High-quality low-dose cardiovascular computed tomography (CCT) in pediatric patients using a 64-slice scanner

Abstract

Background

Cardiovascular computed tomography (CCT) technology is rapidly advancing allowing to perform good quality examinations with a radiation dose as low as 1.2 mSv. However, latest generation scanners are not available in all centers.

Purpose

To estimate radiation dose and image quality in pediatric CCT using a standard 64-slice scanner.

Material and Methods

A total of 100 patients aged 6.9 ± 5.4 years (mean ± standard deviation) who underwent a 64-slice CCT scan using 80, 100, or 120 kVp, were retrospectively evaluated. Radiation effective dose was calculated on the basis of the dose length product. Two independent readers assessed the image quality through signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and a qualitative score (3 = very good, 2 = good, 1 = poor). Non-parametric tests were used.

Results

Fifty-five exams were not electrocardiographically (ECG) triggered, 20 had a prospective ECG triggering, and 25 had retrospective ECG triggering. The median effective dose was 1.3 mSv (interquartile range [IQR] = 0.8–2.7 mSv). Median SNR was 30.6 (IQR = 23.4–33.6) at 120 kVp, 29.4 (IQR = 23.7–34.8) at 100 kVp, and 24.7 (IQR = 19.4–34.3) at 80 kVp. Median CNR was 21.0 (IQR = 14.8–24.4), 19.1 (IQR = 15.6–23.9), and 25.3 (IQR = 19.4–33.4), respectively. Image quality was very good, good, and poor in 56, 39, and 5 patients, respectively. No significant differences were found among voltage groups for SNR (P = 0.486), CNR (P = 0.336), and subjective image quality (P = 0.296). The inter-observer reproducibility was almost perfect (κ = 0.880).

Conclusion

High-quality pediatric CCT can be performed using a 64-slice scanner, with a radiation effective dose close to 2 mSv in about 50% of the cases.

Keywords

Cardiovascular computed tomography pediatric computed tomography congenital heart diseases radiation dose 64-slice CT image quality

Introduction

In the setting of congenital heart diseases (CHD), non-invasive imaging techniques such as echocardiography, cardiac magnetic resonance (CMR), and cardiovascular computed tomography (CCT) play a crucial role in the visualization of cardiovascular structures (1).

Echocardiography, being a ubiquitous and radiation-free technique, represents the first approach for patients having or suspected to have CHD, but it has limitations in defining complex anatomy and reliable imaging of coronary arteries, especially in older children who have a poor acoustic window (1,2). CMR is considered the standard of reference for evaluation of ventricular volumes and valve regurgitation but it still usually requires relatively long imaging times and sedation or anesthesia in children aged < 8 years as well as in developmentally delayed patients of all ages (3 –5). CCT and invasive angiography expose patients to ionization radiation, with potentially more dangerous effects in younger patients. A previous study demonstrated that the radiation exposure from diagnostic catheterization is substantially higher than that from CCT in a pediatric population (1).

In recent years, CCT technology has advanced rapidly. It now provides improved spatial and temporal resolution. Electrocardiographically (ECG)-gated coronary CCT can now be routinely obtained in pediatric patients with a radiation dose as low as 1.2 mSv using dual-source CCT technology (6). Unfortunately, these scanners are still available in few centers. On the other hand, in order to reduce radiation exposure keeping a good image quality, radiologists can apply tailored protocols for CCT in pediatric patients even using 64-slice scanners which are currently a kind of “standard” technology in radiology department.

Thus, our aim was to estimate radiation dose and image quality in pediatric CCT using a 64-slice scanner.

Material and Methods

Patient population

Ethics Committee approval was obtained for this retrospective study. A total of 100 CCT exams of patients aged < 18 years, performed between January 2010 and December 2016 at our Institution, were retrospectively evaluated. Disease distribution in the study population is summarized in Table 1.

Table 1.

Disease distribution in the study population.

