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Digital Tomosynthesis of the Chest: Current and Emerging Applications1 Shinn-Huey S. Chou, MD Greg A. Kicska, MD, PhD Sudhakar N. Pipavath, MD Gautham P. Reddy, MD Abbreviations: DTS = digital tomosynthesis, TEP = tracheoesophageal puncture RadioGraphics 2014; 34:359–372 Published online 10.1148/rg.342135057 Content Code: From the Department of Radiology, University of Washington, 1959 NE Pacific St, UW Mailbox 357115, Seattle, WA 98195-7115. Presented as an education exhibit at the 2012 RSNA Annual Meeting. Received April 12, 2013; revision requested June 7 and received July 18; accepted July 24. For this journalbased SA-CME activity, the authors G.A.K. and S.N.P. have disclosed various financial relationships (see p 371); all other authors, the editor, and reviewers have no relevant relationships to disclose. Address correspondence to S.H.S.C. (e-mail: [email protected]). 1
ONLINE-ONLY SA-CME LEARNING OBJECTIVES After completing this journal-based SACME activity, participants will be able to: ■■Discuss the basic principles of digital tomosynthesis of the chest.
Digital tomosynthesis (DTS) of the chest is a technique whose basic components are similar to those of digital radiography, but that also provides some of the benefits of computed tomography (CT). The major advantages of DTS over conventional chest radiography are improved visibility of the pulmonary parenchyma and depiction of abnormalities such as pulmonary nodules. Calcifications, vessels, airways, and chest wall abnormalities are also much more readily visualized at DTS than at chest radiography. DTS could potentially be combined with chest radiography to follow up known nodules, confirm or rule out suspected nodules seen at radiography, or evaluate individuals who are at high risk for lung cancer or pulmonary metastases. DTS generates coronal “slices” through the chest whose resolution is superior to that of coronal reconstructed CT images, but it is limited by its suboptimal depth resolution and susceptibility to motion; consequently, potential pitfalls in recognizing lesions adjacent to the pleura, diaphragm, central vessels, and mediastinum can occur. However, the radiation dose and projected cost of chest DTS are lower than those of standard chest CT. Besides pulmonary nodule detection, specific applications of DTS that are under investigation include evaluation of pulmonary tuberculous and nontuberculous mycobacterial disease, cystic fibrosis, interstitial lung disease, and asbestos-related thoracic diseases. A basic understanding of chest DTS and of the emerging applications of this technique can prove useful to the radiologist. Online supplemental material is available for this article. ©
RSNA, 2014 • radiographics.rsna.org
■■List
the advantages and limitations of digital tomosynthesis of the chest vis-àvis radiography and CT. ■■Describe
emerging applications of digital tomosynthesis of the chest. See www.rsna.org/education/search/RG.
Scan this code for access to supplemental material on our website.
Introduction
Since the dawn of radiology more than a century ago, chest radiography has formed the basis of first-line imaging evaluation for suspected thoracic disease. Chest radiography is the most commonly performed imaging study in the United States and, likely, worldwide (1). This remains true even with dramatic advances in thoracic imaging technology, including computed tomography (CT) and magnetic resonance imaging (1,2). Today, chest radiographs continue to provide a wealth of vital medical information. However, abnormalities such as pulmonary nodules can go undetected by even the most skillful radiologists. The manifestation of pathologic conditions of the chest is often anatomically minute. A primary reason why these abnormalities are difficult to detect at chest radiography is that the overlap of three-dimensional structures projected on a two-dimensional image leads to decreased contrast resolution, obscuring clinically important lesions and limiting the accuracy of disease detection.
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Figure 1. Use of geometric tomography in a 55-year-old man with a long history of right upper quadrant discomfort. (a) Abdominal radiograph shows a medium-sized calcified cystic lesion (arrow) and two smaller calcified round lesions (arrowheads) in the right upper quadrant, findings that represent cystic echinococcosis of the liver with a mother hydatid cyst and two smaller daughter cysts, respectively. (Courtesy of Charles Rohrmann, MD, University of Washington.) (b) Targeted geometric tomographic image obtained with the in-focus plane at the depth of the mother hydatid cyst (arrow) and one of the daughter cysts (arrowhead) in the liver shows the calcified cyst walls. The more superiorly located daughter cyst is not seen because it lies deep to the plane of interest and is, therefore, blurred. The spine, which is located more posteriorly, is also blurred.
