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Evaluating Y-Dimension Center of Rotation Errors in SPECT Imaging: Insights and Solutions

This paper investigates the Y-dimension alignment issues encountered in SPECT imaging using Tc99m point sources. It discusses problems related to detector sagging, misalignment of the physical and electronic axes, and the impact of source positioning on Y errors. Various measurement methods to assess Y errors are examined, along with the performance of different gamma camera brands. The study highlights potential improvements to calibration processes and suggests that better documentation and technological adjustments could enhance diagnostic accuracy in SPECT imaging.

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Evaluating Y-Dimension Center of Rotation Errors in SPECT Imaging: Insights and Solutions

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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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