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Centre of Rotation: Is there a problem in the Y dimension?

Centre of Rotation: Is there a problem in the Y dimension?. Stephen Brown - Southend Mike Avison - Bradford. Tc99m point source positioned on-axis In the ideal situation the source would appear in the central pixel in both heads. Y axis. Tc99m point source positioned on-axis.

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Centre of Rotation: Is there a problem in the Y dimension?

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  1. Centre of Rotation:Is there a problem in the Y dimension? Stephen Brown - Southend Mike Avison - Bradford

  2. Tc99m point source positioned on-axis In the ideal situation the source would appear in the central pixel in both heads Y axis

  3. Tc99m point source positioned on-axis -the problem of sag (affects y alignment) Y axis

  4. Y axis Y error should be proportional to radius

  5. Y axis Sometimes we used the 3 point IRC source Y error on rear source is less than front source -the radii are more similar for rear source

  6. Actual Variation of Y offset with radius (rear) (mid) (front)

  7. How did we measure IRC Y error • Acquire dual head 360° of data (point source) • Use Display A - FWHM • Place ROI over point, read Y centroid • Enter data in spread sheet • Interfile Export to Park • Automatic processing checks X too • Odyssey IRC test • gives misleading and ill defined results

  8. How did we measure IRC Y error • Acquire dual head 360° of data (point source) • Use Display A - FWHM • Place ROI over point, read Y centroid • Enter data in spread sheet • Interfile Export to Park independent computer • Automatic processing checks X too • Odyssey IRC test • gives misleading and ill defined results

  9. How did we measure IRC Y error? • Acquire dual head 360° of data (point source) • Use Display A - FWHM • Place ROI over point, read Y centroid • Enter data in spread sheet • Interfile Export to Park • Automatic processing checks X too • Odyssey IRC test • gives misleading and ill defined results

  10. Ref. Ref. <1.0 <0.5 <0.5 <1.0 <0.65 <1.5 Is this X or Y or a combination of both? It’s a long way from 5mm so what does it mean? I think I was !!!

  11. Results(heads at max radius 38.6 cm) Camera (collimator) Maximum Y deviation (mm) Front Mid Rear Axis Southend (LEHR) 7.7 5.5 3.6 Axis 1 BRI (LEGP) 5.0 Axis 2 BRI (LEHR) 7.4 5.3 5.2 Axis 1 BRI (MEGP) 7.6 7.1 7.1 mean = 5.7 Acceptance criteria: typically 1 or 2mm maximum Y deviation

  12. Is 5 mm too big? • We are hoping to resolve objects of about 10mm in SPECT of trunk sized volumes • What resolution do we aspire to for DAT scan? • Guidance from professional bodies state • 1mm or 2mm (range) as acceptable • How do other gamma cameras perform?

  13. Is this bad performance? Comparison with other brands of camera

  14. Results - by brand (LE collimators) Camera Max Y deviation (mm) Radius cm Axis (avg) 5.7 38 E Cam 3.1 33 Hawkeye 1.7 33 Argus 4.8 (single head) 33 Forte A 5.4 34 Forte B 5.4 34

  15. Results - by brand: NormalisedNormalised to 33cm radius (LE collimators) Camera Max Y deviation (mm) Axis (avg) 5.0 E Cam 3.1 Hawkeye 1.7 Argus 4.8 (single head) Forte A 5.2 Forte B 5.2

  16. word of warning ... We tested with source on axis Symmetrical constant misalignment (not sag). As the gantry rotates, y-centroids remain fixed therefore no error detected Y axis

  17. Y axis Tc99m point source positioned off-axis • Source off centre in Y vertical direction • as the gantry rotates, y-centroids on each detector move • therefore the error is detected

  18. General Causes of COR errors • Misalignment of electronic and physical axis. (Electronic might change with angle) • Sag of detectors (Physical change with angle) • Detector misalignment • Collimators not fixed firmly • Collimators warp under gravity • Non-linearity

  19. What should we be testing? • Maybe … • acquisition of IRC jig 10cm lat. from iso-centre • radius 33cm • dual head • 180° • 102° non-circular orbit • Write macro to do analysis (X and Y errors) • When you get home try the display A method for Y errors only

  20. Summary 1 • If you are using Odyssey IRC test you are probably being misled into believing performance is much better than it really is. • Philips should supply better documentation.

  21. Summary 2 • Philips should improve correction: • Forte and Axis. (Worst in class). • Our measurements indicate: • If Philips modified the IRC cal. so that it took mean Y offsets for both heads over 360° and used the means to correct data, then offset errors could be reduced to 2.1 mm (Axis) • If Philips derived a variable correction as a function radius and angle: error could be eliminated at COR but linearly increase with radius (to 2.1 mm at 33cm) • This would probably meet all aspirations. • Further improvement would require gantry re-engineering

  22. Axis 1 X centroids raw fitted Y centroids raw mean X error Y error

  23. Axis 2 X centroids raw fitted Y centroids raw mean X error Y error

  24. ECam X centroids raw fitted Y centroids raw mean X error Y error

  25. Hawkeye X centroids raw fitted Y centroids raw mean X error Y error

  26. Forte A X centroids raw fitted Y centroids raw mean X error Y error

  27. Forte B X centroids raw fitted Y centroids raw mean X error Y error

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