Card Set Information
Radiation therapy physics rad science
Physics Test 11/16
: Stationary Anode- the target is embedded in the anode
: Rotating Anode- the target is the rotating disc
Source-to-Image Receptor Distance
When the x-ray tube is centered above the examination table at the standard S.I.D., the x-ray tube is in a ___________ __________ position.
Preferred Detent position
What are the three methods of supporting an x-ray tube?
1. Ceiling support
2. Floor support
3. C-arm support
"With equal intensity in all directions."
When x-rays are produced, they are emitted ISOTROPICALLY, or with equal intensity in all directions.
The x-rays emitted throught the window are called the:
X-rays that escape through the protective housing are called:
What is the purpose of the protective housing? (4)
-Guards against excessive radiation exposure
-Protects against electrocution
-Provides mechanical support for the x-ray tube
-Protects the tube from rough handling
Properly designed housing reduces the level of leakage radiation to less than ______ at _____ when operated at maximum conditions.
The x-ray tube is a special kind of ________ that contains two electrodes: the _______ and the _______.
Early x-ray tube:
(not a vacuum)
The modern x-ray tube:
the Coolidge Tube
(a vacuum tube)
X-ray tubes are designed with a _______ or _______ eclosure.
Glass or Metal enclosure
What type of tube enclosure maintains a constant electric potential between the electrons of the tube current & the enclosure.
Provides longer life & is less likely to fail.
Metal Enclosure Tubes
Two ________ supply separate electron beams to produce two _______ spots.
supply seperate electron beams to produce two
The negative side of the x-ray tube:
What are the two primary parts of the cathode?
a Filament & a Focusing Cup
A coil of wire, similar to that in a toaster, but much smaller (approx. 2mm in diameter & 1 or 2cm long)
An x-ray tube filament emits _________ when it is heated.
The addition of 1% to 2% of thorium to the tungsten filament ________ the efficiency of thermionic emission & _______ tube life.
Prolongs tube life
Emission of electrons from a heated surface
When the current through the filament is sufficiently high, the outer-shell electrons of the filament atoms are "boiled-off" and ejected from the filament.
Filaments are usually made of:
What is the most common cause of tube failure?
Tungsten vaporization with deposition on the inside of the glass or metal enclosure
The filament is embedded in a metal shroud called the:
The focusing cup is _______ charged so that it electrostatically confines the electron beam to a small area of the anode.
Most rotating anode x-ray tubes have _____ filaments
What kind of x-ray tubes are designed to be turned on & off very rapidly?
Part of the Cathode that emits electrons, resulting in a tube current:
The x-ray tube current is adjusted by controlling the _________
The relationship between filament current & x-ray tube current depends on the:
Without a ______ ______, the electron beam is spread beyond the anode because of mutual electrostatic repulsion among the electrons.
Electron cloud near the filament:
Phenomenom of the space charge that makes it difficult for subsequent electrons to be emitted by the filament because of the electrostatic repulsion.
Space Charge Effect
Filament current that has risen to its maximum value because all electrons have been used:
Type of Anode used in general purpose x-ray tubes, because it must be capable of producing high-intensity x-ray beams in a short time.
When an x-ray tube is operated at the saturation current, it is said to be:
Small focal spot is used when better ______ _______ is required.
The ________ focal spot is used when large body parts are imaged
Large Focal Spot
Also used when techniques that produce high heat are required.
What size of focal spot is used to image an ankle or a wrist?
Small focal spot
At approx 400mA and up, only the _______ focal spot is allowed.
Large focal spot
The positive side of the x-ray tube:
Which type of anode is used in general purpose x-ray tubes?
(capable of producing high-intensity x-ray beams in a short time)
The side of the x-ray tube that conducts electricity & radiates heat and contains the target:
What are the three functions of the anode?
1. Electrical conductor
2. Provides mechanical support for the target
3. Thermal dissipater
When the projectile electrons from the cathode interact with the anode, more than _______ of their kinetic energy is converted in to heat.
The area of the anode struck by the electrons from the cathode:
In a rotating anode, the target is the:
What is the material of choice for the target?
What is the limiting factor to focal spot size?
As the size of the focal spot decreases, the heating of the target is concentrated on to a smaller area.
The design incorporated in to x-ray tube targets to allow a large area for heating while a small focal spot is maintained;
Line Focus Principle
The focal spot is the actual _______ source.
Diagnostic x-ray tubes have target angles that vary from approx:
5 to 20 degrees
The smaller the anode angle, the ________ the heel effect.
Absorption of x-rays in the heel of the target, resulting in reduced x-ray intensity to the anode side of the central axis:
the Heel Effect
The imaginary line generated by the centermost x-ray in the beam:
Central Ray of the useful beam
The difference in radiation intensity accross the useful beam of an x-ray field can vary as much as:
The heel effect is important when imaging anatomical structures that differ greatly in _______ or ________.
Thickness or Mass Density
the Heel Effect
Positioning the ________ side of the x-ray tube over the thicker part of the anatomy provides a more uniform exposure.
The heel effect results in ________ effective focal spot & ________ radiation intensity on the anode side of the x-ray beam.
Smaller focal spot
X-rays produced in the anode but not at the focal spot:
Off-focus radiation is reduced by:
A diaphragm in the tube housing near the window of the x-ray tube.
What are the 3 ways an x-ray tube dissipates heat?
- transfer of heat by emission of infrared radiation.
- transfer of energy from one area of an object to another.
- transfer of heat by the movement of a heated substance from one place to another.
Maximum radiographic techniques should never be applied to a _______ anode.
The most frequent cause of abrupt tube failure is electron arcing from the filament to the enclosure because of:
3 reasons for x-ray tube failure:
1. A single excessive exposure
2. Long exposure time causes excessive heating of the anode
3. Vaporization of the filament causes tungsten to coat the enclosure & eventually causes arcing
Produced after a filament is thinned until it eventually breaks after excessive heat by high mA operation for long periods:
(like a blown lightbulb)
________ tube currents and ________ exposure times are possible with the rotating anode.
HIGHER tube currents
SHORTER exposure times