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PART 4. RADIOLOGIC ABNORMALITIES: RECENT DEVELOPMENTS
Improved Detection of Asbest0s-Related Pleural Plaques with Digital Radiography REGINALD GREENE,“ CORNELIA M . SCHAEFER,b AND L. CHRISTINE OLIVER‘ “Department of Radiology Massachusetts General Hospital, and Harvard Medical School Boston, Massachusetts 021 14 bDiagnostische Radiologie I Medizinische Hochschirle Hantiouer Hannouer, Germany “Departmentof Occupational Medicine Massachusetts General Hospitul, and Harvard Medical School Boston. Massachusetts 02114
STORAGE PHOSPHOR-BASED DIGITAL RADIOGRAPHY Computerized axial tomography is making unique contributions to our understanding of asbestos-related pulmonary and pleural abnormality,’.2but projection chest radiography will continue to be the primary survey imaging modality of choice because of its simplicity, wide availability, low cost, and low radiation exposure. As a screening modality, however, traditional projection radiography based on film technology is hampered by low detection sensitivity’; false-positives due to confounding simulators of plaques, such as pleural fat4; masking effects due to pleural abnormalities overlying the site of lung disease; and technical liability to errors in radiographic exposure. The potential benefits of substituting digital for film projection radiography in asbestos screening programs have not been systematically tested. Several substantial advantages may result from the use of storage phosphor-based digital radiography.5 These include ( 1 ) an extraordinarily wide dynamic range of linear response to radiation exposure (i.e., five decades versus two decades of linearity versus film); (2) automatic optical density control, virtually eliminating “overexposure” and “underexposure” errors; (3) image-processing ability to improve contrast and other specific image characteristics; (4) dual-energy image subtraction to provide calcium-weighted “bone” images, and calcium-deleted “soft tissue” images; and (5) future development of computer-based automatic feature extraction and image analysis programs. Despite having a somewhat lower spatial resolution than film has ( 2 . 5 versus 5 line pair/mm), storage phosphor digital radiography has the potential to improve the detection of asbestos-related chest disease. Because the digital imaging process can use standard-sized radiographic cassettes, this new digital imaging modality can continue to utilize most existing radiographic equipment. The radiationsensitive phosphors store the latent image information until it is released by 90
GREENE et al.: DIGITAL RADIOGRAPHY
91
exposure to a helium-neon laser scanner. The resulting phosphorescence is captured by a fiberoptic coupled light guide, and then passed through a photomultiplier tube, amplifier, and analog/digital converter. The final image is made up of 200 pm picture elements with 1,024 grey levels (i.e., 10 bits). This paper describes two unique capabilities of storage phosphors that can improve asbestos surveillance programs for pleural disease: automatic optical density control and dual-energy image subtraction.
AUTOMATIC OPTICAL DENSITY CONTROL Improper exposure, one of the most frequent and potentially serious limitations of traditional chest film radiography, can be totally avoided with storage phosphor digital radiography. The very wide dynamic range of the storage phosMEAN OPTICAL DENSITY OF THE LUNGS (D)
FIGURE 1. Optical density control of digital and film radiographs. In simultaneous exposures designed to optimize film exposures, the mean optical density of digital radiographs (800 measurements) is close to ideal (dashed line) and virtually always within the usable range of optical density. Film radiographs have much greater variation in density. and as many as 28% lie outside of the usable range.
phor detector and the flexible processing capability of the digital format provide automatic optical density control that virtually assures optimum optical density of image hard copy. The digital system can easily adjust system gain according to a preliminary histogram analysis of the pixel values in the image data. The beneficial impact of automatic optical density control is illustrated by a study of the hemithorax in 40 persons obtained with simultaneous exposure of radiographic cassettes, double-loaded with a radiographic film/screen and a storage phosphor. Despite the selection of exposure factors to specifically optimize acquisition of the film images, the mean optical density of the lungs in the digital hard copy was closer to ideal and much more uniform than the film radiographs. Virtually all of the digital radiographs had mean optical densities that fell within the usable range, while as many as 43% of the film radiograph densities fell outside of the usable range6 (FIG. I ) .
