Densitometry imaging physics lecture power point

1. Densitometry and characteristics curve

2. Outlines
• Introduction to densitometry
– Transmittance
– Opacity
– Optical density
– Characteristics curve and its features

3. Densitometry
• It is measurement of the density of a material by determining the
degree to which that material attenuates electromagnetic radiation of a
given energy.
• Because electromagnetic radiation interacts with atomic electrons,
densitometry measurements are element-specific, not isotope-specific.
• A densitometer is a device used to numerically determine the amount
of blackness on the radiograph (i.e., it measures radiographic density).
• The numbers that are displayed by a densitometer are known as
optical density numbers.

4. If we take a small area of an image, it will show three
characteristics
• Transmission
• Opacity
• Density

5. Transmittance
• It measures the fraction of light transmitted by the film
• It can be expressed by the ratio of the transmitted
light(Lt) through the film to the incident light(Li)
• Transmission ratio = light transmitted by the film/ light
incident on a film
= Lt / Li
• It is the reciprocal of opacity
• This ratio usually expressed as Percentage
 Transmission = (Lt / Li) x 100

6. • A perfectly opaque area of an image has zero
transmission and zero percentage transmission
• A perfectly transparent area of an image has a
transmission ratio of one and 100 percent transmission
• It decreases with film blackening or exposure

7. Opacity
• It is the ability of film to stop light
• It is ratio of incident light to transmitted light
• It is reciprocal of transmission
• Opacity = light incident on a film(Li) / light transmitted by
the film(Lt)
• As incident light is always greater than transmitted light,
opacity is always greater than one
• A perfectly opaque area has infinite opacity
• A perfectly transparent area has an opacity of one
• It increases with the exposure

8. Optical density(OD)
• Radiographic density is the amount of overall blackness
produced on the processed image
– It is a measure of film blackness
• Density is expressed as a number that is actually a logarithm,
using the common base 10.
• It the log of the intensity of the incident light divided by the
intensity of the light transmitted through the film.

9. • Note that density is the log10 of opacity (density = log10 of
opacity).
• It is the logarithm (Log10) of the inverse of transmittance.

10. Optical density
Diagnostic Range
• Optical densities can range from 0 to 4 OD. However, the
diagnostic range of optical densities for general
radiography usually falls between 0.5 and 2 OD.
• The number 0.3 is the logarithm of 2. Thus, an increase in
density of 0.3 means an increase in opacity of 2; opacity is
doubled by a density increase of 0.3

11. Light Transmittance and Optical Density
• As the percentage of light transmitted decreases, the optical
density increases; as the percentage of light transmitted increases,
the optical density decreases.
• When 100% of the light is transmitted, the optical density equals
0.
• When 50% of the light is transmitted, the optical density is equal
to 0.3, and when 25% of the light is transmitted, the optical
density equals 0.6.
• For every 0.3 change in optical density, the percentage of light
transmitted has changed by a factor of 2 (log10 of 2 = 0.3).
– A 0.3 increase in optical density results from a decrease in the percentage

12. Examples
• An increase in density from 0.6 to 0.9 decreases the amount of
transmitted light from 25 to 12.5%.
• If a region of a radiograph has an OD of 1.0, this means only 10
percent or 1/10 of the incident light is transmitted through the
radiograph in this region. The opacity of the region would be 10.
• A density of 2 means that Log10 Li / Lt=2. Because the
log of 100 =2
– Li / Lt=100……opacity**
– Transmittance=1/opacity= 1/100
– Thus, for every 100 light photons incident on the film, only 1
photon, or 1%, will be transmitted

13. • Higher density means a blacker film (less
light transmission)
• A density of 2 (1% of light transmitted) is
black when viewed, and a density of 0.25 to
0.3 (50% of light transmitted) is very light.
• A radiograph must have sufficient density to
visualize the anatomic structures of interest.
• A radiograph that is too light has insufficient
density to visualize the structures of the
anatomic part. Conversely, a radiograph that
is too dark has excessive density, and the
anatomic part cannot be well visualized

