Coronary computed tomographic angiography (CCTA) is associated with ionizing radiation, prompting concerns of future cancer risk. Recent studies have reported reduced radiation doses and similar image quality by the selective use of dose reduction techniques, although the clinical penetration of these methods has been limited. In a quality improvement initiative, a comprehensive, standardized radiation dose reduction protocol was implemented, and its effect on radiation dose and image quality was assessed. A total of 449 patients who underwent 64-detector CCTA at 3 centers were prospectively evaluated, and patients were compared before (n = 247) and after (n = 202) the implementation of a standardized body mass index–based and heart rate–based protocol that simultaneously incorporated multiple dose reduction strategies. Median radiation dose decreased from 2.6 mSv (interquartile range 2.0 to 4.2) to 1.3 mSv (interquartile range 0.8 to 1.9) after the implementation of the standardized protocol (p <0.001). On multivariate analysis, reduction in overall radiation dose was observed by numerous dose reduction techniques, with varying efficacy of dose lowering: prospective (vs retrospective) electrocardiographic gating (−82%), reduced tube voltage (−41% for 100 vs 120 kV), lower tube current (−25% per −100 mA), and reduced overall scan length (−6% per −1 cm) (p <0.001 for each). No differences were observed between patients before and after the initiation of the protocol for study interpretability (96% vs 96%, p = 0.66). There was an increase in signal-to-noise ratio after implementing the standardized protocol (11 ± 3 vs 12 ± 4, p <0.01). In conclusion, a quality improvement protocol for CCTA incorporating multiple dose reduction techniques permits significant radiation dose reduction and may improve the safety profile of CCTA.
Multiple dose reduction strategies have been developed to improve the safety profile of coronary computed tomographic angiography (CCTA). These methods include tube current modulation for retrospective electrocardiographically gated studies (reduced tube current outside of diastole when the coronary arteries have less motion and can be optimally imaged), prospective electrocardiographic (ECG) gating (the heart is imaged only during a window in diastole), reduced 100-kV tube voltage (vs standard 120-kV voltage), shorter scan length, and lower tube current. These methods result in significant reductions in radiation dose, are additive in effect, and have been reported to result in similar image quality in multicenter studies. Nevertheless, their combined use has been limited and variable, resulting in radiation doses that remain high, with significant variation among sites. Numerous potential explanations exist to account for the limited penetration of these techniques, including a lack of awareness, concern of inadequate image quality, and uncertainty regarding the appropriate implementation of techniques into everyday practice. To address these issues, we developed a standardized protocol for CCTA as part of a quality improvement initiative that incorporated an array of radiation dose reduction techniques to test the hypothesis that implementation of such a protocol would result in lower radiation dose without sacrificing image quality.
Methods
We prospectively enrolled consecutive patients who underwent CCTA at 3 sites. All adult patients who underwent CCTA were included regardless of body mass index (BMI) or baseline heart rate, and all coronary computed tomographic angiographic studies were interpreted in an intent-to-diagnose manner. Patients who underwent contrast-enhanced computed tomographic scans for indications other than native CCTA were not included (e.g., assessment of coronary artery bypass grafts, pulmonary arteries, or the thoracic aorta). All sites had approval by their respective institutional review boards. Where applicable, sites were compliant with the Health Insurance Portability and Accountability Act.
Each site prospectively enrolled a consecutive and sequential series of patients before and after the initiation of the standardized protocol. Before initiation of the standardized protocol, each site used independent protocols as prescribed by the local site; after initiation, each site used the standardized BMI-based and heart rate–based protocol for all subsequent clinical coronary computed tomographic angiographic studies ( Table 1 ). The standardized protocol was developed in collaboration among the 3 sites as part of a quality improvement initiative and was initiated simultaneously.
| BMI (m/kg 2 ) | Tube Voltage (kV) | Tube Current (mA) |
|---|---|---|
| <25 | 100 | 275–450 |
| 25–29.9 | 100 | 325–550 |
| 30–33.9 | 120 | 375–625 |
| ≥34 | 120 | 500–800 |
The standardized protocol assigned 100-kV tube voltage to all nonobese patients (BMI <30 kg/m 2 ) and 120 kV to all obese patients (BMI ≥30 kg/m 2 ). Tube current was prescribed within standardized ranges on the basis of BMI ( Table 1 ). The z-axis (i.e., scan length) was limited to the minimum necessary to image the entire heart. Furthermore, the standardized protocol mandated prospective ECG gating for all patients with regular heart rates <65 beats/min; exceptions were permitted in cases when the referring physician specifically requested retrospective ECG gating (for assessment of left ventricular function).
Patients received oral and/or intravenous β blockers if needed to achieve a heart rate <65 beats/min and were given sublingual nitroglycerin 0.4 mg immediately before the study. All computed tomographic examinations were performed with the Discovery HD 750 scanner (GE Healthcare, Milwaukee, Wisconsin).
Two sites used a triple phase contrast protocol: 60 ml iodixanol (GE Healthcare, Princeton, New Jersey), followed by 75 ml of a 50:50 mixture of iodixanol and saline, followed by a 50-ml saline flush; 1 site used a dual-phase protocol with 75 ml iodixanol followed by a 50-ml saline flush. The scan parameters included a rotation time of 350 ms, 64 × 0.625 mm collimation, tube voltage of 100 to 120 kV, and tube current of 275 to 800 mA. ECG dose modulation was used for all retrospectively gated studies before and after initiation of the standardized protocol.
