chapter 6- dental x-ray image characteristics

radiolucent

- the portion of the processed image that is dark or black

- a structure that appears black on film lacks density

radiopaque

- the portion of the processed image that appears white

- a structure that appears white on film is dense and absorbs or resists passage of the x-ray beam

in a diagnostic image, the images:

-Have proper density and contrast

-Have sharp outlines

-Are of the same shape and size as the object radiographed

proper density allows all areas to be viewed:

- black areas (air spaces)

- white areas (bone, dentin, enamel)

- gray areas (soft tissue)

density

- mA

- kV

- time

- subject thickness

contrast

kV

kV=long scale contrast:

low contrast

kV=short scale contrast

high contrast

the overall darkness or blackness of a dental image

density

influencing factors of density

- kilovoltage

- milliamperage

- exposure time

- subject thickness

difference in degrees of blackness between adjacent areas

contrast

what kind of film is preferred

that is a compromise between low contrast and high contrast

how does increasing the kilovoltage affect the images contrast

by increasing the mean or average energy of the x-rays and by producing higher energy x-rays

a dental image that has very dark areas and very light areas

high contrast

a dental image that does not have very dark and light ares but instead has many shades of gray

low contrast

the range of useful densities

scales of contrast

an image with only two densities, black and white

short scale contrast (high contrast)

when does short scale contrast (high contrast) occur

with machines functioning at low kVp

what is short scale contrast (high contrast) good for

detecting dental caries

an image with many densities, many shades of gray

long scale contrast (low contrast)

when does long scale contrast (low contrast) occur

with machines functioning at high kVp

what is long scale contrast (low contrast) good for

evaluating periodontal and detecting calculus

what can an aluminum step wedge demonstrate

short scale and long scale contrast

what does the step wedge consist of

uniform layered thickness of an x-ray absorbing material

what kind of densities appear on the step wedge

different densities

what are geometric characteristics

- sharpness

- magnification

- distortion

what are influencing factors of sharpness

- focal spot size

- film composition

- movement

when focal spot size decreases

sharpness increases

when focal spot size increases

sharpness decreases

when crystal size decreases

sharpness increases

when crystal size increases

sharpness decreases

when movement decreases

sharpness increases

when movement increased

sharpness decreases

what are influencing factors of magnification

- target receptor distance

- object receptor distance

when target receptor distance increases

magnification decreases

when target receptor distance decreases

magnification increases

when object receptor distance increases

magnification increases

when object receptor distance decreases

magnification decreases

what are influencing factors of distortion

- object receptor alignment

- angulation

object and receptor parallel =

decreased distortion

object and receptor not parallel =

increased distortion

beam perpendicular to object and receptor =

decrease distortion

beam nit perpendicular to object and receptor =

increase distortion

the capability of the x-ray receptor to reproduce the distinct outlines of an object

sharpness

a certain lack of image sharpness is present in every image- fizzy part of image

penumbra

what does movement cause

a loss of image sharpness

what does the emulsion of faster image contain

larger crystals that produce less image sharpness

larger focal spot =

more penumbra

more penumbra =

loss of sharpness

smaller focal spot =

less penumbra

less penumbra -

sharper image

results from the divergent paths of the x-ray beam as they radiate from the focal spot

magnification

a variation in the true size and shape of object being radiographed

distortion

how does distortion result

from unequal magnification of different parts of the same object because of improper receptor alignment or angulation of the x-ray beam