FIGURE 2. Dual-energy images of asbestos-related disease. (A) Traditional film radiograph of a subject with extensive pleural plaques with calcifications. (B)Storage phosphor digital radiograph of the right hemithorax demonstrates extensive pleural abnormality with a suggestion of calcification obscuring underlying lung parenchyma. (C) Dual-energy calcium-extracted digital image demonstrates the ribcage and extensive pleural calcification along the lateral chest wall and diaphragm. The obscuring effect of the overlying lung disease has been subtracted. (D) Dualenergy soft-tissue-extracted digital image demonstrates long atelectatic strands in the lower lungs without the obscuring effect of the calcifications in overlying plaques and the ribcage. The densities of uncalcified pleural plaques are also seen along the lateral chest wall.
GREENE el al.: DIGITAL RADIOGRAPHY
93
DUAL-ENERGY IMAGE SUBTRACTION The widespread adoption of high-kilovoltage technique and long-scale film for general chest imaging has reduced the detectability of pleural calcifications. Dualenergy imaging can take advantage of the spectral radiation absorption characteristics to separate calcified and noncalcified tissues in the evaluation of asbestosrelated pleural disease. Digital technology makes dual-energy imaging A single exposure of a cassette loaded with two storage phosphor plates. separated by a thin copper sheet, provides a low-energy image from the first plate and a high-energy image from the second plate. The low-energy image is produced
1.o
/Jr \
0.8
0.6
111P
DUAL-ENERGY DIGITAL A, = 0 9 0 ~ 0 0 1
1 SINGLE-ENERGY DIGITAL A,
P42‘
o
=088t001
I
FIGURE 3. Detection of asbestos-related pleural abnormality with dual-energy digital radiography. Estimates of performance (i.e., fitted ROC curves), estimated mean true-pobitive and false-positive fractions ( C standard error), and area under the ROC curve (A,) ? S E demonstrate superior detection with dual-energy digital radiography (7310 observations by eight readers). * = paired t test for eight readers.
predominantly by photoelectric absorption that is sensitive to atomic number (and calcium). The high-energy image is produced by Compton scatter that is insensitive to calcium/soft-tissue differences in atomic number. Weighted subtraction of the pixel values in the high- and low-energy images alternatively produce “calcium-extracted’’ and “soft-tissue-extracted” images from a single patient exposure. The calcium-extracted image removes the distracting effects of overlying soft tissues (such as the structured noise of pulmonary vessels) so that pure calcium-containing structures, like calcified plaques can be more easily visualized. The soft-tissue-extracted image removes the distracting structured noise caused by calcified pleura and normal ribs, so that a more unobstructed view of underlying lung parenchyma is obtained (FIG.2 ) .
ANNALS NEW YORK ACADEMY OF SCIENCES
94
N
c
3U a
a=SINGLE ENERGY DIGITAL RADIOGRAPH I= DUAL-ENERGY DIGITAL RADIOGRAPH
4 = FILM RADIOGRAPH
FIGURE 4. Detection of pleural calcification with dual-energy digital radiography. Paired r tests of ROC areas (A,) of eight individual readers demonstrate that more pleural calcifications are detected with digital than with film radiography (2928 observations by eight readers). CHESTWALL PLEURAL THICKENING CIRCUMSCRIBED
DIFFUSE
I
I
DIAPHRAGM THICKENING DIAPHRAGM
m
y
PLEURAL CALCIFICATION
DlAPH CHESTWALL COSTOPHRENIC OTHER SITES
FIGURE 5. 1LO pleural scoring method. Points are assigned for each positive reporting block in the ILO-1980 classification. Separate points are assigned for the left and right side of the chest according to the points in the shaded triangles. The maximum possible pleural score for any hemithorax is 31 points.
