Wednesday Case of the Day
Physics
Authors: Michael F. McNitt-Gray, PhD,1 Erin Angel, MS,1 Mitchell M. Goodsitt, PhD,2
Clinton V. Wellnitz, MD3
1Department
of Radiological Sciences, David Geffen School of Medicine at UCLA, Los Angeles, CA.
2Department of Radiology, University of Michigan, Ann Arbor.
3Mayo Clinic, Scottsdale, AZ.
History: 32-year-old female (approximately standard-sized female) undergoes CT scan of
abdomen and pelvis after motor vehicle accident. Later she discovers she was 7 weeks
pregnant at time of scan. CT scan info:

GE LightSpeed 16 scanner.

120 kVp; fixed tube current, 400 mA; 0.5-sec rotation time; pitch, 0.9875; 16 x 1.25-mm
collimation; 5-mm reconstructed slice thickness; standard reconstruction filter.

CTDI100 (mGy/100 mAs) in 16-cm-diameter phantom: center = 16.0; peripheral = 17.2.

Scan ranged from top of liver through to pubic symphysis.
Estimate the radiation dose received by the fetus:
a)
0.5–1 mGy.
b)
10–15 mGy.
c)
15–30 mGy.
d)
30–50 mGy.
e)
100–150 mGy.
Gestational
Sac
Uterus
Figure 1. Axial and sagittal views of gestational sac and
uterus from patient who is 7 weeks pregnant at time of CT.
Correct Answer
(c) 15–30 mGy.
Next several slides will go through three different methods of estimating
radiation dose to fetus for this case:
1.
2.
3.
Felmlee, Gray, et al method [AJR 1990 Jan;154(1):185–90]
Using ImPACT dose spreadsheet method (www.impactscan.org)
Monte Carlo simulation method with pregnant patient model (E. Angel, et al,
RSNA 2006)
Final slides will discuss implications of this level of radiation dose on fetus and
recommendations.
Fetal Radiation Dose:
Clinical Context
Most often this calculation is requested in the context of a woman who didn’t
know she was pregnant when she underwent a CT scan; sometimes this is
requested ahead of time when the CT is scheduled for a woman who is
known to be pregnant.
Fetal Radiation Dose:
Scan Protocol and Information
In the scenario presented, the following details were provided:
1.
2.
3.
4.
5.
6.
7.
GE LightSpeed 16 (16–detector row) CT scanner.
120 kVp.
400 mA.
0.5-second rotation time.
Helical scan with pitch = 0.9875.
16 x 1.25-mm collimation (20-mm nominal beam width).
CTDI100 in 16-cm-diameter phantom of 16.0 mGy/100 mAs at center and 17.2
mGy/100 mAs at a peripheral (12:00) position under the above conditions.
8. Because scan ranged from top of liver through to pubic symphysis, the fetus was
definitely in the x-ray beam.
9. The woman was standard female size—approximately 58 kg; 160 cm (128 lb;
5’3”).
10. The gestational age at the time of scan was later determined to be
approximately 7 weeks.
11. No mention of additional risk factors to mother or fetus.
Felmlee, Gray, et al Method
This paper [AJR 1990 Jan;154(1):185–90] first describes the normalized fetaldose ratio (NFDR):
– Defined as the measured fetal-dose contribution from a single CT scan
divided by the CTDI measured (16-cm-diameter phantom) with the
same scanning parameters.
– Determined at various distances from the fetus.
Felmlee, Gray, et al Method
Then the paper describes an integrated normalized fetal-dose ratio
(INFDR):
– Defined as the sum of individual NFDR(d) located from 0 cm to an
offset distance, assuming 1-cm scans at 1-cm incrementation.
– Determined at various distances from the fetus.
Felmlee, Gray, et al Method
For example:
• At z = 0 cm offset (ie, directly over fetus),
– INFDR(0) = 0.255.
• At z = 10 cm offset (say, 10 cm inferior),
– INFDR(10) = 0.453.
• At z = 40 cm offset (say, 40 cm superior),
– INFDR(40) = 0.481.
Scan begins 40 cm superior to location of fetus.
Z-axis location of fetus
Scan ends 10 cm inferior to location of fetus.
Felmlee, Gray, et al Method
Fetal dose (mGy) = CTDI * INFDRE
where:
CTDI = center value from 16-cm phantom.
INFDRE = INFDR0 + INFDRInf + INFDRSup.
–
–
–
–
–
–
–
INFDR0 = NFDR (from Table 3 of Felmlee) at 0 offset.
INFDRInf = INFDR for scan locations inferior to fetal location.
INFDRSup = INFDR for scan locations superior to fetal location.
INFDRSup or Inf = (T/I) [INFDR(M) – INFDR(N–1)]
T = nominal beam width (in mm).
I = table increment per rotation (also in mm).
INFDR(M) = INFDR at the maximum offset distance M (cm) from fetal
location.
– INFDR(N–1) = INFDR at the minimum offset distance N (cm) minus
1 cm from the fetal location.
Felmlee, Gray, et al Method
Under the conditions specified:
1.
2.
3.
4.
CTDI at center position of 16-cm phantom is 32 mGy.
– CTDI at center position of 16-cm phantom was 16.0 mGy/100 mAs.
– 200 mAs.
Superior location is approximately 40 cm superior to fetus.
Inferior location is approximately 10 cm inferior to fetus.
Pitch = 0.9875 = T/I, T = 20 mm, so I = 19.75 mm.
Therefore,
INFDRE = INFDR0 + INFDRInf + INFDRSup
= 0.255 + (20/19.75)*(0.453 − 0.255) + (20/19.75)*(0.481 − 0.255)
= 0.255 + 0.201 + 0.229 = 0.685
Fetal Dose = CTDI * INFDRE = 32 mGy * 0.685
= 21.9 mGy
ImPACT Spreadsheet Method