14. Why is density expressed as a logarithm?
1. Logarithms conveniently express large differences in
numbers on a small scale.
– opacity varies from 1-10000 & by the use of log we can compress this
into 1-4 only.
2. The human eye seems to different tones in a way which is
approximately logarithmic
– The physiological response of the eye to difference in light intensity
is logarithmic
– A density of two looks twice as dark as a density of one
3. Addition & superimposition of density is logarithmic
– If films are superimposed, the resulting density is equal to the sum
of the density of each film

16. CHARACTERISTIC CURVE
• The relationship between exposure and density is plotted as a curve
known as the "characteristic curve“ or
– Density log exposure, or
– D log E curves, or
– Sensitometric curve or
– "H and D curve" (named after F. Hurter and V.C. Driffield, who first
published such a curve in England in 1890).
• The characteristic curve is a graph which illustrates the way in which a
film or film – screen system responds to different levels of exposure.
• The characteristic curve is a plot of Density(D) (y axis) against log
relative exposure (LOG E) (x axis)

17. • Characteristic curves are derived by giving a film a series of
exposures, developing the film, then the series of densities can
then be measured by densitometer and plotting the resulting
density against the known exposure.
X-ray exposure
• The actual exposure the film received may be measured in the laboratory, but
such measurements are not important to use and understand the characteristic
curve.

18. Relative exposure
• It is not easy to measure absolute x-ray exposure, so we produce
relative values of it.
• If a sheet of screen type x-ray film is divided (say) 10 small areas and
each area is exposed with same tube kv and mA but different exposure
times, it is simple matter to relate the x-ray exposure received by each
area.
• The area which has been given the smallest exposure is used as the
baseline and allocated a relative exposure of unity, and the other areas,
subjected to higher exposure values greater than one.
• Relative exposure plotted on the X-axis of the curve is not exact as
given on the film; it is relative to some unit exposure.

19. We take log of relative exposure because
i. It allows very wide range of exposure variation on small scale
to be expressed in a compact graph.
ii. It makes analysis of the curve easier
• An increase in the log relative exposure of 0.3 always
represents a doubling of the relative exposure.

20. How to make a characteristics curve
There are three basic stage involved:
1. Exposing and processing the film.
2. Measuring the densities produced.
3. Plotting the curve.

21. Exposing and processing the film
• To generate a characteristics curve we to irradiate the film or
film-screen system with series of exposures which progress in
known steps so that the relative exposure received by each step
can be recorded.
• The smallest exposure must be such that no measurable effect can
be seen on the film.
• The highest exposure should be greater than that sufficient to
activate every silver halide grain in the emulsion, so that the
maximum possible density produced.

22. Features of The Characteristic Curve
Parts of the curve
• The region to the left of the Toe(A)
1. basic density
2. basic fog
• Toe
• Region between toe and shoulder(C)
• Shoulder
• Region to the right of the shoulder(E)
1. Solarization or Reversal of film

23. The region to the left of the toe
• The base plus fog (b+f) is the density at no exposure, or the density that is
inherent in the film.
– It includes the density of the film base, including its tints and dyes, plus any
fog (development of unexposed grains of silver halide in the emulsion) the
film has experienced.
• The minimum density caused by base and fog in a "fresh" film is about 0.12.
• Processing the film usually adds about OD 0.05–0.10 in fog density.
• Radiographic film base density ranges around OD 0.05–0.10
• The total base plus fog is seldom below OD 0.10 but should not exceed OD 0.22.

25. • Fog is strictly defined as those silver halide grains in the film emulsion
that are developed even though they were not exposed by light or x rays.
• Factors that increase fog are
– Aging of the film
– Improper film storage (high temperature or humidity)
– Poor conditions for the film
– Processing temperature too high
– Overactive developer
– Too long developing time or excessive temperature of development
– Greater film speeds
– Contaminated or exhausted developer solution

26. • Density of base plus fog is often referred to as Gross Fog or Basic Fog
• Gross Fog(<0.2) = fog +density of base
• Therefore, total density on an exposed and developed film will include base
and fog densities .
• To evaluate density produced by the exposure alone, base and fog densities
must be subtracted from the total density.
• Net density = Gross density- Gross fog

27. The toe
• The point where the curve just begins to turn up
• The point on the curve where a minimum amount of
radiation exposure produced a minimum optical density
 Threshold exposure is minimum exposure required that can produce any
amount of net density on the film.
• It represents the first response of the material to radiation
• Changes in exposure intensity in this region have little effect on the
optical density.

