CHARACTERISTICS AND MEASUREMENT OF RADIATION l Oral radiology MCQs
September 27, 2020
CHARACTERISTICS AND MEASUREMENT OF RADIATION l Dental radiology MCQs
Basics
and background
X
rays were accidentally discovered when Roentgen in 1895 noticed
fluorescent glow of crystals on a table near him when he was working with high voltage
[ evacuated light bulb with +ve and -ve electrodes]
Radiation
= the transmission of energy
through space , could be in particulate or electromagnetic form.
Atoms
have protons [+ve] and neutrons [no charge] in the nucleus and electrons [ -ve]
orbiting around the nucleus
An
atom is electrically neutral because the number of electrons = the number of
protons
Two types of xrays are
produced:
Majority
= Bremsstrahlung ( breaking) radiation :
happens when high speed electrons are slowed down as they pass close to or
strike the nucleus
Minority
= characteristic radiation :
If a
K-shell electron is ejected and an L-shell electron drops into the space, the
energy of the x-ray will be equal to the difference in binding energies between
the K- and L-shells
How xrays are generated:
Electrical
energy goes to → a
tungsten filament [ cathode] → filament
heats and releases electrons that pass and hit a target [ anode] generating x
rays
Low
energy soft xrays don’t contribute to the formation of an xray image – they
expose the body to radiation but without reaching the film – they are useless
and must be eliminated
Filtration = The process of removing
soft x rays .
provided
by inherent filtration in the x ray machine + added filtration by aluminum
disks placed in the path of the x-ray beam [ aluminum disks remove the
x-rays that had enough energy to get through the inherent filtration but are
still not energetic enough to contribute to image formation]
x-ray
machine operating at 70 kVp or higher → must
have total filtration of 2.5 mm aluminum
X-ray
machines operating below 70 kVp →need to have a total filtration of 1.5 mm aluminum
Collimation
= to restrict the area of the
head that is exposed to radiation + reduce scatter radiation
Collimator
= lead disk with a hole in the middle -
located in the end of the PID - The size / shape of the hole determines the
size/ shape of the x-ray beam
X-ray
films and processing
Images are stored as an unseen (latent) image that will be
changed to a seen image by processing the film.
Analogue Xray films :
A. Intraoral [ non screen films]
B. Extraoral [ screen films]
Intra oral films:
Consist
of a film base with gelatin emulsion containing silver bromide crystals on
both sides of the film base .
Gelatin
is used as an emulsion medium
because
it is chemically inert
Outer
protective packet has 2 sides:
Front: faces the x-ray tube,
single color (white)
Back: opposite side to x-ray
tube, two colors (white and green) + has the opening tab
Black paper wrapper:
protects the film base from light, saliva and damage during opening.
Lead foil: thin
sheet of lead present at the back of the film to:
prevents
x –rays from going to deeper tissues after passing through the film + prevent
scattered radiation from deeper tissues to come back to the film
Raised dot:
Convex: indicates
front of the film (facing x-ray)
Concave:
indicates back of film
(facing lead foil)
Latent image formation:
Radiolucent objects [ soft tissues ] →
Many x-rays penetrate and expose many silver halide
crystals →maximum silver precipitate
Thick radiolucent objects [ bone ] →
Fewer x-rays penetrate and not as many silver halide
crystals are exposed →fewer
silver precipitate
Radio opaque objects [ amalgam ] →
very few x-rays penetrate → silver halide crystals not exposed → no silver precipitate
0 Comments