Xray Production, Xray Tubes And Xray Generators Flashcards
A device that produces xrays in diagnostic energy range typically contains
- An electron source
- Evacuated path for electron acceleration
- target electrode
- external power source
Contains electron source
Xray tube insert
Provides protective radiation shielding and cools the xray tube insert
Tube housing
Supplies the voltage to accelerate the electrons
Xray generator
Shapes the xray energy spectrum
Xray beam filters
Define the size and shape of the xray field incident on the patient
Collimators
Source of electrons
Cathode
Target of electrons
Anode
Vast majority of interactions are
Collisional
Amount of energy lost by the electron and thus the energy pf the resulting xray is determined by the
Distance between the incident electron and the target nucleus
With increasing distance from the nucleus, the circumference increases, therefore the probability of interaction _______ but the xray energy ________
Interaction increase
Xray energy decrease
As the interaction distance decreases, xray energy increases because of
Greater electron deceleration
Probability distribution of xray photons as a function of photon energy (keV)
Bremsstrahlung spectrum
Shows an inverse linear relationship between the number and the energy of the xrays produced, with the highest xray energy determined by the peak voltage applied across the xray tube
Unfiltered bremsstrahlung spectrum
A typical filtered bremsstrahlung spectrum has no xrays below about
10 keV
Major factors that affect xray production efficiency include
Atomic number of the target material
Kinetic energy of the incident electrons
Most common target material in xray is
Tungsten
Most common target material in mammography is
Molybdenum and rhodium
The innermost shell designated as the ____ shell has the highest electron binding energy
K shell
Xray tube voltages must be greater than ___ kV for tungsten
____ kV for moldybdenum
____ kV for rhodium
69.5 tungsten
20 molybdenum
23 rhodium
Xray tube voltage is set to values from ___ to _____ kV for diagnostic imaging
40 to 150 kV
Voltage for mammo is set typically to
25 to 40 kV
Xray tube current in projection radiography
50-1200 mA
Tube current in continuous flouroscopy
1-5 mA
In pulsed fluoroscopy, tube current is set to
10-50 mA
3 major selectable parameters on the xray generator control panel that determine the xray beam characteristics
kV, mA and exposure time
It has a focusing cup totally insulated from the filament wires so that its voltage is independent of the filament
Biased xray
Xray tube that has a capability of rapidly switching the xray beam on and off
Grid-biased xray tube
A metal target electrode that is maintained at a large positive potential difference relative to the cathode
Anode
Widely used anode material because of its high melting point and high atomic number
Tungsten
Mechanically supports the tungsten insert and efficiently conducts heat from the tungsten target
Copper
Heat insensitive parts in the rotor anode
Non-volatile lubricant
Molybdenum
Infrared radiation emission
Angle of target surface with respect to the central ray im the xray field
Anode angle
Anode angles in xray tubes typically range from
7-20 degrees
12-14 degrees most common
Foreshortening of the focal spot length at the central ray is called the
Line focus principle
A small anode angle (7-9 degrees) is desirable for_____ field of view devices
Small field
Larger anode angles (12-15 degrees) are necessary for
General radiography
Common tools for measuring focal spot size are the
Pinhole camera, slit camera, star pattern and resolution bar pattern
Uses very small circular aperture (10-30 um diameter) in a thin, attenuating metal disk to project a magnified image of the focal spot onto an image receptor
Pinhole camera
Consists of a highly attenuating metal plate with a thin slit
Slit camera
Contains a radial pattern of lead spokes of diminishing width and spacing on a thin plastic disk
Star pattern tool
A simple tool for evaluation of focal spot size. It demonstrates the effective resolution parallel and perpendicular to the A-C axis for a given magnification geometry, determined from the number of bar pattern that can be resolved
Resolution bar pattern
An increase in the size of the focal spot resulting from high tube current and is caused by electron repulsion in the electron beam between the cathode and anode
Focal spot “blooming”
Refers to reduction in the xray beam intensity toward the anode side of the xray field caused by greater attenuation of xrays directed toward the anode side of the field by the anode itself
Heel effect
Heel effect is less prominent with a ______ source to image distance
Longer
Results from electrons that scatter from the anode, and are reaccelerated back to the anode, outside of the focal spot area
Off-focal radiation
Can reduce off-focal radiation by intercepting xrays that are produced away from the focal spot
Small lead collimator
Contains the cathode, anode, rotor assembly and support structures sealed in a glass or metal enclosure under a high vacuum
Xray tube insert
Used to trap gas in the insert and to maintain the vacuum
“Getter” circuit
Adjusts the size and shape of the xray field emerging from the tube port
Collimators
Collimator that automatically limit the field size to the useful area of detector
Positive beam limitation collimators
Grid-biased tube is used in applications such as
Pulsed fluoroscopy and cine-angiography
The output port of mammogram is made of
Beryllium/glass/metal insert material
Dynamic steering of the electron beam within the tube is achieved by _______ to direct the electrons to distinct focal spots on the anode, which can produce slightly different projections and improve data sampling during the CT acquisition
