·Table of Contents
·Methods and Instrumentation
Image Quality comparison of Digital Radiographic Systems for NDTP. Willems, P. Soltani, B. Vaessen
Agfa NDT, Septestraat 27, B-2640 Mortsel, Belgium
Phone: 32-3-4442881, Fax: 32-3-4447655
Here we report on work in progress. We will focus solely on image quality and develop a simple model that allows us to estimate the attainable image quality for a given system and for given exposure conditions. The estimated image quality of digital systems will be contrasted with that of film.
|Fig 1: Workflow for Storage Phosphers (CR)|
For the image quality calculations we made the following assumptions:
Pixel size is 100µ and the scan spot has a FWHM of 56µ. Electronic noise was 0, fixed pattern noise was 0,1% and the swank factor (Poisson excess noise) was 0,588. An overall gain of 19 was assumed; corresponding with a screen gain of 755, an overall collection efficiency of 10% and a PMT quantum efficiency of 25%.
The scintillator layer may consist of a powder phosphor layer or of a vacuum deposited needle crystal layer. Needle crystals reduce light scatter and screens can be made thicker i.e. a higher X-ray absorption can be achieved for the same resolution.
For a direct conversion flat panel a photoconductor is used instead of a scintillator. X-rays are converted directly into charge carriers. A simple collector electrode replaces the photodiode. A bias field is applied to separate the charge carriers and to collect them. For a-selenium systems the resolution is only limited by pixel size.
For the calculation we made the following assumptions:
Direct conversion Flat Panel using a 500µ Selenium layer (Selenium)
Specifications of the Agfa DRC detector: pixel size is 139µ, the fill factor is 86%. We assume that electronic noise per pixel sadd = 2000e and that residual fixed pattern noise is 550ppm. No Poisson excess noise was assumed for radiation energies far above the K-edge of Selenium
Indirect Conversion Flat Panel using a Regular Gd2O2S powder phosphor screen. (GOS Reg)
Pixel size is 126µ, the fill factor is 57% . We assume that electronic noise and that residual fixed pattern noise are equal to II. The diode quantum efficiency was assumed 75%. The swank factor was set to 0,75.
Indirect Conversion Flat Panel using 500µ CsI vapor deposited needle crystal screen (CsI -C)
Pixel size is 143µ, the fill factor is 70%. We assume that electronic noise and that residual fixed pattern noise are equal to II. The diode quantum efficiency was assumed 65%. The swank factor was set to 0,85.
For many NDT applications the highest possible image quality is required. This necessitates the optimization of the exposure conditions with regard to exposed dose, radiation hardness, primary to scatter ratio and geometrical unsharpness. Finally the detector choice will determine the maximum attainable image quality.
A simple equation that relates the minimum detectable image contrast Cmin to the physical parameters characterizing image quality has been proposed .
Cmin is a function both of the exposed dose characterized here by the impacting X-ray flux q0 and the detail aperture A (for a circular detail with radius, A=pr2). The subjective viewing conditions are compounded in the constant k, the observer's threshold criterion. The exposed dose determines the attainable signal to noise ratio at zero spatial frequency SNR(q0,0).
Radiation hardness determines the radiation contrast and hence the signal amplitude.
The contrast is degraded both by radiation scatter (build-up factor (1+S/P) and resolution (resolution degradation factor q(A)). The resolution degradation is due to both geometrical unsharpness and to detector MTF.
When exposure conditions are optimized the remaining critical parameter for detail detection is [q(A).A)] which reduces to the detector MTF. This is why for critical applications, detector resolution should be regarded as the most important decision factor.
|Fig 2: MTF from inherent Unsharpness|
|Fig 3a: MTF Film vs. Digital systems 100kVp, Object 10mm, FFD=1000mm ,Focal spot Film=2mm, Digital=1mm||Fig 3b: MTF Film vs. Digital systems 200kVp, Object 20mm, FFD=1000mm, Focal spot Film=2mm, Digital=1mm|
We have depicted the following situations in fig. 3. The resulting MTF is the product of geometrical unsharpness and detector MTF:
|Fig 3c: MTF Film vs. Digital systems Ir192 , Object 35mm , FFD=750mm , Focal spot =2mm||Fig 4: Mean free path of secondary x-ray photons|
For practical situations the results of the indirect systems and CR will be even worse, since we have postulated that the optical scatter is the dominant source for unsharpness and neglected the X-ray contribution. For medical radiation energies this is a valid assumption but for NDT where gamma sources and linear accelerators are used, the X-ray contribution can not be neglected. The inherent unsharpness for the digital detectors will scale with the mean free path L of the secondary X-ray photons shown in fig. 4
Selenium has a much shorter mean free path than the other digital systems. From this we may expect that CR and the indirect flat panels will get even worse compared to the selenium flat panel at high radiation energies. The selenium resolution may be expected to exceed that of film for linear accelerator energies.
|DQE = (SNRout / SNR in)2||(eq. 2a)|
|DQE(q0,w) = (a. G . q0. MTF)2 [q0 . NNPS(q0,w)]||(eq. 2b)|
We will estimate the DQE values for radiation with a 3,5mm Cu HVL spectrum (220kVp - 8mmCu). These values are then compared with film DQE derived from measurements of film granularity sD and gradient g.
For film the following formula is used:
|Detector||Material||mg/cm2||a||pixel (mu)||fill factor||w (MTF20%)||Nyquist||MTF Nyq.|
|DRC||a - Se||220||4,7%||139||86%||(6,50)||3,6||68%|
|Table 1: Digital Detectors|
|Film||dose D=2||g D=2||sD D=2|
|Table 2: Film Data|
|Fig 5: DQEO 220kVp-8mm Cu|
We see that all films exhibit superior DQE0 values compared to the digital systems. The best results are obtained for the midrange films D4, D5 and D7 , D8 suffers from a granularity that has increased more than speed increase, for D2 and D3 speed is reduced more than the granularity reduction.
For the indirect TFT systems it is obvious that when the scintillator screen is optimized in order to obtain a better resolution (CsI -A , GOS Fine) the DQE0 value is less compared to that of the selenium system. Now let us compare resolution i.e. the spatial frequency where MTF = 20% (shown in table 1). Although a significant improvement has been realized for CsI-A and GOS Fine compared to the thick converter screens, resolution is still far inferior compared to that of Selenium.
The thickest CsI screen (CsI - C) has the highest X-ray absorption and hence its DQE0 value is nearly 3x that of the selenium panel. Now referring to eq. 1, we may quantify the benefit of this high DEQ0 value. Suppose we are looking to a weak detail with large dimensions where resolution degradation is insignificant. To reach the same minimum detectable contrast detectors need equal SNR values. The signal to noise value is related to the DQE by:
|SNR(q0,w) = [q0 . DQE(q0,w) ]1/2||(eq. 4)|
Hence for a detector X with an inferior DQE0 value the exposed dose has to be increased by a factor that equals the square of the ratio DQE0(X) / DQE0(CsI-C). For the selenium panel this results in an increase of dose of 176%. For smaller details the resolution degradation becomes important and the situation is reversed e.g. already for a 500µ; detail both panels need the same dose.
Finally in fig. 6 we show the variation of DQE with spatial frequency. Structurix D7 is compared with the systems of sections 2-3.
|Fig 6: DQE(w) 200kVp - 8mm Cu|
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