Computed radiography

computed radiography introduction

1. COMPUTED RADIOGRAPHY (CR) - AN OVERVIEW AND EVALUATION OF PIPE WALL THICKNESS BASED ON CONTRAST MEASUREMENT C H YADHUKRISHNA (213118008) ILLYAS M K (213118010) MUHAMED MUSTHAFA PULIKKAL (213118015)

2. INTRODUCTION Computed radiography (CR), also commonly known as Photostimulable phosphor (PSP) imaging . It employs reusable imaging plates and associated hardware and software to acquire and to display digital projection radiographs . It is cassette based system like analog film and is more commonly considered to be a bridge between classical radiography and digital radiography CR is used almost exactly like conventional Imaging plate in a cassette which must be processed in CR reader after X-ray exposure for conversion to digital images.

3. CR system consists of Image plate ( photostimulable phosphor) PSP X ray or gamma ray source Image reader/digitizer/scanner Photomultiplier tube (PMT ), ADC ( analoge to digital converter) Computer console or workstation, software, monitors, and a printer . Image reader can be Single-plate readers (each cassette is loaded manually and read separately) Multiple-plate readers (multiple plates—up to 10—can be stacked and loaded automatically )

4. HISTORY Cassette-based computed radiography (CR) systems have continued to evolve in parallel with integrated, instant readout digital radiography (DR) systems. The fundamental innovations in the development of CR was by Kodak ( luckey in 1975) who conceived the storage of an X ray image in a phosphor screen. It required significant technical steps and conceptualization of the application by F uji ( kotera et al 1980) to produce the first medical X ray images F uji , the main developer of CR in the eighties, used BarFBR : phosphor and a cassette-based approach.  

5. PRINCIPLE OF OPERATION In computed radiography , the imaging plates are inserted in a radiographic table’s cassette holder and are exposed to X-rays or gamma rays . T he energy of the incoming radiation is stored in a special phosphor layer , electrons in the phosphor plate are excited into a higher energy state, forming a latent image . Then a specialized machine known as a scanner is then used to read out the latent image from the plate by stimulating it with a very finely focused laser beam . During which the trapped electrons absorb the laser energy and they emit blue light with intensity proportional to the amount of radiation received during the exposure as they return to their ground state .

6. The light is collected by a light guide and transmitted to a highly sensitive analog device known as a photomultiplier tube (PMT) and converted to a digital signal using an analog-to-digital converter (ADC). The generated digital X-ray image can then be digitally stored and viewed on a computer monitor and evaluated. After an imaging plate is read, it is erased by a high-intensity light source that removes residual radiation and can immediately be re-used. Imaging plates can typically be used up to 1000 times or more depending on the application.

7. LIMITATIONS OF CONVENTIONAL RADIOGRAPHY Films cannot tolerate a wide range in radiation exposure (OD - log E curve) One has to choose between good contrast and good latitude Image cannot be adjusted once taken Require space for dark room and film/chemicals storage.. Price of films and chemicals is constantly going up (4-10% per year) Need several exposures to show different tissues (bone vs soft tissue ) Image developing time is more (manual about 60 minutes)

8. STEPS INVOVLVED X ray image received on phosphor plate subjecting to X ray exposure After the image has been captured on an image plate by a standard x-ray system, the plate is taken to an image-plate reader to extract the image. The cassette is loaded (manually or automatically) into the reader. Image extracted from phosphor plate by laser (Raw image) Raw image processed for quality improvement The digital image is then produced in 30-120 seconds and downloaded to an image-processing system, usually a computer workstation, for display and manipulation. Digital image can be either printed burned on CD, or sent to PACS(Picture Archiving and Communication System)

9. COMPONENTS OF COMPUTED RADIOGRAPHY IMAGING PLATES The CR plate has a thin layer of phosphor grains, known as a photostimulable phosphor. The plates are usually 1 mm thick and are coated with Europium activated fluorohalide compounds in crystalline formation embedded in organic binding material. Layers of Imaging Plate   Protective Layer: A thin layer of transparent film that protects the phosphor. Barium Fluorohalide Phosphor Layer : a closely dispersed of fine-grained, Photostimulable phosphor plate is coated with europium-activated , barium Fluoro - halide crystals, that store the latent image until released when re-stimulated during processing (Thickness is about 0.3 mm).

