Radiation dose management for pediatric cardiac computed tomography: a report from the Image Gently ‘Have-A-Heart’ campaign

  • Cynthia K. Rigsby
  • Sarah E. McKenney
  • Kevin D. Hill
  • Anjali Chelliah
  • Andrew J. Einstein
  • B. Kelly Han
  • Joshua D. Robinson
  • Christina L. Sammet
  • Timothy C. Slesnick
  • Donald P. Frush

Abstract

Children with congenital or acquired heart disease can be exposed to relatively high lifetime cumulative doses of ionizing radiation from necessary medical imaging procedures including radiography, fluoroscopic procedures including diagnostic and interventional cardiac catheterizations, electrophysiology examinations, cardiac computed tomography (CT) studies, and nuclear cardiology examinations. Despite the clinical necessity of these imaging studies, the related ionizing radiation exposure could pose an increased lifetime attributable cancer risk. The Image Gently “Have-A-Heart” campaign is promoting the appropriate use of medical imaging studies in children with congenital or acquired heart disease while minimizing radiation exposure. The focus of this manuscript is to provide a comprehensive review of radiation dose management and CT performance in children with congenital or acquired heart disease.

Keywords

Computed tomography Children Congenital heart disease Heart Radiation safety 

Introduction

Diagnostic imaging and image-guided interventions are essential in the care of children with known or suspected congenital or acquired heart disease. Many imaging modalities, including radiography, fluoroscopy/angiography, computed tomography (CT) and nuclear imaging depend on ionizing radiation for image formation. Ionizing radiation in high doses can have biological effects, including carcinogenesis [1]. However, the amount of radiation delivered for diagnostic imaging is typically low-level. Evidence of deoxyribonucleic acid (DNA) damage has been shown to occur with ionizing radiation doses in the medical imaging range [23], but how this relates to the risk of cancer is not known [1]. There have been reports of excess cancer risk in patients exposed to CT as children [45], but the challenges to the methodologies of these studies, including the possibility of reverse causation, highlight the complexities of studying excess cancer risk in populations [6789]. Children are especially vulnerable to the harmful effects of ionizing radiation due to the greater sensitivity of growing tissue and a longer anticipated lifetime to manifest radiation-induced damage as compared to adults [10]. Moreover, because of reduced tissue attenuation, similar exposures in smaller children result in greater doses to organs and tissues than in larger children (i.e. teenagers) and adults.

The scientific and medical communities are engaged in a dialogue regarding the relationship of radiation from modalities such as CT, and cancer risk. Opinions range from a perspective that small amounts of radiation might reduce cancer risk (hormesis), to positions that risk is effectively zero, linearly related to dose, or that risk is greater than linear [45111213141516]. The majority of the expert communities, reflected by reports from the International Commission on Radiological Protection, the United States National Academies and the National Council on Radiation Protection and Measurements [1017181920], subscribe to the position that the risk associated with low-dose ionizing radiation exposure is uncertain, with a linear relationship between dose and risk reasonably fitting the limited available data. This perspective is reflected in the principle of ALARA (dose as low as reasonably achievable) and incorporates both justification and optimization. Justification indicates that imaging is appropriately indicated and that the expected benefits are sufficiently greater than the risks. Optimization entails performing the study so that there is due consideration given to both image quality and radiation exposure, using only as much radiation as is necessary to address the clinical questions. A scientific statement of expert consensus recommendations for multimodality optimization of medical imaging procedures commonly performed in children with congenital or acquired heart disease was written as part of the Image Gently Have-A-Heart campaign [21]. This Image Gently report focuses on radiation dose management and cardiac CT performance in children with congenital or acquired heart disease.

Several factors guide the discussion of CT radiation and risk in children congenital or acquired heart disease: (1) increasing value and use of this modality even in the youngest children [222324], (2) relatively high radiation dose of CT compared with radiography, (3) longer anticipated survival of children relative to adults, with the potential for larger cumulative doses, and (4) less familiarity with the proper performance of pediatric CT for congenital or acquired heart disease among those who care for these patients relatively infrequently [232526272829]. Health care providers as well as patients, parents, other caregivers and the public have a limited and sometimes inaccurate understanding of which imaging examinations use radiation, the potential and effective radiation doses of these exams, and the attendant risks [30313233343536373839]. For these reasons, we review key aspects of cardiac CT performance in children with congenital or acquired heart disease as a reference with respect to radiation dose management for those involved in the care of these children including primary providers, pediatric radiologists, cardiologists, surgeons, physicists, technologists and nurses. Topics include radiation dose metrics, reported cardiac CT dose ranges, considerations for selecting cardiac CT as an imaging modality, techniques for optimum performance of cardiac CT, risk communication with patients and caregivers, and implementation of institutional CT dose monitoring.

Radiation dose metrics

Effective dose is a calculated whole-body metric reported in the SI (International System of Units) unit of Sievert (Sv). Effective dose reflects the relative risk of detrimental biological effects from exposure to ionizing radiation and accounts for the absorbed dose to all cancer-susceptible organs of the body, the relative harm level of the type of ionizing radiation, and the organ radiation sensitivities with respect to cancer susceptibility from ionizing radiation exposure. As a whole-body metric, effective dose is useful for comparing doses across different imaging modalities that employ ionizing radiation (e.g., radiographs, fluoroscopy, CT, nuclear medicine) [4041].

