Radiation Dose to the Fetus for Pregnant Patients Undergoing by wxw48807

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                                                                                    Radiation Dose to the Fetus for
                                                                                    Pregnant Patients Undergoing
                                                                                    Multidetector CT Imaging: Monte
   ORIGINAL RESEARCH




                                                                                    Carlo Simulations Estimating Fetal Dose
                                                                                    for a Range of Gestational Age and
                                                                                    Patient Size1
                       Erin Angel, MS
                                                                                          Purpose:    To use Monte Carlo simulations of a current-technology
                       Clinton V. Wellnitz, MD
                                                                                                      multidetector computed tomographic (CT) scanner to in-
                       Mitchell M. Goodsitt, PhD
                                                                                                      vestigate fetal radiation dose resulting from an abdominal
                       Nazanin Yaghmai, MD                                                            and pelvic examination for a range of actual patient anato-
                       John J. DeMarco, PhD                                                           mies that include variation in gestational age and maternal
                       Christopher H. Cagnon, PhD                                                     size.
                       James W. Sayre, DrPH
                       Dianna D. Cody, PhD                                            Materials and   Institutional review board approval was obtained for this
                       Donna M. Stevens, MS                                               Methods:    HIPAA-compliant retrospective study. Twenty-four mod-
                       Andrew N. Primak, PhD                                                          els of maternal and fetal anatomy were created from image
                       Cynthia H. McCollough, PhD                                                     data from pregnant patients who had previously under-
                                                                                                      gone clinically indicated CT examination. Gestational age
                       Michael F. McNitt-Gray, PhD
                                                                                                      ranged from less than 5 weeks to 36 weeks. Simulated
                                                                                                      helical scans of the abdominal and pelvic region were
                                                                                                      performed, and a normalized dose (in milligrays per 100
                                                                                                      mAs) was calculated for each fetus. Stepwise multiple
                                                                                                      linear regression was performed to analyze the correlation
                                                                                                      of dose with gestational age and anatomic measurements
                                                                                                      of maternal size and fetal location. Results were compared
                                                                                                      with several existing fetal dose estimation methods.

                                                                                           Results:   Normalized fetal dose estimates from the Monte Carlo
                                                                                                      simulations ranged from 7.3 to 14.3 mGy/100 mAs, with
                                                                                                      an average of 10.8 mGy/100 mAs. Previous methods
                                                                                                      yielded values of 10 –14 mGy/100 mAs. The correlation
                                                                                                      between gestational age and fetal dose was not signifi-
                       1
                         From the Departments of Radiology and Radiation On-                          cant (P     .543). Normalized fetal dose decreased lin-
                       cology, David A. Geffen School of Medicine, University of                      early with increasing patient perimeter (R2 0.681, P
                       California at Los Angeles, 924 Westwood Blvd, Suite 650,                       .001), and a two-factor model with patient perimeter
                       Los Angeles, CA 90024 (E.A., N.Y., J.J.D., C.H.C., J.W.S.,
                                                                                                      and fetal depth demonstrated a strong correlation with
                       M.F.M.); Department of Diagnostic Radiology, Mayo Clinic,
                       Scottsdale, Ariz (C.V.W.); Department of Radiology, Uni-                       fetal dose (R2 0.799, P .002).
                       versity of Michigan Hospitals, Ann Arbor, Mich (M.M.G.);
                       Division of Diagnostic Imaging, University of Texas M. D.        Conclusion:   A method for the estimation of fetal dose from models of
                       Anderson Cancer Center, Houston, Tex (D.D.C., D.M.S.);                         actual patient anatomy that represented a range of gesta-
                       and Department of Radiology, Mayo Clinic College of                            tional age and patient size was developed. Fetal dose cor-
                       Medicine, Rochester, Minn (A.N.P., C.H.M.). From the
                                                                                                      related with maternal perimeter and varied more than
                       2006 RSNA Annual Meeting. Received September 24,
                       2007; revision requested December 10; revision received
                                                                                                      previously recognized. This correlation improves when
                       January 28, 2008; accepted April 7; final version ac-                           maternal size and fetal depth are combined.
                       cepted April 21. Supported by grants R01EB004898 and
                       T32EB002101 from the National Institute of Biomedical                           RSNA, 2008
                       Imaging and Bioengineering. Address correspondence
                       to E.A. (e-mail: EAngel@mednet.ucla.edu ).

