Donald P. Frush

Costs and Benefits

Pediatric CTA offers several advantages over other contemporary imaging modalities, including echocardiography, magnetic resonance imaging (MRI), and conventional cardiac catheterization and angiography. First, computed tomography (CT) provides the best global assessment of the lungs and airways, as well as other regional structures, in both congenital⁹ and acquired vascular disorders (e-Figs. 66-1 and 66-2).

Congenital cardiovascular disorders may affect the respiratory system by causing tracheal compression or deviation (e.g., from vascular rings or pulmonary slings) with resulting obstructive effects on the lungs, or by causing air trapping at the parenchymal level, such as diffuse hypoinflation or mosaicism as a result of cardiogenic pulmonary edema. In addition, CT can suggest or demonstrate associated primary abnormalities of the respiratory system, such as pulmonary hypoplasia or tracheomalacia. Although conventional angiography could supply information on the lungs and airway, it is focused primarily on the evaluation of the vascular intraluminal anatomy. In addition, sedation is needed less frequently in younger children for CT than for magnetic resonance (MR) vascular imaging, echocardiography, or conventional angiography.

With newer technology, such as volume MDCT (e.g., 320 detector array single rotation acquisition)¹⁰ or dual-source MDCT (Fig. 66-3),¹¹ a complete examination of the entire chest of an infant can be completed in less than 1 second. With echocardiography, MR vascular imaging, or angiography, imaging times typically exceed 20 minutes and may occur over hours. A reduced examination time means that CT is better tolerated by infants, children, and patients in the intensive care unit who may have a limited ability to hold still or require the examination to be performed quickly for other reasons. The technical quality, including display, also is more consistent with CT compared with echocardiography, which is a more operator-dependent examination, and compared with MR evaluation, where operators select parameters and sequences that may give a different study quality from one examination to the next. CT also allows better patient monitoring than does MRI. In addition, many of the contraindications of MRI (e.g., pacemakers, internal support apparatus, and some surgical devices) are not contraindications with CTA and produce less image artifact than with MR angiography. Unlike with conventional angiography, the multiplanar and three-dimensional capabilities of CTA provide for off-line review of information in virtually any plane. For conventional angiography, this feature is limited to the planes selected during a particular sequence, and for echocardiography, the examination planes or views recorded are the only information available for off-line evaluation. The cost of CT is comparable with that of Doppler echocardiography, in general is less than that of MR, and is much less than conventional angiography.

CTA has some disadvantages. CTA involves the use of radiation, which is an issue with angiocardiography but not with MRI or echocardiography. CT radiation dosages depend on the technique used. In general, CTA can be performed with a dosage similar to or lower than that of a routine chest CT. CTA dose estimates in young children can be less than 1.0 mSv.12,13 With gated technology, employing prospective gating and newer volume scanning, doses also can be substantially lower than with older CT technology and retrospective gating.^13–16 With rare exceptions, properly performed pediatric CTA results in a lower radiation dose than conventional diagnostic angiographic evaluation. Electrocardiographic-gated CTA usually involves a greater radiation dosage than limited diagnostic conventional angiography or a nongated CTA. Intravenous (IV) contrast material is required for CTA but also is required for conventional angiography and for MR angiography. The risk of major adverse effects (e.g., airway spasm and cardiovascular collapse) from iodinated IV contrast material is extremely small in children.^17,18 Unlike with echocardiography, MRI, and conventional angiography, pediatric thoracic CTA typically is used for morphologic assessment, although cardiac function and other hemodynamic information can be obtained.¹⁹ CT for cardiovascular evaluation also is not portable.

Various factors must be taken into consideration in choosing an appropriate imaging algorithm. Individual expertise is a strong consideration, as is the availability of the modality desired. Personnel must be able to perform these examinations, and imaging experts, such as radiologists, may have preferences. Performing CTA on an MDCT device that offers less than 16 slice (e.g., a 4- and 8-slice MDCT device) is problematic, and image quality (both contrast enhancement and multiplanar reformations) is more limited. A CT scan that can be performed within the same day or relative quickly may be preferred compared with waiting several days for an MR evaluation. In general, echocardiography should be the first examination considered, with MR being second, unless contraindications are present.

Technique

Box 66-1 shows the steps taken in performing a pediatric CTA (see e-Fig. 66-2).^6–8

Pediatric cardiovascular CT evaluation is less protocol driven than is cardiovascular CT evaluation for adults. The examination should be constructed to obtain the appropriate diagnostic information while minimizing radiation dosage. Patient preparation includes understanding the clinical indications for the examination and understanding the anatomy in question. Optimal vascular opacification and minimization of streak artifacts depend in part on familiarity with abnormal congenital anatomy, and an understanding of palliative and corrective cardiovascular anatomy is critical.

