Image Quality Parameter with DDAs compared to Film
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Author:  Klaus [ 28.10.2019, 18:56 ]
Post subject:  Image Quality Parameter with DDAs compared to Film

The following table for image quality could be found in the guide for film E94 (Fig. 1 page 5)):

It can be used also for DDA technique:

with CNR (Contrast to Noise Ratio) and SNR (Signal to Noise Ratio). Sometimes Contrast Sensitivity is used, it is 1/CNR.

The unsharpness and contrast is very similar. The biggest difference is the "noise"; with buying the film the noise level is defined with the film class (capacity for dose and film granularity). With DDAs the matrix of the pixel is always in the same position that several images can be captured and accumulated in a computer to overcome the "single shot" dose limit of film (or CR).
The following diagram should show this difference. To compare film and DDAs the film is digitized and the SNR is normalized to a 100µm round aperture (sqared shaped area of 88.6µm x 88.6µm) to have the same SRB.

The measurement was done at BAM in Berlin with different energies and different accumulation times - up to 25 minutes - to get a very high SNRnorm which could be factor 40 better than the best film.

Author:  Klaus [ 28.10.2019, 20:25 ]
Post subject:  Re: Image Quality Parameter with DDAs compared to Film

To demonstrate the influence of noise on detail visibility here is a picture of the Ypsilon Mountain:
SNR = 5

SNR = 80

This is very similar with X-Ray images; here is an example with the BAM 5 weld (see picture in header between Fand f).
1. Film AGFA D2, class C1 EN 584-1, 330s exposure time; no magnification (SNRnorm = 130)

2. PerkinElmer XRD1620 detector (200µm SRB), 100s exposure time; magnification of 3.0 (SNRnorm = 520 - limited by the surface structure of the material beside the weld)

highpass filter with same parameters applied on both images

Author:  Colin Gan [ 17.09.2021, 09:51 ]
Post subject:  Re: Image Quality Parameter with DDAs compared to Film

Hi Klaus:

I have a few questions, again :-D
1. Could the gray value of DR (DDA) be considered as a counterpart to the density of film?
2. Some film standards stipulate that density scale, why do the relevant standards of DR not specify the corresponding gray scale?
3. Is it necessary to make an exposure chart for a DR system?

Thank you for your explanation~

Author:  Klaus [ 17.09.2021, 20:36 ]
Post subject:  Re: Image Quality Parameter with DDAs compared to Film

Hi Colin :cap:
I try to do my best to answer your questions.
1. Mainly not, because there is an inverse logarithmic relationship between gray values of DDAs and film density. As you are using ISee you could convert with the BAM LUT from DDA gray values to film densities.
2. With film the density value corresponds to an exposure dose - in relation to the film sensitivity (film class). With DDAs you have different sensitivity modes which you could change easily; also increase of the internal frame-time would increase the gray value and then there is the energy dependent sensitivity of the scintillator and/or the efficiency of the internal components of the DDA. Finally there is a larger variance in the output value of different DDAs of the same type.
==> There is no fixed relationship between gray value and image quality (noise).
3. No, but you should track the main parameters over lifetime of the DDA as described in E2737 (as there is always a degradation in the system).

Author:  Colin Gan [ 22.09.2021, 02:39 ]
Post subject:  Re: Image Quality Parameter with DDAs compared to Film

Thanks, now I'm moving on to Image Quality Parameters

This forum is amazing!

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