Computered Radiography

Computed Radiography (CR)

• Uses photostimulable storage phosphor inside a cassette

• Evolved in the early 1980s

  • Considered an affordable transition technology to DR

Computed Radiography (CR) systems (two step process)

1) Image Acquisition

2) Image processing and display

Image Acquisition

• First steps of processing the image

• Taking the X-ray/exposure

• Results in latent image (on the cassette/imaging plate)

• Can be used on a table top/grid

• Wider exposure latitude than film/screen

Photostimulable Imaging Plates (functions)

• Transferred to the reader assembly for processing

• Reusable plates

• Rigid sheet of several layers

• Rigid sheet of several layers

• Records and transmits an image from the X-ray beam

• Inserted into cassette (filmness cassette)

Photostimulable Imaging Plates (composed of)

1) Protective layer

2) Phosphor layer

3) Conductor layer

4) Support layer

5) Light-Shleiding layer

6) Backing layer

Protective layer (Photostimulable Imaging Plates)

• First layer

• Insulates the imaging plate from handling trauma

Phosphor layer (Photostimulable Imaging Plates)

• Second layer

• Active component for holding images

Conductor layer (Photostimulable Imaging Plates)

• Third layer

• Grounds plate to fix any electrostatic problems and absorbs light to increase sharpness

Support layer (Photostimulable Imaging Plates)

• Fourth layer

• Base to stop leakage

Light-Shielding layer (Photostimulable Imaging Plates)

• Fifth layer

• Prvents light from erasing data on imaging plate or prevents leaking

• Decreases spatial resolution

Backing layer (Photostimulable Imaging Plates)

• Sixth layer

• Can be lead

Common PSP phosphors

• Barium flurohalide bromides (BaFBr: EU)

• Barium fluorohalide iodides (BAFI: EU)

Europium (EU)

Considered an activator as it maintains presence of electron holes in PSP

Scatter Radiation (PSP)

• More sensitive to scatter both before/after exposure, than radiographic film

• Sensitive to low levels of radiation energy

Latent Image - Production

1) Electron patterns stored in the active layer of the exposed IR. Forming F-centers (exciting # of low energy electrons to form holes in the crystal in the crystal phosphor).

2) Fluorohalides will hold/trap the beam through photoelectric interactions, forming holes at europium sites

Hole Formation (Latent image)

• Fluorohalide crystals trap half of the liberated electrons

• Europium sites contain electrons holes

Reading CR data (steps)

1) Trapped electrons are freed

2) Electrons return to a lower energy state

3) Light captured by photomultiplier (PM) tubes (OR CCD array)

4) PM tube converts light to an analog electric signal

5) Analog electronic signal sent to an analog-to-digital converter (ADC)

6) ADC sends digital data to the computer for additional processing

7) IP erased via exposure to intense light

Reading CR tables (Step 1)

Trapped electrons are freed from IP

• Scanned by a finely focused neon-helium laser beam in raster pattern

Reading CR Data (Step 2)

Electrons returned to lower energy state

• Releases blue/purple light through photostimulable luminescence

PSL

Photostimulable luminescence

Photostimulable luminescence (PSL)

Frees trapped electrons and allows them to return to lower energy level

Reading CR Data (Step three)

Light captured by photomultiplier (PM) tubes

• Sensitive to blue light

• Are photodetectors that convert visible light to electronic signals

Two types of processing

• Point-by-Point Readout

• Line-by-Line Readout

Point-by-Point Processing

Scanned point scan read that moves from side by side

Line-by-Line Readout

Light liberated from the imaging plate is emitted in all directions. Collected by an optical system that directs it to either one or more photodetectors.

Pixel Pitch

Physical distance between pixels (measured from center to center)

Pixel Pitch (dependent on)

• Sampling frequency (Nyquist frequency)(How the laser scans the plate)(Expressed at pixel/mm)

• Matrix size

• Image receptor size

Analog-To-Digital Conversion (steps)

1) Analog signal sampled

2) Quantification

Pixel Bit Depth

• Determines # of density values (affects density/contrast of system)

• Controlled by ADC

Matix size (dependent on)

• Sampling frequency

• Receptor Size (in some systems)

Image File size (affected by)

• Pixel size

• Matix

• Bit depth

CR Imaging Processing (2 steps)

• Pre-processing

• Post-processing

Pre-processing (CR)

• Exposure field recognition

• Histogram analysis

• Gray scale analysis

Post-Processing (CR)

• Frequency processing

• Spatial location processing

Pre-Processing (Exposure Data Recognition)

The computer looks at the distribution of exposure on the plate

• edges of collimation

• orientation on plate

• multiple images

Pre-Processing (Collimator Edge Identification)

• An algorithm that detcts edges of exposure versus non-exposure

• Can sometimes be triggered by prosthetics or implants

Pre-Processing (Histogram Analysis: Data clipping)

Clinically irrelevant data not included in image display

• Determined by VOI

• Different for each body part

• LUT has been appropriated for each body part

Histogram Analysis Errors

• The obtained image data does not match the reference histogram

• The computer cannot find collimated edges

EX: abnormal areas of increased/decreased attenuation (barium in stomach)

EX: If someone has a prosthetic device, it may affect and result in this.

Post-Processing (two approaches)

• Spatial location processing

• Frequency processing

Post-Processing (Examples)

• Edge Enhancement (high band pass-filtering)(Enhances cortical margins of bone, microcalcification, trabeculae)

• Image smoothing (Low band-pass filtering)(Suppresses visible image noise)

• Image stitching (Sitich together multiple images)

Techinical Considerations for CR

• Higher kVps suggested

• Suboptimum mAs exposure creates quantum noise

• CR plate response to radiation is linear and non-limiting

  • Vendor dependent

Grid Use

Should be used more often due to increased sensitivity to scatter

Select proper grid depending on part size

Avoid Moire effect

CR Considerations

• Positioning/part placement is very important

• Symmetrical collimation margins

• Processing to avoid signal loss and reduce image quality

• Correct exposure selection is important