·Table of Contents
·Methods and Instrumentation
CCD Based High Energy Large Field X-ray Digital Radiographic SystemM. Antonakios, V. Lapouge, Ph. Rizo
LETI (CEA-Technologies avancées) 17 rue des Martyrs 38054 Grenoble, FRANCE
|Fig 1: Radiographic system general scheme|
The main requirements for the detection camera are a high sensitivity to the wave length radiation emitted by the converter screen, a low sensitivity of its integrated parts (electronic and CCD) to scattered radiation, and an acquisition rate of few images per second. We choose the HAMAMATSU C4742 camera which have the following characteristics :
It's necessary to frame entirely the converter screen in the CCD window to determine the magnification factor. This dimension is independent of the screen - CCD distance. If we consider the most conservative case (smallest side of the screen, greatest side of the CCD matrix), the magnification factor is equal to .
Optical lens choice
The detection system is build around three elements (fig 2) : a screen converting X-Ray on visible photons and a CCD integrated matrix optically coupled by a lens. The optical calculation allows to determine the theoretical lens to be used and also to optimise the screen/CCD distance in order to move the camera away from the direct X-Ray stream. We finally choose a f17/0.95 lens.
|Fig 2: Detection system scheme|
Scattered radiation measurement
A dose meter probe has been placed in the detection system, in the camera location, in order to evaluate the radiation effect. The detector - X-Ray source distance is 3.7 meters, the conic collimator angle is 15° and a 11 mm lead glass is placed in front of the camera. The accelerator delivers 1750 rd/minute at 1 meter. The scattered radiation after attenuation trough 5 and 10 cm of lead is measured. The following table shows the experimental results.
|No Lead protection||Lead protection 10 cm||Lead protection 20 cm|
|Stream attenuation 5 cm lead||1.03mGy/mn||0.7 mGy/mn||unsignificant|
|Stream attenuation 10 cm lead||0.12 mGy/mn||0.09 mGy/mn||unsignificant|
Sampling rate and image field
In order to verify the previous theoretical measurements about the optical lens, magnification factor and camera position we performed a set of tests. These tests are carried out in a low level visible light (equivalent to the one obtained with an X-Ray stream and converter screen) with a dedicated Image Quality Indicator (IQI). We acquire the image with a 1 second acquisition rate to reduce the effect of the photon noise. Two modes have been tested : the binning 1 mode which gives a 1024 x 1024 pixel image, and the binning 2 mode (superpixel mode) which use 4 adjacent pixels to give a 512 x 640 pixel image. Finally, we obtain a real detectable image field of 770*650 mm and a sampling rate of 0.635 mm/pixel in binning 1 mode and 1.274 mm/pixel in binning 2 mode.
Detection ability and quality image criterions
The defect visibility into the object rediographies referred to the detection ability notion. This parameter defines the smallest X-Ray opacity variation which can be detected. The detection ability depends of some criterions like:
The spatial resolution of an image is measured in line per millimetres. In the case of high energy we evaluate it by measuring the cut off frequency for a step response to obtain the MTF of the system. In all the cases of measurement we reach values close to 0.5 lp/mm.
We compute the contrast factor C by measuring the width evolution of a crack on the image. The formula C = (L - L0)/L, where L0 represent the mean grey level and L the maximum grey level of the crack, gives the contrast factor expressed in %.
To do this, we put in front of the screen a set of lead calibrated cracks (from 0.53 to 5 mm) and acquire images in the full X-Ray stream or in a stream attenuated by 30 cm of concrete.
The following charts (fig 2) shows the contrast evolution.
|Fig 2a: Contrast evolution in full stream||Fig 2b: Contrast evolution in attenuated stream|
Fig 3: Signal on noise ratio evolution
Signal to noise ratio
The S/N ratio according to the exposure time (from 111 to 500 ms) has been measured in the area where the X-ray intensity is maximum. In this case the S/N ratio is basically the same . This measure gives the minimum exposure time to be used to obtain the best S/N ratio. For an X-Ray stream attenuated by 30 cm of concrete, the results shows that there is no evolution of the S/N ratio for an exposure time higher or equal to 1 second (fig 3). Then it is not necessary to work with an exposure time higher than 1 second.
A large field image
Preliminary experimentations have been performed before producing the large field screen. The large detector image has been reconstructed using quarter image (300X400 mm) and moving a small field screen. This allows to generate a large image without a large field converter screen. The following image (fig4) has been recomposed from a set of four quarter images. It represents a concrete bloc (dimensions 800x500 mm) which include some objects buried in 300 mm of concrete.
|Fig 4: Recomposed large field image|
|Fig 5: the experimental transfer angle|
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