X-ray Production, X-ray Tubes, And X-ray Generators Flashcards
These are created from conversion of kinetic energy of electrons into electromagnetic radiation when they are decelerated by interaction with a target material.
X-rays
For diagnostic radiology, a large electric potential difference of how many V is applied between two electrodes in the vacuum?
20,000 to 150,000 V (20 to 150 kV)
On impact with the target, the kinetic energy of electrons is converted to other forms of energy.
The majority of interactions are what?
Whereby energy exchanges with electrons in the target give rise to heat.
Collisional
The unfiltered bremsstrahlung shows an inverse linear relationship between the number and the energy of the x-rays produced.
With highest x-ray energy is determined by what?
By the peak voltage (kV) applied across the X-ray tube.
A typical filtered bremsstrahlung spectrum has no x-rays below how many keV?
About 10 keV
Major factors that affect production efficiency include:
Atomic number of the target material
Kinetic energy of the incident electrons
The most common target material is what?
Tungsten
For 100-keV electrons impinging on tungsten, the approximate ratio of radiative to collisional losses is 0.9%; therefore, more than how many % of the incident electron energy on the target electrode is converted to heat and non useful low-energy electromagnetic radiation?
99%
This provides an environment for the production of bremsstrahlung and characteristic x-rays.
X-ray tube
What are the major X-ray tube components?
Cathode Anode Rotor/ stator Glass or metal envelope Tube port Cable sockets Tube housing
What supplies the power and permits selection of tube voltage, tube current and exposure time?
X-ray generator
Depending upon the type of imaging examination and the characteristics of the anatomy being images, the x-ray tube voltage is set to values ___________ for diagnostic imaging, and _______ got mammography.
40 to 150 kV = Diagnostic imaging
25 to 40 kV = Mammography
This is proportional to the number of electrons per second flowing from the cathode to the anode.
This is measured in milliamperes (mA).
X-ray tube current
For continuous fluoroscopy, the tube current is relatively low ______ mA, and for projection radiography, the tube current is set from ________ mA in conjunction with short exposure times (typically less than 100 ms).
1 to 5 mA = continuous fluoroscopy
50 to 1,200 mA = projection radiography
What are the three major selectable parameters on the x-ray generator control panel that determine the x-ray beam characteristics?
kV, mA, and exposure time
This is the negative electrode in the X-ray tube, comprised of filament or filaments and a focusing cup.
Cathode
A filament is made of tungsten wire wound in a helix, and is electrically connected to the filament circuit, which provides a voltage of approximately how many V and variable current up to how many mA.
10 V
7,000 mA
Most x-ray tubes for diagnostic imaging have two filaments of different lengths, each positioned in a slot machined into the focusing cap, with one end directly connected to the focusing cup, and the other end electrically insulated from the cup by a ceramic insert. Only one filament is energized for an imaging examination.
When energized, the filament circuit heats the filament through electrical resistance, and the process of thermionic emission does what?
It releases electrons from the filament surface at a rate determined by the filament current and corresponding filament temperature.
Output of the x-ray tube is emission-limited.
Which means what?
It means that the filament current determines the X-ray tube current; at any kV, the x-ray flux is proportional to the tube current.
As a large voltage is applied between the cathode and anode in the correct polarity, electrons accelerated into tight distribution and travel to the anode, striking a small area called the what.
Focal spot
The focal spot dimensions are determined by the length of the filament in one direction and the width of electron distribution in the perpendicular direction.
This x-ray tube has a focusing cup totally insulated from the filament wires so that it’s voltage is independent of the filament.
Biased x-ray tube
Because high voltages are applied to the cathode, electrical insulation of the focusing cup and the bias supply voltage is necessary, and can add significant expense to the x-ray system.
Even greater negative applied voltage (about -4,000 V) to the focusing cup actually stops the flow of electrons, providing a means to rapidly switch the x-ray beam on and off; a tube with this capability is called what?
Grid-biased x-ray tube
Grid-biased x-ray tube switching is used by more expensive fluoroscopy systems for pulsed fluoroscopy and angiography to rapidly and precisely turn on and turn off the x-ray beam.
This is a metal target electrode that is maintained at a large positive potential difference relative to the cathode.
Anode
Electrons striking the anode deposit most of their energy as heat, with only a small fractions emitted as x-rays.
Tungsten is the most widely used anode material.
Why?
Because of its high melting point and high atomic number
What anode materials are used in mammographic x-ray tubes.
Molybdenum and rhodium
A simple X-ray tube design has a stationary anode, consisting of a tungsten insert embedded in a block.
The block is made from what material?
Copper
It is a copper block
In fix anode configuration:
Copper serves a dual role which are?
Mechanically supports insert
Efficiently conducts heat from the tungsten target
Modalities or machines that use fix anode x-ray tubes.
Dental x-ray units and some low-output mobile x-ray machines and mobile fluoroscopy systems
These anodes are used for diagnostic x-ray application, mainly because of greater heat loading and higher x-ray intensity output.
