AIM: Ch 6: X-ray Production, X-ray Tubes, and X-ray Generators Flashcards
It supplies the power and permits selection of tube voltage, tube current, and exposure time.
X-ray generator
X-ray tube voltage set for:
a. Diagnostic imaging
b. Mammography
a. 40 to 150 kV
b. 25 to 40 kV
It is measured in milliamperes (mA), and is proportional to the number of electrons per second flowing from the cathode to the anode, where 1 mA = 6.24 x10^15 electrons/s
X-ray tube current
Give the three major selectable parameters on the x-ray generator control panel that determine the x-ray beam characteristics.
kV, mA, exposure time
Voltage, current, and exposure time
It is the negative electrode in the x-ray tube, and comprised of a filament or filaments and a focusing cup
Cathode
T/F: Electrons flow from the cathode to the anode only when the tube voltage is applied between these electrodes.
True
Emission-limited output means that the ____ determines the x-ray tube current
Filament current
When energized, the filament circuit heats the filament through electrical resistance, and the process of ____ releases electrons from the filament surface at a rate determined by the filament current and corresponding filament temperature.
Thermionic emission
As a large voltage is applied between the cathode and anode in the correct polarity, electrons are accelerated into a tight distribution and travel to the anode, striking a small area called the
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.
T/F: 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.
True
For tube voltages 40 kV and lower, a space charge cloud shields the electric field so that further increases in filament current do not increase the tube current. This is known as ____
Space charge-limited
A metal target electrode that is maintained at a large positive potential difference relative to the cathode
Anode
To avoid heat damage to the x-ray tube, two factors must be limited
- Rate of x-ray production (proportional to the tube current)
- At large tube currents, the duration of x-ray production
It is the most widely used anode material because of its high melting point and high atomic number.
Tungsten
An alloy of 10% rhenium and 90% tungsten provides added resistance to surface damage. Tungsten provides greater bremsstrahlung production than elements with lower atomic numbers
Two anode materials in mammographic x-ray tubes
Molybdenum
Rhodium
It contains the electron source and target within an evacuated glass or metal envelope
X-ray tube insert
It provides protective radiation shielding and cools the x-ray tube insert
Tube housing
It supplies the voltage to accelerate the electrons
X-ray generator
They shape the x-ray energy spectrum
X-ray beam filters
They define the size and shape of the x-ray field incident on the patient.
Collimators
SI unit of potential difference is the ____
Volt, V
For diagnostic radiology, a large electric potential difference of ____ is applied between two electrodes (the cathode and the anode) in the vacuum
20,000 to 150,000 V (20 to 150 kV)
The cathode is the ____ of electrons, and the anode, with a positive potential with respect to the cathode, is the ____ of electrons.
source, target
Positive is Always the Target
A common unit of energy is the ____, equal to the energy attained by an electron accelerated across a potential difference of 1 V.
Electron volt (eV)
The vast majority of interactions are ____, whereby energy exchanges with electrons in the target give rise to heat.
Collisional
Electrical (Coulombic) forces attract and decelerate an electron and change its direction, causing a loss of kinetic energy, which is emitted as an x-ray photon of equal energy called a
Bremsstrahlung radiation
T/F: The amount of energy lost by the electron and thus the energy of the resulting x-ray are determined by the distance between the incident electron and the target nucleus, since the Coulombic force is proportional to the inverse of the square of the distance
True
T/F: The probability of electron interactions that result in production of x-ray energy E is dependent on the radial interaction distance, r, from the nucleus, which defines a circumference, 2(pi)r.
True
The ____ bremsstrahlung spectrum (Fig. 6-3A) shows an inverse linear relationship between the number and the energy of the x-rays produced, with the highest x-ray energy determined by the peak voltage (kV) applied across the x-ray tube.
Unfiltered
A typical ____ bremsstrahlung spectrum (Fig. 6-3B) has no x-rays below about 10 keV; the numbers increase to a maximum at about one third to one half the maximal x-ray energy and then decrease to zero as the x-ray energy increases to the maximal x-ray energy.
Filtered
Filtration in this context refers to the removal of x-rays by attenuation in materials that are inherent in the x-ray tube (e.g., the glass window of the tube insert), as well as by materials that are purposefully placed in the beam, such as thin aluminum and copper sheets, to remove lower energy x-rays and adjust the spectrum for optimal low-dose imaging
Give 2 major factors that affect x-ray production efficiency
- Atomic number of the target material
- Kinetic energy of the incident electrons
What determines the energy of the incident electron?
