Transcript Prime Factors Image Quality Lecture Notes Page

Technical Factors or Prime
Factors
Bushong Ch 15
1
PRIME
FACTORS
What is “technique” ?
How does it affect the “image”
2
Exposure Factors – 3 or 4

The four prime exposure factors are:
Voltage = kVp*
Current = mA*
Exposure time = seconds or fractions of a sec*

Source-to-image distance = SID
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3
PRIME FACTORS
• KVP
• MAS
• DISTANCE
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kVp

Kilovolts controls how fast the electrons
are sent across the tube

Controls, quality, penetrability & contrast

Increasing kVp also increases scattered
photons reducing image quality
Does kVp influence OD?

5
kVp
• Low kVp (50 – 60)
• Short scale
• High contrast
• “Bone work”
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kVp
• High kVp (90 – 120)
• Long scale
• Low contrast
• “Chest images”
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mA


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Determines the number of photons, radiation
quantity, OD & patient dose
Changing mA does not change the kinetic
energy of eAvailable mA stations are usually 50, 100,
200, 300, 400 & 600
8
Exposure Time

Should be kept as short as possible, for most
examinations. To minimize the risk of patient
motion
mA X s = mAs
 mAs controls OD

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mAs determines the number of photons in the
primary beam
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Distance

Affects exposure of the IR because of the
Inverse Square Law

SID largely determines the intensity of
photons at the IR

Distance has no effect on radiation quality
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INTENSITY IS SPREAD OUT…
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Inverse Square Law Formula
Intensity #1
Intensity #2
Distance #2 Squared
Distance #1 Squared
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Direct Square Law
• New mAs = New distance 2
Old mAs
Old distance 2
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Focal-Spot Changes
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Tube voltage (kVp)
• Determines the maximum energy
in the beam
• spectrum and affects the quality
of the output spectrum
• Efficiency of x-ray production is
directly related to tube voltage
15
Influencing factors: kVp
15% rule:
 15% kVp = doubling of exposure to the film
 15% kVp = halving of exposure to the film
15% rule will always change the contrast of the
image because kV is the primary method of
changing image contrast.
Remember :
15% change ( ) KVP has the same effect as
doubling or ½ the MAS on density
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kVp Changes
• The kVp setting must be changed by at
least 4% to produce visual changes an
image
17
4% kVp Changes
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Radiographic Technique

Technique charts are based on the “average
patient”

The thicker the part the more x-radiation is
required to penetrate. Calipers should be used

Keep in mind not only the measurement but the
type of tissue you need to penetrate (fat vs
muscle)
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Technique

In general, Soft tissue = low kVp and high
mAs
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Extremity (soft tissue & bone) = low kVp

Chest (high subject contrast) = high kVp
Abdomen (low subject contrast) = middle kVp
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Pathology

Can appear with increased radiolucency or
radiopacity

Some pathology is destructive causing tissue
to be radiolucent

Others can be additive causing tissue to be
radiopaque
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Technique selection – Fixed kVp

