Introduction to digital radiography for process piping

Understanding Digital radiography

(Direct Radiography & Computed Radiography for Industrial Radiography of Process Piping) Babalal/ Charlie Chong March 18th 2017 Pengerang

Babalal/ Charlie Chong

Process Piping

Babalal/ Charlie Chong

Process Piping

Babalal/ Charlie Chong

Babalal/ Charlie Chong

Abstract Developments in digital detector technologies have been taking place and new digital technologies are available for clinical practice. This introduction is intended to give a technical state-of-the-art overview about computed radiography (CR) and digital radiography (DR) detectors.

CR systems use storage-phosphor image plates with a separate image readout process and DR technology converts X-rays into electrical charges by means of a readout process using TFT arrays with and without scintillator.

Digital detectors offer several advantages when compared to analogue detectors/ screen film systems. The knowledge about digital detector technology for use in plain radiograph examinations is thus a fundamental topic to be acquired by industrial radiographic professionals and inspectors.

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DR DR stands for Doctor?

DR stands for Digital Radiography?

DR stands for Direct Radiography?

DR stands for Digital Projection Radiography?

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Chart provides a systematic overview of various types of digital detectors. CCD = charge-coupled device, FPD = flat-panel detector, TFT = thin-film transistor.

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Why DR  No silver based film or chemicals are required to process film.  Reduced film storage costs because images can be stored digitally.  Computed radiography often requires fewer retakes due to under- or over-exposure which can result in lower overall dose to the patient, if you assume a moderate amount of retakes. CR can require up to 30% less dose than film.  Image acquisition is much faster - image previews can be available in less than 10 seconds.  By adjusting image brightness and/or contrast, a wide range of thicknesses may be examined in one exposure, unlike conventional film based radiography, which may require a different exposure or multiple film speeds in one exposure to cover wide thickness range in a component.  Images can be enhanced digitally to aid in interpretation.  Images can be stored on disk or transmitted for off-site review.  Ever growing technology makes the CR more affordable than ever today. With chemicals, dark room storage and staff to organize them, you could own a CR for the same monthly cost while being environmentally conscious, depending upon the size of the radiographic operation.

Babalal/ Charlie Chong

Why DR By adjusting image brightness and/or contrast, a wide range of thicknesses may be examined in one exposure, unlike conventional film based radiography, which may require a different exposure or multiple film speeds in one exposure to cover wide thickness range in a component. Images can be enhanced digitally to aid in interpretation.

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The Advantages of DR over Film Radiography  Lower Doses • Smaller Safety perimeters • Shorter plant shut-downs • Smaller isotopes / longer lifetime • Time efficiency for resources, plant shut down, higher throughput  Easily availability digital images and data  Data and images together on network – Easier and faster analysis of defects  Shorter exposure times ( 10-50 % D7 )  Higher Dynamic Range – Less retakes by bad exposure, different thicknesses in one shot …  Reusable Phosphor plates & No chemicals, no darkroom

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Digital Radiography or DR is an advancement of traditional Radiography. This technique utilizes DDAs (Digital Detector Arrays) instead of Film or CR (Computed Radiography) in order to create an instant Image. The Radiation reaches the DDA, which has passed through the object, converted by (1) a Scintillator into visible light in indirect methods or (2) a photon detector into photoelectric effects in direct methods and then translated into a digital Image. The physics (Angles, Penetration, technique etc.) remain similar and only mild changes are required to make the transition to Digital Radiography.

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Digital radiography is a form of X-ray imaging, where digital X-ray sensors are used instead of traditional photographic film. Advantages include time efficiency through bypassing chemical processing and the ability to digitally transfer and enhance images. Also, less radiation can be used to produce an image of similar contrast to conventional radiography.

Instead of X-ray film, digital radiography uses a digital image capture device. This gives advantages of immediate image preview and availability; elimination of costly film processing steps; a wider dynamic range, which makes it more forgiving for over- and under-exposure; as well as the ability to apply special image processing techniques that enhance overall display quality of the image.