Study interpretability, signal, and noise were determined by experienced level III–certified coronary computed tomographic angiographic imagers, each with experience reading several thousand studies. Diagnostic study quality was graded on per patient and per artery levels. A diagnostic study on a per artery and per patient level was defined as 1 in which the entire artery or coronary artery tree could be adequately assessed for the detection or exclusion of significant coronary artery disease, including all branches ≥1.5 mm in diameter. Conversely, a nondiagnostic study or artery was defined as 1 in which ≥1 segment ≥1.5 mm in diameter could not be accurately assessed for the presence or absence of significant coronary artery disease.
Studies were interpreted using AW 4.4 Advantage Workstations (GE Healthcare, Milwaukee, Wisconsin). The use of axial data sets, maximum intensity projections, curved multiplanar reformats, and volume-rendered reconstructions was at the discretion of each reader. The signal and noise were measured in the aortic root at the level of the left main coronary artery on an axial image in a 1.0-cm region of interest to measure the mean (signal) and standard deviation (noise) in Hounsfield units; additional measurements of signal and noise were made using axial images in the proximal left main, left anterior descending, left circumflex, and right coronary arteries using the largest possible region of interest that fit in the proximal artery. The mean of the signal and noise in the aorta and each of the 4 coronary arteries was used to define the overall study signal and noise; the signal-to-noise ratio represents the ratio of the mean signal to mean noise.
Radiation dose for CCTA was determined by the dose-length product, and this was converted to millisieverts by multiplying this by the conversion factor of 0.014 mSv/(mGy × cm) as has been performed in other recent studies.
Comparisons between groups were performed using Student’s t tests and analysis of variance for continuous variables with normal distributions; the Mann-Whitney U test was used for continuous variables with non-normal distributions. The chi-square test was used for categorical variables.
For multivariate analyses, forward stepwise linear regression models assessed patient or scan characteristics that would be expected to have associations with the radiation dose. Patient characteristics included age, gender, BMI, and heart rate; scan characteristics included prospective versus retrospective ECG gating, tube voltage, tube current, and scan length. To prevent overfitting of the models, only variables with p values <0.10 on univariate linear regression were entered given interactions between patient and scan variables. Because of the non-normal distribution of radiation dose, the dose in millisieverts was converted to the logarithm of millisieverts to obtain a normal distribution for regression analysis. All analyses were performed with SPSS version 18.0 for Windows (SPSS, Inc., Chicago, Illinois). A 2-tailed p value <0.05 was deemed significant.
Results
The 449 patients consisted of 247 and 202 patients before and after the implementation of the standardized protocol at the 3 sites. Patient and scan characteristics are listed in Table 2 . The age, BMI, and heart rate were similar between groups, with more women in the postprotocol cohort.
| Variable | Preprotocol (n = 247) | Postprotocol (n = 202) | p Value |
|---|---|---|---|
| Patient data | |||
| Age (years) | 57 ± 13 | 58 ± 13 | 0.33 |
| Men | 65% | 55% | 0.03 |
| BMI (kg/m 2 ) | 26 ± 5 | 27 ± 5 | 0.13 |
| Heart rate (beats/min) | 57 ± 8 | 57 ± 9 | 0.85 |
| Scan data | |||
| Prospective gating | 89% | 92% | 0.28 |
| 100-kV voltage | 13% | 67% | <0.001 |
| Tube current (mA) | 472 ± 134 | 412 ± 123 | <0.001 |
| Scan length (cm) | 14.1 ± 1.3 | 13.6 ± 1.8 | 0.001 |
| Radiation dose | |||
| Dose-length product (mGy × cm) | 166 (132–248) | 89 (55–117) | <0.001 |
| Dose (mSv) | 2.6 (2.0–4.2) | 1.3 (0.8–1.9) | <0.001 |
| Study quality | |||
| Signal (Hounsfield units) | 440 ± 93 | 492 ± 104 | <0.001 |
| Noise (Hounsfield units) | 43 ± 11 | 42 ± 12 | 0.42 |
| Signal/noise ratio | 11 ± 3 | 12 ± 4 | <0.001 |
| Interpretable study | |||
| Per patient (n = 449) | 96.4% | 95.5% | 0.66 |
| Per artery (n = 1,796) | 99.1% | 98.5% | 0.26 |
There was similar and frequent use of prospective ECG gating between groups, while postprotocol patients had increased use of 100-kV tube voltage, lower tube current, and decreased scan length, as an expected consequence of adherence to the study protocol.
Median radiation dose decreased from 2.6 to 1.3 mSv after the implementation of the standardized protocol (p <0.001; Figure 1 , Table 2 ). Although radiation dose reduction was observed for all techniques, the reduction was most effective by the use of prospective versus retrospective gating, followed by 100- versus 120-kV tube voltage, <450- versus ≥450-mA tube current, and <14- versus ≥14-cm scan length (p <0.001 for all; Figure 2 ). Doses <1 mSv were obtained in 8 of 239 (3%) and 77 of 202 (38%) patients before and after the initiation of the standardized protocol, respectively (p <0.001).
