Download THE RAY TUBE

DEFINITIONS/GLOSSARY
Anode:A positively charged electrode , which acts as a target for the electrons from the
cathode. Electrons interacting wit the anode produce heat and x-rays.
Cathode: A negatively charged electrode that provides a source of electrons.
Filament: Part of a low energy circuit in the cathode that when heated, releases electrons
from their orbits .
Kilovoltage: The amount of electrical energy being applied to the filament milliamperage
describes the number of x-rays produced during the exposure.
Step up transformer: Increases the incoming voltage of 110 or 220 to thousands of volts
(i.e. Kilovolts).
Step down transformer: Reduces the x-ray machine input voltage from 110 or 220V to
10V, in order to prevent the filament of the cathode from burning out.
Timer Swich: Controls the length of exposure.
X-ray Tube: A mechanism consisting of an anode and a cathode in a vacuum that
produces a controlled x-ray beam.
THE X-RAY TUBE
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X-rays are generated in
an x-ray tube. The tube
consists of a cathode
side (negative electrical
charge) and an anode
side (positive electrical
charge).
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An x-ray beam is
generated by passing an
electron beam through a
vacuum between a
cathode (-) and an anode
(+). The positively
charged anode attracts
the rapidly moving,
negatively charged
electrons.
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In the x-ray tube, a stream of fast moving electrons are attracted and directed from
the cathode to the anode. As the electrons collide and interact with the atoms on
the anode target, a great amount of energy is produced. 1% of this energy is in
the form of x-radiation. 99% appears as heat and must be removed from the
anode.
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The tube enclosure is shielded and a series of lead shutters allow the diagnostic
beam to exit.
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The cathode consists of a wire filament that emits electrons when heated. The
temperature of the filament is controlled by the milliamperage (mA) setting on the
control panel of the machine.
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As the mA is increased, the
temperature of the filament is
increased and the filament
produces more electrons. The
period of time during which
the x-rays are permitted to
leave the x-ray tube is
measured in fractions of a
second. The number of
electrons available and the
time period set for their
release from the filament
determines how many x-rays
are produced from the anode.
The mAs thus controls the
total number of x-rays
produced.
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The anode, which attracts the negatively charged electrons, is constructed at an
angle so that the x-rays produced are directed downward (toward the film)
through a window in the metal housing of this x-ray tube. In some machines the
anode spins on a rotor
THE DIFFERENTIAL ABSORPTION OF X-RAYS IN VARIOUS TISSUES.
Radiographic opacity refers to the actual penetrative ability of x-rays to pass through an
object and reach the film. Radiographic opacity of a part is determined by its thickness
and its atomic weight. Therefore, atomic weight and thickness are closely related.
Image formation is dependent on the phenomenon of differential absorption. When x-rays
penetrate tissue, they are not homogeneously absorbed; some tissues absorb x-rays more
efficiently than others. If x-ray absorption were uniform, the resulting radiographic image
would be grey or white. Some of the x-rays are absorbed by the tissues such as bone.
Other x-rays pass through the tissues and produce the diagnostic image on the film. Other
x-rays pass into the tissues and are deflected or scattered in the tissues and may exit onto
the film. These are unrepresentative of the tissue through which they have passed. These
are scattered x-rays and cause distortion of the final image. These scattered rays also
contribute to the radiation hazard of the procedure by virtue of their unpredictable exit
from the animal.
At this point we must differentiate
between different types of subject
tissue density. The term tissue density
is used to describe the degree to which
a patient or object absorbs incident xrays. In the accompanying radiograph
the bone tissue is denser than the
adjacent soft tissue, but the differences
in image tone should be described in
terms of radiolucency and radiopacity.
For example, in this radiograph, the
soft tissues are more radiolucent than
the bones.
Air and fat absorb relatively less
radiation and are consequently
radiolucent, so their images on the film
will appear black and pale grey
repectively. Bone and metal absorb
much more X-radiation and are
radiopaque, so their images are white.
Most soft tissues in the body are composed mainly of water and appear as shades of
grey. The radiopacity of most fluids (blood, urine, transudates, exudates, bile and
cerebrospinal fluid) and non-mineralised non-adipose tissues (muscle, cartilage, tendons,
ligaments, fascia and parenchymatous organs) is the same. Lead and other metals have
high physical density and effective atomic number, which renders them extremely
radiopaque.
THE FIVE RADIOLOGIC OPACITIES SEEN ON RADIOGRAPHS
There are five perceivable degrees of inherent radiopacity they are:
Air
(in the bladder)
Fat
(around intestines)
Soft tissue
Bone
(the kidney) (pedal bone)
Metal
(x-ray marker,R)
These shades of black to white depend on the atomic number of the tissues and their
thickness. Thickness also determines the appearance of overlapping opacities. Two
bones that cross will appear whiter where they cross due to greater thickness. Total
radiopacity depends on thickness and inherent radiopacity.
