DOI: 10.1148/rg.261055034
RadioGraphics 2006;26:79-92
© RSNA, 2006
Dual-Energy Subtraction Chest Radiography: What to Look for beyond Calcified Nodules1
Janet E. Kuhlman, MD, MS,
Jannette Collins, MD, MEd,
Gregory N. Brooks, MD, PhD,
Donald R. Yandow, MD and
Lynn S. Broderick, MD
1 From the Department of Radiology, University of Wisconsin Medical School, Hospital and Clinics, E3/374 CSC, 600 Highland Ave, Madison, WI 53792-3252. Presented as an education exhibit at the 2004 RSNA Annual Meeting. Received March 2, 2005; revision requested April 26 and received May 20; accepted May 23. The University of Wisconsin Department of Radiology receives research support from GE HealthCare. J.E.K. is a stockholder of GE; all remaining authors have no financial relationships to disclose.
Address correspondence to J.E.K. (e-mail: je.kuhlman{at}hosp.wisc.edu).
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Abstract
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Dual-energy subtraction chest radiography is a robust and powerful tool that improves the ability to detect and accurately diagnose a wide variety of thoracic abnormalities on posteroanterior-lateral chest images. Dual-energy subtraction chest radiography has many advantages over conventional chest radiography that facilitate image interpretation. The major advantage of this imaging technique is that it more clearly depicts calcification, which greatly aids in characterizing pulmonary nodules. Dual-energy subtraction images are also helpful in the recognition of hilar and mediastinal masses; the detection of tracheal narrowing and vascular disease; the identification of bone, pleural, and chest wall abnormalities; and the localization of indwelling devices such as stents and catheters. However, dual-energy subtraction imaging also has some limitations of which the radiologist should be aware and requires a somewhat higher radiation dose than does conventional radiography.
© RSNA, 2006
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Introduction
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Dual-energy subtraction chest radiography is very helpful in the detection and diagnosis of thoracic abnormalities. A major advantage of dual-energy imaging over conventional radiography is its superior sensitivity for the detection of calcification within a pulmonary nodule. Other advantages of dual-energy imaging are related to identification of bone and pleural abnormalities; recognition of hilar and mediastinal masses; detection of tracheal narrowing and airway disease; and localization of stents, catheters, and other indwelling devices.
In this article, we review the basic principles of dual-energy subtraction chest radiography. We also discuss and illustrate the advantages and limitations of this imaging modality and provide some suggested indications for its use.
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Basic Principles
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Dual-energy subtraction takes advantage of differences in the degree to which body tissues attenuate low- and high-energy (measured in kilo electron volts) photons. These differences are used to generate tissue-selective images. Bone, because it contains calcium, has a higher attenuation coefficient (ie, absorbs more photons) at lower photon and beam energy. This effect is more pronounced for calcium-containing tissues than for soft tissues, so that structures that contain calcium (including bone) can be removed from images, leaving soft tissues and lung (1,2).
Two types of dual-energy systems are available: a single-exposure system and a dual-exposure system (1,2). In single-exposure systems, one radiograph is obtained by exposing two storage phosphor plates separated by a copper filter (3,4). The front plate receives the whole, unfractionated energy beam, which produces the standard image (4). This plate and the copper filter select out lower-energy photons such that the back plate receives mostly higher-energy photons (3,4). One weighted subtraction is used to produce a bone-selective image, whereas a different weighted subtraction is used to produce a soft tissue–selective image (2–4). One disadvantage of a single-exposure system is the lower signal-to-noise ratio of the tissue-selective subtraction image (2–5).
In dual-exposure systems, two sequential radiographs are obtained at 60 and 120 kV, respectively (2,4,5). The higher-kilovolt exposure is used to produce the standard image. There is a 200-millisecond delay between the two exposures (4). This delay can create misregistration artifacts on the subtracted images due to slight offsets in the alignment of body structures caused by cardiac, respiratory, bowel, and patient motion (2,4). However, dual-exposure systems produce tissue-selective subtraction images with a better signal-to-noise ratio than those produced with single-exposure systems (2). The dual-exposure technique was used to obtain the images in this article.