                         RSNA, 2008


                       220                                                                                          radiology.rsnajnls.org ▪ Radiology: Volume 249: Number 1—October 2008
MEDICAL PHYSICS: Monte Carlo Simulation in Pregnant Patient Models                                                                                        Angel et al




D
       iagnostic computed tomographic                        modified to represent a newly pregnant        edge, this model has only been used for
       (CT) imaging is sometimes neces-                      patient and a patient who was 3 months       the estimation of radiation dose in ex-
       sary in a pregnant patient. When a                    pregnant.                                    periments that involved radiation
radiologist needs to decide if the diagnos-                      Other approaches to estimate fetal       sources other than CT.
tic benefits will outweigh the risks of radi-                 dose are based on Monte Carlo simula-             In summary, existing methods for the
ation, it is important to have a reasonably                  tions of geometric patient models. In        estimation of fetal dose for pregnant pa-
accurate estimate of the radiation dose                      these methods, dose to the uterus is used    tients undergoing CT examinations as-
that the conceptus (fetus or embryo) will                    as an estimate of dose to the fetus at an    sume early term pregnancy in a single-
receive. Furthermore, in cases in which                      early gestational age. One such method is    size patient model with an average, non-
pregnancy is discovered during or after CT                   the ImPACT CTDosimetry dose calcula-         varying maternal anatomy. These dose
examination, the patient and/or physician                    tor (CTDosimetry.xls, version 0.99 ; Im-     estimates do not take into account natural
may request an estimate of the radiation                     PACT, London, England) (4), which is         variations, such as fetal presentation and
dose received by the conceptus. For the                      based on Monte Carlo simulations per-        gestational age. Differences in these at-
remainder of this article, the term fetus will               formed by the National Radiological Pro-     tributes can cause overestimation or un-
be used to refer to either an embryo or a                    tection Board (5) with the use of a geo-     derestimation of up to 100% for fetal dose
fetus and will therefore be used to describe                 metric Medical Internal Radiation Dose       at radiologic examinations (12).
a conceptus at any gestational age.                          (MIRD) phantom model (6). Another                 To overcome these limitations, simu-
     It is not known definitively how                         similar method is the CT-Expo software       lations should model specific scanner and
much radiation dose a fetus receives                         (version 1.5.1; Medizinische Hochschule,     maternal and fetal characteristics. The
during CT examination, because this                          Hannover, Germany) (7,8), which esti-        purpose of this study was to use Monte
cannot be measured directly. Some                            mates organ dose based on simulations        Carlo simulations with a current-technol-
methods to estimate fetal dose exist, but                    performed by Zankl et al at the German       ogy multidetector CT scanner to estimate
these estimates are limited by their sim-                    National Research Center with the Eva        fetal dose at abdominal and pelvic CT ex-
plifying assumptions. Existing fetal dose                    geometric phantom model to represent a       amination with models of pregnant pa-
estimation methods are based on phan-                        standard-size female patient (9,10).         tients that represented a range of gesta-
tom measurements and/or geometric                                The MIRD and Eva phantoms con-           tional ages and maternal sizes.
phantom simulation methods.                                  sist of geometric shapes representing
     The method used to estimate fetal                       human organs for a standard-size pa-
dose developed by Felmlee et al (1) is                       tient model. Although these geometric         Materials and Methods
based on anthropomorphic phantom                             phantoms do not represent pregnant