CT dose reports

Current CT scanners provide a DICOM structured dose report containing the volumetric CT dose index (CTDIvol) and dose–length product (DLP; Fig. 1) [42]. CTDIvol is a measurement of radiation output of a CT scanner that is calculated by irradiating a cylindrical acrylic phantom with either a diameter of 32 cm (models the adult body) or 16 cm (models the adult head) using standard conditions and methods [4344]. Air kerma (in mGy) measurements at the edge and center of the phantom using an ionization chamber are consolidated with a weighted average then scaled by the pitch of the helical scan to calculate the CTDIvol.. CTDIvol is useful for comparing the overall effects of dose modification efforts on a single scanner, and for comparing the difference in radiation output for a specific exam among different CT scanners. The radiation output of a CT scanner might be based on patient-specific information, such as the use of the patient localizer images for tube current modulation based on tissue attenuation [45]. DLP is calculated from the product of CTDIvol and irradiated scan length (z-axis), and is reported in units of mGy·cm. Neither CTDIvol nor DLP is patient-specific because they are both measurements of radiation output and not of patient dose [46]. DLP does not differentiate dose between child and adult cardiac CT scans using the same parameters and z-axis scan length. However, CTDIvol and DLP are widely used dose metrics because these values are automatically generated by the CT scanner both before and after each exam.

Fig. 1

CT dose report page from a cardiac CT examination in a 2-year-old boy. Each scanned series is listed in the report page. Kilovoltage (kV), milliamperes (mAs)/reference mAs for dose modulation purposes, volumetric CT dose index (CTDIvol), dose–length product (DLP), rotation time in seconds (TI) and slice thickness (cSL) are listed for each scanned series. The 32-cm phantom diameter is indicated by the asterisk adjacent to the CTDIvol. The control scan is a single-slice localizer for the test bolus. The CTDIvol for the control scan is less than the DLP because the slice thickness is 5.0 mm. The diagnostic scan was performed as an electrocardiography-triggered high-pitch helical cardiac CT examination with a DLP of 16.2 mGy·cm

Unlike CT scanner radiation output measurements of CTDIvol and DLP, effective dose estimation for CT is not standard and continues to evolve [47]. Effective dose estimation methodologies include physical phantom-based measurements, calculated estimates from the product of DLP and a conversion or k-factor based on the region of the body scanned (estimated effective dose in mSv = k×DLP), and Monte Carlo simulations [41]. These methods have varying degrees of calculated effective dose agreement [404849]. As a result, estimates of effective dose for a CT study performed on one patient might differ substantially because of effective dose calculation methodology alone [4749]. Additionally, and unlike many other body CT applications, there is no consensus on a standard calculation method for cardiac CT effective dose [50]. There are published conversion factors for head, neck, chest, and abdomen and pelvis CT examinations. Chest conversion factors have been used to calculate effective dose for cardiac CT examinations, but the irradiated z-axis scan length for a chest CT examination is generally greater than for a cardiac CT study. Additionally, the electrocardiographic (ECG) synchronization used for cardiac CT is not accounted for by the chest conversion factors. Pediatric cardiac-CT-specific conversion factors have been published, but there are no cardiac conversion factors for children of all sizes, all scanner types or all cardiac CT scan protocols [505152]. However, pediatric cardiothoracic CT conversion factors representing the calculated average of multiple published pediatric cardiac and chest CT conversion factors have the approval of the American Association of Physicists in Medicine and represent a useful alternative to the not yet universally available cardiac-specific CT conversion factors [21]. Last, the generic weighting of radiosensitive organs is included in the effective dose calculation; however, individual patient metrics (such as variation in organ size and location, or age-related sensitivities) are not reflected in this calculation. Steps toward describing the radiation absorbed by the imaged patient have been proposed that include the size-specific dose estimate (SSDE), a metric that involves the anteroposterior and lateral patient dimensions (effective diameter) and patient attenuation metrics (water-equivalent diameter) in adjustments for CTDIvol [5354]. Patient dimensions can be measured from the CT localizer radiographs or from axial images, or they can be directly measured on the patient using calipers. Water-equivalent diameter can be calculated from the CT localizer radiographs or axial CT images. SSDE has not been widely adopted for cardiac CT dose estimation [5556].

Radiation effects

Ionizing radiation is associated with tissue reactions (formerly known as non-stochastic or deterministic effects) and stochastic effects. Tissue reactions such as skin ulceration and hair loss only occur when absorbed radiation doses exceed a threshold value [1]. Tissue reactions can result from cumulative absorbed dose within a short period of time (on the order of days). Tissue reactions are rarely seen in children because reasonable image quality is obtained with relatively low radiation output. In contrast, stochastic effects such as tumorigenesis are probabilistic (i.e. “all-or-nothing”), meaning that it is difficult to predict when or whether an effect will occur and there is no known threshold below which no risk of occurrence can be assumed. The linear no-threshold model assumes that the risk of cancer induction is linearly proportional to organ dose and no amount of incremental radiation exposure is without risk. The linear no-threshold model was determined by extrapolation from epidemiological studies conducted for populations exposed to radiation doses ranging from 5 mSv to 100 mSv [10]. Although the linear no-threshold model is contested, it has been generally accepted based upon the existing epidemiological and radiobiological evidence of the risk of cancer induction in a population from low-dose ionizing radiation [1057].

The carcinogenic risk to the population as a result of a diagnostic imaging procedure can be estimated from the absorbed organ doses, for example using the BEIR (Biological Effects of Ionizing Radiation) VII risk models or the National Cancer Institute’s Radiation Risk Assessment Tool (RadRAT) [1058]. Although these estimates are useful for epidemiological studies, they cannot be used effectively for individual patient risk assessment [59].

