AJR 2004; 183:1233-1238
© American Roentgen Ray Society
Digital Radiography with Dual-Energy Subtraction: Improved Evaluation of Cardiac Calcification
Robert C. Gilkeson1,
R. D. Novak and
Peter Sachs
1 All authors: Department of Radiology, University Hospitals of Cleveland, 11100
Euclid Ave., Cleveland, OH 44106-5000.
Received December 22, 2003;
accepted after revision March 23, 2004.
Address correspondence to R. C. Gilkeson.
Abstract
OBJECTIVE. The objective of our study was to describe cardiac
applications for digital radiography with dual-energy subtraction.
CONCLUSION. Dual-energy subtraction digital radiography offers
potentially important new information in the assessment of coronary artery
disease.
Introduction
Cardiovascular disease is the leading cause of death in the United
States, responsible for approximately 500,000 deaths per year. More than 1
million Americans have angina or heart attacks every year. The increasing
incidence of cardiovascular disease makes accurate and noninvasive imaging of
early cardiovascular disease increasingly important. The relationship between
coronary artery calcification and atherosclerotic heart disease has been well
established [1]. A large body
of literature about the detection of cardiac calcification with standard
film-screen technology and fluoroscopic techniques exists. Although those
techniques have high positive predictive values for the detection of calcium,
they have limited sensitivity for the detection of coronary artery disease
[2]. The enhanced capabilities
of electron beam CT and, more recently, of MDCT to detect coronary calcium
have established these techniques as potential screening tools for coronary
artery disease [3]. Although
these sophisticated techniques have shown considerable promise in the
evaluation of coronary artery disease in high-risk populations, whether
screening the general population with these advanced technologies will be
cost-effective is still unclear.
Along with marked advances in cross-sectional imaging techniques,
considerable improvements in digital radiography have also occurred during the
past decade. Computed radiography and, more recently, digital radiography have
markedly improved imaging of cardiothoracic disease
[4]. Digital technology has
also enabled the use of dual-energy techniques in both computed radiography
and digital radiography systems. With recent advancements in digital
radiography and flat-panel technology, dual-energy subtraction techniques can
produce a conventional high-peak-kilovoltage image and a low-peak-kilovoltage
image with a 200-msec temporal separation. Postprocessing of these two images
results in the presentation of the standard high-peak-kilovoltage image, a
subtracted soft-tissue image that removes overlying bones from the underlying
lung and mediastinum, and a low-energy bone image that optimally displays bone
and calcified thoracic structures
[5].
Research with single-exposure dual-energy subtraction computed radiography
has shown that the subtracted soft-tissue image improves the detection of
parenchymal lung nodules [6].
Our experience has shown that the detection of calcified cardiothoracic
structures is also markedly improved on the low-energy bone image. Although
this capability was initially realized in improved detection of skeletal
abnormalities, we have recently seen substantial improvement in the detection
of cardiac calcification. In addition to being helpful in the detection of
valvular and myocardial calcification, the subtracted bone image is
particularly useful in the detection of coronary artery calcification. In this
report, we summarize our experience with the use of direct digital radiography
with dual-energy subtraction in the improved detection of coronary artery
calcification.
Materials and Methods
A retrospective analysis of patients undergoing direct digital radiography
with dual-energy subtraction was performed from April through September 2003.
Patients were imaged using a direct digital radiography unit (Revolution XRd,
GE Healthcare). With dual-energy subtraction, a conventional 120-kVp image
(high-energy image) was taken. After a 150-msec delay, a second 60-kVp image
(low-energy image) was obtained. After postprocessing of the two images, a
standard 120-kVp image, a subtracted soft-tissue image, and a subtracted bone
image were presented. Hard- and soft-copy images were available for review,
and images were evaluated by one of two dedicated chest radiologists. The
standard posteroanterior and lateral radiographs were interpreted first
followed by analysis of the subtracted soft-tissue and bone images. In
patients with suspected cardiac calcification on the low-energy bone image,
the radiologist was asked to rate visualization of coronary calcification on
the subtracted bone image as equivalent to, better than, or worse than on the
standard posteroanterior and lateral images.
