Lec 4 Image Characteristics Flashcards
Degree of darkening or opacity of an exposed film.
Optical density
Plot of relationship between film optical density and exposure.
Characteristic curve / Hurter-Driffield curve
Overall film density depends on the number of photons absorbed by film emulsion.
Exposure
The thicker the subject, the more the beam is attenuated, the larger the resultant image.
Subject thickness
The greater the density of a structure within the subject, the greater the attenuation of the x-ray beam directed through that subject or area.
Subject density
Dense objects cause radiographic image to be light and are said to be ______?
Radiopaque
Objects with low densities are weak absorbers. They allow most photons to pass through, and they cast a dark area on the film that corresponds to the _______ object.
Radiolucent
Defined as the difference in densities between light and dark regions on a radiograph.
Radiographic contrast
An image that shows both light areas and dark areas has ___________.
high contrast
An image that shows only light gray and dark gray areas has __________.
low contrast
High contrast = ______ grayscale of contrast
Low contrast = ________ grayscale of contrast
- High = short
- Low = long
The range of characteristics of the subject that influences radiographic contrast.
Subject contrast
Describes the inherent capacity of radiographic films to display differences in subject contrast.
Film contrast
Causes fogging of a radiograph — an overall darkening if the image — results in loss of radiographic contrast.
Scattered radiation
The amount of radiation required to produce an image of a standard density.
Radiographic speed
A measure of the range of exposures that can be recorded as distinguishable densities on a film.
Film latitude
The appearance of uneven density of a uniformly exposed radiographic film.
Radiographic noise
The ability of a radiograph to precisely define an edge.
Sharpness
The ability of a radiograph to record separate structures that are close together.
Resolution / resolving power
Image sharpness lost through movement of the film or subject during exposure.
Motion blurring
Loss of image sharpness results in part because photons are not emitted from a point source (focal spot) on the target in the xray tube.
Geometric blurring
Describes the subjective judgement by the clinician of the overall appearance of a radiograph.
Image quality
Combines the features of:
- density
- contrast
- latitude
- sharpness
- resolution
- other parameters
Image quality
The characteristic curve just shows that if you increase the _________, you increase the film’s _____________.
- exposure
- optical density
One can increase the exposure by increasing? (3)
Increasing the milliamperage (mA), peak kilovoltage (kVp), or exposure time
Subject contrast in influenced largely by? (3)
subject’s thickness, density, and atomic number
___ influences radiographic contrast.
kVp
kVp range of ___ to ___ is optimal for dental imaging.
60 to 80
As the kVp of the x-ray beam increases, subject contrast ________ (increases / decreases?).
decreases
Increasing the kVp _________ (increases / decreases?) the overall density of the image.
increases
Measured by the average slope of the diagnostically useful portion of the characteristic curve.
Film contrast
Low ___ and use of ____ will help prevent scatter radiation.
- Low kVp
- collimation
TRUE OR FALSE
A film optimized to display wide latitude records a short range of subject contrasts
FALSE
A film optimized to display wide latitude records a WIDE range of subject contrasts
TRUE OR FALSE
E/F-speed film is preferred because it requires approximately half the exposure time and thus half the radiation dose of D-speed film.
TRUE
Shows as radiographic mottle which can be seen as film graininess, caused by the visibility of silver grains in the film emulsion
Radiographic noise
TRUE OR FALSE
The finer the grain size, the finer the sharpness.
TRUE
______-speed films have finer grains, and _____ films have larger grains
- slow
- faster
TRUE OR FALSE
The larger the focal spot, the greater the loss of image sharpness.
TRUE
A basic measure of the efficiency of an imaging system. It encompasses image contrast, blur, speed, and noise.
Detective quantum efficiency (DQE)
