crdr

Computed and Digital Radiography

Computed Radiography Terms

  • IP: Imaging Plate

  • PD: Photodiode

  • PMT: Photomultiplier Tube

  • PSL: Photostimulable Luminescence

  • PSP: Photostimulable Phosphor

  • SP: Storage Phosphor

  • SPS: Storage Phosphor Screen

Overview of Computed Radiography

  • A "cassette-based" system utilizing a solid-state detector plate instead of traditional film.

  • Requires: CR cassettes & phosphor plates, CR readers, technologist quality control workstation, and viewing means (printer or viewing station).

  • Image plate can be reused and erased thousands of times.

Computed Radiography Image Receptor

  • Combines traditional diagnostic radiology machines with a power computer for digital x-ray images instead of analog.

  • Produces high resolution and quality images, allowing visualization of fine details important for diagnoses, such as compound fractures and mammograms.

  • Known for significantly better contrast than conventional x-ray film-screen systems.

Historical Perspective

  • 1973: George Luckey files a patent for a system to produce images from high energy radiation.

  • 1975: Luckey's patent (USD 3,859,527) approved; Kodak patents first scanned storage phosphor system, leading to modern computed radiography.

  • 1980: Many companies apply for patents based on Luckey's invention.

  • 1981: Fuji Medical Systems commercializes completed CR system.

Major Components of Computed Radiography

  1. Phosphor Imaging Plates: To acquire x-ray image projections.

  2. PIP Reader (Scanner)/CR Reader: To extract electronic latent images.

  3. Workstation: For pre- and post-processing of images.

  4. Digital Electronics: Converts signals to digital format.

Imaging Plate Structure

  • Housed in a rugged cassette similar to screen-film cassettes.

  • Handled similarly to traditional x-ray film.

  • Contains several layers including barcode labels for patient identification.

Barcode Identification Labels

  • Enables technologists to synchronize image information with patient ID barcodes.

  • New exams require scanning patient-identifying barcodes to connect with exam requests.

  • Cassettes labeled with stickers indicating the orientation (top/left or top/right).

CR Cassette Structure and Mechanism

  • Contains a photostimulable storage phosphor imaging plate (PSP) that traps energy from radiation.

  • Original systems comprised of a phosphor storage plate, a reader, and a laser printer.

Imaging Plate Layers

  • Protective Layer: Tough plastic protects the phosphor from handling trauma.

  • Phosphor Layer: Traps electrons during exposure; typically made of barium fluorohalide.

  • Reflective Layer: Sends light forward when read; may be black to reduce light spread.

  • Conductive Layer: Grounds the plate to minimize static electricity and enhance sharpness.

  • Support Layer: Semirigid material providing strength; made of Polyethyleneteraphtalate.

  • Backing Layer: Soft polymer shields the cassette’s back from radiation, made of lead foil.

Photostimulable Luminescence (PSL)

  • Emission of light after stimulation by a light source.

  • Phosphors like Barium Fluorohalide doped with Europium exhibit this property.

  • Europium helps in effectively trapping electrons.

PSL Signal Production Stages

  1. Exposure

  2. Stimulation

    • Laser beam diameter impacts spatial resolution.

  3. Reading

  4. Erasing

CR Reader Functionality

  • No chemicals or darkroom necessary; cassettes are fed into the reader for scanning.

  • Smaller systems for medical offices capable of processing 50-60 imaging plates per hour.

Scan Directions in CR Reader

  • Fast Scan: Movement of the laser across the imaging plate (referred to as 'scan').

  • Slow Scan: Movement of the imaging plate through the reader (referred to as 'translation' or subscan direction).

Optical Features of CR Reader

  • Components include lasers, beam shaping optics, light collecting optics, filters, and photodetectors.

  • The laser provides stimulating light (typically at a wavelength of 633 nm).

  • The beam must maintain a constant size and shape to avoid errors in spatial resolution.

  • Laser scanning involves multiple passes across the imaging plate in a zigzag or raster pattern.

Computer Control of Image Processing

  • Output from PMT provides a time-varying analog signal for processing.

  • Signals are adjusted for amplitude, compression, and shaped before forming images.

  • Analog signals are digitized considering proper sampling and quantization.

  • Images temporarily stored in an image buffer until sent for interpretation or archiving.

Sources of Image Noise

  • Mechanical Defects: Slow/fast scan driver issues.

  • Optical Defects: Laser intensity variations, scattering, and emission light quantas.

  • Computer Defects: Electronic noise, and inadequacies in sampling and quantization.