                                                                                                          Patient Images
measurements and measured CT dose in-                        patient anatomy, ImPACT CTDosim-
                                                                                                          Twenty-four voxelized models of mater-
dex (2). More recently, Hurwitz et al (3)                    etry and CT-Expo software can be used
                                                                                                          nal and fetal anatomy were created on
estimated fetal dose by using physical                       to estimate radiation dose to the uterus,
measurements from internal dosimeters                        which may be used to approximate fetal
in an anthropomorphic phantom that was                       dose during the first 8 weeks of gesta-       Published online
                                                             tion. A patient-based voxelized model of     10.1148/radiol.2491071665
                                                             a patient who is 30 weeks pregnant does      Radiology 2008; 249:220 –227
  Advances in Knowledge                                      exist (11), but, to the best of our knowl-
     Radiation dose to the fetus at CT                                                                    Abbreviation:
                                                                                                          MIRD Medical Internal Radiation Dose
     imaging in pregnant patients was
     estimated with examples of actual                         Implications for Patient Care
                                                                                                          Author contributions:
     pregnant patient anatomy that                                For a typical abdominal and pelvic      Guarantors of integrity of entire study, E.A., D.D.C.,
     represented a range of gestational                           CT examination, the average fetal       M.F.M.; study concepts/study design or data acquisition
                                                                                                          or data analysis/interpretation, all authors; manuscript
     ages.                                                        dose is approximately 24 mGy
                                                                                                          drafting or manuscript revision for important intellectual
     Radiation dose to the fetus corre-                           with a range of 16 –31 mGy, de-         content, all authors; manuscript final version approval, all
     lates with patient size as mea-                              pending on maternal size.               authors; literature research, E.A., M.M.G., D.D.C., C.H.M.,
     sured by the perimeter around                                Fetal dose estimates for a single-      M.F.M.; experimental studies, E.A., C.V.W., M.M.G., N.Y.,
     the mother.                                                  pass abdominal and pelvic acqui-        J.J.D., C.H.C., D.D.C., D.M.S., A.N.P., M.F.M.; statistical
     There was no correlation between                             sition are below the consensus          analysis, E.A., M.M.G., J.W.S., M.F.M.; and manuscript
     gestational age and fetal dose                               levels for negligible risk (50 –150     editing, E.A., C.V.W., M.M.G., J.J.D., C.H.C., D.D.C.,
                                                                                                          D.M.S., A.N.P., C.H.M., M.F.M.
     (P .543).                                                    mGy) and well below the action-
     The average fetal doses were esti-                           able level of 150 mGy.                  Funding:
     mated to be 10.8 mGy per 100                                 Modulation of tube current with         This research was funded by the National Institute of
     mAs for a specific scanner model,                             respect to patient size should be       Biomedical Imaging and Bioengineering (grants R01
                                                                                                          EB004898, T32 EB002101).
     with a range from 7.3 to 14.3                                used to decrease fetal dose for
     mGy/100 mAs.                                                 smaller maternal sizes.                 Authors stated no financial relationship to disclose.