We originally identified a group of 42 patients with findings suspicious
for cardiac calcification on the dual-energy subtracted bone image. In this
initial population of 42 subjects, 33 patients had also undergone MDCT
evaluation of the chest within 6 months of chest radiography. All CT scans
were non-ECG-gated studies using 4- or 16- MDCT. The CT examinations were
obtained for a variety of clinical indications using imaging protocols that
varied considerably in slice thickness, radiographic technique, and the
presence or absence of IV contrast material. The CT studies were analyzed for
the presence of coronary artery, valvular, or myocardial calcification. These
33 patients undergoing both dual-energy subtraction digital radiography and
chest CT were then selected for final analysis. The study group consisted of
14 women and 19 men who ranged in age from 40 to 92 years. The patients had
undergone chest radiography for a variety of reasons including preoperative
surgical evaluation, pneumonia, and chest pain.
Results
Of the 33 patients with cardiac calcification seen on dual-energy
subtraction radiography, all showed evidence of coronary artery calcification;
the calcifications were identified in the left coronary artery distribution in
all the patients (Figs. 1A,
1B) and also in a right
coronary artery distribution in three patients (Figs.
2A,
2B). Furthermore, three
patients also showed evidence of valvular-myocardial calcification: aortic
calcification in one patient, extensive mitral annulus calcification in one
patient (Figs. 3A,
3B), and myocardial
calcification in addition to coronary artery calcification in one patient.
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Fig. 1B. —67-year-old man who underwent dual-energy subtraction
radiography for preoperative evaluation. Subtracted bone image shows evidence
of linear calcification (arrow) in distribution of left coronary
artery, which is consistent with coronary artery calcification.
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Fig. 2B. —58-year-old woman who underwent chest radiography at
admission. Subtracted low-energy bone image shows extensive coronary artery
calcification (arrow) in left coronary artery distribution. Note
extensive right coronary artery calcium (arrowhead) that is not
visible on standard posteroanterior chest radiograph (A).
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Fig. 3A. —78-year-old woman with chest pain and dyspnea. Conventional
posteroanterior digital radiograph shows cardiomegaly and pulmonary edema.
Question of whether area of calcification (arrow) is present in
region of mitral valve is raised.
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Fig. 3B. —78-year-old woman with chest pain and dyspnea. Subtracted
low-energy bone image shows extensive mitral annulus (arrow) and
coronary artery (arrowhead) calcium. Note improved visualization on
subtracted bone image compared with A.
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Of the 33 patients with suspected cardiac calcification on dual-energy
digital radiography, all 33 had confirmed CT evidence of cardiac
calcification. Suspected coronary artery calcification on dual-energy digital
radiography was also confirmed in all 33 patients on CT (Figs.
4A,
4B,
4C), and valvular and
myocardial calcifications were proven in three of the 33 patients. CT
evaluation confirmed that all patients had coronary artery calcification in
both the left and right coronary artery distributions.
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Fig. 4C. —57-year-old man undergoing preoperative assessment for
coronary artery bypass graft surgery. Multiplanar reconstruction of CT scan
shows calcium (arrow) in left coronary artery distribution
corresponding to that shown in B.
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When visualization of coronary artery calcification on the subtracted bone
image was compared with that on the conventional image, all 33 cases were
identified on the bone images, whereas only eight of the 33 cases were
prospectively identified on the standard images. These proportions differed
significantly (z = 6.42; p < 0.0001), which indicates superior
identification of coronary artery calcium using the bone image
[7]. Furthermore, of the eight
patients with coronary calcium identified on the conventional image, the
radiologist preferred the bone image for visualization of calcification in
seven (87.5%) of these eight cases.
Discussion
The association of coronary calcium with atherosclerotic disease has been
well established in the clinical and pathology literature
[7]. Because coronary artery
disease is the leading cause of morbidity and mortality in the United States,
an inexpensive noninvasive screening tool for the detection of coronary
calcium has been a major focus of interest for clinicians and researchers
involved in the assessment of cardiovascular disease. An extensive body of
literature has shown that coronary artery calcium is an independent predictor
of cardiac disease [8].