29. Region between toe and Shoulder (straight-line portion)
• This straight-line region is where the diagnostic or most useful
range of densities is produced and in this region the curve is almost
a straight line.
– In this "straight line“ portion the density is approximately proportional
to the log relative exposure.
– Changes in exposure begin to have a much greater effect on the optical
density
• It is usually fairly straight because the film is reacting in a linear
fashion to exposure in the range of its primary sensitivity.
• The majority of diagnostic-quality information on a radiograph are
within the straight-line portion of the curve.

30. Shoulder region
• There is a point on the
sensitometric curve where
changes in exposure intensity
no longer affect the optical
density.

31. Region to the right of the shoulder
• Maximum density(Dmax): As the film or film-screen system is
subjected to greater and greater exposure, a point is reached
where all of the silver halide grains in the film emulsion are
reduced during development
• It is the maximum density the film capable of recording.
• It is the highest point on the D log E curve.
• In this region characteristics curve has zero gradient, indicates
zero image contrast

32. • Additional exposure beyond Dmax will
result in less density because silver atoms
attached to sensitivity specks will be
ionized again, reversing their charge and
causing them to be repelled from the
speck.
– This process of reversal, or solarization,
reduces the intensity of the latent image and
will produce less density.

33. Solarization/reversal of the film
• When subjected to exposure beyond
required to achieve Dmax; the film starts
to respond in the opposite way to normal,
producing a reduction in image density as
a result of increase in exposure.
• The film is said to be solarized and the
reversal part of the curve is referred to as
the region of solarization
• The true D log E curve is bell-shaped
• Use of reversal properties is being seen in
duplicating the film.

34. Film contrast
• Film contrast is the degree difference in density existing
between various regions on the film.
• An image that has a diagnostic density but no differences in
densities appears as a homogeneous object.
– This appearance indicates that the absorption characteristics of the
object are equal.
• When the absorption characteristics of an object differ, the
image presents with varying densities.
• It is because of these density differences (i.e., radiographic
contrast) that the anatomic tissues are easily differentiated.

35. Cont…
• Radiographic contrast affects the visibility of the structural lines that
make up the recorded image.
• Radiographic contrast depends on subject contrast and on film
contrast.
• Subject contrast depends on the differential attenuation of the x-ray
beam as it passes through the patient.
– It is affected by the thickness, density, and atomic differences of the
subject, the radiation energy (kVp), contrast material, and scatter
radiation.

36. Cont…
• Contrast is defined by the slope of the straight-line portion of the
D log E curve
• The slope of this line mathematically indicates the ratio of the
change in y (optical density) for a unit change in x (log relative
exposure)
• Overall radiographic film contrast is more commonly defined by
the average gradient of the straight-line portion of the D log E
curve between OD 0.25 + b + f and OD 2.50 + b + f.

37. Gradient Point
• The slope of any portion of the D log E curve that
provides information about the contrast produced at that
point
• Gradient points can be determined for any region of the
sensitometric curve, such as the toe, middle, and
shoulder.
• The gradient point can be determined by calculating the
slope of the line (change in optical density divided by the
change in log exposure) at any portion of the curve.

38. Average gradient
• It is the slope of a straight line
joining the useful density
range(0.25 -2.5) on the curve
– It is usually calculated between
density 0.25 and 2.5 above base and
fog

39. Cont…
• The average gradient is calculated as:

40. Cont…
• Steeper the curve higher the
contrast.
– So the maximum contrast shown by the
straight line part of the curve.
• Most radiographic film average
gradients are between 2.5 and 3.5.

