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Components of Radiographic Image Quality. Radiologic Technology 244 created: Fall 2005 Rev 12-01-2009. Review handouts. Main Factors Affecting Recorded Detail. kVp & mAs Technique Selection (Time) Motion Object Unsharpness Focal Spot Size SID (Source to Image Distance)

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components of radiographic image quality

Components of Radiographic ImageQuality

Radiologic Technology 244

created: Fall 2005

Rev 12-01-2009

main factors affecting recorded detail
Main Factors Affecting Recorded Detail
  • kVp & mAs
    • Technique Selection (Time)
  • Motion
  • Object Unsharpness
  • Focal Spot Size
  • SID (Source to Image Distance)
  • OID (Object to Image Distance)
  • Material Unsharpness/ Film Screen

Combo

factors that affect recorded detail
Factors that affectRecorded Detail
  • Geometric unsharpness

OID SID SIZE SHAPE

  • Motion unsharpness (blurring)
  • Intensifying Screens
  • Film Speed / Composition
  • Film – Screen contact
  • Kvp & Mas (density / visibility)
geometric qualities
GEOMETRIC QUALITIES
  • DETAIL
  • DISTORTION
  • MAGNIFICATION
detail
DETAIL
  • The degree of sharpness in an object’s borders and structural details.
  • How “clear” the object looks on the radiograph
recorded detail
Recorded Detail
  • Other names:

-sharpness of detail

-definition

-resolution

-degree of noise

slide8

What are these

What does they

measure?

slide11
Factors Affecting DENSITY
  • PATIENT THICKNESS,PATHOLOGY
  • MAS & KVP
  • SID
slide14
POOR

DETAIL

GOOD DETAIL

motion
Motion
  • Can be voluntary or involuntary
  • Best controlled by short exposure times
  • Use of careful instructions to the pt.
  • Suspension of pt. respiration
  • Immobilization devices
decrease motion unsharpness
Decrease Motion Unsharpness
  • Instruct patient not to move or breath
  • Use Immobilization devices
  • Use Short exposure times
  • Lock equipment in place
focal spot size
Focal Spot Size
  • Smaller x-ray beam width will produce a sharper image.
  • Fine detail = small focal spot (i.e. small bones)
  • General radiography uses large focal spot
  • Beam from penlight size flashlight vs. flood light beam
object unsharpness
Object Unsharpness
  • Main problem is trying to image a 3-D object on a 2-D film.
  • Human body is not straight edges and sharp angles.
  • We must compensate for object unsharpness with factors we can control: focal spot size, SID & OID
sid source to image distance
SID Source to Image Distance
  • The greater the distance between the source of the x-ray (tube) and the image receptor (cassette), the greater the image sharpness.
  • Standard distance = 40 in. most exams
  • Exception = Chest radiography 72 in.
slide28
SID
  • Shine a flashlight on a 3-D object, shadow borders will appear “fuzzy”

On a radiograph it’s called ______________

  • A true border – _____
  • Farther the flashlight from object = sharper borders. Same with radiography.
oid object to image distance
OIDObject to Image Distance
  • The closer the object to the film, the sharper the detail.
  • OID , penumbra , sharpness 
  • OID , penumbra , sharpness 
  • Structures located deep in the body, radiographer must know how to position to get the object closest to the film.
distortion
Distortion
  • Misrepresentation of the true size or shape of an object

MAGNIFICATION (size distortion)

TRUE DISTORTION (shape distortion)

shape distortion
Shape Distortion
  • Misrepresentation of the shape of an object
  • Controlled by alignment of the beam, part (object), & image receptor
  • Influences: Central ray angulation & body part rotation
slide38

Distortion (x-ray beam not centered over object & film)

Distortion (object & film not parallel)

central ray
Central Ray
  • Radiation beam diverges from the tube in a pyramid shape.
  • Photons in the center travel along a straight line – central ray
  • Photons along the beam’s periphery travel at an angle
  • When central ray in angled, image shape is distorted.
slide40

Distortion of multiple objects in same image (right) due to x-ray beam not being centered over objects.

central ray angulation
Central Ray Angulation
  • Body parts are not always 90 degrees from one another
  • Central ray angulation is used to demonstrate certain details that can be hidden by superimposed body parts.
  • Body part rotation or obliquing the body can also help visualize superimposed anatomy.
  • NAME 3 EXAMPLES
magnification caused by
MAGNIFICATIONcaused by:
  • TUBE CLOSE TO THE PART (↓SID)
  • PART FAR FROM THE CASSETTE

