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					                                    Hands-On MultiSlice CT Workshop
                                   LAB A: Radiation Dose Measurements
                                       Mike McNitt-Gray, PhD, DABR
                                          Department of Radiology
                                  David Geffen School of Medicine, UCLA


Radiation Dose
The radiation dose from a CT exam is second in diagnostic radiology only to interventional angiographic
procedures and a cause of significant attention for many medical, professional and regulatory bodies – including
ACR accreditation. Still, the diagnostic benefits of the exam are seen to outweigh the potential risks for most
procedures. The purpose of this portion is to measure exposure and calculate radiation dose in a standard
fashion using phantoms.


Objectives
      To become familiar with radiation dose measurement equipment and techniques as well as several
       associated calculations such as CTDI100, CTDIw, DLP and Effective Dose.
      To address radiation dosimetry issues - and their limitations –in the context of helical and multislice CT
       scanning
      To obtain radiation dose data for typical head and body protocols
      Time permitting, to see the effects of different parameters on the CTDI, DLP and Effective dose
       calculations.


Overview of the Lab
      Get familiar with scanner and measurement equipment
      Clarify definitions of terms necessary for dose calculations: collimation, numbers of channels,
       increment/table movement and pitch
      Select Protocols for measurement (head, body, ped body), focusing on differences between clinical
       protocols and those necessary for making radiation dose measurements.
      Identify which technical factors will make a difference and pay close attention to those
      Set up equipment and perform AXIAL SCANS with NO TABLE INCREMENT
      Record measurements
      Use spreadsheet to apply calculation formulae (CTDI100, CTDIw, DLP, Effective Dose)
      Check against what you expect for each condition
Materials
   Calibrated CTDI (pencil) ionization chamber (typically 10 cm in length)
   Calibrated electrometer
   Acrylic (PMMA) cylindrical phantoms having cylindrical holes (four at 1 cm from the edge and one at the center):
        o    Head CTDI phantom: 16-cm diameter, 15-cm length
        o    Body CTDI phantom: 32-cm diameter, 15-cm length
   Dose calculation Excel spreadsheet (we will use ACR dose spreadsheet for exposure meter).


Definitions
From the ACR CT Accreditation Program Phantom Testing Instructions
    z-axis collimation (T) = the width of the tomographic section along the z-axis imaged by one data channel. In multi-
    detector row (multi-slice) CT scanners, several detector elements may be grouped together to form one data channel.
    Number of data channels (N) = the number of tomographic sections imaged in a single axial scan.
    Increment (I) = the table increment per axial scan or the table increment per rotation of the x-ray tube in a helical
    scan.


