Optical density (OD) refers to the degree of blackening on a radiographic film. It has a numeric value and can range from completely transparent to completely opaque. The primary factor that controls OD is the milliampere-seconds (mAs) setting, as OD increases directly with mAs. Doubling the mAs will double the OD. Other factors like kilovoltage (kVp), intensifying screens, distance and anatomy can also affect OD. To maintain OD, the technique factors of mAs and kVp must be adjusted based on the inverse square law and square root relationships. Reciprocity law states OD will remain constant if mAs is constant, regardless of exposure time.

1. Film Screen
Image
Acquisition

2. X-ray film sensitive to
radiation
• Density: Overall
blackening of film
• Density depends on
amount of radiation
reaching film
• Film gets DARKER as
exposure increase
What is
DENSITY?
2

3. Lets Talk about optical density
3

4. “
Optical density is the degree of
blackening of the finished radiograph.
OD has a numeric value and can be
present in varying degrees, from
completely black, in which no light is
transmitted through the radiograph,
to almost clear.
4

5. ⮚ In medical imaging, many
problems involve an image being
“too dark” or “too light.” A
radiograph that is too dark has
a high OD caused by
overexposure. This situation
results when too much x-
radiation reaches the image
receptor.

6. ⮚ A radiograph that is too light has been
exposed to too little x-radiation, resulting
in underexposure and a low OD.
6

7. Check which is under/over exposed
7

8. FACTORS
mAs and KVP
8

9. mas
❖ When distance is fixed, however, as is usually the case, the
mAs value becomes the primary variable technique factor
used to control OD. OD increases directly with mAs,
which means that if the OD is to be increased
❖ on a radiograph, the mAs setting must be increased
accordingly.Optical density can be affected by other
factors, but the mAs value becomes the factor of choice
for its control
9

10. “
A wise penguin once said
10

11. ❖ A change in mAs of
approximately 30% is required to
produce a visible change in OD.
As a general rule, when only the
mAs setting ischanged, it should
be halved or doubled). If a
significant change is not required,
a repeat examination probably is
not required.

12. I
A C
I
B
I
12
Optical density is determined principally by the mAs value, as
shown by these phantom radiographs of the abdomen taken at
70 kVp. A, 10 mAs. B, Plus 25%, 12.5 mAs. C, Plus 50%, 15 mAs.

13. OPTICAL DENSITY - OD has a precise numeric
value that can be calculated if the level of light
incident on a process film (Io) and the level of light
transmitted through that film (It) are measured.
13
100%
LIGHT
PHOTON
(Io)
100%
LIGHT
PHOTON
(Io)

14. DENSITOMER
ER

15. FACTORS AFFECTING RADIOGRAPHIC DENSITY
INFLUENCING FACTOR FOR DENSITY’
❖ Focal film distance
❖ kVp
❖ Intensifying screen
❖ Film processing
❖ Film emulsion
❖ Heel effect
❖ Pathology
15

16. X-RAY QUANTITY
X-RAY INTENSITY
❖ The intensity of
the x-ray beam of
an x-ray imaging
system is
measured in
milligray in air
(mGya) [formerly
milliroentgen
(mR)] and is
called the x-ray
quantity.
❖ Another term,
radiation
exposure, is often
used instead of x-
ray intensity or x-ray
quantity. All have
the same meaning,
and all are
measured in mGya
(mR).
❖ The mGya (mR) is
a measure of the
number of ion
16
❖ Ionization of air
increases as the
number of x-rays in
the beam increases.
The relationship
between the x-ray
quantity as measured
in mGya (mR) and
the number of x-rays
in the beam is not
always one to one.
Some small
variations are related
to the effective x-ray
energy.

17. 17

18. 18
Milliampere Seconds (mAs). X-ray quantity is
directly proportional to the mAs. When mAs
is doubled,the number of electrons striking
the tube target is doubled, and therefore the
number of x-rays emitted is doubled.
mAs is directly proportional to the density.
When Density is extremly increases it reduce
contrast, then if its extremly decrease,
contrast also decrease
Quantity = mAs
mAs = density

19. MAS
When distance is fixed, however, as is usually the
case, the mAs value becomes the primary variable
technique factor used to control OD. OD increases
directly with mAs, which means that if the OD is to
be increased
on a radiograph, the mAs setting must be increased
accordingly.
19

20. 20
Changes in mAs value have a direct effect on
optical density (OD). A, Original image. B,
Decrease in OD when the mAs value is decreased
by half. C, Increase in OD when the mAs value is
doubled.

21. 21
TECHNIQUE /DENISTY
CHANGES

22. 22
Normal chest radiograph taken at 100 cm source-to-image receptor distance
(SID). B, If the exposure technique factors are not changed, a similar radiograph at 90 cmSID (A) will be
overexposed, and at 180 cm SID (C), will be underexposed.
⮚ Optical density can be controlled in radiography by two major factors: mAs and SID. A
significant number of problems would arise if the SID were continually changed.
Therefore, SID usually is fixed at 90 cm for mobile examinations, 100 cm for table]
studies, and 180 cm for upright chest examinations. Figure 13-7 illustrates the change
in OD that occurs at these SIDs when other exposure technique factors remain constant.