(↑ OID)

Compensate for MAG : ↑ OID by ↑ SID =

“increase SID 7” for every 1” OID”

size distortion sid
Size Distortion & SID
  • Major influences: SID & OID
  • As SID , magnification 
  • Standardized SID’s allow radiologist to assume certain amt. of magnification factors are present
  • Must note deviations from standard SID
slide45

In terms of recorded detail and magnification, the best image is produced with a small OID and a large SID.

size distortion oid
Size Distortion & OID
  • If source is kept constant, OID will affect magnification
  • As OID , magnification 
  • The farther the object is from the film, the more magnification
measuring of magnification1
Measuring % of Magnification
  • What is the % of mag when you have a 72” SID and

4” OID?

  • DO the math…………
material unsharpness
Material Unsharpness
  • Equipment used can contribute to image unsharpness
  • Fast film/screen combinations = decrease in image sharpness
  • Slower film/screen combinations = increase in image sharpness
intensifying screens
Intensifying screens
  • Lower patient dose
  • Changes resolution of image
  • slow screens less LIGHT = better detail
  • Faster – less detail (more blurring on edges)
intensifying screens review
Intensifying Screens: Review
  • Located inside the cassette (film holder)
  • Calcium Tungstate
    • Blue to purple light
  • Rare Earth
    • Green & Ultraviolet light
poor screen contact
POOR SCREEN CONTACT
  • FOAM BACKING HELPS TO PLACE INTENSIFYING SCREENS IN DIRECT CONTACT WITH THE FILM – NO GAPS
  • IF GAPS – MORE LIGHT CAN BE EMITTED IN SPACE, CAUSING THE IMAGE TO BE OF POOR DETAIL
screen speed
Screen Speed
  • Efficiency of a screen in converting x-rays to light is Screen Speed.
spectral matching f s
Spectral Matching (F/S)
  • What does it mean?
  • Name the two types of screen phosphors
  • What light spectrum do they emit?
spectral sensitivity
Spectral Sensitivity

Film is designed to be sensitive to the color of light emitted by the intensifying screens.

  • Blue LIGHT– Conventional Calcium Tungstate screen
  • Green, Yellow-Green LIGHT
  • – Rare Earth screen
safe lights
Safe lights
  • What wattage bulb?
  • Distance from counter top?
review of film characteristics
Review of Film Characteristics
  • Size of silver halide crystals & emulsion thickness determine speed of film and degree of resolution

Speed – the response to photons

  • Resolution – the detail seen
slide70
What are these
  • What are they made of
film speed crystal size
Film Speed / Crystal size
  • Larger crystals or Thicker crystal layer

Faster response= less detail, and

less exposure (chest x-ray)

  • Finer crystals / thinner crystal layer

=Slower response, greater detail, more exposure (extremity)

image on film
IMAGE ON FILM
  • SINGLE EMULSION = BETTER DETAIL
  • DOUBLE EMULISON = LESS DETAIL
  • PARALLAX

With double emulsion – an image is created on both emulsions – then superimposed – slight blurring of edges

slide74

QUANTUM MOTTLEFilm grain, or graininess, refers to the tiny black spots that make up the visible image, one grain from each silver halide crystal exposedMORE COMMON IN CR SYSTEMS NOWNOT ENOUGH PHOTONS TO CREATE IMAGE

factors affecting mas
Factors Affecting mAs
  • LIST 6 factors
factors affecting mas1
Factors Affecting mAs
  • Patient factors: size of pt., density of tissue, pathology
  • kVp
  • Distance - how
  • Grids
  • Film/Screen Combinations
  • Processing
slide79
a densitometer,
  • measures film blackness.
  • Film blackness is the relationship of the intensity of the light that hits the film from the view box (incident intensity) to the intensity of the light transmitted through the film (transmitted intensity).
  • These measurements plotted on a graph produce a characteristic curve.The limitations of the human eye determine the useful density range in diagnostic radiography.
  • The diagnostically useful range of densities is 0.25 to 2.5.
  • The later module on exposure calculation considers this in more detail.
main factors affecting recorded detail1
Main Factors Affecting Recorded Detail
  • kVp & mAs
  • Motion
  • Object Unsharpness
  • Focal Spot Size
  • SID (Source to Image Distance)
  • OID (Object to Image Distance)
  • Material Unsharpness/ Film Screen

Combo

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