Pediatric cardiac CT dose ranges

Table 1 provides a broad overview of selected published effective dose ranges for pediatric cardiac CT studies [2428405051525660616263646566676869707172]. Notably the data in this table reflect inconsistencies in the way that effective dose is calculated for cardiac CT studies. Examinations are performed in patients of different ages using different scanner manufacturers with varying ECG-synchronization schemes and tube potentials. Dose calculation methodology and weighting factors between studies also vary. However, general trends are present [2428405051525660616263646566676869707172]. Most exams are performed at 80 kV, with the tube current reflecting age- or weight-based adjustments. The effective doses generally increase with age and range from 0.2 mSv to 9.6 mSv for children younger than 1 year, with larger ranges for children between 1 year and 10 years of 0.5–28 mSv, and 0.4–10.3 mSv for older children. Prospectively ECG-triggered exams have overall lower effective doses (0.05–5.8 mSv) than retrospectively ECG-gated exams (0.5–28 mSv).

Table 1

Selected published cardiac CT effective dose ranges

Patient agea

Number of patients

Exam type

Scanner type(s)

Tube potential (kV)

Effective dose (mSv)a

Effective dose MOSFET (mSv)a

Weighting factor

DLP (32-cm phantom, mGy·cm)a

Source

10.7±6.2 yrs

50

RG CT

Siemens

64-slice MDCT

64-slice DSCT

128-slice DSCT

80, 100, 120

6.1 (2.5–10.6)

ICRP 103 [4060]

211.5 (64.0–648.0)

Ghoshhajra[61]

15

PT CT

2.2 (0.9–3.4)

136.0 (43.3–188.0)

30

HPH CT

0.9 (0.6–1.8)

49.0 (18.0–82.0)

0–6 yrs

25

RG CT, PT CT, HPH CT

2.3 (1.4–4.3)

53.0 (28.0–91.8)

6–12 yrs

27

1.8 (0.6–4.7)

82.0 (23.8–205.5)

12–18 yrs

43

2.9 (1.2–10.3)

193.0 (71.8–657.8)

1.2 (0.02–5.4) yrs

30

PT CT

Siemens

128-slice DSCT

80

0.26±0.16 (0.05–0.8)

ICRP 60 [626364]

5.7±4.8 (1–22)

Paul [65]

0–3 yrs

10

PT & RG CT

Siemens

16-slice MDCT

64-slice MDCT

128-slice DSCT

80, 100, 120

2.2 (0.4–4.9)

ICRP 103 [4060]

Westra [56]

3–8 yrs

11

4.7 (0.8–14.4)

8–15 yrs

10

2.5 (0.1–11.3)

>15 yrs

11

2.6 (0.4–7.9)

132 (1–361) days

110

cCT

Siemens

64-slice DSCT

80

0.5±0.2 (0.2–0.9)

ICRP 60 [626364]

8±6 (4–18)

Ben Saad [66]

32

RG CT

80

1.3±0.6 (0.6–2.8)

21±9 (10–39)

0.4 (0–6) yrs

108

PT CT

Siemens

64-slice DSCT

80

0.36±0.12

GSF 30/91 [2867]

Goo [68]

108

cCT

80

0.99±0.23

<2 yrs

29

cCT

Toshiba

64-slice MDCT

80

2.1 (1.1–10.6)

ICRP 60[626469]

66 (39–272)

Han [24]

32

HPH CT

Siemens

128-slice DSCT

80

0.29 (0.1–1.9)

7 (3–50)

0–4.5 yrs

RG CT

40-slice MDCTb

***

1.9±0.7 (0.8–8)

***

28±11 (10–52)

Young [70]

PT CT

Not specifiedb

***

0.7±0.2 (0.3–1.3)

***

11±3.3 (3–16)

cCT

***

0.9±0.2 (0.54–1.4)

***

12±4 (6–22)

1.3 (0.2–6) yrs

35

PT CT

Siemens

64-slice DSCT

80

0.38±0.09 (0.24–0.58)

ICRP 60 [6264]

19.86±6.27 (10–32)

Cheng [71]

1 yr

Phantom

RG CT (120 bpm)

Toshiba

320 MDCT

100

1.6

2.7±0.04

ICRP 103 [4060]

64.8

Podberesky [52]

PT CT (120 bpm)

2.3

4.5±0.05

90.4

5 yrs

RG CT (60–120 bpm)

0.5

1.2±0.03

16.1

PT CT (60–120 bpm)

1.6

4.9±0.09

50.5

5 yrs

Phantom

RG CT

GE

16-slice MDCT

80, 120

8.7–28

7.4–25.7

ICRP 60 [6264]

411.88–1,344.02

Hollingsworth [51]

5 yrs

Phantom

RG CT (40 bpm)

GE

64-slice MDCT

100

16.45

ICRP 103 [40]

Huang [72]

Phantom

(90 bpm)

11.81

1 yr

Phantom

RG CT (90–150 bpm)

GE

64-slice MDCT

80

5.8–9.6

ICRP 103 [40]

Trattner [50]

1 yr

Phantom

PT CT (90–150 bpm)

