Managing patient dose in MSCT by ert634

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									Managing patient dose in MSCT




               John Damilakis, PhD
      Assist. Professor of Medical Physics
    University of Crete, Iraklion, Crete, Greece
                damilaki@med.uoc.gr
Aim


• Are the doses from a MSCT examination high?


• What are the radiation risks?


• How can we manage patient dose?
               Patient sample (n = 250)

30 %




20 %




10 %




       18-28   28-38   38-48    48-58      58-68   68-78   >78
                       Age group (years)
                               Scans per patient

            40
Patient fraction (%)




          10




                       1   2      3         4      5   6   7
                                 Number of scans
                      Scans per anatomic region
              2



             1.5
# of scans




              1



             0.5




                   HEAD   THORAX   LUMBAR   ABDOMEN    LOWER
                                   SPINE              ABDOMEN
                                Mean number of scans per patient

                          2.5
                                                             Total
No of scans per patient




                                                             Contrast



                          1.5




                          0.5



                                     Males        Females
                           Anatomic regions scanned
              70

               60

               50
Patient fraction (%)




                10


                       1          2          3               4
                                      Number of anatomic regions scanned
           Effective dose per scan and anatomic
                           region
      12



      9
mSv




      6



      3




            HEAD   THORAX   LUMBAR   ABDOMEN    UPPER
                            SPINE              ABDOMEN
           Mean effective dose per patient

      15
                                        per scan
                                        per patient
mSv




      9




       3




               Males         Females
 Are CT doses comparable to
    background radiation ?

Average background dose :

       3 mSv / year (chronic exposure)


Average CT dose :

      14 mSv / examination (acute exposure)
                  Normalized effective dose vs. age
                            150                                                                      450
                                                                                                                                           Chest




                                                                                   Normalized dose
         Normalized dose
                                                            Head - neck
                            140                                                                      400

                            130                                                                      350

                            120                                                                      300

                            110                                                                      250

                            100                                                                      200

                            90                                                                       150
                                  0   2   4   6   8   10    12     14   16   18                            0   2   4   6   8   10   12     14   16   18
                                                                 Patient age                                                             Patient age


                            750                                                                      950




                                                                                  Normalized dose
          Normalized dose




                            680                            Abdomen - Pelvis                          880                                  Trunk
                            610                                                                      810

                            540                                                                      740

                            470                                                                      670

                            400                                                                      600

                            330                                                                      530
                                  0   2   4   6   8   10     12    14   16   18                            0   2   4   6   8   10   12     14   16   18

                                                                 Patient age                                                             Patient age



Medical Physics (in press)
                                              Normalized effective dose vs age
                                        750                              5-year-old         Abdominal scan
Normalized effective dose (µSv / mGy)




                                        680                120 kV, 35 mAs, BC = 24 mm, pitch = 1, rsw = 5.0 mm

                                        610
                                                  580 µSv/mGy                    ED = ND x CTDI = 580 x 6.4 = 3.7 mSv

                                        540


                                        470


                                        400


                                                                    5y
                                        330

                                              0       2         4    6       8       10    12     14    16      18
                                                                                                         Patient age (y)
               Effective dose per scan from
                pediatric MSCT (abdomen)
      20


      16
mSv




      12


      8


       4



           2     4   6   8   10   12        14      16       18
                                       Patient Age (years)
             Effective dose per scan from
           pediatric CT (head & neck / trunk)
      20
                                                 trunk
      16
mSv




      12
                                                 head
      8


       4



            2   4   6   8   10   12   14    16      18
                                       Patient Age (yr)
    Thyroid doses from head & neck CT




                                                                   Normalized thyroid dose
     Normalized thyroid dose   0.4                                                           0.4
                                              Brain                                                         Paranasal sinuses
                               0.3                                                           0.3


                               0.2                                                           0.2


                               0.1                                                           0.1


                               0.0                                                           0.0


                                      0   5     10           15                                     0   5       10          15




                                                                   Normalized thyroid dose
                               0.08                                                          1.50
     Normalized thyroid dose




                                              Inner ear                                                      Neck
                               0.06                                                          1.45


                               0.04                                                          1.40


                               0.02                                                          1.35


                               0.00                                                          1.30


                                      0   5     10           15                                     0   5       10          15

                                                     Patient Age                                                     Patient Age

European Radiology, 2006 Sep 21; [Epub ahead of print]
                  Thyroid dose per scan from
                pediatric head and neck MSCT
           50
                                                ral
                                         ,   spi
                                       ck
           40
                                     Ne
Dose mGy




           30


           20


           10
                                     Brain, spiral
                                                         Brain, seq
                2   4   6   8   10      12        14      16     18
                                                  Patient Age (yr)
Aim


• Are the doses from a MSCT examination high?


