Digital Radiography An Imaging Technique Engineering Essay

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Digital radiography is an imaging technique in which radiographic images are created using image capturing devices instead of photographic film. In traditional radiography, film serves as both detector and storage medium. But in digital radiography, detectors as only used as imaging device and a separate storage medium is used. It consists of mainly generation, processing, archiving and presentation of image. The biggest advantage of this technique is that it a system can be implemented which can store digital images and through the latest advancement in communications, these images can be made available anywhere and anytime without the risk of losing the images.

GENERAL PRINCIPLE

When the detector is exposed to radiation, the energy is absorbed by the detector. This absorbed energy is then converted into electrical impulses which are processed by an image processor to generate gray scale gradations that implies the amount of radiation energy absorbed at each area. The images are then sent to a storage media for archiving process. The archived image is accessible through any of the workstations connected to the network. This digital image gives the edge in inspection by allowing zooming, panning, inverting the gray scale and measuring dimensions. Hard copy of these images can also be made with the use of adequate printers but the highest efficiency is only achieved when the inspection id done by viewing the images on high definition display.

TYPES

Digital radiography can be classified based on the type of detector and image readout process used. The basic classifications are:

Computed Radiography (CR)

Direct Radiography

Direct Radiography can be further classified based on readout process. They are:

Direct conversion

Based on the type of detector being used, direct conversion can be divided into:

Selenium drum

Photoconductor-Flat Panel Detector (FPD)

Indirect conversion

Based on the type of detector being used, indirect conversion can be divided into:

Scintillator-Charged Couple Device (CCD)

Scintillator-Thin Film Transistor (TFT)

COMPUTED RADIOGRAPHY

Computed Radiography (CR) uses storage-phosphor imaging plates instead of conventional films. The detection process and readout process is separate in CR. The image plate (IP) is coated with a layer of photostimulable crystals. The crystal contains bromide, chlorine or iodine. IP is made of resin and usually the phosphor crystals are cast into the resin in an unstructured way. This IPs replaces the conventional films in cassette.

When exposed to X-ray, the energy is absorbed by the electrons within the phosphor crystals and moves to high energy levels. Based on the crystal used, the storage time changes. But it can be stored for several hours. Mostly, the readout process is started immediately after the exposure because the stored energy dissipates over time.

A separate readout process is required for CR. The exposed IP is scanned with laser beam. This laser beam causes the high energy electrons to relax and move to lower energy levels and the stored energy is emitted as light which is having a wavelength different from that of the scanning laser. This light is then detected by a photo-multiplier tube or photo diodes which converts it into electrical charges.

Implementation of CR is easy as it is cassette based and can be integrated into existing radiographic devices. But the resolution of CR is lower compared to conventional radiography.

DIRECT RADIOGRAPHY

Direct radiography(DR) is similar to CR in all aspects except for the fact that while CR uses a cassette with an IP to capture the energy and then later convert it to image using a separate readout process, DR directly captures the image onto a detector without the use of a cassette.

DIRECT CONVERSION

A photoconductor is used in direct conversion to convert X-ray into electrical charges. Amorphous selenium, lead oxide, thallium bromide and gadolinium compounds are used as the photoconductor materials. They are having high intrinsic special resolution which can help in achieving high good special resolution, pixel size and matrix of the resulting image. The resolution is limited only by the recording and readout devices being used in this method. The most commonly used photoconductor is amorphous selenium. Selenium has high X-ray absorption efficiency and good photoconductivity. This makes it the ideal to be part of imaging device that can produce electrical charges when exposed to X-ray. DR with selenium are either equipped with a selenium drum or a flat panel detector.

Selenium drum detectors use a rotating drum that is selenium dotted and an analog to digital convertor. The selenium drum gets charged when exposed to X-ray radiation. The charge of each particle depends on the amount of X-ray it is exposed to. The drum is kept on rotating and the charge is recorded by an analog to digital convertor. The images produced by selenium drum detectors are of higher quality compared to the conventional film and CR systems but the mechanical design causes glitches and hinders mobility.

Flat panel detectors use a panel coated with amorphous selenium. The selenium gets charged when exposed to X-ray just like in a drum detector but here the panel is stationary. A Thin Film Transistor (TFT) array is linked to the flat panels which can directly readout the image. The image quality is same as that of drum detector and this system is more mobile.

INDIRECT CONVERSION

In indirect conversion, the X-ray is converted into light by a scintillator and then this light is recorded by a Charged Couple Device (CCD) or an array of photo diodes. The most commonly used indirect conversion methods are CCD systems and TFT array.

CCD is a system consists of coupled capacitors. It is a light sensitive sensor. A scintillator such as TI-doped cesium iodide is used to convert the X-ray into light and this light. The CCD converts this light into electrical charges and it is recorded. Several CCD chips are combined to create larger detector areas. CCD systems can be either lens coupled or slot scan. Several CCD chips form a large area and the area of the projected light is reduced by the use of optical lenses in a lens coupled system. Because of this the number of photons reaching the CCD array is reduced resulting in lower signal-to-noise ratio. Slot scan system needs a special X-ray tube that creates a collimated fan-shaped beam. The CCD array will be moving along with the beam and will have a matching detector width. Lens coupled CCD systems are inferior to that slot system because of lower signal to noise ratio.

Some indirect detection methods use a TFT array for the readout of images. A sandwich construction is used in this case. It consists of a scintillator layer, an amorphous silicon layer and a TFT array. The scintillator converts the X-ray into visible light as in a CCD system, but here the amorphous silicon layer converts the light into electrical charges and it is readout by the TFT array. These are flat panel detectors. Almost real time results can be obtained by this kind of detectors. The delay is always less than 10 seconds. The image quality produced is far higher than all the other methods.

CONCLUSION

The digital radiographic techniques are a new frontier in the field of inspection. They not only reduce the time required for the inspection, but also give better image results. The consumables required are also less. Amount of X-ray required for getting the result is far less than that in conventional techniques. This creates both safety and economic advantages. The problem created in digital radiography is because of dead pixels. The dead pixels appear as black dots in the image and can be misinterpreted for defects. Software can be eliminated due to these dead pixels but eventually the detector has to be replaced which can be expensive.

The digital radiographic techniques creates images with high quality, the detectors have long life, low X-ray exposure required and the need for retake will be drastically reduced. Because of these techniques the efficiency of the inspection department will be increased exponentially. More images can be created within limited time and inspection can be done with software. Image distribution without the loss of data can be done easily.

Overall, digital radiography is the tomorrow in inspection field.

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