Un3 Computer Radiography

Computed Radiography

the generic term applied to an imaging system made up of PSP screen, CR reader, and digital electronics

The invisible image on the image receptor immediately after patient exposure and prior to development is called a(n):

aerial image

Photostimuable Storage Phosphor (PSP) screen

Acquires the aerial image (stores x-ray energy):

CR reader

Extracts the latent image and converts it to a manifest image:

Digital Electronics

digital component of the system that converts the analog signals(electronic latent image) to a digital manifest image

what's are the similarities of the screen film imaging vs. computed radiography (CR)?

+both use an xray sensitive image receptor

+both in protective cassettes

+both can be used interchangeable with any xray imaging system

+both carry a latent image that must be made visible by processing

what's the difference between screen film imaging vs. computed radiography (CR)?

they differ in processing:

-in film imaging they use chemical processing to produce image

-in CR there's a CR reader, converts analog into digital manifest image

when was CR introduced and by who?

chimerically introduced in 1983 by Fuji

what. is CR technology based on?

based on certain halide phosphors having energy storage and excitation characteristics known as photostimuable lumunance(PSL)

what halide based phosphors does CR technology use?

halide crystals that are embedded in PSP screens which enables the phosphors

what is the characteristics phosphor or property?

photostimuable luminance(PSL)

what does photostimuable luminance (PSL) do?

**The condition of light being emitted from the phosphors in the PSP

1-enables phosphors to store xray energy temporarily

2-release that energy upon excitation by a laser beam

(when xrays strike the PSP screen, its interact w/ the crystals & then crystals will store some of that xray energy temporarily. when the screen is scanned with a red laser beam the phosphors will release that energy in a form of light)

Causes the phosphors to emit light:

red laser beam

where is the PSP screen located?

PSP screen is positioned within the cassette or imaging plate (IP)

What phosphor material is most common for PSP screens?

barium fluorohalide

-in CR for the PSP screen the barium fluorohalide crystals are suspended in turbid structure that's been hardened

how much energy is stored in the barium fluorohalide crystals when PSP screen is exposed to xrays

a fraction of that energy is stored in the barium fluorohalide crystals

how is the IP processed?

by a scanning system or reader

Amplifies the analog signal:

photomultiplier

what does the scanning system that processes the IP do?

1-extracts PSP screen from the cassette

2-moves the screen across a high intensity scanning laser(red) beam

3-barium fluorohalide crystals release energy in form of blue violet light, emitted via photostimuable luminescence (PSL)

4-light energy/signal (analog) is intensified by a photomultiplier tube

5-the analog signal is changed into an electric/digital signal by an analog to digital converter

Converts light into a digital form:

analog to digital converter

Erases the imaging plate so it can be used again:

intense white light

how long is the CR image processing?

20 seconds

what allows the plate to be reused thousands of times? (approx up to 10k times before a new phosphor is need)

a high intensity white light dumps all the remaining energy traps

what are the PSP layers?

1-protective layer

2-phosphor or active layer

3-support layer

4-reflective layer

5-lead backscatter control

what does the protective layer do?

a very thing tough clear plastic that protects the phosphor layer

what is the phosphor layer?

contains the barium flurorohalid crystals which trap energy til stimulated by red laser beam which then releases the blue violet light and that becomes amplified

what contains a light absorbing dye to prevent light spread?

barium fluorohalide

with a light absorbing dye is the system fast or slow?

SLOW

= increased detail

=increased patient exposure(still very minimal)

what does the support layer do?

a semi rigid material that gives the imagine sheet some strength, supports the phosphor layer

What is the purpose of the reflective layer found in the PSP screen?

Redirects light towards photomultiplier tube so no light is wasted.

what does lead backscatter control do ?

absorbs backscatter, its the backing layer, a soft polymer that protects the back of cassette. this is also where the bar code reader is placed.

What does pixel stand for?

picture element

digital images are a metric of pixels where each pixel is assigned a value which determines what?

how much darkness(density) will be assigned to that little dot or square.

A high sensitivity (S) value and the condition of light being emitted from the phosphors in the PSP demonstrates:

under exposure (over400-means most rays were absorbed by patient-bad)

what is the difference between the S and EI number?

S number is inversely proportional to radiation striking detector

EI is directly proportional to radiation striking the detector

an S number under 100 indicates what?

overexposure (patient has been too exposed + scatter radiation)

an EI number lower than 2.0 is ?

underexposure

an EI number higher than 2.4 is ?

overexposure

What is the major disadvantage of computed radiography?

radiation patient exposure may be excessive