10. Light reflective Layer : This layer increases the intensity of light being emitted from the crystals by reflecting it back toward the reader, instead of it being absorbed . Conductive Layer : This is a light absorbing layer, made up of conductive needle-like crystals that absorb any unreflected light as well as any electrostatic charges. Polyester Support Layer : Made from a polyester material, this layer gives structural rigidity and a base for the coating of all of the other layers. Polyester is used because of its excellent stabilities well as its durability and flexibility. Light Shielding Layer : This is a carbon particle layer that prevents the light from leaking from the rear of the imaging plate. Backing Layer : This is a protective layer made from a soft polymer that prevents scratching when the plates are stacked during the manufacturing process.

11. PROTECTIVE LAYER BARIUM FLUOROHALIDE PHOPHOR LAYER REFLETIVE LAYER CONDUCTIVE LAYER POLYSTER BASE SUPPORT LAYER LIGHT SHIELDING LAYER BACKING LAYER

12. X-RAY EXPOSURE When the X-ray is absorbed by the material, absorbed energy excites the europium atoms, causing ionization of Eu atom. The electrons are raised to higher energy state in the conduction band. Where electrons travel freely until they are trapped in a so called F-centre in a metastable state with an energy level slightly below that of conduction band but higher than that of the valence band. The number of trapped electrons per unit area is proportional to intensity X-rays incident at each location. These trapped electrons constitute the latent image. Due to the thermal motion the trapped electrons will slowly be liberated from the traps, and so at room temperature the image should, however, be readable up to 8 hours after exposure

13. IMAGE SCANNING Plate is scanned by the laser causing the emission of blue light from the plate which is then detected by the photomultiplier. Plate is erased with light (not laser) and returned to the cassette

14. PHENOMENON OF LASER LIGHT The cassette is placed in the reader where the Image Plate is extracted and raster scanned with a highly focused and intense laser light of low energy (~2 eV). Laser light is absorbed at the F- center ( Farbzentren center ) and, thus, stimulates the trapped electrons up to conduction band where they are free to move to Europium atom thereby leaving high energy conduction band to lower energy valence band. When these electrons become reabsorbed by trivalent Europium, trivalent Europium is transferred back into divalent Europium atom. This involves the liberation of high energy (~3 eV) and this is done by emission of green light. Since, there is a larger energy difference between conduction band, valence band and F- center , the green light emitted has a higher energy than the laser light needed to stimulate the trapped electrons.

15. The difference in wavelength between the two light is critical for the detection of emitted light. Incident x-rays form an “electron” latent image in a metastable “F” center site that can be stimulated with a low energy laser beam (~2.0 eV), producing the desired luminescent signals (~3.0 eV ).

16. PHOTOMULTIPLIER TUBE A photomultiplier tube (PMT) useful for light detection of very weak signals, is a photo emissive device in which the absorption of a photon results in the emission of an electron. These detectors work by amplifying the electrons generated by a photocathode exposed to a photon flux.

17. ERASING PROCESS Residual latent image information is erased by using an intense light (consisting of wavelengths that remove electrons in traps without stimulating further electron trapping), and the image plate is reinserted into the cassette for reuse. Exposure to a bright fluorescent light removes the remaining information in 10-15 sec . Ghosting - if the residual latent image remained so it should be removed by flooding with intense white light.

18. ADVANTAGES CR technology can be considered as the digital replacement of conventional X-ray film . Post-processing , manipulation & storage of images is easy . Repeat examinations are reduced due to wide exposure latitude. Image Plate is reusable . Ability to produce consistent high quality images . Ability to deliver images quickly to those who need to make critical decisions.

19. Decreased time to acquire images thereby increasing patient throughput in medical diagnosis. Increased Radiologist reading capacity and the ability to have instant comparison images along with previous reports. Ability to print on paper or make CD media instead of film, economically profitable. Increased savings : no film, chemicals, dark room and storage room required Computer processing of raw image brightness, contrast, sharpness enhancement, zooming, measurements… Reduction in exposure and processing time Software based evaluation and reporting Digital magnification allows better detail viewing Special analysis tools: e.g. for automated wall thickness measurement Global data exchange via a local network and/or the Internet

20. DISADVANTAGES   Initial cost Additional cost (Service and maintenance) Plate is sensitive to fogging: use grid, need to be erased daily Dose-creep : since, exposure latitude is wide, high exposure technique is used which increases the patient dose which is called dose-creep.