Measurements
1. Head Radiation Dose using Head Phantom Scanned with Head protocol
    a) Purpose: to perform CTDI100 measurements on the scanner for a head protocol.
    b) Methods- Scanning and Measurements: Set up the Head (16 cm diameter) acrylic CTDI phantom in a
       head holder and position at isocenter in the scanner. Align it so that it is centered longitudinally as well.
       Set up the pencil ion chamber so that it can be inserted into one of the holes in the phantom and connect
       it to the electrometer so that measurements can be made easily and repeatedly.
    c) Using an axial technique for head scanning (or use the manufacturer’s quoted radiation dose exposure
       conditions) with no table incrementation. Even if clinical protocol is helical, perform an axial scan.
    d) Perform the scan with the pencil chamber placed in the center and again at the 12:00 position within the
       phantom. Record the technical factors under which the exposures were made as well as the resulting
       exposures.
    e) Analysis: For each exposure condition, use the ACR Dose calculator spreadsheet to calculate the
       CTDI100, CTDIw, DLP, and Effective Dose. Report the expected radiation dose from a head CT scan.
       Note: the ACR Dose calculator uses the following values for its spreadsheet:
        i)   Assumed total scan length of 17.5 cm for calculating the dose-length product (DLP) in mGy-cm
                DLP (mGy-cm) = CTDIvol (mGy) • total scan length (cm)
        ii) A value of k=.0023 (mSv • mGy-1 • cm-1) for calculating the estimated Effective Dose (E)*
                E (mSv) = k (mSv • mGy-1 • cm-1) • DLP (mGy-cm)
                    (from European Guidelines on Quality Criteria for Computed Tomography, EUR 16262 EN, May
                    1999)
2. Pediatric Body Radiation Dose using Head Phantom Scanned with Pediatric Body protocol
   a) Purpose: to perform CTDI100 measurements on the scanner for a pediatric body protocol.
   b) Methods- Scanning and Measurements: Still use the Head (16 cm diameter) acrylic CTDI phantom, but
      place on the patient table and position at isocenter in the scanner. Keep it aligned it so that it is centered
      longitudinally as well. Set up the pencil ion chamber so that it can be inserted into one of the holes in
      the phantom and connect it to the electrometer so that measurements can be made easily and repeatedly.
   c) Using an axial technique for ped body scanning with no table incrementation. Even if clinical
      protocol is helical, perform an axial scan.
   d) Perform the scan with the pencil chamber placed in the center and again at the 12:00 position within the
      phantom. Record the technical factors under which the exposures were made as well as the resulting
      exposures.
   e) Analysis: For each exposure condition, use the ACR Dose calculator spreadsheet to calculate the
      CTDI100, CTDIw, DLP, and Effective Dose. Report the expected radiation dose from a pediatric
      abdomen CT scan. Note: the ACR Dose calculator uses the following values for its spreadsheet:
      i)        Assumed total scan length of 15 cm for calculating the dose-length product (DLP) in mGy-cm
      ii)       A value of k=.0081 * 2.6 (mSv • mGy-1 • cm-1) for calculating the estimated Effective Dose (E)* in the
            case of a 5 year old pediatric patient’s abdomen region.
                     (Shrimpton and Wall, Radiation Protection and Dosimetry 2000: 90:249-252.)
3. Adult Body Radiation Dose using Body Phantom Scanned with Body protocol
   a) Purpose: to perform radiation dose measurements on the scanner using for an adult body protocol.
   b) Methods- Scanning and Measurements: Set up the Body (32 cm diameter) acrylic CTDI phantom at
      isocenter in the scanner. Align it so that it is centered longitudinally as well. Set up the pencil ion
      chamber so that it can be inserted into one of the holes in the phantom and connect it to the electrometer
      so that measurements can be made easily and repeatedly.
   c) Using an axial technique for body scanning (or use the manufacturer’s quoted radiation dose exposure
      conditions) with no table incrementation. Even if clinical protocol is helical, perform an axial scan.
   d) Perform the scan with the pencil chamber placed in the center and again at the 12:00 position within the
      phantom. Record the technical factors under which the exposures were made as well as the resulting
      exposures.
   e) Analysis: For each exposure condition, use the ACR Dose calculator spreadsheet to calculate the
      CTDI100, CTDIw, DLP, and Effective Dose. Report the expected radiation dose from a body CT scan.
      i)        Assumed total scan length of 25 cm for calculating the dose-length product (DLP) in mGy-cm
      ii)       A value of k=.015 (mSv • mGy-1 • cm-1) for calculating the estimated Effective Dose (E)* for an abdomen
            scan.
            (from European Guidelines on Quality Criteria for Computed Tomography, EUR 16262 EN, May 1999)
      *It is important to note that alternative methods and conversion coefficients exist to calculate Effective Dose. This is an estimate only, and
      can differ from other estimates by as much as a factor of 2. This estimate is NOT the dose for any given individual, but rather, for a
      standardized anthropomorphic phantom, representative of the “whole-body-equivalent” radiation detriment (risk) associated with the
      “partial-body” CT exam. These values can be used to optimize protocols, and as a broad indication of the relative risk of the CT exam
      compared to background radiation or exams from other modalities. [From ACR Phantom Testing Instructions]
Body (left) and Head (right) dosimetry phantoms   Pencil ionization chamber




Photograph depicting setup of electrometer, pencil chamber and body (32 cm diam) phantom. Pencil chamber
can be inserted into any of the center, 12:00, 3:00, 6:00 or 9:00 positions of the phantom. For the ACR CT
accreditation program, measurements are made at center and 12:00 positions only.
Radiation Dose Formulae:
          CT is unique in its geometry and usage. Therefore, there have been many radiation dose descriptors

developed just for CT. In addition to the Multiple Scan Average Dose descriptor (MSAD- [1,2,3]), one of the

original parameters for measuring radiation dose from CT was the Computed Tomography Dose Index (CTDI)

[1,2, 4]. This was defined as the radiation dose, normalized to beam width, measured from 14 contiguous

slices:

                           CTDI = (1/nT) 7T-7T Dsingle(z) dz                       (eq. 1)

          Where:

                   n is the number of slices per scan,

                   T is the width of the interval equal to the selected slice thickness, and

                   Dsingle(z) is the dose at point z on any line parallel to the z (rotational) axis for a single axial scan.



