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Digital Radiography

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Digital Radiography

  1. 1. DIGITAL RADIOGRAPHY Upakar Paudel B.Sc. MIT Final year UCMS BHAIRAHAWA NEPAL
  2. 2. INTRODUCTION •Since the clinical use of x-rays in 1895,majority of radiographic examinations have been carried out by the conventional method. •The beam is projected through the patient and the transmitted beam, which has information about the body structures, is made to strike the cassette containing the film and the intensifying screens. This way the latent image is produced.
  3. 3. The latent image can be made visible and permanent by processing it with suitable chemicals. This conventional method of obtaining radiographs has dominated the field of radiography for many years. But it has been realized that the film-screen system has its own limitations.
  4. 4. LIMITATIONS OF CONVENTIONAL RADIOGRAPHY 1. After the film has been exposed,the information contents cannot be enhanced. 2. If the radiograph is too dark or too light,it has to be repeated. This results in extra exposure to the patient. 3. The completion of the examination is delayed as the film has to be processed to convert the latent image into a permanent one.
  5. 5. 4. A magnifying glass may be required to see very small structures in detail. 5. Copied radiographs have an inferior quality as compared to original ones. 6. The film is a physical object and so it requires considerable space for storage. 7. Films can only be in one place at a time and they also get deteriorated with passage of time.
  6. 6. HISTORICAL DEVELOPMENTS •Initially,radiographs were obtained in the conventional manner and then put through some kind of digitization process to obtain a digital image. •In the 1960s,computed axial tomography was invented by Godfrey Hounsfeild. Here, x-rays were not detected by a film but by detectors the computer analyzed the signals from the detectors and reconstructed an image which was displayed on a T.V. monitor.
  7. 7. •With subsequent development in digital imaging,other modalities such as M.R.I. and U.S. appeared. •In 1982, the first computed radiography system was developed by the Fuji film corp. This used photostimulable phosphors as image receptors. •In 1990, direct capture radiography or FLAT PANEL SYSTEM started, which used amorphous Si or Se as detectors.
  8. 8. COMPUTED RADIOGRAPHY •PRINCIPLE: •In the C.R. system we use an imaging plate made of a photostimulable phosphor. •The cassette is exposed to x-rays in a similar fashion as the conventional cassette. •The latent image is produced in the phosphor layer of the imaging plate. •Then the cassette is transferred to the reader system where the imaging plate is scanned with a red helium-neon [633mm] beam.
  9. 9. This stimulates luminescence proportional to the x- ray energy absorbed. These light signals are converted into electrical signals by using photomultiplier tubes. These electrical signals are converted into digital information by an ANALOG TO DIGITAL CONVERTER. The digitized data is transferred to the digital image processor in the computer, from where it can be processed and viewed on the monitor.
  10. 10. COMPONENTS OF THE C.R. SYSTEM • The C.R. system comprises of: 1. An imaging plate 2. An image reader 3. An image processor 4. An image recorder
  11. 11. THE IMAGING PLATE •It consists of a polyester base over which a layer of photostimulable phosphor [europium doped barium fluoro bromide crystals- BaFBr:Eu 2] is coated. •A protective layer composed of fluorinated polymer material is applied over it. A supporting layer which prevents the reflection if light is also applied. •Next is the backing layer. This prevents the scratching on the imaging plates during storage and transfer. Therefore it has a protective action.
  12. 12. •The next is the bar-code table which contains the number assigned to the imaging plate. •This bar-code provides a mechanism for associating each imaging plate with patient identification, related examination and positioning information. •The imaging plate is flexible and less than 1mm thick.
  13. 13. OTHER CHARACTERISTICS OF IMAGING PLATES •It retains the image for 24 hours, but some degradation may occur with passage of time. •Imaging plate shows a linear response to the intensity of x-ray exposure over a broad range. •It shows superior performance capability i.e. it provides more information. •It is available in the same sizes as conventional cassettes.
  14. 14. • High resolution imaging plates are also available which help in reducing the radiation dose to the patient considerably. • Imaging plates are reusable and thousands of exposures can be made on it.
  15. 15. THE IMAGE READER •The image reader converts the continuous analog information [latent image] into a digital format. •In the reader the imaging plate is scanned sequentially by a red helium-neon [633mm] laser beam. •The laser beam induces photostimulable luminescence from the phosphor. The intensity of the emitted luminescence is proportional to the amount of x-ray energy absorbed in the crystal layer.
  16. 16. •This emitted light is directed by highly efficient light guides to the photomultiplier tubes, where it is converted into electrical signals. •The electrical signals are sampled and digitized by an A.D.C. •The digital data is stored on the hard disk of a work station from where it can be processed, viewed, printed or distributed via a network to peripheral stations.
  17. 17. • The image reader has a capacity to read 110 plates per hour. • Therefore one reader can serve several radiographic rooms and the data input is stored on an easy image workstation.
  18. 18. THE IMAGE RECORDER • The work station provides a DICOM compliant output which maybe directed to a laser printer for hard copies, or networked to other viewing stations.