41. Cont…
• If the average gradient of the film used is greater than 1, the
film will exaggerate subject contrast and, the higher the
average gradient, the greater this exaggeration will be.
• A film with average gradient of 1 will not change subject
contrast; a film with an average gradient of less than 1 will
decrease subject contrast.
• Because contrast is very important in radiology, x-ray films
all have an average gradient of greater than 1.

42. Cont…
• Toe and shoulder gradients are less than <1.0 and therefore not
only fail to amplify the contrast but actually decrease it.
• For radiographic films under most conditions, contrast is
maximized when the density range is within the range of
diagnostic densities (OD 0.5–2.5).
– When the diagnostic densities are below or above this range, the
contrast will be decreased.

43. • The contrast must exist within the diagnostic range of the film
if it is to be visualized; in other words, within the straight-line
portion of the curve
• If average gradient
– >1 :exaggerates subject contrast
– Typical for x-ray for film
– =1 :no change in subject contrast
– <1 : decreases subject contrast

44. Film gamma
• It is the maximum slope of the characteristic
curve
• It is a measure of the maximum change of
film density for a certain change of exposure.
• This corresponds to the part of the
characteristic curve with the steepest slope
• In radiology the concept of film gamma is of
little value because the maximum slope
(steepest) portion of the characteristic curve is
usually very short.

45. Film Speed
• The amount of density (degree of blackening) a film produces
for a given amount of exposure.
• It is determined by the film’s sensitivity to exposure.
• Film speed is directly related to thickness of emulsion layer,
crystal size and the number of sensitivity specks

46. Speed point
• The speed point of a film is that point on the D log E curve
where a density of OD 1.0 + B+F is achieved.
• The American National Standards Institute (ANSI) specifies x-
ray film speed as the exposure required to reach OD 1.00.
• However, many users add base plus fog to this standard.

47. Speed point
• The speed of the radiographic film
typically is determined by locating the
point on a sensitometric curve that
corresponds to the optical density of 1.0
plus base+fog.
• This optical density point is used because
it is within the straight line portion of the
curve.
• The speed point serves as a standard
method of indicating film speed.

48. Relative speed
• Relative film speed can be determined by using the reciprocal
of the exposure required to produce a given density:
• For medical x-ray films, relative film speed is usually compared
at a density of 1.0 above base and fog.

50. Speed exposure point
• Speed exposure point indicates the intensity of exposure
needed to produce a density of 1.0 plus B + F (speed point).
• A faster-speed film is positioned to the left (closer to the y-
axis) of slower-speed film.
• Speed of the film is determined by the amount of exposure
(log of exposure) needed to produce an optical density of 1.0
plus B + F, regardless of the shape of the sensitometric curve

51. • When comparing film types, the
radiographer must determine
what log of exposure produces the
speed point.
• This can be determined by
drawing a line from the
sensitometric curve speed point to
the area on the x-axis (log of
exposure) that produced the
optical density at 1.0 plus B +F
• A film that has a speed exposure
point of 1.5 is faster than a film
having a speed exposure point of
2.0

52. • The shape of the characteristic curve is
controlled by film contrast; the film speed
determines the location of the curve on
the log exposure scale
• Film A produces all density levels with
less exposure than film B requires for the
same density.
– Film A is more sensitive to exposure, or
faster. Film B is less sensitive, or slower.

53. Uses of characteristics curve
• To compare different types of films
• To determine average gradient and therefore subject contrast
amplification
• To find film exposure, latitude
• To find the speed

54. References
• Christensen's physics of diagnostic radiology.-4th ed./Thomas S.
Curry III, James E. Dowdey, Robert C. Murry, Jr.
• Principles of Radiographic Imaging: An Art and a Science, Sixth
Edition/Richard R. Carlton, Arlene M. Adler, and Vesna Balac
• Radiographic imaging and Exposure: Fifth Edition/Terri L. Fauber,
EdD, RT(R)(M)
• www.radiologykey.com
• Other different websites