Objective:
To evaluate the role of an ultra-low-dose dual-source CT coronary angiography (CTCA) scan with high pitch for delimiting the range of the subsequent standard CTCA scan.
Methods:
30 patients with an indication for CTCA were prospectively examined using a two-scan dual-source CTCA protocol (2.0 × 64.0 × 0.6 mm; pitch, 3.4; rotation time of 280 ms; 100 kV): Scan 1 was acquired with one-fifth of the tube current suggested by the automatic exposure control software [CareDose 4D™ (Siemens Healthcare, Erlangen, Germany) using 100 kV and 370 mAs as a reference] with the scan length from the tracheal bifurcation to the diaphragmatic border. Scan 2 was acquired with standard tube current extending with reduced scan length based on Scan 1. Nine central coronary artery segments were analysed qualitatively on both scans.
Results:
Scan 2 (105.1 ± 10.1 mm) was significantly shorter than Scan 1 (127.0 ± 8.7 mm). Image quality scores were significantly better for Scan 2. However, in 5 of 6 (83%) patients with stenotic coronary artery disease, a stenosis was already detected in Scan 1 and in 13 of 24 (54%) patients with non-stenotic coronary arteries, a stenosis was already excluded by Scan 1. Using Scan 2 as reference, the positive- and negative-predictive value of Scan 1 was 83% (5 of 6 patients) and 100% (13 of 13 patients), respectively.
Conclusion:
An ultra-low-dose CTCA planning scan enables a reliable scan length reduction of the following standard CTCA scan and allows for correct diagnosis in a substantial proportion of patients.
Advances in knowledge:
Further dose reductions are possible owing to a change in the individual patient''s imaging strategy as a prior ultra-low-dose CTCA scan may already rule out the presence of a stenosis or may lead to a direct transferal to an invasive catheter procedure.In recent years, dramatic advances in CT technology have led to the establishment of CT coronary angiography (CTCA) as a non-invasive imaging modality with robust image quality for the detection of coronary artery stenosis.
1,2 A major drawback of CT is the radiation exposure, which may be as high as 20 mSv.
3,4 Several techniques are available to reduce the radiation dose to the patient, including electrocardiography (ECG)-based tube current modulation, automatic exposure control and prospective ECG gating.
5–7 State-of-the-art dual-source CT scanners, which use two radiation sources and detectors, provide markedly better resolution and, in conjunction with fast table advancement, enable image acquisition of the entire heart in a single heartbeat.
8 This technique requires no overlapping acquisition and—under ideal conditions, that is, in patients with low heart rates—can reduce radiation exposure to <1 mSv.
9While these techniques can already substantially lower the radiation exposure of patients undergoing CTCA, there is potential for further reduction by optimally planning the scan length in the
z-axis. An anteroposterior view acquired for localization of the imaging volume provides only a general idea of the course of the coronary arteries within the cardiac silhouette. Therefore, in order to ensure coverage of the entire coronary system, most examiners define the scan length using the tracheal bifurcation as the upper limit and the lateral diaphragmatic recess as the lower limit.
10 In many cases, this strategy results in a longer scan and higher radiation exposure than is actually needed. An option for more accurate delimitation of the scan length is to use the axial slices of a prior calcium scan for orientation.
11,12 Alternatively, an accurate definition of the necessary scan length is achieved by acquiring a contrast-enhanced ultra-low-dose planning scan that might allow for a simultaneous diagnostic evaluation of at least the larger, proximal coronary artery segments, that is, those segments that are potentially amenable to a catheter-based intervention. We hypothesized that an ultra-low-dose planning scan can reduce the overall radiation exposure of CTCA: patients in whom the planning scan already excludes a stenosis would not need the subsequent diagnostic scan and patients in whom the planning scan detects at least one stenosis can directly undergo invasive cardiac catheterization.The aim of our study was to investigate the use of a high-pitch ultra-low-dose dual-source CTCA scan for delimiting the scan range of the subsequent diagnostic CTCA, and to assess how such a scan might reduce radiation exposure and modify the imaging strategy in an individual patient.
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