• What are the radiation risks?


• How can we manage patient dose?
Biological effects of radiation
Stochastic effects : Carcinogenesis

   As dose increases, the probability of the effect

occurring increases.

  Stochastic effects are assumed to have no threshold.


Deterministic effects : Opacities

   They are characterized by a threshold dose, below

which the effect does not occur.
                                      Risk coefficients for fatal cancer
Risk per Unit Dose (% per Sv)

                                15                                           5-year-old
                                     13.5 % per Sv
                                                               Dose from an abdominal scan: 3.7 mSv
                                12

                                                               Risk = 13.5 x 3.7 x 10-3 % = 0.05 %
                                9


                                6


                                3

                                         5y
                                           10        20   30    40      50       60     70       80
                                                                              Age at acute exposure (yr)
                                     Source : BEIR V
                            Risk of radiation-induced fatal
                            cancer from MSCT (abdomen)

                     0.30

                     0.25
Estimated Risk (%)




                     0.20

                     0.15


                     0.10


                     0.05



                            10   20   30   40   50   60      70       80
                                                     Age at acute exposure (yr)
 The probability of radiogenic risk for cancer is not negligible


 The number of CT examinations is increasing worldwide

                                            ?
         33
Number
of CT
examinations
(millions)



        3.6

                 1980              1998     2007     Year

  Variety of examinations is increasing
Aim


• Are the doses from a MSCT examination high?


• What are the risks?


• How can we manage patient dose?
How can we manage patient dose?

    Justification

    Proper selection of scanning parameters


    Use of technologic innovations

    Protection of radiosensitive organs
           Justification




BENEFITS
                       RISKS
           Justification




                       RISKS

BENEFITS
 How do we know if CT is the most
     appropriate examination ?
An ACR committee has developed criteria for determining


appropriate imaging    examinations   for    diagnosis   and


treatment of specified medical conditions.

These criteria are intended to guide radiologists, radiation


oncologists, and referring physicians in making decisions


regarding radiologic imaging and treatment.
www.acr.org
How can we manage patient dose?

    Justification

    Proper selection of scanning parameters


    Use of technologic innovations

    Protection of radiosensitive organs
  Parameters that affect CT dose

Beam shaping filter     Collimation



                           kV, mAs



     Filtration
                        Detection system efficiency




       Scanning length, Reconstruction slice width, Pitch,
                 Scanner geometry, Algorithms
  A comprehensive evaluation of the dosimetric characteristics

of a CT scanner is needed.

  Some years ago, we used to follow rules for an optimized

CT dose reduction in patients.

  A dose-effective use of any scanner can only be established

with on-site measurements of its dosimetric characteristics.
Rotation Time
Do we optimize a MSCT examination by

 selecting short or long rotation time ?


A: The shortest rotation time should be selected to

   minimize motion artifacts.



   Rotation time decreased          mA increased
                   mA & automatic change from small to large focal spot


                       10.5
nCTDIw (mGy/100 mAs)




                       9.5


                        8.5


                       7.5


                                                        210
                              0   100             200         300   400          500
                                                                    Tube current (mA)

           European Radiology, 16:2575-85, 2006
Do we optimize a MSCT examination by

 selecting short or long rotation time ?


 When a high tube load is required, an increased

 rotation time should be preferred in order to

 avoid the automatic selection of the large focal

 spot.
Pitch
Do we optimize a MSCT examination by

     selecting high or low pitch value ?

A: Higher pitch is associated with a reduced dose to the

  patient because of a shorter exposure time.



                             mAs
                 mAseff =
                             pitch
                            High or low pitch value ?