Disease	Patients (n)
Known or suspected coronary abnormality
Anomalous origins	18
Coronary fistula	6
Complex congenital heart disease
Tetralogy of Fallot	20
Transposition of great arteries	10
Truncus arteriosus	4
Aortic disease
Stent evaluation in coarctation	18
Interrupted or hypoplastic arch	2
Pulmonary artery disease
Pulmonary artery stenosis with stent	11
Subvalvar pulmonary stenosis	3
Major aorto-pulmonary collateral arteries in pulmonary atresia	3
Total	100

Image acquisition

At our department, all CCT studies, including those performed in pediatric patients, are performed under the direct responsibility of a cardiovascular radiologist. To minimize technical errors, technicians are carefully instructed by the radiologist on a case-by-case basis.

The CCT examinations were performed on a 64-slice CT scanner (Somatom 64, Siemens Healthcare, Erlangen, Germany). Patients aged less than 3 years needed sedation. The administration of sedative drugs happened shortly before the CT exam. The anesthesiologist monitored the patient conditions during the procedure and evaluated the patient status after the exam. Midazolam was administered intravenously (dose of 0.1–1.1 mg/kg), orally (dose of 0.5–0.6 mg/kg), or intramuscularly (only in one patient, dose of 0.2 mg/kg). The administration route of ketamine was intravenous (dose of 0.9–1.9 mg/kg) or intramuscular (dose of 3.3–5.4 mg/kg). Propofol was intravenously administered at a dose of 0.5–2.4 mg/kg. The variability of the administered doses depended on age, comorbidities, and known drug response of the patient.

A tailored CCT protocol was performed according to the clinical question. In the majority of patients (n=80), the unenhanced scan was waived to reduce the radiation dose. A bolus of contrast material of 5–60 mL (Iopamiro 370, Bracco Imaging S.p.A., Milan, Italy) followed by saline solution in the range of 10–60 mL was intravenously injected by means of a power injector (Empower CTA, EZEM, Westbury, NY, USA) at a flow rate of 1.5–3.0 mL/s according to the patient’s characteristics and the clinical question.

When investigating cardiac anatomy or coronary arteries, a test-bolus technique was used. A time-attenuation curve was obtained by measuring the enhancement within a region of interest (ROI) positioned in a ascending aorta or in the pulmonary trunk according to the clinical question. The contrast arrival time was determined from the time to peak enhancement and was used to estimate the scan delay for a full-bolus diagnostic CCT (7).

To acquire an angiogram, we used the bolus tracking technique, based on real-time monitoring of the main bolus during injection to acquire a series of dynamic low-dose monitoring scans at the level of the vessel of interest. The trigger threshold inside the ROI was set at 100 HU above the baseline. The delay between each monitoring scan acquisition was 1.25 s. As soon as the threshold was reached, the table moved to the cranial start position. During this interval the contrast material concentration increased to the desired level of enhancement (8).

The CCT were either performed without ECG synchronization or using a prospective or a retrospective ECG-gating depending on the patient’s heart rate and rhythm and on whether an evaluation of myocardial function was indicated (9). Tube voltage was set at 80, 100, or 120 kVp; tube current was set between 36 and 100 mAs, according to body size. The gantry rotation time was 0.33 s; pitch was 0.2–0.5. The reconstruction parameters were set as follows: section thickness = 0.75 mm; reconstruction interval = 0.45mm; matrix size = 512 × 512; and field of view = 250 mm. Two-dimensional images were then transferred to a workstation (Multimodality, Siemens Healthcare, Erlangen, Germany) for obtaining off-line three-dimensional reconstructions.

Radiation exposure

The dose length product (DLP) was retrieved for each patient. The effective dose (ED) in mSv was calculated as DLP*k. The conversion factor for the chest, k (measured in mSv/mGy/cm), varied with age and was estimated from the International Commission on Radiological Protection publication 103 recommendation (10,11). The ED was then evaluated for four patient age groups (newborns < 1 year, 1–5 years, 6–10 years, and 11–17 years), and according the tube voltage used (kVp).

Image quality assessment

Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were calculated for each scan using the following formulas: SNR = HU_{left ventricle}/SD_air; CNR = [(HU_{left ventricle} − HU_myocardium)/SD_air].

