The measurement of the characteristic responses of film to exposure and processing and is accomplished by exposing and processing a film and then measuring and evaluating the resulting densities (How a film responds to radiation)
sensitometric methods are useful for...
evaluating technical factor exposure systems, films, intensifying screens and processing equipment
two devices that are used to produce a uniform range of densities on a film
device required to provide an accurate reading of the amount of light transmitted through the film
Device constructed of uniform absorbers of increasing thicknesses and made of aluminum or tissue-equivalent plastic. (The thinner the step, the more the density changes).
Penetrometer (step wedge)
Numeric calculation that compares the amount of light transmitted thru an area of radiographic film to the amount of light orginally striking (incident) the film.
Optical Density Formula
OD= log10 (I o/I t) where:
I o= intensity of incident light
I t= intensity of transmitted light
when using a log scale with a base of 10, every .3 increment increase represents
the percent of light transmitted has changed by a factor of 2 or a doubling in exposure
Graphic relationship between the amount of exposure and the resultant density on film
Additional exposure beyond Dmax will result in ___ because silver atoms attached to sensitivity specks will be ___, reversing their charge and causing them to be repelled from the speck.
Ability to accurately image an object
(also called detail, sharpness, definition, resolving power)
Determined by locating point on curve that corresponds to optical density of 1.0 plus B+F
Speed Exposure Point
Indicated intensity of exposure (log exposure) needed to produce a density of 1.0 plus B+F (point where you draw line from curve speed point to area on x-axis)
The OD specified for determining speed is 1.0 above B+F density and the speed is measured in reciprocal roentgens (1/R)
Difference between adjacent densities on a film. Defined by slope of straight-line portion of curve
toe gradient is between?
Slope of any part of curve
between OD .25 and 1.0
between OD 1.0 and 2.0
between OD 2.0 and 2.5
Range of exposures that produce ODs within the straight line region of curve. Formula = Eh-Ei
why is a penetrometer not recommended for quality control of film processors
vast number of variables in x-ray generating equipment
why is penetrometer useful for monitoring x-ray equip and film/intensifying screen combos
reproduces variables of the clinical situation
designed to expose a reproducible, uniform, optical step wedge onto a film with a controlled-intensity light source
21 step wedges increase density by what percent? what factor?
factor of 1.41 or the square root of 2
Because of the rigid control of the densities produced on the film, other variables that may affect processor quality control monitoring are eliminated, thus the use of a ____ for monitoring quality control of automatic processors is acceptable.
sensitometer produced step wedges
sensitometric film strips should be fed into a processor how? why?
either long edge or axis of the step wedge parallel to entrance rollers or with light edge entering processor first.
exhausted reducing agents and bromide ions may be carried backward on the emulsion as film is driven through rollers
____ strips are produced by laser, dry imaging (when images are transferred to films_ and PACS to calibrate flat-panel monitors
parts of a densitometer
calibrated uniform light source
stage for placing film
light aperture to control light from source
sensor arm with optical sensor
densitometers should be calibrated before each reading to determine...
the amount of light the light source is emitting
how are density readings made by a densitometer
comparing amount of light emitted by source with amount of light transmitted through the film
how is the densitometer calibrated or zeroed for reading
pushing sensor arm so that sensor is in contact with light source to eliminate inverse squ. law
If a radiograph has an OD of 1.0 this means only what percentage of light is transmitted to the radiograph in this region? If the OD is increased to 1.3, then the opacity is ___ and the light transmitted through the film is ___?
Density at no exposure, or density that is inherent in film (tint)
base + fog
includes tints and dyes in base plus any fog film has experienced
straight line portion is usually fairly straight because...
film is reacting in a linear fashion to exposure in the range of its primary sensitivity
range of diagnostic densities
low OD .25-.50 and high OD 2.0-3.0
majority will be between .5 and 1.25
(these densities are within the straight line portion)
controls heavy black tone later in development process?
affects which part of the curve?
point on curve where all silver halides have a full complement of silver atoms and cannot accept more
extremely high or long exposures can cause silver halide crystals to become overloaded and ____ results
determined by size of halide crystals
because small crystals darken small areas and large crystals darken large areas, therefore information smaller than the crystal cannot be visualized
speed is controlled by the activity of ____ because it affects the toe of the D log E curve.
position of the toe determines when straight line portion begins and indicates overall speed
factors affecting film sensitivity other than crystal size
number of sensitivity specks
thickness of emulsion layer
relationship between film speed and emulsion thickness
one photon can interact with many crystals therefore if more crystals are in a given area, more interactions will occur
how is difference in film speed calculated
antilog (log E1- logE2)
where log E1 is the log exposure of first film
formula for relative speed
exposure in R needed to produce speed point density (OD 1.0 +b+f)
only ___ degrees in developer temp. will result in a visible change in film density
higher temp increases activity level of reducing agents thereby producing greater density on the film
chemical activity factors affecting film speed
bromide ion barrier not formed properly (insufficient restrainer or overreplenishment)
or it is too strong (excess restrainer or underreplenishment)
For the same film, a change in density will affect contrast only when...
above or below the straight line portion of the D log E curve
hydroquinone establishes which region of the curve and affects what other portion
in radiographic films, doubling the radiographic exposure will...
produce more than a doubling of radiographic opacity
diagnostic range of radiographic film
when densities are above or below this range contrast is decreased