Dual-energy subtraction chest images are displayed in a "trio" format that includes a standard unsubtracted image, a soft tissue–selective image, and a bone-selective image. These three images may be viewed in a scrollable stack or side by side for simultaneous comparison (1,4).
Dual-energy subtraction is performed on the posteroanterior view only (2,4). Fischbach et al (5), using a dual-exposure system based on phantom entrance doses, reported that for a standard posteroanterior-lateral chest examination performed digitally with a flat-panel detector, the dose for the lateral view (680 µGy) was approximately six times that for the posteroanterior view (110 µGy); therefore, adding a second posteroanterior exposure for the purpose of dual-energy subtraction increased the total examination dose by only about 14%. Compared with conventional film-screen systems, dual-energy systems require slightly higher radiation doses for the posteroanterior view to maintain the quality and signal-to-noise ratio of the posteroanterior subtraction images (4,6). It has been suggested that, to improve the quality of the posteroanterior subtraction images while limiting radiation dose when performing a dual-energy examination, the dose used for the lateral view be decreased while increasing the dose used for the posteroanterior view by the same amount. Doing so would keep the total radiation dose for the examination within the same range as for a conventional posteroanterior-lateral chest examination (4).
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Advantages
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Dual-energy subtraction imaging allows better visualization of a variety of entities, including nodules, bone lesions, vascular disease, pleural disease, mediastinal and hilar masses, tracheal and airway abnormalities, complex chest disease, and indwelling devices.
Nodules
Dual-energy subtraction removes overlying bone structures to create soft tissue–selective images. Overlying bone structures can obscure lung nodules; thus, soft tissue–selective images allow soft-tissue structures, including nodules that underlie the bone, to be seen more clearly. In addition, dual-energy subtraction generates bone-selective images that allow increased detection of calcium (Figs 1, 2) (1,2,4,5). Consequently, dual-energy subtraction improves the ability to detect both calcified and noncalcified (Fig 3) nodules (1,3,6–11), regardless of the level of experience of the viewing radiologist (1,6,10).
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Figure 1a. (a) Standard unsubtracted image obtained in a 79-year-old woman with a history of smoking shows a possible nodule (arrow) in the right upper lobe. (b) Bone-selective image shows that the "nodule" (arrow) represents the calcified costochondral junction of the first right rib.
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Figure 1b. (a) Standard unsubtracted image obtained in a 79-year-old woman with a history of smoking shows a possible nodule (arrow) in the right upper lobe. (b) Bone-selective image shows that the "nodule" (arrow) represents the calcified costochondral junction of the first right rib.
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Figure 2a. Lung nodule in a 79-year-old man with rales. (a) Standard unsubtracted image shows an indeterminate nodule (arrowhead) in the left lung. (b) Bone-selective image reveals that the nodule (arrowhead) measures 5 mm in diameter and represents a calcified granuloma. No computed tomography (CT) is required for further evaluation of such a nodule.
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Figure 2b. Lung nodule in a 79-year-old man with rales. (a) Standard unsubtracted image shows an indeterminate nodule (arrowhead) in the left lung. (b) Bone-selective image reveals that the nodule (arrowhead) measures 5 mm in diameter and represents a calcified granuloma. No computed tomography (CT) is required for further evaluation of such a nodule.
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Figure 3a. (a) Standard unsubtracted image obtained prior to surgery in a 59-year-old man with a history of smoking. (b) Soft-tissue–selective image reveals a left apical mass (arrow), a finding that is more easily visualized because the overlying bone structures, including the clavicle, have been removed. (c) Axial CT scan helps confirm a left apical lung cancer.