Patient motion needs to be controlled (e-Fig. 66-4), which may require sedation of younger children. Breath holding is preferred during CTA but is not a requisite, especially with volume scanning and other fast (e.g., dual-source, high-pitch) scanning technology. If the child is intubated, an inspiratory hold during the CTA evaluation is recommended. Metal (e.g., pacemakers, intracardiac leads, sternal wires, stents, clips, and valves) and arms (Fig. 66-5) may cause streak artifact. Monitors and associated leads should be positioned outside the scan field if possible to minimize streak artifact across the chest. Knowing the location and caliber of angiocatheters used for IV contrast administration will help determine the rate of administration and the time it takes contrast material to reach the heart.²⁰ Finally, because cardiac CT evaluation may involve patients with left-to-right shunts or admixture lesions, extreme care must be taken to avoid injection of air or a thrombus through the line when contrast is administered. This situation is rarely, if ever, a problem with adult cardiac evaluation.

Technical considerations regarding the IV contrast material include type and concentration, dosage, rate of administration, and timing of scan onset after administration.²¹ In general, low osmolar, nonionic contrast media are recommended for contrast-enhanced CT scanning in the pediatric population. The iodine concentration varies but usually is around 300 mg/mL. For newborns and small children, higher concentrations such as 370 mg/mL will afford better enhancement of their smaller vessels. A dosage of 1.5 mL/kg is usually adequate, with a maximum total volume of 125 mL. With gated cardiac evaluation, the contrast dose can be smaller (e.g., <100 mL). If a second CT scan must be performed while the patient remains on the scanner, then 1 to 1.5 mL/kg of additional contrast can be administered. In this case, the maximal dosage would be 3.0 mL/kg, which is still reasonable for a single examination.

The rate of administration depends on whether a manual injection or a power injector is used and also on the size of the angiocatheter. Most peripherally inserted central catheters are not amenable to contrast administration for CTA. Suggested rates of administration are 1.5 mL/sec for a 24-gauge angiocatheter, 2.0 to 2.5 mL/sec for a 22-gauge angiocatheter, and 3.0 to 4.0 mL/sec for a 20-gauge angiocatheter. When possible, contrast should be administered through a power injector, which gives a more predictable and consistent enhancement curve. However, performing manual administration as quickly as possible can provide adequate enhancement when power injection is not appropriate. Use of central venous catheters for power administration varies based on individual practice and preferences.

The timing of contrast administration with respect to scanning initiation is a critical factor in CTA. In general, scanning is started either during administration of the contrast or immediately after administration is completed. Timing depends on the structures that need to be opacified. Later scan initiation is used for opacification of the thoracic aorta and major branches or systemic venous structures, whereas earlier scan initiation is appropriate for right-sided structures, especially pulmonary arteries. An empiric delay can be used, but because of the wide range of sizes that may be encountered in pediatric CTA (i.e., 1.0 to 100 kg), a single recommendation is impossible. A range of delays could be determined depending on the rate of administration. In general, most cardiac CTA, even in the smallest child, does not begin before 5 seconds after the onset of contrast administration. For large children, the empiric delay may be as long as 50 seconds. Bolus tracking technology, which obviates the need for this estimation, can be used to start the CTA. Alternatively, a test bolus can be administered and the arrival of contrast to the desired location (for cardiac CT, the right side versus the left side of the heart) can be used (Fig. 66-6).

This test bolus technique works even in neonates. A test bolus of 10% of the total expected volume is used, which requires a tuberculin syringe and minimization of catheter and tubing dead space for a child receiving a test bolus of less than 1.0 mL. The arrival of the test bolus at the appropriate side of the heart is timed, and this time is used to set the delay for the start of scanning. In neonates, usually about 2 to 3 seconds is added to this arrival of the test bolus for optimal enhancement. Tracking without a test bolus is simple to perform, and a test bolus rarely is used for nongated CTA in children. Fontan circuits may require special contrast administration techniques.²²

Scan techniques include adjustment of technical parameters such as the number of detector rows, the thickness of detectors, the gantry cycle time, tube current (milliamperes [mA]), and peak kilovoltage (kVp). Parameters such as scan thickness and interval are adjustable after the scan volume is obtained (these parameters usually are in protocols). Suggestions for a CTA technique are provided in Table 66-1.²¹

