NPS problem

Submitted by Meijing on Thu, 2014-12-04 01:59

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FindDQE

Hi,
I have used findDQE software to test the DQE of three different cameras. I collected images on TF20 operating at 120 kV. Although the MTF and DQE curves seem very good, the NPS curves decrease sharpely from 2 to ~0, which are not reasonable.
I have added the image here. the three diffrence color curves indicate three kinds of cameras. In the above are MTF curves, middle DQE curves and bottom NPS curves. You can see the unnormal data.
The dose are about 42e-/A2, a little small than 50e-/A2 you have asked in paper. I have pondered over the issues for a long time. But I just cannot find the reason. Can you help me to figure out the problem?

rsruskin Thu, 2014-12-04 02:27

Hi Meijing,

The NPS in column 3 of the output text file is the raw NPS, which is *not* used in calculating the DQE. The DQE is calculated from a smoothed version of the NPS (fitted using splines), called NPS-fit.

The splines should attenuate that crazy spike at the start of the NPS. The spike is artifactual; I'm not 100% sure why, but sometimes the very low frequencies contain way more power than expected/the NPS measurement is very noisy.

Can you show me what your plot of NPS-fit (column 4 of the output) looks like? I assume that the plot you've attached shows NPS (column 3 of the output).

Rachel

Meijing Thu, 2014-12-04 03:48

Thank you so much for your timely reply. yes, you're right.it's the NPS instead of NPS-fit. But the DQE was got from the software not calculated by myself with the NPS.

I have added the NPS-fit at the top of this topic. IS it right? thanks again.