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PRELUDE TO DIGITAL RADIOGRAPHY
Dr. V R. Ravindran
The evolution of radiography from the conventional film based technique
to the latest Digital technology is through a few generations of
developments. The film-less radiography concept began from the
fluoroscopy technique of seventies. In this technique, the X-rays
penetrating through the object is directly interacting with the
fluorescent screen which is coated with X-ray scintillators like,
CsI, NaI, Gd2O2S, etc. The scintillator screen converts the X-rays
into visible photons and forms the radiographic image. Initially
the image was directly viewed and the quality was very poor to reveal
details. Later this technology was improved by introducing image
intensifier system to enhance the brightness and quality of the image.
CCTV systems were integrated with X-ray intensifiers to view the
images in the TV monitor away from the radiation zone. The X-ray
image intensifier system was later coupled with CCD Camera and image
digitiser system to improve the quality. The fiber optic technology
was also introduced in the system to achieve noise free coupling of
image data from the phosphor screen to the CCD camera. But the image
quality and resolution obtained were much below that of film radiography.
COMPUTED RADIOGRAPHY (CR)
In this advanced X-ray imaging technique, a photo-stimulable phosphor
plate basically Barium Fluorobromide (BaFBr:Eu), is used for imaging
in a way similar to film radiography. The phosphor atoms on
irradiation with X-rays get excited to higher energy levels proportional
to the incident intensity. The electrons get trapped at certain energy
levels forming a latent image and subsequent release of energy can
occur by photo-stimulated emission on scanning with a laser beam. The
emitted light is picked up by photodiodes or photomultiplier tubes to
generate the digital image (Fig.1). The two step process and the
conversion by laser scanning method introduce noise and the quality
of image is just comparable to that of film radiography. The plate
is reusable but the two step process doesn't give much attraction to
the technique for real-time inspection and automation in industrial
radiography.
CCD BASED RADIOGRAPHY SYSTEM
Charge Coupled Device (CCD) is commonly used as image acquisition
component of digital photography. In CCD based digital radiography
system high resolution small area CCD detector typically 2 to 4 cm2
is used which is much smaller than the area of interest in the
radiographic projection. Suitable optics is used to reduce the
size of the input image produced by the phosphor screen to suit the
area of CCD detector (Fig. 2). Demagnification is inefficient and
transfer efficiency from the scintillator to the CCD is also very poor.
CCD detector is sensitive to radiation damage and if not shielded properly
noise level increases. The thermal noise within the CCD itself can degrade
the image quality, so cooled CCD detector is used in latest systems. The
CCD based system is a transitory technology and flat panel is the
preferred digital radiography system due to its very high quality,
robustness and compact design.
FLAT PANEL DETECTORS
The state of art technology is direct digital X-ray imaging based on
flat panel detector (FPD) systems in which the image is displayed
directly on a computer without intermediate imaging optics or
mechanical scanning. The incident x-rays are converted into electric
charge and then to digital image through a large area panel sensor
(Fig.3&4). Compared to earlier technologies the FPD provides high
quality digital images better
than film radiography with better signal to noise ratio.
Two distinct technologies are available for
flat panel detectors: "direct conversion" and "indirect conversion".
In the direct conversion detectors X-ray energy is converted
directly into electric charge and in the indirect conversion
detectors X-ray energy is first converted to light photons by a
scintillator and subsequently into electric charge by adjacent
semiconductor layers. Amorphous selenium based flat panels are
direct conversion type and amorphous silicon as well as CMOS
based flat panels are the presently available indirect conversion
detectors.
The size of the individual sensor restricts the pixel size in the
digital X-ray image. Presently with the advanced chip technology
FPD detectors are commercially available with pixel size varying
from 400 microns to as low as 39 microns and sensor area 20cm x 20cm
to 35cm x 43cm. There is a one to one correspondence between the
size of the individual sensor and the out put image pixel. The
spatial resolution of the electronic image depends on the captured
signal profile and pixel size. Also the resulting digital image has
a gray level dynamic range of 12 to 16 bit which provides high
sensitivity for radiography application. The risks involved are
uncertainty in the life time due to lack of significant data and
the high investment as it is an emerging technology. Besides this
there remains a problem of dead pixels inherent to the production
process. Every panel shows a few pixels more or less do not work
and appears as black spots in the raw image. In most of the cases
software can eliminate the effect of dead pixels by means of the
offset image subtraction and filtering.
