Dual-source and dual-energy CT
Dual-source CT (Dual-Source CT, DSCT) uses two X-ray tubes and two detector systems offset by approximately 90 degrees relative to each other. This provides a tremendous gantry rotation speed (0.25 s) and a temporal resolution of 66 ms. The main advantage for pediatrics is the ability to perform cardiac imaging at high heart rates (HR) without retrospective ECG gating, acquiring data as a continuous volume in the systolic phase.
Dual-energy CT (Dual-Energy CT, DECT) acquires data simultaneously at different tube voltages (typically 80 and 140 kV). Different tissues attenuate radiation differently at different potentials, allowing mathematical algorithms to subtract bone structures or create virtual unenhanced images. This eliminates the need for a real non-contrast phase and reduces the radiation dose by half.
Parameter optimization and radiation dose
Increasing radiation exposure in a growing organism is unacceptable. All CT settings must be based on the child's age, weight, or body diameter.
- Tube voltage (kVp): The use of classic 120 kV for routine chest CT in children is no longer recommended. Reducing the voltage to 80 kV does not degrade quality, but increases the contrast of iodine-containing agents. This allows a smaller volume of contrast to be administered.
- Tube current (mAs): Children have less adipose tissue (natural tissue contrast), so they tolerate image noise more poorly. A simple reduction in tube current will lead to a loss of diagnostic information, which is why mAs often needs to be proportionally increased in young children.
- Automatic tube current modulation (ATCM): Adapts the current in real time. However, the effectiveness of ATCM in infants is debatable due to the "round" cross-sectional shape of the body. Additionally, older bismuth shields for protecting radiosensitive organs falsely increase the current when ATCM is enabled. Nowadays, to protect the breast and thyroid glands, the radiation output in the anteroposterior projection is simply reduced at the hardware level.
Basic pediatric protocols
Tomograph settings are strictly tied to the patient's body weight and clinical task.
- Chest: Helical mode. For children under 9 kg, 70 kV and 70 ref. mAs are used; from 10 kg, 80 kV and 50 ref. mAs. The pitch is high (1.9), and the delay equals the injection time + 5 seconds.
- Abdomen: Helical mode. Standardly 100 kV, 120 ref. mAs. The scan delay after contrast administration (Omnipaque 300) is 25 seconds after the end of the injection.
- Head and hydrocephalus evaluation: Sequential (Sequential) mode is used, with 120 kV and high mAs (200–300). When checking shunt function, no contrast agent is administered.
Preparing the child for scanning
Modern tomographs work very quickly, so children over 3 years old often cope with the procedure without sedation. Play therapy, virtual room tours, or toy CT models are used for adaptation. If the child cannot hold their breath, switching them to calm, shallow breathing is recommended.
For infants, the gold standard is the "feed-and-wrap approach" (feed-and-wrap approach). The examination is performed during natural sleep. Sedation or general anesthesia is used only in uncooperative patients or with severe developmental delay.
Alternatives: MRI and dental CBCT
Where standard CT is excessive or carries unjustified risks, other cross-sectional imaging methods are used:
- MRI. Is the preferred method in children due to the absence of ionizing radiation (exception: lungs, cortical bone, acute trauma). The main drawback is long scanning times. It requires strict control of peristaltic artifacts (by administering glucagon) and respiratory artifacts (using ECG leads or liver dome navigators).
- Cone-beam computed tomography (CBCT). Used in orthodontics and maxillofacial surgery. Provides accurate 3D bone imaging without projection distortions typical of panoramic radiography, but is unable to display soft tissues. The most important rule of CBCT is that the field of view (FOV) must be strictly limited to the area of interest to protect the salivary and thyroid glands.
Briefly: answers to topical questions
What are the absolute contraindications to MRI in children?
Sources explicitly state that pacemakers and other implants can be a contraindication to MRI. Specific absolute contraindications exclusively for pediatric age are not described in the materials.
MRI is the preferred imaging method in children, except for the examination of the lung parenchyma, cortical bone layer, certain cardiovascular malformations, and acute trauma; these conditions are not named absolute contraindications.
What pharmacological agents are used to suppress peristaltic artifacts during abdominal MRI?
To suppress peristalsis during abdominal MRI in children, intravenous administration of hyoscine (Hyoscine) is used.
How are the dose and injection rate of an iodine-containing contrast agent calculated in pediatric CT?
The dose of an iodine-containing contrast agent in pediatric CT is calculated by body weight. Sources indicate the use of Omnipaque 300 (Omnipaque 300) in the following dosages:
- Chest CT — 2 ml/kg, maximum 75 ml;
- Abdominal CT — 2 ml/kg, maximum 100 ml;
- Head CT — 1 ml/kg, maximum 50 ml.
An automatic injector is used for administration. The specific injection rate is not specified in the sources. The scan delay for chest CT equals the injection duration plus 5 seconds, and for abdominal CT — the end of the injection plus 25 seconds; for routine head CT, there is no delay.
To remember the relationship between parameters and contrast: "Lower voltage (kV) — brighter iodine." Reducing from 120 to 80 kV not only protects the child from unnecessary radiation, but also increases the conspicuity of the contrast agent.
Class and exam questions
Why is it not possible to simply lower mAs (tube current) in children to reduce radiation exposure?
Reducing mAs proportionally increases image noise. Children have little visceral fat separating tissues, so noise drastically reduces diagnostic value; mAs often has to be increased to compensate.
Should bismuth organ-shielding devices be used during CT?
In modern devices with automatic tube current modulation (ATCM), their use is not recommended because the shield causes the system to falsely overestimate the dose. The devices themselves reduce power when passing over the thyroid and mammary glands.
What is the essence of virtual unenhanced imaging?
Dual-energy CT (DECT) allows a mathematical algorithm to remove iodine from an image acquired in the contrast phase. This creates a "virtual" unenhanced scan and eliminates the need to perform a real scan before contrast administration.
What else to study on this topic
- Features of iterative reconstructions in pediatrics
- CT protocols for assessing the maxillofacial region (low-dose 3D CT)
- Strategies for suppressing macroscopic motion artifacts in MRI
- Principle of operation of automatic tube current modulation (ATCM)
- Interpretation of iodine distribution maps in suspected PE
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