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Original Research |
1 Department of Radiology, University Hospital Basel, Petersgraben 4, Basel
CH-4054, Switzerland.
2 Department of Cardiology, University Hospital Basel, Basel, Switzerland.
Received December 31, 2004;
accepted after revision February 9, 2005.
Address correspondence to J. Bremerich.
Abstract
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SUBJECTS AND METHODS. Patients with suspected coronary artery disease (n = 166) were examined with contrast-enhanced MDCT (16 x 0.75 mm focused on the heart) during injection of contrast material (80 mL injected at a rate of 4 mL/sec) followed by saline (20 mL injected at 4 mL/sec). Retrospectively gated images were reconstructed at a 1-mm slice thickness and a 0.5-mm increment with isotropic voxels of 1 mm3. Images were reviewed for extracardiac findings, which were then classified as none, minor, or major with respect to their impact on patient management and treatment. In a different group of patients (n = 20), chest scans (16 x 1.5 mm) were used for measuring volumes of displayed body structures on wholechest scans, coronary artery MDCT images, and coronary artery MDCT images reconstructed with the maximum field of view.
RESULTS. Extracardiac findings were detected in 41 patients (24.7%). Findings were classified as minor (19.9%) or major (4.8%). Among the major findings, which had an immediate impact on patient management and treatment, were bronchial carcinoma and pulmonary emboli. Volume analysis revealed that 35.5% of the total chest volume was displayed on dedicated coronary artery MDCT focused on the heart, whereas 70.3% of the chest was visible when coronary artery MDCT raw data were reconstructed with the maximal field of view (p < 0.001).
CONCLUSION. Coronary artery MDCT can reveal important findings and disease in extracardiac structures. Thus, the entire examination should be reconstructed with the maximum field of view and should be reviewed by a qualified radiologist.
Keywords: cardiac imaging • chest • CT coronary arteriography • lung diseases • MDCT
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Coronary artery CT images can be acquired on MDCT scanners with protocols optimized for high spatial resolution and with raw data reconstruction synchronized to the cardiac cycle. MDCT scanners are multipurpose imaging tools for which applications, such as imaging of lung cancer [4], emphysema [5], pulmonary emboli [6], and aortic aneurysm [7], are well established. Thus, one would expect to see signs of such diseases when located close to the heart and contained within the reconstructed field of view.
Risk factors for coronary artery disease are well established. Some risk factors such as age, male sex, and smoking are also risk factors for other diseases such as bronchial carcinoma [8]. Smoking also plays a major role in emphysema because it can contribute to the imbalance between proteases released from smoke-induced inflammatory cells in the lower respiratory tract and the antiproteolytic defenses of the lung, resulting in destruction of lung matrix and development of enlarged air spaces that characterize emphysema [9].
Thus, a patient referred for suspected coronary artery disease may have clinically inapparent lung cancer or other relevant diseases. On coronary artery CT, findings suggestive of disease may be visible when located close to the heart. The interpreting radiologist is responsible for identifying and reporting such findings displayed on coronary artery CT.
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CT data were acquired on a 16-MDCT scanner (Sensation 16, Siemens Medical Solutions). To calculate the bolus arrival time for the contrast-enhanced scan, 20 mL of contrast medium (300 mg I/mL; Ultravist 300, Schering) was injected at a rate of 4 mL/sec with a power injector (Missouri, Ulrich), followed by a chaser bolus of 20 mL of saline in the antecubital vein. CT attenuation values were measured in the ascending aorta to identify the first slice with strong enhancement and to calculate the scanning delay time. Subsequently, the contrast medium bolus (80 mL) was injected (rate, 4 mL/sec) followed by a 20-mL saline chaser (rate, 4 mL/sec). The collimation was 16 x 0.75 mm; table feed, 2.8 mm/rotation; and effective tube current, 400 mAs at 120 kV. The typical scan duration was 25 sec. No ß-blocker was administered to modulate heart rate.