Rotating anodes
This design spreads the heat over a much larger area than does the stationary anode design, permitting much larger tube currents and exposure duration.
The rotating anode is beveled disk mounted on a _______ assembly supported by bearings of X-ray tube insert.
Rotor
The rotor consists of what?
Copper bars arrange around a cylindrical iron core
A donut-shaped stator device is comprised of what?
It surrounds the rotor and is mounted outside of the X-ray tube insert.
Electromagnets
Alternating currents (AC), the periodic reversal of electron movement in a conductor, passes through the stator windings and produces a rotating magnetic field.
Rotation speeds are how many revolutions per minute (rpm)?
3,000 to 3,600 (low speed) or 9,000 to 10,000 (high speed) rpm
A _______ focal spot allows a greater x-ray beam intensity but cause a loss of spatial resolution that increases with distance of the imaged object from the image receptor.
Smaller or large?
Larger
This is defined as the angle of the target surface with respect to the central ray (central axis) in he x-ray field.
Anode angle
Anode angles in diagnostic X-ray tubes typically range from 7 to 20 degrees.
What degree of angles are most common?
12- to 15- degree angles
What are the major factors affected by the anode angle?
Effective focal spot size
Tube output intensity
X-ray field coverage provided at a given focal spot to detector distance
The actual focal spot size is the area on the anode that is struck by electrons, and is primarily determined by what?
By the length of the of the cathode filament and the width of the focusing cup slot
Foreshortening of the focal spot length at the central ray is called the what?
Line focus principle
An ability to have a smaller effective focal spot size for a large actual spot increases the power loading for smaller effective focal spot size
The optimal anode angle depends on the clinical imaging application.
A small anode angle for small field-of-view devices, such as some small fluoroscopy detectors, where field coverage is limited by the image receptor diameter (e.g. 23 cm).
Small anode angle are approximately how many degrees?
7 to 9 degrees
Larger angle anodes are necessary for general radiographic imaging to achieve sufficiently large field area coverage at typical spot-to-detector distances such as 100 cm.
Large anode angle are approximately how many degrees?
12 to 15 degrees
Toward the anode side of the field, the projected length of the focal spot _______ , whereas it ________ towards the cathode side of the field.
Shorten or lengthen?
Shortens
Lengthens
Common tools for measuring focal spot size are the:
Pinhole camera
Slit camera
Star pattern
Resolution bar pattern
This uses a very small circular aperture (10 to 30 um diameter) in a thin, highly attenuating metal (e.g., lead, tungsten, or gold) disk to project a magnified image of the focal spot onto an image receptor.
Pinhole camera
This consists of highly attenuating metal (usually tungsten) plate with a thin slit, typically 10 um wide.
Slit camera
This contains a radial pattern of lead spokes of diminishing width and spacing on a thin plastic disk.
Star pattern test tool
This is a simple tool for evaluation of focal spot size.
These images demonstrates the effective resolution parallel and perpendicular to the A-C axis for a given magnification geometry.
Resolution bar pattern
The refers to a reduction in the x-ray beam intensity in the anode side, caused by the greater attenuation of x-rays directed toward the anode side of the field by the anode itself.
Heel effect
The heel effect is less prominent with a longer source-to-image distance (SID)
These results from electrons that scatter from the anode, and are re-accelerated back to the anode, outside of the focal spot area.
Off-focal radiation
These electrons cause low-intensity x-ray emission over the entire face of the anode, increasing patient exposure, causing geometric blurring, reducing image contrast, and increasing random noise.
What is a “getter” circuit?
It is used to trap gas in the insert and to maintain the vacuum
Mammography tubes use what type of material in the port?
In order to minimize absorption of the low-energy x-rays used in mammography.
Beryllium
This contains the cathode, anode, rotor assembly , and support structures sealed in a glass or metal enclosure under a high vacuum.
X-ray tube insert
This supports, insulates , and protects the X-ray tube insert from the environment.
X-ray tube housing
Lead shielding inside the housing attenuates nearly all x-rays that are not directed to the tube port. A small fraction of these x-rays, know as what?
It penetrates the housing.
Leakage radiation
This adjust the size and shape of the x-ray field emerging from the tube port.
Collimators
What is the most commonly used added filter material?
Aluminum
Other common materials include copper and plastic (e.g., acrylic)
These are used to change the spatial pattern of the x-ray intensity incident on the patient, so as to deliver a more uniform x-ray exposure to the detector.
Compensation (equalization) filters
What is the principal function of an x-ray generator?
Is to provide current at a high voltage to an X-ray tube
Electrical power available to a hospital or clinic provides up to about 480 V, much lower than the 20,000 to 150,000 V needed for x-ray production.
Transformers are principal components of x-ray generators; they convert low voltage into high voltage through a process called what?
