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Radiation Dose Management (PDF)

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					The Exposure Index
and Its Standardization

Ulrich Neitzel
Philips Medical Systems – Clinical Science
Hamburg, Germany

ulrich.neitzel@philips.com
     Content

     • The Exposure Index (EI)
              – What is it?
              – What is it good for?
              – What are its limitations?
              – How can it be improved?




Philips Medical Systems, Ulrich Neitzel, 2006-02-15   2
     Conventional and Digital X-ray

        • Conventional X-ray:

               Fixed dose requirement
               due to fixed film speed



        • Digital X-ray:

               Wide dynamic range
               = variable speed
               = wide range of possible doses

Philips Medical Systems, Ulrich Neitzel, 2006-02-15   3
     Wide Dynamic Range: A Two-Edged Sword




    Potential for dose reduction                      How to control   Risk of overexposure
                                                        the dose?
Philips Medical Systems, Ulrich Neitzel, 2006-02-15                                           4
     Dose Monitoring

     • ALARA Principle
       Dose „As low As Reasonably Achievable“
     • EU-Directive 97/43/EURATOM
              – Justification, Optimization
              – Patient Dose Monitoring
              – Diagnostic Reference Values




Philips Medical Systems, Ulrich Neitzel, 2006-02-15   5
     Dose-Related Quantities

                                                      Tube
       Philips DigitalDiagnost:                       Filter
                                                      Collimator
     • Kerma-area product (KAP)
     • Entrance skin exposure (ESE)
     • Effective dose
                                                               Grid
     • Detector exposure (air kerma)                           AEC
                                                               Detector
     • Exposure index (EI)

Philips Medical Systems, Ulrich Neitzel, 2006-02-15                       6
     Why is the KAP not enough?

     • The KAP does not indicate approriateness of
       exposure level in the individual case.
     • Under- and overexposed images (in sense of noise
       level) cannot be identified from the KAP value.
     • In radiography, KAP gives only an upper limit of dose
       in a statistical (average) sense.
            ⇒ comparison to DRL


Philips Medical Systems, Ulrich Neitzel, 2006-02-15            7
     What is the EI good for?

     • Feedback to radiographer about appropriate
       exposure level in clinical routine („speedometer“)
     • Comparison with prescribed „speed classes“ for
       radiographic examinations
              – e.g. in Germany most examinations should be done
                with „400 speed“

     • Quality control tool for medical physicist
       (e.g. for dose check @ constancy test)

Philips Medical Systems, Ulrich Neitzel, 2006-02-15                8
     EI as Dosemeter
                                                                           2500
     • If properly calibrated,                                                        y = 228.859214x + 37.378536
                                                                                                2
                                                                                               R = 0.999936
       digital detectors can be                                            2000

       used as „dosemeters“




                                                             Pixel Value
                                                                           1500

     • Flat-field exposure                            EI ⇔
                                                                           1000
                                                                                                              Data
     • Known (calibrated)                                                                                     Linear (Data)
                                                                            500
       beam quality
                                                                              0
                                                                                  0        2        4       6      8          10   12
              ⇒ Exposure in µGy                                                                      Air Kerma [µGy]

             can be calculated from
              the Exposure Index

Philips Medical Systems, Ulrich Neitzel, 2006-02-15                                                                                 9
     Be aware!!
     The Exposure Index
              – is related to detector exposure,
                NOT to patient exposure
              – does not replace patient dose related parameters (like
                dose-area-product, entrance skin exposure)



                        Both are necessary for dose-conscious work


Philips Medical Systems, Ulrich Neitzel, 2006-02-15                      10
              Determination of Exposure Index
  Air kerma




                                                                               EI
                                                                               EI

                  y
                           x
                                                         Detector signal     Exposure Index
                Air kerma distribution
                                                      („raw“ pixel values)



Philips Medical Systems, Ulrich Neitzel, 2006-02-15                                      11
     Determination of Exposure Index

                            ROI-based




                                                              Characteristic
                                                              Characteristic
                                                               pixel value
                                                               pixel value     EI
                                                                               EI
                                                                 (VOI)
                                                                  (VOI)



                                                      L
                          Histogram-based

                                                          S
Philips Medical Systems, Ulrich Neitzel, 2006-02-15                                 12
     Determination of “Value of Interest (VOI)“
     Different concepts:
     • ROI based
              – e.g. mean value of center 25% area
     • Histogram based
              – e.g. mean of relevant pixel value range
     • Combination
              – Histogam evaluation of subarea
                                                          L


                                                              S
Philips Medical Systems, Ulrich Neitzel, 2006-02-15               13
     Now to the Practice....

