RAD 370: Digital Imaging - Ch 4 PowerPoint

Conventional Radiography

• Cassette / film

• wet processing with chemicals

• intensifying screen that emits light

• film exposed to light

• narrow exposure latitude

CT Scanning

• 1971

  • head only

  • digital technology, not f/s

• 1975

  • Whole body scanner

Photostimulable Phosphor Equipment used in

CR

PSP Equipment

• backed by aluminum sheet / lead

  • absorbs backscatter x-ray photons

• Antistatic Material

  • usually “felt” material - protect against static electricity build up, dust collection, and mechanical damage

PPCSLB

Layers of CR Imaging Plate Construction

Thin, tough, clear plastic that protects the active phosphor layer

• made of carbon fiber or magnesium

• no image producing capabilities

1. Protective Layer

Layer of Photo-Stimulable Phosphor (PSP) composed of

• Barium Fluorohalides

• Europium Activators

A. Ability to store and release energy

B. Responsible for storing “latent” image/creating latent image from transmitted x-ray photons exiting the patient

2. Phosphor or Active Layer

Layer of material that grounds the IP from electrostatic charge

• Increases spatial resolution by absorbing light

• reduces static electricity

3. Conductive layer (A.K.A. Antistatic Layer)

Semi-rigid material that gives the imaging sheet some strength

4. Support Layer

Sends light in a forward direction when released in the reader

• prevent unwanted light from affecting the unexposed latent image

5. Light Shield (Reflective Layer)

Soft Polymer that protects the back of the cassette

• prevent unwanted background radiation from affecting the unexposed latent image

6. Backing Layer

How long will the trapped signal remain in PSP?

hours, even days but begins to deteriorate almost immediately

Signal Loss (image fading) becomes apparent

approx. 8 hours

• but trapped signal is never completely lost

CR imaging plates should be processed

as soon as possible after exposure to prevent loss of latent image

Two important factors considered when selecting the PSP imaging cassette

• Type

• Size

The two types of imaging plates (PSP)

• Standard Resolution

• High Resolution

Imaging plates containing a thinner phosphor layer

High Resolution

To acquire and form the image in CR, the remnant beam interacts with electrons in

barium fluorohalide crystals within the imaging plate

• stimulates / gives energy to electrons in the crystals

Stimulated electrons within the crystals are now trapped in an area of the crystal known as

color or phosphor center

Two types of CR readers

• Point Scan (older)

• Line Scan

Scanner that has

• scans one pixel at a time

• photomultiplier (collect and multiply the light photons and convert them into electrons)

• ADC (changes energy to a digital format)

• slower

Point Scan

Scanner that has

• simultaneous stimulation of the imaging plate one line at a time

• acquisition of the photo-stimulated luminescence (PSL) occurs with a charge-coupled device (CCD) linear array photodetector

• faster

Line Scan

When being scanned, a red laser hits the imaging plate releasing a

blue, visible light

The released blue, visible light is captured by the

photomultiplier tube to create the manifest image (visible image)

After acquiring the image from the IP, it is

erased with a bright white (strong fluorescent) light and returned to cassette

The two scan directions

• Fast Scan

• Slow Scan

the movement of the laser across the imaging plate

(also known as the “scan)

Fast Scan direction

the movement of the imaging plate through the reader

(also known as the “translation” or “subscan direction”)

Slow Scan direction

Once an exposure has been taken using an imaging plate

cassette is fed into a reader that

• removes the imaging plate

• scans it with a laser to release stored electrons

The imaging plate must be processed to release the

latent (invisible) image from the radiation detector IP

the CR laser reader uses a

red helium neon laser light to scan info from the IP

( Red Ruby Laser )

The IP travels through ______ inside the CR reader

rollers

In a fast scan direction, the red laser light scans in a

raster pattern at 2 eV

Because PSPs are sensitive to scatter,

the erasure process should be repeated at least once a week

CR Plate Erasure Error

Ghosting Erros

• double image

  • second image partially outside collimated field

• Remnants of lead marker

• caused by incomplete erasure of imaging plate

Phosphor layer thickness and pixel size determine

Resolution in PSP

Raw Data (Pre-Processing) is

converted into digital data using the ADC

Quantization assigns

each pixel a number

• each number has an assigned gray scale value

Bit Depth

number of gray shades that can be used to define each pixel

Grayscale

digital term for “scale of contrast”

Moire Pattern

wavy artifact lines caused by grid lines running parallel to laser scanning motion

The relationship between the height of the lead strips and the space between the lead strips

Ratio H/D

The higher the ratio, the more critical the positioning is, so

a high grid ratio is not an appropriate choice for mobile radiography

Shuttering

removes background or distracting light surrounding an image

• postexposure image manipulation

• black background can be added (to screen image)

The CR processing system uses a “stitching application” to

connect the two images into 1 for the radiologist to view

Image Inversion

changes the image contrast from negative to positive

• swap white/black

• bones black

• used often for visualizing tubes or lines

Image Subtraction

technique used in interventional radiology to clearly visualize blood vessels in a bony or dense soft tissue environment

• removes bones to look for blockages or obstructions

Any undesirable objects on the processed image other than those caused by scatter and fog

Artifact

If an unused PSP has been stored for 48 hours

it should be erased prior to use to avoid image fog

occurs when image darkens due to sources other than primary x-ray beam interactions

fog

four common types of artifacts

• imaging plate

• plate reader

• imaging processing

• printer

• cracks in imaging plate appear as lucent areas

• if static exists because of low humidity, hair can cling to IP

Imaging plate artifacts

White lines that are parallel to the direction of plate travel are caused by dirt, dust, or scratches on the light guide

Plate Reader Artifacts

Tiny increased brightness artifacts

dirt/dust