Abstract
Recent advances in coronary computed tomography angiography (CCTA) increased its utilization as a tool for non-invasive detection of coronary artery disease (CAD). The aim of this analysis is to determine the impact of adopting new radiation dose-reducing techniques on the radiation exposure in consecutive patients undergoing CCTA. We included 1341 consecutive patients who underwent CCTA to rule out CAD between January 2007 and December 2013. New dose-reducing techniques were adopted in July 2010 in 1034 patients. These included high pitch scanning, 100 KVP (Kilovoltage Peak) imaging, and iterative reconstruction. The total radiation dose was calculated for each scan from the dose length product multiplied by conversion factor (0.014). The annual median radiation doses were compared over the study period. After the adoption of the new scanning techniques (n = 578), 53% of the scans were done with high pitch scanning, 46% with prospective gating, and 1% with retrospective gating. This was associated with >90% reduction in the radiation doses with a median radiation dose of 2.7, 1.5, 1.7, and 1.5 mSv (Millisievert) in 2010, 2011, 2012, and 2013, respectively. A total of 15 and 66% of the CCTA scans had a radiation dose of <1 and 2 mSv, respectively. There was no difference in the frequency of non-diagnostic studies or imaging quality before and after July 2010. Our analysis demonstrates that, in the current era, low-radiation CCTA can be routinely done in clinical practice.
Introduction
In recent years, coronary computed tomography angiography (CCTA) has emerged as a novel tool for the non-invasive detection of coronary artery disease (CAD).1,2 Multiple studies have demonstrated its high sensitivity and a negative predictive value for the detection and exclusion of CAD.3,4 However, CCTA was limited by the use of high-radiation doses, which may lead to downstream malignancies in the future.5,6 In addition, a multicenter study demonstrated significant variability in the radiation exposure associated with CCTA, even among users of the same scanners.6
In the past few years, all vendors developed new tools that allow to minimize the radiation exposure associated with CCTA, like prospective gating, 100 KVP (Kilovoltage Peak) imaging, iterative reconstruction, high pitch scanning and volumetric imaging.7,8 Two single-center studies suggested that the utilization of these tools would result in a significant reduction in the associated radiation dose in a highly selected subgroup of patients.9,10 However, there are limited data on whether these tools can be utilized in all consecutive patients undergoing CCTA. Thus, the aim of this analysis is to determine whether low-radiation doses can be routinely achieved in all consecutive patients referred for CCTA.
Methods
We included consecutive patients who underwent coronary CT angiography for the exclusion of CAD between September 2006 and December 2013. Patients younger than 18 years old were excluded from this analysis. In addition, patients with known CAD (prior myocardial infarction, prior angioplasty, and coronary artery bypass grafting), atrial fibrillation, and patients undergoing non-coronary cardiac CT were excluded from this analysis.
Coronary computed tomography angiography image acquisition
All CCTA scans were performed on either a 64-multidetector row scanner (High Definition Scanner, General Electric, Milwaukee, WI, USA) or a dual source cardiac CT system with high pitch scanning capability (Siemens Healthcare, Erlangen, Germany). Patients were in normal sinus rhythm and were capable of the breath-hold needed for CCTA. For patients presenting with baseline heart rates >65 b.p.m., beta-blockers were administered to slow the heart rate. Following a scout radiograph, contrast timing was determined using a test bolus (15 mL contrast) to detect optimal time for peak contrast opacification in the ascending aorta. Nitroglycerine sublingually was administered immediately before contrast injection. During CCTA acquisition, 60–80 mL of iodinated contrast was injected followed by a 20–30 mL saline flush. All CCTA scans were attended and sometimes performed by a consultant cardiologist with special training in CCTA.