     • Using the EI for monitoring the exposure levels in
       routine clinical work
     • Example Philips DigitalDiagnost




Philips Medical Systems, Ulrich Neitzel, 2006-02-15         14
     Digital Diagnost
     Exposure Index (EI)
                                                      Exposure   Detector
                                                        Index    Exposure
                                                          EI       [µGy]

                                                        160         6.3
     • Monitors exposure incident on                    200         5.0
       detector
                                                        250         4.0
     • Derived from pixel values of the                 320         3.1
       actual image                                     400         2.5

     • Scaled identically to film speed                 500         2.0
                                                        630         1.6
                                                        800         1.3
                                                        1000        1.0
                                                        1250        0.8
                                                        1600       0.63
                                                        2000        0.5
Philips Medical Systems, Ulrich Neitzel, 2006-02-15                       15
     Clinical Dose Monitoring

     • Feedback about dose-relevant
       parameters
              – Examination settings
                (kV, mAs, ms, filter, ...)
              – Exposure Index (EI) and
                Kerma-Area Product (KAP)

     • Displayed at acquisition workstation
     • Included in the DICOM image
       header



Philips Medical Systems, Ulrich Neitzel, 2006-02-15   16
     Exposure Log
     • Philips Digital Diagnost:
       Log file contains all relevant parameters for each exposure:

              –     date, time

                                                      #acquisitionNumber




                                                                                                                                                                                                          generatorareadose
                                                                                                  PatientNumber




                                                                                                                                                                generatormas
              –     pat ID, image #




                                                                                                                                                                               generatorms
                                                                                                                                                  generatorkv




                                                                                                                                                                                                                                   geodatagrid
                                                                                                                                                                                             geodatasid
                                                                                                                       description




                                                                                                                                      organ
                                                                            date



                                                                                       time
              –     exam type



                                                                                                                  EI
                                                      360                  24.05.00   15:00:40   1221 800 Fem ur / Knee ap           EXTREM ITY     66               8,9 13,4                 992 2,8736                       yes
                                                      361                  24.05.00   15:01:58   1221 1000 Fem ur / Knee lat         EXTREM ITY     66               4,3   6,5                992 1,5528                       yes

              –     kV, mAs, ms, SID                  362
                                                      364
                                                      363
                                                                           24.05.00
                                                                           24.05.00
                                                                           24.05.00
                                                                                      15:04:08
                                                                                      15:15:07
                                                                                      15:15:40
                                                                                                 1221 630 Fem ur / Knee lat
                                                                                                 1219 630 Knee lat
                                                                                                 1219 800 Knee ap
                                                                                                                                     EXTREM ITY
                                                                                                                                     KNEE
                                                                                                                                     KNEE
                                                                                                                                                    73
                                                                                                                                                    66
                                                                                                                                                    66
                                                                                                                                                                     5,6
                                                                                                                                                                     2,7
                                                                                                                                                                     2,9
                                                                                                                                                                           9,4
                                                                                                                                                                           4,0
                                                                                                                                                                           4,3
                                                                                                                                                                                              992 2,8412
                                                                                                                                                                                               -1 0,8166
                                                                                                                                                                                               -1 0,8264
                                                                                                                                                                                                                               yes
                                                                                                                                                                                                                               yes
                                                                                                                                                                                                                               yes

              –     grid, filter
                                                      365                  24.05.00   15:21:09   1220 500 W rist pa                  EXTREM ITY     48               3,1 12,2                 989 0,0614                       yes
                                                      366                  24.05.00   15:22:14   1220 125 W rist lat                 EXTREM ITY     60               6,2 17,9                 989 0,3561                       yes
                                                      367                  24.05.00   15:32:37   1222 500 Finger ap                  EXTREM ITY     46               2,9 12,2                 989 0,0257                       no


              –     field size
                                                      368                  24.05.00   15:33:30   1222 320 Finger ap                  EXTREM ITY     46               2,9 12,2                 989 0,2343                       no
                                                      369                  24.05.00   15:34:47   1222 320 Finger lat                 EXTREM ITY     46               2,9 12,2                 989 0,0442                       no
                                                      370                  24.05.00   15:36:07   1222 250 Finger oblique             EXTREM ITY     46               2,9 12,2                 989 0,0663                       no
                                                      263                  25.05.00   09:22:09   1147 320 Lower leg / Ankle lat      EXTREM ITY     60               2,9   8,5               1276 0,4444                       no

