In ASME Sec-v Article 1 Digital Radiography is defined as follows
Digital radiography (DR): all radiography methods whereby image data is stored in a digital format.
Typical Conventional Film Based Radiography Process:
Above image shows a typical setup in conventional radiography where a radioisotope positioned in front of the test object is used as a source of radiation sometimes X-ray units are also used as a source of energy.
A loaded cassette is positioned at the back of the Test object. The film(unexposed) is made of a Polyester base with silver bromide and emulsion of gelatin coating bonded with the polyester base using an adhesive on both sides and is sandwiched between Lead Screens.
Depending upon the material and its density the film is exposed for a calculated period of time. When transmitted Radiation hits this film. The film gets exposed making a latent image of the test item and further processing of this film gives the permanent radiographic image.
This exposed film goes through the film development process and a visible permanent image is obtained.
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Several major disadvantages of the conventional film-based radiography include the following:
- Low efficiency leading to longer exposure times;
- Radiographic films are not reusable;
- Considerable film processing facilities are required, chemicals used are not environmentally friendly;
- Exposer to evolution time is high;
- Considerable time is required to develop the film and interpret the results;
- Workers are exposed to hazardous chemicals during film development;
- Storage of radiographs needs specific environment conditions ;
- There are storage and retrieval costs for radiographs after inspection;
- Radiographs are prone to aging, X-ray film deteriorates over time;
- Separate setup for interpretation and evaluation of Radiographs;
- Interpretation of radiographs is subjective;
- There is difficulty in full automation.
These disadvantages can be overcome by using Digital radiography
Analog Image: an image produced by a continuously variable physical process (for example, exposure of film).
Digital Image: an image composed of discrete pixels each of which is characterized by a digitally represented luminance level.
pixel: the smallest addressable element in an electronic image.
pixel size: the length and width of a pixel.
There are two types of Digital Radiography
- Computed radiography(CR)
- Cassette less Digital Radiography(DR)
Computed Radiography (CR) (photostimulated luminescence method)
In computed radiography, the same source of radiation (Gamma or X-ray) can be used with the same radiography techniques used in Conventional film-based radiography. The major change in the process is the Image recording medium which allows the use of all means of computer processing to produce a digital image with traditional film exposures using different digitizing methods.
It is a two-step radiographic imaging process.
- Step 1. a storage phosphor imaging plate is exposed to penetrating radiation;
- Step 2. the luminescence from the plate’s photostimulable luminescent phosphor is detected, digitized, and displayed on a monitor.
An imaging plate consists of flexible polymer support coated with a sensitive layer. On top, it is covered with a thin transparent protective layer. The sensitive layer of the most common systems consists of a mixture of BaFBr crystallites doped with europium and a binder. The ANTI-HALO layer prevents the layer from passing through and allowing the reflected light to pass through the phosphor.
X ray or gamma-ray radiation results in activation of F-centers in the crystallites, which result in the emission of blue light photons upon stimulation with red light photons through a process known as photostimulated luminescence. After exposed imaging plates have to be scanned by a laser scanner to obtain a digital image radiograph. The residual information stored in the F-centers can be erased by exposure to bright white light and the imaging plate can be reused up to 1000 times depending on the handling conditions.
The digital image radiograph obtained can be directly viewed on a work station. The workstation needs to be a high-speed computer so that it can process the data quickly using the specific software. Moreover, the monitor should be of a high resolution of 3.5 megapixels or above.
CR provides extra beneficial activities like digital film archiving, quantitative evaluation, image processing, automatic image evaluation, remote image transfer and production of reference catalogs for flaw evaluation.
SOME COMPARISONS FILM AND COMPUTED RADIOGRAPHY
Cassette-less Digital Radiography
As the name says they don’t involve any image plates or Cassettes. DR systems use imaging devices that remain in the original, natural, or existing place or position and produce an image with a delay that is generally no more than about 10 s. This system consists of sensors that send a digital image directly to a computer
The cassette-less digital radiography system can be further broken down into
- indirect conversion system
- direct conversion system/ non-scintillation system
Direct conversion system / Non-scintillation system
The system is comprised of an amorphous selenium-based semiconductor and a TFT (Thin flat-panel transistor). this system works by applying a high voltage charge to the top surface nanoseconds before the X-ray exposure is made. the interaction between the incident X-ray photons and the high voltage charge causes the selenium atoms to release their electrons, which are then to be collected and processed by the TFT Layer.
The TFT (Thin Flat Panel Transistor) layer is comprised of an array or matrix of very small detector elements (DELs). Each DEL is comprised of a capture element or pixel detector, a storage capacitor that stores the electric charge produced by a capture element and a thin film transistor or switch which opens and closes the release of electric charge leaving each Del, thus producing the digital image.
Indirect Conversion System
The indirect conversion of digital radiography systems is further broken down into two categories. This category is the thin flat panel transistor (TFT) and the charged coupling device (CDC) system.
Indirect TFT Systems
Indirect TFT systems are composed of three main layers.
- Scintillation layer/light layer on top- the incoming Incident X-rays photons gets converted into light photons. Each photon that strikes the scintillation layer creates a small burst of energy. The scintillation layer is comprised of ionic compound cesium iodide or chemical element gadolinium. However, cesium iodide is preferred over gadolinium since they can be formed into very small rods that produce a smaller and more focused light beam which increases the spatial resolution.
spatial resolution: the size of the smallest detectable element of the digitized image.
Scintillation: the process or state of emitting flashes of light.
- Photodiode layer in middle: it is comprised of semiconducting material known as amorphous silicon. This layer converts the incoming light photons from the scintillation layer into an electric charge.
- TFT Array layer at the bottom: the electric charge produced by the photodiode layer is transferred to a thin film transistor or TFT layer. It is comprised of an array or matrix of very small detector elements (DELs). Each Del Is comprised of a capture element or pixel detector, a storage capacitor that stores the electric charge Produced by a capture element and a thin film transistor or switch which opens and closes the release of electric Charge leaving each Del, thus producing the digital image.
CCD (charged coupled devices) System
In the CCD (charged coupled devices) system, there is no photocathode or TFT. The scintillation layers within CCD systems are optically coupled to each CCD sensor chip by lenses or optical fibers. the CCD is able to both convert light photons into electric signals as well as send the electrical signal to the computer for processing.
Advantages of digital radiographic systems (DIR)
- reduced radiation dosage and exposures, resulting in less risk to the operator and less disruption to other operations.
- It reduces radiographic inspection time and improves productivity.
- eliminates film processing chemicals, chemical disposal, and storage costs.
- Digital radiographs are not degradable.
- It is easily customized for field radiography in a portable package.
- It allows analysis using image processing and defect detection algorithms.
- Storage costs are minimized, as all images are stored on hard disks or optical media such as CD-ROMs or DVD-RAMs. Images can also be accessed via a network and even emailed to experts for real-time verification.
- Reusable imaging plates mean that savings can be generated, as one plate can be used many times. Significant cost savings from the use of DIR systems have been reported by industry. With the advancement of image intensifier systems, imaging plates, flat-panel detectors, and fast multimedia computers, DIR is finding increased applications.
IAEA RADIATION TECHNOLOGY REPORTS No. 2 Design, Development and Optimization of a Low-Cost System for Digital Industrial Radiology
ASME BPVC SEC-V
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