For each anatomic part there is an optimum
kVp

mAs is varied based on part thickness or
pathological condition
22
Image Quality
Bushong Ch. 16
Objectives
• Image Quality – Factors
• Geometric Factors
• Subject Factors
• Artifacts
Image Quality
• Is the exactness of the representation of
the patient’s anatomy
• 3 major factors affecting image quality that
is under the control of the technologist:
Image Receptor selection/use, Geometric
factors & Subject factors.
Judging Image Quality
• The most important characteristic of
radiographic quality are: Spatial
Resolution, Contrast Resolution, Noise &
Artifacts
Main Factors Affecting
Recorded Detail
• kVp & mAs
• Motion
• Object
Unsharpness
• SID (Source to
Image Distance)
• OID (Object to
Image Distance)
• Material
Unsharpness/ Film
Screen Combo
• Focal Spot Size
• MTF (modulation
transfer function)
Recorded Detail
• Other names:
- detail
-sharpness of detail
-definition
-resolution
-degree of noise
- visibility of detail
Resolution
• Is the ability to image two separate objects
and visually distinguish one from the other.
• Spatial resolution is the ability to image
small objects that have high subject
contrast. Ex: bone-soft tissue interface,
breast calcifications, calcified lung nodule
• Conventional radiography has excellent
spatial resolution
RESOLUTION TEST
TOOLS
LINE PAIRS/ MM
Depicts how well you
can see the differences
in structures
More lines=more detail
Measuring Resolution for an x-ray
imaging system
SMPTE Test Pattern
• In 1985 the Society of Motion Picture and
Television Engineers (SMPTE) published
a recommended practice (RP-122).
• Specifications for Medical Diagnostic
Imaging Test Patterns for Television
Monitors and Hard-copy Cameras.
SMPTE Test Pattern
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
Focal spot size of the cathode
Line-focus principle
Modulation Transfer Function
• The ability of a system to record available
spatial frequencies.
• The sum of the components in a recording
system cannot be greater than the system
as a whole.
• When any component’s function is
compromised because of some type of
interference, the overall quality of the
system is affected.
Contrast Resolution
• Is the ability to distinguish anatomic
structures of similar subject contrast. Ex:
liver-spleen, gray matter-white matter
• Magnetic Resonance Imaging has the
highest contrast resolution
• Computed Tomography is excellent as
well
MRI
CT
The contrast of an object is expressed relative to
its surrounding background.
That is what determines its visibility.
Radiographic Contrast
• Is the product of image receptor contrast
and subject contrast
“Noise”
• Borrowed from electrical engineering
• Audio noise = hum or flutter heard from a
stereo
• Video noise = “snow” on a TV
• Radiographic noise = random fluctuation
on the OD of the image
QUANTUM MOTTLE
Not enough PHOTONS – can create a mottled or grainy
image - MORE COMMON IN CR SYSTEMS
Radiographic noise
Radiographic Noise
• Four components:
• Film graininess, structure mottle, quantum
mottle & scatter radiation
Radiographic Noise
• Film graininess – distribution & size of the
silver halide grains in the emulsion
• Structure mottle – speed of the intensifying
screen. Phosphor size & DQE/CE
• Not under the control of the technologist
Image Noise
• Speckled background on the image
• Caused when fast screens and high kVp
techniques are used. Noise reduces image
contrast
• The percentage of x-rays absorbed by the
screen is the detective quantum efficiency
(DQE)
• The amount of light emitted for each x-ray
absorbed is the conversion efficiency (CE)
Quantum Mottle
• An image produced with just a few x-rays
will have more quantum mottle.
• The use of very fast intensifying screens
or not enough mAs or kVp will increase
quantum mottle
Quantum mottle
Screen Speed
• Efficiency of a screen in converting x-rays to
light is Screen Speed.
Speed
• Fast image receptors
– ? Noise
? Spatial resolution
– ? Contrast resolution
• Slow image receptors
– ? Noise
? Spatial resolution
– ? Contrast resolution
–
See pg 274 for answers
Speed
• Low noise = fast or slow speed?
• High contrast resolution = fast or slow
speed?
• Reduced spatial resolution = fast or slow
speed?
PARALLAX –
each emulsion has an image
single image
overlaped edges
edge sharp
less sharp
Other Film Factors
Characteristic Curve
• Is used to describe the relationship
between OD and radiation exposure
What is the useful OD range?
Characteristic curve
of radiographic film
The latitude of an image receptor is the exposure
range over which it responds with diagnostically
useful OD.
• Depending on the
manufacturing
characteristics
radiographic film will
respond differently to
radiation exposure
F/S vs Digital
Dynamic Range
Unexposed film
• Appears like a frosted glass window
• ODs of unexposed film are due to base
density and fog density
• Base density – tint added to the base to
reduce eye strain and crossover. Has a
densitometer value of approximately 0.1
CROSSOVER
• Reducing crossover
by adding a dye to the
base
Unexposed film
• Fog Density – inadvertent exposure of film
during storage, chemical contamination,
improper processing, radiation exposure,
etc.
• Fog density contributes to reduction of
radiographic contrast
• Fog density should not exceed 0.1
• Base + fog OD = 0.1 to 0.3
2 principal characteristics of any image
are Spatial & Contrast Resolution
• Spatial resolution
– Resolution is the ability to image two separate
objects and visually distinguish one from the
other
– Spatial resolution is the ability to image small