              –     EI, DAP                           262
                                                      333
                                                      381
                                                                           25.05.00
                                                                           25.05.00
                                                                           26.05.00
                                                                                      09:23:29
                                                                                      10:45:22
                                                                                      08:45:33
                                                                                                 1147 400 Lower leg / Ankle ap
                                                                                                 1200 250 Ankle lat
                                                                                                 1273 630 Hand pa
                                                                                                                                     EXTREM ITY
                                                                                                                                     ANKLE
                                                                                                                                     HAND
                                                                                                                                                    60
                                                                                                                                                    55
                                                                                                                                                    46
                                                                                                                                                                     2,9   8,5
                                                                                                                                                                     4,9 15,8
                                                                                                                                                                     3,9 16,3
                                                                                                                                                                                             1276 0,4751
                                                                                                                                                                                             1119 0,6563
                                                                                                                                                                                             1182 0,2067
                                                                                                                                                                                                                               no
                                                                                                                                                                                                                               no
                                                                                                                                                                                                                               no

              –     processing type
                                                      383                  26.05.00   08:47:28   1273 800 Hand lat                   HAND           48               3,9 15,3                1182 0,1618                       no
                                                      382                  26.05.00   08:47:59   1273 400 Hand lat                   HAND           48               3,9 15,3                1182 0,1618                       no
                                                      384                  26.05.00   08:57:35   1276 800 Lum bar spine ap           LSPINE         77              23,7 42,3                1153 6,9593                       yes


              –     ….
                                                      385                  26.05.00   08:59:10   1276 800 Lum bar spine ap           LSPINE         77              28,7 51,1                1153 8,4388                       yes
                                                      386                  26.05.00   09:01:56   1276 630 Lum bar spine lat          LSPINE         96              61,5 135,3                 -1 27,5011                      yes




Philips Medical Systems, Ulrich Neitzel, 2006-02-15                                                                                                                                                                           17
    Exposure Index: Free Technique
             Hand pa, 46 kV, 3.9 mAs
                                                                                                Median value:
                                                                                                 EI = 320
                                                                        Exposure index
                                                                                                Std. Deviation:
          1000
                                                                                                    ± 70%


           100                                                                                   äquivalent „400"


            10                                                          fixed mAs value




              1
                                      50              100         150        200          250
                                                            Image #

Philips Medical Systems, Ulrich Neitzel, 2006-02-15                                                               18
    Exposure Index: AEC Technique
             Pelvis ap, 77 kV, AEC
                                                                                       Mean value:
             1000                                                                       EI = 400

                                                                                         Mean
                                                                Exposure index
                                                                                       deviation :
               100
                                                                                        ± 25%


                10

                                                                mAs value

                  1
                                               100     200        300            400
                                                      Image #

Philips Medical Systems, Ulrich Neitzel, 2006-02-15                                                  19
     Exposure Index: AEC Technique
             LWS ap, 77 kV, AEC with adjustments
                                                      AEC adjustments
                                                                         Exposure index          Even small
            1000
                          1600                                                                  adjustments/
                          800
                                                                                          400   changes can
                                                                                                be detected
              100



                10

                                                                         mAs value
                  1
                                                50               100           150
                                                               Image #


Philips Medical Systems, Ulrich Neitzel, 2006-02-15                                                        20
     Exposure Index Today
     .... like European currencies before the EURO



                                                             Philips
     Kodak
                                                      Fuji




                                                             ???
                             Agfa
Philips Medical Systems, Ulrich Neitzel, 2006-02-15                    21
     The EI “Currencies”
                                                Name       Symbol   Range (typ.)   Value @2.5 µGy

  Agfa (CR)                                Logarithmic                              Depends on
                                                            LgM      1.6 – 2.2
                                             median                                selected speed
  Canon (DR)                                Reached                                 Depends on
                                                            REX          ?
                                            Exposure                                 processing
  Fuji (CR)                                  Sensitivity     S       200 – 800          200
  Kodak (CR)                                 Exposure
                                                             EI     1300 – 1800        1500
                                               Index
  Philips (DR)                               Exposure
                                                             EI      200 – 800          400
                                               Index
  Siemens (DR)                               Exposure
                                                            EXI      200 – 800         380(?)
                                               Index

Philips Medical Systems, Ulrich Neitzel, 2006-02-15                                                 22
     Exposure Index Scales
                                                 inverse
                                                     linear          proportional
                                                                         logarithmic


                            1.25 Gy 800
                            1.25 µGy
                                 µ                    1600    190     1100   1.3     1.6
                                                                                     1.3



                            2.5 Gy
                            2.5 µ
                                µGy 400               800     380     1400   1.6       1.6
                                                                                       1.9



                             5µGy
                             5 µGy            200     400     760
                                                              1700    1700   1.9       1.9
                                                                                       2.2