Geometric tomography, introduced in 1931, was one of the earliest cross-sectional imaging attempts to overcome this limitation of singleprojection radiography (2–4). In this analog tomographic technique, the x-ray tube moves through a limited acquisition angle around the patient, with continuous film exposure (3,4). Objects in the predetermined plane of interest (in-focus plane) are depicted in sharp detail, whereas objects outside the plane are blurred (Fig 1). Common indications for thoracic geometric tomography included cavitation in pulmonary tuberculosis, calcification in pulmonary nodules and lymph nodes, and disease of the sternum and central airways (2). A limitation of this technique is the persistence of residual blurring caused by objects in front of and behind the in-focus plane, often obscuring soft-tissue abnormalities that have low intrinsic contrast resolution. Moreover, each x-ray tube sweep and exposure of the entire thickness of the patient generates only a single image (2,3); the process must be repeated if multiple images at different focal depths are required, increasing the radiation dose to the patient. Ultimately, geometric
tomography of the chest was unable to compete with CT, which gradually supplanted it in the late 1970s and early 1980s (2,3). At CT, cross-sectional images are reconstructed with little or no contamination by objects outside the in-focus plane. This is possible because, unlike with geometric tomography, the x-ray tube rotates 360° around the patient. Compared with geometric tomography, however, CT of the chest results in increased radiation. With the advent of digital radiography and flat-panel detectors, development of digital tomosynthesis (DTS) became technically feasible and practical. DTS is based on principles similar to those of geometric tomography (3,4). DTS images represent sections through the anatomy that are generated and reconstructed from a set of projection radiographs. The use of DTS for thoracic imaging offers some of the advantages of both chest radiography and CT. DTS of the chest improves conspicuity of thoracic abnormalities compared with radiography by removing overlapping clutter. It also provides higher resolution in the coronal plane and a lower radiation dose to patients than does standard chest CT. Clinical
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Figure 2. Photograph shows a commercial DTS unit (VolumeRAD) manufactured by GE Healthcare (Milwaukee, Wis). The x-ray tube is attached to a motorized crane, which moves the tube in a straight line parallel to the plane of the flat-panel detector while angling the tube so that it remains directed at the detector. The detector is similar to that used in a standard digital radiography unit.
implementation of this novel technology has become a reality, with commercially available U.S. Food and Drug Administration–approved systems (3,5). Therefore, a basic understanding of DTS and its potential applications in thoracic imaging can prove useful to the radiologist. In this article, we discuss chest DTS in terms of basic principles, fundamentals of image interpretation, advantages and limitations, potential applications, and suggested indications, with supporting evidence from the medical literature.
Chest DTS Systems
The basic components of a chest DTS system are similar to those of a digital radiography system and include a conventional x-ray tube, a digital flat-panel detector, and a grid (Fig 2). One major difference is that a chest DTS system includes a motorized crane for the x-ray tube. This computer-controlled crane allows the x-ray tube to tilt at various angles so that it can remain directed at the detector while sweeping over a prescribed linear path parallel to the detector (3). Chest DTS has some features of both projection radiography and CT. Similar to projection radiography, chest DTS makes use of a digital flat-panel detector instead of a CT detector array. This flat-panel detector provides higher in-plane spatial resolution—most commonly a 200 × 200-µm pixel size, compared with a pixel size of approximately 500 × 500 µm at CT. As with CT, multiple images are acquired at various angles (3); however, instead of a full 180°-plus fan beam angle being used to reconstruct an in-focus plane, a total acquisition angle ranging from 35° to 60° and approximately 60 images are needed for chest DTS. Unlike breast DTS, which uses an isocentric system, chest DTS uses a parallel-path system in which the x-ray tube travels in a linear
motion (3). Moreover, unlike with geometric tomography, which delivers continuous exposure, the chest DTS detector receives pulsed x-ray exposures at different tube angles (3,4). This allows reconstruction of an image that represents a specific in-focus plane; however, because an acquisition angle of less than 180° is used, the image is contaminated by structures behind and in front of the in-focus plane. When a broader angle is used for image acquisition, computerized reconstruction can more effectively separate in-plane from out-of-plane structures. With a commercially available DTS system, the tube typically sweeps in a vertical path along the anatomy of interest over a preset total acquisition angle, with the patient either upright (Movie 1 [online]) or supine (Movie 2 [online]). About 60 raw projection images are obtained with VolumeRAD, the commercial DTS system from GE Healthcare (Movie 3 [online]). The exposure time for each projection is short (~10 msec). Total image acquisition takes place during a single 10–12-second breath hold in inspiration (4).
