We retrospectively analyzed 30 consecutive patients (M:F = 20:10; mean age, 57.9 ± 11.4 years) who presented at our hospital with atypical chest pain and underwent both CCTA and subsequent echocardiography between January 2009 and March 2009. All patients underwent CCTA and echocardiography on the same day. Exclusion criteria were unstable angina or acute coronary syndrome, previous history of percutaneous coronary stent implantation or coronary bypass graft surgery, atrial fibrillation or other cardiac arrhythmia, hypersensitivity to iodinated contrast media, absolute contraindication to β-blocker, impaired renal function (serum creatinine level > 1.5 mg/dL), pregnancy or lactation, and the inability to hold the breath for 15 seconds. Table 1 lists the clinical characteristics of the patients. All patients signed informed consent forms for CCTA and echocardiography, and the Institutional Review Board approved this study.

Prior to CCTA examination, the blood pressure and heart rate of each patient were measured. In all patients with a resting heart rate exceeding 65 beats per minute, an oral dose of short-acting beta-blocker (propranolol 10-40 mg) was administered one hour before CT scan in order to obtain a heart rate below 65 beats per minute. No additional intravenous beta-blocker was administered at the time of the examinations. Vital signs including blood pressure, heart rate, oxygen saturation of arterial blood, and ECG were monitored in the scanning room.

Coronary CT angiography examinations were performed on a 128-slice MDCT (Definition As+, Siemens Medical Solution, Germany) with a rotation time of 300 msec, and a slice collimation of 128 × 0.6 mm using a continuous helical scan mindose® technique. The CT scan was triggered automatically by the arrival of the main contrast bolus (automatic bolus tracking) technique. A region of interest was placed into the ascending aorta, and image acquisition began 6 seconds after the signal density level reached the predefined threshold of 100 Hounsfield units (HU). For all CT examinations, a dual-head power injector (Stellant D; Medrad, Indianola, IA) was used to administer a dual-phase bolus at a rate of 5 mL per second: At first, 70 mL of iopromide (ultravist 370, BSP, Germany) was administered and this was followed by the administration of 20 mL of an 80% to 20% mix of ultravist 370 and saline. Retrospective CCTA with mindose® was performed with the following parameters: helical scanning, 0.16-0.22 pitch, use of ECG-dependent tube current modulation (tube current of 160 mA during 50-80% of R-R interval and minimal tube current of 6.4 mA during the remainder of the R-R interval based on the patient's age and weight/height: 100 kVp when BMI was less than 25, 120 kVp when BMI was 25 or more).

Firstly, images were reconstructed at 50-80% of the R-R interval in 5% increments to obtain optimal coronary artery images and then other images were reconstructed at 10-100% of the R-R interval in 10% increments for the assessment of LV function and volume. For the reconstruction of axial images obtained by CT scan, we used a slice thickness of 0.7 mm and a slice width of 0.4 mm with the use of a small- or medium-sized cardiac field of view. A medium-hard algorithm was used for reconstruction and all CT data sets were transferred to a dedicated workstation (Leonard, Siemens Medical Solutions). All different 10 phase images were assessed if they could have a good enough quality for discriminating between the lumen and endocardium from two radiologists by consensus on a short axis image.

Left ventricular function and volume were evaluated by two independent radiologists at 10-100% of the R-R interval using dedicated software (Circulation, Siemens Medical Solutions) with a semiautomated LV endocardial and epicardial contour detection technique. Inter-observer variability was analyzed by comparing the parameter measurements of two radiologists with seven years and two years experience in cardiac imaging, respectively.

Two-dimensional transthoracic echocardiography (2D-TTE) was performed with one machine (Acuson Sequoia, Siemens Medical Solution, USA) as a standard reference for the evaluation of LV function and volume. All patients were placed in the left lateral decubitus position for the examination. Images were obtained in the standard 4- and 2-chamber apical views with a 3.5-MHz transducer by a cardiologist with 10 years of experience and blinded to the MDCT data. LV function and volume were estimated according to the modified Simpson's method (10).

The dose-length product (DLP, measured in milligray-centimeters) is defined as the volume CT dose index multiplied by the scan length and is an indicator of the integrated radiation dose of the entire CT examination. DLP was displayed on the dose report on the CT scanner and was recorded. A reasonable approximation of the effective radiation dose (E) of coronary CT angiography was calculated by multiplying DLP by a conversion coefficient for the chest and coronary arteries (k = 0.014 mSv/[mGy × cm]) (11).

All statistical analyses were performed with statistical software (SPSS, version 12.0 for Windows; SPSS, Chicago, IL). Continuous data were presented as the mean ± standard deviation (SD) and compared using the paired two-tailed Student's t-test. Agreement for the LVEF and volume by MDCT and TTE was assessed by Pearson's correlation coefficient for linear regression and Bland-Altman analysis. The Pearson's correlation coefficient was valued as follows: poor = 0; slight = 0.01-0.20; fair = 0.21-0.40; moderate = 0.41-0.60; good = 0.61-0.80; excellent = 0.80-1.00. Inter-observer variability of LVEF and LV were measured by two MDCT readers was also assessed using Pearson's correlation.

The average LVEF calculated by MDCT was 64.9 ± 9.60%, as opposed to 65.8 ± 8.68% by TTE (p = 0.53). The evaluation of LVEF by linear regression analysis demonstrated good correlation between MDCT and TTE (r² = 0.68. p < 0.001) (Fig. 1A). A Bland-Altman analysis showed a mean difference of 0.9 ± 0.92% for LVEF between MDCT and TTE (Fig. 1B).

The average LVEDV by MDCT was 122.2 ± 22.01 mL, compared to 114.0 ± 19.19 mL by TTE. The average LVEDV measured by MDCT was significantly higher than LDEDV by TTE (p = 0.001). A linear regression analysis showed a good correlation between MDCT and 2D-echocardiography for the assessment of LVEDV (r² = 0.82, p < 0.001) (Fig. 2A). For the Bland-Altman analysis, a mean difference of 9.0 ± 2.82 mL was observed between MDCT and TTE (Fig. 2B).

For MDCT, the average LVESV was 43.9 ± 20.45 mL, while in comparison, was 41.4 ± 20.73 mL by TTE. Also the average LVESV measured by MDCT was significantly higher than by TTE (p = 0.001). Moreover, the comparison of the correlation coefficients between the two modalities for the assessment of LVESV was good (r² = 0.97, p < 0.001) (Fig. 3A). The Bland-Altman analysis showed a mean difference of 2.5 ± 0.55 mL between MDCT and TTE (Fig. 3B).

The inter-observer agreements for LVEF, LVEDV, and LVESV by MDCT were 0.90, 0.94 and 0.97, respectively.

All 10 of the different phase images assessed by the two radiologists by consensus were of good image quality for differentiating the lumen from the endocardium, and the mean radiation doses were 285.2 ± 132.1 mGy × cm in DLP (3.99 ± 1.85 mSv) and 16.8 ± 7.79 mGy for the CTDI vol.