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Figure 3b. (a) Standard unsubtracted image obtained prior to surgery in a 59-year-old man with a history of smoking. (b) Soft-tissue–selective image reveals a left apical mass (arrow), a finding that is more easily visualized because the overlying bone structures, including the clavicle, have been removed. (c) Axial CT scan helps confirm a left apical lung cancer.
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Figure 3c. (a) Standard unsubtracted image obtained prior to surgery in a 59-year-old man with a history of smoking. (b) Soft-tissue–selective image reveals a left apical mass (arrow), a finding that is more easily visualized because the overlying bone structures, including the clavicle, have been removed. (c) Axial CT scan helps confirm a left apical lung cancer.
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Improved accuracy and confidence results in fewer recommendations for follow-up radiography or CT for benign calcified nodules and more expeditious evaluation of noncalcified, potentially malignant nodules (Figs 1–3) (6).
Bone Lesions
Dual-energy subtraction improves the detection and characterization of bone lesions (3,5,12). Dual-energy bone-selective images make bone diseases more conspicuous than they are on conventional radiographs. These diseases include bone metastases (Fig 4); primary bone tumors, both benign and malignant; rib fractures; rib erosions; bone islands; and postsurgical changes (5).
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Figure 4a. (a) Standard unsubtracted image obtained in a 55-year-old man with prostate cancer and possible lung nodules. The image demonstrates what appear to be nodular opacities (arrows) in the lungs. (b) Bone-selective image reveals that the multiple "lung nodules" seen in a represent bone metastases. No lung metastases are present. Note the misregistration artifacts due to cardiac pulsation and motion along the border of the left side of the heart (black streak) and along the aorta (white streak). This type of artifact may be seen on dual-energy subtracted images that are obtained with a dual-exposure technique.
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Figure 4b. (a) Standard unsubtracted image obtained in a 55-year-old man with prostate cancer and possible lung nodules. The image demonstrates what appear to be nodular opacities (arrows) in the lungs. (b) Bone-selective image reveals that the multiple "lung nodules" seen in a represent bone metastases. No lung metastases are present. Note the misregistration artifacts due to cardiac pulsation and motion along the border of the left side of the heart (black streak) and along the aorta (white streak). This type of artifact may be seen on dual-energy subtracted images that are obtained with a dual-exposure technique.
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Vascular Disease
Dual-energy bone-selective images highlight vascular calcifications and aid in the detection of atherosclerotic disease of the coronary arteries, aorta, great vessels, subclavian arteries, and carotid arteries (Figs 5, 6). Dual-energy subtraction also improves the visualization of cardiac calcifications, including valvular, pericardial, and myocardial calcifications (Fig 5).
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Figure 5a. (a) Standard unsubtracted image obtained in an 84-year-old man with congestive heart failure. (b) Bone-selective image more clearly depicts extensive calcification (arrows) in the left ventricular wall. Right-sided pleural calcification is also seen. Note the misregistration artifact line along a pacemaker wire, another example of motion artifact that is seen on dual-energy images obtained with the dual-exposure technique. During the short delay between the two exposures, cardiac pulsation can cause motion of the pacemaker wire and subsequent misregistration during subtraction. (c) Axial CT scan helps confirm myocardial calcification in the left ventricular wall and a left ventricular apical aneurysm (arrow) due to a previous myocardial infarction. Note also the right-sided pleural calcification.
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Figure 5b. (a) Standard unsubtracted image obtained in an 84-year-old man with congestive heart failure. (b) Bone-selective image more clearly depicts extensive calcification (arrows) in the left ventricular wall. Right-sided pleural calcification is also seen. Note the misregistration artifact line along a pacemaker wire, another example of motion artifact that is seen on dual-energy images obtained with the dual-exposure technique. During the short delay between the two exposures, cardiac pulsation can cause motion of the pacemaker wire and subsequent misregistration during subtraction. (c) Axial CT scan helps confirm myocardial calcification in the left ventricular wall and a left ventricular apical aneurysm (arrow) due to a previous myocardial infarction. Note also the right-sided pleural calcification.
