Dual-energy Subtraction Chest Radiography

PICTORIAL ESSAY Dual-energy Subtraction Chest Radiography Application in Cardiovascular Imaging Kianoush Ansari-Gilani, MD,* Yasmeen K. Tandon, MD,* David W. Jordan, PhD,† Leslie Ciancibello, RT,† David L. Wilson, PhD,† and Robert C. Gilkeson, MD* Downloaded from http://journals.lww.com/thoracicimaging by BhDMf5ePHKbH4TTImqenVHwXmMsAVI5gi362E2/Y11FAr5wmpB+5Cdu0wtIm6Dxa on 07/02/2020 Abstract: The chest radiograph is the most frequently performed imaging in radiology and by including the heart and central vessels can suggest the presence of cardiovascular disease. Dual-energy subtraction radiography of the chest provides improved detection of a wide variety of cardiovascular pathologies including coronary artery disease, valvular pathologies, and pericardial disease given the presence of calcification in many subtypes of these diseases. We review the principles of dual-energy subtraction radiography and demonstrate its added value in the assessment of cardiovascular disease. Key Words: dual-energy, subtraction, chest radiograph, cardiovascular (J Thorac Imaging 2020;35:W75–W81) D ual-energy subtraction radiography (DESR) of the chest has been used for the assessment of noncalcified and calcified chest pathologies of both cardiovascular or noncardiovascular origin.1 Noncardiovascular applications include better detection of lung nodules, hilar and mediastinal masses, and tracheal lesions. Assessment of bone, pleural, and chest wall pathologies has also been discussed in the literature.1,2 The presence of calcium is a well-recognized biomarker of cardiovascular disease. Coronary artery disease, aortic and mitral valve stenosis, and constrictive pericarditis are characterized by the presence of calcium. DESR enables detection of calcified structures with high sensitivity,3 which can be therefore advantageous in detecting abnormal calcium in the detection of cardiovascular disease. Although computed tomography (CT) is almost universally available, its higher radiation dose and relative cost limits its routine utility as a screening tool.3 Even though DESR is not as widely available as conventional radiography, its lower cost and lower radiation dose compared with CT can provide a promising future for more widespread use in the assessment of the diseases. In this article, we review the basic principles and physics of DESR. The 2 types of dual-energy radiography systems with their advantages and disadvantages will be discussed. We also discuss and illustrate the advantages of this imaging modality in the assessment of cardiovascular From the *University Hospital Cleveland Medical Center; and †Department of Radiology, University Hospitals Cleveland Medical Center & Case Western Reserve University, Cleveland, OH. The authors declare no conflicts of interest. Correspondence to: Kianoush Ansari-Gilani, MD, Radiology Department, University Hospital Cleveland Medical Center, 11100 Euclid Avenue, Cleveland, OH 44124 (e-mail: [email protected]). Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved. DOI: 10.1097/RTI.0000000000000472 J Thorac Imaging  Volume 35, Number 3, May 2020 pathologies. Some of the limitations and artifacts associated with DESR will also be reviewed. BASIC PRINCIPLES AND PHYSICS In dual-energy radiography, tissue decomposition is carried out on the basis of using 2 different x-ray energies to exaggerate the difference between 2 different materials, which otherwise have significant overlap in imaging appearance. As linear attenuation coefficients (LACs) are energy-dependent, using 2 different energies results in 2 different LACs, which can be used to decompose the imaged structures (Fig. 1).4 DESR is based on the differences in the degree of body tissue attenuation of low-energy and high-energy photons. As a general rule, tissues containing elements that have higher atomic number (such as bone with large amount of calcium, which has a higher atomic number) attenuate x-ray beams more efficiently than tissues that contain lower atomic number elements (eg, soft tissue which has lower atomic number elements such as oxygen, hydrogen, carbon, and nitrogen). Because of the different LACs mentioned above, low-energy photons are more attenuated than the high-energy photons in different body tissues, and this is especially more pronounced in bone that has calcium with higher atomic number and therefore has higher attenuation coefficient (Fig. 1). The higher