Image Reconstruction
Geometric tomography was able to produce only a single in-focus plane per set of x-ray tube sweeps. DTS represents a substantial improvement over the analog-based tomography system in that it creates a voxel dataset by using filtered back projection on commercial chest DTS systems (3,6). This allows an arbitrary number of images to be reconstructed at multiple depths from a single set of projection images that encompasses the entire thickness of the patient in a single sweep (3,4,6). DTS images have an in-plane resolution similar to the detector resolution (200 × 200 µm). The concept of “slice thickness” does not exist because objects in front of and behind the in-focus plane
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Figure 3. Improved conspicuity of osseous structures of the chest wall at DTS. (a) On a reconstructed DTS “slice,” or in-focus plane, obtained at the level of the sternum with appropriate windowing, the borders of the sternum are accentuated and distinctly visible. The sternum is typically obscured on conventional anteroposterior chest radiographs. (b) Reconstructed DTS image shows improved visibility of the spinous processes (white arrow) and pedicles of the vertebrae (black arrows), which can be assessed at each individual vertebral level. The spinous processes and pedicles are not in the same coronal plane. (c) Chest radiograph depicts several spinous processes and pedicles, but the sternum is virtually obscured by overlapping mediastinal structures.
will also be present, albeit blurred. The farther an object is from the in-focus plane, the more blurred it will be, and the larger the acquisition angle between projection images, the more rapidly these objects will blur with respect to their distance from the in-focus plane.
Fundamentals of Image Interpretation
The interpretation of chest DTS images is similar to that of chest radiographs, except that DTS generates a greater number of images for review and provides increased conspicuity of many structures. On the other hand, scrolling through images while paying particular attention to certain structures is akin to the assessment of a chest CT scan (Movie 4 [online]). Major anatomic structures that are evaluated at chest DTS include the thoracic wall and osseous structures, mediastinum, vasculature, airways, pleura, lungs, and upper abdomen. DTS markedly improves the visibility of anatomic components that are harder to assess and more easily overlooked at conventional radiography. Examples include osseous structures such as the sternum, anterior ribs, pedicles, and spinous
processes (Fig 3), and vessels such as the pulmonary arteries (Fig 4). Normal vascular branches can be visible to within 1 cm of the pleura (7). Details of the airways and lung parenchyma are also much better visualized at chest DTS and should be carefully scrutinized during image interpretation (Fig 5) (8,9). The quality of the examination technique is also determined by evaluating all of the aforementioned structures (10). Clear and crisp delineation of the diaphragm confirms adequate suspension of respiration, as do distinct outlines of the airways, vessels, and interlobar fissures. As with radiography, large structures oriented perpendicular to the imaging plane (eg, the diaphragm) are more clearly resolved than are structures oriented parallel to the imaging plane (eg, the anterior and posterior surfaces of the heart).
Advantages and Limitations of DTS
Chest DTS offers distinct advantages over chest radiography in the appraisal of intra- and extrapulmonary lesions. Multiple chest DTS studies have demonstrated improvements in diagnostic accuracy, reader confidence, and interobserver
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Figure 4. Improved conspicuity of vasculature at DTS. (a) DTS image shows improved visibility of the pulmonary vessels, including the left pulmonary artery (white arrowhead), left lower lobe pulmonary artery (white arrow), right inferior pulmonary vein (black arrowhead), and right interlobar pulmonary artery (black arrow). (b) DTS image shows improved visibility of the outline of the descending aorta (arrowheads). (c) Chest radiograph obtained for comparison.