2.9–5.8

cCT non-gated cardiac CT, DLP dose–length product, DSCT dual-source CT, HPH CTprospective ECG-triggered high-pitch helical scan, ICRP International Commission on Radiological Protection, MDCT multi-detector CT, MOSFET metal-oxide-semiconductor field-effect transistor, PT CT prospective ECG-triggered axial scan, RG CT retrospective ECG-gated helical scan, yrs years

aMean +/- standard deviation, when available (range from minimum to maximum value)

bScanner manufacturer not reported

***Parameter not reported

Modality choices in pediatric cardiovascular imaging

Echocardiography remains the primary imaging modality in patients with congenital or acquired heart disease for most clinical questions, with excellent diagnostic accuracy [7374]. Catheter-based angiography and cardiac magnetic resonance imaging (MRI) have been used as adjunct imaging modalities when echocardiography yields an incomplete examination. With recent advances in CT technology, cardiac CT has been increasingly used for evaluation of congenital or acquired heart disease [2375]. Each of these imaging modalities has benefits and limitations, so careful consideration should be given to choosing the optimal imaging modality for a given clinical question while minimizing risks (Table 2) [76].

Table 2

Strengths and weaknesses of cardiac CT, cardiac MRI, echocardiography and cardiac catheterization in children

Modality

Spatial resolution

Temporal resolution

Availability

Exam time

Invasiveness

Cost

Need for contrast media

Field-of-view limitation

Contra-indications

Porta-bility

Sedation

Ease of monitoring patients during exam

Cardiac CT

++

+

++

+

++

Intermediate

Yes

No

Yes

Occasionally

++

Cardiac MRI

+

++

+

++

++

Intermediate

Often

No

Yes

Typically below age 8

+

Echocardiography

++

+++

+++

++

+

Intermediate

Rarely

Yes

No

+++

Rarely

+++

Catheterization

+++

+++

++

+++

+++

High

Yes

No

Yes

Typically for all pediatric patients

+++

+ low, ++ moderate, +++ high,  not available

Echocardiography is widely available, portable, has no known biological effects, and has excellent ability to delineate intracardiac anatomy and function [777879]. Imaging thoracic vascular anatomy, such as systemic and pulmonary venous return, distal branch pulmonary arteries, and some aortic arch anomalies, however, can be more difficult, particularly as children age and their acoustic windows become more limited [80]. Echocardiography also has limited ability to quantify right ventricular function (whether in the sub-pulmonary or systemic position) and valvular regurgitation [81].

When echocardiography is insufficient, cardiac MRI is often used [79]. Cardiac MRI provides excellent visualization of both intracardiac and extracardiac vascular anatomy, as well as assessment of valvular regurgitation and intracardiac shunting with phase-contrast imaging, and is considered the gold standard for quantifying ventricular volumes and function [8283]. Cardiac MRI provides myocardial characterization data and can be used to assess myocardial perfusion at rest or with stress [848586]. However, cardiac MRI has limitations (Table 2). The cost of cardiac MRI studies tends to be higher than either echocardiography or cardiac CT, although not as high as invasive procedures such as cardiac catheterization. Most comprehensive cardiac MRI examinations in children with congenital or acquired heart disease take 30–60 minutes to complete [87]. Patient cooperation is paramount to achieve a high-quality study, so children younger than 8 years and those with developmental delays might require sedation or anesthesia for the study, which entails several potential short- and long-term risks [8788]. Recent studies suggesting potential neurotoxicity of anesthetic agents on neurologic, cognitive and social development of neonates and young children remain inconclusive but motivate the desire to minimize anesthetic exposure [8990]. Gadolinium-based contrast agents are often used for detailed vascular definition or myocardial characterization [9192]. Gadolinium use has been linked to nephrogenic systemic fibrosis in patients with pre-existing kidney disease [93]. Recent studies have shown brain deposition of linear gadolinium chelates in the dentate nucleus and globus pallidus with an unknown clinical significance of this finding [94]. Increasingly, post-procedural patients with congenital or acquired heart disease have metallic implants that can cause artifacts on cardiac MRI or, as with some implanted devices, preclude cardiac MRI entirely. While cardiac MRI can be performed with newer pacemakers or defibrillators, artifacts from the leads may significantly impair exam quality [95].

Cardiac catheterization provides excellent spatial resolution for delineation of vascular structures and allows direct measurement of saturation and pressure data. Thus it provides unique hemodynamic insights not currently possible with noninvasive imaging modalities. However catheterization is invasive, involves ionizing radiation, and for many patients requires general anesthesia. Catheterization is reserved primarily for interventional procedures and cases where direct hemodynamic measurements are needed for clinical decision-making rather than solely for diagnostic imaging purposes [96].

Cardiac CT has a number of advantages compared to other imaging modalities for evaluating children with congenital or acquired heart disease. CT has excellent spatial resolution, though less than the spatial resolution of catheter angiography [76]. CT scanners are fast and can provide complete pediatric cardiovascular datasets in less than 1 second or in a single heartbeat, so many scans can be performed without the need for sedation or breath-holding [2497]. In addition to cardiovascular information, cardiac CT also provides excellent visualization of the airways, pulmonary parenchyma, bones and soft tissues.

Despite these advantages, cardiac CT does have risks and limitations. As previously discussed, the ionizing radiation required for CT in children has been linked to a potential small increase in risk of malignancy [4512131415]. Although functional and volumetric data can be obtained, the temporal resolution of CT is less than either echocardiography or cardiac MRI, which can impact ventricular functional analysis [98]. For nearly all cardiac CT scans, injection of iodinated contrast material is required for vessel opacification, but the risks of adverse reactions to iodinated contrast are low, especially in children and in patients with normal kidney function [99].