Radiation dose–reduction program
In July 2010, the new radiation reduction tools became available in our center and were strictly utilized for every CCTA. A radiation reduction protocol was developed and included minimizing longitudinal scan range; use of sufficient beta-blockers to control heart rate and heart rate variability; using 100 KVP for patients with body mass index <30 kg/m2; use of high pitch scanning when possible; and minimizing the milliamperage used and applying iterative reconstruction in all cases in addition to direct supervision of the CCTA by a consultant physician.5
Estimation of radiation dose
Radiation doses were estimated from the scanner provided protocol summary that contain the dose–length product for each image series, which integrated estimated absorbed radiation in the x-, y-, and z-directions based on the CT dose–index volume. The effective radiation dose was derived from the summed dose–length product multiplied by the European Working Group for Guidelines on Quality Criteria in Computed Tomography conversion coefficient (κ = 0.014 mSv (Millisievert)/mGy × cm).6
Image quality assessment
Physicians rated the quality of each image on a per-patient basis at the time of scan interpretation. This is adopted from the prior published criteria.5 Excellent (score = 1) was defined as the complete absence of motion artefacts, excellent signal-to-noise ratio, and clear delineation of vessel walls, with the ability to assess luminal stenosis as well as plaque characteristics. Good (score = 2) was defined as non-limiting motion artefacts, reduced signal-to-noise ratio, and/or calcifications are present, with preserved ability to assess luminal stenosis as well as plaque characteristics. Fair (score = 3) was defined as reduced image quality due to any combination of noise, motion, poor contrast enhancement, or calcium that significantly impairs ease of interpretation, but image quality is sufficient to rule out significant stenosis. Non-diagnostic (score = 4) was defined as reduced image quality that precludes adequate assessment of stenosis in the majority of vessels.
Statistical analysis
Patients were grouped into two groups, before and after July 2010. Group comparisons were conducted between the two groups using χ2 testing or Students' t-testing when appropriate.
Results
A total of 1341 patients underwent CCTA in the study period with 307 patients (23%) before July 2010. The baseline characteristics of the included patients are summarized in Table 1. After July 2010, patients undergoing CCTA had a lower risk profile with less frequent hypertension, diabetes, and dyslipidaemia. However, there was no difference in the body mass index of the patients before or after the implementation of the low-radiation CCTA program (Table 1).
Baseline characteristics of the study population
| Before July 2010 | After July 2010 | P-value | |
|---|---|---|---|
| Mean age (years) | 50 ± 12 | 49 ± 12 | 0.433 |
| Male gender | 69% | 58% | <0.0001 |
| Hypertension | 66% | 52% | <0.0001 |
| Diabetes | 44% | 36% | 0.006 |
| Dyslipidaemia | 70% | 56% | <0.0001 |
| Smoking | 28% | 18% | 0.028 |
| Mean body mass index (kg/m2) | 32 ± 7 | 31 ± 6 | 0.685 |
| Before July 2010 | After July 2010 | P-value | |
|---|---|---|---|
| Mean age (years) | 50 ± 12 | 49 ± 12 | 0.433 |
| Male gender | 69% | 58% | <0.0001 |
| Hypertension | 66% | 52% | <0.0001 |
| Diabetes | 44% | 36% | 0.006 |
| Dyslipidaemia | 70% | 56% | <0.0001 |
| Smoking | 28% | 18% | 0.028 |
| Mean body mass index (kg/m2) | 32 ± 7 | 31 ± 6 | 0.685 |
Group comparisons are made before and after the implementation of the low-radiation imaging protocols.
Baseline characteristics of the study population
| Before July 2010 | After July 2010 | P-value | |
|---|---|---|---|
| Mean age (years) | 50 ± 12 | 49 ± 12 | 0.433 |
| Male gender | 69% | 58% | <0.0001 |
| Hypertension | 66% | 52% | <0.0001 |
| Diabetes | 44% | 36% | 0.006 |
| Dyslipidaemia | 70% | 56% | <0.0001 |
| Smoking | 28% | 18% | 0.028 |
| Mean body mass index (kg/m2) | 32 ± 7 | 31 ± 6 | 0.685 |
| Before July 2010 | After July 2010 | P-value | |
|---|---|---|---|
| Mean age (years) | 50 ± 12 | 49 ± 12 | 0.433 |
| Male gender | 69% | 58% | <0.0001 |
| Hypertension | 66% | 52% | <0.0001 |
| Diabetes | 44% | 36% | 0.006 |
| Dyslipidaemia | 70% | 56% | <0.0001 |
| Smoking | 28% | 18% | 0.028 |
| Mean body mass index (kg/m2) | 32 ± 7 | 31 ± 6 | 0.685 |
Group comparisons are made before and after the implementation of the low-radiation imaging protocols.
