D EFINITIONS /G LOSSARY
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.
T HE XRAY 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).
•
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
T HE D IFFERENTIAL A BSORPTION OF XRAYS IN V ARIOUS T ISSUES .
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.
T
F
R
O
R

There are five perceivable degrees of inherent radiopacity they are:
Air
(in the bladder)
Fat
(around intestines)
Soft tissue
(the kidney)
Bone
(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.

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.
A bone x-ray is used to:
• 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.
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.

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.
R ADIOLOGY 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
V
Term Definition
Ventral The body area situated toward the underside of quadrupeds.
M Medial Toward the median plane, or midline.
L
Cr
Cd
R
Pa
Lateral
Caudal
Away from the median plane or midline.
Cranial Structures situated towards the head.
Structures situated towards the tail.
Rostral Areas on the head situated toward the nose.
Palmar
Situated on the caudal aspect of the front limb, distal to the antebrachial joint.
Pl
Pr
Plantar
Situated on the caudal aspect of the rear limb, distal to the tarsocrural joint.
Proximal
Situated closer to the point of attachment.
Di
O
Distal Situated away from the point of attachment.
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 SYNONOMOUS TERM
Dorsopalmar View of the Foot, Fetlock or Distal Cannon 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
Tangential view of Carpus
Posterior View
Tunnel or Skyline View