Radiology: Volume 249: Number 1—October 2008 ▪ radiology.rsnajnls.org                                                                                             221
MEDICAL PHYSICS: Monte Carlo Simulation in Pregnant Patient Models                                                                                                Angel et al




the basis of image data from a cohort of                  mm. The patients in the cohort were                        patient data by one of two board-certi-
31 pregnant patients who previously un-                   originally scanned with one of the                         fied radiologists (C.V.W., N.Y.). The
derwent clinically indicated abdominal                    following scanners: HighSpeed CT/I,                        radiologist contoured the uterus, gesta-
and pelvic CT examination. The original                   LightSpeed QX/i, LightSpeed Ultra,                         tional sac (when visible), and fetus
CT examinations were performed unre-                      LightSpeed PRO, LightSpeed 16,                             (when visible), which resulted in a set of
lated to this study and were often in                     or LightSpeed VCT (GE Healthcare, Wau-                     voxels that were labeled as belonging to
emergency and/or trauma patients (such                    kesha, Wis). Oral contrast agent had                       these three tissue groups. Images were
as those who were injured in a motor                      been used in 13 of the 24 CT examina-                      subsampled from a 512 512 matrix to
vehicle accident). Institutional review                   tions. The cranial-caudal length of each                   a 128 128 matrix to reduce simulation
board approval was obtained for this                      voxelized patient model was dependent                      time.
Health Insurance Portability and Ac-                      on the length of CT image data avail-                          Voxels within the fetus were mod-
countability Act– compliant study for                     able. The average length of image data                     eled as soft tissue or soft bone, de-
the use of the anonymized CT images,                      was 46.2 cm, and all of the image series                   pending on the CT number (in
as well as for additional anonymized pa-                  included, at a minimum, the patient                        Hounsfield units). The voxels in the
tient data (ie, patient weight and gesta-                 anatomy from the lower thorax to the                       gestational sac were modeled as wa-
tional age). The 24 patient models were                   pubic symphysis.                                           ter, and the voxels in the uterus were
selected to represent a range of gesta-                       Two anatomic measures were ob-                         modeled as soft tissue. Each voxel of
tional ages; seven cases were excluded                    tained from each patient’s images by one                   the mother outside these fetal regions
because of a lack of sufficient volumetric                 of the authors (E.A.), who had been                        was modeled as one of six tissue types
image data (images did not fully cover                    trained in reviewing CT image data.                        (lung, fat, water, muscle, bone, or air)
the uterus) or because gestational ages                   These measures were used to assess the                     (13). The process of creating voxel-
were similar to those of other cases al-                  size of the mother and the location of the                 ized models from the original patient
ready in the cohort. The set of 24 pa-                    fetus within the mother. The patient size                  images is illustrated in Figure 3.
tients included gestational ages ranging                  metric was the outer (skin) perimeter of
from less than 5 weeks to 36 weeks,                       the patient on the image containing the                    Multidetector CT Source Model
with an average gestational age of 20                     three-dimensional geometric centroid of                    A previously validated multidetector CT
weeks. The cohort included six patients                   the fetus. The perimeter was determined                    source model was created by using
in the first trimester of pregnancy, 10                    by using a semiautomated threshold-                        Monte Carlo simulation– based tech-
patients in the second trimester, and                     based contouring technique. Fetal depth                    niques (14–18). The CT scanner mod-
eight patients in the third trimester. Ex-                was measured manually as the distance                      eled was a LightSpeed 16 (GE Medical
amples of these CT images are shown in                    from the anterior skin surface to the most                 Systems). The scanner’s x-ray spec-
Figure 1. The 24 models created as part                   anterior part of the fetus. The patient pe-                trum, filtration, and geometry were ob-
of this study are publicly available at                   rimeter and fetal depth measures are il-                   tained from the manufacturer and con-
http://medqia.org/ mcnitt/FetalModels.                    lustrated in Figure 2.                                     firmed through measurements. The x-ray
    The original image data were acquired                                                                            spectrum was initially described as a func-
between January 2002 and February 2006                    Patient Models                                             tion of the number of photons at 1-keV
with 120-kVp acquisition protocols, with                  To estimate fetal dose, three regions of                   energy intervals, which was then con-
image thickness varying from 1.25 to 10                   interest were identified for each set of                    verted to a cumulative distribution func-


 Figure 1




 Figure 1: CT images in three patients in the cohort at gestational age of 7 weeks (left), 24 weeks (middle), and 36 weeks (right) that demonstrate early term, midterm,
 and late-term pregnancy, respectively.


222                                                                                                      radiology.rsnajnls.org ▪ Radiology: Volume 249: Number 1—October 2008
MEDICAL PHYSICS: Monte Carlo Simulation in Pregnant Patient Models                                                                                                   Angel et al