  19. 19. ARCHIVAL OF C.R. IMAGES •A 12 bit output of the A.D.C. is converted into 10 bits within the reader; discarding the information which is irrelevant to the exam being performed. •This change reduces the size of the image data files, increasing the speed of the system and also increasing the storage space. •For bulk and long storage, optical discs, jukebox system, storage shelves etc. may be used.
  20. 20. ADVANTAGES OF C.R. SYSTEM •No special equipment is required. •The exposure latitude is wider and so more information from the x-ray beam can be extracted as compared to a conventionally acquired image. •Repeats are extremely few and that too due to positioning and not exposure factors. •All types of radiography is possible with the C.R. system.
  21. 21. •The image displayed on the monitor can be manipulated in a variety of ways: contrast enhancement, edge enhancement, black/white reversal etc. •The process of filing the images does not require separate rooms etc. and is relatively easier. •The acquired image can be transferred to many monitors for viewing in separate places.
  22. 22. LIMITATIONS OF THE C.R. SYSTEM •Lesser spatial resolution as compared to conventional radiography. •C.R. systems are not inherently low dose systems as compared to the conventional rare earth screen- film systems. •Radiological technologists receive no direct feedback on the accuracy of their selection of exposure factors as the resultant images are of consistent quality regardless of the exposure. This may lead to undesirable and undetected over exposure to the patient.
  23. 23. DIRECT RADIOGRAPHY • FLAT PANEL DETECTOR SYSTEMS: • This system uses x-ray detectors of photoconductive materials such as amorphous Se or Si for direct acquisition of projection radiographs.
  24. 24. METHODS • Essentially,two methods have been developed for direct capture radiographs: • Indirect method • Direct method
  25. 25. INDIRECT METHOD • Here we use CsI scintillation phosphors coated over an active matrix array of amorphous silicon photodiodes. • The x-ray beam emerging from the patient interacts with the cesium iodide producing light. • This light interacts with the amorphous silicon producing electrical charge.
  26. 26. •Thin film transistors store the signal until read out, one pixel at a time.
  27. 27. DIRECT METHOD •In this case we do not use the phosphor coating, thus eliminating the intermediate light producing step. •Hence amorphous selenium directly acts as the x- ray detector. •The x-ray beam directly interacts with a thin layer of amorphous selenium creating electron-hole pairs, which being charged, travel directly to the electrodes. •From here, the charge pattern is read out to form the image.
  28. 28. • The advantage of the amorphous selenium approach is that there is no light spreading in the phosphor and so there is improved spatial resolution. • On the other hand, the cesium iodide phosphor has a high detective quantum efficiency and so it results in lower radiation dose.
  29. 29. CONSTRUCTION AND WORKING OF A D.R. SYSTEM • The physical dimensions of the detector array are 40 x 50 x 4 cms with 2560 x 3072 pixel matrix. • The array consists of a glass substrate onto which a layer of amorphous silicon is evaporated. • The matrix is covered with a cesium iodide scintillator layer.
  30. 30. •The amorphous silicon is structured in a matrix of individual photo sensors and switching elements, either a thin film transistor or a diode which allows the connections of the sensor with the read out line in column direction. •Thin film transistors or switching diodes are controlled via address lines in the horizontal direction, in order to read out the single charge values of photodiodes.
  31. 31. • These signals are multiplexed and converted into digital signals by an A.D.C. inside the detector housing. • The 2-D image data is directly transferred to the image processing computer via an optic fiber link. • So the image is available in digital form shortly after the exposure has been made.
  32. 32. CHARACTERISTICS OF AMORPHOUS SILICON • It is a good photo detector in thin film form. • Its easy to deposit on large glass substrates. • They are very sensitive to light with an efficiency close to 100%
  33. 33. ADVANTAGES OF FLAT PANEL DETECTOR SYSTEM •Less radiation dose to the patient. •The examination becomes quick as no cassettes have to be fetched from the storage area, taken to the examination site, or to the processing unit after exposure. •Radiography as well as fluoroscopy can be performed. •Post processing can be done.
  34. 34. DISADVANTAGES OF F.P.D. SYSTEMS • Due to its inflexibility, portable or ward radiography is not possible. • Different equipment is required for different kinds of work. • They are quite costly.
  35. 35. DIGITAL FLUOROSCOPY •It provides real time viewing of anatomic structures. As maximum image detail is required, so image brightness must be high. •Image intensifier was developed to replace the conventional fluoroscopic screen. •With the introduction if computer technology into fluoroscopy,digital images with better detail can be obtained.
  36. 36. EQUIPMENT • D.F. requires the same fluoroscopy equipment in addition to a computer, 2 video monitors, and a more complex operating console. • A high voltage generator. • A video system. • A charge couple device.
  37. 37. ADVANTAGES • Less radiation dose as compared to the I.I.T.V. system. • Better image quality.
  38. 38. DEVELOPMENTS IN D.F. SYSTEM • Flat panel detector system has replaced the I.I.T.V. SYSTEM. • X-rays passing through the patient are converted into electrical signals by the F.P.D.s. These are then passed through the amplifier and ADC where they are converted into digital signals.
  39. 39. • The digital image data is directly transferred to an image storage PC via an optic fiber link at the rate of 30 f/s • This system permits high speed digital image acquisition, processing and display. • Images are of excellent resolution.

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