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Answer to Question #10753 Submitted to "Ask the Experts"

Category: Medical and Dental Patient Issues — Diagnostic X Ray and CT

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

Q

I received a CT (computerized tomography) scan of the abdomen and pelvis with and without contrast and the DLP (dose length product) was 3,047 mGy-cm for the three scans, and the CTDI (computed tomography dose index) was 16.80 mGy! What is the radiation dose in mSv for this exam (machine GE LIGHTSPEED VCTXT)?

I also received a CT scan of the head with contrast. What is the radiation dose in mSv for this exam?

A

The information provided and displayed for a CT scan includes the CTDI (vol) and the DLP. Both of these displayed numbers are really radiation machine output metrics and not patient specific dose information.

Both the dose distribution and the dose equivalent distribution are not uniform in a patient who has a CT scan. To meaningfully ask for a dose or dose equivalent would require that a location be specified. For example, you could meaningfully ask what is the dose or dose equivalent at 2 cm depth below the umbilicus.

The CTDI is calculated from a weighted average of the peripheral doses and center line dose that is delivered to a 32 cm homogenous plastic phantom. When a patient is scanned, the displayed CTDI, which is calculated by the CT scanner, represents the weighted average dose that a 32 cm diameter cylindrical phantom would have received if the plastic phantom had been scanned with the same radiographic parameters as the patient. The greater the physical difference of the patient from a 32 cm homogeneous plastic phantom, the greater the difference of the patient’s dose from the displayed CTDI. In pediatric protocols a 16 cm diameter plastic phantom is used.

The displayed DLP is calculated from the product of the scan length and the weighted average of dose to a 32 cm plastic phantom if the phantom had been scanned with the exact CT protocol with which the patient was scanned.

Of interest to a patient would be information about the effective dose equivalent (EDE), which is relevant to overall radiation risk. The literature and many textbooks contain “typical” EDEs for CT scans. The typical values are calculated for an assumed dose distribution in standard man. The extent that a patient’s dose distribution differs from the assumed dose distribution, and the extent to which the patient differs in geometry from the mathematical model of standard man that is used in the calculation, determines how closely the actual patient EDE approaches the calculated typical value available in the literature.

In order to accurately calculate the EDE to a specific patient from a specific diagnostic imaging procedure, you would need to have a data set that represented the entire patient tissue contour and inhomogeneities. You would also need x-ray beam data in order to have a computer perform a dose distribution calculation just as is done in treatment planning in radiation oncology. This is not done in diagnostic imaging. 

The CT output metrics such as CTDI and DLPs still remain useful in managing patient radiation exposure because there is an “expected” range of values of the output metrics for a scan type regardless of the patient size and x-ray beam spectra. A pre- or post-scan CTDI that is significantly greater than the range that is normally seen, should alert the operator to consult with a radiologist.

Wayne A. Wiatrowski, PhD, FACR, CHP
Clinical Associate Professor

Editor’s Note: What Dr. Wiatrowski is saying is that we cannot calculate your specific radiation dose. But we can provide you with typical effective doses (or effective dose equivalents) for the procedures you underwent. For a pelvis CT scan, a typical effective dose is 14 mSv and a typical effective dose from a CT scan of the head is 2.2 mSv.

Kent Lambert, CHP

Answer posted on 18 September 2013. 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.