Assessment of subjective image quality was independently performed by two observers with eight and four years of experience in cardiovascular imaging. Image analysis was performed individually and image series were evaluated in a random order. Scans were classified using a three-level scale (3 = very good; 2 = good; and 1 = poor). The two readers agreed on the following criteria:

‐ images were judged as very good when all structures were well visualized without artifacts;

‐ images were judged as good when almost all structures were well visualized even though some artifacts were visible;

‐ images were judged as poor when the vascular structures were not well visualized due to the presence of large artifacts.

Statistical analysis

Statistical analyses were performed using statistical software (SPSS for Windows v.21.0, SPSS Inc., Chicago, IL). Parametric variables were expressed as mean and standard deviation (SD), whereas non-parametric variables were expressed as median and interquartile ranges (IQR). Overall comparisons among groups were performed using the Kruskal–Wallis test; the paired post-hoc analysis of the two groups was performed using the Mann–Whitney U test. A P value < 0.05 was considered as significant.

The inter-observer agreement about the image quality qualitative assessment was evaluated using the Cohen κ, with the following interpretation of the κ values: <0.20 = slight agreement; 0.21−0.40 = fair agreement; 0.41−0.60 = moderate agreement; 0.61−0.80 = substantial agreement; 0.81−1.0 = almost perfect agreement (12).

Results

Study population

The study population included 100 patients (63 boys, 37 girls), aged 6.9 ± 5.4 years (mean ± SD). The most common primary indications for CCT were complex CHD (n = 34), pulmonary arteries abnormalities (n = 22), and coronary arteries abnormalities (n = 24). A detailed list of indications is provided in Table 1.

Radiation exposure

The overall median ED was 1.3 mSv (IQR = 0.8–2.7 mSv). Concerning the subgroup analysis, the median ED was 1.0 mSv (IQR =0.6–1.4 mSv) for exams performed at 80 kVp, 1.9 mSv (1.1–3.5 mSv) for exams performed at 100 kVp, and 5.1 mSv (3.6–6.0) for exams performed at 120 kVp (P < 0.001). Post-hoc analysis is reported in Table 2. Examples of CCT images obtained at 80, 100, and 120 kVp are shown in Figs. 1 –4.

Table 2.

Effective dose by tube voltage.

	Effective dose (mSv)	P value
80 kVp (n = 49)	1.0 (0.6–1.4)	<0.001
100 kVp (n = 35)	1.9 (1.1–3.5)
120 kVp (n = 16)	5.1 (3.6–6.0)
80 kVp vs. 100 kVp		<0.001
80 kVp vs. 120 kVp		<0.001
100 kVp vs. 120 kVp		0.003

Data of effective dose are medians and interquartile ranges in parentheses. The Kruskal–Wallis test was used for the overall comparison while the Mann–Whitney U test was used for the post-hoc analysis.

Fig. 1.

Cardiac CT of a one-month-old newborn with a double outlet right ventricle. Prospectively ECG-triggered scan for the definition of coronary and intra-cardiac anatomy was performed. Effective dose was 0.3 mSv using 80 kVp. On the left panel, the abnormal origin of left main coronary artery is shown. On the right panel, a 3D reconstruction of great vessels anatomy is shown (double outlet right ventricle).

Fig. 2.

Cardiac CT of a two-year-old with tetralogy of Fallot with sub-pulmonary stenosis surgically treated. Retrospectively ECG-triggered scan for the definition of coronary and intra-cardiac anatomy was performed before a new surgical procedure. Effective dose was 1.1 mSv using 80 kVp. Anomalous left coronary origin from right sinus passed anterior of right ventricle outflow tract is shown (arrows).

Fig. 3.

Cardiac CT of an eight-year-old girl with tetralogy of Fallot treated with a pulmonary conduit. An angiographic not ECG-triggered scan was performed for evaluating pulmonary stents. Effective dose was 1.1 mSv using 100 kVp. On the left panel, a thrombosis of the pulmonary conduit due to endocarditis is shown. On the right panel, a 3D reconstruction of pulmonary conduit and pulmonary arteries is shown.