               75
CTDIv (mGy)




               70


               65
                                                                         mAs
                                                           mAseff =
                                                                         pitch
               60




                    0.2         0.4            0.6   0.8   1.0            1.2
                                                                 Pitch

              European Radiology, 16:2575-85, 2006
              z – overscanning (overranging)


z-overscanning




                                       In spiral CT, the tissue volume
z-overscanning
                                       of patient   irradiated differs

                                       from the volume imaged.


  Medical Physics 32:1621-1629, 2005
                               z – overscanning (mm)
                                                           Pitch = 1.5
                         BC = 16 x 1.5
               100

                                                           Pitch = 1.0
Overscanning




                75
                                                           Pitch = 0.5
                50


                25




                     0         2               4   6   8         10
                                                           RSW (mm)
         Medical Physics, 32:1621-1629, 2005
                               z – overscanning (mm)

                         BC = 16 x 0.75
               100
Overscanning




                75


                50                                         Pitch = 1.5

                                                           Pitch = 1.0
                25                                         Pitch = 0.5


                     0         2               4   6   8         10
                                                           RSW (mm)
         Medical Physics, 32:1621-1629, 2005
                               z - overscanning




van der Molen, A. J. et al. Radiology 2006;0:2421051350
                              z - overscanning




van der Molen, A. J. et al. Radiology 2006;0:2421051350
                   z - overscanning
When radiosensitive organs are marginally included in


the examination field, proper selection of BC, RSW and


pitch is needed to restrict z - overscanning.


The relative contribution of the extra exposure due to


z–overscanning may be considerable especially when the


planned image volume is limited.
               z - overscanning

Thick beam collimation (24 mm) increases patient

dose due to z-overscanning.


For a given beam collimation, an increase in

the RSW increases patient dose. High pitch

values increase dose due to z-overscanning.
Do we optimize a MSCT examination by

   selecting high or low pitch value ?


 High pitch values increase dose due to:


 • automatic selection of focal spot size

 • z-overscanning
Beam Collimation
 Do we optimize a MSCT examination by

selecting wide or narrow beam collimation ?


 A: The wider the beam the smaller the percentage of

    wasted radiation due to overbeaming. Therefore, we

    optimize a MSCT examination by selecting     wide

    collimation.
         Overbeaming




          Overbeaming =
          wasted radiation




z axis                       z axis
                   Overbeaming




The wider the beam the smaller the percentage of wasted radiation.

However, wide collimation limits the width of the thinnest sections

                    that can be reconstructed.
              Beam Collimation

Narrow collimations should be avoided as they

are less dose effective, unless their use is dictated

by the clinical need for thin reconstructed slices.


Scans at thick BC’s are to be preferred on the

basis of protecting the patient from radiation.
                    Beam Collimation
Thick beam collimation (24 mm) increases patient dose due to overscanning




                D
                O
                S        OVERSCANNING              OVERBEAMING
                E
16 x 1.5




                D
                O
                S
                         OVERBEAMING               OVERSCANNING
                E
16 x 0.75
Recommended beam configuration

for Siemens Sensation 16:


• Head examinations: 16 x 1.5 mm

                                    16 x 1.5




• Body examinations: 16 x 0.75 mm

                                    16 x 0.75

Medical Physics, in press
Do we optimize a CT examination by selecting

      wide or narrow beam collimation ?


 Overbeaming and z - overscanning are two

 competing effects regarding patient radiation

 burden.

 • Head examinations: 16 x 1.5 mm

 • Body examinations: 16 x 0.75 mm
What is the proper selection of scanning

 parameters to avoid motion artifacts ?

Motion artifacts can be avoided by selecting:


• short rotation time

• high pitch value

• wide beam collimation


However, the dose to the patient increases !
      ‘Standard’ rules to reduce dose

• Scan minimal length


Efforts must be made to

restrict the scan length to

that clinically essential.


• Reduce mAs without compromising image quality

• Reduce number of multiple scans
How can we manage patient dose?

