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?

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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

-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

Actual Variation of Y offset with radius

(rear)

(mid)

(front)

- 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

- 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

- 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 !!!

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

mean = 5.7

Acceptance criteria: typically 1 or 2mm maximum Y deviation

- 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

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

CameraMax Y deviation (mm)

Axis(avg)5.0

E Cam 3.1

Hawkeye1.7

Argus4.8(single head)

Forte A5.2

Forte B5.2

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

- 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

- 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

- 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.

- 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