Figure 5. Improved conspicuity of the central airways at DTS. (a) Reconstructed DTS image clearly delineates the carina (arrow) and the branching bronchi, including the right upper lobe bronchus (arrowhead) and bronchus intermedius. (b) Chest radiograph obtained for comparison.
agreement compared with radiography (11–14). Use of DTS to verify suspected findings of pulmonary lesions on chest radiographs can reduce CT utilization by about 75% (12). Specific advantages of chest DTS over radiography, most notably in the detection of pulmonary nodules, are discussed in the following paragraphs. Because a commercial DTS system is “piggybacked” on a digital radiography unit, a DTS study can be performed immediately and conve-
niently after the acquisition of chest radiographs to verify suspected findings without having to move the patient (6). By providing improved lesion detection and localization, chest DTS may obviate lateral chest radiography, which has traditionally been supplementary for confirming or localizing findings at anteroposterior radiography (4,5). The spatial resolution of DTS exceeds that of CT (Fig 6) because DTS yields coronal images and utilizes a higher-resolution detector compared
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Figure 6. Superior spatial resolution of DTS. Chest radiograph (magnified view) (a), DTS image (b), and coronal chest CT image (c) obtained in a 69-year-old woman in whom a lung nodule was incidentally detected at radiography of the thoracic spine show a 6-mm pulmonary nodule (circle) in the left upper lobe. The nodule is more conspicuous on the DTS image than on the radiograph and has higher contrast resolution on the CT image. The spatial resolution of DTS and radiography are identical, at approximately 200 × 200 µm. CT demonstrates superior contrast resolution but at a reduced spatial resolution of 500 × 500 µm. The lung nodule demonstrated stability at follow-up imaging performed over a period of more than 2 years, indicating that the nodule likely had a benign cause.
with CT, whereas CT spatial resolution is limited for the most part by the z-axis resolution of its detectors. Furthermore, with most CT scanners, coronal imaging of the chest requires image reconstruction, whereas a DTS system utilizes the full resolution of the detection panel (reported matrix sizes include 2022 × 2022 pixels and 2880 × 2880 pixels) and does not require retrospective postprocessing for coronal imaging (7,15). The estimated effective radiation dose for a chest DTS study is 0.08–0.13 mSv (4,16), whereas the effective doses for posteroanterior and left lateral chest radiography are approximately 0.02 mSv and 0.04–0.05 mSv, respectively (16). Therefore, the total radiation dose for a chest DTS study is approximately twice that for standard two-view digital chest radiography (0.06 mSv). However, compared with typical chest CT, which involves an estimated effective dose of 1–4 mSv, chest DTS requires a much smaller radiation dose (11,16,17). CT techniques in which a very low dose (effective dose,
Digital Tomosynthesis of the Chest: Current and Emerging Applications1 Shinn-Huey S. Chou, MD Greg A. Kicska, MD, PhD Sudhakar N. Pipavath, MD Gautham P. Reddy, MD Abbreviations: DTS = digital tomosynthesis, TEP = tracheoesophageal puncture RadioGraphics 2014; 34:359–372 Published online 10.1148/rg.342135057 Content Code: From the Department of Radiology, University of Washington, 1959 NE Pacific St, UW Mailbox 357115, Seattle, WA 98195-7115. Presented as an education exhibit at the 2012 RSNA Annual Meeting. Received April 12, 2013; revision requested June 7 and received July 18; accepted July 24. For this journalbased SA-CME activity, the authors G.A.K. and S.N.P. have disclosed various financial relationships (see p 371); all other authors, the editor, and reviewers have no relevant relationships to disclose. Address correspondence to S.H.S.C. (e-mail: [email protected]). 1
ONLINE-ONLY SA-CME LEARNING OBJECTIVES After completing this journal-based SACME activity, participants will be able to: ■■Discuss the basic principles of digital tomosynthesis of the chest.
Digital tomosynthesis (DTS) of the chest is a technique whose basic components are similar to those of digital radiography, but that also provides some of the benefits of computed tomography (CT). The major advantages of DTS over conventional chest radiography are improved visibility of the pulmonary parenchyma and depiction of abnormalities such as pulmonary nodules. Calcifications, vessels, airways, and chest wall abnormalities are also much more readily visualized at DTS than at chest radiography. DTS could potentially be combined with chest radiography to follow up known nodules, confirm or rule out suspected nodules seen at radiography, or evaluate individuals who are at high risk for lung cancer or pulmonary metastases. DTS generates coronal “slices” through the chest whose resolution is superior to that of coronal reconstructed CT images, but it is limited by its suboptimal depth resolution and susceptibility to motion; consequently, potential pitfalls in recognizing lesions adjacent to the pleura, diaphragm, central vessels, and mediastinum can occur. However, the radiation dose and projected cost of chest DTS are lower than those of standard chest CT. Besides pulmonary nodule detection, specific applications of DTS that are under investigation include evaluation of pulmonary tuberculous and nontuberculous mycobacterial disease, cystic fibrosis, interstitial lung disease, and asbestos-related thoracic diseases. A basic understanding of chest DTS and of the emerging applications of this technique can prove useful to the radiologist. Online supplemental material is available for this article. ©
RSNA, 2014 • radiographics.rsna.org
■■List
the advantages and limitations of digital tomosynthesis of the chest vis-àvis radiography and CT. ■■Describe
emerging applications of digital tomosynthesis of the chest. See www.rsna.org/education/search/RG.
