Daily Bulletin 2015

Researchers Develop Size-Specific Fetal CT Dose Calculator for Pregnant Patients

Tuesday, Dec. 01, 2015

A method to easily and accurately estimate size-specific fetal dose for CT exams using tube current modulation (TCM) on pregnant patients was presented by researchers at a Monday session.

Presenter Kyle McMillan, Ph.D., of the University of California, Los Angeles (UCLA), and colleagues developed patient size-specific, scanner-independent conversion coefficients for CT dose index volume (CTDIvol) to fetal dose. The coefficients were developed for abdominal and pelvic CT exams of pregnant patients who had a gestational age ranging from 12 to 36 weeks.

Kyle McMillan, Ph.D.

Kyle McMillan, Ph.D.

Plugging the patient size as measured by water-equivalency displacement and the CTDIvol into a formula developed by this research will calculate the patient-specific dose, said Dr. McMillan in the standing-room only session of 200-plus people.

"That's a heck of a lot easier than doing a Monte Carlo simulation," Dr. McMillan said. "A doctor may take these measurements and generate a patient-specific dose estimate, which may provide some comfort to patients."

Dr. McMillan conducted the research as part of his doctoral dissertation under the supervision of Michael McNitt-Gray, Ph.D., of UCLA, who also attended the RSNA 2015 session.

Estimating Patient-Specific Dose

Fetal dose estimates have been limited to fixed tube current CT exams of pregnant patients even though nearly all other CT exams are performed using TCM. The UCLA researchers collected data to determine normalized CTDIvol for TCM scans and incorporated the water-equivalent diameter (WED) measurement available on most CT equipment to calculate a patient-specific dosage.

Researchers created models of fetal anatomy from image data from 18 pregnant women who had undergone abdominal/pelvic CT. They estimated fetal doses for each model using Monte Carlo simulations of fixed tube current CT scans with a 64-slice MDCT scanner.

Predicted TCM schemes were generated for each pregnant patient model using a validated method accounting for patient attenuation and scanner limits to determine TCM functions for each voxelized model, Dr. McMillan said. The validated method involves yet-to-be published research that is part of Dr. McMillan's doctoral dissertation. Fetal doses were normalized by scan ‑specific 32 cm CTDIvol values based on the average tube current across the entire scan (scanner-reported CTDIvol) to obtain scan technique-independent CTDIvol-to-fetal-dose conversion coefficients for each patient.

WED can be reliably incorporated into the dose measurement so radiologists can generate patient-specific dose, instead of relying on models. The study found a positive correlation between the normalized dose and the WED. Using models when determining doses for organs is acceptable, but a patient-specific calculation for fetal dose provides more accurate information since the fetus is always growing, Dr. McMillan said.

"A reasonably accurate dose to the fetus can be obtained by knowing simply the scanner output and the size of the patient expressed in this water-equivalent diameter," Dr. McNitt-Gray said.

Although using WED in the dose calculation is not standard on all scanners, Dr. McNitt-Gray hopes that research showing the usefulness of the function will encourage physicians to request the measure and persuade manufacturers to incorporate it into all scanners.

Question of the Day:

I want to change the MR imaging parameters for a new protocol based on a research paper I read, but the console says the SAR is too high. What is SAR?

Tip of the day:

Patients who have many follow-up head CTs should be assessed for dose to the eyes as they swiftly become at risk for cataracts.

The RSNA 2015 Daily Bulletin is owned and published by the Radiological Society of North America, Inc., 820 Jorie Blvd., Oak Brook, IL 60523.