This index was suggested by the FDA and incorporated into the Code of Federal regulations [5].

          However, to be measured according to the definition, only 14 slices could be measured and one had to

measure the radiation dose profile – typically done with TLDs or film, neither of which were very convenient.

Measurements of exposure could be done with a pencil ionization chamber, but its fixed length of 100 mm

meant that only 14 slices of 7 mm thickness could be measured using that chamber alone. To measure CTDI

for thinner nominal slices, sometimes lead sleeves were used to cover the part of the chamber that exceeded 14

slice widths.

          To overcome the limitations of CTDI with 14 slices, another radiation dose index – CTDI100 was

developed. This index relaxed the constraint on 14 slices and allowed the calculation of the index for 100 mm

along the length of an entire pencil ionization chamber [3], regardless of the nominal slice width being used.

This index is defined therefore as [6]:
       CTDI100 = (1/NT) 5cm-5cm Dsingle(z) dz                              (eq. 2)

       Where,

       N is the number of acquired slices per scan – also referred to as the number of data channels used during

       acquisition, and

       T is the nominal width of each acquired slice (which is not necessarily the same as the nominal width of

       the reconstructed slice width).

       (Note: the product of N *T is meant to reflect the total nominal width of the x-ray beam collimation

during acquisition. Therefore, for a multidetector scanner that is using 4 channels (rows) of 1.25 mm each for

scan acquisition, then, regardless of the reconstructed slice width, NT = 4*1.25 mm = 5 mm; similarly, if the

same scanner is using 4 channels of 5 mm width for scan acquisition, then, regardless of reconstructed slice

width, NT = 20mm).



       And because the ionization chamber measures an integrated exposure along its 100 mm length, this is

equivalent to:



       CTDI100 = (f*C*E*L)/(NT)                             (eq. 3)



       Where:

       f = conversion factor from exposure to dose in air, use 0.87 rad/R

       C = calibration factor for electrometer

       E = measured value of exposure in R acquired from a single 360 degree rotation using a beam profile of

       NT (as defined above).
L = active length of pencil ion chamber

        and N and T are defined for equation (2).



        Thus, the exposure measurement, performed with one axial scan in one of the PMMA phantoms for

which CTDI is defined, results in a calculated dose index, CTDI100. This index can be measured and calculated

for the center location as well as at least one of the peripheral positions (1 cm below the surface) within the

phantom to describe the within scan plane variations as well.

        CTDIw was created to represent a dose index that provides a weighted average of the center and

peripheral contributions to dose within the scan plane [6]. This index is used to overcome the limitations of

CTDI100 and provides a single index that takes into account variation due to position within the scan plane. The

definition is:

        CTDIw = (1/3) (CTDI100)center + (2/3) (CTDI100)periphery             (eq. 4)

        One final CTDI descriptor takes into account the parameters that are related to a specific imaging

protocol, the helical pitch or axial scan spacing, is defined as CTDIvol:

        CTDIvol = CTDIw * NT/I                                               (eq. 5)



        Where: N and T are as defined above and represent the total collimated width of the x-ray beam and

        I = table speed for rotation for a helical scan OR

           Spacing between acquisitions for axial scans.

        For helical scans, NT/I = 1/pitch, so that

        CTDIvol = CTDIw /pitch                                               (eq. 6)
        Where pitch is defined as: Table distance traveled in one 360 degree rotation/ Total collimated width of

x-ray beam



        Another dose descriptor that is related to CTDI and is commonly reported on CT scanners and in the

literature is the Dose Length Product – DLP [6]. This value is simply the CTDIvol multiplied by the length of

the scan and is given in units of mGy*cm.



        DLP = CTDIvol * scan length in cm; units are mGy*cm.                    (eq. 7)

This descriptor is used in one approach to obtain an estimate of effective dose that will be described later.