objects that have high subject contrast (eg.
bone-soft tissue interface, calcified lung
nodules)
– Determined by focal-spot size and other
factors that contribute to blur
– Diagnostic x-ray has excellent spatial
resolution. It is measured in line pairs per mm.
Other factors affecting the
finished radiograph
• The concentration of processing chemicals
• The degree of chemistry agitation during
development
• Development time
• Development temperature
Image Quality in Digital
Matrix size is determined by . . .
• Receptor size (Field of View: FOV)
• Pixel size
• CR - Sampling frequency
• DR - DEL size
Spatial Resolution determined by:
􀁹 Pixel size.
• CR- sampling frequency
• DR – DEL size
• 􀁹 There are relationships between
• Pixel size
• Receptor size
• Matrix size
• 􀁹 pixel size = larger matrix
• 􀁹 receptor size = larger matrix
• Spatial resolution is not related the amount of exposure
Sampling Frequency
• The sampling frequency is the rate at
• which the laser extracts the image data
• from the plate.
Signal Sampling Frequency
Good sampling
under sampling
Nyquist Frequency
• The Nyquist Frequency will be ½ of the
• sampling frequency.
• A plate that is scanned using a sampling
frequency of 10 pixels per millimeter would not
be able to demonstrate more than 5 line pairs
per millimeter based upon the Nyquist
Frequency.
• The Nyquist Frequency allows the
determination of the spatial resolution for
a given sampling frequency.
Geometric Factors
• Producing high quality radiographs.
Technologists must maximize geometric
conditions
• Three principal geometric conditions affect
radiographic quality: Magnification,
Distortion & Focal-spot blur.
•
Review table 16-4, pg. 295 for summary
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
Magnification
• All image on the radiograph are larger
than the object they represent.
• For most exams minimizing magnification
is desired. There are a few exams where
some magnification can be helpful
• TUBE CLOSE TO THE PART (SID),
• PART FAR FROM THE CASSETTE (OID)
Minimizing Magnification
• Large SID: use as large a source-to-image
receptor distance as possible
• Small OID: place the object as close to the
Image receptor as possible
• In terms of recorded detail and magnification,
the best image is produced with a small OID and
a large 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
The position of the tube (SID) to IR
Will influence how the structures appear
on the image The farther away – the
less magnified ↑SID ↓ MAGNIFICATION
SID
• Shine a flashlight on a 3-D object, shadow
borders will appear “fuzzy”
-On a radiograph called Penumbra
• Penumbra (fuzziness) obscures true
border – umbra
• Farther the flashlight from object =
sharper borders. Same with radiography.
OID
Object 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.
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
The position of the structure in the body
will influence how magnified it will be
seen on the image
The farther away – the more magnified
Minimal magnification
small OID
Magnification large OID
40” SID VS 72” SID
Magnification Factor
source-to-image receptor distance
• MF =
source-to-object distance
– SOD difficult to measure accurately
*usually an estimated value
• MF =
SID
SOD
Finding SOD
• SID – OID = SOD
• SID = 100 cm
• OID = 7 cm
• What is the SOD?
Finding magnification of the heart
on a lateral CXR
• SID – OID = SOD
• SID = 72 inches
• OID = 8 inches (estimated)
• What is the SOD?
• What is the Mag Factor?
Distortion
• Misrepresentation of the true size or shape
of an object
-MAGNIFICATION (size distortion)
-TRUE DISTORTION (shape
distortion)
• Shape distortion: unequal magnification of
different portions of the same object
Shape Distortion
• Depends on:
• Object thickness
• Object position
• Object shape
Object Thickness
• Thick objects have more OID and are
more distorted than thinner structures
Object Position
• If the object plane and the image plane are
parallel, the image is not distorted
• CR perpendicular
to the part
Position Distortion
• Foreshortened = anatomy at an incline to
the CR displays smaller than true size
D&E =
shape distortion
(foreshortening of part)
Position Distortion
• Elongation: anatomy at an incline and
lateral to the central axis
– Could be
foreshortened as well
Elongation
Foreshortened
Normal
Position Distortion
• Spatial distortion = anatomy positioned at
various OIDs but superimposed, only one
can be seen
Position Distortion – Irregular
Anatomy
• Anatomy or objects can cause
considerable distortion when imaged off
the central axis
Focal-Spot Blur
• Dependent on the size of the effective
focal spot
•
Smaller the effective focal spot = less
blur and better spatial resolution
• Focal-spot blur is the most important
factor in determining spatial resolution
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
ANODE
ANODE
Focal spot size – determined by filament in cathode
& surface area used at anode
THE SMALLER THE BEAM TOWARDS THE
PATIENT - THE BETTER THE DETAIL OF
THE IMAGE PRODUCED
Focal-spot blur is caused by the effective
size of the focal spot, which is larger to the
cathode side of the image.
Focal-spot blur is small when the object-toimage receptor distance (OID) is small.
Image Quality
Subject Factors
• Patient thickness • Subject contrast
• Effective atomic
number
• Tissue mass
density
• Object shape
• kVp
Object shape
Objects with structure
having a form that
coincides with the
x-ray beam has
maximum detail
Patient 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
Blurring of image
due to patient
movement during
exposure.
Questions?