differential attenuation of bone as a function of photon energy, as compared with soft tissue, allows the ability to decompose the 2 images taken at different x-ray energies into soft tissue only and bone-only images.5,6 This is carried out by calculating the amount of calcium in any imaged pixel and then subtracting it from the original image to produce the bone and soft tissue–specific information in the imaged pixel. TYPES OF DUAL-ENERGY RADIOGRAPHY SYSTEMS Currently, there are 2 clinical systems available for dual-energy radiography5,6: Single-exposure system: in this system, which is also called sandwich or passive detector system, one radiograph is obtained by exposing 2 storage phosphor plates that are separated by a copper filter.1 Copper filter absorbs lower x-ray energies. The imaging plates are stacked and aligned, and therefore, with one exposure, 2 images are obtained. This eliminates motion artifact. As the energy separation in this system solely relies on the detectors and as only one exposure is used (with no major variation in the energy of exposed beam), the energy separation between the 2 image pairs remains small, and therefore signal to noise ratio is relatively low at typical patient exposure (Fig. 2). Dual-exposure system: in this system, a flat-panel detector with a fast readout capability is used. The first acquisition www.thoracicimaging.com Copyright r 2020 Wolters Kluwer Health, Inc. All rights reserved. | W75 Ansari-Gilani et al J Thorac Imaging  Volume 35, Number 3, May 2020 especially of the aorta. The energy separation, on the other hand, and therefore signal to noise ratio for any given patient exposure is relatively higher in this technique, as, by default, 2 different exposures using two sets of energies are used (Fig. 3). We used the dual-exposure technique to obtain the images in this article. Dual-energy subtraction chest radiographs are displayed as 3 separate images (standard or conventional, soft tissue–selective image, and bone-selective image) and always carried out in posterior-anterior (PA) projection. A standard lateral radiograph is usually added (total of 4 images). It has been suggested to reduce the dose of the lateral view to compensate for a slight increase in radiation dose caused by dual exposure.1,7 CARDIOVASCULAR APPLICATIONS FIGURE 1. The effect of the difference in attenuation coefficient on the attenuation of high and low-energy beams. There is much greater attenuation of bone at low energies (60 kVP) and almost similar attenuation of bone and soft tissue at higher energies (120 kVp). The effective energy of a spectrum is equal to the energy of a monoenergetic x-ray photon with the same overall attenuation. Shown are typical x-ray “effective energies” for spectra generated at 60 kVp (green) and 12 kVp (purple). occurs with the high (120 kV) energy beam, and then a fast image readout is performed. This is immediately followed by the low (60 kV) energy beam acquisition, and then another fast image readout. An ∼200-ms delay is present between the 2 exposures/readouts,7 which can result in motion artifact due to voluntary and involuntary patient motion including respiratory motion, heart motion, and pulsation of the great vessels, DESR can be used in the detection of cardiovascular and noncardiovascular pathologies. Noncardiovascular applications include better detection of calcified and noncalcified lung nodules, hilar and mediastinal masses, tracheal lesions, and bone, pleural, and chest wall pathologies and have been previously discussed in the literature.1,2 We will discuss the cardiovascular application of DESR in this article. As most of the radiographically detectable cardiovascular pathologies have calcification, bone selective images are the most sensitive to depict those pathologies. Valvular Disease Aortic valve stenosis is the most common valvulopathy. Whether it is caused by a degenerative process in the tricuspid or bicuspid aortic valve, calcification is a constant feature of the disease.8 Multiple studies have established aortic valve calcification as a sensitive marker of aortic valve stenosis. There is a strong correlation between the amount of aortic valve calcium quantified on CT and the echocardiographic assessment of aortic valve stenosis.9,10 The bone-selective image of DESR is able to better depict aortic valve calcification (Fig. 4) when compared with the standard image. However, as calcification of the aortic FIGURE 