Voltage applied to the x-ray tube
T/F: A Kb x-ray is more energetic than a Ka x-ray.
True
T/F: K-shell transitions are the only relevant transition in the image formation process
True
Characteristic x-rays other than those generated by K-shell transitions are too low in energy for any useful contributions to the image formation process and are undesirable for diagnostic imaging.
T/F: Characteristic K x-rays are produced only when the electrons impinging on the target exceed the binding energy of a K-shell electron.
True
X-ray tube voltages must therefore be greater than 69.5 kV for W targets, 20 kV for Mo targets, and 23 kV for Rh targets to produce K characteristic x-rays.
T/F: In terms of intensity, as the x-ray tube voltage increases, so does the ratio of characteristic to bremsstrahlung x-rays.
True
It provides an environment for the production of bremsstrahlung and characteristic x-rays
X-ray tube
Give the usual tube current for the following studies:
a. Continuous fluoroscopy
b. Pulsed fluoroscopy
b. Projection radiography
a. 1 to 5 mA
b. 10 to 50 mA
c. 50 to 1,200 mA
T/F: Often, the product of the tube current and exposure time are considered as one entity, the mAs (milliampere-second; technically, mAs is a product of two units but, in common usage, it serves as a quantity)
True
It is made of tungsten wire wound in a helix, and is electrically connected to the filament circuit, which provides a voltage of approximately 10 V and variable current up to 7,000 mA (7 A)
Filament
T/F: At any kV, the x-ray flux is proportional to the tube current.
True
T/F: Higher tube voltages also produce slightly higher tube current for a much higher filament current.
False
Higher tube voltages also produce slightly higher tube current for the same filament current.
A ____ x-ray tube has a focusing cup totally insulated from the filament wires so that its voltage is independent of the filament.
biased
Isolation of the focusing cup from the filament and application of a negative bias voltage (−100 V) reduces electron distribution further by increasing the repelling electric fields surrounding the filament and modifying the electron trajectories
A tube with a capability of applying greater negative applied voltage (about −4,000 V) to the focusing cup to stop the flow of electrons, and providing a means to rapidly switch the x-ray beam on and off is called
Grid biased or grid pulsed 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 eliminates the build-up delay and turnoff lag of x-ray tubes switched at the generator, which cause motion artifacts and produce lower average x-ray energies and unnecessary patient dose.
T/F: Electrons striking the anode deposit most of their energy as x-rays, with only a small fraction emitted as heat.
False
Electrons striking the anode deposit most of their energy as heat, with only a small fraction emitted as x-rays.
Two components of a stationary anode
Tungsten insert
Copper block
However, the small area of the focal spot on the stationary anode limits the tube current and x-ray output that can be sustained without damage from excessive temperature.
Stationary versus Rotating Anodes:
a. Dental x-ray units and some low-output mobile x-ray machines
b. Interventional fluoroscopy and CT
c. Mobile fluoroscopy
a. Stationary
b. Rotating
c. Stationary
This anode configuration is used for most diagnostic x-ray applications, mainly because of greater heat loading and higher x-ray intensity output.
Rotating anodes
T/F: A large focal spot, which permits high x-ray output and short exposure times, should be used in situations where motion is expected to be a problem and geometric magnification is small (the object is close to the image receptor).
True
It is defined as the angle of the target surface with respect to the central ray (central axis) in the x-ray field
Anode angle
Anode angles in diagnostic x-ray tubes typically range from ____ degrees, with ____-degree angles being most common
7 to 20, 12- to 15
Give 3 major factors affected by the anode angle
- Effective focal spot size
- Tube output intensity, and
- X-ray field coverage provided at a given focal spot to detector distance
It is the area on the anode that is struck by electrons, and is primarily determined by the length of the cathode filament and the width of the focusing cup slot.
Actual focal spot
T/F: The projected length of the focal spot area at the x-ray field central ray is much smaller, because of geometric foreshortening of the distribution from the anode surface.
True
Foreshortening of the focal spot length at the central ray is called the ____.