Scan this code for access to supplemental material on our website.
Introduction
Since the dawn of radiology more than a century ago, chest radiography has formed the basis of first-line imaging evaluation for suspected thoracic disease. Chest radiography is the most commonly performed imaging study in the United States and, likely, worldwide (1). This remains true even with dramatic advances in thoracic imaging technology, including computed tomography (CT) and magnetic resonance imaging (1,2). Today, chest radiographs continue to provide a wealth of vital medical information. However, abnormalities such as pulmonary nodules can go undetected by even the most skillful radiologists. The manifestation of pathologic conditions of the chest is often anatomically minute. A primary reason why these abnormalities are difficult to detect at chest radiography is that the overlap of three-dimensional structures projected on a two-dimensional image leads to decreased contrast resolution, obscuring clinically important lesions and limiting the accuracy of disease detection.
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Figure 1. Use of geometric tomography in a 55-year-old man with a long history of right upper quadrant discomfort. (a) Abdominal radiograph shows a medium-sized calcified cystic lesion (arrow) and two smaller calcified round lesions (arrowheads) in the right upper quadrant, findings that represent cystic echinococcosis of the liver with a mother hydatid cyst and two smaller daughter cysts, respectively. (Courtesy of Charles Rohrmann, MD, University of Washington.) (b) Targeted geometric tomographic image obtained with the in-focus plane at the depth of the mother hydatid cyst (arrow) and one of the daughter cysts (arrowhead) in the liver shows the calcified cyst walls. The more superiorly located daughter cyst is not seen because it lies deep to the plane of interest and is, therefore, blurred. The spine, which is located more posteriorly, is also blurred.
Geometric tomography, introduced in 1931, was one of the earliest cross-sectional imaging attempts to overcome this limitation of singleprojection radiography (2–4). In this analog tomographic technique, the x-ray tube moves through a limited acquisition angle around the patient, with continuous film exposure (3,4). Objects in the predetermined plane of interest (in-focus plane) are depicted in sharp detail, whereas objects outside the plane are blurred (Fig 1). Common indications for thoracic geometric tomography included cavitation in pulmonary tuberculosis, calcification in pulmonary nodules and lymph nodes, and disease of the sternum and central airways (2). A limitation of this technique is the persistence of residual blurring caused by objects in front of and behind the in-focus plane, often obscuring soft-tissue abnormalities that have low intrinsic contrast resolution. Moreover, each x-ray tube sweep and exposure of the entire thickness of the patient generates only a single image (2,3); the process must be repeated if multiple images at different focal depths are required, increasing the radiation dose to the patient. Ultimately, geometric
tomography of the chest was unable to compete with CT, which gradually supplanted it in the late 1970s and early 1980s (2,3). At CT, cross-sectional images are reconstructed with little or no contamination by objects outside the in-focus plane. This is possible because, unlike with geometric tomography, the x-ray tube rotates 360° around the patient. Compared with geometric tomography, however, CT of the chest results in increased radiation. With the advent of digital radiography and flat-panel detectors, development of digital tomosynthesis (DTS) became technically feasible and practical. DTS is based on principles similar to those of geometric tomography (3,4). DTS images represent sections through the anatomy that are generated and reconstructed from a set of projection radiographs. The use of DTS for thoracic imaging offers some of the advantages of both chest radiography and CT. DTS of the chest improves conspicuity of thoracic abnormalities compared with radiography by removing overlapping clutter. It also provides higher resolution in the coronal plane and a lower radiation dose to patients than does standard chest CT. Clinical
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Figure 2. Photograph shows a commercial DTS unit (VolumeRAD) manufactured by GE Healthcare (Milwaukee, Wis). The x-ray tube is attached to a motorized crane, which moves the tube in a straight line parallel to the plane of the flat-panel detector while angling the tube so that it remains directed at the detector. The detector is similar to that used in a standard digital radiography unit.
implementation of this novel technology has become a reality, with commercially available U.S. Food and Drug Administration–approved systems (3,5). Therefore, a basic understanding of DTS and its potential applications in thoracic imaging can prove useful to the radiologist. In this article, we discuss chest DTS in terms of basic principles, fundamentals of image interpretation, advantages and limitations, potential applications, and suggested indications, with supporting evidence from the medical literature.
