Centre of Rotation:Is there a problem in the Y

dimension?

Stephen Brown - Southend

Mike Avison - Bradford

Y axis

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

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

Y axis

Actual Variation of Y offset with radius

0

2

4

6

8

10

0 20 40 60

Detector radius

Max peak2peak y-deviation

(mm)

Spot 1

Spot 2

Spot 3

Linear (Spot 3)

Linear (Spot 2)

Linear (Spot 1)

(mid)(rear)

(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

IRC Y Deviants form mean MEGP

-1.5

-1

-0.5

0

0.5

1

1.5

0 20 40 60 80 100 120

Projections

Co

ord

ina

te f

o c

en

tro

id

Spot 1

Spot 2

Spot 3

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

<0.5<0.5<0.65

<1.0<1.0<1.5

Ref. Ref.

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

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

mean = 5.7

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 A5.4 34

Forte B5.4 34

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

Range of Y COR (IRC) Error

0

1

2

3

4

5

6

Axis Forte ECam Hawkeye

Camera

mm

Y axis

word of warning ...

Symmetrical constant misalignment (not sag).

As the gantry rotates, y-centroids remain fixed therefore no error detected

We tested with source on 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– 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

raw fitted

Y centroids raw

mean

X error Y error

Axis 2X centroids

raw fitted

Y centroids raw

mean

X error Y error

ECamX centroids

raw fitted

Y centroids raw

mean

X error Y error

HawkeyeX centroids

raw fitted

Y centroids raw

mean

X error Y error

Forte AX centroids

raw fitted

Y centroids raw

mean

X error Y error

Forte BX centroids

raw fitted

Y centroids raw

mean

X error Y error