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DIRECT RADIOGRPHY Direct Conversion Indirect Conversion

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Direct Radiography’s Flat Panel Detectors

Flat-panel structure

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Direct Radiography’s Flat Panel Detectors Flat panel detector used in digital radiography. FPDs are further classified in two main categories: 1.

Indirect FPDs Amorphous silicon (a-Si) is the most common material of commercial FPDs. Combining a-Si detectors with a scintillator in the detector’s outer layer, which is made from Caesium iodide (CsI) or Gadolinium oxysulfide (Gd2O2S), converts X-rays to light. Because of this conversion the a-Si detector is considered an indirect imaging device. The light is channeled through the a-Si photodiode layer where it is converted to a digital output signal. The digital signal is then read out by thin film transistors (TFTs) or fiber-coupled CCDs. The image data file is sent to a computer for display.

2.

Direct FPDs. Amorphous selenium (a-Se) FPDs are known as “direct” detectors because X-ray photons are converted directly into charge. The outer layer of the flat panel in this design is typically a high-voltage bias electrode. X-ray photons create electron-hole pairs in a-Se, and the transit of these electrons and holes depends on the potential of the bias voltage charge. As the holes are replaced with electrons, the resultant charge pattern in the selenium layer is read out by a TFT array, active matrix array, electrometer probes or microplasma line addressing.

Babalal/ Charlie Chong

1.Indirect FPDs Amorphous silicon (a-Si) is the most common material of commercial FPDs. Combining a-Si detectors with a scintillator in the detector’s outer layer, which is made from Caesium iodide (CsI) or Gadolinium oxysulfide (Gd2O2S), converts X-rays to light. Because of this conversion the a-Si detector is considered an indirect imaging device. The light is channeled through the a-Si photodiode layer where it is converted to a digital output signal. The digital signal is then read out by thin film transistors (TFTs) or fiber-coupled CCDs. The image data file is sent to a computer for display.

Note: X ray → CsI/ Gd2O2S (light) → a-Si → TFT Array

Keywords: Scintillator a-Si detector - TFT The best known application of thin-film transistors is in TFT LCDs, an implementation of LCD technology. Transistors are embedded within the panel itself, reducing crosstalk between pixels and improving image stability.

As of 2008, many color LCD TVs and monitors use this technology. TFT panels are frequently used in digital radiography applications in general radiography. A TFT is used in both direct and indirect capture[jargon] as a base for the image receptor in medical radiography.

AMOLED (active-matrix organic light-emitting diode) screens also contain a TFT layer.

The most beneficial aspect[neutrality is disputed] of TFT technology is its use of a separate transistor for each pixel on the display. Because each transistor is small, the amount of charge needed to control it is also small. This allows for very fast re-drawing of the display. https://en.wikipedia.org/wiki/Thin-film_transistor

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Indirect Conversion with a CCD. A CCD is a light-sensitive sensor for recording images that consists of an integrated circuit containing an array of linked or coupled capacitors. X-ray energy is converted into light by a scintillator such as Tl-doped cesium iodide. The amount of light emitted is then recorded by the CCD, and the light is converted into electrical charges. Because the detector area cannot be larger than the CCD chip, it is necessary to combine several chips to create larger detector areas.

CCDs can be used for radiography as part of either a lens-coupled CCD system or a slot-scan CCD system. In lens-coupled CCD systems, an array consisting of several CCD chips forms a detector area similar to that of a flat-panel detector. Optical lenses are needed to reduce the area of the projected light to fit the CCD array (Fig 5a). One drawback of the lens system is a decrease in the number of photons reaching the CCD, resulting in a lower signal-to-noise ratio and relatively low quantum efficiency.