        These CTDI descriptors are obviously meant to serve as an index of radiation dose due to CT scanning

and are not meant to serve as an accurate estimate of the radiation dose incurred by an individual patient. While

the phantom measurements are meant to be reflective of an attenuation environment somewhat similar to a

patient, the homogeneous PMMA phantom does not simulate the different tissue types and heterogeneities of a

real patient. In addition, the CTDI100 calculation only uses the f factor (from equation 3) to convert from

exposure to dose in air; other tissues have different f factors. The f factor is determined by the ratio of the mass

energy absorption coefficient of a tissue to that of air (f=.87* [((en)t/t)/ ((en)a/a)] in units of rad/R), where:

        (en)t /t is the mass energy absorption coefficient of the tissue (e.g. bone, lung , soft tissue), and

        (en)a/a is the mass energy absorption coefficient of air

The mass energy absorption coefficient depends not only on the tissue, but also on the energy of the photons,

especially in the energy range utilized by X-ray CT. Thus, the CTDI100 calculation presents a very simplified

condition for measuring radiation dose.
       Effective Dose
       Effective dose [7,8] (formerly referred to as the effective dose equivalent [9]) takes into account where

the radiation dose is being absorbed (e.g. which tissue has absorbed that radiation dose) and attempts to reflect

the equivalent whole body dose that results in a stochastic risk that is equivalent to the stochastic risk from the

actual absorbed dose to those tissues irradiated in a nonuniform, partial body irradiation such as a CT scan. It is

a weighted average of organ doses as described in equation 8, where the weighting factors are set for each

radiosensitive organ in ICRP 60 [7]. Effective dose is measured in Sievert - Sv or rem. The conversion

between Sv and rem is 100 rem = 1 Sv.


                                E = T(wT*wR*DT,R)                                    (eq 8)
       Where:
       E = the Effective dose
       wT= tissue weighting factor
       wR= radiation weighting coefficient (1 for x-ray)
       DT,R= average absorbed dose to tissue T from radiation type R (here only x-rays)
       T is the subscript for each radiosensitive tissue

       and R is the subscript for each type of radiation (here only x-rays are present).




       While methods to calculate the effective dose have been established (ICRP 26 [9] and ICRP 60[7])these

methods depend heavily on the ability to estimate the dose to radiosensitive organs from the CT procedure (DT,R

above). However, determining the radiation dose to these organs is problematic and direct measurement is not

possible.

       One method of note to estimate the effective dose involves conversion factors for a general anatomic

region as described by the European Guidelines for Quality Critera for Computed Tomography [6] which are

based on the work of Jessen et al [10]. In this approach, the CTDIvol and distance are used to estimate the DLP,

which is then multiplied by a region-specific conversion factor to estimate the effective dose. These conversion
factors range from .0023 mSv/mGy*cm for the head region, .017 for the chest region and .019 mSv/mGy*cm

for the pelvis. This approach obviously does not take into account any patient specific or even exam specific

factors, but provides an easily estimated value of effective dose. This is the approach used in the accompanying

spreadsheets employed by the ACR CT accreditation program.

       Therefore, with the correct specification of the imaging protocol and the measurement of exposure at

center and periphery, one can calculate CTDI100 at center and periphery, CTDIw, DLP for an assumed length of

exam and an estimate of the effective dose resulting from the specified imaging protocol – with some

understanding of the limitations of the measurements and calculations. These measurements and calculations

can be done for both head and body examinations.
FROM: ACR CT Accreditation program worksheets

Radiation Dosimetry
Section 10 – Radiation Dosimetry (Adult Head)                                     Adult Head (Cerebellum) technique

                      *** CTDI data must be acquired using a single axial (non-spiral) scan ***
                                                                                                                         Film
                                                                                                               2
Use: 16-cm diameter PMMA CTDI phantom                                   Measured                  Calculated            Page:
                                                                                                                         Box
kVp
mA
Exposure Time per rotation (s)
Z axis collimation (T) 1                                              _______ mm
# data channels used (N) 1
Axial (A): Table Increment (mm) 1 = (I)
or
Helical (H): Table Speed (mm/rotation) 1 = (I)
Active Chamber Length                                                 _______ mm
                              3
Chamber Correction Factor
Isocenter
    Measurement 1 (note units)
    Measurement 2 (note units)
    Measurement 3 (note units)                                                                                           3:10
    Average of above 3 measurements (note units)
    Head CTDI at isocenter in phantom (mGy)
12 o’clock position
    Measurement 1 (note units)
    Measurement 2 (note units)
    Measurement 3 (note units)
    Average of above 3 measurements (note units)
    Head CTDI at 12 o’clock position in phantom (mGy)
CTDIw (mGy)
      Clinical Exam Dose Estimates (using measured CTDIw and site’s Adult Head protocol from Table 1)
CTDIvol (mGy)                                                      = CTDIw • N • T / I
DLP (mGy-cm)                                                         = CTDIvol • 17.5
Effective Dose (mSv)                                                 = DLP • 0.0023
1
  See definitions on page 6 of the Phantom Testing Instructions.
2
  See instruction manual for specific definitions and calculation methods. Calculations may be made with the Dose Calculator
spreadsheet sent with the Full Application materials. The completed spreadsheet page may be submitted for this page.
3
  Physicist should record the model, serial number, and calibration date of the ionization chamber and electrometer in their own records.
FROM: ACR CT Accreditation program worksheets