The isotropic images obtained with 16-slice and higher MDCT scanners provide excellent multiplanar reformations and three-dimensional evaluation, which show complicated anatomy in a way to which clinical care providers (e.g., cardiac surgeons) are accustomed (e-Figs. 66-7 to 66-9). The thinnest detector option should be chosen for pediatric CTA, because thinner slices improve the quality of reformations. The fastest gantry cycle time is recommended to minimize motion artifact and to obtain the fastest scanning time possible, which is important in children who may be anxious or otherwise fidgety. Pitch, essentially representing table speed (mm/second) divided by effective collimation (mm), generally is in the range of 1 to 1.5 for nongated CTA. Tube current, including tube current modulation technology,²³ varies depending on patient size; recommendations can be found in Table 66-1.

Because contrast is relatively high in cardiothoracic CTA, consideration should be given to lowering the peak kilovoltage.8,24 This step will improve image contrast and, although some increased noise occurs, the increase in contrast is greater than the increase in noise in small children, which produces an improved contrast-to-noise ratio and better image quality. Therefore 80 kVp (Fig. 66-10) for newborn through young school-age children and 100 kVp for older but still young school-age children (up to about 10 years assuming normal body habitus) is acceptable. A value of 120 kVp can be used for older children.

With advances in image processing time and storage and the availability of off-line workstations, evaluation of the pediatric heart and great vessels in multiple perspectives is now performed rapidly. Images should be reconstructed at the narrowest slice thickness possible (submillimeter) and at intervals just under the slice thickness. These reconstructions generally are performed with use of low noise kernels.

Cardiac gated CTA became more practical after the availability of 16-detector row technology with improved spatial resolution. As previously noted, prospective gating reduces radiation dose. Technical considerations have been reviewed recently¹³; a detailed description of gated techniques is beyond the intent of this chapter. Even more than with the nongated angiographic technique, presence at the scanner by the imaging expert usually is necessary to determine contrast details, particularly with the timing bolus, and scan parameters (e.g., scan range, tube current, and peak kilovoltage), as well as performance of postacquisition processing. Applications include improved depiction of the great vessels (e.g., for aortic dissection). However, evaluation in children usually is focused on coronary artery anatomy. Recently, dual-source design (i.e., two x-ray tubes) has improved temporal resolution (obviating β-blockage of heart rates, even to heart rates around 120 beats/min) (see Figs. 66-10 and 66-11).²⁵ Pediatric applications of coronary artery CTA include congenital abnormalities related to the number, site of origin, or course of the arteries, as well as postoperative effects on coronary arteries (e.g., after correction of transposition of the great vessels). In addition, the effect of systemic disorders, such as Kawasaki disease, on coronary arteries (and development of stenosis or aneurysm) increasingly is reported using cardiac gated techniques. As with adults, intracardiac anatomy is better displayed with use of gating, although dual-source high-pitch technology can provide improved intracardiac detail compared with non–high pitch and single-source scanners. Applying adult techniques to pediatric scanning can result in excessively high radiation dosages—as high as 30 millisieverts (mSv)—which is approximately 10 to 20 times the dosage of a normal chest CT examination in a child. Dosages and image quality for gated cardiac evaluation should reflect reduced-dosage techniques for other body applications in infants and children, with consideration of lower tube current and kilovoltage (see Fig. 66-6).

The use of bismuth breast shields to reduce breast dose has lowered radiation dose and maintained image quality for pediatric chest CT,²⁶ although the benefit of shielding is debated given the availability of other techniques for dose reduction.²⁷ The use of organ-based dose modulation, where the tube current is reduced over an arc to reduce surface dose such as for the breasts, has yet to be systematically studied in pediatric chest CT. In addition, iterative reconstruction technology²⁸ likely will provide new opportunities to reduce dose or improve image quality for pediatric thoracic cardiovascular CTA.

Conclusion

CT evaluation of intrathoracic cardiovascular anatomy has increased dramatically in both adults and children during the past decade. In certain pediatric populations, including very young children, technical challenges need to be considered before and during performance of cardiac CT angiography. Considerations include proper patient preparation and individualization of scanning techniques to address the specific clinical questions, as well as considerations unique to that child (such as size). These considerations also will minimize radiation exposure. Despite these challenges, excellent cardiac angiographic evaluation can be obtained even in the most challenging cases.

Suggested Readings

References

References

Full references for this chapter can be found on www.expertconsult.com.