Radiography with Photostimulable Storage Phosphor Equipment

Photostimulable Storage Phosphor (PSP) Equipment

• PSP systems differ from conventional radiography in that the cassette is a light-proof container that protects the imaging plate from light and handling.
• The imaging plate stores an image formed by incident x-ray photons exciting phosphors.
• Cassetteless systems use PSP technology without a cassette.

Cassette and Imaging Plate

• The cassette is made of durable, lightweight plastic backed by aluminum or lead to absorb backscatter x-ray photons.
• The imaging plate is a thin sheet of plastic with several layers:
  • Protective layer: clear plastic for protection.
  • Phosphor layer: traps electrons during exposure.
  • Reflective layer: sends light in a forward direction when released.
  • Conductive layer: absorbs static electricity.
  • Color layer: absorbs stimulating light but reflects emitted light.
  • Support layer: semi-rigid material for strength.
  • Backing layer: soft polymer for protection.

Image Acquisition and Formation

• The patient is radiographed in the same way as conventional radiography.
• The remnant beam interacts with electrons in the barium fluorohalide crystals in the imaging plate.
• The interaction gives energy to electrons, trapping them in the crystal.
• The trapped signal remains for hours or days and can be released with a laser.

The Reader

• There are two types of PSP readers: point scan and line scan.
• Point scan readers use an optical stage, scanning laser beam, and photomultiplier.
• Line scan readers use a charge-coupled device (CCD) linear array photodetector.
• The reader scans the imaging plate with a laser to release the stored electrons.

Using the Laser to Read the Imaging Plate

• The laser beam is shaped to ensure consistent beam size and speed.
• A beam deflector moves the laser beam rapidly back and forth across the imaging plate.
• The light collection optics direct the released phosphor energy to an optical filter and photodetector.
• The scanning process results in the conversion of light into an electrical signal.

Digitizing the Signal

• The electrical signal is sampled and digitized to represent a specific location within the image matrix.
• The digitized signal is displayed as a specific brightness.

Spatial Resolution

• Resolution is determined by phosphor layer thickness and pixel size.
• The thinner the phosphor layer, the higher the resolution.
• Factors affecting PSP resolution include laser beam spot size, translation speed, sampling frequency, and laser beam sweep.

Erasing the Image

• The process of reading the image returns most electrons to a lower energy state, effectively removing the image from the plate.
• Imaging plates are extremely sensitive to scatter radiation and should be erased regularly.

Preprocessing, Processing, and Forwarding the Image

• Once the imaging plate has been read, the signal is sent to the computer for preprocessing.
• The technologist can review the image, manipulate it if necessary, and send it to the quality control station and PACS.

Part Selection

• The proper body part and position must be selected for the proper conversion to take place.
• Selecting the improper part and/or position can result in a suboptimal image.

Technical Factors

• Kilovoltage peak (kVp), milliamperage/second (mAs), and distance are chosen in the same manner as for film/screen radiography.
• kVp is selected for penetration and tissue type.
• mAs is selected according to the number of photons required for the body part.

Equipment Selection

• Imaging plate selection: two types of imaging plates, standard and high resolution.
• Grid selection: frequency, ratio, and focus.
• High-resolution imaging plates are limited to smaller cassette sizes and are used for extremities and mammography.

Imaging Plate Artifacts

• Cracks in the imaging plate appear as areas of lucency on the image.
• Adhesive tape residue can leave marks on the imaging plate.
• Static can cause hair to cling to the imaging plate.
• Backscatter can cause dark line artifacts.

Plate Reader Artifacts

• Intermittent appearance of extraneous line patterns can be caused by problems in the electronics of the plate reader.

• Horizontal white lines can be caused by dirt on the light guide in the plate reader.### Plate Reader Artifacts

• Insufficient erasure settings can result in residual images left on the imaging plate before the next exposure.

• A faulty erasure lamp can also cause residual images.

• The amount and location of residual images affect the results.

• Plate readers can produce artifacts if multiple imaging plates are loaded in a single cassette and only one is extracted, leading to multiple exposures.

Grid Artifacts

• The orientation of stationary grids relative to the laser scan direction is critical to prevent moiré pattern errors.
• Grid lines must be perpendicular to the laser scan direction.
• Moving grids do not typically produce moiré patterns due to blurred grid lines.
• Vendors specify grid frequencies for their equipment to prevent moiré artifacts.

Printer Artifacts

• Debris on the mirror in the laser printer can cause fine white lines to appear on the image.
• Service personnel need to clean the printer to prevent this artifact.

Operator Errors

• Insufficient collimation leads to improper calculation of the exposure indicator and misrepresentation of the displayed image.
• Exposing the back of the cassette instead of the tube side can cause artifacts from hinges or other hardware.
• Care should be taken to expose only the tube side of the cassette.
• Underexposure produces quantum mottle, while overexposure reduces contrast.
• Proper selection of technical factors is crucial for patient dose, image quality, and production of light from the imaging plate.