Exposure Contrast Flashcards
Receptor eposure/Optical density
Amount of overall blackness produced on the processed image
In digital, represented by exposure index/ S-Value
High EI = High exposure
Low EI = Low exposure
High S = Low exposure
Low S = High exposure
Receptor exposure controlling/influencing factors
mAs - Determines quantity of xrays produced and eit the xray tube, has direct relationship with RE, governed by reciprocity law
kVp - manages quality of xrays, xrays wih high energy (short wave length, high frequency) are able to penetrate body & reach IR, Low energy = Long wave length, low frequency, exponential relationship with RE (small inrease in kVp, big increase in RE), governed by 15% rule
SID - Has inverse square relationship with RE, if SID is doubled, beam will have 1/4 its original intenity, changing SID means you have to adjust mAS, Beam intensity = new distance^2/old distance^2
Beam intensity equation
Old intensity/new intensity = new distance ^2/ old distance^2
Grids
Absorb scatter radiation = less radiation striking IR/detector
As grid ratio increases, RE decreases
mAs must be increased to compensate for decrease in RE
Beam restriction
Decreases receptor exposure by limiting the size of xray beam, unless mAs is increased to compensate
Decreases receptor exposure by limiting the area of the patient being struck by photons
Reduces the amount of scatter radiation being produced, reducing receptor exposure by fog
Anatomy and pathology
Anatomy effects receptor exposure through its variation of atomic number, tissue thickness, tissue density
Pathologic changes affect receptor exposure by altering tissue integrity, atomic number, tissue density, tissue thickness
Anode heel effect
Xray intensity varies along the longitudinal axis of the beam
Exposure greatest near cathode end\Exposure less near the anode end, due to photons being absorbed by the heel
Thicker anatomy should be placed under the cathode side
Filtration
Negligible effect on receptor exposure, its purpose is primarily for radiation protection
As filtration increases, RE decreases
Contrast
Visible difference between two selected areas of brightness in the displayed radiographic image
Effect of kVp on contrast using penetrometer
High kVp = long scale, low contrast
Low kVp = Short scale, High contrast
kVp
influences contrast, but computer processing sets displayed contrast
Grids
Allow most primary radiation through & absorb scatter radiation
Less fog = less shades of gray
Collimation
Open collimation = more matter for xrays to interact with
Allows for more scatter production
Scatter decreases contrast
Collimating to necessary anatomy, reduces scatter production
Filtration
Aluminum filtration used to absorb low energy xrays
Reduces patient skin dose
Filtration increase, contrast decreases by average energy of beam
Subject contrast
Difference in tissue densities being xrayed
Breast tissue
Similar density = low subject contrast, need to increase image contrast by decreasing kVp
Chest
Large differences in tissue densities = High subject contrast
Need to decrease image contrast by increasing kVp
Recorded detail
Sharpness of the structural edges in the images, smallest detail that can be detected
Also referred to as definition, sharpness, spatial resolution
Recorded detail controlling and influencing factors
Pixel size - smaller pixels provide greater spatial resolution
Pixel pitch - space between pixels, less space between pixels provides greater spatial resolution
OID
Distance from anatomic part being imaged to the image receptor
Shortest possible OID should be used
Increased OID causes magnification of the image, resulting in loss of recorded detail
SID
Increased SID compensates for increased OID
Decreases magnification and increase in recorded detail
Focal spot size
Small focal spot = better detail
Contrast resolution
ability of the imaging system to distinguish the smallest change in exposure or signal that can be detected
provides visibility of small objects having similar shades of gray
controlled by bit depth, the number of bits per pixel (the larger the bit depth, the greater the number of levels of gray possible in an image)
Distortion
Any geometric misrepresentation of an anatomic structure on an image receptor
Two types of distortion: size and shape
Controlling factors
Size
Magnification
Influencing factors
OID
SID
SOD
Causes anatomic structure to appear larger on the image than in reality
As SID increases, the amount of magnification decreases due to less beam divergence.
A shorter SID will cause more magnification to occur.
