IMAGE QUALITY

IMAGE QUALITY 

The second basic factor determining

image clarity is image quality. The quality

of the radiographic image may be defined

as the ability of the film to record each

point in the object as a point on the film.

In radiology this point-for-point reproduction

is never perfect, largely because of

the diffusion of light by intensifying

screens. There is no general agreement as

to what should be included in a discussion

of image quality. Our approach will involve

almost no mathematics .

Image quality is influenced by

l. radiographic mottle

2. sharpness

3. resolution

Radiographic Mottle

If an x-ray film is mounted between intensifying

screens and exposed to a uniform x-ray beam to produce a density of

about 1.0, the resulting film will not show

uniform density but will have an irregular

mottled appearance. This mottled appearance

(caused by small density differences)

is easily detected by the unaided eye, and

may be seen, if looked for, in any area of

"uniform" density in a radiograph exposed

with screens (e.g., the area of a chest film

not covered by the patient). This uneven

texture, or mottle, seen on a film that

"should" show perfectly uniform density is

called radiographic mottle.

Radiographicmottle has three components:

Radiographic mottle

SCREEN MOTTILE

STRUCTURE MOTTILE

QUANTUM MOTTILE

FILM GRAINESS

Only quantum mottle is of any importance

in diagnostic radiology.

Film Graininess.

Film graininess makes no contribution to the radiographic mottle

observed in clinical radiology. Film graininess

can be seen when the film is examined

with a lens producing magnification of 5 to

10 x . With this magnification, the image

is seen to be made up of a nonhomogeneous

arrangement of silver grains in gelatin.

Because a radiograph is almost never

viewed at an enlargement of 5 x, film

graininess is not seen

  Screen Mottle.

Screen mottle has two

components, structure mottle (unimportant)

and quantum mottle (important).

Structure mottle is caused by defects in the

intensifying screen, such as varying thickness

or physical imperfections in the phosphor

layer. Such screen in�egularities can

occur, but the quality control used in screen

manufacturing is so good that structure

mottle may be dismissed as making no contribution

to radiographic mottle.

Quantum mottle is the only important

cause of radiographic mottle. Quantum refers

to a discrete unit of energy and, in this

discussion, it may be considered as the energY

carried by one x-ray photon. An x-ray

beam may be thought of as containing a

certain number of x-ray photons, or an

equivalent number of quanta.

By showing a pattern of mottle, or nonuniform

density, the x-ray film is telling us

that it has "seen" a nonuniform pattern of

light on the surface of the intensifying

screen. The nonuniform pattern of light

on the screen is caused by fluctuations in

the number of photons (or quanta) per

square millimeter in the beam that arrived

at the screen. What this means is that a

"uniform" beam of x rays is not uniform

at all. Suppose a "uniform" x-ray beam

could be frozen in space and cut into cross

sections. If the number of x-ray photons

per square millimeter were counted, it

would be unlikely that any two square mm

would contain exactly the same number of

photons. The "uniform" beam is not uniform.

The actual number of x-ray photons per

mm2 obeys the law of probability, because

the emission of x rays by the x-ray tube is

a random event. The average number of

photons per mm2 can be calculated by adding

the number in each mm2 and dividing

by the number of squares. It will then be

found that the actual number in any square

will almost never be the average value, but

that all numbers will fall within a certain

range (percent fluctuation) of the average.

Quantum mottle is caused by the statistical

fluctuation in the number of quanta

per unit area absorbed by the intensifying

screen.