What is Bone X-ray (Radiography)?
An x-ray (radiograph) is a painless medical test that helps diagnose and treat medical
conditions. Radiography involves exposing a part of the body to a small dose of ionizing
radiation to produce pictures of the inside of the body. X-rays are the oldest and most
frequently used form of medical imaging.
A bone x-ray makes images of any bone in the body.
What are some common uses of the procedure?
A bone x-ray is used to:
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determine whether a bone has been fractured or if a joint is dislocated.
ensure that a fracture has been properly aligned and stabilized for healing
following treatment.
determine whether there is a build up of fluid in the joint or around a bone.
guide orthopedic surgery, such as spinal repair, joint replacement and fracture
reductions.
evaluate injury or damage from conditions such as infection, arthritis, abnormal
bone growths or other bone diseases.
assist in the detection and diagnosis of cancer.
locate foreign objects.
evaluate changes in bones.
How does the procedure work?
X-rays are a form of radiation like light or radio waves. X-rays pass through most
objects, including the body. Once it is carefully aimed at the part of the body being
examined, an x-ray machine produces a small burst of radiation that passes through the
body, recording an image on photographic film or a special image recording plate.
Different parts of the body absorb the x-rays in varying degrees. Dense bone absorbs
much of the radiation while soft tissue, such as muscle, fat and organs, allow more of the
x-rays to pass through them. As a result, bones appear white on the x-ray, soft tissue
shows up in shades of gray and air appears black.
X-ray images are maintained as hard film copy (much like a photographic negative) or,
more likely, as a digital image that is stored electronically. These stored images are easily
accessible and are sometimes compared to current x-ray images for diagnosis and disease
management.
What are the limitations of Bone X-ray?
While x-ray images are among the clearest, most detailed views of bone, they provide
little information about the adjacent soft tissues.
An MRI may be more useful in identifying ligament tears and joint effusions in knee or
shoulder injuries and in imaging the spine, because both the bones and the spinal cord can
be evaluated. MRI can also detect a bone bruise when no crack is visible on x-ray images.
Ultrasound imaging, which uses sound waves instead of ionizing radiation, has also been
useful for injuries around joints.
Additionally, in the field, the machines most veterinarians use are not large enough or
strong enough to penetrate thick areas of the equine body. Field x-rays on horses are
generally limited to the distal limb. For x-ray imaging of other areas of the equine body,
it is usually necessary to transport the horse to a large veterinary facility such as those on
a university vet school campus.
RADIOLOGY NOMENCLATURE
The purpose of this section on nomenclature is to provide an overview of the various
terminologies used in equine radiology. While attempts have been made to develop a
universal system of descriptive terminology for radiographs, many different terms are
currently used. Some of these terms are more widely accepted than others, and it is our
intent to review many of the terms currently in use.
The following drawing shows a horse with different aspects of its limbs labeled
according to the terms used to describe them:
Abbreviation Term
Definition
V
Ventral
The body area situated toward the underside of quadrupeds.
M
Medial
Toward the median plane, or midline.
L
Lateral
Away from the median plane or midline.
Cr
Cranial
Structures situated towards the head.
Cd
Caudal
Structures situated towards the tail.
R
Rostral
Areas on the head situated toward the nose.
Pa
Palmar
Situated on the caudal aspect of the front limb, distal to the
antebrachial joint.
Pl
Plantar
Situated on the caudal aspect of the rear limb, distal to the
tarsocrural joint.
Pr
Proximal
Situated closer to the point of attachment.
Di
Distal
Situated away from the point of attachment.
O
Oblique
Slanting, inclined, used to offset an area that would be
superimposed over another area.
D
Dorsal
The body area situated toward the back or topline of
quadrupeds, opposite of ventral
In radiography of the equine limb, it is important to express the view radiographed by
naming the point of entry of the x-ray beam first, and the point of exit second. Thus,
terms such as DV (dorsoventral), AP (anteroposterior), Lateromedial, Dorsopalmar, etc.,
are interchangeably used by many equine practitioners. The following table demonstrates
these common terms, their meaning, and equivalent synonyms. Keep in mind that some
of these terms are included solely because of the fact that they are used commonly by
some clinicians. The use of some of these terms is strictly for the purpose of providing a
definition, not an opinion as to their correctness.
TERM USED
Dorsopalmar View of the Foot, Fetlock or Distal Cannon
SYNONOMOUS TERM
Anteroposterior (AP)
Dorsopalmar View of the Third Phalanx and Navicular Bone Dorsoventral (DV)
Dorsoplantar View of the Foot, Fetlock or Distal Cannon
Anteroposterior (AP)
Dorsoplantar View of the Third Phalanx and Navicular Bone Dorsoventral (DV)
Flexor View of the Foot
Posterior View
Tangential view of Carpus
Tunnel or Skyline View