  Figure 2
                                                             tion for implementation within the                         which resulted in a first half-value layer
                                                             Monte Carlo source model (14–18).                          at isocenter of approximately 7.8 mm
                                                             The multidetector CT model included                        aluminum. The simulated scan had a
                                                             the bowtie filter and beam collima-                         helical source path with a pitch of 1.0
                                                             tion. The multidetector CT model was                       and had 16 detector rows of 1.25 mm
                                                             designed to simulate acquisition param-                    (resulting in 16    1.25-mm total nomi-
                                                             eters selected by the user, including                      nal collimation). Although the actual
                                                             beam energy (kilovoltage peak), source                     scanner modeled does not allow these
                                                             path (ie, helical or axial), pitch, total                  particular acquisition parameters (pitch
                                                             nominal beam collimation, and scan                         of 1.0 is not available), the simulation
                                                             length.                                                    parameters represent a typical abdomi-
                                                                                                                        nal and pelvic examination, and the re-
                                                             Monte Carlo Simulation of Abdominal and                    sults can be scaled to any other pitch
                                                             Pelvic CT Examination                                      value. Simulated examinations were per-
  Figure 2: CT image with semiautomated con-                 The simulated CT examination covered the                   formed in all 24 voxelized phantoms. No
  tour (dashed line) around the perimeter of the             abdominal and pelvic region. The total                     tube current–time product value was
  mother on central image in a 35-week-old fetus, as         length of the simulated examination (in-                   specified; all doses were tallied and re-
  well as the assessment of fetal depth (distance            cluding image data and z-axis overscan)                    ported on a normalized basis of milli-
  from skin to fetus [double arrow]).                        was equivalent to the length of the orig-                  grays per 100 mAs.
                                                             inal patient image data. The simulated                         The simulation involves modeling
                                                             abdominal and pelvic examination had a                     the x-ray photon transport through the
                                                             beam energy and filtration combination                      voxelized patients and tallying dose in
                                                             of 120 kVp and body bow tie filter,                         the voxels that were labeled as uterus,


  Figure 3




  Figure 3: CT images and voxelized models created from patient images of an early pregnancy (7 weeks) and a later pregnancy (35 weeks). Pink represents the uterus,
  yellow represents the gestational sac, and red represents the fetus (when visible). In the voxelized model, fetal bone is white, and tissues outside the radiologist’s con-
  tours range from blue to green.


Radiology: Volume 249: Number 1—October 2008 ▪ radiology.rsnajnls.org                                                                                                       223
MEDICAL PHYSICS: Monte Carlo Simulation in Pregnant Patient Models                                                                                                   Angel et al




                                                   Table 1
gestational sac, and fetus. All simula-
tions used the Monte Carlo N-Particle                Patient Model Measurements and Radiation Dose per 100 mAs to Fetus according to
eXtended code (version 2.5.0; Radia-                 Gestational Age of 24 Voxelized Models
tion Safety Information Computational
                                                                                                                                                    Normalized Fetal Dose
Center, Oak Ridge, Tenn) created at                  Gestational Age (wk)              Maternal Perimeter (cm)               Fetal Depth (cm)       (mGy/100 mAs)
the Los Alamos National Laboratory
(19,20). Each Monte Carlo simulation                      5.0                          123                                   10.6                    7.3*
was performed with 1–10 million simu-                     5.0                           89                                    4.2                   11.8†
lated x-ray photons, which were enough                    5.0                           88                                    7.6                   10.3†
to ensure less than 1% statistical error                  6.6                          102                                   10.9                    8.8†
within the tally region of interest (ap-                  7.1                           90                                    5.9                   12.6†
                                                         12.1                           88                                    4.6                   14.2
proximately 2 hours run time on an av-
                                                         14.3                          105                                    6.5                   10.9
erage desktop computer).
                                                         14.9                           93                                    7.1                   11.2
Dose Calculations                                        17.0                           94                                    7.7                   11.3
                                                         17.1                           87                                    6.7                   12.2
The average radiation dose to the fetus
                                                         18.5                           87                                    5.6                   14.3
was tallied when the fetus was distin-                   20.3                          112                                    8.0                    8.5
guishable from the gestational sac and                   22.0                          108                                    4.7                   11.1
uterus. If the fetus was not visible, the                23.7                          118                                    6.3                    8.1
average dose was tallied in the gesta-                   24.0                           95                                    5.6                   11.5
tional sac. In one voxelized model, with                 24.4                           94                                    6.6                   11.5
a gestational age less than 5 weeks, the                 25.0                           92                                    2.5                   12.3
gestational sac was not visible; in this                 27.0                           89                                    9.0                   11.2
case, the dose was tallied in the uterus.                27.4                          104                                    3.6                   11.9
The average dose was computed by av-                     27.4                          122                                    6.0                    8.6
eraging the values of dose absorbed in                   28.3                          119                                    5.5                    9.5
the voxels within the tally region. Ab-                  29.4                          105                                    3.5                    9.7
sorbed dose within a voxel was com-                      35.0                          100                                    5.1                   10.4
puted from the collision kerma, calcu-                   35.9                          119                                    3.4                    8.9
lated from the Monte Carlo N-Particle                Note.—The means for gestational age, maternal perimeter, fetal depth, and normalized fetal dose were 19.7 weeks, 101 cm,
eXtended track-length estimate of en-                6.1 cm, and 10.8 mGy/100 mAs, respectively.
ergy fluence, and multiplied by the ma-               * Uterus dose was calculated because the fetus and gestational sac were not visible.
terial-specific and energy-dependent                  †
                                                         Gestational sac dose was calculated because the fetus was not visible.
mass-energy absorption coefficient. The
mass-energy absorption coefficients were
taken from the tables of Hubbell and
Seltzer (21). All reported dose results
were normalized per 100 mAs.                       Table 2

