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This presentation is a professional collaboration of

development time prepared by:

Rex Christensen

Terri Jurkiewicz

and

Diane Kawamura

Computed Radiography

Chapter 3

Physics and Technology

Today’s Humor: More Medical Anomalies

What is Computed Radiography?

Or what is CR?

It is x-rays passing thru the patient

being picked up by a digital detector.

Photostimulable luminescence (PSL) is

seen due to phosphors being excited by

the x-rays.

Other names for CR are SPR storage

phosphor radiography and DSP digital

storage phosphor radiography

CR Technology

The Imaging plate

The imaging reader

The display

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History

1983 CR started being used for

diagnostic radiography.

Fuji started with FCR 101 followed by

Kodak, and Agfa.

Now there are four manufacturers with

the addition of Konica.

Imaging System . . . . . . .

Four Steps:

1. Image Acquistion

2. Image plate

scanning and erasure

3. Image processing

4. Image display

+ implementation issues:

Use of grids, radiation exposure,

and technique selection.

PSP

The photostimulable phosphor on the

imaging plate must have good

absorption efficiency of x-rays and

must be stimulated by a helium-neon

laser.

It must also be compatible with the

photomultiplier tube and luminescence

must be shorter the 1 microsecond.

Phosphors must be able to store the

latent image for several hours without

compromising the signal from the IP

Phosphors

Phosphors meeting the above criteria are:

Barium Fluoro Halide:Europium

Halide can be chlorine, bromine, or

iodine or a mixture of them.

The phosphor is usually doped with

Europium because it is an activator

which increases the efficiency of the

PSL.

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Latent Image Formation

Electrons move from a ground state to

the higher energy state when the

europium atoms are bombarded by x-

rays.

The x-ray’s travel to the F-Center (F

comes from farbe meaning color)

The number of trapped electrons are

equal to the absorbed radiation.

PHOTOSTIMULABLE LUMINESCENCE

Compton and

photoelectric x-ray

interactions occur with

outer-shell electrons

Outer-shell electrons

are sent into an

excited, metastable

state

X-ray interaction with a

photostimulable

phosphor results in

excitation of electrons

into a metastable state.

PHOTOSTIMULABLE LUMINESCENCE

Over time, the

metastable electrons

return to the ground

state on their own

When metastable

electrons return to

their ground state,

visible light is emitted.

PHOTOSTIMULABLE LUMINESCENCE

Exposing the phosphor to intense infrared light

from a laser accelerates or stimulates the

process of the electrons returning to the ground

state.

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PHOTOSTIMULABLE LUMINESCENCE

The storage phosphor screen appears white

because the small photostimulable phosphor

particles scatter light excessively

• Turbid is the term used to describe the excessive

scattering of light

The photostimulable phosphor particles are

randomly positioned throughout the binder

Laser Light

The lasers today

are semiconductor

lasers that produce light

with a 680nm

wavelength,

compared to the

older He Ne lasers

that produce light at

633nm which was used

in the earlier CR units.

Computer Radiography (CR)

plate is photostimulable phosphor

radiation traps electrons in high energy states

higher states form latent image

H i g h e r E n e r g y

E l e c t r o n

S t a t e

L o w e r E n e r g y

E l e c t r o n

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X - R a y

P h o t o n

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P h o t o n p u m p s

e l e c t r o n t o

h i g h e r e n e r g y s t a t e

Reading Imaging Plate reader scans plate

with laser

laser releases electrons trapped in high energy states

electrons fall to low energy states

electrons give up energy as visible light

light intensity is measure of incident radiation

Laser Beam

Higher Energy

Elect ron

St at e

Lower Energy

Elect ron

St at e

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Lower Energy

Electron State

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Fluoroescent Bulbs Erase the Plate

The IP plate is erased using high intensity

light to remove residual energy.

Analog to Digital Image

The light emission is then collected by a

collection device and sent to the

photomultiplier tube which then

produces an electrical signal, it is

digitized by the ADC and sent to the

computer for processing.