    Justification

    Proper selection of scanning parameters


    Use of technologic innovations

    Protection of radiosensitive organs
             mA modulation: Performance
                    evaluation


                              a
                                   b



                                       a
                            OR =
                                       b



Submitted for publication
                                  mA modulation
                             Oval ratio               % Dose Reduction

                                                                          40
             2.5                                                          35




                                                                                % Dose Reduction
                                                                          30
              2                                                           25
Oval ratio




                                                                          20
             1.5                                                          15
                                                                         10
                                                                          5
              1                                                           0
                                                                         -5
             0.5                                                         - 10
                   0   150       300      450   600       750     900


                                Anatomic position (mm)
                                      10-year-old




                                                    % Dose Reduction
                                      5-year-old
Oval ratio




                                      1-year-old




                                       neonate



             Anatomic position (mm)
              mA modulation




Helical Mode 16x1.5    Sequential Mode 12x1.5
                 mA modulation

The dose reduction achieved with tube mA modulation

is not substantial for neonates and young children.



mA-modulation should be considered as a complementary

means to reduce dose and should not replace other

dose reduction methods, especially in young children.
       Software tools for noise simulation
    What is the effect of a possible reduction of mA on image quality?




By courtesy of IMP, Erlangen
       Software tools for noise simulation




By courtesy of IMP, Erlangen
How can we manage patient dose?

     Justification

     Proper selection of scanning parameters


     Use of technologic innovations

     Protection of radiosensitive organs
        Bismuth

        shielding




Eur Radiol 16:2334-2340, 2006
    Eye lens dose from pediatric CT

Dose to the eye lens per scan: 0.07 Gy in CT scanning

of sinuses and 0.13 Gy in CT of orbital trauma.

NCRP Publication 87, 2000


The threshold for ophthalmologically detectable opacities

has been reported to be 0.5–1.3 Gy. These values refer to

adult individuals and therefore may be lower in infants.

ICRP 60, 1990 & NRPB Vol 7, Nr 3, 1996
                      Εye bismuth shielding




               Dose reduction factors (%) of eye lens dose
   CT examination                 Infants    1 year   5 years   10 years   15 years

   Scanning of orbits                 33.1    35.7     37.4       37.1      35.2

   Scanning of the head               31.4    32.8     33.1       34.7      33.0

   Angled scan. excl. orbits          <1       <1       <1        <1         <1


Medical Physics 32, 1024-1030, 2005
    Protection of radiosensitive organs
                  Eye Shielding

A considerable reduction in eye lens dose may be

achieved by using orbital bismuth shielding during

pediatric head CT scans. However, this shielding

should not be used in children when the eyes are

excluded from the primarily exposed region.
         z – overscanning and eye lens dose
        In helical mode, the proximity of eye lenses to the

        boundaries of planned image volume in combination

        with the additional exposure due to z-overscanning,

        can result in a significant increase in the lens dose.




Medical Physics, 33:2472-2478, 2006
                                       z – overscanning and paediatric patients
  Normalized eye lens dose (mGy/mGy)


                                       2.0

                                                                                       axial scanning
                                                                                       helical, pitch = 1
                                       1.5
                                                                                       helical, pitch = 0.5

                                       1.0



                                       0.5




                                             -1    0               1               2                3
                                                  Distance from first scan line (cm)
Medical Physics, 33:2472-2478, 2006
     What is the distance of eye lens from the
     first slice of the volume to be imaged ?
              Category                Number of axial   Number of helical
                                       examinations       examinations
      I (distance = -1 to 0 cm)             12                 6


      II ( distance = 0 to 1cm)             21                 12


     III (distance = 1 to 2cm)              9                  6


     IV (distance = 2 to 3 cm)              3                  3


                 Total                      45                 27


Medical Physics, 33:2472-2478, 2006
     Protection of radiosensitive organs

   z – overscanning and paediatric patients


It is more dose efficient to use axial mode acquisition

rather than helical scan for pediatric head studies, if

there are no overriding clinical considerations.
Messages to take home
 Radiation dose from MSCT examinations is not

 comparable to background radiation.



 The probability of radiogenic risk for cancer

 from MSCT examinations is not negligible.
Messages to take home
 A dose-effective use of a MSCT scanner can

 only be established with on-site measurements

 of its dosimetric characteristics.


 The relative contribution of the extra exposure

 due to z-overscanning may be considerable.
Messages to take home
 ‘Standard rules’ can be used to reduce dose

      Justify CT examinations

      Scan minimal length

      Reduce mAs without compromizing quality

      Reduce number of multiple scans

      Avoid radiosensitive organs
                     HANDOUTS:

URL ADDRESS: http://medicalphysics.med.uoc.gr/handouts/

								
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