Slot-scan CCD systems make use of a special x-ray tube with a tungsten anode. The patient is scanned with a collimated fan-shaped beam, which is linked to a simultaneously moving CCD detector array having a matching detector width (Fig 5b). The combination of a small collimated beam and a concordant detector reduces the impact of scattered radiation in the image, since much of this radiation will escape without detection. In addition, the relatively low quantum efficiency of slot-scan CCD systems, which is comparable to that of CR systems, can be offset by the resulting lower image noise. The exposure time to the patient is about 20 msec, and the readout process takes about 1.3 seconds. Because of the need for fixed installation, slot-scan CCD systems are dedicated to chest radiography, mammography, or dental radiography.

Studies dealing with CCD-based digital general radiography are rare. Phantom studies have been conducted to investigate slot-scan CCD systems and compare them with screen-film combinations and various digital detectors. In all of these studies, CCD-based systems were comparable to flat-panel detectors in terms of image quality and allowed slightly superior low-contrast visualization. Clinical studies performed with slot-scan detectors are mainly concentrating on applications in mammography and digital dental radiography. The performance of lens-coupled CCD systems is somewhat inferior to that of slot-scan systems because of their technical principle, substantially lower quantum efficiency, and lower signal-to-noise ratio.

Babalal/ Charlie Chong

http://pubs.rsna.org/doi/full/10.1148/rg.273065075

Fig 5. CCD-based indirect conversion DR system. (a) Drawing illustrates a lens-coupled CCD-based system.

The incident x-ray energy is converted into light by a scintillator. The emitted light has to be bundled by an optical lens to fit the size of the CCD chip, which subsequently converts the light energy into electrical charges. (b) Drawing illustrates a slot-scan CCD-based system. The patient is scanned with a fan-shaped beam of xrays. A simultaneously moving CCD detector of the same size collects the emitted light and converts the light energy into electrical charges.

Babalal/ Charlie Chong

http://pubs.rsna.org/doi/full/10.1148/rg.273065075

Fig 5a. CCD-based indirect conversion DR system. (a) Drawing illustrates a lens-coupled CCD-based system.

The incident x-ray energy is converted into light by a scintillator. The emitted light has to be bundled by an optical lens to fit the size of the CCD chip, which subsequently converts the light energy into electrical charges. (b) Drawing illustrates a slot-scan CCD-based system. The patient is scanned with a fan-shaped beam of xrays. A simultaneously moving CCD detector of the same size collects the emitted light and converts the light energy into electrical charges.

Babalal/ Charlie Chong

http://pubs.rsna.org/doi/full/10.1148/rg.273065075

Indirect Conversion with a Flat-Panel Detector.

Indirect conversion DR systems are “sandwich” constructions consisting of a (1) scintillator layer, (2) an amorphous silicon photodiode circuitry layer, and (3) a TFT array.

When x-ray photons reach the scintillator, visible light proportional to the incident energy is emitted and then recorded by an array of photodiodes and converted to electrical charges. These charges are then read out by a TFT array similar to that of direct conversion DR systems (Fig 6).

Drawing illustrates an amorphous silicon–based indirect conversion DR system. X-ray energy is converted into visible light in a scintillator layer. The emitted light is then converted into electrical charges by an array of silicon-based photodiodes and read out by a TFT array.

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The scintillators usually consist of CsI or Gd2O2S. Gd2O2S crystals are cast into a binding material and are unstructured scintillators having a structure similar to that of storage phosphors.

The advantage of CsI-based scintillators is that the crystals can be shaped into 5–10-Οm-wide needles, which can be arranged perpendicular to the surface of the detector. This structured array of scintillator needles reduces the diffusion of light within the scintillator layer (5,39,40). As a result, thicker scintillator layers can be used, thereby increasing the strength of the emitted light and leading to better optical properties and higher quantum efficiency.

One further advantage of flat-panel detectors is their small size, which allows integration into existing bucky tables or thorax stands. Because CsI-based flat-panel detectors are highly vulnerable to mechanical load because of their fine structure, these systems cannot be used outside of fixed installations and therefore lack mobility. Portable flat-panel detector systems make use of Gd2O2S-based scintillators, which are as resistant to mechanical stress as are storage phosphors. Any defects that occur in the detector may cause a complete breakdown of the imaging system, making contingency imaging devices necessary.

