Computed radiography

A general introduction to Computed Radiography to Radiography Students and Radiology Resident or those interested in Medical Imaging.

1. Anjan Dangal B.Sc.MIT 2 nd Year National Academy of Medical Sciences, Bir Hospital, Kathmandu, Nepal

2. Introduction It is an Conventional Projection Radiography in which the image is acquired in digital format using the Imaging Plate Rather than film.

3. Historical Development CT, MRI and Ultrasound were all digital since their introduction. But General Radiography remained Analog. In 1981, at International Conference of Radiology at Brussels, Fuji presented the concept of CR employing Photostimulable Phosphor ( special luminescent material that stores x ray energy and emits light proportional to the stored x ray energy when stimulated by the energy such as visible light from a laser) In 1983, CR first Clinically used in Japan.

4. CR Cassette Looks like conventional Radiography cassette. Made of durable light weight plastic material Backed by thin sheet of Aluminium that absorbs x ray Instead of Intensifying Screen inside, there is antistatic material that protects against static electricity build up, dust collection and mechanical damage to plate.

5. Imaging Plate In CR, Image in captured in thin sheet of plastic known as imaging Plate. Has several layers: Protective layer Phosphor/ Active layer Reflective layer Conductive layer Color layer Support layer Backing layer

6. Protective Layer : thin, tough, clear plastic that protects the phosphor layer. Phosphor/ Active layer : This is a layer of photostimulable phosphor that“traps ” electrons during exposure. It is usually made of phosphors from the barium fluorohalide family (e.g., barium fluorohalide , chlorohalide , or bromohalide crystals ). This layer may also contain a dye that differentially absorbs the stimulating light to prevent as much spread as possible and functions much the same as dye added to conventional radiographic screens .

7. Reflective layer. This is a layer that sends light in a forward direction when released in the cassette reader. This layer may be black to reduce the spread of stimulating light and the escape of emitted light . conductive layer. This is a layer of material that absorbs and reduce static electricity. color layer. Newer plates may contain a color layer, located between the active layer and the support, that absorbs the stimulating light but reflects emitted light. A support layer. This is a semi rigid material that gives the imaging sheet some strength . A backing layer. This is a soft polymer that protects the back of the cassette .

8. Doping Barium Flurohallide with Europium When Pure crystals of Barium flurohallide are dopted with small amount of Europium, Crystals develop a tiny defect called metastable sites of F center.( From German Farbzentren or Color center). F centres acts like electronic holes in the crystals that can trap electrons.

9. Acquiring and forming the latent Image Remnant beam interacts with the electron in the barium flurohallide crystals within IP. Gives energy to the electron in the crystal, allowing them to enter in conductive layer, where they are trapped in the area in the cystal known as color or phosphor center. The trapped signal will remain for hour or even for a day.* According to the American Association of Medical Physicist in Medicine, it will lose about 25% of the stored signal between 10 min and 8 hrs after an exposure resulting in the loss of the energy through spontaneous phosphorescence .

10. The Reader No chemicals and dark room Necessary. Cassette is fed into reader. Removes IP and scans plate with the laser to release the stored energy.

11. Laser in Imaging Plate The extracted imaging plate is scanned with red laser and gives energy to trapped electron and energizes them. Extra energy allows the trapped energy to escape active layer where they emit blue light at energy of 3eV. Laser scans IP multiple times. Scanning produces lines of light intensity information that are detected by photomultiplier tube that amplifies and send it to digitizer. Typical Through output is 50/hr.

12. Imaging Plate Movement

13. PhotoMultiplier Tube When the laser beam in the CR reader stimulates the PSP plate, the visible light emitted from it is directed through a light-channeling guide onto a photocathode layer on the input side of a PM tube. electrons are emitted from the photocathode . But, this electronic signal is too small to be detected by most other types of electronic devices, so it needs to be amplified . This is accomplished through a series of dynode plates.

14. Dynodes Plates. Dynodes are electrodes which can be switched back and forth between positive and negative charges . With each strike, the dynode plate switches from positive to negative, repelling the stream of electrons toward the next plate, but with increasing effect such that the number of electrons in the stream is multiplied . Each collision of an incident electron with a dynode plate releases about 5 electrons from the plate. By having the electron beam pass through 10–12 of these dynodes in succession, the electronic signal can be amplified by more than a million times.

15. Digitizing the Signal It is assigning the numerical value to each light photon. Each phosphor storage is scanned and the electrons eneters the digitizer where the analog image is converted into square matrix and assigns each matrix a number based on the amount of signal . More Pixel= greater resolution. No of Photons detected determines the Grey level. Grey level will determine the Quality of Image.

