Non-invasive coronary angiography using 16- and 64-slice multidetector computed tomography (MDCT) has become a reliable tool for visualisation of coronary arteries. It has been proved to be especially beneficial for ruling out coronary artery disease (CAD) in selected patient collectives.1 The high negative predictive value to rule out CAD has prompted current European Society of Cardiology guidelines on management of stable angina to recommend MDCT for patients with a low pre-test probability of CAD and inconclusive stress testing.2 Dual-source CT (DSCT) was introduced in 2006 to overcome the occurrence of motion artefacts at heart rates of >65 bpm, caused by limited temporal resolution1 3 4: DSCT uses two x-ray tubes and two corresponding detector units mounted onto a single gantry. It achieves a temporal resolution of 83 ms, which allows for visualisation of coronary arteries at raised heart rates. DSCT thus reduces the number of coronary segments that cannot be assessed owing to motion.1 5 6

Because computed tomography (CT) is the major source of ionising radiation in medicine, it is clinically important to study radiation exposure in coronary computed tomography angiography (CTA).7 Moreover, dual isotope myocardial perfusion scintigraphy and coronary CTA are associated with the highest amount of radiation dose among diagnostic techniques for detecting the presence of relevant CAD, and for cardiac CT, the improvement of spatial resolution by increasing the number of detectors has been equivalent to an additional increase in radiation dose exposure.8 9 The clinical acceptability of DSCT is therefore closely related to radiation dose exposure in routine use, and one prerequisite for a wider acceptance of this technique is a mean radiation dose for coronary CTA comparable with that for invasive angiography, or even lower.

Initial phantom measurements showed that the use of DSCT does not necessarily implicate an increase of radiation dose exposure, although two x-ray tubes are used, but there are still no comparative data on radiation dose exposure of DSCT in clinical practice.10

The purpose of our study was to compare the effective dose estimates (EDs) of different DSCT protocols with those of 16- and 64-slice MDCT.

Techniques of radiation dose reduction in coronary CTA

Electrocardiogram-triggered tube current modulation (ECG pulsing) and automatic exposure control () are considered the most common methods for reducing radiation dose exposure in coronary CTA.11^–14 For conventional CT scanners, ECG pulsing limits full tube current to a predefined interval of the cardiac cycle, usually in diastole.15 By contrast, tube current is variable over the entire cardiac cycle for DSCT and may be set individually for each patient.5 16 This technique reduces radiation dose by 34–42% in phantom measurements and by up to 50% in patient studies for 16- and 64-slice MDCT, so it may lead to further reduction of radiation dose for DSCT.1 10 14 15 17

AEC (CARE Dose 4D, Siemens Healthcare, Forchheim, Germany) automatically adjusts tube current according to the size and attenuation of the body region being scanned in order to use the lowest radiation dose possible.13 18 19 The tube current modulation is based on attenuation information obtained from the localiser scan, and it is performed on the basis of a reference patient (“typical adult” weighing 70–80 kg).19

For ECG pulsing in 16- and 64-slice MDCT, the minimum level of CT tube current between intervals of maximum radiation is defined as 20% of the maximum tube current. The introduction of a low radiation dose protocol, which limits the minimum tube current to 4% of the maximum current level (MinDose, Siemens Healthcare) promises a further reduction of radiation exposure in clinical practice.20

When 16- and 64-slice MDCT are used, the table pitch is almost constant at 0.2–0.24. For DSCT, it can be adjusted from 0.2 to 0.5, according to the patient’s heart rate. Scan time and related radiation dose exposure are directly linked to the table pitch, so its modification may lead to a reduction of radiation dose exposure for CTA.21 In contrast, higher heart rates lead to increasing dose values for single-source MDCT.22

Another approach to reduce ED values is to decrease CT tube voltage from 120 kV to 100 kV. Previous studies reported the promising result of a 25–35% reduction of radiation dose exposure, so we included a subgroup of 38 patients examined with a 100 kV scan protocol.9 23