Image generation with flat-panel detectors is almost a real-time process, with a time lapse between exposure and image display of less than 10 seconds. Consequently, these systems are highly productive, and more patients can be examined in the same amount of time than with other radiographic devices.

Many clinical studies have shown indirect conversion flat-panel detectors to provide superior image quality.

Studies comparing indirect conversion flat-panel detectors with conventional screen-film combinations, storagephosphor image plates, or other digital detectors have verified that flat-panel detectors offer the best image quality and low-contrast performance of all digital detectors and, so far, are superior to conventional screen-film combinations.

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Indirect FPDs Amorphous silicon

Caesium iodide (CsI) or Gadolinium oxysulfide (Gd2O2S)

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http://www.slac.stanford.edu/econf/C0604032/talks/SNIC_Roos.pdf

Babalal/ Charlie Chong

http://4nsi.com/assets/files/trc-pdfs/2009/Dr.%20Fahrad%20Ghelmansarai.pdf

Babalal/ Charlie Chong

http://4nsi.com/assets/files/trc-pdfs/2009/Dr.%20Fahrad%20Ghelmansarai.pdf

Babalal/ Charlie Chong

http://4nsi.com/assets/files/trc-pdfs/2009/Dr.%20Fahrad%20Ghelmansarai.pdf

Babalal/ Charlie Chong

http://4nsi.com/assets/files/trc-pdfs/2009/Dr.%20Fahrad%20Ghelmansarai.pdf

Babalal/ Charlie Chong

http://pocketdentistry.com/6-direct-digital-imaging/

Babalal/ Charlie Chong

https://www.slideshare.net/islamkotb/medical-x-ray-image-sensors-1

GdO2S2 amorphous structure

Cesium Iodide CsI columnar structure

Electron microscope images of scintillators used in indirect conversion of x-rays; a) GdO2S2 amorphous structure; b) CsI columnar structure. The amorphous structure of the GdO2S2 and columnar structure of the CsI can be clearly seen. (Courtesy of Varian X-ray Products).

Babalal/ Charlie Chong

http://pocketdentistry.com/6-direct-digital-imaging/

Examples of indirect conversions using a scintillator and a thin film transistor (TFT) array; a) GdO2S2 scintillator; b) columnar CsI scintillator.

Babalal/ Charlie Chong

http://pocketdentistry.com/6-direct-digital-imaging/

Examples of indirect conversions using a scintillator and a thin film transistor (TFT) array; a) GdO2S2 scintillator; b) columnar CsI scintillator.

Babalal/ Charlie Chong

http://pocketdentistry.com/6-direct-digital-imaging/

Cross-section of an indirect TFT detector using CsI structured phosphor shows the conversion of X-rays first into light, traveling through the structured phosphor to a photodiode etched on the TFT array, and the creation of a proportional charge stored in the local capacitor Babalal/ Charlie Chong

http://pocketdentistry.com/6-direct-digital-imaging/

a-Si flat panel with read-out electronics

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https://www.slideshare.net/islamkotb/medical-x-ray-image-sensors-1

a-Si detector after housing

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https://www.slideshare.net/islamkotb/medical-x-ray-image-sensors-1

Main properties of scintillators influencing in imaging system performance are: a) b) c) d)

Conversion Efficiency X-ray quantum efficiency Light conversion per X-ray photon MTF / spatial resolution Light output spectrum Light decay / after glow

CsI:Tl and Gd2o2S:Tb (Gadox) are mainly used in the FPDs.

Babalal/ Charlie Chong

2.Direct FPDs. Amorphous selenium (a-Se) FPDs are known as “direct” detectors because X-ray photons are converted directly into charge. The outer layer of the flat panel in this design is typically a high-voltage bias electrode. X-ray photons create electron-hole pairs in a-Se, and the transit of these electrons and holes depends on the potential of the bias voltage charge. As the holes are replaced with electrons, the resultant charge pattern in the selenium layer is read out by a TFT array, active matrix array, electrometer probes or microplasma line addressing.

