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.