Digital Radiography and PACS: chapter 4

What does the aluminum or lead do in a cassette?

Absorbs back scatter x ray photons

Felt; protects against static electricity buildup, dust collection and mechanical damage to plate.

What is the anti static material usually in PSP cassette and what's its functions?

Image plate definition

Thin sheet of plastic that records the image.

What are the layers of the imaging plate?

Protective

Phosphor

reflective

conductive

color

support

backing

Protective layer definition

Thin, tough, clear plastic that protects the phosphor layer

Phosphor layer definition

Layer that traps electrons during exposure. Also may contain a dye that deferentially absorbs the stimulating light to prevent as much spread as possible and functions much the same as dye added to conventional radiographic screens.

What is the phosphor layer made up of?

Photostimulable phosphor that is part of the barium fluorohalide family

Reflective layer definition

Layer that sends light in a forward direction when released in the cassette reader. May be black to reduce the spread of stimulating light and the escape of emitted light. Some detail is lost in this process.

Conductive Layer definition

Material that absorbs and reduces static electricity

Color Layer definition

Absorbs the stimulating light but reflects emitted light

Where is the color layer located?

Between the active and support layer

Support layer definition

gives the imaging sheet strength

Backing Layer definition

Soft polyer that protects back of cassette

What is the purpose of the barcode label

Allows technologist to match the image information with the patient identifying barcode on the exam request

What is the difference in image acquisition with film and PSP?

How the image is recorded

How is the image formed in PSP systems?

In PSP the remnant beam interacts with electrons in the barium fluorohalide crystals contained within the plate. The stimulated electrons in the crystals are trapped in the phosphor center.

What are the two types of PSP readers?

Point scan and Line scan

Point Scan reader

only a single laser point radiates the imaging plate at any point in time with this type of reader

What does a point scan reader have?

Optical stage, scanning laser beam, translation mechanics, light pick up guides, a photomultiplier, signal transformer or light amplifier and an analog to digital converter.

Line Scan reader

Based on simultaneous stimulation of plate one line at a time.

Acquisition of PLS occurs with a CCD linear array photodetector.

PSP image capture

1. Remnant photons interact with electrons in the barium fluorohalide crystals in plate

2. Electrons gain energy and become trapped

3. Cassette is fed into reader

4. Plate is removed from reader

5. Plate is scanned with a helium laser beam in a raster (Zig Zag) pattern

6. Laser gives energy to trapped electrons

7. Trapped electrons can now escape the active layer and emit visible blue light

8. Photodetectror detects lines of light intensity info

9. Photodetector amplifies the light and sends it to Analog to digital converter

10. Electrical signal is sampled and digitized to represent a specific location within the image matrix and displays as a specific brightness

11. Most of the electrons return to a lower energy state.

12. Plate is released by flooding it with light to remove any electrons still trapped

What are the two scan directions of the reader?

Fast scan and slow scan

Fast scan definition

Movement of laser across imaging plate

Slow scan definition

movement of plate through reader. AKA translation or subscan direction

What does the Laser function in doing?

Creates and amplifies a narrow intense beam of coherent light.

What does Laser stand for

Light Amplification of Stimulated Emission of Radiation

Why is the laser coherent?

All photons are traveling in the same direction and frequency.

Why does the laser require a constant power source?

To prevent output fluctuations

What kind of laser scans the imaging plate?

helium laser beam or solid state laser diodes.

How long is the wavelength for the laser beam?

633 nanometers

Translation

When the laser scans the plate multiple times

What does the scanning process produce?

Lines of light intensity info detected by a photodetector

What must be coordinated with the translation speed? Why?

the scan direction of the laser or the spacing of the scan lines will be affected

More angle of the laser beam will result in what?

More elliptical shape of the beam

If the beam shape were ignored what would happen?

The output of the screen would differ from middle to edges.

What does the photodetector do?

amplifies the light and sends it to an ADC

What does it mean to digitize a light signal from a photodetector?

Assigning a numerical value to each light photon

Factors affecting PSP resolution

Laser beam spot size, translation speed, sampling frequency, and the laser beam sweep in patient beam readers.

How often should plates be run under erase

once a week

What does erasure mode allow

surface of plate to be scanned without recoding generated signals

Why is it important to select the proper body part and position

So the proper conversions can take place

What is the kVp range required for radiography?

45-120

What is the k edge of phosphor imaging plate range? And what is the equivalent in kVp

30-50 keV- 60-110

why would we increase kVp

to decrease mAs

What happens when insufficient light is produced?

the image will appear grainy

What two factors should be considered when selecting the PSP imaging cassette?

Type and size

Grid selection factors

frequency, ratio, and focus

What does a high resolution plate contain

Thinner phosphor layer than standard plates

A thinner phosphor layer will ____ the sharpness of the image. Why?

Increase; the amount of light spreading laterally is reduced

What happens when the light spreads laterally?

Image appears blurry

What are high resolution plates limited to? What exams are they most often used for?

smaller cassettes, small extremities and mammo

If the matrix remains constant

FOV decreases

Pixel size decreases

spatial resolution of image increases

Miore effect definition

A wavy artifact that causes grid lines running parallel to laser scanning motion

Grid frequency

refers to the number of grid lines per centimeter or lines per inch

What is the typical grid frequency range?

80-152

Grid ratio

the relationship between the height of the lead strips and space

Shuttering is a postexposure image manipulation technique that

adds a black background to the original collimation edges

Why is the image recognition phase important?

Establishes the parameters that will determine collimation borders and edges and histogram formation

What are the 4 types of artifact?

Image plate

Plate reader

Image processing

Printer

Image Plate artifacts

cracks on plate within the reader, adhesive tape, hair that clings to cassette and lead coating

Plate reader Artifacts

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

Printer artifacts

Fine white lines that may appear on the image because of debris on the mirror in the laser printer