Note: X ray → a-Se (Charge) → TFT Array

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Figure 4. Amorphous selenium–based direct conversion DR systems. (a) Drawing illustrates a selenium drum–based system. A rotating selenium-dotted drum with a positive electrical surface charge is exposed to x-rays. Alteration of the charge pattern of the drum surface is proportional to the incident x-rays. The charge pattern is then converted into a digital image by an analog-to-digital (A/D) converter. (b) Drawing illustrates a selenium-based flat-panel detector system. Incident x-ray energy is directly converted into electrical charges within the fixed photo-conductor layer and read out by a linked TFT array beneath the detective layer.

Babalal/ Charlie Chong

http://pubs.rsna.org/doi/full/10.1148/rg.273065075

Direct Conversion.

Direct conversion requires a photoconductor that converts x-ray photons into electrical charges by setting electrons free. Typical photoconductor materials include amorphous selenium, lead iodide, lead oxide, thallium bromide, and gadolinium compounds. The most commonly used element is selenium.

All of these elements have a high intrinsic spatial resolution. As a result, the pixel size, matrix, and spatial resolution of direct conversion detectors are not limited by the detector material itself, but only by the recording and readout devices used.

Selenium-based direct conversion DR systems are equipped with either a selenium drum or a flat-panel detector. In the former case, a rotating selenium-dotted drum, which has a positive electrical surface charge, is exposed to x-rays. During exposure, a charge pattern proportional to that of the incident x-rays is generated on the drum surface and is recorded during rotation by an analog-to-digital converter (Fig 4a).

Amorphous selenium–based direct conversion DR systems. (a) Drawing illustrates a selenium drum–based system. A rotating selenium-dotted drum with a positive electrical surface charge is exposed to x-rays. Alteration of the charge pattern of the drum surface is proportional to the incident x-rays. The charge pattern is then converted into a digital image by an analog-todigital (A/D) converter. (b) Drawing illustrates a selenium-based flat-panel detector system.

Incident x-ray energy is directly converted into electrical charges within the fixed photoconductor layer and read out by a linked TFT array beneath the detective layer.

Babalal/ Charlie Chong

http://pubs.rsna.org/doi/full/10.1148/rg.273065075

Several clinical studies have confirmed that selenium drum detectors provide good image quality that is superior to that provided by screen-film or CR systems. However, because of their mechanical design, selenium drum detectors are dedicated thorax stand systems with no mobility at all.

A newer generation of direct conversion DR systems make use of selenium-based flat-panel detectors. These detectors make use of a layer of selenium with a corresponding underlying array of thin-film transistors (TFTs).

The principle of converting x-rays into electrical charges is similar to that with the selenium drum, except that the charge pattern is recorded by the TFT array, which accumulates and stores the energy of the electrons (Fig 4b).

One advantage of these systems is greater clinical usefulness, since the detectors can be mounted on thorax stands and bucky tables. To date, there have been only a few clinical studies conducted with selenium-based flat-panel detectors. However, these studies indicate that the image quality provided by selenium-based flatpanel detectors is equivalent to that provided by other flat-panel detectors and selenium drum detectors.

Another promising clinical application of selenium-based flat-panel detectors is in the field of mammography (33).

Babalal/ Charlie Chong

http://pubs.rsna.org/doi/full/10.1148/rg.273065075

Direct FPDs. Amorphous selenium (a-Se)

Typical photoconductor materials include amorphous selenium, lead iodide, lead oxide, thallium bromide, and gadolinium compounds. The most commonly used element is selenium.