In accordance to the low-radiation protocol, the higher radiation retrospective gating protocol was rarely used after July 2010 as presented in Table 2 (<1%). Most patients were scanned using the lower radiation protocols (prospective gating 46% and high pitch scanning 52%) and more often with 100 KVP imaging (Figure 1)
Scanning parameters before and after the implementation of the low-radiation imaging protocols
| Before July 2010 | After July 2010 | P-value | |
|---|---|---|---|
| Mean heart rate (b.p.m.) | 65 ± 10 | 62 ± 8 | 0.274 |
| Mode of image acquisition | |||
| Prospective gating | 10% | 46% | <0.0001 |
| Retrospective gating | 90% | 1% | |
| High pitch scanning | 0% | 53% | |
| Before July 2010 | After July 2010 | P-value | |
|---|---|---|---|
| Mean heart rate (b.p.m.) | 65 ± 10 | 62 ± 8 | 0.274 |
| Mode of image acquisition | |||
| Prospective gating | 10% | 46% | <0.0001 |
| Retrospective gating | 90% | 1% | |
| High pitch scanning | 0% | 53% | |
Scanning parameters before and after the implementation of the low-radiation imaging protocols
| Before July 2010 | After July 2010 | P-value | |
|---|---|---|---|
| Mean heart rate (b.p.m.) | 65 ± 10 | 62 ± 8 | 0.274 |
| Mode of image acquisition | |||
| Prospective gating | 10% | 46% | <0.0001 |
| Retrospective gating | 90% | 1% | |
| High pitch scanning | 0% | 53% | |
| Before July 2010 | After July 2010 | P-value | |
|---|---|---|---|
| Mean heart rate (b.p.m.) | 65 ± 10 | 62 ± 8 | 0.274 |
| Mode of image acquisition | |||
| Prospective gating | 10% | 46% | <0.0001 |
| Retrospective gating | 90% | 1% | |
| High pitch scanning | 0% | 53% | |
Utilization of the low tube voltage before and after the implementation of a low-radiation-dose imaging protocol. As per the protocol, more patients are imaged with the lower voltage despite no change in the body mass index of the patients before and after July 2010.
Adherence to the low-radiation protocol resulted in a significant drop in the radiation dose from a median of 17.7–1.6 mSv after July 2010 (Figure 2). Most patients after July 2010 had a radiation dose of <2 mSv as shown in Figure 3. Only 6.2% of the study population had a radiation dose of >5 mSv after July 2010, compared with 91% prior to July 2010 (P < 0.0001). Most importantly, there were no differences in the image quality before or after the implementation of the low-radiation protocol or in the rate of poor quality scans (Figure 4).
Median annual radiation dose. There was >90% drop in the radiation dose after the implementation of low-radiation-dose imaging protocol (P < 0.0001). It was also sustained for 4 years.
Percentage of patients according to the radiation dose (P < 0.0001). Coronary computed tomography angiography with a radiation dose of <5 mSv was rarely possible before July 2010.
Image quality before and after the implementation of low-radiation-dose imaging protocol. There were no differences in the image quality before or after the implementation of the low-radiation protocol or in the rate of poor quality scans.
Discussion
Our analysis shows that, in the current era, low-radiation CCTA can be routinely used in consecutive patients without prior CAD. This is possible with the utilization of the new imaging radiation lowering techniques and tools that allow the minimization of radiation exposure without impacting the image quality or the diagnostic accuracy of the study.
Our analysis is in agreement with prior analysis from the advanced consortium of cardiovascular imaging analysis from the state of Michigan, which showed that utilizing radiation exposure lowering measures would allow for routine low-dose CCTA. The median radiation dose in the consortium was 4.8 mSv at the end of the study.11 In our analysis, we took this further and showed that the radiation dose can be lowered further to <2 mSv. This is achieved primarily by applying iterative reconstruction in every patient and selecting the imaging protocol that is associated with the least radiation exposure according to the patient's clinical status. In addition, the presence of the physician by the scanner console in every case allowed for adjusting the scanning parameters and achieving this result.
In conclusion, we have shown that in the current CCTA can be and should be routinely done in consecutive patients with low-radiation doses.
Conflict of interest: none declared.
