The coronary artery calcium score (CS), an independent predictor of cardiovascular events, can be obtained from a stand-alone nonenhanced computed tomography (CT) scan (CSCT) or as an additional nonenhanced procedure before contrast-enhanced coronary CT angiography (CCTA). We evaluated the accuracy of a novel fully automatic tool for computing CS from the CCTA examination. One hundred thirty-six consecutive symptomatic patients (aged 59 ± 11 years, 40% female) without known coronary artery disease who underwent both 256-row CSCT and CCTA were studied. Original scan reconstruction (slice thickness) was maintained (3 mm for CSCT and 0.67 mm for CCTA). CS was computed from CCTA by an automatic tool (COR Analyzer, rcadia Medical Imaging, Haifa, Israel) and compared with CS results obtained by standard assessment of nonenhanced CSCT (HeartBeat CS, Philips, Cleveland, Ohio). We also compared both methods for classification into 5 commonly used CS categories (0, 1 to 10, 11 to 100, 101 to 400, >400 Agatston units). All scans were of diagnostic quality. CS obtained by the COR Analyzer from CCTA classified 111 of 136 (82%) of patients into identical categories as CS by CSCT and 24 of remaining 25 into an adjacent category. Overall, CS values from CCTA showed high correlation with CS values from CSCT (Spearman rank correlation = 0.95, p <0.0001). In conclusion, CS values automatically computed from 256-row CCTA correlated highly with standard CS values obtained from nonenhanced CSCT. CS obtained directly from CCTA may obviate the need for an additional scan and attendant radiation.
In patients referred for coronary computed tomography angiography (CCTA) a coronary artery calcium score (CS) is commonly calculated as a separate nonenhanced scan (CSCT) before CCTA in view of the large body of prognostic data related to the CS. Elevated CS is associated with increased prevalence of obstructive coronary artery disease and elevated risk of adverse cardiac events in both asymptomatic and symptomatic subjects and may be preferable to other risk-assessment tools in clinical practice. To limit radiation exposure and address a specific clinical scenario, the procedure may be limited to a single scan depending on the clinical scenario: CCTA for symptomatic patients or stand-alone CSCT for asymptomatic patients. Until recently, the high attenuation from contrast medium within the coronary vessel lumen precluded assessment of CS from CCTA with standard CS software. Moreover, CS from CCTA might yield CS values different from those derived from conventional CSCT. In this study, we evaluated the accuracy of a novel fully automatic tool for computing CS directly from CCTA.
Methods
This retrospective, single-center study was approved by the institutional review board with waiver of informed consent. The cohort included consecutive adults (aged >21 years) in sinus rhythm, without previously diagnosed coronary artery disease and none of the exclusion criteria (listed subsequently) who were referred to our institution by their treating physician for 256-slice CCTA during a 2-month period. Study exclusion criteria were known stenosis (and/or previous revascularization) or CCTA exclusion criteria (unable to cooperate, pregnancy, iodine allergy, renal function impairment, creatinine clearance <60 ml/min).
A structured interview on the day of CCTA detailed common risk factors: (1) diabetes mellitus (patient history and/or treatment with insulin or oral hypoglycemic agents), (2) hypercholesterolemia, (3) systemic hypertension, (4) family history of coronary artery disease (in first-degree relatives <55 [male] or <65 [female] years of age), and (5) smoking history.
All scans were performed using a 256-row scanner (Brilliance iCT, Philips Healthcare, Cleveland, Ohio), which has a longitudinal coverage of 8 cm, variable rotation time (minimum 0.27 seconds) and a 120-kW generator. CS (in Agatston units, AU) was assessed initially as a CSCT using prospective electrocardiographic (ECG) triggering, tube voltage of 120 kV and a tube current of 55 mAs as standard (100 mAs in obese patients). CCTA was performed with either prospectively triggered “step-and-shoot” scans (Step & Shoot Cardiac, Philips Healthcare) or with helical retrospective ECG gating. Oral and/or intravenous β blockers were used to lower heart rate when >70 beats/min. Sublingual nitroglycerine (0.4 mg) was given before CCTA to all patients with systolic blood pressure of ≥110 mm Hg and no clinical contraindications, and a contrast enhanced scan was then performed using a bolus of 71 ± 11 ml (range 50 to 100) Iohexol (Omnipaque 350 mg I/ml, GE Healthcare, Princeton, New Jersey) injected into an antecubital vein at a flow rate of 5 to 6 ml/s, followed by a mixed 50% contrast/saline injection and then a 20- to 30-ml saline chaser bolus. Both modes of scans were performed at 120 kV with a slice collimation of 128 × 0.625 mm with a dynamic dual focal spot (providing 256-slice acquisition) and a rotation time of 0.27 or 0.33 seconds. Helical scans (retrospective ECG gating) were performed with an effective tube current (rotation time product normalized by the pitch) in the range of 900 to 1,500 mAs depending on body mass index and body habitus and a pitch of 0.14 to 0.18. ECG based tube-current modulation was used with retrospective gating. The step-and-shoot scans were performed in patients with stable heart rhythm and heart rate <65 beats/min with a tube current – x-ray on time product of 160 to 300 mAs. Radiation exposure was assessed as dose-length-product (product of scan length and CT dose index [CTDI vol ]). Reconstruction was performed using a window centered at 75% of the R-R interval as default. For heart rates >70 beats/min, an earlier reconstruction phase (usually ≤45%) was frequently used with retrospective gating.