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Figure 5c. (a) Standard unsubtracted image obtained in an 84-year-old man with congestive heart failure. (b) Bone-selective image more clearly depicts extensive calcification (arrows) in the left ventricular wall. Right-sided pleural calcification is also seen. Note the misregistration artifact line along a pacemaker wire, another example of motion artifact that is seen on dual-energy images obtained with the dual-exposure technique. During the short delay between the two exposures, cardiac pulsation can cause motion of the pacemaker wire and subsequent misregistration during subtraction. (c) Axial CT scan helps confirm myocardial calcification in the left ventricular wall and a left ventricular apical aneurysm (arrow) due to a previous myocardial infarction. Note also the right-sided pleural calcification.
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Figure 6a. (a) Standard unsubtracted image obtained in a 70-year-old woman with a supraglottic tumor. (b) Bone-selective image demonstrates a thin rim of calcification (arrow) at the edge of a right-sided paratracheal mass, findings that suggest a vascular aneurysm. (c) Axial CT scan shows an aberrant right subclavian artery aneurysm with wall calcification.
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Figure 6b. (a) Standard unsubtracted image obtained in a 70-year-old woman with a supraglottic tumor. (b) Bone-selective image demonstrates a thin rim of calcification (arrow) at the edge of a right-sided paratracheal mass, findings that suggest a vascular aneurysm. (c) Axial CT scan shows an aberrant right subclavian artery aneurysm with wall calcification.
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Figure 6c. (a) Standard unsubtracted image obtained in a 70-year-old woman with a supraglottic tumor. (b) Bone-selective image demonstrates a thin rim of calcification (arrow) at the edge of a right-sided paratracheal mass, findings that suggest a vascular aneurysm. (c) Axial CT scan shows an aberrant right subclavian artery aneurysm with wall calcification.
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Pleural Disease
With dual-energy subtraction, the radiologist can more confidently identify pleural calcifications (Fig 5) and calcification in pleural plaques (Fig 7), thus eliminating the need for CT and allowing a confident diagnosis of asbestos-related pleural plaques to be made without further imaging (4–6).
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Figure 7a. (a) Standard unsubtracted image obtained in a 56-year-old woman with shortness of breath, airway compromise, and partial right middle lobe atelectasis. (b) Bone-selective image shows a calcified pleural plaque on the left hemidiaphragm (right arrow), healed posterior right rib fractures (arrowheads), and a calcified nodule in the right lower lobe (left arrow). (c) Soft-tissue–selective image shows a tracheal stenosis (arrow). (d) Axial CT scan helps confirm a tracheal stenosis at the thoracic inlet.
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Figure 7b. (a) Standard unsubtracted image obtained in a 56-year-old woman with shortness of breath, airway compromise, and partial right middle lobe atelectasis. (b) Bone-selective image shows a calcified pleural plaque on the left hemidiaphragm (right arrow), healed posterior right rib fractures (arrowheads), and a calcified nodule in the right lower lobe (left arrow). (c) Soft-tissue–selective image shows a tracheal stenosis (arrow). (d) Axial CT scan helps confirm a tracheal stenosis at the thoracic inlet.
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Figure 7c. (a) Standard unsubtracted image obtained in a 56-year-old woman with shortness of breath, airway compromise, and partial right middle lobe atelectasis. (b) Bone-selective image shows a calcified pleural plaque on the left hemidiaphragm (right arrow), healed posterior right rib fractures (arrowheads), and a calcified nodule in the right lower lobe (left arrow). (c) Soft-tissue–selective image shows a tracheal stenosis (arrow). (d) Axial CT scan helps confirm a tracheal stenosis at the thoracic inlet.