                                                     Radiation Dose to Fetus as Estimated with Monte Carlo Technique Compared with
Statistical Analysis                                 That of Three Existing Fetal Dose Estimation Techniques
The fetal dose estimates from the co-
                                                                                                                                                   Normalized Fetal Dose
hort of voxelized patient models were
                                                                                                                                                      (mGy/100 mAs)
tested for correlation with each of three
                                                     Dose Estimation Method                                             No. of Patient Models     Average      Range
independent measures described above:
gestational age, perimeter of the                        Monte Carlo                                                    24                        10.8           7.3–14.3
mother, and fetal depth. Stepwise mul-                   Felmlee et al (1) (recommended        20% size range)           1                        11.3           9.0–13.6
tiple linear regression analysis was per-                ImPACT                                                          1                        12.0           NA
formed to assess the partial correlation                 CT-Expo                                                         1                        11.7           NA
of each variable, and P values were cal-             Note.—NA        not applicable.
culated to determine statistical signifi-
cance. A P value less than .05 indicated
a significant difference.                         this study. However, there are several                               estimation of fetal dose was that of Felm-
                                                 existing fetal dose estimation methods                               lee et al (1). For these calculations, the
Existing Methods                                 against which the results of this study                              fetus was assumed to be axially centered,
There is no reference standard for fetal         can be compared.                                                     and contiguous axial scans were used in-
dose that can be used as comparison for              The first comparison method for the                               stead of helical scans, with a pitch of 1.0.

224                                                                                                     radiology.rsnajnls.org ▪ Radiology: Volume 249: Number 1—October 2008
MEDICAL PHYSICS: Monte Carlo Simulation in Pregnant Patient Models                                                                       Angel et al




Although this technique does not address                     These methods were used to estimate        regression analysis (R2    0.017) indi-
gestational age and produces a single fetal                  dose to the uterus and represent fetal     cated no significant correlation (r
dose result, Felmlee et al state that a dose                 dose for gestational ages approximately      0.130; P    .543) between gestational
estimate may vary by 20% due to varia-                       less than or equal to 8 weeks in a stan-   age and fetal dose.
tions in patient size. The estimated fetal                   dard-size patient. The perimeter of the        Fetal dose as a function of the pe-
dose and a 20% range were compared                           hermaphrodite MIRD phantom used with       rimeter of the mother is shown in
with the Monte Carlo results from this                       the ImPACT software was approximately      Figure 5; the relationship between the
study. The abdomen of the anthropo-                          99 cm at the pelvis (6). The Eva geomet-   perimeter (P) (in centimeters) of the
morphic phantom used for the Felmlee                         ric phantom used with the CT-Expo soft-    mother and fetal dose (DOf) (in milli-
technique had a perimeter of approxi-                        ware has an anteroposterior size of 18.8   grays per 100 mAs) can be repre-
mately 88 cm.                                                cm (7) and an estimated perimeter of 93    sented as:
    The second and third fetal dose esti-                    cm. The ImPACT and CT-Expo software
mation methods used for comparison                           each produced a single fetal dose esti-          DOf      0.122(P)     23.11,     (1)
were the ImPACT dose calculator (4) and                      mate, which was compared with the re-
CT-Expo software (7,8), which are both                       sults from this study.                     which has an R2 value of 0.681, indicat-
based on Monte Carlo simulations (5,10).                         For each dose comparison method,       ing a significant linear correlation (r
                                                             the CT acquisition parameters (tube          0.825; P      .001) between maternal
                                                             voltage, collimation, etc) were matched    size and fetal dose. The standard error
  Figure 4
                                                             as closely as possible to the protocol     of the estimate for the perimeter model
                                                             simulated in this study. The scan length   was 1.053.
                                                             for the abdominal and pelvic examina-          A two-variable model including pa-
                                                             tion was selected anatomically to span     tient perimeter (in centimeters) and fe-
                                                             from the lower thorax to inferior of the   tal depth (DEf) (in centimeters) pro-
                                                             pubic symphysis. This corresponded to      vides a better correlation with fetal dose
                                                             a 50-cm scan length for the Felmlee        (milligrays per 100 mAs), with an R2
                                                             technique, a 50-cm scan length for the     value of 0.799 (r      0.894; P .002).
                                                             ImPACT technique, and a 48-cm scan         The two-variable model allows fetal
                                                             length for the CT-Expo technique.          dose to be estimated by:

