Patient population: This retrospective observational study was approved by the institutional review board and conducted in accordance with the 1964 Declaration of Helsinki. Data of the consecutive 55 patients (17 men, 38 women; mean age, 68 years ± 10 [standard deviation]) of whole heart dynamic CCTA during a cardiac cycle obtained by dose modulated scan and of adenosine-stress MPS were retrospectively analyzed. All patients had been clinically referred for suspected CAD. The patient exclusion criteria were as follows: (i) previous coronary bypass surgery; (ii) contraindications to nitroglycerin or contrast media; (iii) heart rate >75 bpm; (iv) body weight >80 kg. All adenosine-stress MPS were performed within 3 months after the CCTA.
Dose modulated dynamic CCTA: All scans were performed by a 320-row CT scanner. A dynamic CCTA scan was continuously performed in 75% of the R-R interval (mid-diastole) for 8 to 11 cardiac cycles with prospective electrocardiography gating axial scans after a 10 second injection of contrast media (259 mgI/kg) [6]. One scan of the dynamic CCTA was performed as a boost scan for standard CCTA at the peak phase of the ascending aorta, which was determined by a test bolus injection (Figure 1). The acquisition parameters as follows; slice thickness, 0.5 mm; slice intervals, 0.25 mm; iterative reconstruction of AIDR 3D, Standard; Beam collimation, 320 × 0.5 mm; tube voltage, 100 kVp; tube current (mA), auto exposure control.
Analysis of CT-iFR: Firstly, Dynamic CCTA datasets were computed into about 100 datasets by motion coherence image processing. Secondary, time-density curves (TDCs) of ascending aorta and distal sites of coronary arteries were obtained by the average of CT numbers at region of interests during dynamic scan (Figure 2). Thirdly, the slope of the linear part of TDCs were obtained as the up slope. Finally, the CT-iFR was defined as the ratio of the up slope of coronary to aorta. CT-iFRs were measured at the distal sites of right coronary artery (RCA), left anterior descending artery (LAD), and left circumflex artery (LCX).
Adenosine-stress MPS: The adenosine-stress MPS (0.12 mg/kg/min; rest scan, 4 hours later than stress scan) using 111 MBq of thallium-201 was performed in all patients to identify as the reference standard of myocardial ischemia due to the functional coronary stenosis. Data was acquired using a hybrid single-photon emission computed tomography (SPECT-CT) with low-energy general-purpose collimators centered on a 73 keV photo peak with a 23.5 % window. Sixty projection images were acquired over 360 degree (6 degree per step, 35 s/step), with a 64 × 64 matrix. The two radiologists made a consensus judgment regarding the presence of ischemia or infarct in the RCA, LAD, and LCX territories for each case [7].
Statistical analysis: The Shapiro-Wilk test was employed to assess normality of data distribution. Due to normal data distribution, descriptive statistics are provided as mean and the standard deviation. CT-iFRs between the patients with and without functional coronary stenosis are compared by the Mann-Whitney U test. Diagnostic performances for detecting ischemia of CT-iFR and conventional CCTA were compared by the receiver-operating-characteristic (ROC) analyses. The differences of ROCs were tested by DeLong’s test. All statistical analyses were conducted using GraphPad Prism (version 8.1.2 for Mac OS, GraphPad Software, La Jolla, California, USA) with statistical significance set at P <0.05.