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Figure 7d. (a) Standard unsubtracted image obtained in a 56-year-old woman with shortness of breath, airway compromise, and partial right middle lobe atelectasis. (b) Bone-selective image shows a calcified pleural plaque on the left hemidiaphragm (right arrow), healed posterior right rib fractures (arrowheads), and a calcified nodule in the right lower lobe (left arrow). (c) Soft-tissue–selective image shows a tracheal stenosis (arrow). (d) Axial CT scan helps confirm a tracheal stenosis at the thoracic inlet.
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Mediastinal and Hilar Masses
Dual-energy soft-tissue–selective images appear similar to traditional tomograms and have some of the same advantages. Soft-tissue–selective images help in detecting size, shape, and changes in opacity in the hila and improve the ability to identify mediastinal and hilar disease (Fig 8).
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Figure 8a. (a) Standard unsubtracted image obtained in a 54-year-old woman with a history of smoking. (b) Soft-tissue–selective image more clearly demonstrates increased opacity (arrow) in the right infrahilar region. (c) Bone-selective image shows a malpositioned left central venous catheter. Arrow indicates the tip of the catheter, located in the neck. Note the artifacts along the aortic arch (white streak), the border of the left side of the heart (black streak), and the left hemidiaphragm and stomach bubble (parallel white and black streaks) due to misregistration during the subtraction process. (d) Axial CT scan shows a right infrahilar mass that proved to be lung cancer.
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Figure 8b. (a) Standard unsubtracted image obtained in a 54-year-old woman with a history of smoking. (b) Soft-tissue–selective image more clearly demonstrates increased opacity (arrow) in the right infrahilar region. (c) Bone-selective image shows a malpositioned left central venous catheter. Arrow indicates the tip of the catheter, located in the neck. Note the artifacts along the aortic arch (white streak), the border of the left side of the heart (black streak), and the left hemidiaphragm and stomach bubble (parallel white and black streaks) due to misregistration during the subtraction process. (d) Axial CT scan shows a right infrahilar mass that proved to be lung cancer.
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Figure 8c. (a) Standard unsubtracted image obtained in a 54-year-old woman with a history of smoking. (b) Soft-tissue–selective image more clearly demonstrates increased opacity (arrow) in the right infrahilar region. (c) Bone-selective image shows a malpositioned left central venous catheter. Arrow indicates the tip of the catheter, located in the neck. Note the artifacts along the aortic arch (white streak), the border of the left side of the heart (black streak), and the left hemidiaphragm and stomach bubble (parallel white and black streaks) due to misregistration during the subtraction process. (d) Axial CT scan shows a right infrahilar mass that proved to be lung cancer.
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Figure 8d. (a) Standard unsubtracted image obtained in a 54-year-old woman with a history of smoking. (b) Soft-tissue–selective image more clearly demonstrates increased opacity (arrow) in the right infrahilar region. (c) Bone-selective image shows a malpositioned left central venous catheter. Arrow indicates the tip of the catheter, located in the neck. Note the artifacts along the aortic arch (white streak), the border of the left side of the heart (black streak), and the left hemidiaphragm and stomach bubble (parallel white and black streaks) due to misregistration during the subtraction process. (d) Axial CT scan shows a right infrahilar mass that proved to be lung cancer.
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Tracheal and Airway Abnormalities
Tracheal abnormalities are easier to detect on images obtained with dual-energy subtraction, which can remove the thoracic spine that overlies and obscures the airway on standard posteroanterior chest radiographs (Fig 7) (3). Soft-tissue–selective images readily demonstrate tracheal stenoses and tracheal narrowings due to extrinsic compression by adjacent masses or adenopathy.
Complex Chest Disease
Removal of overlapping structures on dual-energy subtraction images allows the detection of lesions in difficult-to-see areas (eg, the apices, behind the heart, and beneath the clavicles and ribs) (1,4). Dual-energy subtraction also makes subtle changes more conspicuous in the setting of complex multifocal disease (Fig 9). In addition, temporal subtraction techniques can be used to aid in discerning subtle changes from one examination to the next (1,4).