Electromagnetic induction
This is a phenomenon in which a changing magnetic field induces an electrical potential difference (voltage) in a nearby conductor and also in which a voltage is induced in a conductor moving through a stationary magnetic field.
Electromagnetic induction
These use the principle of electromagnetic induction to change the voltage of an electrical power source.
Transformers
A basic electrical component known as what?
This will allow current flow in one direction.
Rectifier
Clever use of diodes arranged in a bridge rectifier circuit can do what?
It can route the flow of electrons through an AC circuit to create a direct current (DC), a unidirectional movement of electrons in which the voltage polarity never reverses.
It is the ratio of energy production or expenditure per unit time.
Power
The SI unit of power is the watt (W) defined as 1 joule (J) of energy per second.
The maximum voltage at any point in the circuit for a tapped transformer of 150 kV is ________ kV , relative to ground.
-75 kV or +75 kV
Modules of the X-ray generator include:
The high-voltage power circuit
The stator circuit
Tthe filament circuit
The focal spot selector
Automatic exposure control (AEC) circuit
This is defined as the difference between the peak voltage and the minimum voltage, divided by the peak voltage and multiplied by 100%.
Voltage ripple
This is also known as the automatic exposure control (AEC) system.
It is often used instead of manual exposure time setting in radiography.
These measure the actual amount of radiation incident on the image reception (i.e., screen-film or digital radiography detector) and terminate x-ray production when the proper radiation exposure is obtained.
Phototimer
This is the maximal power that an X-ray tube focal spot can accept or the generator can deliver.
Power rating of an X-ray tube or generator
Most medium focal spot dimensions (0.6 mm x 0.6 mm) have moderate power ratings (30 to 50 kW),
Smaller focal spots (0.3 mm x 0.3 mm) have low power ratings (5 to 15 kW),
The highest generator power ratings are found in interventional radiology and cardiolovascular suites, while modern multirow detector CT scanners have 85- to 100-kW generators.
How high are the highest generator power ratings?
80 to 120 kW
General radiographic or radiographic/fluoroscopic systems use generators providing 30 to 80 kW.
Lower powered generators (5 to 30 kW) are found in mobile radiography and fluoroscopy systems, dental x-ray systems, and other systems that have fixed anode X-ray tubes.
This is a traditional unit the provides a simple way of expressing the energy deposition on and dissipation from the anode of an X-ray tube.
Heat unit (HU)
What is the standard unit of energy?
The joule (J)
This shows the anode heat loading for various input powers (kW or HU/s) as the X-ray tube is operating, taking into account the cooling that simultaneously occurs.
Anode heating and cooling chart
For low-input power techniques, the heat loading increases to a plague, where the cooling rate equals the heating rate.
At higher power inputs, heating exceeds cooling, and after a certain time, a limit is reached where x-ray production must be stopped and anode cooling must be allowed prior to using the X-ray tube again.
With larger anode heat capacities, the anode cooling curve is steeper because higher temperatures result in more rapid cooling (“black-body” effect)
What is the “black-body” effect?
The radiative cooling rate is proportional to T4, for absolute temperature T
The output of an X-ray tube is often described by the terms:
Quality, quantity, and exposure
This describes the penetrability of an x-ray beam, with higher energy x-ray photons having larger half-value layer (HVL)
Quality
This refers to the number of photons comprising the beam.
Quantity
This is nearly proportional to the energy fluence of the x-ray beam.
Exposure
X-ray production efficiency, exposure, quality and quantity are determined by six major factors.
What are these factors?
X-ray tube target material Tube voltage Tue current Exposure time Beam filtration Generator waveform
Anode target material affects the ______ of bremsstrahlung radiation production, with output exposure roughly proportional to atomic number.
Efficiency
The target material affects the quantity of bremsstrahlung photons and the quality of the characteristics radiation.
This determines the maximum energy in the bremsstrahlung spectrum and affects the quality of the output spectrum.
Tube voltage (kV)
An increase in kV increases the efficiency of x-ray production and the quantity and quality of the x-ray beam.
This is proportional to the number of electrons flowing from the anode per unit time.
Tube current (mA)
The exposure of the beam for a given kV and filtration is proportional to the tube current.
This modifies the quantity and quality of the x-ray beam by preferentially removing the low-energy photons in the spectrum.
Beam filtration
This reduces the number of photons (quantity) and increases the average energy , also increasing the quality.
Type of filtration that includes the thickness (1 to 2 mm) of the glass or metal insert at the X-ray tube port.
Inherent filtration
Glass and aluminum have similar attenuation properties and effectively attenuate all x-rays in the spectrum below 15 keV.
This refers to sheets of metal intentionally placed in the beam to change its effective energy.
Added filtration
In general DR, added filters attenuate the low-energy x-rays in the spectrum that have almost no chance of penetrating the patient and reaching the x-ray detector.
Voltage induces in the second winding is proportional to the voltage on the primary winding and the ratio of the number of turns in the two windings.
This is according to what Law?
Law of transformation
