Ch. 21 Carlton

Computed Radiography (CR)

  • Evolved in early 1980s, utilizing storage phosphors.

  • Pioneered by Eastman Kodak and Fuji Medical Systems.

  • Several vendors & reimbursement decreasing.

  • Affordable transition to DR.

CR Systems

  • Two-step process: acquisition, image processing and display.

  • Uses photostimulable imaging plates (PSP).

  • Image acquisition, Latent image production & reading CR data.

Photostimulable Imaging Plates (PSP)

  • Reusable, multi-layered rigid sheets.

  • Record and transmit image.

  • Inserted into filmless cassettes and processed in reader assembly.

Common PSP Phosphors

  • Barium fluorohalide Bromides (BaFBr:Eu).

  • Barium fluorohalide Iodides (BaFI:Eu).

  • Europium (Eu) acts as an activator, maintaining electron holes.

K-Edge Attenuation

  • Optimal between 35 and 50 keV (35 keV).

  • Average energy of 80 kVp beam.

  • PSPs absorb more low-energy radiation than film.

  • Requires more exposure if kVp is outside the optimal range.

Scatter Radiation

  • PSPs are more sensitive to scatter before and after exposure.

  • Sensitive to low levels of radiation energy.

Image Acquisition

  • X-rays expose cassettes, storing the latent image in the IP.

  • Can be used tabletop or with a grid.

  • Rules of positioning stay the same

  • Wider exposure latitude than film, avoid overexposure. (More room for error in selection of kvp)

Latent Image Production

  • Electron pattern stored in the active layer of IP, creating electron traps or F-centers.

  • Fluorohalides absorb the beam through photoelectric interactions.

    • Energy transferred to photoelectrons

    • Several photoelectrons liberated

    • More electrons freed by photoelectrons

  • Liberated electrons have extra energy

  • Flourohalides trap electrons to create holes at Europium sites.

  • Latent image consists of millions of trapped electron holes.

Important Note

  • CR plates are highly sensitive to post-exposure radiation.

  • Latent image loses approximately 25% of its energy in 8 hours; process right after exposure.

Reading CR Data

  • Trapped electrons are freed by a neon-helium laser beam in a raster pattern.

  • Electrons return to a lower energy state, emitting blue-purple light (photostimulable luminescence).

  • Light captured by photomultiplier (PM) tubes, converted to an analog signal.

  • Analog signal converted to digital data by ADC for processing.

  • IP erased via intense light exposure.

  • Two types of IP processing

    • point-by-point

    • line-by-line readout.

Pixel Pitch and Size

  • Inversely related to spatial resolution.

  • Sampling frequency expressed as pixels/mm (Nyquist frequency).

  • Dependent on matrix size and image receptor size.

  • Higher sampling frequencies increase resolution.

Pixel Bit Depth

  • Determines the number of density values, affecting density and contrast.

  • Controlled by ADC.

Matrix Size

  • Dependent on sampling frequency and plate size.

  • Image file size affected by pixel size, matrix, and bit depth.

Data Manipulation (Preprocessing)

  • Raw data is located and prepared.

  • Includes anatomical part selection, orientation on IP, and number of projections.

CR Image Processing

  • Two-step process: pre-processing and post-processing.

  • Pre-processing includes exposure field recognition, histogram analysis, and grayscale analysis.

  • Post-processing includes frequency processing and spatial location processing.

Exposure Field Recognition

  • Also known as exposure data recognition (EDR).

  • Computer analyzes exposure distribution, collimation edges, orientation, and multiple images.

  • Improper placement leads to histogram analysis errors.

Histogram Analysis Errors

  • Occur when obtained data does not match the reference histogram.

  • Caused by inability to find collimated edges, prosthetic devices, or abnormal attenuation areas.

Histogram Analysis: Data Clipping

  • Clinically irrelevant data is excluded from image display based on VOI.

  • Different for each body part.

  • LUT has appropriate contrast for each body part.

Look-Up Table Adjustments

  • Image display adjusted via LUT, changing optical density or contrast. Graph of processed pixel values

  • Enhances pathologies, similar to changing the DlogE curve of film emulsion.

Histogram Equalization

  • Examples include normal chest x-ray, bone-enhanced, and soft tissue images.

  • Adheres to American College of Radiology (ACR) standards.

Post-Processing

  • Spatial Location Processing and Frequency Processing.

  • Edge Enhancement (High band-pass filtering): Enhances cortical margins of bone, micro-calcifications, trabeculae.

  • Image Smoothing (Low band-pass filtering): Suppresses visible image noise.

  • Image Stitching: Used for scoliosis or leg length studies.

Technical Considerations for CR

  • Higher kVp's suggested.

    • Kvp outside suggest range can produce too much compton for CR

  • Suboptimum mAs exposures create quantum noise.

  • CR plate response to radiation is linear and nonlimiting.

  • Vendor Dependent:

    • indirect with exposure

    • direct with exposure

Grid Use

  • Grids should be used more often due to increased sensitivity to scatter.

  • Select proper grid depending on part size (Chest greater24–26 cm).

  • Avoid Moire effect.