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# Centre of Rotation: Is there a problem in the Y dimension? PowerPoint PPT Presentation

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.

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

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)

### 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 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.

<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

### 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?

Comparison with other brands of camera

### Results - by brand (LE collimators)

CameraMax Y deviation (mm) Radius cm

Axis(avg)5.738

E Cam 3.133

Hawkeye1.733

Forte A5.434

Forte B5.434

### Results - by brand: NormalisedNormalised to 33cm radius (LE collimators)

CameraMax Y deviation (mm)

Axis(avg)5.0

E Cam 3.1

Hawkeye1.7

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

### 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

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