Contrast Flashcards
Image contrast
Differences between adjacent optical densities/gray shades, or range of shades available in image
Functions to make details visible
Dynamic range
Describes the concept of contrast as it is displayed on soft copy monitor for DR
Proper term for range of brightness
3 types of contrast to consider
Radiographic, image, or displayed
Subject contrast
Image receptor, inherent, or detector
Radiographic contrast
The contrast on finished image, contrast that we see in the final image
Subject contrast
Range of differences in the quantity of radiation transmitted by a particular part as a result of the different
absorption characteristics of the tissues and structures of that part
Helps determine radiographic contrast
Low contrast, long scale
High range of grays, mostly appear gray, also known as decreased contrast
High contrast, short scale
Low range of grays, appears mostly black and white, also known as increased contrast
Image receptor contrast
the contrast that is
built into the image receptor system by the manufacturer
Major control of image contrast in film screen
kVp
Lower kVp
Lower energy=cant penetrate through the body, less shades of gray
Long wavelength
High kVp
More energy=can penetrate through body, more shades of gray
short wavelength
Optimum kVp values
Extremities - 60-80
Chest- 110-125
Ribs & cervical - 60-75
Thoracic spine - 75 for AP, 90 for lateral
Lumbar Spine – 80 for AP and 90
to 115 for lateral
Skull - 85
Digital imaging
kVp will influence, but not control displayed contrast
Computer processing will correct shade values
Factors affecting subject contrast
Internal:
anatomic part - homogenous vs heterogenous
Pathology - changes tissue density
External:
Radiation quality
Contrast Media
Scatter Radiation
Fogging
Chest vs abdomen
Chest has high subject contrast because there’s only bone and air. Abdomen has low subject contrast because there are lots of tissue, and fluids
Tissue densities
Tooth enamel - most dense tissue
bone
solid organs
hollow organs (full)
muscle
hollow organs (empty)
cartilage
fat
gas
Pathology
Changes subject contrast because it can change normal tissue density
Edemas and hemorrhaging tend to decrease subject contrast
Radiation quality
As quality of radiation increases, subject contrast decreases
causes radiographic contrast to decrease
Radiation quality is affected by:
kVp - affects maximum energy of beam
beam filtration - affects average energy of beam
type of xray generator - affects average energy of beam
Beam filtration
Removing low energy xrays, reduce patient dose, increase energy of beam to pass through patient
Contrast media
Often used in areas where tissue densities are more homogenous to increase subject contrast
2 basic types of contrast media
Positive - absorbs xrays, causing decrease in optical density
Negative - allows xrays to pass through with minimal absorption increasing optical density
Scatter radiation
Causes fogging or noise
decreases subject contrast = decrease in radiographic contrast
The more scatter removed, the higher the contrast
Fogging
will decrease contrast by adding extra shades of gray
IR contrast factors in film/screen
Film emulsion(speed) - faster speed emulsions yield higher inherent contrast
Film development - Proper time, temp, concentration, pH are essential to contrast. Any change will decrease inherent contrast
Intensifying Screen/Scintillator Speed - the faster the speed of the
screen, the higher the inherent contrast
Sensitometry
Study of relationship between the intensity y of radiation exposure (light or
x-rays) and the optical density that is
produced
Developed by Hurter & Driffield
Sensitometric curve
Optical density readings are plotted on y-axis and the log exposure on the x-axis
3 main parts of the sensitometric curve
toe - indicates base + fog
Straight line portion - contains useful optical densities
Shoulder - indicates saturation point or maximum optical density
Slope - used to indicate the inherent contrast
Calculation of film gamma
2 slope values that can be utilized to measure inherent contrasts are:
1. Film gamma - maximum slope measured at the steepest point
Normal ranges are 2.0 to 3.5
Subjective and not as accurate
2. Film average gradient - mean slope between the optical densities of 0.25-2.0 above base + fog
Better indicator
Average gradient range - 2.5-3.5
Film speed or sensitivity
relative exposure necessary to produce an optical density of
1.0
Exposure latitude
Range of exposures over which xray film respond with optical densities between 0.25 and 2.0 above the base + fog
High contrast film has narrow latitude
Solarization or image reversal
A decrease in optical density as exposure increases
Caused by rebromination
Used in duplicating film, pre exposed to solarization point