23. 23
90 cm 100 cm 180 cm
A B C
120kVp 5mAs 120kVp 5mAs 120kVp 5mAs

24. DISTANCE
❖ X-ray intensity varies inversely with the square of the
distance from the x-ray tube target.
❖ This relationship is known as the INVERSE SQUARE
LAW
24
where I1 and I2 are the x-ray intensities
at distances d1 and d2, respectively.

25. When SID is increased, mAs must be
increased by SID2 to maintain constant
exposure to the image receptor.
❖ Compensating for a change in SID by
changing mAs by the factor SID2 is
known as the SQUARE LAW, a corollary
to the inverse square law.

26. KILOVOLT PEAK (KVP)
❑ Kilovolt Peak (kVp). X-ray quantity varies rapidly with
changes in kVp. The change in x-ray quantity is
proportional to the square of the ratio of the kVp; in other
words, if kVp were doubled, the x-ray intensity would
increase by a factor of 4.
❑ Mathematically, this is expressed as follows:
26

27. 27
KVP AND INTENSITY
RELATIONSHIP
❖ In practice, a slightly different situation prevails. Radiographic technique
factors must be selected from a relatively narrow range of values, from
approximately 40 to 150 kVp. Theoretically, doubling the x-ray intensity
by kVp manipulation alone requires an increase of 40% in kVp.
❖ This relationship is not adopted clinically because as] kVp is increased, the
penetrability of the x-ray beam is increased, and relatively fewer x-rays are
absorbed in the patient. More x-rays go through the patient and interact with
the image receptor. Consequently, to maintain a constant exposure of the
image receptor, an increase of 15% in kVp should be accompanied by a
reduction of one half in mAs.

28. 28
A
B
C
Normal chest radiograph taken at 70 kVp (B).
If the kilovoltage is increased by 15% to 80 kVp (A), overexposure
occurs. Similarly, at 15% less, 60 kVp (C), the radiograph
is underexposed.

29. 29
❑ Technique changes involving kVp
become complicated. A change in
kVp affects penetration, scatter
radiation, patient radiation dose,
and especially contrast.
❑ Image contrast is affected when
kVp is changed to adjust OD.
This makes it much more difficult
to optimize OD with kVp. It takes
the eye of an experienced
radiologic technologist to
determine whether OD is the only
factor to be changed or if contrast
also should be changed to
optimize the radiographic image.

30. KVP AND INTENSITY RELATIONSHIP
30
❖ Note that by increasing kVp and reducing mAs so that
image receptor exposure remains constant, the patient dose
is reduced significantly.
❖ The disadvantage of such a technique adjustment is
reduced image contrast when screen film is the image
receptor. There is no change in contrast when using
digital image receptors.

31. RECIPROCITY LAW:
❑ One would think that the OD
on a radiograph would
depend strictly on the total
exposure (mAs) and would be
independent of the time of
exposure. This, in fact, is the
reciprocity law. Whether a
radiograph is made with short
exposure time or long
exposure time, the reciprocity
law states that the OD will be
the same if the mAs value is
constant.
❑ The reciprocity law holds
for direct exposure with x-
rays, but it does not hold
for exposure of film by the
visible light from
radiographic intensifying
screens. Consequently, the
reciprocity law fails for
screen-film exposures at
exposure times less than
approximately 10 ms or
longer than approximately
2 s.
31

32. 32
DENSITY
.25 TO -2.5
The straight line of the
H&D curve

33. 33
The angle of the target can cause
a density variation up to 45%
between the cathode and the
anode. Density is always greater
at the cathode end. It is more
pronounced as:
Short SID’s
Large image receptor/open wide collimator
X-ray tubes having small target angle (12o or less)
ANODE HEEL EFFECT

34. 34
FILM EMULSION
Speed. Screen-film IRs are available with
different speeds. Speed is the sensitivity of the
screen-film combination to x-rays and light.
Usually, a manufacturer offers several different
IRs of different speeds that result
from different film emulsions and different
intensifying screen phosphors.
For direct-exposure film, speed is principally a
function of the concentration and the total
number of silver halide crystals. For screen
film, silver halide grain size, shape, and
concentration are the principal determinants of
film speed.

35. 35
FILM EMULSION
Compared with earlier technology, current
emulsions contain less silver yet produce
the same optical density (OD) per unit
exposure. This more efficient use of silver
in the emulsion is called the covering
power of the emulsion.

36. 36
I.S.
Intensifying screen amplify the action of x-rays through their property of
fluorescence.
Fluorescent light is responsible for more than 98% of the image density.
All other factors remaining constant, an increase in screen speed will result
in an increase in optical density.
Screen speed and optical density are directly proportional.
Screen speed and patient dose are inversely proportional.
Screen speed and x-ray tube heat production are inversely related.
Compensations for changes in relative speed can be made by adjusting
mAs: