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112 terms

sensitometry

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Sensitometry
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
sensitometer
penetrometer
device required to provide an accurate reading of the amount of light transmitted through the film
densitometer
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)
Optical Density
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
Sensitometric curve
Base + Fog
AKA "Toe" on Curve
Density at no exposure, or density that is inherent in film (tint)
Straight Line Portion
Portion between toe and shoulder
(gamma)
Dmax
maximum density film is capable of recording (highest point of curve)
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.
less density
ionized again
Reduces the intensity of the latent image and will produce less density
reversal or solarization
Film Characteristics
Resolution, Speed, Contrast, Latitude
Ability to accurately image an object
resolution
(also called detail, sharpness, definition, resolving power)
Speed
Amount of density a film produces for a given amount of exposure.
Speed Point
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)
Relative Speed
The OD specified for determining speed is 1.0 above B+F density and the speed is measured in reciprocal roentgens (1/R)
Relative Speed Formula
1/exposure in R needed to produce speed point density (OD 1.0+ B+F)
Film Contrast
Difference between adjacent densities on a film. Defined by slope of straight-line portion of curve
Gradient Point?
toe gradient is between?
middle gradient?
upper gradient?
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
Average Gradient Formula
D2-D1/E2-E1
D1=
OD .25 +B+F
D2=
OD 2.0 +B+F
E=
exposure that produces D1 and D2
Exposure Latitude
Range of exposures that produce ODs within the straight line region of curve. Formula = Eh-Ei
alternate name for penetrometer
step wedge
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
sensitometer
which uses x-rays to produce a step wedge:
penetrometer
sensitometer
penentrometer
type of light best for sensitometer source
pulsed stroboscopic
optical step wedges are available in what 2 step versions
11 and 21
11 step wedge increase density by what percent ? which is a factor of ___.
100
2
21 step wedges increase density by what percent? what factor?
41 percent
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
sensitometric
device that provides a readout of the amount of blackening (density) on a film
densitometer
parts of a densitometer
calibrated uniform light source
stage for placing film
light aperture to control light from source
sensor arm with optical sensor
readout display
calibration control
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
numbers displayed by a densitometer are know as
optical density numbers
radiographic film densities range from....
0.0 to 4.0
ability of a film to stop light
opacity
density is the log10 of
opacity
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 ___?
10
doubled
halved
increments of .3 changes represent...
doubling or halving of opacity
Alternate name for D log E curves
characteristic, sensitometric, Hurter and Driffield (H&D)
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
radiographic film base density range
OD .05-.10
amount of OD that processing typically adds to a film in fog
.05-.10
Total b + f is rarely below OD __ and usually does not exceed ___
.10
.22
fog may be caused by..
heat, chemical fumes, light, x-radiation
natural amounts of radiation occurring on film is termed...
age fog
most of a film's fog is produced by...
processing
controls subtle gray tones in early development process
phenidone
phenidone controls which portion of the curve?
toe
portion of curve between toe and shoulder
straight line portion
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?
hydroquinone
shoulder
maximum density film is capable of recording
Dmax
highest point of D log E curve
Dmax
point on curve where all silver halides have a full complement of silver atoms and cannot accept more
Dmax
extremely high or long exposures can cause silver halide crystals to become overloaded and ____ results
reciprocity failure
measured by the ability to see pairs of lines
resolution
unit of resolution
line pairs per mm
lp/mm
determined by size of halide crystals
resolution
because small crystals darken small areas and large crystals darken large areas, therefore information smaller than the crystal cannot be visualized
relationship between film resolution and crystal size
inverse
(smaller crystal= higher resolution)
poor resolution
graininess
graininess controlled by..
size of intensifying screen phosphors rather than crystal size
determined by film's sensitivity to exposure
speed
speed is controlled by the activity of ____ because it affects the toe of the D log E curve.
phenidone
position of the toe determines when straight line portion begins and indicates overall speed
film sensitivity is determined primarily by...
size of silver halide crystals
factors affecting film sensitivity other than crystal size
number of sensitivity specks
thickness of emulsion layer
relationship between film speed and crystal size
directly related
(larger crystal-faster speed)
relationship between film speed and number of sensitivity specks
directly related
relationship between film speed and emulsion thickness
directly related
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
1
----------------------------------------------------------
exposure in R needed to produce speed point density (OD 1.0 +b+f)
film speed is affected by...
immersion time, solution temp., chemical activity
only ___ degrees in developer temp. will result in a visible change in film density
.5
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
contrast is controlled by level of activity of ____
hydroquinone
hydroquinone establishes which region of the curve and affects what other portion
shoulder
straight line
measure of the slope of the straight line portion at speed point 1.0
gamma
radiographic film contrast is most commonly defined by what gradient
average
most radiographic film average gradients are between....
2.5 and 3.5
part of the curve that is the diagnostic range
straight line portion
if slope of straight line portion is 45 deg. the avg gradient will measure ___
1.0
True or false:
doubling exposure will double density
false
in radiographic films, doubling the radiographic exposure will...
produce more than a doubling of radiographic opacity
used in quality control to indicate contrast
contrast index
a contrast index is ....
two density points subtracted from each other
difference between densities
contrast
changes in the average gradient between the toe and shoulder are actually changes in ___
contrast
for OD measurements above or below the optimal contrast, contrast is ____
decreased
diagnostic range of radiographic film
low .12-.50
high 2.0-3.0
when densities are above or below this range contrast is decreased
contrast must exist with in the _____ to be visualized
straight line portion
range of exposures that will produce densities within the diagnostic range
latitude
formula for latitude
Eh-El
h=high
l=low
value of Eh?
El?
Eh=OD2.50 exposure point
El= OD .25 exposure point
relationship between latitude and contrast
inverse
as contrast increases, latitude decreases
___ can change without altering the average gradient
speed
High contrast= ___ latitude, ___ patient dose
narrow, high
Low contrast = ____ latitude, ___patient dose
wide, low
slow speed= ___ patient dose
high