Babalal/ Charlie Chong

http://www.slac.stanford.edu/econf/C0604032/talks/SNIC_Roos.pdf

Direct FPDs. Amorphous selenium (a-Se)

Amorphous selenium–based direct conversion DR systems. (a) Drawing illustrates a selenium drum–based system. A rotating selenium-dotted drum with a positive electrical surface charge is exposed to x-rays. Alteration of the charge pattern of the drum surface is proportional to the incident x-rays. The charge pattern is then converted into a digital image by an analog-to-digital (A/D) converter. (b) Drawing illustrates a selenium-based flat-panel detector system. Incident x-ray energy is directly converted into electrical charges within the fixed photo-conductor layer and read out by a linked TFT array beneath the detective layer.

Babalal/ Charlie Chong

http://pubs.rsna.org/doi/full/10.1148/rg.273065075

Indirect a-Si FPD construction was needed due to inefficient absorption of xrays by silicon. But some materials like Selenium have much better stopping power. Se also has photo-conductive properties. This resulted in construction of Direct type FPD. • • • • • • • •

On absorption of x-ray energy, a-Se produces charge pairs.

No separate scintillator is required.

With a bias voltage, electron and holes travel to respective electrodes.

TFT structure is similar to indirect type and storage capacitor for each pixel is provided.

Lateral spread of light is absent for a-Se FPD (there is no scintillator). The resolution is determined by pixel geometry.

Se has a limited stopping power and therefore it mainly used for low energy X-ray (up to about 100 – 150 kV).

At higher X-ray energy, Se layer thickness would become impractically thick. Hence, alternative materials like CdTe, HgI2 and PbI2 are being explored as they have better stopping power.

Babalal/ Charlie Chong

http://4nsi.com/assets/files/trc-pdfs/2009/Dr.%20Fahrad%20Ghelmansarai.pdf

Direct Conversion With direct exposure sensors there is no scintillator. Instead, the x-rays are directly converted to a charge in the material through the photoelectric effect. This occurs in both aSi and aSe, although the conversion is not very efficient in the former.

Currently, the material of choice for direct conversion is aSe. X-ray exposure produces a charge (positive and negative) in the selenium, the size of which is proportional to the quantity of xrays incident upon it. By placing a high voltage (kilovoltage) across the aSe, these charges can be moved to the surface to produce an electric current. This current is stored in a matrix of discrete capacitors that can then be read.

Babalal/ Charlie Chong

http://www.slac.stanford.edu/econf/C0604032/talks/SNIC_Roos.pdf

Direct FPDs. Amorphous selenium (a-Se)

Babalal/ Charlie Chong

http://pocketdentistry.com/6-direct-digital-imaging/

Babalal/ Charlie Chong

https://www.dovepress.com/energy-resolved-x-ray-detectors-the-future-of-diagnostic-imaging-peer-reviewed-fulltext-article-RMI

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https://www.studyblue.com/notes/note/n/chapter-3-projection-radiography-i/deck/6202111

Taxonomy of digital radiography technologies

TFT Array

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TFT Array

Computed RADIOGRPHY

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Computed Computed Radiography Cassette Based Computed Radiography utilizes a reusable plate high-density line-scan solid state detector with photostimulable phosphor in place of the film.

After X-ray exposure the plate (sheet) is placed in a special scanner where the latent formed image is retrieved point by point and digitized, using laser light scanning. The digitized images are stored and displayed on the computer screen.

This method is halfway between old film-based technology and current direct digital imaging technology. It is similar to the film process because it involves the same image support handling but differs in that the chemical development process is replaced by scanning. This is not much faster than film processing and the resolution and sensitivity performances are contested. PSP has been described as having an advantage of fitting within any pre-existing equipment without modification because it replaces the existing film; however, it includes extra costs for the scanner and replacement of scratched plates.

Note: CR = Computed Radiography = Cassette Radiography

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The high-density line-scan solid state detector is composed of a photostimulable barium fluorobromide doped with europium (BaFBr:Eu) or caesium bromide (CsBr) phosphor. The phosphor detector records the X-ray energy during exposure and is scanned by a laser diode to excite the stored energy which is released and read out by a digital image capture array of a CCD.

photostimulable phosphor

Babalal/ Charlie Chong

https://quizlet.com/14410560/chapter-15-computed-radiography-flash-cards/

Drawing illustrates a CR system based on storage-phosphor image plates. Image generation is separated into two steps. First, the image plate (IP) is exposed to x-ray energy, part of which is stored within the detective layer of the plate. Second, the image plate is scanned with a laser beam, so that the stored energy is set free and light is emitted. An array of photomultipliers collects the light, which is converted into electrical charges by an analog-to-digital (A/D) converter.