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Figure 9a. (a, b) Standard unsubtracted images obtained in July 2004 (a) and August 2004 (b) in a 73-year-old woman with non–small cell lung cancer. (c) Soft-tissue–selective image obtained in July 2004 shows extensive meta-static pleural disease. (d) Soft-tissue–selective image obtained in August 2004 shows interval growth of a right hilar mass (arrow) amid the extensive metastatic pleural disease. Dual-energy soft-tissue–selective images help sort out complex, overlapping, and multifocal disease. CT helped confirm an enlarging right hilar mass, along with extensive tumoral involvement of the right pleura.
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Figure 9b. (a, b) Standard unsubtracted images obtained in July 2004 (a) and August 2004 (b) in a 73-year-old woman with non–small cell lung cancer. (c) Soft-tissue–selective image obtained in July 2004 shows extensive meta-static pleural disease. (d) Soft-tissue–selective image obtained in August 2004 shows interval growth of a right hilar mass (arrow) amid the extensive metastatic pleural disease. Dual-energy soft-tissue–selective images help sort out complex, overlapping, and multifocal disease. CT helped confirm an enlarging right hilar mass, along with extensive tumoral involvement of the right pleura.
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Figure 9c. (a, b) Standard unsubtracted images obtained in July 2004 (a) and August 2004 (b) in a 73-year-old woman with non–small cell lung cancer. (c) Soft-tissue–selective image obtained in July 2004 shows extensive meta-static pleural disease. (d) Soft-tissue–selective image obtained in August 2004 shows interval growth of a right hilar mass (arrow) amid the extensive metastatic pleural disease. Dual-energy soft-tissue–selective images help sort out complex, overlapping, and multifocal disease. CT helped confirm an enlarging right hilar mass, along with extensive tumoral involvement of the right pleura.
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Figure 9d. (a, b) Standard unsubtracted images obtained in July 2004 (a) and August 2004 (b) in a 73-year-old woman with non–small cell lung cancer. (c) Soft-tissue–selective image obtained in July 2004 shows extensive meta-static pleural disease. (d) Soft-tissue–selective image obtained in August 2004 shows interval growth of a right hilar mass (arrow) amid the extensive metastatic pleural disease. Dual-energy soft-tissue–selective images help sort out complex, overlapping, and multifocal disease. CT helped confirm an enlarging right hilar mass, along with extensive tumoral involvement of the right pleura.
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Indwelling Devices
Dual-energy subtraction improves the visualization of foreign matter. Often, indwelling devices and foreign bodies are best seen on bone-selective images; these entities include breast implants, surgical clips, catheters and catheter fragments, and vascular stents (including coronary artery stents) (Figs 8 , 10–12).
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Figure 10a. (a) Standard unsubtracted image obtained in a 41-year-old man with rectal cancer and chest pain. (b) Bone-selective image more clearly shows a fractured left central venous catheter with an embolized catheter fragment (arrows) in the left pulmonary artery. Indwelling devices are often more conspicuous and easier to evaluate at dual-energy subtraction imaging.
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Figure 10b. (a) Standard unsubtracted image obtained in a 41-year-old man with rectal cancer and chest pain. (b) Bone-selective image more clearly shows a fractured left central venous catheter with an embolized catheter fragment (arrows) in the left pulmonary artery. Indwelling devices are often more conspicuous and easier to evaluate at dual-energy subtraction imaging.
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Figure 11a. (a) Standard unsubtracted image obtained in a 63-year-old woman with breast cancer. (b) Bone-selective image more clearly demonstrates bilateral breast implants. A coronary artery stent (arrow) is also noted.
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Figure 11b. (a) Standard unsubtracted image obtained in a 63-year-old woman with breast cancer. (b) Bone-selective image more clearly demonstrates bilateral breast implants. A coronary artery stent (arrow) is also noted.
