(KIN) Unit 4 Ch 21 - CR and PSP Plates

What are the two types of digital radiography systems?

computed radiography and digital radiography

what is computed radiography

photostimulable phosphor imaging plates (PSP IP)

-developed in 1980s-pased out 2010

-uses technology of storing phophors

-analog imaging=film/screen

what is digital radiography?

- flat-panel detectors (indirect conversion and direct conversion)

-charged couple devices (CCD)

-complementary metal oxide semiconductor (CMOS)

what is computed radiography's 2 step process?

1. acquisition

2. image processing and display

What is being phased out and what is replacing it??

CR (computed radiography) is being phased out and replaced by photostimulable phosphor imaging plates (PSP IP)

the cassette has 2 uses. what are they

1. protects IP from damage

2. provides ligthtweight, sturdy vehicle for IP transportation and placement under patient (IP is flexible and fragile)

what is the cassettes construction?

frame, front panel, anti-static felt, back panel, and a memory chip or bar code

Cassette Construction: Frame

aluminum, plastic, or steel

Cassette Construction: Front Panel

radiolucent carbon fiber

Cassette Construction: Anti-static Felt

protects against static build-up and provides cushion

Cassette Construction: Back Panel

lined with lead foil (backscatter)

Cassette Construction: Memory Chip or Bar Code

to match patient information with cassette

What is the PSP IP construction (describe it, don't tell me what it contains)

rigid sheet of several layers which records and transmits image from xray beam inserted into cassette

PSP Plate Construction: what are the layers?

protective layer, phosphor layer, reflective layer, base layer, light shielding layer, conductor/anti static layer, backing layer

PSP Plate Construction: Protective Layer

protects phosphor from mechanical wear and cleaning solutions

-provides a smooth surface that improves signal to noise ratio (S/N ratio)

PSP Plate Construction: Phosphor layer

- where the image is trapped; considered the active layer; has a scintillator

-USES barium fluorohalide (bromide and iodide) with europium activators

- "conventional powder" where the light goes in all directions

OR

-uses europium-doped cesium bromide

-needle shaped crystals - channels light up and down

PSP Plate Construction: reflective layer

reflects light forward so that it can be captured in the image reader device (IRD)

PSP Plate Construction: Base layer

provides support and strength for the IP

PSP Plate Construction: Conductor layer (or antistatic layer)

conducts static electricity away from phosphor layer because potential cause for artifacts; grounds the plate

PSP Plate Construction: light shield layer

prevents extraneous light from hitting phosphor layer

PSP Plate Construction: backing layer

thin sheet of lead foil that absorbs backscatter and is a protective layer

the phosphor layer. of the PSP IP uses what 2 PSPs (a powder or a crystal)

barium fluorohalide (bromide and iodide) with europium activators

- "conventional powder" [light in all directions]

OR

europium-doped cesium bromide

-needle shaped crystals [channels light up and down]

within the phosphor layer, what does the scintillator do?

it's an object that fluoresces (emits light) when hit by ionizing radiation

how much more sensitive are CR phosphor plates to scatter and background radiation than conventional film

10 times

a fog can be produced in how little of time for PSP IPs

as little as 24 hrs - this is why it is the best practice to erase any cassette prior to use if there is a chance it has been in storage for 2 days or more

what are the 3 steps to latent image production?

1. upon exposure, X-rays strike the cassette and phosphor layer

2. some X-rays get absorbed (photoelectric absorption)

3. this absorption of energy ejects photoelectrons, results in "holes" in the phosphor

during the latent image production, in step 2, it says "some X-rays get absorbed (photoelectric absorption)"...what other 2 absorb some of the beam through photoelectric interactions?

fluorohalides or cesium bromide

what are the 2 outcomes for the freed electrons from latent image production?

they immediately return to their original position <--- fluorescence occurs

OR become trapped in "F centers" <--- traps e- in excited state

why are F centers present?

because of europium

in the F centers, what do the freed electrons that get trapped there contain?

the latent image (hidden, not visible image yet)

how long does it take for a latent image to lose 25% of its energy?

8 hours - this is why it's important to [rocess cassette quickly after exposure

how do we read PSP image data?

how long does it take to read the image?

they're scanned in an image reader device (IRD)

a minute and a half per cassette

Steps to reading PSP image data: FIRST step

latent image is scanned by high intensity helium-neon or solid state red laser beam in a raster pattern, which reflects off a mirror onto the IP as the IP is transported through the rollers using fast scan direction or slow scan direction

Steps to reading PSP image data: SECOND step

when the laser hits the phosphor layer, the trapped electrons in the F centers are freed and return to a lower energy state

-(energy is released as a blue-purple light called photo stimulated luminescence (PSL))

Steps to reading PSP image data: THIRD step

this light is captured by photodetector (or photomultiplier tube or charged couple device (CCD)

Steps to reading PSP image data: FOURTH step

photodetector (photomultiplier [PM]) tube or charge coupled device (CCD) converts light to electrons

Steps to reading PSP image data: FIFTH step

an analog to digital converter (ADC) converts electrons to digital data and receives electron stream from photodetector and converts it to a digital signal to be sent to computer

Steps to reading PSP image data: SIXTH step

digital signal data is sent to the computer for additional processing

Steps to reading PSP image data: SEVENTH step

imaging plate is then completely erased by exposure to intense white light (analog in ADC, digital OUT)

if there is photostimulated luminescence (PSL) divergence, what can decrease the amount of divergence?

CsBr:Eu (cesium bromide with europium) - because phosphor is needle-shaped and increases resolution

what is fast san direction?

aka Laser scan direction; laser light moving back and forth across IP (raster pattern)

what is slow scan direction?

aka Plate scan direction; direction of IP movement as it is transported through the rollers

define luminescence

refers to any emission of light in general

define fluorescence

refers to the immediate emission of light under stimulation

define phosphorescence

is the delayed emission of light some time after the original stimulus/exposure has occurred..."glowing in the dark"

define stimulated phosphorescence

requires re-stimulation

define charged couple device

very similar to CCDs used as IRs

define photomultiplier tube

a photocathode plate attached to an electronic amplifier

-dynodes are a series of plates that can be switched back/forth between relative positive and negative charge to continuously accelerate and multiply the electron stream

define photocathode

a later of material that releases electrons when light strikes it, through the photoelectric effect

in terms of the latent image, WHAT is proportional to the number of xray photons that interact with the imaging plate

the number of electrons trapped