Babalal/ Charlie Chong

http://pubs.rsna.org/doi/full/10.1148/rg.273065075

Note: Computed radiography (CR) uses very similar equipment to conventional radiography except that in place of a film to create the image, an imaging plate (IP) made of photostimulable phosphor is used. The imaging plate is housed in a special cassette and placed under the body part or object to be examined and the x-ray exposure is made. Hence, instead of taking an exposed film into a darkroom for developing in chemical tanks or an automatic film processor, the imaging plate is run through a special laser scanner, or CR reader, that reads and digitizes the image. The digital image can then be viewed and enhanced using software that has functions very similar to other conventional digital image-processing software, such as contrast, brightness, filtration and zoom.

Computed radiography (CR) is often distinguished from Direct Radiography (DR). CR and DR have many similarities. Both CR and DR use a medium to capture x-ray energy and both produce a digital image that can be enhanced for soft copy diagnosis or further review. Both CR and DR can also present an image within seconds of exposure. CR generally involves the use of a cassette that houses the imaging plate similar to traditional film-screen systems, whereas DR typically captures the image directly onto a flat panel detector without the use of a cassette.

Image processing or enhancement can be applied on DR images as well as CR images due to the digital format of each. There are many different types of DR detectors in use in medicine and industry. Each type has its own merits and distinctions and may be applied to certain imaging requirements based on these attributes

Babalal/ Charlie Chong

CR systems make use of image plates having a detective layer of photostimulable crystals that contain different halogenides such as bromide, chlorine, or iodine (e.g. BaFBr:Eu2+). The phosphor crystals are usually cast into plates into resin material in an unstructured way (unstructured scintillators). Image plates replace the conventional films in the cassette.

The exposure process with storage-phosphor image plates is illustrated in Figure 3. During exposure, x-ray energy is absorbed and temporarily stored by these crystals by bringing electrons to higher energy levels. In this way, x-ray energy can be stored for several hours, depending on the specific physical properties of the phosphor crystals used. However, the readout process should start immediately after exposure because the amount of stored energy decreases over time.

The readout process is a separate step that follows exposure of the image plate (Fig 3). When the detective layer is scanned pixel by pixel with a high-energy laser beam of a specific wave length (flying-spot scanner), stored energy is set free as emitted light having a wave length different from that of the laser beam. This light is collected by photodiodes and converted digitally into an image. The whole readout process for a 14 × 17-inch image plate takes about 30–40 seconds. Thus, a maximum workload of 90–120 image plates per hour is theoretically possible.

The advantages of storage-phosphor systems include a wide dynamic range, which leads to reduced rates of failed x-ray exposure. Because CR systems are cassette based, they can easily be integrated into existing radiographic devices, are highly mobile, and are easy to use for bedside examinations and immobile patients, making these systems flexible in routine clinical use. Furthermore, if a single image plate shows defects, it can easily be replaced by the radiographer with no need for specialized equipment or service personnel.

Spatial resolution with storage-phosphor image plates is usually lower than that with conventional screen-film combinations. However, several studies have shown that the diagnostic value of storage-phosphor radiography is at least equivalent to that of screen-film radiography. Still, compared with more modern digital detectors (eg, flat-panel detectors), storage-phosphor plates tend to be inferior in terms of image quality and diagnostic value, depending on the developmental stage of the storage-phosphor system being investigated.

Babalal/ Charlie Chong

http://pubs.rsna.org/doi/full/10.1148/rg.273065075

Drawing illustrates a CR system based on storagephosphor image plates.

Image generation is separated into two steps.