Chest DTS Systems
The basic components of a chest DTS system are similar to those of a digital radiography system and include a conventional x-ray tube, a digital flat-panel detector, and a grid (Fig 2). One major difference is that a chest DTS system includes a motorized crane for the x-ray tube. This computer-controlled crane allows the x-ray tube to tilt at various angles so that it can remain directed at the detector while sweeping over a prescribed linear path parallel to the detector (3). Chest DTS has some features of both projection radiography and CT. Similar to projection radiography, chest DTS makes use of a digital flat-panel detector instead of a CT detector array. This flat-panel detector provides higher in-plane spatial resolution—most commonly a 200 × 200-µm pixel size, compared with a pixel size of approximately 500 × 500 µm at CT. As with CT, multiple images are acquired at various angles (3); however, instead of a full 180°-plus fan beam angle being used to reconstruct an in-focus plane, a total acquisition angle ranging from 35° to 60° and approximately 60 images are needed for chest DTS. Unlike breast DTS, which uses an isocentric system, chest DTS uses a parallel-path system in which the x-ray tube travels in a linear
motion (3). Moreover, unlike with geometric tomography, which delivers continuous exposure, the chest DTS detector receives pulsed x-ray exposures at different tube angles (3,4). This allows reconstruction of an image that represents a specific in-focus plane; however, because an acquisition angle of less than 180° is used, the image is contaminated by structures behind and in front of the in-focus plane. When a broader angle is used for image acquisition, computerized reconstruction can more effectively separate in-plane from out-of-plane structures. With a commercially available DTS system, the tube typically sweeps in a vertical path along the anatomy of interest over a preset total acquisition angle, with the patient either upright (Movie 1 [online]) or supine (Movie 2 [online]). About 60 raw projection images are obtained with VolumeRAD, the commercial DTS system from GE Healthcare (Movie 3 [online]). The exposure time for each projection is short (~10 msec). Total image acquisition takes place during a single 10–12-second breath hold in inspiration (4).
Image Reconstruction
Geometric tomography was able to produce only a single in-focus plane per set of x-ray tube sweeps. DTS represents a substantial improvement over the analog-based tomography system in that it creates a voxel dataset by using filtered back projection on commercial chest DTS systems (3,6). This allows an arbitrary number of images to be reconstructed at multiple depths from a single set of projection images that encompasses the entire thickness of the patient in a single sweep (3,4,6). DTS images have an in-plane resolution similar to the detector resolution (200 × 200 µm). The concept of “slice thickness” does not exist because objects in front of and behind the in-focus plane
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Figure 3. Improved conspicuity of osseous structures of the chest wall at DTS. (a) On a reconstructed DTS “slice,” or in-focus plane, obtained at the level of the sternum with appropriate windowing, the borders of the sternum are accentuated and distinctly visible. The sternum is typically obscured on conventional anteroposterior chest radiographs. (b) Reconstructed DTS image shows improved visibility of the spinous processes (white arrow) and pedicles of the vertebrae (black arrows), which can be assessed at each individual vertebral level. The spinous processes and pedicles are not in the same coronal plane. (c) Chest radiograph depicts several spinous processes and pedicles, but the sternum is virtually obscured by overlapping mediastinal structures.
will also be present, albeit blurred. The farther an object is from the in-focus plane, the more blurred it will be, and the larger the acquisition angle between projection images, the more rapidly these objects will blur with respect to their distance from the in-focus plane.