Standard CS was obtained by assessment of CSCT using semiautomatic software (HeartBeat CS) available on the Extended Brilliance Workstation (EBW, Philips Healthcare). Traditional CS scoring on a nonenhanced scan involves (1) reconstruction of images with 3-mm slice thickness, (2) calcium segmentation, performed semiautomatically by selecting all pixels >130 HU in the coronary arteries, and (3) Scoring (by the software), measured slice by slice by summing the 2-dimensional area of each lesion in the slice multiplied by a factor determined by the maximal intensity of the calcium lesion in that slice.
Reconstruction of CCTA images were initially performed with a slice thickness of 0.67 mm and spacing (z axis) of 0.4 mm. The reconstructed images were then sent to the COR Analyzer II workstation. The COR Analyzer software (rcadia Medical Imaging, Haifa, Israel) is a fully automatic tool developed to diagnose coronary stenosis from CCTA. The software classifies the coronary artery system into 3 main arteries and 10 coronary segments. Each artery is classified by the software to 1 of 3 categories: significantly stenotic (≥50% diameter stenosis), without significant stenosis (<50%), or indecisive (expert reading required). Additionally, the software version tested in the current study includes an additional module (COR CS module) that uses a novel algorithm involving the following steps to allow CS assessment from CCTA: (1) calcium segmentation is performed by comparing images to a model of intensity distribution within the artery lumen. The model is adaptive to the local vessel geometry and to the local contrast intensity and noise estimation. (2) A “virtual” CS (nonenhanced, 3-mm slice thickness) study is simulated on the basis of the original CCTA, and segmented calcium (this step is patent pending) is marked. (3) The standard Agatston scoring is applied to the virtual CS study. The total CS score is then presented in Agatston units.
Baseline characteristics and imaging and scanning parameters were recorded using descriptive statistics. CS obtained from CSCT (with traditional semiautomatic analysis) and that from CCTA (using the COR Analyzer) were compared using Bland Altman analysis and (nonparametric) Spearman rank correlation. We also compared both methods for classification into 5 commonly used CS categories (0, 1 to 10, 11 to 100, 101 to 400, and >400 AU). A p value <0.05 was considered significant. Statistical analysis was performed using Statistix 8 software package (Analytical Software, Tallahassee, Florida).
Results
One hundred thirty-six patients met study inclusion criteria and underwent both CSCT and CCTA. Their baseline characteristics are presented in Table 1 . All scans were of diagnostic quality, and CCTA scanning parameters are presented in Table 2 . Mean CS from traditional nonenhanced CSCT was 251 ± 424 AU (range 0 to 2,384, median = 63, interquartile range = 3, 276). Mean CS by CCTA using the COR Analyzer was 244 ± 397 AU (range 0 to 2,351, median = 72, interquartile range = 4, 301). The COR Analyzer analysis of the CCTA classified 111 of 136 patients (82%) into identical CS categories as the standard CSCT analysis and 24 of the remaining 25 into an adjacent CS category ( Table 3 ). Thus a good correlation was maintained for all CS categories (regardless of scanning mode). Overall, CS values from CCTA and CSCT correlated well (Spearman rank correlation = 0.95, p <0.0001), and Bland Altman analysis showed good correlation for all CS values ( Figures 1 and 2 ).
| Variable | |
|---|---|
| Age (yrs) | 60 ± 13 |
| Female gender | 55 (40%) |
| Diabetes mellitus | 31 (23%) |
| Hyperlipidemia ∗ | 90 (66%) |
| Hypertension † | 65 (48%) |
| Family history for coronary disease | 29 (21%) |
| Smoker | 42 (31%) |
∗ Total serum cholesterol level ≥5 mmol/L or treatment with lipid-lowering drugs.
† blood pressure >140/90 mm Hg or treatment with antihypertensive medication.
| Heart rate (beats/min), mean ± SD (range) | 61 ± 9 (44–85) |
| Contrast material (ml), mean ± SD (range) | 71 ± 11 (50–100) |
| Dose length product (mGy × cm), mean ± SD (range) | 663 ± 315 (232–1,716) |
| Step-and-shoot mode, n | 38 of 136 (28%) |
| No. of Patients in Each CS Category Using CSCT | ||||||
|---|---|---|---|---|---|---|
| No. of patients in each CS category using CCTA | 0 | 1–10 | 11–100 | 101–400 | >400 | |
| 0 | 24 | 4 | 0 | 0 | 0 | |
| 1–10 | 5 | 6 | 3 | 0 | 0 | |
| 11–100 | 1 | 1 | 29 | 2 | 0 | |
| 101–400 | 0 | 0 | 5 | 26 | 2 | |
| >400 | 0 | 0 | 0 | 2 | 26 | |
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