                            10 µ
                               µGy
                            10 Gy             100     200     1520
                                                              2000    2000   2.2       2.5
                                                                                       2.2


                                             EI       S        EXI      EI    lgM     lgM
                                          (Philips) (Fuji) (Siemens) (Kodak) (Agfa) (Agfa)
                                                                            SC= 200 SC= 400
Philips Medical Systems, Ulrich Neitzel, 2006-02-15                                           23
     Exposure Index: Limitations

     • The EI is a nice tool for dose monitoring – where is
       the problem?
        – Different scales
        – Different algorithms
        – Different calibrations

                       EI values between modalities/vendors
                            are not directly comparable

Philips Medical Systems, Ulrich Neitzel, 2006-02-15           24
     So what is necessary?

     • Unification of EI of DR and CR
     • Unification of EI of different vendors
     • Unification of calibration conditions



                                                   Introduce the
                                               Exposure Index € or $!

Philips Medical Systems, Ulrich Neitzel, 2006-02-15                     25
     Standardization Activities

     • AAPM (American Association of Physicists)
       – Task Group 116 “DR Exposure Index”

     • DIN NAR (NormenAusschuss Radiologie)
       – AK Dosisindikator



                      IEC New Work Item Proposal (2006)


Philips Medical Systems, Ulrich Neitzel, 2006-02-15       26
     Standardized EI: Concept (WIP)

     • Shall reflect detector exposure
     • Derived from pixel value (VOI)
     • Lin scale proportional to detector signal / dose
     • Still open: radiation quality for calibration
     • Still open: scale
              – AAPM: directly in µGy
              – DIN: proportional, but not directly µGy
     • Still open: scale resolution / precision

Philips Medical Systems, Ulrich Neitzel, 2006-02-15       27
     Relative EI
     • Additional “relative” or “deviation” index
     • Indicates EI deviation from an established optimal value
     • Optimal value can be dependent on
       – Examination type
       – Detector type
       – Hospital

                                             -2 -1       0      +1 +2

                                                      EI =100
Philips Medical Systems, Ulrich Neitzel, 2006-02-15                     28
     Precision and Nominal Values

     • Which precision is appropriate for stating EIs ?

 continuous                  ..200....................273........................................491.................................800...


   ISO R‘20                    200 220 250 280 320 360 400 450 500 560 630 710 800


   ISO R‘10                    200               250           320              400              500              630             800

DIN 6867-10
(“classes“)
                               200                                              400                                               800



Philips Medical Systems, Ulrich Neitzel, 2006-02-15                                                                                           29
     DICOM Integration
     • Full integration into digital environment
              –     Dose information included in DICOM header
              –     Dose information printed on film hardcopy
              –     Dose feedback to RIS via MPPS
              –     Dose structured report (DICOM Suppl. 94)
                                         (BWLM)
                                                                DICOM Print       Printer
                                        Worklist
                                                       Philips Digital
                           RIS                           Diagnost
                                                                         STORE
                                                                                      PACS
                                           (MPPS)

              Radiation Safety                        Dose SR        Storage Commit (SC)
             Reporting System
Philips Medical Systems, Ulrich Neitzel, 2006-02-15                                          30
     DICOM Radiation Dose Report

     • New DICOM Supplement 94 (Nov 2005)
              – „Diagnostic X-ray radiation dose reporting (Dose SR)“

     • DICOM Structured Report object
     • Provides mechanism for extensive dose reporting
     • Can be independent of PACS or RIS




Philips Medical Systems, Ulrich Neitzel, 2006-02-15                     31
     Exposure Index and DICOM

       Where should the value(s) be stored?

             Tag              Attribute Name          Description

      0018,1152 Exposure                              Tube current-time product in mAs
      0018,115E Image Area Dose Product               Kerma-area product in dGy· cm²
      0018,1404 Exposures on plate                    Number of exposures on plate
      0018,1405 Relative X-ray Exposure               Relative exposure on plate
      0018,6000 Sensitivity                           Detector sensitivity
      0040,0302 Entrance Dose                         Entrance dose on the patient [dGy]

Philips Medical Systems, Ulrich Neitzel, 2006-02-15                                        32
     EI Standardization

     • Terms and definitions
     • Basic concept
     • Scale and calibration point                    Possible
                                                      standardization
     • Precision and nominal values                   issues
     • Required/attainable accuracy
     • DICOM documentation
     • VOI determination                              Probably not
     • Algorithmic details                            standardization
                                                      issues

Philips Medical Systems, Ulrich Neitzel, 2006-02-15                     33

				
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