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					    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

 -the   problem of sag (affects y alignment)




                                               Y axis
Y error should be proportional to radius




                                       Y axis
 Sometimes we used the 3 point IRC source




                                            Y axis




Y error on rear source is less than front source
-the radii are more similar for rear source
               Actual Variation of Y offset with radius


              10                                          Spot 1   (rear)
                                                          Spot 2   (mid)
               8                                          Spot 3   (front)
                                                          Linear (Spot 3)
    M ax
peak2peak
               6                                          Linear (Spot 2)
                                                          Linear (Spot 1)
y-deviation
   (mm)        4

               2
                           Detector radius
               0
                   0       20         40         60
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
                                    IRC Y Deviants form mean MEGP

                         1.5



                           1
Coordinate fo centroid




                         0.5

                                                                              Spot 1
                           0                                                  Spot 2
                                0   20    40       60        80   100   120   Spot 3

                         -0.5



                          -1



                         -1.5
                                               Projections
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
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
                            Ref.         Ref.
                            <0.5        <1.0
                            <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 !!!
                          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           mean = 5.7
Axis 2 BRI       (LEHR)      7.4     5.3   5.2

Axis 1 BRI       (MEGP)      7.6     7.1   7.1




Acceptance criteria: typically 1 or 2mm maximum Y deviation
           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?
Is this bad performance?

Comparison with other brands of
           camera
          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
   Results - by brand: Normalised
             Normalised 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
                Range of Y COR (IRC) Error

     6

     5

     4
mm




     3

     2

     1

     0
         Axis          Forte            ECam   Hawkeye
                               Camera
            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
       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




                                                      Y axis
  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
    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
               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.
                   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
Axis 1
         X centroids   Y centroids
             raw           raw
            fitted        mean




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




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




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




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




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




            X error      Y error

				
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posted:10/26/2011
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