Imaging scenarios where cardiac CT might be useful

In light of the risks and benefits and strengths and weaknesses associated with each of the anatomical imaging modalities, cardiac CT is the current optimal study for several indications [26]. Because of the small caliber of coronary arteries in young children, cardiac CT has particular advantages over cardiac MRI and echocardiography for coronary imaging. It is possible with relatively low radiation exposure to visualize coronary artery origins, proximal through distal courses including evidence of anomalous origins, acute origin angulations, or intramural segments in congenital or acquired anomalies, and in infants with complex anomalies [100101102]. Patients with repaired or palliated congenital heart disease have a higher incidence of coronary artery abnormalities. Current recommendations include imaging of coronary arteries “at least once in adulthood” for anyone who has undergone coronary artery manipulation (e.g., as part of the Ross procedure or arterial switch operation) [103]. In patients with acquired coronary disease, such as Kawasaki disease, cardiac CT can identify areas of coronary artery dilation or stenosis [104105]. The coronary luminal caliber can be assessed for evidence of vasculopathy, as seen in patients with transplant coronary artery disease, and those with concerns for atherosclerotic plaque, such as in children with familial hypercholesterolemia or older adults with congenital or acquired heart disease [103106107]. Beyond coronary artery evaluation, cardiac CT is an excellent imaging modality in patients with suspected thoracic arterial abnormalities or thromboembolism [108]. Cardiac CT provides clear imaging of vascular rings, in which airway evaluation is also a key clinical question, as well as other aortic arch anomalies, branch pulmonary artery and pulmonary vein assessment, and evaluation for aortopulmonary collaterals [109110111112].

Another common indication for cardiac CT arises in critically ill pediatric patients or those who are at high risk for anesthetic complications, including children with single-ventricle physiology, severe outflow tract obstruction and Williams syndrome [113]. Because cardiac CT scans can now be typically performed in less than 1 second, cardiac CT provides a rapid diagnostic option for critically ill children compared to cardiac MRI and invasive angiography. Cardiac CT can be successfully performed without sedation in infants and young children, making this an attractive alternative to cardiac MRI in this age group [114]. Finally, it might be advantageous to image children with intrathoracic metal, including but not limited to pacemakers and defibrillators, by cardiac CT rather than cardiac MRI [115].

Pediatric cardiac CT technique optimization

Recent technological advances and growing indications have increased the utility of cardiovascular CT and drive the need to optimize image quality [232627116]. Consultation between cardiovascular imager and the referring provider is critical to clearly establish the goals of the cardiac CT study. Scan protocols targeted to clinical indication and patient size should be designed, implemented and audited with the combined input of medical physicists, cardiovascular imagers, technologists and application specialists to optimize scanner performance in order to obtain the highest-quality diagnostic images while minimizing radiation exposure. An experienced cardiovascular imaging physician should be at the scanner when needed to optimize the scan protocol for the particulars of the patient and clinical question.

Evaluation of extracardiac thoracic vasculature including the aorta, pulmonary arteries, and pulmonary and systemic veins can generally be performed without ECG synchronization [117118119120]. If evaluating the aortic or pulmonary roots, ECG synchronization is needed to eliminate pulsation artifacts [121]. ECG synchronization is necessary for evaluation of ventricular function, volumetry, detailed coronary artery analysis, and dedicated evaluation of intracardiac anatomy [117]. If ECG synchronization is used, the lowest radiation doses can generally be delivered to a child with a slow and steady heart rate. Consequently, administration of heart-rate-lowering medication such as a beta-blocker alone or with the addition of phenylephrine as an adjunct to support blood pressure should be contemplated [122123124125]. The use of calcium channel blockers for heart rate lowering might be useful if there are contraindications to beta-blocker administration [126]. Sublingual nitroglycerin to achieve coronary vasodilatation has been used to improve coronary CT quality in adults. Although not universally adopted for coronary CT in children, this approach could be considered if evaluating for coronary artery stenosis or distal coronary detail [25105127128].

Children with pacemakers should be evaluated prior to an ECG-synchronized examination to determine whether pacemaker rate or mode adjustment is needed to avoid heartrate irregularity, which can result in scan artifacts [129]. ECG synchronization can be either prospectively or retrospectively performed. The optimal phase of the cardiac cycle to freeze cardiac motion during image acquisition should be targeted and is generally at end-systole for heart rates above 80 bpm and at mid-diastole for lower heart rates [125130131132]. Prospective ECG-triggering can either be performed using an axial “step-and-shoot” method, high-pitch helical scanning, or with volumetric scanning. For one vendor, axial step-and-shoot imaging is not available but rather prospective ECG-triggering is generally performed with low-pitch helical scanning. For prospectively triggered acquisitions, the scanner predicts the time of the R wave and then triggers the scan acquisition at the time during the cardiac cycle designated by the operator [133]. This technique allows for dose modulation with radiation delivered in only a predefined narrow window of the cardiac cycle and can lead to a dose reduction of 40–69% as compared to retrospective ECG-gating (Fig. 2) [5271134135136137]. If there is heart rate variability, image degradation might be seen with prospective ECG-triggering, so retrospective ECG-gating or prospective ECG-triggering with added temporal padding should be utilized. Retrospective ECG-gating might also diminish the effects of respiratory or patient motion. Retrospective ECG-gating involves backward-looking measurement of R wave timing, with the X-ray beam turned on during the entire cardiac cycle. To decrease dose, beam intensity is modulated with the optimal phase of the cardiac cycle to freeze cardiac motion targeted during the relatively greater radiation dose with a significantly reduced radiation dose delivered during the remainder of the cardiac cycle [133] (Fig. 2). Retrospective-ECG gating should also generally be used when performing volumetric or functional analysis. The narrowest acquisition window suitable for the diagnostic scenario and patient characteristics should be used; a wider acquisition window of relatively greater dose might be needed for patients with irregular heart rates.