AMORPHOUS SILICON FPD
In this type of indirect FPD amorphous silicon (a-Si) photodiode
is deposited onto a two dimensional array of thin film transistor (TFT)
and a scintillator coating of cesium iodide (CsI) or Gadolinium
Oxysulphide (Gd2O2S:Tb) as the top layer.
When X-ray strikes the
scintillator, visible light photons are emitted with intensity
proportional to that of the incident radiation. These light photons
are converted into electric charge by the array of a-Si photodiodes
(Fig. 5). The charges collected at each detector is amplified and
quantized to a digital code value for that pixel by the underlying
readout electronics based on field-effect transistor (FET). In this
process the light emitted by the interaction can spread to adjacent
pixels due to scatter and reduce the spatial resolution. In the latest
systems structured scintillator crystals are grown over the a-Si
detectors to reduce the problem and could achieve image quality much
better than that of film radiograph.
AMORPHOUS SELENIUM FPD
The FPD based on amorphous Selenium (a-Se) is a direct X-ray imaging
system without the use of a scintillator layer.
The primary advantage
of the system is that the light scatter problem is totally avoided
due to direct conversion of X-ray into electronic charge.
The device employs a uniform and continuous a-Se layer deposited over
the two dimensional array of a-Si TFT for charge collection and FET
based readout electronics (Fig. 6). The a-Se FPD of 35cm x 43cm
area are commercially available with a pixel pitch of 139 microns.
In comparison to other types of FPD, the a-Se panel gives very high
modulation transfer function (MTF) and spatial resolution but its
application is limited to X-ray energy less than 150keV. The
system is mainly used for medical application.
COMPLEMENTARY METAL OXIDE SEMICONDUCTOR (CMOS) FPD
CMOS chips are used as detector component in FPD based digital
radiography.
These sensors are built into array of photodiodes
with chip image processing capability and acts as matrix addressed
photodiode arrays like a-Si panels. The top layer scintillator
screen converts the X-rays into light photons and the CMOS pixel
in direct contact with it converts the light into electric charge.
Each CMOS pixel is configured with its own amplifier and the on
chip resident control circuitry performs digital conversion and
data is directly sent to the computer. CMOS FPD systems with
spatial resolution upto 10 microns and panel area of 20cm x 20cm
have been reported (Fig. 7). It is claimed that the CMOS FPD
can be used for high energy X-ray applications also.
REAL-TIME DR SYSTEM
The FPD based digital radiography system installed and operationalised
at VSSC in 2003 for real-time digital radiography of space vehicle
components is shown in Fig.8. This real-time digital radiography
system is based on an a-Si FPD, which is integrated with a 450kV X-ray
machine and an indigenously developed four-axis object manipulator.
A 225 kV mini-focus X-ray machine of focal spot size 0.2mm is also
attached to the system for high resolution radiography and projection
radiographic inspection of small components like electronic devices.
This FPD is the first generation type with sensor size 400 microns
and sensor area 20cm × 20cm. The output image is of 512 × 512 pixel
with a gray level dynamic range of 16-bit. The scintillator coating
used in this FPD is Gd2O2S:Tb, so that it can be used for X-rays
upto 450KV. A shutter system with remote control is provided for
the X-ray machine to protect the FPD from direct X-ray exposure.
Necessary software is incorporated for image processing and analysis.
It is also linked with an indigenously developed Relational Data
Base Management System for NDE (RDBMS), so that the inspection
results of the large number of components routinely tested are easily
handled. The linearity of the detector response with respect to the
incident X-ray provides wide dynamic range for radiography (Fig.9).
The MTF studies showed that this FPD gives a spatial resolution of
2.5 line pairs per mm (Fig.8) which is sufficient for practical
radiographic NDT of solid propellant grains and other rocket
components. A few sample digital radiographs recorded in the
system are presented in Fig. 11 - 14.
CONCLUSION
A perspective of digital radiography technology and the performance
of the system operationalised are presented here. The flat panel
detector based real time digital radiography system is the best
alternative presently available to replace the conventional film
radiography. The system provides a very fast and cost effective
inspection for the critical components. The system with its digital
image processing support provides a very good tool for quantitative NDE.
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