Fig. 4.

Cardiac CT of a 14-year-old girl with tetralogy of Fallot and pulmonary stenosis. An angiographic not ECG-triggered scan was performed to evaluate pulmonary stents. The effective dose was 2 mSv using 120 kVp. On the left panel, a maximum intensity projection of both stents is shown. On the right panel, a 3D reconstruction of pulmonary arteries is shown.

Comparing not ECG-triggered, prospectively ECG-triggered, and retrospectively ECG-gated examinations, median ED were 1.1 (0.6–3.1) mSv, 1.3 (1.0–1.8) mSv and 1.7 (1.3–3.5) mSv, respectively (P < 0.036). Post-hoc analysis is reported in Table 3.

Table 3.

Effective dose for not ECG-synchronized examinations, prospectively ECG-triggered examinations, and retrospectively ECG-gated examinations.

	Effective dose (mSv)	P value
A Not ECG-synchronized (n = 55)	1.1 (0.6–3.1)	0.036
B Prospective ECG-triggering (n = 25)	1.3 (1.0–1.8)
C Retrospective ECG-gating (n = 20)	1.7 (1.3–3.5)
A vs. B		0.347
A vs. C		0.022
B vs. C		0.027

Finally, a significantly different ED was also found among the age groups (P < 0.001). Post-hoc analysis is reported in Table 4.

Table 4.

Effective dose by patient age.

	Effective dose (mSv)	P value
A Newborn (n = 15)	1.0 (0.5–1.3)	<0.001
B 1–5 years (n = 25)	0.8 (0.6–1.2)
C 6–10 years (n = 27)	1.6 (1.1–2.4)
D 11–17 years (n = 33)	3.4 (1.5–5.1)
A vs. B		0.639
A vs. C		0.038
A vs. D		0.020
B vs. C		0.003
B vs. D		<0.001
C vs. D		0.081

Image quality

Overall, the image quality was judged by the first reader (most expert) to be very good in 56 examinations (56%), good in 39 (39%), and poor in five (5%). The agreement between the two readers was almost perfect (κ = 0.880). A non-significant difference in terms of subjective image quality was found, both overall and among the three groups of kilovoltage used (p≥0.296). In terms of objective image quality, the SNR was 30.6 (IQR = 23.4–33.6), 29.4 (IQR = 23.7–34.8), and 24.7 (IQR = 19.4–34.3), at 120, 100, and 80 kVp, respectively (P = 0.486). The median CNR was 21.0 (IQR = 14.8–24.4), 19.1 (IQR = 15.6–23.9), and 25.3 (IQR = 19.4–33.4), at 120, 100 and 80 kVp, respectively (P = 0.336).

There were not significant differences according to SNR and CNR in the different age-group patients. In particular, the median of SNR was 30.5 (IQR = 22.5–36.6), 25.2 (IQR = 15.1–30.9), 24.9 (IQR = 20.6–41.1), and 24.4 (IQR = 20.7–30.9) in newborn, 1–5, 6–10, and 11–17 age groups, respectively (P = 0.227). The mean CNR was 19.4 (IQR = 24.8–23.5), 20.1 (IQR = 10.8–20.9), 23.3 (IQR = 13.7–41.1), and 18.4 (IQR = 10.7–20.8) in newborn, 1–5, 6–10, and 11–17 age groups, respectively (P = 0.101).

Discussion

In this study, we assessed radiation exposure and image quality of 64-slice CCT in a consecutive series of 100 pediatric patients. Using a tailored dose-saving protocol with careful radiologist’s supervision of technical performance, the overall median ED was limited to 1.3 mSv, including both prospective and retrospective ECG-triggering. Image quality was very good or good in 95/100 patients. Of note, the inter-observer reproducibility for the qualitative evaluation of image quality was almost perfect.

Our results can be favorably compared with those obtained by other authors. Tsai et al. (13) described an average ED for pediatric CCT of 2.6, 2.1, and 2.0 mSv for 16-, 64-, and 128-slice CCT, respectively, using a prospectively triggered acquisition. Conversely, the dose estimated in other studies was 6.8 mSv (10) or 12 mSv (14) using a 64-slice scan with retrospective ECG-gating.

