Pediatric heart scan delivers high radiation dose

May 27, 2005

The total effective dose from a pediatric gated cardiac CT angiography exam can be twice as high as that of an adult scan. Young female patients, in particular, are at risk because of the radiosensitivity of their breast tissue. Radiologists should wait until further research defines dose-reduction strategies before routinely applying this technique in children, according to Duke University researchers.

The total effective dose from a pediatric gated cardiac CT angiography exam can be twice as high as that of an adult scan. Young female patients, in particular, are at risk because of the radiosensitivity of their breast tissue. Radiologists should wait until further research defines dose-reduction strategies before routinely applying this technique in children, according to Duke University researchers.

Dr. Caroline Hollingsworth, an assistant professor of radiology at Duke, and colleagues used a 16-slice scanner and a five-year-old anthropomorphic phantom to establish a range of radiation doses for pediatric gated cardiac CTA. They reported their findings at the 2004 RSNA meeting.

The researchers scanned from the carina to apex, with retrospective gating (100 bpm heart rate simulator) and multiphase reconstruction at 0.625 mm. Other parameters included 0.5-sec rotation speed, 0.275 pitch, 120 kVp (at 120, 220, and 330 mA), and 80 kVp (at 385 mA).

Organ doses were measured using MOSFET technology, which allows direct measurement of absorbed organ dose. Without this technology, only estimates of dose, such as skin entrance dose, can be performed, Hollingsworth said.

The researchers also recorded the CT dose index and dose length product, two parameters automatically calculated and displayed by most new scanners. CT dose index is a measure of exposure per slice, while dose length product is a measure of total radiation exposure for the whole series of images. The combination of the two measurements provided the investigators with an approximation of effective dose, which is the risk of cancer relative to natural background radiation.

The CT dose index and dose length product ranged up to 68.89 mGy and 676.03 mGy/cm, respectively, for the highest mA scan. These two parameters are meant to be reference points, rather than absolute numbers, Hollingsworth said. If the numbers go up as settings are changed, it merely indicates that the dose has increased, not necessarily to an alarming degree. The effective dose tells the story.

The total effective dose values for each protocol, including the timing bolus of 2.7 mSv, were:

  • 28.4 mSv - 330 mA/120 kVp

  • 19.9 mSv - 220 mA/120 kVp

  • 10.1 mSv - 110 mA/120 kVp

  • 13.3 mSv - 385 mA/80 kVp

Individual organ doses were also high. Between the low and high mA settings, the breast received from 35 to 120 mSv, while bone marrow (rib and spine) received between 10 and 70 mSv.

The protocol using 80 kVp at 385 mA produced less radiation dose to all organs than the 120 kVp at 220 mA exam. Both settings were calibrated to produce the same noise.

At 385 mA, the breast and the bone marrow/spine had an effective radiation dose of 53 mSv and 19 mSv, respectively. At 220 mA, the numbers increased to 78 mSv and 41 mSv, respectively.

"We were surprised at the high doses," Hollingsworth said.

Pediatric chest CT scans range from 1 to 4 mSv, while chest x-rays range from 0.01 to 0.005 mSv. An adult gated CTA exam averages 12 mSv, which is roughly equivalent to four years' exposure to natural background radiation.

The effective dose to children may actually be higher because of limitations in the standards set by the International Commission on Radiological Protection (ICRP), Hollingsworth said.

ICRP 60, the current standard, does not take into account specific weighted factors for increased sensitivities of children, who are more radiosensitive than adults. New guidelines (ICRP 26), which include a heavier weighting factor for children, are currently under review and should be finalized soon, she said.

Nonetheless, researchers used preliminary ICRP 26 numbers to project total effective dose values for pediatric gated cardiac CTA exams. Total dose rose significantly when the heavier weighting factors were used (includes an increased timing bolus of 7.3 mSv):

  • 46.1 mSv - 330 mA/120 kVp

  • 33.1 mSv - 220 mA/120 kVp

  • 18.4 mSv - 110 mA/120 kVp

  • 23.9 mSv - 385 mA/80 kVp

The study has several limitations. Investigators used only one scanner and examined only one age group. They also did not evaluate image quality. In addition, they did not have access to dose modulation techniques, which have been reported to reduce the radiation dose by 30% to 50%.

Cardiac CTA use in children is appealing. It is noninvasive and fast, has relatively high spatial and temporal resolution, and decreases need for sedation, Hollingsworth said.

"However, our youngest patients are up to 10 times more radiosensitive than adults. Further evaluation of lower kVp exam with noise-adjusted mA is justified," she said.

For more information from the Diagnostic Imaging archives:

CT angiography safely visualizes pediatric vascular anomalies

Cardiac CT's triple ruleout: Is it hype or real benefit?

Dose concerns limit trauma CT in pediatrics

Radiation dose challenges PE diagnosis in women