The Exposure Index and Its Standardization

Transcription

1 The Exposure Index and Its Standardization Ulrich Neitzel Philips Medical Systems Clinical Science Hamburg, Germany

2 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,

3 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,

4 Wide Dynamic Range: A Two-Edged Sword Potential for dose reduction How to control the dose? Risk of overexposure Philips Medical Systems, Ulrich Neitzel,

5 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,

6 Dose-Related Quantities Philips DigitalDiagnost: Kerma-area product (KAP) Tube Filter Collimator Entrance skin exposure (ESE) Effective dose Detector exposure (air kerma) Exposure index (EI) Grid AEC Detector Philips Medical Systems, Ulrich Neitzel,

7 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,

8 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 constancy test) Philips Medical Systems, Ulrich Neitzel,

9 EI as Dosemeter If properly calibrated, digital detectors can be used as dosemeters Flat-field exposure Known (calibrated) beam quality EI Pixel Value y = x R 2 = Data Linear (Data) 0 Exposure in µgy can be calculated from the Exposure Index Air Kerma [µgy] Philips Medical Systems, Ulrich Neitzel,

10 Be aware!! The Exposure Index is relatedto 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,

11 Determination of Exposure Index Air kerma EI y x Air kerma distribution Detector signal ( raw pixel values) Exposure Index Philips Medical Systems, Ulrich Neitzel,

12 Determination of Exposure Index ROI-based Characteristic pixel value (VOI) EI Histogram-based L Philips Medical Systems, Ulrich Neitzel, S

13 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,

14 Now to the Practice... Using the EI for monitoring the exposure levels in routine clinical work Example Philips DigitalDiagnost Philips Medical Systems, Ulrich Neitzel,

15 Digital Diagnost Exposure Index (EI) Exposure Index EI Detector Exposure [µgy] Monitors exposure incident on detector Derived from pixel values of the actual image Scaled identically to film speed Philips Medical Systems, Ulrich Neitzel,

16 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,

17 Exposure Log Philips Digital Diagnost: Log file contains all relevant parameters for each exposure: date, time pat ID, image # exam type kv, mas, ms, SID grid, filter field size EI, DAP processing type. #acquisitionnumber date time PatientNumber EI description :00: Femur / Knee ap EXTREMITY 66 8,9 13, ,8736 yes :01: Femur / Knee lat EXTREMITY 66 4,3 6, ,5528 yes :04: Femur / Knee lat EXTREMITY 73 5,6 9, ,8412 yes :15: Knee lat KNEE 66 2,7 4,0-1 0,8166 yes :15: Knee ap KNEE 66 2,9 4,3-1 0,8264 yes :21: Wrist pa EXTREMITY 48 3,1 12, ,0614 yes :22: Wrist lat EXTREMITY 60 6,2 17, ,3561 yes :32: Finger ap EXTREMITY 46 2,9 12, ,0257 no :33: Finger ap EXTREMITY 46 2,9 12, ,2343 no :34: Finger lat EXTREMITY 46 2,9 12, ,0442 no :36: Finger oblique EXTREMITY 46 2,9 12, ,0663 no :22: Lower leg / Ankle lat EXTREMITY 60 2,9 8, ,4444 no :23: Lower leg / Ankle ap EXTREMITY 60 2,9 8, ,4751 no :45: Ankle lat ANKLE 55 4,9 15, ,6563 no :45: Hand pa HAND 46 3,9 16, ,2067 no :47: Hand lat HAND 48 3,9 15, ,1618 no :47: Hand lat HAND 48 3,9 15, ,1618 no :57: Lumbar spine ap LSPINE 77 23,7 42, ,9593 yes :59: Lumbar spine ap LSPINE 77 28,7 51, ,4388 yes :01: Lumbar spine lat LSPINE 96 61,5 135,3-1 27,5011 yes organ generatorkv generatormas generatorms geodatasid generatorareadose geodatagrid Philips Medical Systems, Ulrich Neitzel,

18 Exposure Index: Free Technique Hand pa, 46 kv, 3.9 mas 1000 Exposure index Median value: EI = 320 Std. Deviation: ± 70% 100 äquivalent 400" 10 fixed mas value Image # Philips Medical Systems, Ulrich Neitzel,

19 Exposure Index: AEC Technique Pelvis ap, 77 kv, AEC 1000 Mean value: EI = Exposure index Mean deviation : ± 25% 10 mas value Image # Philips Medical Systems, Ulrich Neitzel,

20 Exposure Index: AEC Technique LWS ap, 77 kv, AEC with adjustments AEC adjustments Exposure index 400 Even small adjustments/ changes can be detected 10 1 mas value Image # Philips Medical Systems, Ulrich Neitzel,

21 Exposure Index Today... like European currencies before the EURO Philips Kodak Fuji Agfa Philips Medical Systems, Ulrich Neitzel, ???

22 The EI Currencies Name Symbol Range (typ.) µgy Agfa (CR) Logarithmic median LgM Depends on selected speed Canon (DR) Reached Exposure REX? Depends on processing Fuji (CR) Sensitivity S Kodak (CR) Exposure Index EI Philips (DR) Exposure Index EI Siemens (DR) Exposure Index EXI (?) Philips Medical Systems, Ulrich Neitzel,

23 Exposure Index Scales inverse linear proportional logarithmic 1.25 µgy µgy µgy µgy µgy EI (Philips) S (Fuji) EXI (Siemens) EI (Kodak) lgm (Agfa) SC= 200 lgm (Agfa) SC= 400 Philips Medical Systems, Ulrich Neitzel,

24 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,

25 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,

26 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,

27 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,

28 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 EI =100 Philips Medical Systems, Ulrich Neitzel,

29 Precision and Nominal Values Which precision is appropriate for stating EIs? continuous ISO R ISO R 10 DIN ( classes ) Philips Medical Systems, Ulrich Neitzel,

30 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) RIS (BWLM) Worklist (MPPS) Philips Digital Diagnost DICOM Print STORE Printer PACS Radiation Safety Reporting System Dose SR Storage Commit (SC) Philips Medical Systems, Ulrich Neitzel,

31 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,

32 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, , ,6000 Exposures on plate Relative X-ray Exposure Sensitivity Number of exposures on plate Relative exposure on plate Detector sensitivity 0040,0302 Entrance Dose Entrance dose on the patient [dgy] Philips Medical Systems, Ulrich Neitzel,

33 EI Standardization Terms and definitions Basic concept Scale and calibration point Precision and nominal values Required/attainable accuracy DICOM documentation VOI determination Algorithmic details Possible standardization issues Probably not standardization issues Philips Medical Systems, Ulrich Neitzel,

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