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

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