Line focus principle
An ability to have a smaller effective focal spot size for a large actual focal spot increases the power loadings for smaller effective focal spot sizes
T/F: As the anode angle decreases (approaches 0 degrees), the effective focal spot becomes smaller for the same actual focal area, providing lesser spatial resolution of the object when there is geometric image magnification.
False.
As the anode angle decreases (approaches 0 degrees), the effective focal spot becomes smaller for the same actual focal area, providing BETTER spatial resolution of the object when there is geometric image magnification.
T/F: For larger actual focal areas, greater x-ray output intensity with shorter exposure times is not possible.
False
For larger actual focal areas, greater x-ray output intensity with shorter exposure times IS possible.
Given a large anode angle and long filament length, describe the following
a. Field coverage
b. Effective focal spot
c. Power loading
d. Disadvantages: (2)
a. Large
b. Large
c. Good
d. Geometric blurring and image degradation
T/F: The effective focal spot length varies with the position in the image plane, in the anode-cathode (A–C) direction. Toward the anode side of the field, the projected length of the focal spot shortens, whereas it lengthens towards the cathode side of the field
True
It uses a very small circular aperture (10 to 30 mm 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. Positioned on the central axis between the x-ray source and detector, an image of the focal spot is recorded.
Pinhole camera
It consists of a highly attenuating metal (usually tungsten) plate with a thin slit, typically 10 mm wide. In use, this device is positioned above the image receptor, with the center of the slit on the central axis, and with the slit either parallel or perpendicular to the A-C axis.
Slit camera
It contains a radial pattern of lead spokes of diminishing width and spacing on a thin plastic disk
Star pattern test tool
It is a simple tool for evaluation of focal spot size wherein the images demonstrate the effective resolution parallel and perpendicular to the A-C axis for a given magnification geometry, determined from the number of the bar pattern that can be resolved.
Resolution bar pattern
It is an increase in the size of the focal spot resulting from high tube current (mA), and is caused by electron repulsion in the electron beam between the cathode and anode.
Focal spot “blooming”
It is most pronounced at LOW kVs
It is a slight decrease in the measured size with increasing kV (electron repulsion and spreading in the x-ray tube is reduced)
Focal spot “thinning”
It refers to a reduction in the x-ray beam intensity toward the anode side of the x-ray field, caused by the greater attenuation of x-rays directed toward the anode side of the field by the anode itself
Heel effect
T/F: The heel effect is less prominent with a shorter source-to-image distance (SID)
False
The heel effect is less prominent with a LONGER source-to-image distance (SID)
The L in HeeL is for less and longer
It results from electrons that scatter from the anode, and are re-accelerated back to the anode, outside of the focal spot area. 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.
Off-focal radiation
It 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 is used to trap gas in the insert and to maintain the vacuum.
“Getter” circuit
Mammography tubes use ____ in the port to minimize absorption of the low-energy x-rays used in mammography.
Beryllium (Z = 4)
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, known as ____, penetrates the housing.
Leakage radiation
Federal regulations (21 CFR 1020.31) require that the light field and x-ray field be aligned so that the sum of the misalignments, along either the length or the width of the field, is within ____ of the SID.
2%
This automatically limit the field size to the useful area of the detector.
Positive beam limitation (PBL) collimators
Adjustment to a smaller field area is possible; however, a larger field area requires disabling the PBL circuit
It has a focusing cup that is electrically isolated from the cathode filament and maintained at a more negative voltage. When the bias voltage is sufficiently large, the resulting electric field stops the tube current.
Grid-biased tube
Grid biasing requires approximately ____ applied to the focusing cup with respect to the filament to switch the x-ray tube current off
−4,000 V
The following include the main differences between a dedicated mammography tube and a conventional x-ray tube:
a. Target materials (molybdenum, rhodium, and tungsten)
b. Output port (beryllium versus glass or metal insert material)
c. Smaller effective focal spot sizes (typically 0.3 and 0.1 mm nominal focal spot sizes)
d. Use of grounded anodes
e. All of the above
e. All of the above
In an advanced CT x-ray tube, circulating oil rapidly removes excess heat from the anode. The electron beam can be rapidly deflected between two focal spots; this is known as a ____
Flying focal spot
The following maximizes x-ray tube life, except:
a. Minimize filament boost “prep” time (the first detent of two on the x-ray exposure switch) especially when high mA is used
b. Use higher tube current with shorter exposure times to arrive at the desired mAs if possible
c. Avoid extended or repeated operation of the x-ray tube with high technique (kV and mAs) factors
d. Always follow the manufacturer’s recommended warm-up procedure.
e. Limit rotor start and stop operations
f. All of the above maximizes x-ray tube life
b. Use LOWER tube current with LONGER exposure times to arrive at the desired mAs if possible
It is the removal of x-rays as the beam passes through a layer of material.
