Centre of rotation is there a problem in the y dimension
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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.

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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

-the problem of sag (affects y alignment)

Y axis


Y axis

Y error should be proportional to radius


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



How did we measure irc y error
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


How did we measure irc y error1
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 error2
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.

<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 !!!


Results heads at max radius 38 6 cm
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


Is 5 mm too big
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

Is this bad performance?

Comparison with other brands of camera


Results by brand le collimators
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
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


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


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


General causes of cor errors
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
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
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
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

raw

fitted

Y centroids

raw

mean

X error

Y error


Axis 2

X centroids

raw

fitted

Y centroids

raw

mean

X error

Y error


ECam

X centroids

raw

fitted

Y centroids

raw

mean

X error

Y error


Hawkeye

X centroids

raw

fitted

Y centroids

raw

mean

X error

Y error


Forte A

X centroids

raw

fitted

Y centroids

raw

mean

X error

Y error


Forte B

X centroids

raw

fitted

Y centroids

raw

mean

X error

Y error


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