METHODS

Patient collective

The study was a retrospective data analysis concerning scan protocol, radiation dose, body surface area, mean heart rate, age and sex for 292 patients who had been examined for suspected or known CAD. Fifty-six patients had been examined on a 16-slice MDCT scanner equipped with a STRATON tube (Siemens Somatom Sensation 16, Siemens Healthcare), 38 patients on a 64-slice MDCT scanner (Siemens Somatom Sensation 64, Siemens Healthcare) and 202 patients had been investigated on a DSCT scanner system (Siemens Somatom Definition, Siemens Healthcare). All patients investigated on the 16-slice MDCT and on the DSCT scanner were selected at Kerckhoff Heart Centre in Bad Nauheim, Germany, whereas the patients examined on the 64-slice MDCT scanner were selected at Bethanien Heart Centre in Frankfurt am Main, Germany. Only patients with sinus rhythm, in stable clinical condition and without contraindications for the administration of iodinated contrast agents were investigated. Implanted stents, coronary bypass vessels, or valve prostheses were not regarded as exclusion criteria. Written informed consent was obtained from all patients for the coronary CTA examination.

For evaluation purposes, patients included into the study were separated into six groups. Table 1 gives the examination settings and scan protocols for all patient groups. Table 2 lists patient characteristics.

Scan protocols

The patients investigated on 16- and 64-slice MDCT were given 5–20 mg of metoprolol intravenously 5 min before scanning if their heart rate was >65 bpm. No patient investigated by DSCT received any heart rate modulation by oral or intravenous administration of β blockers before the scanning procedure. However, 78 patients (38.6% of the DSCT subgroup) were receiving continuous β-blocker treatment. For comparability purposes, the scan protocols were identical for all scanner types used in the study: After placing an intravenous line in an antecubital vein and connecting the ECG electrodes in standard position, all patients received 0.8 mg of isosorbide dinitrate sublingually immediately before scanning. After a topogram scan, a test bolus approach was performed to determine the contrast agent transit time (10 ml of iopamidole contrast, containing 370 mg iodine/ml, followed by 50 ml of isotonic saline, both at 5 ml/s) by calculating the time between beginning of the contrast agent injection and maximum enhancement in the ascending aorta. For CTA, the individual delay was calculated by adding an additional time delay of 2 s, and 60–80 ml of contrast agent (depending on scan time) were injected at 5 ml/s, followed by 50 ml of saline.

For patients without bypass grafts, the starting point of the CTA scan was defined as the level of the tracheal bifurcation, for patients after coronary artery bypass grafting surgery, all CTA scans started slightly cranial to the aortic arch. The end points of all CTA scans were defined individually for each patient in terms of full coverage of the whole heart.

Calculation of radiation dose estimates

The ED was chosen as the best measure to assess and compare the radiation dose exposure. The ED reflects the non-uniform radiation absorption of partial body exposure relative to a whole-body radiation dose.24 25 It is calculated using information on the radiation dose to individual organs and the relative risk or biological sensitivity assigned to each organ or tissue; it is measured in units of milliSievert (mSv). By choosing ED as the analysis variable, a direct comparison is possible of the potential risk of different examinations using x-ray in diagnostic radiology and cardiology.24^–28 For ED calculation, the dose–length product (DLP) was multiplied with a conversion factor k (units: mSV/mGy/cm) specific for the body scanned (0.017 mSV/mGy/cm): ED  = k×DLP.25^–28

The DLP includes the total scan volume and varies from patient to patient (eg, total scan length and scan width).25^–27 It has a maximum deviation of 10–15% from the mean, so it provides a reasonable calculation of radiation dose estimates.25 EDs were calculated by multiplying the dose–area product and a conversion factor k of 0.10 mSV/mGy/cm² for both men and women, according to the conversion factors published by Le Heron for lateral and posterior–anterior radiation exposures in the chest area.29

Statistical analysis

Values for ED are presented as mean values (SD). The Mann–Whitney test for unpaired non-parametric data, the Wilcoxon test for paired non-parametric data, and the Friedman test for n samples of paired non-parametric data were used for statistical analysis of the correlation between ED and heart rate or body surface area. Statistical analysis was performed using SPSS version 15.0 (SPSS; Chicago, Illinois, USA).

RESULTS

Radiation dose parameters and heart rates

Radiation dose parameters (volume CT dose index (CTDIvol), DLP and scan length z) were available for all patients investigated, and there were no scan failures in the study. The mean (SD) heart rate was 64 (11) bpm for patients examined by 16-slice MDCT (subgroup 1) and 55 (6) bpm for patients examined by 64-slice MDCT (subgroup 2). Patients investigated by DSCT had a mean heart rate of 62 (11) bpm, which was significantly higher than the heart rate in the 64-slice MDCT subgroup (p<0.05). The differences of mean heart rates between subgroups 3–6 were not statistically significant, nor was the difference between subgroup 1 and any subgroup investigated by DSCT.