GREENE et al.: DIGITAL RADIOGRAPHY
95
The potential for improved detection of asbestos-related pleural disease with posteroanterior (PA) digital radiography is illustrated by a comparison with traditional chest films in detecting pleural findings previously documented by multiview radiography (PA, lateral, and both oblique views) using ILO- 1980 riter ria.^^'^ Single PA views with dual-energy digital radiography significantly outperformed conventional film-based PA radiographs in the overall detection of pleural thickening and pleural calcification (FIG.3). In this receiver operating characteristics (ROC) study of the hemithorax in 61 patients with proven asbestos-related disease (52 with pleural abnormality and 9 normals), each of eight individual radiologists performed better with PA dual-energy digital radiographs than with PA chest
- MULTIPLE- VIEW
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RADIOGRAPHY 6
5
2 Y
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PART 4. RADIOLOGIC ABNORMALITIES: RECENT DEVELOPMENTS
Improved Detection of Asbest0s-Related Pleural Plaques with Digital Radiography REGINALD GREENE,“ CORNELIA M . SCHAEFER,b AND L. CHRISTINE OLIVER‘ “Department of Radiology Massachusetts General Hospital, and Harvard Medical School Boston, Massachusetts 021 14 bDiagnostische Radiologie I Medizinische Hochschirle Hantiouer Hannouer, Germany “Departmentof Occupational Medicine Massachusetts General Hospitul, and Harvard Medical School Boston. Massachusetts 02114
STORAGE PHOSPHOR-BASED DIGITAL RADIOGRAPHY Computerized axial tomography is making unique contributions to our understanding of asbestos-related pulmonary and pleural abnormality,’.2but projection chest radiography will continue to be the primary survey imaging modality of choice because of its simplicity, wide availability, low cost, and low radiation exposure. As a screening modality, however, traditional projection radiography based on film technology is hampered by low detection sensitivity’; false-positives due to confounding simulators of plaques, such as pleural fat4; masking effects due to pleural abnormalities overlying the site of lung disease; and technical liability to errors in radiographic exposure. The potential benefits of substituting digital for film projection radiography in asbestos screening programs have not been systematically tested. Several substantial advantages may result from the use of storage phosphor-based digital radiography.5 These include ( 1 ) an extraordinarily wide dynamic range of linear response to radiation exposure (i.e., five decades versus two decades of linearity versus film); (2) automatic optical density control, virtually eliminating “overexposure” and “underexposure” errors; (3) image-processing ability to improve contrast and other specific image characteristics; (4) dual-energy image subtraction to provide calcium-weighted “bone” images, and calcium-deleted “soft tissue” images; and (5) future development of computer-based automatic feature extraction and image analysis programs. Despite having a somewhat lower spatial resolution than film has ( 2 . 5 versus 5 line pair/mm), storage phosphor digital radiography has the potential to improve the detection of asbestos-related chest disease. Because the digital imaging process can use standard-sized radiographic cassettes, this new digital imaging modality can continue to utilize most existing radiographic equipment. The radiationsensitive phosphors store the latent image information until it is released by 90
GREENE et al.: DIGITAL RADIOGRAPHY
91
exposure to a helium-neon laser scanner. The resulting phosphorescence is captured by a fiberoptic coupled light guide, and then passed through a photomultiplier tube, amplifier, and analog/digital converter. The final image is made up of 200 pm picture elements with 1,024 grey levels (i.e., 10 bits). This paper describes two unique capabilities of storage phosphors that can improve asbestos surveillance programs for pleural disease: automatic optical density control and dual-energy image subtraction.
AUTOMATIC OPTICAL DENSITY CONTROL Improper exposure, one of the most frequent and potentially serious limitations of traditional chest film radiography, can be totally avoided with storage phosphor digital radiography. The very wide dynamic range of the storage phosMEAN OPTICAL DENSITY OF THE LUNGS (D)
FIGURE 1. Optical density control of digital and film radiographs. In simultaneous exposures designed to optimize film exposures, the mean optical density of digital radiographs (800 measurements) is close to ideal (dashed line) and virtually always within the usable range of optical density. Film radiographs have much greater variation in density. and as many as 28% lie outside of the usable range.
phor detector and the flexible processing capability of the digital format provide automatic optical density control that virtually assures optimum optical density of image hard copy. The digital system can easily adjust system gain according to a preliminary histogram analysis of the pixel values in the image data. The beneficial impact of automatic optical density control is illustrated by a study of the hemithorax in 40 persons obtained with simultaneous exposure of radiographic cassettes, double-loaded with a radiographic film/screen and a storage phosphor. Despite the selection of exposure factors to specifically optimize acquisition of the film images, the mean optical density of the lungs in the digital hard copy was closer to ideal and much more uniform than the film radiographs. Virtually all of the digital radiographs had mean optical densities that fell within the usable range, while as many as 43% of the film radiograph densities fell outside of the usable range6 (FIG. I ) .