First, the image plate (IP) is exposed to x-ray energy, part of which is stored within the detective layer of the plate.

Second, the image plate is scanned with a laser beam, so that the stored energy is set free and light is emitted.

An array of photomultipliers collects the light, which is converted into electrical charges by an analog-todigital (A/D) converter.

Babalal/ Charlie Chong

Babalal/ Charlie Chong

Babalal/ Charlie Chong

http://www.acutech.gr/media/pdf/Succesful%20conversion%20from%20film%20to%20CR.pdf

DR Versus CR DR stands for Direct Radiography?

CR stands for Computed Radiography?

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Computed Radiography and Digital Radiography: A Comparison

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Computed Radiography and Digital Radiography: A Comparison Computed Radiography (CR) The Pros  CR costs significantly less than DR in every aspect. From equipment, install, and ongoing servicing.  CR units are compatible with many different X-Ray devices; they are quite versatile in that they’re able to convert many analog units.  CR’s can render an image digitally in less than a minute, and once it is done, storage is simple and space efficient.  Least labor intensive of X-Ray conversions.

The Cons  Much slower than DR.  Compared with more modern digital detectors (e.g. flat-panel detectors), storage-phosphor plates tend to be inferior in terms of image quality and diagnostic value, depending on the developmental stage of the storage-phosphor system being investigated

Digital Radiography (DR) The Pros  Digital is replacing analog at an exponentially fast rate, and eventually, it will be harder for those with analog systems to find parts and to find people that are trained to use them.  If you replace your analog with DR, you most likely won’t even need to remodel your X-Ray space.  DR’s can render an image in about 5 seconds.  New systems sometimes come with software that is far more advanced than analog.

The Cons  Far more expensive than CR.  Labor intensive, meaning that install can take up to four days to complete.

Babalal/ Charlie Chong

Fluoroscopy, CR & DR CR and DR should not be confused with fluoroscopy, where there is a continuous beam of radiation, and the images appear on the screen like on a TV. This is the system many people are familiar with, where the image of the article being x-rayed is viewed in real time on a monitor or display. Fluorosopes until recently have used a device called an image intensifier to enhance the analog output of the real time x-ray image from a fluorescent screen, viewing the output with a video or CCD camera and digitally enhancing the video to reduce the noise inherent in the system; the latest fluroscopes now use flat detectors read at up to 60 frames per second to yield a realtime image via a dedicated imaging computer.

Babalal/ Charlie Chong

Digital Radiography Quality Control

Babalal/ Charlie Chong

Quality Controls -Light scatter within the II reduces contrast in the image. Shown is an image of a radioopaque lead disk and a substantial signal underneath, as illustrated by the digital horizontal profile values plotted across the image through the center of the disk. Ideally, the signal would be at zero, while in the figure a value of about 60 occurs. This has a deleterious impact on image contrast and quantitative accuracy. Vignetting is also illustrated; it is the fall-off of light intensity at the periphery of the image relative to the center, caused by light scatter loss at the edge of the image

Vignetting

Babalal/ Charlie Chong

Geometric Effects

Babalal/ Charlie Chong

Geometric Effects

Babalal/ Charlie Chong

Geometrical Distortion Comparison of geometrical distortion and brightness non- uniformity between a 12 inch image intensifier and 8 inch flat panel detector.

Babalal/ Charlie Chong

More Reading Flat-panel detectors: how much better are they? https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2663651/ Best Practices in Digital Radiography https://www.asrt.org/docs/default-source/publications/whitepapers/asrt12_bstpracdigradwhp_final.pdf

Advances in Digital Radiography: Physical Principles and System Overview

http://pubs.rsna.org/doi/full/10.1148/rg.273065075

Babalal/ Charlie Chong

DR Images

Babalal/ Charlie Chong

DR Images

Babalal/ Charlie Chong

DR Images

Babalal/ Charlie Chong

CCD- Charged Coupled Device

Babalal/ Charlie Chong

Babalal/ Charlie Chong