Furthermore, angiographically normal coronary arteries have lower amounts of
coronary artery calcium than diseased segments, and the amount of calcium is
predictive of the degree of stenosis
[9].
There is a large body of literature that focuses on the detection of
coronary calcium on standard film-screen radiography. Although the positive
predictive value of coronary artery disease when calcium is seen on
conventional radiography is encouragingly high, the sensitivity of
conventional radiography for this purpose is poor. In one study, the
sensitivity of standard chest radiography was only 42%
[10]. In a study by Agatston
[11], sensitivity of
conventional radiography was only 52% compared with electron beam CT. Analysis
of the areas of calcification also showed that visualization of coronary
calcium requires a threshold measurement of 522 H on chest radiography
compared with 99 H on CT.
Cardiac fluoroscopy has also been used to detect calcium. Large-population
studies indicate a correlation between calcium detected on cardiac fluoroscopy
and coronary artery disease
[12]. Although this research
shows encouraging data for the positive predictive value of fluoroscopy in the
detection of coronary artery calcium, correlation with angiography shows that
fluoroscopy is an insensitive method in predicting angiographically
significant coronary artery disease
[13].
Dual-energy radiography has a number of potentially important applications
in the evaluation of cardiothoracic disease. Dual-energy subtraction
techniques have been used in the evaluation of pulmonary nodules
[6], asbestos-related pleural
plaques [14], and breast
microcalcifications [15].
Several early reports have also described efforts to improve evaluation of
coronary artery calcification with dual-energy techniques. In a phantom study
performed by Molloi et al.
[16], dual-energy subtraction
techniques were used during videofluoroscopy to evaluate coronary calcium in
both phantoms and excised segments of human coronary artery. The subtracted
dual-energy data showed excellent sensitivity in the detection of coronary
calcium and accurate quantification of the amount of coronary artery calcium.
A different dual-energy subtraction technique was used by Detrano et al.
[17] to improve the detection
of coronary artery calcification on fluoroscopy. In that article, the authors
described improved detection of moving structures (i.e., calcified segments of
coronary artery) during fluoroscopy by subtracting stationary calcified
structures such as the ribs and spine. Although those researchers reported
encouraging results, the technique has not found widespread clinical
application.
Our work using dual-energy subtraction techniques with digital radiography
has shown exciting preliminary results, but additional prospective studies are
required to establish its future clinical validity. Our early experience
indicates that further understanding and optimization of dual-energy
technology are needed to reproducibly visualize cardiac calcification (Figs.
5A,
5B,
5C). Although this early
experience suggests that dual-energy technology may be able to detect
relatively low levels of coronary calcium on the basis of a CT-derived
coronary artery calcium score (Figs.
6A,
6B,
6C), the nongated nature and
variable imaging protocols of the CT scans obtained in this study did not
allow absolute quantification of coronary artery calcium. Limitations of the
present dual-energy technique is evidenced by the difference in visualization
of right and left coronary artery calcifications on dual-energy radiography,
despite the fact that all patients had CT evidence of coronary artery
calcification in both coronary arteries.
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Fig. 5C. —40-year-old with history of thoracic radiation and
cardiomyopathy. Subtracted left anterior oblique low-energy bone image shows
extensive curvilinear calcification (arrows) in left atrial
distribution.
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Fig. 6B. —57-year-old man undergoing preoperative chest radiography.
Subtracted bone image shows subtle linear density (arrow) in region
of left coronary artery distribution, consistent with coronary artery
calcification.
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Fig. 6C. —57-year-old man undergoing preoperative chest radiography.
Representative axial non-ECG-gated MDCT scan with cursors placed around left
coronary artery for coronary artery calcium evaluation shows calcified plaque
in left coronary artery. Coronary artery calcium score was 78. LAD = left
anterior descending artery.
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Additional work is needed to define specific quantitative thresholds for
the detection of coronary calcification on dual-energy subtraction digital
radiography studies. However, our early experience points to an exciting
potential for dual-energy subtraction digital radiography to provide a
low-cost noninvasive tool for the evaluation of atherosclerotic cardiac
disease. Further research is needed to define the full potential of
dual-energy subtraction radiography in the evaluation of cardiovascular
disease.
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Abstract
Introduction
