# Centre of Rotation

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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
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
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
• 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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 views: 5 posted: 10/26/2011 language: English pages: 31