Effective dose estimates for DSCT

The use of a DSCT scan protocol with a minimum of 20% of the full tube current and a wide ECG pulsing window of 35–70% of the cardiac cycle led to the highest mean ED of 21.3 (8.4) mSv. For the protocol with a minimum tube current of 4% a wide ECG pulsing window yielded a mean ED of 15.4 (7.3) mSv. Narrowing the ECG pulsing window to a minimum of 110 ms in the diastolic phase—for example, from 70% to 70% of the RR cycle, led to a significant decrease of the calculated radiation dose estimate to a mean of 9.6 (3.6) mSv (p<0.05). The lowest radiation dose estimate was achieved by reducing the tube voltage to 100 kV, using a narrow ECG pulsing window of 110 ms in the diastolic phase and a minimum tube current of 4% of the maximum tube current level: The mean ED in this subgroup was 3.8 (1.7) mSv, and it was significantly lower than the calculated radiation doses of all other patient subgroups examined on a DSCT scanner (p<0.05 for comparison of groups 5 and 6, table 3).

Effective dose estimates for DSCT at different heart rates

For the assessment of the influence of different heart rates on radiation dose, patient groups Nos 5 and 6 (120 kV/100 kV tube voltage, ECG pulsing window of 110 ms in the diastolic phase, minimum tube current of 4%) were both subdivided into three groups at different heart rate ranges: <60 bpm, 60–70 bpm and >70 bpm.

While scan protocol parameters were identical for all patients, radiation dose estimates differed: for both groups there was an inverse correlation between heart rate and radiation dose, resulting in a reduction of radiation dose at raised heart rates. This effect was not statistically significant for patients scanned with a tube voltage of 120 kV, but for 100 kV statistical significance was achieved (table 4, figs 1 and 2).

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Figure 1

Regression analysis of radiation dose depending on heart rate for 120 kV tube voltage; r = −0.131; r²  = 0.017; p = 0.192.

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Figure 2

Regression analysis for radiation dose depending on heart rate for 100 kV tube voltage, r = −0.455, r² = 0.207, p = 0.022.

Effective dose estimates for DSCT in comparison with 16- and 64-slice MDCT

The mean (SD) estimated radiation dose of all patients investigated on a DSCT scanner was 11.4 (7.2) mSv. The mean estimated radiation dose was 9.8 (1.8) mSv for 16-slice MDCT and 8.6 (2.8) mSv for 64-slice MDCT (table 5). If ECG pulsing was activated and if a narrow ECG pulsing window of 110 ms and a low radiation dose protocol was used, similar radiation dose estimates were achieved for DSCT, 16- and 64-slice MDCT. If heart rates above 60 bpm are not regarded as an exclusion criterion, DSCT potentially allows non-invasive coronary angiography at a lower radiation dose than both 16- and 64-slice MDCT. Importantly, all patients examined under a 100 kV DSCT protocol showed significantly lower estimated radiation dose values than patients scanned on a 16- or 64-slice MDCT, regardless of heart rate.

DISCUSSION

This study demonstrates that DSCT has the potential to reduce radiation dose exposure, if the scan protocol is adjusted appropriately. Our study provides estimates of radiation dose for dual-source coronary CTA for different protocols and 16- and 64-slice MDCT. It demonstrates that a radiation dose estimate as low as 7.8 mSv can be achieved, under use of AEC and ECG pulsing with a tube current reduction to 4% of the maximum tube current, if a default setting of 120 kV tube voltage is implemented. If a 100 kV scanning protocol is used, this estimated value may be reduced even further to an estimated dose value of 3.8 mSv. These values suggest that DSCT has the potential to reduce radiation doses from coronary CTA in daily practice. But according to our data, DSCT does not provide for a substantial reduction of radiation dose compared with either 16-slice or 64-slice MDCT. A mean radiation dose estimate of 9.4 mSv was calculated for DSCT, using a scan protocol with full tube current for a time instant of 110 ms (70% to 70% of the cardiac cycle) and a minimum tube current of 4%. This is suitable for datasets requiring reconstructions of raw data only in the diastolic phase and thus assumes a heart rate <70 bpm. As this is not substantially lower than the calculated radiation dose exposure for patients investigated on 16-or 64-slice MDCT in our patient series, it has to be noted that the ECG pulsing window for investigations on both 16- and 64-slice MDCT was relatively wide at 50–70%.