Images were reconstructed with the multisegment algorithm provided by the manufacturer with retrospective ECG gating starting 400, 450, or 500 msec before the gating signal. Images were reconstructed with a 1-mm slice thickness and a 0.5-mm increment to obtain overlapping isotropic voxels of 1 x 1 x 1 mm3.
Coronary artery MDCT images focused on the heart were reviewed offline at a workstation (Leonardo, Siemens Medical Solutions) by an experienced radiologist. Among the three data sets with different reconstruction intervals (400, 450, or 500 msec before the gating signal), the radiologist chose the images with the least motion artifact for evaluation of lung, chest wall, and spine. All images were reviewed in standard soft-tissue (width, 300 H; center, 30 H), lung (width, 1,400 H; center, -500 H), and bone (width, 2,500 H; center, 800 H) window settings. All extracardiac findings were reported and scored as none; minor, meaning that no immediate workup or treatment was necessary; or major, meaning that immediate workup, treatment, or both were required.
For analysis of imaged body volume, chest MDCT scans of 20 consecutive patients referred to our institution for extracardiac disease, such as suspected pulmonary embolism, neoplasm, or interstitial lung disease, were reviewed. Chest MDCT scans were acquired with a collimation of 16 x 1.5 mm and were reconstructed with 3-mm slice thickness and 1.5-mm increment. Images were transferred to a workstation (Leonardo) for offline analysis. Three different imaged body volumes were calculated: first, chest MDCT, which included images from the apex to the base of the lungs and regions of interest were placed around the outer surface of the chest wall. Second, coronary artery MDCT from the level of the carina to the most caudal part of the heart was evaluated with the field of view focused on the heart. Third, coronary artery MDCTFOVmax, images were examined, which had the same extension in the z direction as coronary artery MDCT but a larger field of view in the x-y direction (maximum field of view [FOVmax]), containing the outer surface of the chest wall. The purpose of the coronary artery MDCTFOVmax measurements was to simulate a typical coronary artery CT acquisition from the level of the carina to the base of the heart but with atypical raw data reconstruction displaying the entire chest at the imaged levels to allow visualization of extracardiac findings in all lung structures available from the raw data.
Values are expressed as means ± SEM. Comparisons among volume measurements were made with a one-way matched analysis of variance. If the analysis showed an overall p value of less than 0.05, Bonferroni's test was implemented as a post-hoc test. A p value of less than 0.05 was considered significant.
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Extracardiac findings were present in 41 patients (24.7%). These were classified as minor and major in 33 and eight patients, respectively, as shown in Figure 1. A detailed list of all findings is given in Table 1. Among the minor findings were anatomic variations such as arteria lusoria and acquired diseases such as aortic aneurysm and pulmonary fibrosis, as illustrated in Figures 2A, 2B, and 2C. Most important were major findings that had an immediate impact on workup, treatment, or both. One patient presented with recurrent chest pain and dyspnea unrelated to physical activity. Coronary artery MDCT provided good enhancement of coronary and pulmonary arteries, as shown in Figures 3A, 3B, and 3C, and revealed peripheral pulmonary emboli. Symptoms resolved in this patient once anticoagulation therapy was initiated. In two patients, pulmonary masses were discovered in the upper left lobes, as shown in Figures 4A, 4B, and 4C. Both masses had spiculations and were highly suggestive of lung cancer; the histologic diagnosis was adenocarcinoma in both patients. Lung cancer was not known before coronary artery MDCT, and patients had no symptoms suggestive of malignant pulmonary disease.
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The volume measurements based on the chest MDCT, coronary artery MDCT, and coronary artery MDCTFOVmax images revealed 18.3 ± 4.4 L, 12.8 ± 3.3 L, and 6.5 ± 1.6 L, respectively. The differences among these volumes were statistically significant (p < 0.001) (Fig. 5). Thus, with typical coronary artery CT settings, only 35.5% of the total chest volume was displayed. When the same raw data, however, were reconstructed with a maximal field of view (MDCTFOVmax), 70.3% of the total chest volume was displayed (p < 0.001). These volume measurements indicate that at least one set of images should be reconstructed with the maximum field of view for the assessment of extracardiac findings because such reconstruction increases the displayed chest volume significantly without additional radiation exposure.
