|Lavin| Chapter 2: Anatomy of the X-Ray Machine
List methods of heat dissipation within the x-ray tube housing.
-Copper at base of Tungsten target - surrounding the glass tube with oil within the metal housing to pull heat from the target and/or circulated through heat exchanger -using target made of material other than tungsten
List the five elements necessary for x-ray production.
1. Source of electrons 2. A method of accelerating the electrons 3. An obstacle-free path for the passage of high-speed electrons 4. A target in which the electrons can interact. releasing energy in the form of x-rays 5. An envelope (tube) to provide a vacuum environment, eliminating the air molecule obstacles from the electron stream and preventing rapid oxidation of the elements
Tungsten
A common metal used in the filament of a cathode
Heel Effect
A decrease of x-ray intensity on the anode side of the x-ray beam caused by the anode target angle
Define "heel effect".
A decrease of x-ray intensity on the anode side of the x-ray beam caused by the anode target area.
Glass Envelope
A glass vacuum tube that contains the anode and cathode of the x-ray tube
X-ray Tube
A mechanism consisting of an anode and cathode in a vacuum that produces a controlled x-ray beam
Molybdenum
A metal commonly used in focusing cups because of its high melting point and poor conduction of heat.
Half-Wave Rectification
A method of converting alternating to direct current in which half of the current is lost
Alloy
A mixture of metals
Cathode
A negatively charged electrode that provides a source of electrons
Stationary Anode
A nonmoving anode, usually found in dental and small portable radiography units
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.
Anode
A positively charged electrode that acts as a target for the electrons from the cathode. Electrons interacting with the anode produce heat and x-rays.
Focusing Cup
A recessed area where the filament lies, directing the electrons toward the anode
Collimator
A restricting device used to control the size of the primary x-ray beam
Line-voltage Compensator
Adjusts the incoming voltage to the autotransformer so that the voltage remains constant
Valve Tubes
Allow the glow of electrons in one direction only. Commonly called self-rectifiers.
Rotating Anode
An anode that turns on an axis to increase x-ray production while dissipating heat
Target
Anode
List the possible areas of x-ray tube failure.
Cathode Failure, Anode Bearing Failure, Anode Target Failure, Glass-Envelope Damage, Tube Housing Abnormalities
Timer Switch
Controls the length of exposure
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
True or False: A small amount of air within the glass envelope is beneficial because it helps dissipate the heat.
FALSE- Air molecules interfere with the path of electrons, thus decreasing the number of electrons reaching the target
Define and describe the focal spot.
Focal Spot is the small area of the target with which electrons collide on the anode. Size of the focal spot has a large effect on the formation of the x-ray image. Smaller FP = clearer image. Larger FP = shadowed and unsharp
List the electrical components of an x-ray machine.
High-Voltage Circuit, Low-Voltage (Filament) Circuit, Timer Switch
Step-up Transformer
Increases the incoming voltage of 110 or 220 V to thousands of volts (i.e. kilovolts)
Filament
Part of a low energy circuit in the cathode that, when heated, releases electrons from their orbits
Penumbra
Partial outer shadow of an object being imaged by illumination
Rectification
Process of changing alternating current and direct current
Autotransformer
Provides a variable yet predetermined voltage to the high-voltage step-up transformer. It acts as the kilo voltage selector
State and define the methods of rectification.
Rectification is the process of changing alternating current to direct current. The x-ray tube may perform its own rectification, known as half-wave rectification. As this happens, one half of the current is lost and a marked increase in heat ocurrs at the anode. If the anode becomes too hot, it may form an electron cloud and pass a current from the anode to the cathode. Rectifiers allow the flow of electrons in one direction only.
Step-down Transformer
Reduces the x-ray machine input voltage of 110 or 220 V to 10V to prevent burnout of the cathode filament
List and describe the two types of anodes.
Rotating Anode: disk shaped and rotates on an axis through the center of the tube. has bevelled edge, composed of tungsten or similar to withstand higher temperatures. Stationary Anode: "fixed" anode, found in dental and small protable radiography units, have a small capacity for x-ray production, and inability to withstand large amounts of heat.
Kilovoltage
The amount of electrical energy being applied to the anode and cathode to accelerate the electrons rom the cathode to the anode [kV]=1000 volts [V]
Milliamperage [mA]
The amount of electrical energy being applied to the filament. Milliamperage describes the number of x-rays produced during the exposure
Actual focal spot
The are of the focal spot consisting of a coiled wire that is perpendicular to the surface of the target.
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
Describe the basic construction of the anode.
The basic construction of the anode consists of a beveled target placed on a cylindric base.
Console
The control panel of the x-ray machine
Define the 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-focus Principle
The effect of making the focal spot size appear smaller when viewed from the position of the film because of the angle of the target to the electron stream
Acceleration
The increase in speed over time.
Kilovoltage Peak [kVp]
The peak energy of the x-rays, which determines the quality (penetrating power) of the x-ray beam.
State the purpose and construction of the cathode.