The improved temporal resolution of 83 ms for DSCT allows one to increase the table pitch to 0.5 at raised heart rates. According to a study of phantom measurements, this decrease in scan time translates into a linear decrease in patient dose.10 This theorem is supported by our data, showing a decrease in radiation dose exposure if low heart rates (mean radiation dose estimate was 10.1 mSv for patients <60 bpm) are compared with raised heart rates (mean radiation dose estimate was 7.8 mSv for patients ⩾70 bpm). However, it has to be taken into account, that the number of patients with raised heart rates (>70 bpm) in our patient series is very small (n = 14). Since CT scanning requires a low table pitch for patients with low heart rates to avoid discontinuities of anatomical coverage, one essential benefit of DSCT technique thus appears to be partly neglected by our data.30 Moreover, early data from Stolzmann et al showed that no relevant decrease in radiation dose at raised heart rates was achieved with the use of dedicated scanning protocols. A decrease in radiation dose at raised heart rates was simply noted by using an ECG pulsing-based scanning protocol limiting the tube current to 20% of the full tube current outside the ECG pulsing window. It must be noted that this reduction in radiation dose could not be seen if a modified scanning protocol with a limitation of the tube current to only 4% was used.20 While data on different ECG pulsing windows were not provided by McCollough et al, Stolzmann et al widened the ECG pulsing window to 30–80% of the cardiac cycle for patients with heart rates >70 bpm. This approach may partly explain the absence of influence of different heart rates on effective radiation dose estimates in the study of Stolzmann et al. Since we did not modify the ECG pulsing window for heart rates >70 bpm in our study, a certain influence of the table pitch on effective radiation dose should be taken into account.

Importantly, the average estimated radiation dose for patients investigated by DSCT by Stolzmann et al was 7.8 mSv, which is in the same range as our group 5 (9.6 (3.6) mSv) and thus appears to mark a reasonable value for coronary CTA under use of radiation-reducing scan protocols. Nonetheless, it depends on the investigator which examination protocol to choose for optimal reduction of radiation. This means a trade-off between dose reduction and image quality. A low-dose protocol with 4% of full tube current can achieve radiation doses of 3.8 mSv at the cost of impaired image quality beyond the narrow borders of the ECG pulsing window. Therefore, analysis of left ventricular function and correction for artefacts by dedicated image reconstruction is not possible at heart rates of >70 bpm.20

In agreement with Hausleiter et al and Leschka et al, our study demonstrates that the reduction of tube voltage from 120 kV to 100 kV has an enormous potential to greatly reduce radiation dose exposure in the clinical routine.9 23 31 Nonetheless, it must be remembered that, for image quality reasons, a low tube voltage protocol is only feasible in patients with a low body mass index. Consequently, a careful selection of patients suitable for 100 kV scanning is required, although the recently published PROTECTION I study states that algorithms for radiation dose saving are not necessarily associated with decreased image quality.31

We must acknowledge several limitations of our study. First, we did not perform a prospective study with the aim of an intraindividual comparison of radiation dose values in different scanning modalities. Unlike standard radiological MDCT examinations of the chest, abdomen, or pelvis, there is no justifying clinical indication to perform several coronary CTA investigations in one single patient, apart from a few individual exceptions, either for a comparison of different scanners or for follow-up examinations.13 18 32 Therefore, this retrospective data analysis was only able to compare different patient groups.

Another limitation of this study is that image quality was not evaluated systematically. Nonetheless, all investigations provided diagnostic image quality to evaluate all coronary segments. In addition, we did not assess the feasibility and potential for radiation dose saving of prospectively ECG-triggered coronary CTA, which is supposed to allow accurate detection of CAD at very low radiation doses in carefully selected patient series.33 34

In accordance with the recently published PROTECTION I study, cardiac CT remains associated with a considerable radiation dose, depending among other factors on the CT systems used as well as the handling operator and their experience in cardiac CT scanning.31

Our data show that calculated radiation dose estimates of DSCT are no lower than those calculated for 16- and 64-slice MDCT, if standard protocols are used. However, patient subgroups at heart rates of >70 bpm receive a lower radiation dose from DSCT.

Dose optimisation thus remains a highly important concern that must be dealt with by cardiologists, radiologists and manufacturers of MDCT scanners.