Fading

This is the time it takes for a latent

image to disappear.

That is why it is important to have the

image plate “read” as soon as possible

after the exposure.

The PSL decreases by 25% within 8

hours between exposure and reading.

PSP Layers

A high resolution PSP layer is thinner

than the standard resolution layer, and

has a sharper image due to less lateral

spread of the laser light.

The thicker phosphor layer IP’s have

faster speed than the high resolution

IP’s.

High resolution IP’s will be used for

small parts to increase the resolution.

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The plate consists of:

A PSP layer on a base for support.

Two protective layers

Electroconductor layer

Light shielding layer.

Film Screen vs. IP Layers

The purpose of the electroconductive

layer is to reduce any static electricity

problems when the IP is transported

into the reader.

Imaging Plates

Housing the imaging plate is a

lightweight aluminum or aluminum

honeycomb panel on the back which is

designed to prevent backscatter

radiation, and a radiolucent front.

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Scanning the IP

There are two main types of scanning

mechanisms:

1. Point Scan Reader

2. Line Scan Reader

Seeram P. 55

Bushong P. 419-421

Workstation

Allows:

Input of patient information

Region of exposure

Image preview

Image processing

Quality assurance

Image printing

Sending images to PACS

Workstation

Consists of :

Processing computer

Monitor (CRT or LCD)

Keyboard

Mouse

Some may have

barcode readers and

magnetic card reader.

Digital Image Processing

Picture is of colliding galaxies taken from Hubble telescope

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Pre-Processing Operations

Pre-processing is necessary to make

corrections on the data received from

the imaging plate.

1. Exposure field recognition: This

pertains to the size of the field of view.

That is why centering, and collimation is

so important.

Histogram

2. Histogram analysis- the computer

analyzes the exposure field and makes

an analysis of what was imaged. So if it

was centered incorrectly the image will

be light or dark.

Measured histogram is the actual

histogram of the image taken

Stored histogram is the predetermined

histogram for the study (ie forearm

predetermined average histogram.

Histogram

A histogram is a graph of the number of pixels in

the entire image or part of the image having the

same gray levels (density values) plotted as a

function of the gray levels

Grayscale Rendition

3. Grayscale rendition- It is the determination

of the minimum and maximum penetration

and sets the luminance presentation

according to those values obtained during the

exposure.

This is where the look up table comes into

play.

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Exposure Recognition

It is the visual indicator for the

technologist letting him/her know

whether the exposure was appropriate.

This can be used as a form of quality

control to optimize the quality of the

image and limit the exposure rate to the

patient.

Exposure Indicators

Fuji Medical- Sensitivity number (S

number)

Agfa Medical- log of Median (lgM)

Kodak-Exposure Index (EI)

They each use a different name for the

same thing – this is the exposure

recognition indicator.

Post Processing

These enable the technologist or

radiologist to manipulate the image to

optimize it for viewing purposes.

Increase/decrease contrast-brightness,

sharpen the image, smooth the image,

enlarge an area, lighten or darken the

image, remove the bone, or soft tissue.

All of this is possible with post

processing.

Edge Enhancement - Smoothing

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Contrast Manipulation

W 226

W 100 W 500

- =

Post-contrast Image Pre-contrast Image - Mask

Subtraction Image

Contrast Enhancement

Accomplished by windowing Window width encompasses the range of

densities within an image

A narrow window is comparable to the use of a high-contrast radiographic film

Image contrast is increased

A narrow window is valuable when subtle difference in subject density need to be better visualized

Narrow window increases image noise and the densities outside of the narrow window are not visualized

Digital Imaging Processing

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Contrast Enhancement

Increasing the width of the window allows

more of the gray scale to be visualized or

more latitude in the densities of the image

Digital Imaging Processing

Windowing

Windowing is a digital image processing

technique that also changes the contrast and

brightness of an image.