Fig. 2

Electrocardiogram (ECG) tracings depict prospective ECG-triggering and retrospective ECG-gating. The heart rate of 60 beats per minute was simulated by a cardiac trigger device incorporated into the scanner’s cardiac monitor. a Prospectively ECG-triggered tracing shows the X-ray beam on (dark blue bars) for only a targeted portion of the cardiac cycle centered at 65% of the R-R interval with temporal padding (light blue bars) to include 60–70% of the R-R interval. Prospective ECG-triggering allows for image reconstruction only during the portion of the cardiac cycle when the X-ray beam is on. bWith retrospective ECG-gating, the X-ray beam is on for the entire R-R interval, but the radiation dose is modulated up (dark blue bars) for a targeted portion of the cardiac cycle centered at 65% of the R-R interval with padding (light blue tall bars) to include 60–70% of the R-R interval. The dose during the remainder of the cardiac cycle is generally in the range of 20% of the full dose (light blue shorter baseline). Retrospective ECG-gating allows for reconstruction of images in any phase of the cardiac cycle, but image quality is best at the targeted portion

Evaluation of extracardiac vasculature and coronary artery origins using high-pitch dual-source or wide-detector scanners can generally be performed in a freely breathing child [5661]. Scan acquisition with breath-holding or under anesthesia for suspended respiration is ideal for detailed distal coronary or intracardiac evaluation [138].

Tube potential (in units of kV) and tube current–time product (in units of mAs) should be built into protocols for each study based on clinical indication and body size because radiation dose and image quality need to be balanced. These parameters might need to be adjusted for individual cases to further optimize quality and radiation dose. The lowest tube potential that will yield adequate image quality for body size should be used. A lower tube potential provides the dual advantage of not only providing a higher contrast-to-noise ratio that accentuates iodinated cardiovascular contrast enhancement in CT, but can also be used to lower contrast amounts and radiation dose [139]. Generally 80 kV (or 70 kV if available) is adequate for scanning most infants and children through the first decade of life [140141142]. In larger children or adolescents, 100 kV might be needed, especially if examining subtle details such as coronary artery stenosis. Automatic tube current modulation should be employed to decrease radiation dose while maintaining constant image noise and preserving image quality. For adequate performance of automatic tube current modulation, the child should be centered in the gantry [143144145]. Applying iterative reconstruction techniques prospectively allows a lower radiation dose to be delivered relative to standard protocols and also decreases image noise, thus improving image quality [146147148149150]. The z-axis scan range should be minimized to cover only the anatomy of interest. There is generally no need to scan the entire chest.

Homogeneous vascular opacification of the structure(s) of interest is paramount to cardiac CT. The iodinated contrast injection rate is determined by the size of the intravenous cannula and total allowable volume of contrast agent and can be in the range of 0.5–6 mL/s with a contrast dose of 1–3 mL/kg up to a maximum contrast amount of approximately 125 mL in an adult-size patient [108151]. Power injection is the most reproducible method of intravenous contrast administration. Optimum scan acquisition delay can be evaluated with bolus tracking using a region of interest with automatic scan triggering when a predetermined Hounsfield unit threshold is met, a timing bolus, or a manual scan trigger when opacification of the structure(s) of interest is visualized during dynamic evaluation. If a timing bolus is used, an empiric 2- to 3-second diagnostic delay is added to the time of peak test bolus opacification of the structure of interest for neonates and up to 8 seconds for older children/adolescents [152]. An empiric scan delay without bolus tracking can also be used but might not be as reliable. A single scan acquisition should be timed so that all necessary structures are simultaneously opacified without additional pre-contrast or delayed post-contrast images whenever possible [117]. Contrast administration can be either biphasic, typically consisting first of full-strength contrast or a contrast/normal saline mix followed by a normal saline flush, or triphasic, typically consisting first of full-strength contrast followed by a contrast/normal saline mix and a normal saline flush. The normal saline flush helps to clear the intravenous catheter tubing and central systemic veins of dense contrast material that might cause streak artifact in young children because smaller total amounts of contrast are administered in these patients. Care should be taken to avoid any air in the intravenous line during contrast administration in children with right-to-left shunts. Also, in children with intracardiac shunts, there is generally rapid and near simultaneous opacification of all involved cardiac chambers, which needs to be considered when planning the timing of the diagnostic scan.

Special consideration should be taken when imaging the child with the Fontan circulation and total cavopulmonary connection. The specific Fontan anatomy must be well understood, with the plan to opacity the superior and inferior vena cavae and the pulmonary arteries to avoid the pitfall of unopacified blood mimicking thrombus in the Fontan pathway. This can be achieved with simultaneous dual-injection of contrast agent in upper and a lower extremity veins and early phase imaging performed to assess for thrombus in the pulmonary arteries [153]. Delayed imaging can be performed as needed to assess for thrombus in the inferior vena cava and Fontan baffle if not optimally opacified on the initial study. Alternatively, contrast can be injected into an upper extremity vein with a single delayed scan acquisition timed to the equilibrium phase of contrast enhancement at least 70 seconds after the initiation of contrast injection, when the pulmonary arteries and the superior and inferior vena cavae are all opacified [108151].

