Final Review: Spacial Resolution and Distortion Flashcards
Two geometric properties:
-spacial resolution
-distortion
Spacial resolution is referred to several terms:
-definition
-sharpness
-recorded detail
A misrepresentation in an image of an objects size or shape is called
distortion
what happens to recorded detail when there is less distortion
recorded detail increases
what happens to the recorded detail as the magnification increases
recorded detail decreases
what are the two types of distortion
size and shape distortion
size distortion is also called:
magnification
is called elongation when a structure appears longer in an image than the structure really is, and foreshortened when the structure appears shorter
Shape distortion
How can magnification be kept minimal?
increase in SID and decrease in OID
what law describes how the xray beam diverges, the principle that accounts for magnification and unsharpness
Divergence or penumbra law
what is kept minimized as possible to reduce magnification
OID
increases the OID about 4in and increases the magnification of the image
use of a grid
What kind pf distortion is present on every radiograph?
Magnification- because there will always be OID
Ability of imaging system to image structures as separate and distinct
Spacial Resolution
Dependent on matrix size, pixel size, and grayscale bit depth
Spacial Resolution
what happens to spacial resolution when there is an increases matrix and decreased pixel size
increased spacial resolution
what wavelength is high spacial frequency
short wavelength
what wavelength is low spacial frequency
long wavelength
what represents your rows and columns
matrix
what represents your individual squares
pixels
Compare digital and film to dynamic range
Digital has more dynamic range and film has less dynamic range
unit of resolution
lp/mm
Penumbra
-unsharpness or blur
high spacial resolutions is expressed as
high frequency
More shades of gray, better spacial resolution(more detail)
Gray Scale
noise
Unwanted on image
Assessing Recorded Detail
Spacial Resolution
Spacial Frequency
Noise
-Ability of imaging system to accurately display objects in two dimensions
-penumbra vs. umbra
Spacial Resolution
-High frequency signal
-determined by measuring distance between pairs of lines distinct from one another
spacial frequency
what happens to signal when noise is decreased
increases
umbra
sharpness
Measures accuracy of image to actual image
-range from 0-1
- 1 represents “perfect”
reproduction of original
MTF
As higher spacial frequencies occurs what happens to MTF
falls dramatically
Bit Depth
how many shades of gray that pixel can hold
Ability to capture and display subtle energy differences in area of interest
-responsible for low energy, shades of gray on image
Low Contrast Resolution (LCR)
Background information received by image receptor
noise
Types of Noise:
System Noise
Ambient Noise
Quantum Noise
Best Spacial Resolution
small focal spot
increased SID
decreased OID
Resolution as a function of exposure time and dynamic motion of body part
Temporal Resolution
Extremely low exposure times optimize TR
**I dont think we went over this but just put it in here just in case lolll
-artifact if sampling frequency inst working
-occurs when Nyquist criterion not met
Aliasing(moire pattern)
Factors affecting recorded detail
Eliminate motion
reduce oid
reduce focal spot size
use detector with smallest detector element (DEL)
reduce intensifying screen phosphor size and concentration (film)
increase SID
-line focus principle
-creates a penumbra and inherent loss of sharpness due to focal spot geometry and distance -a major factor in spacial resolution
Focal Spot Size
-some degree of image unsharpness occurs due to shape and size of patient anatomy
-related to beam divergence and incongruence with anatomical structures
Attenuation/Absorption Unsharpness
Image processing system limits recorded detail dependent on:
-matrix(large)
-pixel (small)
-bit depth (large)
Motion
-voluntary (communication)
-Involuntary motion( exposure time reduction and immobilization)
-equipment motion
how to reduce voluntary motion
best reduced through effective communication
how to reduce involuntary motion
-best reduced through short exposure time
-immobilization
Anatomic details must be recorded accurately and with the greatest amount of sharpness
Recorded Detail
Refers to the distinctness or sharpness of the structural lines that make up the recorded film image
Recorded Detail
What affects distortion?
CR alignment of the tube, part, and image receptor
Radiographic misrepresentation of either the size or shape of the anatomic part
Distortion
an increase in the objects image size compared to its true or actual size
-SID and OID affect magnification
Size distortion
A misrepresentation of an objects image shape
-elongation and foreshortening
Shape Distortion
Radiographic distances
-controlled by SID and OID
-Magnification only
electronic magnification/minification
post processing can resize image
SID is a major factor for magnification
longer SID yields less magnification
Longer SID’s advocated by experts
Decreases entrance skin exposure
SID
OID is a critical distance for magnification and spacial resolution
OID should be minimized, whenever possible, to minimize magnification and improve resolution
OID
Unwanted exposure to the image receptor resulting in fog
-a result of Compton
interactions
-provides no useful
information
-scatter of fog decreases
image contrast
Scatter
in medical radiography, image size is always larger than object size
O=I/M
O=object size
I=image size
M=magnification factor
M= SID/SOD
magnification factor
Role of central ray
Alignment
-central ray
-anatomical part
-Image Receptor
Angulation
-Degree
-Direction
if an angle is needed, what is better to angle for better spacial resolution
better spacial resolution is achieved by angling body part rather than angling central ray
what is an example of purposeful distortion
Radiographic positioning
SOD=
SID-OID
As the primary beam passes through the patient it will lose some of its orginal energy. This reduction in the energy of the primary beam is known as :
Beam Attenuation
To maintain exposure to IR:
increase mA and proportionally decrease time
increase time and proportionally decrease mA
increase kvp 15% and decrease mAs by a half
Decrease kvp 15% and increase mAs by two times
To increase or decrease exposure to IR what do you do to mA, exposure time or kVp
Increase:
increase mA, exposure time or kVp
Decrease:
decrease mA, exposure time and lVp
increases penetration and decreases absorption
increasing kvp
decreases penetration and increases absorption
decreasing kVp
what type of wavelength is there when the kvp is higher
shorter the wavelength
and lower the kvp, longer the wavelength
Doing what to the OID will increase the exposure to the IR, decrease contrast and magnification, and increase recorded detail/ spacial resolution
Decreasing OID
Distance between the anatomic part and IR will affect:
-radiation intensity reaching the ir
-amount of scatter radiation reaching the ir
-magnification
-recorded detail/spacial resolution
Practice:
penumbra equation
magnification factor
