image quality

IMAGE QUALITY

Radiographic Quality

Radiographic Quality refers to the fidelity

with which the anatomic structures being

examined are imaged on the film.

Three main factors:

Film Factors

Geometric Factors

Subject Factors

Characteristic of radiographic quality:

Spatial Resolution (Recorded Detail)

Contrast Resolution (Visibility of Detail)

Noise (Visibility of Detail)

Artifacts

Spatial ResolutionResolution is the ability to image two

separate objects and visually distinguish one from the other.

Spatial Resolution is the ability to image small structures that have high subject contrast such as bone-soft tissue interface.

When all of the factors are correct, conventional radiography has excellent spatial resolution.

Contrast Resolution

Contrast resolution is the ability to

distinguish structures with similar subject

contrast such as liver-spleen, fat-muscle.

Computed tomography and MRI have

excellent contrast resolution.

Conventional radiography is fair to poor.

Noise

Noise is an undesirable fluctuation in

optical density of the image.

Lower noise results in a better

radiographic image because it improves

contrast resolution.

Two major types:

Film Graininess- no control over

Quantum Mottle- some control over

Film Graininess Film graininess refers to the distribution

in size and space of the silver halide grains in the film emulsion.

Similar to structure mottle that refers to the size and shape of the phosphors in the intensifying screens.

Inherent in image receptor, and are not under the control of technologist, and they contribute very little to radiographic noise.

Quantum Mottle Quantum mottle refers to the random nature

by which x-rays interact with the image receptor.

Principal contributor to radiographic noise.

Image produced with few x rays will have higher QM than image produced with from large number of x rays. use of very fast intensifying screens results in increased QM.

The use of high mAs , low kVp settings and of slow image receptors reduces quantum mottle.

Quantum Mottle

Very fast screens have higher quantum

mottle because it takes fewer x-rays to

make the image.

Speed

Resolution and noise are intimately

connected with speed.

While the speed of the images receptor is

not apparent on the image, it influences

both resolution and noise.

Radiographic Quality Rules

Fast Image receptors have high noise and

low spatial and contrast resolution.

High spatial and contrast resolution

require low noise and slow image

receptors.

Low noise accompanies slow image

receptors with high spatial and contrast

resolution.

Film Factors of Quality

Characteristic curve

Density

Contrast

Latitude

Processing

Time

Temperature

Sensitometry Sensitometry is the study of the

relationship between the intensity of exposure of the film and the blackness after the film is processed.

Unexposed film is clear with a blue tint after processing.

Properly exposed film appears with various shades of gray.

Heavily Exposed film is black after processing.

Sensitometry

Two principles involved.

Exposure of the film

Amount of light transmitted through the

processed film of optical density.

Used to describe the relationship of

radiation exposure and blackness or

optical density on the film.

Characteristic Curve

This relationship is

called the

characteristic curve

or H & D curve of

the film.

H & D stands for

Hurter and Driffield.

Parts of the Characteristic Curve

Toe and shoulder

low and high ,

exposure levels,

where large

changes in

exposure results in

small changes in

OD.

Parts of the Characteristic Curve

The straight line or

intermediate area is

where very small

changes in exposure

results in large changes

in density.

This is the important

part of the curve in

radiography,where

properly exposed

radiographs appear.

Log Relative Exposure (LRE)

X-ray films responds to a

wide range of exposure

from 1 mR to 1000 mR.

It is not the absolute

exposure that is of interest

but rather the change in

OD over each exposure

interval

Exposure is represented

on logarithmic

manner(log relative

exposurer).

Optical Density It is not enough to say that OD is the

degree of blackening of a radiograph,orthat a clear area of the radiograph represent low OD and a black area represent high OD.

OD density has a pre size numeric value that can be calculated if the level of light incident on a processed film(Io)and the level of light transmitted through that film(It) are measured.

OD=log10 Io/It

Optical Density Range

The optical density

range is from 0.0 to

4.0 corresponding to

clear and absolute

black repectively.

Useful range in

general radiography

is from 0.5 to 2.25.

Image range is 0.5 to

1.25 OD

Fog density and base density

Most unexposed and processed film has an OD in the range of 0.1 to 0.3,corresponding to 79% and 50% transmission, respectively.these ODs of unexposed film are due to base density and fog density.

Base density is inherent in the base of the film and is due to the composite of the base and s the tint added to the base.

Fog density results from development of silver grains that contain no useful information.higher fog density reduces the contrast of the image.

Base fog or base density

The tint of the base

of the film and the

inadvertent

exposure of the

during processing.

Range is from 0.1 to

0.3. Should be never

above 0.30 most is

.21 OD

Items that Impact Base Fog

Film storage

Film exposure to wrong spectrum of light

or light intensity.

Chemical contamination.

Improper processing.

High Base fog levels reduce contrast.

Contrast

The variations in the OD in the

radiograph is called radiographic

contrast.

Marked differences in OD----High

contrast radiograph.

OD differences are small----Low

contrast radiograph

Radiographic Contrast is the combined

result of image receptor contrast and

subject contrast.

Image receptor contrast refers to the

contrast inherent in the film and

influenced by the processing of the film.

Contrast

Subject contrast is determined by the size,

shape and x-ray attenuating

characteristics of the subject being

examined and the energy (kVp) of the x-

ray beam.

Image Receptor Contrast

Inherent to the film and screen combination but is influenced by:

Range of Optical Density

Film Processing Technique

Film type is determined by the type of intensifying screens used.

Film-screen images always have higher contrast compared with direct exposure images.

Image Receptor Contrast

The slope of the straight line portion of the H & D curve is the receptor contrast.

The average gradient is a straight line drawn between the densities of 0.25 and 2.00 + base fog.

Average Gradient

The average

gradient is a straight

line drawn between

0.25 OD and 2.0 OD

above base plus

fog.

This is the useful

range of optical

density in on most

radiographs.

Speed

Speed is the ability

of the receptor to

respond to low x-ray

exposure.

The H & D curve is

useful in comparing

speed when

selecting film or

screens.

Speed

A relative number of 100 given to Par

Speed Calcium Tungstate Screens.

High Speed Calcium Tungstate has a

speed of 200. Half of the exposure is

needed to produce the same image.

Rare earth screen film combinations

range is speed from 80 to 1600.

Speed

By knowing the Speed, sometimes

referred to as the Relative Speed Value, it

is easy to convert the technical factors for

one speed to another speed.

When image receptors are replaced,a

change in mAs setting may be necessary

to maintain the same OD,e.g if image

receptor speed is doubled, the mAs must

be halved.no change is required in kVp.

LATITUDE

Latitude can be

observed on the H

& D curve.

Latitude refers to

the range of

exposure that will

produce a

diagnostic range

OD.

Latitude

Latitude and Contrast are inversely proportional.

Wide latitude has a wide gray scale or low contrast. (B)

Narrow latitude has a short scale or high contrast. (A)

Latitude

Latitude is designed into some screen and

film combinations. With wide latitude, the

error factor in technique is wider.

Latitude can also be impacted by the

technical factors.

Film Processing

Radiographic

Quality is impacted

by film processing

parameters.

The developer must

be at the proper

concentration and

at the correct

temperature.

Film Processing

The film must also

spend the correct

amount of time in

the developer.

This is the time &

temperature relationship.

Processing

Speed and base fog increase with the

temperature.

Contrast will increase to a point and then

drop with the base fog increase.

Manufactures set processing parameters

to optimize speed, contrast and low base

fog.

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