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

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

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

5. Y axis Y error should be proportional to radius

6. 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 Y axis

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

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 –Automatic processing checks X too Odyssey IRC test –gives misleading and ill defined results

11. 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

13. 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

14. <0.5 <0.65 <1.0 <1.5 Ref. 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 !!!

15. Results (heads at max radius 38.6 cm) Camera (collimator)Maximum Y deviation (mm) FrontMidRear Axis Southend(LEHR)7.75.53.6 Axis 1 BRI(LEGP)5.0 Axis 2 BRI(LEHR)7.45.35.2 Axis 1 BRI (MEGP)7.67.17.1 Acceptance criteria: typically 1 or 2mm maximum Y deviation mean = 5.7

16. 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?

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

18. Results - by brand (LE collimators) CameraMax Y deviation (mm) Radius cm Axis(avg)5.738 E Cam 3.133 Hawkeye1.733 Argus4.8(single head)33 Forte A5.434 Forte B5.434

19. Results - by brand: Normalised Normalised to 33cm radius (LE collimators) CameraMax Y deviation (mm) Axis(avg)5.0 E Cam 3.1 Hawkeye1.7 Argus4.8(single head) Forte A5.2 Forte B5.2

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

22. 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

23. 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

24. 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

25. 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.

26. 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

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

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

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

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

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

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