Frank Nachtrab1May, 4 th 2006Medipix2 Meeting Physikalisches Institut Universität...

Frank Nachtrab1 May, 4th 2006Medipix2 Meeting

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Optimising THL-Masks for the Medipix-Quad

May, 4th 2006

Frank Nachtrabfrank.nachtrab@physik.uni-erlangen.de

Physikalisches Institut, Abt. IV Friedrich-Alexander-Universität Erlangen

Frank Nachtrab2 May, 4th 2006Medipix2 Meeting

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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

Frank Nachtrab3 May, 4th 2006Medipix2 Meeting

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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

Frank Nachtrab4 May, 4th 2006Medipix2 Meeting

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Equalisation with Noise

Frank Nachtrab5 May, 4th 2006Medipix2 Meeting

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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

Frank Nachtrab6 May, 4th 2006Medipix2 Meeting

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Histogram of Flatfields: THL-maskbit 0, 7; different THS→ calculate width and distance of distributions

Equalisation using Flatfield Images

Frank Nachtrab7 May, 4th 2006Medipix2 Meeting

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Optimum THS - Quad

Linear fit of right boundary of bit0-distribution vs THS→ OptimumTHS (for each Chip on the Quad!)

Frank Nachtrab8 May, 4th 2006Medipix2 Meeting

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Mask quality - Definition

Countrate independent parameter for mask quality!(lower is better; zero for a perfect detector)

Frank Nachtrab9 May, 4th 2006Medipix2 Meeting

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Mask quality Single Chip

● All THL-Masks worse for higher thresholds● Flatfieldmasks better by a factor of ~ 1.35

Frank Nachtrab10 May, 4th 2006Medipix2 Meeting

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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

Frank Nachtrab11 May, 4th 2006Medipix2 Meeting

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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?

Frank Nachtrab12 May, 4th 2006Medipix2 Meeting

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