Familiarity with scan technology and attention to contributions of parameters to radiation dose and image appearance can provide both dose-conscious and high-quality cardiac CT studies over a wide variety of indications and sizes in children (Figs. 3 and 4).

Fig. 3

Six-month-old, 8.7-kg girl with congenitally corrected (levo-) transposition of the great arteries, large ventricular septal defect (VSD), and pulmonary atresia, status post right Blalock-Taussig shunt and with ductus arteriosus left patent. Cardiac CT examination was requested to delineate intracardiac anatomical relationships and to generate 3-D images prior to a biventricular repair (atrial switch with Rastelli procedure). a Axial image shows the leftward and anterior moderately dilated ascending aorta (A) and rightward and posterior main pulmonary artery (P). There is mild focal narrowing (arrow) of the left pulmonary artery at the insertion site of the patent ductus arteriosus. R right pulmonary artery. b Axial image shows mesocardia with an anteriorly directed apex. There is atrioventricular discordance with the right atrium (RA) connected to the rightward and anterior subpulmonary left ventricle (LV) and the left atrium (LA) connected to the leftward and posterior hypertrophied systemic right ventricle (RV). Note the large VSD (V). c Sagittal oblique reformation shows the aorta (A) arising from the hypertrophied systemic right ventricle (RV). The proximal aspect of the patent ductus arteriosus is seen (arrow). The contrast-enhanced electrocardiographic (ECG)-gated cardiac CT was performed on a GE LightSpeed VCT XTe scanner (GE Healthcare, Waukesha, WI) with suspended respiration and utilizing the following parameters: 80 kV, 120 mA, gantry rotation time of 0.35 s, and with prospective systolic ECG-triggering using a step-and-shoot protocol with two 4-cm acquired slabs. Systolic-phase ECG-triggering was used to freeze cardiac motion as the heart rate at the time of study was 124–125 beats per minute. “Smart Prep” bolus tracking was used to time the initiation of scan acquisition, with repetitive monitoring scans performed at the level of the proximal descending aorta that measured the contrast enhancement in Hounsfield units (HU) within a prescribed region of interest within the aortic lumen. When the contrast threshold exceeded 220 HU, the scan was manually initiated. A bolus of 22 mL (2.5 mL/kg) of iodinated contrast (Iohexol, Omnipaque 350; GE Healthcare, Princeton, NJ) was administered via power injector through a 22-gauge right antecubital intravenous cannula at 4 mL/s, followed by 20 mL of saline at the same rate. CTDIvol(32-cm phantom) was 0.79 mGy and DLP was 8.3 mGy·cm. Images were reconstructed with iterative reconstruction (ASIR 70%). CTDI vol volumetric CT dose index, DLP dose–length product

Fig. 4

Fifteen-year-old boy with no medical history following successful cardiopulmonary resuscitation for a syncopal and unresponsive episode that occurred while running in gym class. Transthoracic echocardiography was suspicious for an anomalous left main coronary artery from the right sinus of Valsalva. Cardiac CT was requested and confirmed this diagnosis. a Axial 2-D and (b) 3-D reformatted images. The anomalous left main coronary artery (aarrowhead and barrow) arises from the leftward aspect of the right sinus of Valsalva near the sinotubular junction. The left main coronary artery origin is acutely angled relative to the aortic root (A) and the proximal course between the aorta and right ventricular outflow tract/main pulmonary artery (P) is anteroposteriorly narrowed. c–e Endoluminal 3-D rendered images. The left main coronary artery ostium (L in c and e) is small and elliptical relative to the right coronary artery ostium (R in d). Imaging findings suggest a proximal intramural left main coronary artery course. The expected intramural left main coronary artery course (black arrow in e) is in close proximity to the right/left commissural post (* in e). The anomalous left main coronary artery origin from the right sinus of Valsalva with a small orifice and proximal intramural course was confirmed surgically. The boy underwent creation of a neo-ostium in the left sinus of Valsalva rather than left main coronary artery unroofing to avoid damaging the adjacent right/left commissural post. Fifty milligrams of metoprolol (approximately 1 mg/kg) was administered orally for heart rate uniformity control approximately 2 hours prior to imaging to achieve a slow and steady heart rate so that a prospectively ECG-triggered examination could be performed. The contrast-enhanced CT study was performed without sedation on a second-generation Siemens SOMATOM Definition Flash dual-source scanner (Siemens Healthineers, Malvern, PA) using the following parameters: 100 kVp, 70 mAs, automatic exposure control (CAREDose4D). The scan was prospectively ECG-triggered and timed to mid-diastole using a high-pitch (3.2) helical single heart beat acquisition; heart rate at the time of the study was regular and 55 beats per minute. A test bolus of 10 mL iodinated contrast (Iohexol, Omnipaque 350; GE Healthcare, Princeton, NJ) was administered with imaging at the level of the carina performed every 2 seconds beginning 10 seconds after initiation of contrast injection until peak test bolus contrast was achieved in the ascending aorta at 20 seconds. Seventy milliliters of iodinated contrast agent was then injected through an 18-gauge right upper extremity intravenous cannula at a rate of 6 mL/s using a triphasic injection of 60 mL full-strength contrast, followed by 20 mL of a 50% contrast/50% normal saline mix, and then 20 mL normal saline, with the diagnostic scan onset at 28 seconds following initiation of contrast injection. CTDIvol (32-cm phantom) was 3.93 mGy and DLP was 54.2 mGy·cm. Images were reconstructed with a soft-tissue kernel and iterative reconstruction (SAFIRE) strength 2. CTDI vol volumetric CT dose index, DLP dose–length product, ECG electrocardiogram

