Optical Density Flashcards
Radiographic quality
fidelity with which an anatomic
structure is represented in a
radiograph.
Radiographic imaging
multistep process in which the
measured parameter is transferred to
different types of information
carriers along the imaging chain.
The pathway for imaging processing consist of:
Image Acquisition
* Image Processing
* Image Display
* Image Analysis
* Image Archiving
Image acquisition
consists of all of
the steps leading up to and
including x-ray exposure to an
image receptor
Image processing
converts the
latent image into a manifest
image, accomplished by chemical processing or digital processing
Image display
allows the final
image to be seen using either a
viewbox illuminator (film-based)
or computer monitor
Image analysis
analyzing
the image to determine if proper
image quality is present
Major controller factor for optical density
mAs is the major controller of optical density, kVp influences optical density
Image archiving
storing image for future reference
Photographic properties
in film/screen imaging, optial density y and contrast are
considered photographic
properties, determine visibilty of detail
Geometric properties
recorded detail and distortion are considered geometric properties, dependent on geometry of xray beam and IR
Function of optical density
provide information in the radiographic image
3 possible definitions
Optical density is directly proportional to the amount of radiation absorbed by film emulsion
Optical density
Amount of blackening on a finished image
Densitometer
Measures optical density
Low optical density = more light transmitted through area
High optical density = less light transmitted through area
Photometer
Measures pixel brightness by measuring luminance
Higher the number, greater the amount of luminance
Diagnostically useful range of optical densities
0.5-2.5
Major factors of optical density
mAs
kVp
SID
Part thickness/tissue density
mAs
Has direct linear relationship to optical density/IR exposure
25-30% change in mAs is required to cause visible change in optical density (only in film)
Reciprocity failure can occur using extremely short or long exposure times, typically due to intensifying screen phosphors to produce the appropriate amount of light
Reciprocity failure results in underexposure of IR
Reciprocity failure is not seen in DR
kVp
Increase in kVp causes OD/IR exposure to increase because beam is more penetrating and more scatter is produced
kVp and OD have an exponential or logarithmic mathematical relationship
15% rule: increase of 15% in kVp will double OD
SID
SID and OD have an inverse square relationship. Density gets cut by 4 times if SID doubled
If SID cut in half, cut OD by 1/4
mAs and part thickness
For every 5 cm change in thickness, mAs should change by a factor of 2
Secondary factors affecting optical density
Development/processing
Field size
grid type/ratio
image receptor system speed
OID
Fogging/noise
Pathology
xray generator type
filtrtion
anode heel effect
Field size
Larger field size means increase in OD compared to smaller field size taken with same part nd same technical factors
More scatter created with larger field size
scatter adds to overall OD
IF field size reduced significantly, mAs should be increased by 30-50%
Developer processing
An increase in developer time,
temperature, pH,
concentration, or
replenishment rate will
increase optical density and
vice versa
Grid ratio
Higher grid ratio will decrease OD unless a compensation is made, using the bucky factors of the grid
IR system speed
Faster speed, greater OD and vice versa
OID
Increase OID, a decrease of OD occurs due to reducion of scatter hitting IR like air gap technique
Fogging
Fog is non-informational optical density
any fog resent increases overall OD
Caused by age, light exposure, processing parameters higher than specified
Pathology
Different diseases and conditions affect OD because they can change the normal tissue density present in region
2 basic categories of pathologies
Atropjic or destructive pathologies - decrease normal tissue density and require decrease in mAs by 30%
Additive diseases – these increase normal tissue density and require a
compensation of increasing mAs by 30%
Type of xray generator
Constant potential
equipment (3-phase & high
frequency) generators will
emit more radiation/mAs
than single phase
generators, thereby
increasing IR exposure.
* The difference is
approximately 40%
Filtration
Adding filtration decreases OD since radiation is absorbed from the beam
Relationship depends on type of filtering material
Anode heel effect
Increase of radiation on cathode side, affects OD on cathode side
More OD on cathode side vs anode side
Inherent optical densities
Optical densities in film based imaging
Found in film without any exposure taking place and come in 2 types
Base density- optical density caused by blue tint added to film base, value of 0.05
Fog density- development of silver grains that contain no useful information