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CT of the heart is being performed with increasing frequency, and many coronary artery CT examinations are interpreted by cardiologists who are primarily concerned with the cardiac portion of the examination. These scans, however, include information about the lung, mediastinum, chest wall, and spine. This issue is relevant because evidence regarding lung cancer screening by CT shows that this technology detects earlier stage and smaller lung tumors with greater frequency than other techniques. CT is a well-established tool for imaging diseases of the lung and chest wall and can play a role as a screening technique for lung cancer [12]. To date, however, no trials have shown that CT screening leads to a reduction in lung cancer mortality [13].
Because coronary artery CT examinations are being performed more frequently, it is possible that malpractice litigation containing various kinds of allegations of radiologic negligence will emerge and increase in number as well. Berlin [14] elaborated on the potential legal ramifications of CT screening. Lung cancer may be overlooked on CT images. Kakinuma et al. [15] reported that half the lung tumors detected on helical CT were present in retrospect on a prior CT screening examination. White et al. [16], focusing on missed carcinomas on chest CT, found that an experienced thoracic radiologist—even knowing prospectively that each CT scan in a study group included a missed cancer—was able to identify the missed lesion in fewer that half the cases. Wechsler et al. [17] documented a miss rate of 13.5% in emergency body CT scans and highlighted the importance of radiologic training and experience in the interpretation of CT scans.
Can all the available information about extracardiac structures be extracted from coronary artery CT? Typically not; the reason is that image reconstruction from the raw data sets focuses on the heart and does not include the entire right lung and chest wall at the scanned level. These structures, however, can be visualized without further radiation exposure by reconstruction of at least one data set with a larger field-of-view setting. The current study showed that 70.3% of the chest volume was visible on coronary artery CT reconstructed with a large field of view and that typically only 35.5% of the chest volume is contained in the reconstructed field of view. Thus, image reconstruction with a larger field of view containing the entire chest at the scanned levels might reveal findings that are not visible on images focused on the heart. In a recent study Hong et al. [18] assessed the effect of various sizes of reconstructed fields of view on image quality and interpretation. Those investigators analyzed unenhanced CT images acquired for coronary calcium scoring and found no significant differences between the image sets reconstructed with different fields of view with respect to coronary calcium score, lesion volume, and image noise.
The role of coronary artery CT as a diagnostic tool in coronary artery disease is not yet clearly defined. In asymptomatic patients with intermediate risk of future cardiac events, unenhanced low-dose MDCT scans may be obtained to initiate therapeutic strategies for risk factor modification [1]. Risk factors for coronary artery disease such as smoking, male sex, and age overlap with risk factors of other chest diseases such as bronchial carcinoma. Because of the high negative predictive value, coronary artery CT may be justified in symptomatic patients with a low to moderate pretest probability of coronary artery disease. Extensive calcifications and stents, however, may hinder the assessment of the coronary artery lumen.
The following limitation of the current study must be mentioned: Volume measurements of the entire chest, structures displayed on coronary artery MDCT focused on the heart, and structures displayed on coronary artery MDCTFOVmax were not done on coronary artery CT scans. These measurements were done retrospectively on chest CT scans acquired for various indications such as pulmonary embolism, bronchial carcinoma, aortic aneurysm, or inflammatory pulmonary disease. We preferred this approach rather than acquiring a CT scan of the entire chest to avoid additional radiation exposure for the study group of patients who underwent coronary artery CT.
In conclusion, coronary artery MDCT can reveal important findings and abnormalities in extracardiac structures contained in the scanned volume. Disclosure of such findings may require image reconstruction with a field of view that contains the entire chest at that level. Thus, the entire examination should be reconstructed with the maximum field of view and should be reviewed by a qualified radiologist.
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