Radiology Chapter 2

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Anatomy of the X-Ray Machine

Acceleration

the increase in speed over time

Actual Focal Point

the area of the focal spot consisting of coiled wire that is perpendicular to the surface of the target

Alloy

a mixture of metals

Anode

a positively charge electrode that acts as a target for the electrons from the cathode. Electrons interacting with the anode produce heat and x-rays

Arcing

a phenomenon in which metal deposits on the inner wall of the envelope act as a secondary anode, thereby attracting electrons from the cathode.

Autotransformer

provides a variable yet predetermined voltage to the high-voltage step-up transformer. It acts as the kilovoltage selector

Cathode

a negatively charged electrode that provides a source of electrons

Collimator

a restricting device used to control the size of the primary beam

Effective Focal Spot

the area of the focal spot that is visible through the x-ray tube window and directed toward the x-ray film

Filament

part of a lower-energy circuit in the cathode that, when heated, releases electrons from their orbit

Focal Spot

the small area of the target that with which electrons collide on the anode

Focusing Cup

a recessed area where the filament lies, directing the electrons toward the anode

Full-wave rectification

creates an almost constant electrical potential across the x-ray tube, converting the positive electrical current pulses to 120 times per second compared with the normal rate of 60 times per second

Glass Envelope

a glass vacuum tube that contains the anode and cathode of the x-ray tube

Half-wave rectification

a method of converting alternating to direct current in which half of the current is lost

Heel Effect

a decrease of x-ray intensity on the anode side of the x-ray beam caused by the anode target angle

Kilovoltage

the amount of electrical energy being applied to the anode and cathode to accelerate the electrons from the cathode to the anode

1 kilovolt=

1000 volts

Line-focus principle

the effect of making the actual focal spot size appear smaller when viewed from the position of the film because of the angle of the target to the electron stream

Line-voltage compensator

adjusts the incoming line voltage to the autotransformer so that the voltage remains constant

Milliamperage (mA)

the amount of electrical energy being applied to the filament. Milliamperage describes the number of x-rays produced during the exposure.

Molybdenum

a metal commonly used in focusing cups because of its high melting point and poor conduction of heat

Penumbra

partial outer shadow of an object being imaged by illumination

Rectification

process of changing alternating current to direct current

Rotating anode

an anode that turns on an axis to increase x-ray production while dissipating heat

Stationary anode

a nonmoving anode, usually found in dental and small portable radiography units

Step-down Transformer

reduces the x-ray machine input voltage from 110 to 220 V to thousands of volts (i.e kilovolts)

Target

anode

Timer Switch

controls the length of exposure

Tungsten

a common metal used in the filament of a cathode

Valve tubes

allow the flow of electrons in one direction only. Commonly called self-rectifiers

X-ray tube

a mechanism consisting of an anode and a cathode in a vacuum that produces a controlled x-ray beam

Filaments located in an x-ray tube

emit electrons when heated

The anode's target

is composed of tungsten
reaches temperatures in excess of 1000C during x-ray production
usually has a copper base

Limitations of the stationary anode

it is unable to withstand large amounts of heat

How can the technician help to prolong the life of the filament in the x-ray tube

enter the proper exposure settings in the control panel before the final positioning of the animal

Which of the following are possible effects of excessive heat within an x-ray tube

bearing failure and decreased anode speed
roughened target surface
arcing

Veterinary patients have a tendency to move while being positioned for radiographs to be taken. The radiographer should help to safely prevent artifacts of movement by

using the shortest exposure time possible

Recommendation to reduce unnecessary irradiation of the patient or persons restraining the patient and to decrease scatter radiation

adjustment of the collimator so that the smallest field size possible is used

X-ray tube ratings are based on target angle, focal spot size, electrical current

rectification

the advantages of using an x-ray machine with a three-phase generator as opposed to a single-phase generator include

use of shorter exposure times because more power is available to the x-ray tube per unit time

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