FIGURE 2. Dual-energy images of asbestos-related disease. (A) Traditional film radiograph of a subject with extensive pleural plaques with calcifications. (B)Storage phosphor digital radiograph of the right hemithorax demonstrates extensive pleural abnormality with a suggestion of calcification obscuring underlying lung parenchyma. (C) Dual-energy calcium-extracted digital image demonstrates the ribcage and extensive pleural calcification along the lateral chest wall and diaphragm. The obscuring effect of the overlying lung disease has been subtracted. (D) Dualenergy soft-tissue-extracted digital image demonstrates long atelectatic strands in the lower lungs without the obscuring effect of the calcifications in overlying plaques and the ribcage. The densities of uncalcified pleural plaques are also seen along the lateral chest wall.
GREENE el al.: DIGITAL RADIOGRAPHY
93
DUAL-ENERGY IMAGE SUBTRACTION The widespread adoption of high-kilovoltage technique and long-scale film for general chest imaging has reduced the detectability of pleural calcifications. Dualenergy imaging can take advantage of the spectral radiation absorption characteristics to separate calcified and noncalcified tissues in the evaluation of asbestosrelated pleural disease. Digital technology makes dual-energy imaging A single exposure of a cassette loaded with two storage phosphor plates. separated by a thin copper sheet, provides a low-energy image from the first plate and a high-energy image from the second plate. The low-energy image is produced
1.o
/Jr \
0.8
0.6
111P
DUAL-ENERGY DIGITAL A, = 0 9 0 ~ 0 0 1
1 SINGLE-ENERGY DIGITAL A,
P42‘
o
=088t001
I
FIGURE 3. Detection of asbestos-related pleural abnormality with dual-energy digital radiography. Estimates of performance (i.e., fitted ROC curves), estimated mean true-pobitive and false-positive fractions ( C standard error), and area under the ROC curve (A,) ? S E demonstrate superior detection with dual-energy digital radiography (7310 observations by eight readers). * = paired t test for eight readers.
predominantly by photoelectric absorption that is sensitive to atomic number (and calcium). The high-energy image is produced by Compton scatter that is insensitive to calcium/soft-tissue differences in atomic number. Weighted subtraction of the pixel values in the high- and low-energy images alternatively produce “calcium-extracted’’ and “soft-tissue-extracted” images from a single patient exposure. The calcium-extracted image removes the distracting effects of overlying soft tissues (such as the structured noise of pulmonary vessels) so that pure calcium-containing structures, like calcified plaques can be more easily visualized. The soft-tissue-extracted image removes the distracting structured noise caused by calcified pleura and normal ribs, so that a more unobstructed view of underlying lung parenchyma is obtained (FIG.2 ) .
ANNALS NEW YORK ACADEMY OF SCIENCES
94
N
c
3U a
a=SINGLE ENERGY DIGITAL RADIOGRAPH I= DUAL-ENERGY DIGITAL RADIOGRAPH
4 = FILM RADIOGRAPH
FIGURE 4. Detection of pleural calcification with dual-energy digital radiography. Paired r tests of ROC areas (A,) of eight individual readers demonstrate that more pleural calcifications are detected with digital than with film radiography (2928 observations by eight readers). CHESTWALL PLEURAL THICKENING CIRCUMSCRIBED
DIFFUSE
I
I
DIAPHRAGM THICKENING DIAPHRAGM
m
y
PLEURAL CALCIFICATION
DlAPH CHESTWALL COSTOPHRENIC OTHER SITES
FIGURE 5. 1LO pleural scoring method. Points are assigned for each positive reporting block in the ILO-1980 classification. Separate points are assigned for the left and right side of the chest according to the points in the shaded triangles. The maximum possible pleural score for any hemithorax is 31 points.
GREENE et al.: DIGITAL RADIOGRAPHY
95
The potential for improved detection of asbestos-related pleural disease with posteroanterior (PA) digital radiography is illustrated by a comparison with traditional chest films in detecting pleural findings previously documented by multiview radiography (PA, lateral, and both oblique views) using ILO- 1980 riter ria.^^'^ Single PA views with dual-energy digital radiography significantly outperformed conventional film-based PA radiographs in the overall detection of pleural thickening and pleural calcification (FIG.3). In this receiver operating characteristics (ROC) study of the hemithorax in 61 patients with proven asbestos-related disease (52 with pleural abnormality and 9 normals), each of eight individual radiologists performed better with PA dual-energy digital radiographs than with PA chest
- MULTIPLE- VIEW
-
RADIOGRAPHY 6
5
2 Y
4
W
a
0
5:
aa
__ __ __
3
__
2
-_
1
__
0
-
PA RADIOGRAPHY
1 I
P