Fundamentals of Image Interpretation
The interpretation of chest DTS images is similar to that of chest radiographs, except that DTS generates a greater number of images for review and provides increased conspicuity of many structures. On the other hand, scrolling through images while paying particular attention to certain structures is akin to the assessment of a chest CT scan (Movie 4 [online]). Major anatomic structures that are evaluated at chest DTS include the thoracic wall and osseous structures, mediastinum, vasculature, airways, pleura, lungs, and upper abdomen. DTS markedly improves the visibility of anatomic components that are harder to assess and more easily overlooked at conventional radiography. Examples include osseous structures such as the sternum, anterior ribs, pedicles, and spinous
processes (Fig 3), and vessels such as the pulmonary arteries (Fig 4). Normal vascular branches can be visible to within 1 cm of the pleura (7). Details of the airways and lung parenchyma are also much better visualized at chest DTS and should be carefully scrutinized during image interpretation (Fig 5) (8,9). The quality of the examination technique is also determined by evaluating all of the aforementioned structures (10). Clear and crisp delineation of the diaphragm confirms adequate suspension of respiration, as do distinct outlines of the airways, vessels, and interlobar fissures. As with radiography, large structures oriented perpendicular to the imaging plane (eg, the diaphragm) are more clearly resolved than are structures oriented parallel to the imaging plane (eg, the anterior and posterior surfaces of the heart).
Advantages and Limitations of DTS
Chest DTS offers distinct advantages over chest radiography in the appraisal of intra- and extrapulmonary lesions. Multiple chest DTS studies have demonstrated improvements in diagnostic accuracy, reader confidence, and interobserver
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Figure 4. Improved conspicuity of vasculature at DTS. (a) DTS image shows improved visibility of the pulmonary vessels, including the left pulmonary artery (white arrowhead), left lower lobe pulmonary artery (white arrow), right inferior pulmonary vein (black arrowhead), and right interlobar pulmonary artery (black arrow). (b) DTS image shows improved visibility of the outline of the descending aorta (arrowheads). (c) Chest radiograph obtained for comparison.
Figure 5. Improved conspicuity of the central airways at DTS. (a) Reconstructed DTS image clearly delineates the carina (arrow) and the branching bronchi, including the right upper lobe bronchus (arrowhead) and bronchus intermedius. (b) Chest radiograph obtained for comparison.
agreement compared with radiography (11–14). Use of DTS to verify suspected findings of pulmonary lesions on chest radiographs can reduce CT utilization by about 75% (12). Specific advantages of chest DTS over radiography, most notably in the detection of pulmonary nodules, are discussed in the following paragraphs. Because a commercial DTS system is “piggybacked” on a digital radiography unit, a DTS study can be performed immediately and conve-
niently after the acquisition of chest radiographs to verify suspected findings without having to move the patient (6). By providing improved lesion detection and localization, chest DTS may obviate lateral chest radiography, which has traditionally been supplementary for confirming or localizing findings at anteroposterior radiography (4,5). The spatial resolution of DTS exceeds that of CT (Fig 6) because DTS yields coronal images and utilizes a higher-resolution detector compared
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Figure 6. Superior spatial resolution of DTS. Chest radiograph (magnified view) (a), DTS image (b), and coronal chest CT image (c) obtained in a 69-year-old woman in whom a lung nodule was incidentally detected at radiography of the thoracic spine show a 6-mm pulmonary nodule (circle) in the left upper lobe. The nodule is more conspicuous on the DTS image than on the radiograph and has higher contrast resolution on the CT image. The spatial resolution of DTS and radiography are identical, at approximately 200 × 200 µm. CT demonstrates superior contrast resolution but at a reduced spatial resolution of 500 × 500 µm. The lung nodule demonstrated stability at follow-up imaging performed over a period of more than 2 years, indicating that the nodule likely had a benign cause.
with CT, whereas CT spatial resolution is limited for the most part by the z-axis resolution of its detectors. Furthermore, with most CT scanners, coronal imaging of the chest requires image reconstruction, whereas a DTS system utilizes the full resolution of the detection panel (reported matrix sizes include 2022 × 2022 pixels and 2880 × 2880 pixels) and does not require retrospective postprocessing for coronal imaging (7,15). The estimated effective radiation dose for a chest DTS study is 0.08–0.13 mSv (4,16), whereas the effective doses for posteroanterior and left lateral chest radiography are approximately 0.02 mSv and 0.04–0.05 mSv, respectively (16). Therefore, the total radiation dose for a chest DTS study is approximately twice that for standard two-view digital chest radiography (0.06 mSv). However, compared with typical chest CT, which involves an estimated effective dose of 1–4 mSv, chest DTS requires a much smaller radiation dose (11,16,17). CT techniques in which a very low dose (effective dose,