Results are based on Monte Carlo simulations performed by National
Radiological Protection Board (NRPB) as described in NRPB report 250.
–



Original work done in 1991.
Modern scanners are “matched” to original scanners based on CTDI values
measured in air and in phantom.
Helical scans modeled using pitch values:
–

(Jones DG, Shrimpton PC. Survey of CT practice in the UK: Part 3. Normalized organ doses
calculated using Monte Carlo techniques. National Radiological Protection Board, NRPBR250, 1991).
(table travel per rotation)/(total nominal beam collimation).
MIRD patient model:
–
–
Hermaphrodite standard man representation.
Geometrically shaped organs, including uterus.
ImPACT Spreadsheet Method
To obtain an estimate of fetal dose:
1. Choose scanner manufacturer and model.
o
2.
Choose scanner parameters.
o
o
3.
4.
Matched to a scanner make and model from original simulations.
kVp, body region (which selects the bowtie filter), mAs, beam collimation, pitch.
Choose start/end location of scan on patient model.
Provides an estimate of dose to all ICRP 60 organs, including uterus.
For early gestational age, estimate to uterus is approximately equal to fetal
dose.
See www.impactscan.org for more details on this method.
ImPACT Spreadsheet Method
90
80
70
60
50
40
Start location
30
20
10
0
-10
Uterus
End location
ImPACT Spreadsheet Method
Uterus Dose
in mGy
Monte Carlo Simulation Method
(Angel et al, RSNA 2006)

Monte Carlo simulations based on:
– Detailed description of scanner.
– Detailed description of pregnant patient.
– Simulate arbitrary scan protocol.
Monte Carlo Simulation Method
(Angel et al, RSNA 2006)
Monte Carlo model:
–
–
–
–
MCNPX software from Los Alamos National Lab.
Uses a detailed voxelized model of pregnant patient (next slide).
Description of scanner geometry (fan angle, SID, etc).
Simulates photon transport from modified x-ray source through patient model.
Modified CT x-ray source:
–
–
Source model previously published in:
• Jarry, et al. Phys Med Biol 2003;48(16):2645–63.
• DeMarco, et al. Phys Med Biol 2005;50(17):3989–4004.
Detailed descriptions of source:
• X-ray spectra (from manufacturer).
• Beam filtration including the bowtie filter (from manufacturer).
• Beam collimation in the longitudinal (z) direction.
• Source movement for helical scan of arbitrary pitch.
Agreement between simulation and measurement shown to be within
3.5% in phantom (Jarry et al and DeMarco et al).
Monte Carlo Simulation Method:
Creating a Voxelized Model
Using CT image data from patients who were scanned for clinical
indications (such as motor vehicle accident), create voxelized models for
simulation.
Radiologist contoured uterus and gestational sac (when visible).
Original Image
Contoured Image
Voxelized Model
Monte Carlo Simulation Method:
Modeling the CT Source
Modeled GE LightSpeed 16
120 kVp, body bowtie filter
4 x 5-mm nominal beam collimation
Dose reported on a per-100-mAs basis
Monte Carlo Simulation Method:
Modeling the Scan
Abdomen/pelvis scan:
Just above liver to below pubic symphysis
Helical scan, pitch = 1 (for reference)
Can be adjusted for pitch = 0.9875
Only one series
Monte Carlo Simulation Method:
Results
In our inventory of cases, we had two patients who were 7
weeks pregnant:
 Patient 1 was 134 pounds (no info on height, but
perimeter was approximately 95 cm at level of fetus).
 Patient 2 was 137 pounds (no info on height, but
perimeter was approximately 86 cm at level of fetus).

Estimated fetal doses for 200-mAs 0.9875-pitch scan:
– Patient 1: 17.8 mGy
– Patient 2: 24.3 mGy
Comparing Results
Felmlee, Gray, et al (standard-sized patient):
21.9 mGy
ImPACT method (standard-sized patient):
29 mGy
Monte Carlo method (Angel et al):
17.8–24.3 mGy
Implications of Results
With estimated radiation dose to fetus in the 15–30-mGy range, what are the
implications for a 7-week-old fetus?
While there may be a wide variation among individual patients and in estimates
from different methods:
Fetal doses from a single abdomen/pelvis CT exam are still below the
consensus levels for negligible risk (50–150 mGy) and well below “actionable”
levels of 150 mGy.
References:
McCollough CH, Schueler BA, Atwell TD, et al. Radiation exposure and
pregnancy: when should we be concerned? RadioGraphics 2007;27(4):909–17.
Wagner, et al. Exposure of the pregnant patient to diagnostic radiations.
Madison, WI: Medical Physics Publishing, 1997.