Informing patients and families about pediatric cardiac CT

Optimal informed decision-making for pediatric cardiac CT is a collaborative family-centered process that must acknowledge both the unique patient characteristics and the wide range of public knowledge and perceptions regarding the risks of exposure to ionizing radiation [163336154]. There is evidence that families prefer to be informed of risks related to medical imaging procedures, though this might not be applicable to all settings [155]. Accordingly, discussion of the risk/benefit profile of a cardiac CT exam is a multi-disciplinary responsibility that can extend across the entire congenital heart center enterprise [29]. The many opportunities for an effective dialogue are shared by referring physicians, interpreting cardiologists and radiologists, imaging technologists, medical physicists, nurses, and, in some cases, the anesthesiologists and intensivists providing supportive care. Discussion of the use of ionizing radiation for cardiac CT should be a shared responsibility of the primary cardiologist, cardiac surgeon or caregiver and cardiovascular imager, with the imaging team best prepared to address specific concerns relating to radiation dose [156]. For an uncomplicated exam, written or web-based materials should be adequate and the technologist or imaging nurse will play a large role in the dialogue with the patient and family [29157158]. Cardiac CT exams that may require a larger ionizing radiation dose (e.g., dynamic or functional imaging or dual-phase acquisitions), administration of heart-rate-lowering medications, anesthesia, studies obtained in critically ill children, and patients undergoing contrast exposure in the setting of kidney disease or iodinated contrast allergy warrant more extensive dialogue. Such discussions should incorporate input from the referring cardiology, intensive care or cardiothoracic surgery services [89123124]. Nonetheless, pediatric-specific standardized dose thresholds for triggering formal discussions, or even written consent, have not been reached by expert consensus [159]. Many team members might contribute to the collective process, individualizing the conversation to a specific child and the related exam indications, protocol, risks and imaging alternatives.

Implementing a CT dose management program

Enterprise radiation dose management must incorporate both CT protocol development and optimization to comply with hospital accrediting standards and should include a regular audit of protocol compliance [160]. This audit should assess how well CT protocols are meeting expectations related to dose and quality, with review of designated outliers or areas of practice outside established standards. Although there are numerous ways to conduct these reviews, the current trend is to adopt software solutions that collect, archive and analyze CT radiation dose data that afford monitoring of both the individual patient and practice population performance. Knowing what data to monitor (amid an increasingly large amount of available data), establishing what is practice beyond standards, and what follow-through to initiate in response to outlier data are some of the challenges for a dose-management program. Other challenges include frequency of review, accountable parties, inclusion of all services that use these modalities, how to incorporate impact of evolving dose management strategies (e.g., iterative reconstruction on the CT dose profile for the practice), evolving dose metrics (e.g., the move away from effective dose and toward organ dose), and lack of available U.S. national benchmarks for dose ranges in children [161].

Dose-management analytics can be institution-specific, but certain data elements and variability should be uniformly captured to ensure adherence to standardized protocols and enable cross-institutional analysis. These data elements include (1) appropriate CT utilization analysis [162], (2) evaluation of size-specific protocol use, (3) assessment of correct CT technical performance (e.g., correctly functioning automatic dose modulation features), (4) monitoring of CT protocol dose consistency across the scanners and operators in the enterprise, and (6) comparison of delivered radiation doses with established standards, with variation from established standards adequately assessed and addressed.

While these principles are generally applicable to all CT radiation dose management, they are particularly important in pediatric cardiac CT. Imaging children introduces variability in patient size that should be reflected in the cardiac CT protocols and the configuration of equipment. However the default settings provided by the manufacturers might not be optimally designed for pediatrics and can return suboptimal image quality or higher radiation exposures than necessary [163]. An additional layer of complexity is introduced in pediatric cardiac imaging as the complex balance between image quality, contrast timing, and cardiac/respiratory gating can affect radiation dose. Anatomical variability can lead to substantial variation in dose or quality for the same CT protocol. For this reason, constant vigilance in reviewing radiation exposure data is needed to ensure that quality and dose are optimal in pediatric cardiac CT.

Conclusion

Children with congenital or acquired heart disease might require lifelong imaging surveillance. Advances in technology have made pediatric cardiac CT the best imaging modality choice in certain scenarios because detailed images can be obtained quickly and often without sedation. Discussion among caregivers regarding the relative risks and benefits of cardiac CT versus other imaging modalities should lead to the most appropriate imaging study being performed. Understanding cardiac CT technical parameters and how to apply them to children of various sizes and heart rates is necessary to optimize image quality at the lowest radiation dose. Instituting a dose-management program helps ensure regulatory compliance and should aim to achieve consistently high-quality images with appropriate radiation exposure.

Notes

Compliance with ethical standards

Conflicts of interest

C.K. Rigsby and J.D. Robinson are supported by grant NHLBI R01 HL115828 from the National Heart, Lung, and Blood Institute. K.D. Hill is supported by UL1 TR001117 from the National Center for Advancing Translational Sciences. A.J. Einstein is supported by grant R01 HL10971 from the National Heart, Lung, and Blood Institute and has received research grants to Columbia University from GE Healthcare, Philips Healthcare, and Toshiba America Medical Systems. C.L. Sammet is a member of Bayer HealthCare Informatics Global Advisory Board. S.E. McKenney, A. Chelliah, B.K. Han, T.C. Slesnick and D.P. Frush report no conflicts of interest.

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