                                                                                                        DOf      0.119(P)     0.029(DEf)
                                                               Results
  Figure 4: Graph of radiation dose to fetus ver-                                                                                    24.56. (2)
  sus gestational age. Gestational age does not sig-
                                                             Dose Calculation                               The standard error of the estimate
  nificantly correlate with normalized fetal dose
  (R 2 0.017; P .543).                                       Table 1 shows the gestational age, pe-     for the two-variable model was 0.855.
                                                             rimeter of the mother, fetal depth, and
                                                             the estimated fetal dose for each of the
                                                             24 patient models. The average fetal
                                                                                                         Discussion
                                                             dose was 10.8 mGy/100 mAs            1.8   Radiation dose to the fetus from mul-
  Figure 5
                                                             (standard deviation). The minimum,         tidetector CT imaging in pregnant pa-
                                                             maximum, and median fetal doses were       tients was estimated for a range of
                                                             7.3, 14.3, and 11.2 mGy/100 mAs, re-       actual pregnant patient anatomies,
                                                             spectively.                                yielding a mean value of 10.8 mGy/100
                                                                                                        mAs with a range of 7.3–14.3 mGy/
                                                             Comparison with Existing Methods           100 mAs.
                                                             The results of the existing methods are        Patient size (perimeter) had a signif-
                                                             shown in Table 2; the Felmlee et al        icant correlation with fetal dose (P
                                                             method produced estimates of 11.3          .001). This size metric was chosen be-
                                                             mGy/100 mAs (the 20% range being           cause it can be easily measured in the
                                                             equal to 9.0 –13.6), while the ImPACT      clinic by using a tape measure around
                                                             and CT-Expo methods yielded estimates      the estimated center of the fetus. The
                                                             of 12.0 and 11.7 mGy/100 mAs, respec-      fetal depth also correlated well with fe-
  Figure 5: Graph of radiation dose to fetus ver-            tively.                                    tal dose, resulting in a two-variable
  sus perimeter of mother. Perimeter of mother cor-                                                     model for the estimation of fetal dose.
  relates linearly with normalized fetal dose (R2            Regression Analysis                        Fetal depth may be measured on an ul-
  0.681; P .001; standard error of the estimate
                                                             Fetal dose as a function of gestational    trasonographic image of the patient or,
  1.053).
                                                             age alone is shown in Figure 4. Linear     if available, directly from a CT image.

