CRDR SF123

• Overexposing the patient in CR or DR imaging

  1. Computer will still display the image as acceptable based on the histogram stored in memory for each examination type

  2. Slight increase in use of factors to reduce repeat exams has increased patient dose
• Selecting incorrect anatomic menu selection in CR or DR imaging

  1. Displayed image may have slight change in contrast/density based on the histogram of the selected exam

  2. Can be easily corrected by the RT through window and level adjustment
• Exposing the CR cassette twice (different areas of the IP)

  1. Apply four-sided collimation and use lead blockers to protect unexposed and exposed areas

  2. Automatic processing may display the image as too dark, can be rescaled after manual postprocessing of window and level
• Failing to remove metallic objects from the patient when using AEC

  1. Metal objects will absorb a lot of x-rays and affect the intensity reaching the image receptor

  2. X-rays will continually be produced until enough radiation is received, leading to patient overexposure

  3. A backup mAs or timer is set to limit time/intensity of exposure to avoid overexposing the patient

  
• Anatomic menu selection


  In CR/DR, the RT selects the anatomy to be examined and various projections, which prompts the system to select a dedicated histogram analysis and automatically adjust the gray scale
• Exposure field recognition


  • In CR, the computer algorithm automatically detects the exposed part of the IP and displays it as a visible image
  • Same happens with direct/indirect digital receptors

  
• Image histogram


  • A plot of the frequency of appearance of a given object characteristic
  • Digital radiographic imaging systems can store and analyze characteristic image histograms for each radiographic projection

  
• Collimation and partition


  If the x-ray exposure field is not properly collimated, sized, and positioned, exposure field recognition errors may occur, leading to histogram analysis errors and very dark, light or noisy images

  
• Automatic Exposure Control (AEC)


  • A device that measures the quantity of radiation reaching the image receptor and automatically terminates the exposure when the required radiation intensity is reached
  • A backup timer is set to 1.5 times the expected exposure time in case the AEC fails to terminate

  
• Imaging Plate (IP)


  The PSP screen is housed in a rugged cassette that appears similar to a screen-film cassette. In this form as an image receptor, the PSP screen-film cassette is called an imaging plate (IP).
• Latent image in PSP

  The latent image can remain "stored" in the photostimulable phosphor (PSP) plate for 8 hours before information is lost.
• Imaging plates


  • Should be used soon after the erase cycle has been completed
  • The PSP is sufficiently sensitive that it can become fogged by background radiation
  • Background radiation is approximately 40 mR-80 mR per day
  • Cassette-based imaging plates are sensitive to levels of radiation as low as 60 mR
  • Make sure to erase the image plate if it has been unused for more than 48 hours
• Computed radiography cassettes


  • Are highly sensitive to background radiation and scatter
  • If a CR cassette has not been used for several days, it should be inserted into the reader for re-erasure

  
• Imaging plate


  • Is highly sensitive to scattered radiation
  • Cassettes should be kept away from radiation fields, because radiation leads to a deterioration of image quality
  • No object should be kept on a cassette when it is in a radiation field, because the object will be imprinted on the image, leading to unwarranted repeating of the radiograph
  • The cassette must be protected from all sources of ionizing radiation, including scatter radiation, as well as heat and humidity

  
• A cassette seen to be lying unidentified in the radiography room should be erased before use. If the cassette was previously exposed or kept in a field of radiation, it has to be put through the process of erasure before reuse because it will hamper the quality of the image and cause erroneous artifacts.

  
• Increasing kVp


  Increases the scatter radiation that reaches the image receptor

  
• Production of scatter Radiation

  1. As x-ray energy is increased, the absolute number of Compton interactions decreases, but the number of photoelectric interactions decreases much more rapidly

  2. Therefore, the relative number of x-rays that undergo Compton scattering increases

  
• Kilovoltage, which is one of the factors that affect the level of scatter radiation, can be controlled by the radiologic technologist
• When kVp is increased, the level of scatter radiation also increases, leading to reduced image contrast

  
• Scattered x-rays


  • Emitted in all directions from the patient

  
• Image of a long bone in cross section

  • Using only transmitted, unscattered x-rays would be very sharp
  • Using only scatter radiation and no transmitted x-rays would be dull gray
  • Using both transmitted and scattered x-rays would have moderate contrast

  
• Beam restrictors & Grids 


  Devices that reduce the amount of scatter radiation that reaches the image receptor

  
• Types of beam-restricting devices

  • Aperture diaphragm
  • Cones or cylinders
  • Variable-aperture collimator
• Aperture diaphragm


  A lead or leadlined metal diaphragm attached to the x-ray tube head with an opening designed to cover just less than the size of the image receptor used
• Cones and cylinders


  Radiographic extension devices that restrict the useful beam to the required size
• Radiographic grids


  A carefully fabricated section of radiopaque material (grid strip) alternating with radiolucent material (interspace material), positioned between the patient and the image receptor to transmit only x-rays whose direction is on a straight line from the x-ray tube target to the image receptor, absorbing scatter radiation

  
• Scatter radiation that reaches the image receptor is part of the image-forming process, and the x-rays that are scattered forward do contribute to the image

  
• Main advantages of CR and DR imaging over film-screen imaging

  0% repeat chance or reduction of repeat examination
• With CR/DR imaging, even when RT overexposes px (too much kVP or mAs), the computer will still display the image as an acceptable image based on?

  Histogram stored in the memory
• It is common in film-screen iamging to perform two exposures in a single cassette.
• If metal objects are present in the px's body during exposure, it will absorb a lot of x-rays and affect the intensity of radiation that reaches IR. Therefore, x-rays will continually be produced until enough radiation is received.
• How to prevent overexposure to patient and increased dosage due to metal object?

  Set up a back-up mAs or timer to limit time or intensity of exposure
• Ohm's law formula


  V=IR