Answer to Question #13538 Submitted to "Ask the Experts"

I had a high-resolution chest computed tomography (HRCT) scan a few years back. Since then the radiation dose from the scan has been a great concern for me. I read on the internet that HRCT doses are much higher than from a standard CT. I also found a paper online that reported the CT dose index (CTDI) value of a HRCT scan could reach as high as 140 mGy under the setting of 140 kVp and 510 mAs. These numbers really freak me.

1. What was the likely CTDI of my scan? Can you also give me the likely dose length product (DLP)?
2. Am I doomed to get cancer due to me having the scan?
3. Is the information I found accurate?

I want to begin by reassuring you that the risks associated with HRCT are too low to measure or do not exist. In HRCT, the x rays are collimated to a much thinner slice width than in conventional CT scans, typically less than 1.5 mm compared to 5–10 mm. If the images are taken contiguously, without any gap, the effective dose can be higher than for conventional CT scans. However, most facilities use a gap of 10–20 mm between the thin slices, which results in lower doses to the patient from HRCT than from conventional CT. HRCT is used to evaluate the structure of the lung for fibrosis, emphysema, or similar conditions. Whether a physician orders a conventional CT scan or a HRCT depends on the reason for the scan. 

There are different methods used in CT scanning. Sequential acquisition refers to the more traditional axial scans in which the table is stationary while the x-ray tube spins around the patient, and then the table moves to the location for the next image. In helical scanning, the table moves while the x-ray tube is rotating. The term volumetric acquisition is most often used to refer to CT scans performed with a multidetector CT (MDCT) scanner that can obtain multiple slices at the same time and reconstruct the images into a three-dimensional representation of the patient. 

The dose information provided from CT scans can be confusing. The CT scanner provides the dose-length-product (in mGy-cm) and the CT for the volume (CTDIvol in mGy). Both of these values are based on the output of the CT scanner, not the dose to the patient. The values can be used to estimate an effective dose from each scan. Effective dose can be used to compare the long-term risk of cancer from partial-body irradiation to whole-body irradiation, and it accounts for the differing sensitivity of the tissues and organs exposed. However, effective dose estimates cannot be made for an individual patient. Effective dose estimates are calculated using weighting factors that are averaged across populations. The effective dose depends on the anatomy imaged as well as the DLP. 

The effective doses from HRCT scans can vary widely depending on the x-ray parameters used, slice thickness and gap between slices. In a British Journal of Radiology article, the DLP values for HRCT varied from 22–537 mGy cm. This would correspond to a range of effective doses between 0.308 mSv and 7.518 mSv. As noted in the Health Physics Society (HPS) Position Statement on Radiation Risk in Perspective, for doses this low, there is no consistent evidence that there are health risks.

For more information, I recommend you read the HPS Fact Sheet on Radiation Exposure from Medical Exams and Procedures. If you need more specific information about the CT scans you had, you should contact the imaging center and ask to speak with the medical physicist. He/she would be able to review your scans and answer your questions.

Deirdre H. Elder, MS, CHP, CMLSO