Section 11 – Radiation Dosimetry (Pediatric Body)                                 Pediatric Abdomen (5 y.o.) technique

                      *** CTDI data must be acquired using a single axial (non-spiral) scan ***
                                                                                                                         Film
                                                                                                               2
Use: 16-cm diameter PMMA CTDI phantom                                   Measured                  Calculated            Page:
                                                                                                                         Box
kVp
mA
Exposure Time per rotation (s)
Z axis collimation (T) 1                                              _______ mm
                            1
# data channels used (N)
Axial (A): Table Increment (mm) 1 = (I)
or
Helical (H): Table Speed (mm/rotation) 1 = (I)
Active Chamber Length                                                 _______ mm
                                3
Chamber Correction Factor
Isocenter
     Measurement 1 (note units)
     Measurement 2 (note units)
     Measurement 3 (note units)                                                                                         3:11
     Average of above 3 measurements (note units)
     Ped Body CTDI at isocenter in phantom (mGy)
12 o’clock position
     Measurement 1 (note units)
     Measurement 2 (note units)
     Measurement 3 (note units)
     Average of above 3 measurements (note units)
     Ped Body CTDI at 12:00 position in phantom (mGy)
CTDIw (mGy)
    Clinical Exam Dose Estimates (using measured CTDIw and site’s Pediatric Abdomen protocol from Table 1)
CTDIvol (mGy)                                                      = CTDIw • N • T / I
DLP (mGy-cm)                                                          = CTDIvol • 15
Effective Dose (mSv)                                              = DLP • 0.0081 • 2.6
1
  See definitions on page 6 of the Phantom Testing Instructions.
2
  See instruction manual for specific definitions and calculation methods. Calculations may be made with the Dose Calculator
spreadsheet sent with the Full Application materials. The completed spreadsheet page may be submitted for this page.
3
  Physicist should record the model, serial number, and calibration date of the ionization chamber and electrometer in their own records.
FROM: ACR CT Accreditation program worksheets

Section 12 – Radiation Dosimetry (Adult Body)                                              Adult Abdomen technique

