Answer to Question #14761 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment and Shielding — Shielding

The following question was answered by an expert in the appropriate field:

Q

Very recently, I took a chest computed tomography (CT) scan. The technologist did not offer me a protective lead apron, and I forgot to ask. I would like to know if it is a mistake by both of us to neglect the protective apron. I am getting very nervous about this now. Also, the CT dose index to a volume (CTDIvol) was 4.36 milligray (mGy) and dose length product (DLP) was 150 milligray-centimeter (mGy-cm). What does that mean? Are those doses really high numbers?

A

Lead protection is no longer routinely used on patients. When the lead is not in the image, it offers negligible or no protection to the patient. Improved technology has decreased radiation use in imaging which has decreased by 95%. However, if the lead is in the image, it can cover important anatomy leading to repeat imaging, and can lead to higher output from the x-ray tube due to automatic exposure controls. When the lead blocks some of the x rays from reaching the detector, the x-ray tube emits more x rays so the detector receives enough for good images. This automatic exposure control ensures each patient receives just enough radiation to produce a usable image.

CT dose metrics can be confusing. For CT scans, the machine indicates the CTDIvol in mGy and the DLP in mGy-cm. Neither of those are patient dose—they are measures of the radiation that was output by the CT scanner. The CTDIvol is a dose value that was measured at a specific location in a phantom (an acrylic cylinder used for CT quality assurance testing) and the DLP is the CTDIvol multiplied by the length of the scan. The CTDIvol is very useful for comparing the radiation output during different types of scans and between different CT scanners.

You indicate that the CTDI was 4.36 mGy and the DLP was 150 mGy-cm. When I compare these values to the achievable doses and diagnostic reference levels reported for adult CT exams in the United States as reported in Radiology, your dose metrics were on the low side. For a small adult, the achievable dose (50th percentile) CTDI is 5 mGy and the DLP is 186 mGy-cm. The dose used for your chest CT was below the average for a small adult.

You do not need to be nervous about the imaging; your technologist followed the latest guidelines and used an appropriate protocol.

Deirdre H. Elder, MS, DABHP, CMLSO

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 14 December 2022. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.