Donald P. Frush, M.D.
Division Of Pediatric Radiology, Duke University Medical Center

Multislice (multidetector) CT (MDCT) is a significant technologic advancement which has resulted in improved imaging capabilities in children. However, this technology is quite complicated and confusing and performing optimal or even adequate CT examinations requires an understanding of the basic technology as well as a familiarity with the appropriate range of techniques, including intravenous contrast administration and adjustments of various scanning parameters. This lecture will discuss the basic technology of multidetector CT (and summarize the differences from single slice CT such as cone beam configuration and detector technology), and provide appropriate technical guidelines for infants and children.

Multidetector technology involves use of more than one (generally 4, although most recently 8) rows of detectors. The beam is also conical, versus the fan shaped beam for single slice systems. These factors contribute to the potential for increased radiation from multidetector studies. With even greater numbers of rows (8, 16) of detectors, faster coverage of similar areas, or thinner slices can be obtained compared than with 4 detector row systems. Multidetector technology provides substantial benefits in terms of optimizing IV contrast material enhancement, including CT angiography, even over large areas. However, because of this technology, there are vast and complex options which must be selected for scan design. Importantly, some of these options may provide an unnecessary radiation dose.

Scan Parameters for Children: While multidetector CT can provide additional benefits in terms of imaging in children, the complexity of the technology may also result in inappropriate techniques been used. Basically, because of selections of section thickness, the detector configurations, gantry rotations cycle, tube current (mA), kilovoltage (kVp), and table speed, literally hundreds of options exist for any individual pediatric examination. Some of these options, in terms of radiation dose, can be inappropriate. Strategies for minimizing dose include only doing appropriate CT examinations, focusing the examination to the clinical question, and minimizing multiphase IV contrast examinations (for example, a precontrast examination is almost never necessary) should not be overlooked. The individual contributions in radiation dose by the variety of parameters will also be reviewed. Guidelines for appropriate, size-based scanning using multidetector CT in children will be provided, and a color-coded system for size based scanning (and the role in simplifying pediatric CT with reduction in scan error) will be emphasized.

Parameters which should be adjusted include tube current. Tube current should be adjusted based on scan indication (e.g., lower current with lower signal-to-noise when only large abnormalities need to be assessed - such as evaluation of much of lung pathology or small bowel obstruction -, versus higher signal-to-noise in settings where small or low contrast abnormalities need to be distinguished - for example microabscess in the liver). In general, the thickest slice, and fastest table speed which are acceptable should be selected.

The current status of CT dose and cancer risk will also be discussed briefly. Basically, the relationship between cancer and low-level radiation, as found with CT, is much closer than has been previously thought, and can overlap with repeated CT examinations. That is why it is especially important to be aware of CT radiation in children. This population has an increased risk for a variety of reasons (greater organ sensitivity, longer lifetime to manifest changes).

IV Contrast Material: Because scanning can be obtained much faster, it is especially important to attend to appropriate timing of IV contrast agents in scanning in children. What is known about multidetector CT and scan delays with intravenous contrast will be reviewed, including data from this institution. In general with a rate of about 2.0 mL/sec with a power injector (manual bolus techniques are also often employed with an average rate of just under 1.5 mL/sec) rate of the average manual bolus), scanning the chest 0-15 seconds and the abdomen about 25 seconds following completion of IV contrast provides excellent enhancement and diagnostic information. The technique of multiregion scanning (neck, chest, abdomen, pelvis) with MDCT will be summarized. This fast technology obviates the old practice of split boluses.

One of the greatest benefits of MDCT is with CT angiography. In this lecture, the technical difficulties with children (need for sedation, limited breath holding, small total volumes of contrast material, small vessel anatomy, difficulties regulating the rate of contrast administration with manual injection) will be discussed and strategies to minimize the potential impact of these complexities will be reviewed. In this respect, MDCT yields thin slices and rapid acquisition. A step-by-step method of performing pediatric CT angiography will be provided.

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Last Updated:
Thursday, 21 August, 2003