of the image.

Masses and Nodules

Digital chest radiography has not resulted in problems in the detection of small lung nodules [4,7,8,10,11]. Nodules superimposed on the mediastinum have been shown to be easier to detect on digital compared to screen-film chest radiographs [10,12]. Because the contrast scale on digital chest radiographs can be changed, it may be more difficult to tell if a nodule is calcified. However, when techniques of image processing are standardized, this problem does not occur. The detection equivalence of lung nodules is seen when appropriate image processing is used [12]. Inappropriate image processing of digital chest radiographs, however, can decrease the visibility of lung nodules. The con-spicuity of lung nodules is equivalent with high-kilovolt-peak, high-contrast images. On the other hand, if the image processing produces a low-contrast image or if a low-kilovolt-peak image results in increased conspicuity of the ribs, nodules may be less visible (Fig. 6).

The detection of large lung masses and large areas of pneumonia may appear different on digital images when regional density equalization algorithms are used. These density equalization algorithms may partially equalize their density so they may not appear as dense for their size as they would on screen-film images.

Detection of Calcium in Lung Nodules. Digital image processing allows one to change the contrast of the chest image. Calcium in lung nodules is detected by two findings: first, the identification of the irregular shape of the calcifications and second by the greater density of nodule for its size than would be expected for a noncalcified nodule. While the first sign remains unchanged with digital images, image processing can change the contrast of a nodule compared to its background, making it appear more radiodense or less radio-dense. If techniques for image processing are standardized, there is usually no problem, but if one varies the image processing technique for different patients, it may be confusing to the radiologist.

Figure 6 Effect of edge and contrast enhancement on lung nodule detection. Non-small-cell primary lung cancer in the periphery of the left upper lobe shown (a) without and (b) with minimal edge and contrast enhancement. The mass is more easily seen with the edge and contrast enhancement.

Figure 6 Effect of edge and contrast enhancement on lung nodule detection. Non-small-cell primary lung cancer in the periphery of the left upper lobe shown (a) without and (b) with minimal edge and contrast enhancement. The mass is more easily seen with the edge and contrast enhancement.

Figure 7 Assessment of lines and tubes using digital radiography. Retrocardiac tubes shown with DRC plus edge enhancement (a) and black/white inversion (b). (c) Same patient, different day. Film is underexposed. Noise is more visible on local view shown in d. (d) Same image as c, but a local view. The noise is visible, but the edges of the lines can be seen. (e) Image blurring has been used to decrease visible noise. (f) Same image as (e), but a local view. Lines blurred by image smoothing, and less noise is visible. The edges of the lines are harder to see as compared to those in d.

Figure 7 Assessment of lines and tubes using digital radiography. Retrocardiac tubes shown with DRC plus edge enhancement (a) and black/white inversion (b). (c) Same patient, different day. Film is underexposed. Noise is more visible on local view shown in d. (d) Same image as c, but a local view. The noise is visible, but the edges of the lines can be seen. (e) Image blurring has been used to decrease visible noise. (f) Same image as (e), but a local view. Lines blurred by image smoothing, and less noise is visible. The edges of the lines are harder to see as compared to those in d.

Requirements for the Mediastinum

In the mediastinum and chest wall, there is much more water density material to absorb and scatter the X-rays. The absorption difference between normal structures and the findings of mediastinal disease (such as lymph nodes) is relatively low. For this reason, one would ideally want to have high contrast to see the anatomic changes in the mediastinum and a higher exposure than we need to see the lungs. Digital chest radiographs have been shown to enhance the visibility of mediastinal structures [10,13].

Special Requirements for Seeing Tubes and Devices in the Mediastinum. Especially in bedside examinations, identifying the location of tubes and lines within the mediastinum is of great importance since a misdirected line may have serious consequences for the patient. Digital chest radiography allows one to enhance the identification of the locations of these tubes and lines using three different methods (Fig. 7a-c). The first is to increase the optical density of the image so that the mediastinum is projected in the maximum contrast density of the film or monitor one is using. The second is to black/ white invert the image. The third is to use density equalization algorithms. If the film is properly exposed, any of these techniques will usually suffice. If the image is underexposed, the necessary information may not have been captured.

Digital radiography software allows one to smooth the image to make the noise that occurs in regions of low exposure less visible. This smoothing may also smooth the edges of tubes and lines making them less visible (Fig. 7c-f). For this reason, we prefer to have a somewhat noisy-appearing mediastinum on our images since that increases the chance that we will be able to identify the edges of the tubes and lines.

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