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Frank Nachtrab1 May, 4th 2006Medipix2 Meeting
Phy
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nstit
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Uni
vers
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Optimising THL-Masks for the Medipix-Quad
May, 4th 2006
Frank [email protected]
Physikalisches Institut, Abt. IV Friedrich-Alexander-Universität Erlangen
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Frank Nachtrab2 May, 4th 2006Medipix2 Meeting
Phy
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Uni
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Medipix2 CT setup in Erlangen
● Three translation stages (1 µm resolution)
● One rotation stage(0.1 degree resolution)
● MUROS 2.1● Distances:
– Focus-detector: 768mm– Focus-object plate: 635mm
● Tungsten X-ray tube: – standard diagnostic tube– tube voltage 40 – 125 kV
translationstagesrotation stage with
object plate
X-ray tube
medipix2detector
MUROS 2.1
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Frank Nachtrab3 May, 4th 2006Medipix2 Meeting
Phy
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Uni
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Ring artefacts in CT
Problem: variations in lower threshold lead to differing counting behavior for every pixel (‘gain’)
→ ring artefacts in computed tomography
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Frank Nachtrab4 May, 4th 2006Medipix2 Meeting
Phy
sika
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nstit
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Uni
vers
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Equalisation with Noise
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Frank Nachtrab5 May, 4th 2006Medipix2 Meeting
Phy
sika
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nstit
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Uni
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Flatfield method: Advantages
Equalisation using
electronic noise flatfield images
- spectrum not taken into acount
- optimised for THL of noise
- only electronic properties
- time ~ hours
+ optimised for given spectrum
+ optimised for given THL
+ sensor inhomogenities considered
+ rather fast method
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Frank Nachtrab6 May, 4th 2006Medipix2 Meeting
Phy
sika
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nstit
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Histogram of Flatfields: THL-maskbit 0, 7; different THS→ calculate width and distance of distributions
Equalisation using Flatfield Images
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Frank Nachtrab7 May, 4th 2006Medipix2 Meeting
Phy
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Uni
vers
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Optimum THS - Quad
Linear fit of right boundary of bit0-distribution vs THS→ OptimumTHS (for each Chip on the Quad!)
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Frank Nachtrab8 May, 4th 2006Medipix2 Meeting
Phy
sika
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nstit
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Uni
vers
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Mask quality - Definition
Countrate independent parameter for mask quality!(lower is better; zero for a perfect detector)
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Frank Nachtrab9 May, 4th 2006Medipix2 Meeting
Phy
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nstit
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Uni
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Mask quality Single Chip
● All THL-Masks worse for higher thresholds● Flatfieldmasks better by a factor of ~ 1.35
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Frank Nachtrab10 May, 4th 2006Medipix2 Meeting
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nstit
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Mask quality Quad
standard mask
flatfield mask
Chip 1 0.089 0.055
Chip 2 0.082 0.058
Chip 3 0.067 0.057
Chip 4 0.067 0.059
Overall 0.113 0.096
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Frank Nachtrab11 May, 4th 2006Medipix2 Meeting
Phy
sika
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nstit
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Outlook
● five different THS Values instead of three → quadratic fit?
● tune FBK settings after generation of mask for better global homogeneity
● automate procedure → Pixelman plugin?
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Frank Nachtrab12 May, 4th 2006Medipix2 Meeting
Phy
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nstit
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Uni
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