2. Single-exposure system: x-ray generator produces a single-energy beam (120 kVp). Two image plates are used with a copper filter sandwiched between the 2 layers. This produces 2 sets of images: low-energy image by the front plate and high-energy image by the backplate. The low-energy image has higher bone-tissue contrast than the high-energy image. For the final images, either the bone signal is nulled for a “soft tissue only” image or the soft tissue is nulled for a “bone-only” image. W76 | www.thoracicimaging.com Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved. Copyright r 2020 Wolters Kluwer Health, Inc. All rights reserved. J Thorac Imaging  Volume 35, Number 3, May 2020 Cardiovascular Applications of Dual-energy Radiographs FIGURE 3. Dual-exposure system: x-ray generator produces two beams (60 and 120 kVp) and 2 image acquisitions. The first acquisition occurs with the low (60 kVp) energy beam, then by low-energy image readout. This is followed immediately by the high (120 kVp) energy beam acquisition, then by high-energy image readout. Approximately, a 200-ms delay is present between the 2 exposures/ readouts (65+135 ms). The low-energy image has higher bone-tissue contrast than the high-energy image. Again, for the final images, either the bone signal is nulled for a “soft tissue only” image or the soft tissue is nulled for a “bone-only” image. valve on the PA view usually overlaps with thoracic vertebral bodies and sternum, the distinction of the 2 can be challenging. Slightly oblique images or the presence of scoliosis in the thoracic spine improves detection of aortic valve calcification (Fig. 4E). Mitral annular calcification (MAC) is a chronic process involving the fibrous annulus of the mitral valve. It is a very common finding in the older population.11 Although annular calcification is associated with mitral regurgitation, the amount of regurgitation is usually trivial, and most patients are asymptomatic.11 Contrary to aortic valve calcification, calcification in the mitral valve annulus is not significantly associated with mitral valve stenosis.12 In contrast to the aortic valve calcification, MAC can be more readily seen on PA bone selective image due to more lateral location of the mitral valve (Fig. 5). MAC is a very common entity with an estimated prevalence of 10%.13 It appears as a semicircular calcification on radiograph due to more severe involvement of the posterior annulus. Unlike typical MAC, caseous mitral annulus calcification is more commonly symptomatic, causing mitral stenosis, mitral regurgitation, left ventricular outflow obstruction, or systemic embolization.1,14 It can be better characterized on DESR, demonstrating a more mass-like appearance on imaging (Fig. 6). Mitral valve leaflet calcification, on the other hand, is less common and may be found in patients with a history of rheumatic fever, noninflammatory calcific disease, and chronic renal failure, and it is difficult to be seen on radiograph.15 Coronary Artery Disease Coronary artery calcification is associated with increased risk for a major adverse cardiac event.16 The coronary calcium score is a strong predictor of coronary artery disease and coronary events and provides predictive information beyond that provided by standard risk factors.17 Although coronary artery calcification is rarely seen on a standard chest radiograph,18 bone-selective image of DESR can improve the detection of coronary artery calcification. DESR has shown promising results in detecting patients at high risk of cardiovascular disease and with high FIGURE 4. A 21-year-old man with shortness of breath. Conventional chest radiograph (A) is unremarkable. Bone-selective image (B) shows a subtle area of calcification in the expected location of the aortic valve (red arrow). There is a curvilinear black area near the left ventricular apex (blue arrow), which is due to pulsation/motion artifact. Reconstructed coronal gated computed tomography of the chest (C) confirms calcification in the aortic valve (arrow). Parasternal long-axis transthoracic echocardiogram with color (D) shows aliasing at the level of the aortic valve (arrow) due to the gradient across the aortic valve confirming the presence of aortic stenosis. The presence of aortic valve calcification is better appreciated in the presence of thoracic scoliosis (arrow in E), as there is less overlap of the aortic valve and thoracic spine. Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved. www.thoracicimaging.com Copyright r 2020 Wolters Kluwer Health, Inc. All rights reserved. | W77 Ansari-Gilani et al J Thorac Imaging  Volume 35, Number 3, May 2020 FIGURE 5. A 64-year-old man for routine assessment before renal transplant. Conventional chest radiograph (A) is unremarkable. Boneselective image (B) shows a curvilinear area of calcification in the expected location of the mitral valve (arrow) in keeping with mitral annulus calcification. Coronal reconstructed gated CT of the chest (C) shown in maximum intensity projection confirms the presence of significant mitral annulus calcification (arrow). FIGURE 6. A 75-year-old man with caseous mitral annulus calcification (CMAC) and severe mitral regurgitation. Conventional chest radiograph (A) shows an enlarged cardiac silhouette size with no other significant abnormal findings. Bone-selective image (B) shows a round mass-like calcification (arrow), which corresponds to the patient’s known CMAC. C, Transesophageal echocardiogram in midesophageal 4-chamber view shows a mass-like echogenicity attached to the posterior aspect of the annulus and posterior mitral valve (*), causing significant posterior shadowing (arrows) due to calcification and in keeping with patients with known CMAC. FIGURE 7. An 87-year-old man with severe 3-vessel coronary artery disease. Conventional chest radiograph (A) is unremarkable except for mild cardiomegaly. Bone-selective image (B) shows extensive coronary artery calcifications. This includes a short area of calcification in the left main coronary artery (red arrow), a slightly curvilinear but horizontal calcification in the left anterior descending coronary artery (blue arrow), and slightly curvilinear but vertical calcification in the left circumflex artery (yellow arrow). Right coronary artery calcification is projecting over the spine and has a vertical/oblique course (green arrow). W78 | www.thoracicimaging.com Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved. Copyright r 2020 Wolters Kluwer Health, Inc. All rights reserved. J Thorac Imaging  Volume 35, Number 3, May 2020 Cardiovascular Applications of Dual-energy Radiographs FIGURE 8. A 55-year-old man with a history of coronary artery disease status-post percutaneous coronary intervention. Conventional chest radiograph (A) is unremarkable. Bone-selective image (B) shows the left anterior descending coronary artery stent (arrow). FIGURE 9. A 57-year-old man with a prior history of pericarditis. Conventional chest radiograph (A) shows right lower lung consolidation and pleural effusion with an enlarged cardiac silhouette. Bone-selective image (B) shows a linear area of calcification in the inferior aspect of the cardiac silhouette (arrow), which corresponds to pericardial calcification inferior to the right ventricle in the reconstructed coronal nongated computed tomography of the chest (C, arrow). FIGURE 10. A 61-year-old woman with a known history of pulmonary artery hypertension. Conventional chest radiograph (A) shows enlarged cardiac silhouette and prominent main and right pulmonary arteries (arrows). Bone-selective image (B) shows calcification in the right main pulmonary artery (red arrows) in keeping with the patient’s known pulmonary artery hypertension. Aortic arch calcification (blue arrow in B) is also visible. Dark lines adjacent to the border of the right atrium, left ventricles, and aortic arch (yellow arrows in B) are due to pulsation/motion artifact. Coronal reconstructed CT of the chest (C) shown in maximum intensity projection confirms the presence of calcification in the pulmonary artery (red arrows) in keeping with the patient’s known pulmonary artery hypertension. Aortic arch calcification can also be seen (blue arrow in C). Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved. www.thoracicimaging.com Copyright r 2020 Wolters Kluwer Health, Inc. All rights reserved. | W79 Ansari-Gilani et al J Thorac Imaging  Volume 35, Number 3, May 2020 FIGURE 11. A 52-year-old man with a long history of end-stage renal disease and hemodialysis. Conventional chest radiograph (A) shows left more than right basilar opacities with left pleural effusion. Bone-selective image (B) shows significant improvement in visualization of a femoral approach central venous catheter, which is terminating in the expected location of the junction of the superior vena cava (SVC) and right atrium (red arrow). There is also a focal