                      *** CTDI data must be acquired using a single axial (non-spiral) scan ***
                                                                                                                         Film
                                                                                                               2
Use: 32-cm diameter PMMA CTDI phantom                                   Measured                  Calculated            Page:
                                                                                                                         Box
kVp
mA
Exposure Time per rotation (s)
Z axis collimation (T) 1                                              _______ mm
                            1
# data channels used (N)
Axial (A): Table Increment (mm) 1 = (I)
or
Helical (H): Table Speed (mm/rotation) 1 = (I)
Active Chamber Length                                                 _______ mm
                                3
Chamber Correction Factor
Isocenter
    Measurement 1 (note units)
    Measurement 2 (note units)
    Measurement 3 (note units)                                                                                           3:12
    Average of above 3 measurements (note units)
    Body CTDI at isocenter in phantom (mGy)
12 o’clock position
    Measurement 1 (note units)
    Measurement 2 (note units)
    Measurement 3 (note units)
    Average of above 3 measurements (note units)
    Body CTDI at 12 o’clock position in phantom (mGy)
CTDIw (mGy)
     Clinical Exam Dose Estimates (using measured CTDIw and site’s Adult Abdomen protocol from Table 1)
CTDIvol (mGy)                                                      = CTDIw • N • T / I
DLP (mGy-cm)                                                          = CTDIvol • 25
Effective Dose (mSv)                                                  = DLP • 0.015
1
  See definitions on page 6 of the Phantom Testing Instructions.
2
  See instruction manual for specific definitions and calculation methods. Calculations may be made with the Dose Calculator
spreadsheet sent with the Full Application materials. The completed spreadsheet page may be submitted for this page.
3
  Physicist should record the model, serial number, and calibration date of the ionization chamber and electrometer in their own records.
References
   (1)  AAPM, Standardized methods for measuring diagnostic x-ray exposure. Report No. 31, 1990, AAPM, New
        York (available at www.aapm.org/pubs/reports).
   (2) AAPM, Specification and acceptance testing of computed tomography scanners. Report No. 39, 1993, AAPM,
        New York. (available at www.aapm.org/pubs/reports).
   (3) Jucius RA, Kambic GX. Radiation dosimetry in computed tomography. Appl. Opt. Instrum. Eng. Med.
        1977;127:286-295.
   (4) Shope TB, Gagne RM and Johnson GC. A method for describing the doses delivered by transmission x-ray
        computed tomography. Med Phys 1991;8 (4): 488-495.
   (5) Department of Health and Human Services, Food and Drug Administration. 21 CFR Part 1020: Diagnostic x-
        ray systems and their major components; amendments to performance standard; Final rule. Federal Register
        1984, 49, 171.
   (6) European Guidelines on Quality Criteria for Computed Tomography (EUR 16262 EN, May 1999). (available at
        www.drs.dk/guidelines/ct/quality/index.htm)
   (7) International Council on Radiation Protection, 1990 Recommendations of the International Commission on
        Radiological Protection, Publication 60. Annals of the ICRP 1991; 21. Pergammon Press, Oxford.
   (8) McCollough CM, Schueler BA. Calculation of effective dose. Med Phys 2000; 27:838-844.
   (9) International Council on Radiation Protection, Recommendations of the International Commission on
        Radiological Protection, 1977, Publication 26, Annals of the ICRP 1(3), reprinted with additions in 1987.
        Pergammon Press, Oxford. Superseded by ICRP Publication 60.
   (10) Jessen et al., “Dosimetry for optimisation of patient protection in computed tomography.” Applied Radiation
        and Isotopes 50:165-172 (1999).

   Other CT Dose Related References
   (11) Rothenberg LN, Pentlow KS. .CT dosimetry and radiation safety. In: Goldman and Fowlkes ed. Categorical
        Course in Diagnostic Radiology Physics: CT and US Cross-Sectional Imaging. RSNA:Oak Brook, IL, 2000;
        171-188.
   (12) McNitt-Gray MF, The AAPM/RSNA Physics Tutorial for Residents: Radiation Dose in CT,
        Radiographics, 2002 (in press).
   (13) Nagel, ed., et al. Radiation Exposure in Computed Tomography, 2nd edition, COCIR:Hamburg, Germany
        (2000) (available through cocir@zvei.org)
   (14) McCollough CH, Zink FE. Performance evaluation of a multi-slice CT system. Med Phys. 1999
        Nov;26(11):2223-30
   (15) Feigal DW. FDA Public Health Notification: Reducing Radiation Risk from Computed Tomography for
        Pediatric and Small Adult Patients. Center for Devices and Radiological Health            Food and Drug
        Administration. Available at: http://www.fda.gov/cdrh/safety/110201-ct.html
   (16) Shrimpton PC, Wall, .Reference doses for paediatric computed tomography. Radiation Protection and
        Dosimetry 2000;90:249-252.
   (17) Jones DG and Shrimpton PC. Survey of CT practice in the UK. Part 3: Normalised organ doses calculated using
        Monte Carlo techniques. NRPB R-250, 1992. National Radiological Protection Board, Chilton, Oxon, England.
   (18) Jones DG, Shrimpton PC. Normalised Organ Doses For X-Ray Computed Tomography Calculated using Monte
        Carlo Techniques.      NRPB SR-250, 1992. National Radiological Protection Board, Chilton, Oxon,
        England.(computer software report).
   (19) Shrimpton PC, Jones DG. Normalised Organ Doses For X-Ray Computed Tomography Calculated using Monte
        Carlo Techniques And A Mathematical Anthropomorphic Phantom. Radiation Protection Dosimetry 1993; 49:
        241-243.
   (20) Imaging Performance Assessment of CT (ImPACT) CT Patient Dosimetry Calculator, version 0.99m, Created
        07/01/2002, Available at http://www.impactscan.org/ctdosimetry.htm.
   (21) Kalender WA, Schmidt B, Zankl M, Schmidt M. A PC program for estimating organ dose and effective dose
        values in computed tomography, Eur. Radiol 1999;9:555-562.

				
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