16. Erasing Not all electrons have came back to the initial phase after reading. System automatically erase the plate by flooding it with light to remove any electron still trapped after initial plate reading.

17. Dynamic range Excellent linear response to the intensity of X-ray. Informations available in high and low exposure regions too. Wider

18. Wider Dyanamic Range

19. Spatial Resolution Amount of detail present in any image is known as spatial resolution. Phosphor layer thickness and Pixel size determines Film Screen Radiography 10 lp /mm and CR 2.5-5 lp /mm

20. Speed CR system “speeds” are a reflection of the amount of photostimulable luminescence (PSL) given off by the imaging plate while being scanned by the laser. For example, Fuji Medical Systems reports that a 1-mR exposure at 80 kVp and a source-to-image distance of 72 inches will result in a luminescence value of 200, hence the “speed” number. In CR, most cassettes have the same “speed”; however, there are special extremity or chest cassettes that produce greater resolution. These are typically 100 relative “speed.” Great care must be taken when converting to a CR system from a film/screen system to adjust technical factors to reflect the new “speed.”

21. Advantage of CR Improved diagnostic and expanded diagnostic scope. X ray dosage reduction. Repeat rate reduction Teleradiographic transmission Picture Archival and Communication System possible.

22. Recent Development in CR Dual Sided reading : there are two sets of detectors in the reader that capture light from both sides of the plate upon stimulation by the laser beam. Combined with a thicker phosphor layer on the plates, the signal to-noise ratio is improved in these systems . New Line scan Reader: have increased the speed of processing. They use a laser line source and a shaping lens to refine the beam into a fine line rather than a point. Stimulated light is then emitted from the phosphor line-by-line, captured by a lens array and fed to a CCD photo detector array . many newer CR readers now use solid-state semiconductor laser diodes rather than helium-neon gas lasers . These emit a slightly different wavelength of light, but are more reliable and consistent in the long run.

23. Efficiency of CR Phosphors Any receptor system using a phosphor or scintillation layer to convert x-rays into light must be good at absorbing x-rays, converting their energy into light and emitting that light efficiently . Absorption efficiency is the ratio of x-ray photons absorbed by the phosphor crystals to the x-ray photons incident upon the phosphor layer . Elements with high atomic numbers, thicker phosphor layer are good in Absorption. The more x-rays are absorbed by the phosphor layer, the more light is emitted when the plate is stimulated by a laser beam in the digital reader (processor ). Conversion efficiency is the percentage of energy from absorbed x-ray photons that is converted into light rather than into infrared or heat energy. Emission efficiency is the ability of the light produced by the phosphor crystals to escape the phosphor layer and reach the light guides in the CR reader that direct it to the light detector, a photomultiplier tube.

24. Background and Scatter radiation A pproximately 10 times more sensitive than the older film/screen.

25. Summary The basic CR imaging cycle has three steps ( 1) expose, (2) readout, and (3) erase . When the SPS is exposed to the X-ray the energy of the incident radiation is absorbed and excites electrons to high-energy levels. These excited electrons remain trapped at unstable energy levels of the atom . This trapped energy can be released if stimulated by additional light energy of the proper wavelength by the process of photostimulated luminescence (PSL ). the SPS is scanned in a separate CR reader device

26. A red laser beam scans the photostimulable screen stimulating the emission of the blue light photons. Stored energy is set free with the emission of blue light equivalent to the absorbed remnant beam. Blue light is captured by photodiode and converted to electric charge. Analog to digital converter converts it into a corresponding digital image. Residual latent image electron is flushed with high intensity white light and erased without reintroducing electrons from the ground energy level.

27. Bibliography Carter C, Veale B, Cassette-Based Equipment: The Computed radiography Cassette, Imaging Plate and Reader , Digital Radiography and PACS, Texas, 2011 , 63-76. Ballinger P, Frank E, Computed radiography, Merill’s Radiographic Positioning and Radiologic procedure , Ohio, 2007 Vol : 3, 360-371 Lanca L, Silva A, Digital Radiography Detectors: A technical overview, Digital Imaging Systems for plain Radiography, Portugal , 2013, 12-3 Carrol Q, , Creating the Digital Image: Radiography in Digital Age, Illinois, 2011, 369

28. 1. Do CR Imaging plate have a single Emulsion layer or Double layer of Phosphor ? Ans : They do have only one single Emulsion surface and must be placed facing forward in the Cassette. 2. Do CR Imaging plates glow during X ray exposure of not? Ans : Yes they do and this can be demonstrated by Removing Phosphor CR plate and exposing it with x rays in darkened room.