In recent years, the advent of new generation scanners implied a drastic decrease in radiation exposure, also in the pediatric setting. The study published by Han et al. (3) analyzed a cohort of 70 pediatric patients and found an average ED of 1.7 mSv for retrospectively ECG-gated CCT and 0.9 mSv for prospectively ECG-triggered CCT. These findings highlighted the increasing role of CCT in the pediatric setting, in particular as a tool that allows to avoid diagnostic angiography or to limit or overcoming the diagnostic phase of interventional angiography. In fact, Lee et al. (15) demonstrated that in a population of 14 neonates with complex CHD referred for diagnostic cardiac catheterization after initial assessment with echocardiography and CCT, none of them required additional diagnostic imaging. This is a goal in terms of reduction of radiation dose considering the 13.4 mSv that are required on average for a diagnostic catheterization (1).

Our study showed that using a “standard” 64-slice scanner high-quality images can be obtained with a relatively low radiation exposure, fulfilling the diagnostic aim of the examination. This is an important clinical finding considering that 64-slice CT units remain the most available type of CT scanner in the majority of radiology departments (16 –18).

Rapid technological development resulted in accelerated technical and functional obsolescence of imaging equipments, creating a need for renewal (19). A dramatic change in this scenario, in the current era of “spending review” by public health systems, is not expected.

Thus, implementing CCT dose-saving protocols on 64-slice scanners should be considered as mandatory, especially in the pediatric population.

Importantly, since no differences were found in terms of SNR and CNR among the different tube voltages used (120, 100, or 80 kVp), our experience suggests that an 80-kVp protocol could be adequate to image most pediatric patients.

The results in terms of CNR deserve a particular comment. Although the difference among the groups was not statistically significant, probably due to the small sample size combined with data distribution, a higher CNR at the lowest tube voltage (80 kVp) was observed (median 25.3 vs. 21.0 for 120 kVp and 19.1 for 100 kVp). This possible increase in CNR could be explained with the higher contrast effect of the iodinated contrast material at lower voltages (20). Notably, the agreement in image quality evaluation between the two observers was not conditioned by the voltage used.

The approach here presented (tailored protocols under strict control by the cardiovascular radiologist) could be generally applied also to late-generation CT scanners, allowing for a lower and lower dose exposure, in particular in the pediatric population. All these results are inverting the traditional way of thinking about the comparison between CCT and CMR in pediatric patients. Most probably, CMR will no longer be an easy winner because of being radiation-free. As radiation doses go lower and lower, towards 0.1–0.2 mSv, examination time and need for sedation, spatial resolution as well as image quality related to movement artifacts can play in favor of CCT. In addition, when considering the probability of multiple CMR examinations, the potential gadolinium accumulation in the brain should be taken into account (21).

The results of this study should be interpreted in view of its limitations. First, we should consider the retrospective study design. However, we included the whole consecutive series of pediatric patients who underwent CCT at our institution in the study period (starting from the installation of the 64-slice unit). Thus, the study reports what happened in real clinical life. Second, the number of patients is rather small as CMR is still preferred in the evaluation of CHD patients.

In conclusion, CCT is a valuable imaging modality when evaluating pediatric patients with a large spectrum of known or suspected cardiovascular abnormalities. Using dose-saving techniques, CCT protocols tailored to the pediatric population allowed for performing high-quality CCT in children with a relatively low radiation exposure also using a “standard” 64-slices scanner.

Footnotes

Declaration of Conflicting Interests

The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: F Secchi and G Di Leo have been sponsored to congresses by Bracco Imaging SpA (Milan, Italy). F Sardanelli received research grants from Bayer Healthcare (Berlin, Germany) and Bracco Imaging SpA (Milan, Italy); moreover is member of advisory board for Bracco Imaging SpA (Milan, Italy) and General Electric Healthcare (Buc, France).

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by local research funds of the IRCCS Policlinico San Donato, a Clinical Research Hospital partially funded by the Italian Ministry of Health.

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