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Figure 12a. (a) Standard unsubtracted image obtained prior to peripheral vascular surgery in a 70-year-old woman. (b) Bone-selective image more clearly shows a metallic wire fragment (arrow) in the region of the superior vena cava.
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Figure 12b. (a) Standard unsubtracted image obtained prior to peripheral vascular surgery in a 70-year-old woman. (b) Bone-selective image more clearly shows a metallic wire fragment (arrow) in the region of the superior vena cava.
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Limitations
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Artifacts
Misregistration artifacts occur on dual-energy subtraction images that are obtained with a dual-exposure technique. As mentioned earlier, the 200-millisecond delay between the two exposures can cause artifacts on the subtracted images due to slight offsets in the alignment of body structures caused by various kinds of motion. These artifacts are seen as black or white lines along the diaphragm, cardiac silhouette, aortic arch, bowel, and pulmonary arteries, as well as along indwelling devices such as pacemaker wires (Figs 4, 5, 8) (4,5). These artifact lines can be seen on one or both subtraction images but are usually most conspicuous on the bone-selective image.
Pitfalls
Dual-energy subtraction images can be misinterpreted. It is possible to fail to properly identify a pulmonary nodule that is not seen on the soft-tissue–selective image, and one should not discount a nodule just because it is seen only on the standard image (6). It has been reported that, with single-exposure systems, occasionally some nodules are more visible on the standard unsubtracted image than on the soft-tissue– or bone-selective image because of the decreased signal-to-noise ratio of the subtracted images obtained with these systems (6,11).
One must also be careful not to misdiagnose a noncalcified nodule as a calcified nodule. Dual-energy subtraction is not foolproof. Occasionally, a calcium-containing structure can be superim-posed on a lung nodule in such a way that the opacity from the calcium appears to be located in the nodule, when in fact the nodule is not calcified and may not be benign (Fig 13). In addition, with dual-exposure systems, misregistration motion artifacts seen around small, tortuous pulmonary vessels can be mistaken for small calcified nodules (5).
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Figure 13a. (a) Standard unsubtracted image obtained in a 53-year-old man with colon cancer who had undergone right pneumonectomy. (b) Soft-tissue–selective image reveals a nodule in the left lower lobe. (c) On a bone-selective image, the nodule (arrow) appears to be calcified, a finding that could lead to misdiagnosis of the nodule as a benign lesion. (d) Axial CT scan shows that the nodule is not calcified, but, in fact, represents a metastasis. The appearance of calcification within the nodule in c may have been due to the calcified costochondral junction superimposed over the nodule.
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Figure 13b. (a) Standard unsubtracted image obtained in a 53-year-old man with colon cancer who had undergone right pneumonectomy. (b) Soft-tissue–selective image reveals a nodule in the left lower lobe. (c) On a bone-selective image, the nodule (arrow) appears to be calcified, a finding that could lead to misdiagnosis of the nodule as a benign lesion. (d) Axial CT scan shows that the nodule is not calcified, but, in fact, represents a metastasis. The appearance of calcification within the nodule in c may have been due to the calcified costochondral junction superimposed over the nodule.
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Figure 13c. (a) Standard unsubtracted image obtained in a 53-year-old man with colon cancer who had undergone right pneumonectomy. (b) Soft-tissue–selective image reveals a nodule in the left lower lobe. (c) On a bone-selective image, the nodule (arrow) appears to be calcified, a finding that could lead to misdiagnosis of the nodule as a benign lesion. (d) Axial CT scan shows that the nodule is not calcified, but, in fact, represents a metastasis. The appearance of calcification within the nodule in c may have been due to the calcified costochondral junction superimposed over the nodule.