The illustration shows the range of pixel values

(gray levels) and displayed image contrast range

on a digital image.

Windowing

A digital image is made up of numbers.

• The range of number is the window width

(WW)

• The center of the range is defined as the

window level (WL).

The WW controls image contrast.

The WL controls the brightness.

Windowing

The displayed WW and

WL values are always

shown on the image.

Narrow WW provides

higher image contrast

(short-scale contrast) and

a wide WW will show an

image with less contrast.

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Windowing

• If the WL is increased, the

image becomes darker

since more of the lower

numbers will be displayed.

Exposure Indicators

It is a numerical value that is used to

demonstrate the exposure.

As previously state the different

companies use different indicators.

S-Number

EI (exposure indicator)

LgM (log of median values)

EI Guidelines for Quality Control

The challenge is to minimize the

radiation exposure to the patient, while

maintaining optimal exposure image

quality.

The exposure indicator is what allows

the technologist to accomplish this.

Spatial Resolution

Size of the pixels help to determine the

resolution

of an image

. The smaller

the pixels the

better the

resolution.

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Resolution

One pixel

The number of pixels in a given area

defines the resolution of an image

1”

500 x 1,000 pixels http://escholarship.cdlib.org/rtennant/presentations/2002bcla/16

Dynamic Range (bit-depth)

1 bit 8 bit grayscale 8 bit color 24 bit color

(GIF) (GIF) (JPEG)

1 bit = black or white

8 bits = 256 shades

16 bits = thousands

24 bits = millions

36 bits = billions http://escholarship.cdlib.org/rtennant/

presentations/2002bcla/17

The bit depth has an effect on the number of

shades of gray, hence the density resolution of

the image.

Noise

Electronic Noise (System): You cannot

control the system noise, it is inherent in

the machine.

Quantum Mottle (noise): This is

determined by the number of photons

hitting the detector. When the number

of photons is too low there is more

noise, and the reverse high number of

photons lower noise.

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Modulation Transfer Function MTF is the spatial frequency response of an

imaging system or a component; it is the contrast at a given spatial frequency relative to low frequencies.

Spatial frequency is typically measured in cycles or line pairs per millimeter (lp/mm)

High spatial frequencies correspond to fine image detail. The more extended the response, the finer the detail-- the sharper the image.

Digital Imaging Processing DQE

Detective Quantum Efficiency-

The detector receives the exposure

converting it to an image. The DQE is a

measurement of the efficiency in which

the detector performs this task.

It includes the signal to noise ratio and

the system noise in the value. You want

no loss of information.

Perfect detector is 1 or 100%

Artifacts

Digital artifacts come from many

sources.

Hardware, software, and objects

imaged.

Many artifacts arise from the imaging

plate –it is handled frequently, and

should be cleaned often.

Incorrect storage of IP and use of grids.

CR reader is another source of artifacts.

Poor Screen Cleaning

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CR IP in upside down

Dirty Reader

CQI

Continous Quality Improvement was

established by JCAHO (Joint

Commision on Accreditation of

Healthcare Organizations). It states

that every employee plays a vital role in

continuous quality improvement.

Quality Assurance

This is the system that assures quality

patient care. Administrative dealing

mainly with assessment measures or

outcome.

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Quality Control

This is technical aspect dealing mainly

with the quality and maintenance of

equipment and dose to the patient.

The point of CQI, QA and QC is to

maintain and improve the quality of

service, dose, images, and diagnosis of

the patient all at the lowest cost

possible.

Test Tools for CR

Inspect the Imaging Plate

Test the monitor for resolution, contrast,

brightness, linearity, collimation, etc.

Wire mesh can be used for contact,

antiscatter grid, densitometer for hard

copy images, and phantoms

The End of Chapter 3

Picture of Earth from

Hubble telescope

Thanks to

http://www.ceessenti

als.net/article11.html

for some of the images

obtained in this ppt.