calcification in SVC (blue arrow in B), which can be better seen in axial noncontrast computed tomography of the chest (arrow in C) and in keeping with patients with known chronic SVC occlusion. calcium score3; however, its role in patients with lower calcium score is unclear.3 The location and course of the calcification, as seen on chest radiograph, can be helpful in locating the abnormal coronary artery. The left main coronary artery is usually seen as a small area of linear calcification adjacent to the left side of the thoracic spine, while left anterior descending and circumflex calcification are respectively seen as horizontal and vertical curvilinear calcifications to the left side of the spine (Fig. 7). Right coronary artery calcification is seen as a curvilinear area of calcification with a horizontal/oblique course overlying the thoracic spine or immediately to the right of the spine (Fig. 7). Coronary artery stents are also better seen on the boneselective image of DESR given their metallic content (and therefore higher atomic number) (Fig. 8). Pericardial and Myocardial Calcification Pericardial calcification is more frequently detected due to more common use of chest CT and can be due to more common causes such as cardiac surgery or radiation treatment versus less common causes such as viral pericarditis, trauma, malignancy, and connective tissue diseases.19 Tuberculosis is a less frequent cause in developed countries. It can be associated with constrictive pericarditis.19 Pericardial calcification is seen more commonly adjacent to the right ventricle and right atrioventricular groove20 (Fig. 9). Myocardial calcification is usually seen as a sequela of prior infarction, especially in the presence of myocardial aneurysm or pseudoaneurysm, and is seen more commonly over the left ventricle.20 While standard PA or AP (anterior-posterior), chest radiograph can show pericardial or myocardial calcification, and bone-selective image of DESR can increase the conspicuity of the calcification in more subtle cases (Fig. 9). Miscellaneous Cardiovascular Applications Vascular, either arterial or venous, calcification including pulmonary artery calcification can also be better seen on the bone-selective image of DESR (Fig. 10). Calcified venous or arterial thrombus and lines and tubes are also better seen on bone-selective images (Fig. 11). LIMITATIONS AND PITFALLS Misregistration artifact can be seen on dual-energy subtraction images that are obtained by dual-exposure technique. This is due to 200 ms delay between the 2 exposures/readout and, as can be predicted, happens more frequently adjacent to moving organs such as the diaphragm, heart, aorta, or pacemaker wires1,7 (Figs. 4, 10). Misregistration artifacts can be seen as white or black lines and are most pronounced on the bone-selective images. Reviewing the standard image is helpful to understand the cause of these extra lines/interfaces and to correctly interpret the study. Respiratory motion artifact can also occur in DESR. While a generalized haziness is seen on respiratory motion degraded conventional image, the effect of motion would be exaggerated and more pronounced on subtracted images. Again, reviewing the conventional image is helpful to better interpret the study in these scenarios. DESR is associated with a slight increase in radiation dose.1 It cannot be applied to the lateral chest radiographs or portable x-rays. The increase in radiation dose, however, is minimal.1 Another limitation associated with DESR systems is their limited availability, as compared with ubiquitously available conventional radiography units. The cost of obtaining a radiograph using this system can also be potentially slightly higher, which might limit its use. As described above, the detection of midline calcified pathologies can be challenging due to the overlapping thoracic spine and sternum. CONCLUSIONS Dual-energy subtraction chest radiography improves the radiologist’s ability to detect and more accurately diagnose a wide variety of cardiothoracic pathologies, both cardiovascular and noncardiovascular in origin. Bone-selective images make the calcification more conspicuous and better delineate many calcified cardiovascular pathologies. This technique, however, has some limitations including misregistration artifact and slightly higher radiation dose. REFERENCES 1. Kuhlman JE, Collins J, Brooks GN, et al. 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