The purpose of the cathode is to provide a source of electrons and direct these electrons toward the anode. (Fig 2-2, pg 11)
State the purpose of the x-ray tube.
The purpose of the x-ray tube is to produce a controlled x-ray beam
Focal Spot
The small area of the target with which electrons collide on the anode.
Describe the anatomy of the x-ray tube.
The x-ray tube consists of a cathode side (with negative electrical charge) and an anode side (with a positive electrical charge) encased in a glass envelope, which is evacuated to form a vacuum. (Fig 2-1, pg 11)
Give reasons for the use of tungsten, molybdenum, and copper in the construction of the x-ray tube
Tungsten: common element used in the filament of a cathode and target of an anode due to its high melting point and high atomic number (more protons= same electrons) as well as its ability to produce heat. Molybdenum: is a substance used mostly in the production of the focusing cup because of its high melting poing and is a poor conductor of heat. Copper: material used at the base of the target on an anode, which acts as a conductor of heat and draws the heat away from the tungsten target. (protection from 1000 degrees C which would melt the tungsten)
How can the technician help to prolong the life of the filament in the x-ray tube? a. enter the proper exposure settings in the control panel before the final positioning of the animal b. leave the x-ray unit on at all times to ensure that the gilament is heated when the radiograph is requested c. always leave the x-ray unit in the standby mode d. the technician can do nothing because filament defects are largely the fault of the manufacturer
a. Enter the proper exposure settings in the control panel before the final positioning of the animal
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: a. using the shortest exposure time possible b. altering the direction in which the x-rays move c. selecting a longer exposure time than is recommended d. sedating all patients before taking radiographs
a. using the shortest exposure time possible
Which of the following are limitations of the stationary anode? a. the target is made of tungsten b. it is unable to withstand large amounts of heat c. if the target becomes pitted, radiographs appear darker d. it is limited to a larger focal spot to accommodate higher temperatures
b. it is unable to withstand large amounts of heat
The advantages of using an x-ray machine with three-phase generator as opposed to a single-phase generator include: a. creation of more low-energy electrons bombarding the target, thus producing less heat b. use of shorter exposure times because more power is available to the x-ray tube per unit time c. production of more low-energy x-rays so that radiation quality is increased d. generation of considerably higher intensity of the x-radiation
b. use of shorter exposure times because more power is available to the x-ray tube per unit
X-ray tube ratings are based on target angle, focal spot size, electrical current (single- or three-phase operation), and: a. rectification b. its alloy composition c. anode speed d. type of filament
c. anode speed
Which of the following is recommended to reduce unnecessary irradiation of the patient or persons restraining the patient and to decrease scatter radiation? a. opening the collimator as wide as possible b. placement of a lead apron over the area of interest on the patient c. section of full-wave rectification as opposed to half-wave rectification on the control panel d. adjustments of the collimator so that the smallest field size possible is used
d. Adjustments of the collimator so that the smallest field size possible is used
The anode's target: a. is composed of tungsten b. reaches temperatures in excess of 1000 degrees C during x-ray production c. usually has a copper base d. all of the above
d. all of the above
Which of the following are possible effects of excessive heat within the x-ray tube? a. bearing failure and decreased anode speed b. roughened target surface c. arcing d. all of the above
d. all of the above
Filaments located in an x-ray tube: a. are made of molybdenum b. must have a low melting point and low atomic number c. are found in the anode d. emit electrons when heated
d. emit electrons when heated
Describe x-ray tube rating and the three-phase generator.
x-ray tube rating is based on four factors: focal spot size, target angle, anode speed, and electrical current (single or three phase operation). Each type of x-ray tube has an individual tube rating which dictate the maximum combinations of kVp, mA, and time that can safely be used without overloading the tube. This rating is expressed in kilowatts. Three-phase generator is present on most modern table-based machines which produces an almost constant electrical potential difference between the anode and the cathode. Advantaged of 3-phase generator is 1)more power is available to the x-ray tube per unit time, and therefore shorter exposure can be used, 2) Intensity of the x-radiation generated is considerably higher, 3) Radiation quality is greater because it contains fewer low-energy x-rays, 4) Tube utilization is more efficient because rhe target is not subjected to bombardment of low-energy electrons, which creates only heat in the anode target area.
State the purpose of the autotransformer, step-up transformer, line-voltage compensator, step-down transformer, and timer switch.
| High-Voltage Curcuit | Autotransformer: Provides a variable yet predetermined voltage to the high-voltage step-up transformer. It acts as the kilovoltage selector. Step-up Transformer: Increases the incoming voltage of 110-220 V to thousands of volts Line-Voltage Compensator: associated with the autotransformer and adjusts the incoming line voltage to the autotransformer so that the primary coil voltage remains constant | Low-Voltage (filament) Circuit | Step-down Transformer: placed between the cathode filament and the x-ray machine input voltage. Reduces the x-ray machine input voltage from 110-220V down to 10V to prevent burnout of the cathode filament. | Timer Switch | :