Filtration
Inherent filtration includes the thickness of ____ of the glass or metal insert at the x-ray tube port.
1 to 2 mm
Glass (primarily ____) and aluminum have similar attenuation properties and effectively attenuate all x-rays in the spectrum below about ____
Silicon dioxide, SiO2
15 keV
Inherent filtration includes the following, except:
a. Inherent filtration includes the thickness (1 to 2 mm) of the glass or metal insert at the x-ray tube port.
b. Attenuation by housing oil
c. Field light mirror in the collimator assembly
d. All of the above
e. None of the above
d. All of the above are part of inherent filtration
It refers to sheets of metal intentionally placed in the beam to change its effective energy
Added filtration
It is the most commonly used added filter material
Aluminum (Al)
They 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
These are useful for lateral projections in cervical-thoracic spine imaging, where the incident fluence is increased to match the increased tissue thickness encountered (e.g., to provide a low incident flux to the thin neck area and a high incident flux to the thick shoulders)
Wedge filters
These are used in CT to reduce dose to the periphery of the patient, where x-ray paths are shorter and fewer x-rays are required.
Bow-tie filters
Transformers are principal components of x-ray generators; they convert low voltage into high voltage through a process called ____. It 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
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.
T/F: The magnitude of the induced voltage is proportional to the rate of change of the magnetic field strength.
True
T/F: Electrical current, such as the electrons flowing through a wire, produces a magnetic field whose magnitude (strength) is proportional to the magnitude of the current
True
T/F: For a coiled wire geometry, superimposition of the magnetic fields from adjacent turns of the wire decreases the amplitude of the overall magnetic field (the magnetic fields penetrate the wire’s insulation), and therefore the magnetic field strength is inversely proportional to the number of wire turns.
False
For a coiled wire geometry, superimposition of the magnetic fields from adjacent turns of the wire increases the amplitude of the overall magnetic field (the magnetic fields penetrate the wire’s insulation), and therefore the magnetic field strength is proportional to the number of wire turns.
What kind of transformer circuit is used in a diagnostic x-ray generator?
Step-up transformer circuit
Based on the Law of Transformers,
For Ns > Np, a “step-up” transformer increases the secondary voltage
For Ns < Np, a “step-down” transformer decreases the secondary voltage
For Ns = Np, an “isolation” transformer produces a secondary voltage equal to the primary voltage
A basic electrical component known as a ____ will allow current to flow in one direction only
Rectifier
A ____ is a device with two terminals. When a voltage is applied between the terminals with a specific polarity, there is very little resistance and a large current flows; when the same voltage is applied with the opposite polarity, little or no current flows.
Diode
Clever use of diodes arranged in a ____ 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.
Bridge rectifier circuit
It is the rate of energy production or expenditure per unit time
Power
SI unit of power
Watt = J/s
For electrical devices, power is equal to the product of ____
Voltage and current
In an operator console, if this is used, exposure time is not set
Automatic exposure control (AEC) circuit
The focal spot size (i.e., large or small) is usually determined by the ____
mA setting
Generator design which is now the contemporary state-of-the-art choice for diagnostic x-ray systems. This uses a high frequency alternating waveform (up to 50,000 Hz) for efficient conversion of low to high voltage by a step-up transformer.
High-frequency x-ray generator
It measures the difference between the reference voltage (a calibrated value proportional to the requested kV) and the actual kV measured across the tube by a voltage divider (the kV sense circuit)
Voltage comparator
The mA is regulated in an analogous manner to the kV, with a ____ sensing the actual mA (the voltage across a resistor is proportional to the current) and comparing it with a reference voltage.
Resistor circuit
In an electronic waveform, ____ is defined as the difference between the peak voltage and the minimum voltage, divided by the peak voltage and multiplied by 100%:
Voltage ripple
High-frequency generators have a ripple that is kV and mA dependent
It is used as a safety mechanism to terminate the exposure in the event of an exposure switch or timer failure.