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Figure 13d. (a) Standard unsubtracted image obtained in a 53-year-old man with colon cancer who had undergone right pneumonectomy. (b) Soft-tissue–selective image reveals a nodule in the left lower lobe. (c) On a bone-selective image, the nodule (arrow) appears to be calcified, a finding that could lead to misdiagnosis of the nodule as a benign lesion. (d) Axial CT scan shows that the nodule is not calcified, but, in fact, represents a metastasis. The appearance of calcification within the nodule in c may have been due to the calcified costochondral junction superimposed over the nodule.
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Caution must also be exercised to avoid misinterpreting a partially or eccentrically calcified nodule as benign. As with conventional radiography, the pattern of calcification in a nodule seen on dual-energy subtraction images must conform to a benign pattern for the nodule to be deemed benign. Eccentrically or partially calcified lesions may still be malignant.
Other Limitations
Dual-energy subtraction, which is performed only on the posteroanterior view, does not obviate lateral imaging (Fig 14). Some abnormalities are identified or localized only on the lateral view. Dual-energy subtraction is most helpful when a lung lesion is obscured by overlying bone structures. If a nodule is obscured by an overlapping soft-tissue structure, dual-energy subtraction may not be beneficial. Finally, dual-energy subtraction techniques are not used for portable chest radiography.
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Figure 14a. (a, b) Standard unsubtracted (a) and bone-selective (b) images obtained in a 39-year-old man with a heart murmur. (c) Lateral image demonstrates aortic valve calcifications (arrow) associated with aortic stenosis. Dual-energy subtraction is performed on the posteroanterior view only. However, this does not eliminate the need for a lateral view, which alone depicted the aortic valve calcifications in this case.
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Figure 14b. (a, b) Standard unsubtracted (a) and bone-selective (b) images obtained in a 39-year-old man with a heart murmur. (c) Lateral image demonstrates aortic valve calcifications (arrow) associated with aortic stenosis. Dual-energy subtraction is performed on the posteroanterior view only. However, this does not eliminate the need for a lateral view, which alone depicted the aortic valve calcifications in this case.
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Figure 14c. (a, b) Standard unsubtracted (a) and bone-selective (b) images obtained in a 39-year-old man with a heart murmur. (c) Lateral image demonstrates aortic valve calcifications (arrow) associated with aortic stenosis. Dual-energy subtraction is performed on the posteroanterior view only. However, this does not eliminate the need for a lateral view, which alone depicted the aortic valve calcifications in this case.
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Indications
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Suggested indications for dual-energy subtraction imaging include preoperative or pre–organ transplantation evaluation; confirmed cancer or a history of cancer; an indeterminate pulmonary nodule; a history of smoking; and a history of tuberculosis or a positive skin test.
Because of the incremental increase in radiation dose, we generally do not perform a dual-energy subtraction chest examination at our institution if the patient has undergone such an examination or chest CT within the past 30 days. We also do not perform dual-energy subtraction imaging in patients 16 years of age or younger.
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Conclusions
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Dual-energy subtraction chest radiography improves the radiologist’s ability to detect and accurately diagnose a wide variety of chest lesions. The major advantage of this technique is that it makes calcification more conspicuous, an essential aid in characterizing pulmonary nodules. Dual-energy subtraction images are also helpful in recognizing hilar and mediastinal masses; detecting tracheal narrowing and vascular disease; identifying bone, pleural, and chest wall abnormalities; and localizing stents, catheters, and other indwelling devices. Despite its many advantages, however, dual-energy subtraction imaging has some limitations of which the radiologist should be aware and requires a slightly higher radiation dose than does conventional radiography.
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References
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- Dobbins JT, Warp RJ. Dual-energy methods for tissue discrimination in chest radiography. In: Samei E, Flynn MJ, eds. RSNA categorical course in diagnostic radiology physics: advances in digital radiography. Oak Brook, Ill: Radiological Society of North America, 2003; 173–179.
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B. F. Branstetter IV
Basics of Imaging Informatics: Part 1
Radiology,
June 1, 2007;
243(3):
656 - 667.
[Abstract]
[Full Text]
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Abstract
Introduction
