Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
Main Factors Affecting Recorded Detail
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
l kVp & mAs
l Technique Selection (Time)
l Motion
l Object Unsharpness
l Focal Spot Size
l SID (Source to Image Distance)
l OID (Object to Image Distance)
l Material Unsharpness/ Film Screen
Combo
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
Factors that affectRecorded Detail
l Geometric unsharpness
OID SID SIZE SHAPE
l Motion unsharpness (blurring)
l Intensifying Screens
l Film Speed / Composition
l Film – Screen contact
l Kvp & Mas (density / visibility)
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
GEOMETRIC QUALITIES
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
l DETAIL
l DISTORTION
l MAGNIFICATION
DETAIL
The degree of sharpness in an object’s borders and structural details.
How “clear” the object looks on the radiograph
Recorded Detail
l Other names:
-sharpness of detail resolution
-definition degree of noise
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
Factors Affecting DENSITY
l PATIENT THICKNESS,PATHOLOGY MAS & KVP SID
Motion
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
l Can be voluntary or involuntary
l Best controlled by short exposure times
l Use of careful instructions to the pt.
l Suspension of pt. respiration
l Immobilization devices
Decrease Motion Unsharpness
l Instruct patient not to move or breath
l Use Immobilization devices
l Use Short exposure times
l Lock equipment in place
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
Focal Spot Size
l Smaller x-ray beam width will produce a sharper image.
l Fine detail = small focal spot (i.e. small bones)
l General radiography uses large focal spot
l Beam from penlight size flashlight vs. flood light beam
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
FOCAL SPOT ANGLE
Object Unsharpness
l Main problem is trying to image a 3-D object on a 2-D film.
l Human body is not straight edges and sharp angles.
l We must compensate for object unsharpness with factors we can control: focal spot size, SID & OID
SID Source to Image Distance
l The greater the distance between the source of the x-ray (tube) and the image receptor (cassette), the greater the image sharpness.
l Standard distance = 40 in. most exams Exception = Chest radiography 72 in.SID
l Shine a flashlight on a 3-D object, shadow borders will appear “fuzzy”
On a radiograph it’s called ______
l A true border – _____
l Farther the flashlight from object = sharper borders. Same with radiography.
OIDObject to Image Distance
l The closer the object to the film, the sharper the detail.
l OID ê, penumbra ê, sharpness é
l OID é, penumbra é, sharpness ê
l Structures located deep in the body, radiographer must know how to position to get the object closest to the film.
Distortion Misrepresentation of the true size or shape of an object
MAGNIFICATION (size distortion) TRUE DISTORTION (shape distortion)
Shape Distortion
l Misrepresentation of the shape of an object
l Controlled by alignment of the beam, part (object), & image receptor
l Influences: Central ray angulation & body part rotation Elongation Foreshortened Normal
Central Ray
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
l Radiation beam diverges from the tube in a pyramid shape.
l Photons in the center travel along a straight line – CR
l Photons along the beam’s periphery travel at an angle
l When central ray in angled, image shape is distorted.
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
Central Ray Angulation
l Body parts are not always 90 degrees from one another
l Central ray angulation is used to demonstrate certain details that can be hidden by superimposed body parts.
l Body part rotation or obliquing the body can also help visualize superimposed anatomy.
l NAME 3 EXAMPLES
MAGNIFICATIONcaused by:
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
l TUBE CLOSE TO THE PART (↓SID)
l PART FAR FROM THE CASSETTE
(↑ OID)
Compensate for MAG : ↑ OID by ↑ SID =
“increase SID 7” for every 1” OID
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
Size Distortion & SID
l Major influences: SID & OID
l As SID é, magnification ê
l Standardized SID’s allow radiologist to assume certain amt. of magnification factors are present
l Must note deviations from standard SID What can be done to improve the detail with a large OID
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
Use a smaller FS
40” SID VS 72” SID
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
Size Distortion & OID
l If source is kept constant, OID will affect magnification
l As OID ê, magnification ê
l The farther the object is from the film, the more magnification
How can it be measured? Measuring % of Magnification
SID
SOD
Measuring % of Magnification
What is the % of mag when you have a 72” SID and4” OID? DO the math…………
Material Unsharpness
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
l Equipment used can contribute to image unsharpness
l Fast film/screen combinations = decrease in image sharpness
l Slower film/screen combinations = increase in image sharpness
Intensifying screens
l Lower patient dose
l Changes resolution of image
l slow screens less LIGHT = better detail
l Faster – less detail (more blurring on edges)
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
Intensifying Screens: Review
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
l Located inside the cassette (film holder)
l Calcium Tungstate
l Blue to purple light
l Rare Earth
l Green & Ultraviolet light
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
POOR SCREEN CONTACT
l FOAM BACKING HELPS TO PLACE INTENSIFYING SCREENS IN DIRECT CONTACT WITH THE FILM – NO GAPS
l IF GAPS – MORE LIGHT CAN BE EMITTED IN SPACE, CAUSING THE IMAGE TO BE OF POOR DETAIL
WIRE MESH FOR SCREEN CONTACT TEST
Screen Speed
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
l Efficiency of a screen in converting x-rays to light is Screen Speed.
Spectral Matching (F/S)
l What does it mean?
l Name the two types of screen phosphors
l What light spectrum do they emit?
Spectral Sensitivity
Film is designed to be sensitive to the color of light emitted by the intensifying screens.
l Blue LIGHT– Conventional Calcium Tungstate screen
l Green, Yellow-Green LIGHT
l – Rare Earth screen
Spectral Matching (F/S systems)
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
Safe lights What wattage bulb? Distance from counter top?
Review of Film Characteristics
l Size of silver halide crystals & emulsion thickness determine speed of film and degree of resolution
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
Speed – the response to photons
l Resolution – the detail seen
l What are these
l What are they made ofFilm Speed / Crystal size
l Larger crystals or Thicker crystal layer
Faster response= less detail, and
less exposure (chest x-ray)
l Finer crystals / thinner crystal layer
=Slower response, greater detail, more exposure (extremity)
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
IMAGE ON FILM
l SINGLE EMULSION = BETTER DETAIL DOUBLE EMULISON = LESS DETAIL
l PARALLAX With double emulsion – an image is created on both emulsions – then superimposed – slight blurring of edges
Extremity vs Regular cassettes
QUANTUM MOTTLE Film grain, or graininess, refers to the tiny black spots that make up the visible image, one grain from each silver halide crystal exposed
MORE COMMON IN CR SYSTEMS NOW - NOT ENOUGH PHOTONS TO CREATE IMAGE
Factors Affecting mAs
LIST 6 Factors Affecting mAs
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
l Patient factors: size of pt., density of tissue, pathology
l kVp
l Distance - how
l Grids
l Film/Screen Combinations
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
l Processing
Technique /Denisty CHANGES Log denisty H & D curve
l a densitometer, measures film blackness.
l 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).
l These measurements plotted on a graph produce a characteristic curve.
The limitations of the human eye determine the useful density range in diagnostic radiography.
l The diagnostically useful range of densities is 0.25 to 2.5.
l The later module on exposure calculation considers this in more detail.
Film latitude ? What does it mean how does it plot on the curve?
Main Factors Affecting Recorded Detail
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1
l kVp & mAs
l Motion
l Object Unsharpness
l Focal Spot Size
l SID (Source to Image Distance)
l OID (Object to Image Distance)
l Material Unsharpness/ Film Screen
Combo
Components of the Radiolgraphic Image Detail and Distortion RT 244 pg 1