Countdown or backup timer
Fastest switching method, with minimal turn-on–turnoff “lag”
Grid-controlled x-ray tube
It is often used instead of manual exposure time settings in radiography; which measures the actual amount of radiation incident on the image receptor (i.e., screen-film or digital radiography detector) and terminate x-ray production when the proper radiation exposure is obtained.
Phototimer, also known as the automatic exposure control (AEC) system
It 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
It is a traditional unit that provides a simple way of expressing the energy deposition on and dissipation from the anode of an x-ray tube
Heat unit
For three-phase, high-frequency, and constantpotential generators, because of their lower voltage ripple and higher average voltage, a multiplicative factor of ____ compensates for this difference in the HU equation below
1.35 to 1.40
It is the SI unit of energy
Joule (J)
The energy, in joules, deposited in the anode is calculated as follows:
It is the constant voltage that would deliver the same power as a varying voltage waveform (e.g., for a single phase generator)
Root-mean-square voltage, VRMS
It 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
With larger anode heat capacities, the anode cooling curve is steeper because higher temperatures result in more rapid cooling (also known as the ____, where the radiative cooling rate is proportional to T4, for absolute temperature T)
“Black-body” effect
Quality, quantity, and exposure of x-ray tube output:
It describes the penetrability of an x-ray beam
Quality
With higher energy x-ray photons having a larger half-value layer (HVL) and higher “quality.”
Quality, quantity, and exposure of x-ray tube output:
It refers to the number of photons comprising the beam
Quantity
Quality, quantity, and exposure of x-ray tube output:
It is nearly proportional to the energy fluence of the x-ray beam.
Exposure
Factors affecting x-ray emission:
It affects the efficiency of bremsstrahlung radiation production, with output exposure roughly proportional to atomic number. It also affects the quantity of bremsstrahlung photons and the quality of the characteristic radiation.
Anode target material
Factors affecting x-ray emission:
It determines the maximum energy in the bremsstrahlung spectrum and affects the quality of the output spectrum. In addition, the efficiency of x-ray production is directly related to this factor.
Tube voltage (kV)
Exposure is approximately proportional to the square of the kV in the diagnostic energy range.
Factors affecting x-ray emission:
T/F: An increase in kV increases the efficiency of x-ray production and the quantity and quality of the x-ray beam.
True
Changes in the kV must be compensated by corresponding changes in mAs to maintain the same exposure.
Factors affecting x-ray emission:
It is proportional to the number of electrons flowing from the cathode to the anode per unit time.
Tube current (mA)
The quantity of x-rays is directly proportional to the product of tube current and exposure time (mAs), as shown in Figure 6-31.
T/F: The exposure of the beam for a given kV and filtration is proportional to the tube current.
True
Factors affecting x-ray emission:
This modifies the quantity and quality of the x-ray beam by preferentially removing the low-energy photons in the spectrum. This reduces the number of photons (quantity) and increases the average energy, also increasing the quality
Beam filtration
T/F: It is not necessary to know the HVL of the beam and the normalized x-ray tube intensity per mAs (in addition to the geometry) to calculate the incident exposure and radiation dose to the patient. One simply uses kV and mAs for determining the “proper technique” or to estimate the dose to the patient without this information.
False
It is necessary to know the HVL of the beam and the normalized x-ray tube intensity per mAs (in addition to the geometry) to calculate the incident exposure and radiation dose to the patient. One cannot simply use kV and mAs for determining the “proper technique” or to estimate the dose to the patient without this information.
Factors affecting x-ray emission:
It affects the quality of the emitted x-ray spectrum
Generator waveform
Both the quality and quantity of the x-ray spectrum are affected
Factors affecting x-ray emission:
T/F: The x-ray quality is approximately proportional to Ztarget x kV^2 x mAs.
False
The x-ray QUANTITY is approximately proportional to Ztarget x kV^2 x mAs.
T/F: The x-ray quality depends on the kV, the generator waveform, and the tube filtration.
True
T/F: Exposure is independent from both the quantity and quality of the x-ray beam
False
Exposure depends on both the quantity and quality of the x-ray beam
Compensation for changes in kV with radiographic techniques requires adjustments of mAs on the order of the fourth to fifth power of the kV ratio, because kV determines quantity, quality, and transmission through the object, whereas mAs determines ____ only.
Quantity