Radiology: Volume 249: Number 1—October 2008 ▪ radiology.rsnajnls.org                                                                          225
MEDICAL PHYSICS: Monte Carlo Simulation in Pregnant Patient Models                                                                            Angel et al




There was no correlation between ges-            should be scaled proportional to the in-        only one multidetector CT scanner model
tational age and fetal dose. There was           verse of pitch (22). For example, to es-        was used in this study. We expect that
insufficient information available on the         timate the dose for an examination with         there may be substantial differences in
height or weight of patients to investi-         a pitch of 1.375 (with all other technical      radiation dose per unit of tube current–
gate correlations of these patient size          parameters constant), the results from          time product with the various makes
metrics with fetal dose.                         this study can be divided by 1.375.             and models of CT scanners due to dif-
     When we compared existing dose                   To illustrate a dose calculation using     ferences in x-ray beam filtration
estimation methods with the results of           all of the parameters that were found to        schemes. The differences between
this study, we found that the Felmlee            be important in this study, consider an         these schemes may not be easily taken
et al technique for the estimation of ra-        abdominal and pelvic helical CT exami-          into account; therefore, we are cur-
diation dose produced a good estimate            nation in a pregnant patient with a pe-         rently extending our Monte Carlo
of fetal dose for an average-size patient.       rimeter of 101 cm and a fetal depth of 7        source models to those from other man-
The ImPACT and CT-Expo fetal dose                cm with the following acquisition proto-        ufacturers. This study also simulated
estimates were slightly conservative be-         col: GE LightSpeed 16, 120 kVp, pitch           only constant tube current scans and
cause fetal dose was slightly overesti-          of 1.375, 300 mAs (220 effective mAs),          therefore did not assess any dose sav-
mated relative to that estimated with            and 16       1.25-mm total nominal colli-       ings that may be offered by using tube
the methods and patient cohort used in           mation. By substituting the mother’s pe-        current modulation methods. This is
this study. The estimates with the               rimeter and the fetal location in Equa-         also the subject of ongoing evaluations.
ImPACT and CT-Expo methods were                  tion (2) and multiplying the result by a            A method for the estimation of fetal
still within the range of the doses esti-        factor of 3 (300 mAs divided by 100             dose from models of actual patient anat-
mated for the patients modeled in this           mAs) and dividing by the 1.375 pitch,           omy that represented a range of gesta-
study.                                           one obtains a fetal dose of approxi-            tional age and patient size was devel-
     Of note, however, is that the results       mately 27 mGy. (Note that the gesta-            oped. The mean fetal dose estimated
demonstrate a range of fetal doses that          tional age is not a factor in this esti-        across the patient cohort was 10.8
varied by nearly a factor of two (7.3–           mate.)                                          mGy/100 mAs, which is consistent with
14.3 mGy/100 mAs) among the 24 pa-                    These results indicate that, while         estimates from existing methods. How-
tients modeled. This difference in fetal         there may be wide variation among indi-         ever, the results from this study show a
dose exceeded the range suggested by             vidual patients, fetal doses from a single      larger spread of values than previously
Felmlee et al.                                   abdominal and pelvic CT examination             recognized, thus demonstrating the im-
     The results from this study are re-         are still below the consensus levels or         portance of accounting for patient size
ported as normalized dose per 100 mAs            negligible risk (50 –150 mGy) and well          in fetal dose estimates. Fetal dose cor-
to allow for dose estimations of exami-          below the “actionable” level of 150 mGy         related with maternal perimeter, and
nations with any tube current–time               (23). This should not be interpreted as         this correlation improved when both
product. For example, for an examina-            CT imaging in pregnant patients should          maternal size and fetal depth were ac-
tion with 300 mAs, the normalized re-            be performed without adequate clinical          counted for.
sults from this study should be tripled.         justification. It must also be considered        Acknowledgment: This work was made possi-
The results can also be used to estimate         that the dose estimates from this study         ble by assistance from Mohammad Ghatali, MD.
fetal dose for CT protocols utilizing size-      represent a single CT acquisition. Many
dependent tube current–time product              clinical examination protocols require
                                                                                                 References
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                                                                                                   1. Felmlee JP, Gray JE, Leetzow ML, Price JC.
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226                                                                                  radiology.rsnajnls.org ▪ Radiology: Volume 249: Number 1—October 2008
MEDICAL PHYSICS: Monte Carlo Simulation in Pregnant Patient Models                                                                                     Angel et al




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