External electromagnetic deflection coils
Recommendations to maximize xray tube life:
- Minimize filament boost “prep” time especially when high mA is used
- use lower tube current with longer exposure time to arrive at the desired mAs if possible
- avoid extended or repeated operation of the xray tube with high technique factors (kV and mAs)
- always follow the manufacturer’s recommended warm-up procedure
- limit rotor start and stop operations; 30-40 sec delay should be used
Removal of xrays as the beam passes thru a layer of material
Filtration
Filtration includes
Inherent filtration of the xray
And added filtration
Includes the thickness (1-2 mm) of the glass or metal insert at the xray tube port
Inherent filtration
Glass used for filtration
Silicon dioxide
Inherent filtration include attenuation by
Housing oil and field light mirror in the collimator assembly
It refers to sheets of metal intentionally placed in the beam to change its effective energy
Added filtration
What element is the most commonly used added filter material
Aluminum
Others include copper and plastic
In mammography, thin filters of ____, _____ and ____ are used to transmit bremsstrahlung xrays in the intermediate energy range (15-25 keV)
Molybdenum
Rhodium
Silver
Filters that are used to change the spatial pattern of the xray intensity incident on the patient, so as to deliver a more uniform xray exposure to the detector
Compensation (equalization) filters
Filters that are useful for lateral projections in cervical-thoracic spine imaging, where the incident fluence is increased to match the increased tissue thickness encountered
Wedge filters
Filters that are used in ct to reduce dose to the periphery of the patient, where the xray paths are shorter and fewer xrays are required
“Bow-tie” filters
Principal function of an xray generator is to provide current at _____ voltage to an xray tube
High
Transformers convert low voltage into high voltage thru a process called
Electromagnetic induction
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 thru a stationary magnetic field
Electromagnetic induction
Magnitude of the induced voltage is proportional/inversely proportional to the rate of change of the magnetic field strength
Proportional
Electric current, such as the electrons flowing thru a wire,
Produces a magnetic field whose magnitude (strength ) is proportional/inversely proportional to the magnitude of the current
Proportional
Uses the principle of electromagnetic induction to change the voltage of an electrical power source
Transformer
Direction of the current is dependent on the
Direction of the magnetic field
The polarity and magnetic field strength are determined by the
Amplitude and direction of the current
A basic transformer consist of
Iron core
Primary winding circuit
Secondary winding circuit
Allows current to flow in one direction only
Rectifier
Can route the flow of electrons thru an AC circuit to create a direct current (DC) a unidirectional movement of electrons in which the voltage polarity never reverses
Bridge rectifier circuit
A device with 2 terminals
Diode
Rate of energy production or expenditure per unit time
Power
Power is equal to the product of
Voltage and current
SI unit for power
Watts- 1 joule/sec
Defined as 1 joule per coulomb
Volt
Defined as 1 coulomb/sec
Ampere
The focal spot size is usually determined by what setting
mA setting
Measures the actual amount of radiation incident in the image receptor and terminate xray production when the proper radiation exposure is obtained
Phototimer/ automatic exposure control
Maximal power that an xray tube focal spot can accept or the generator can deliver
Power rating
Traditional unit that provides a simple way of expressing the energy deposition on and dissipation from the anode of an xray tube
Heat unit
Constant voltage that would deliver the same power as a varying voltage waveform
Root-mean-square voltage
Describes the penetrability of an xray beam with a higher energy xray photons having a larger half value layer and higher quality
Quality
Refers to number of photons comprising the beam
Quantity
It is nearly proportional to the energy fluence of the xray beam
Exposure
Xray production efficiency, exposure, quality and quantity are determined by 6 major factors
Xray tube target material Tube voltage Tube current Exposure time Beam filtration Generator waveform
Affects the efficiency of bremsstrahlung radiation
Anode target material
Affects the quality of output spectrum
Tube voltage
It is proportional to the number of electrons flowing from the cathode to the anode per unit time
Tube current
It modifies the quantity and quality of xray beam by preferentially removing the low energy photons in the spectrum. It reduces the number of photons (quantity) and increases the average energy, also increasing the quality
Beam filtration
Affects the quality of the emitted xray spectrum
Generator waveform
Xray quality depends on the
kV, generator waveform and tube filtration
Compensation for changes in kV with radiographic techniques requires adjustments of mAs on the order of ___ to ____ power of the kV ratio
4th to 5th
kV determines
Quantity, quality and transmission
mAs determines
Quantity only
Production of a 2 dimensional image from a 3 dimensional object
Radiography
Magnification would always be greater than ____
1
For most radiographic examinations, the source to image distance is fixed at 100cm, except for upright chest radiography where the SID is typically set to
183 cm or 72 inches
Higher object magnification increases the width of the ____ and reduces the spatial resolution of the image
Edge gradient
Film is composed of a thin plastic base coated on one or both sides with a layer of high-sensitive emulsion consisting of
Silver halide
It is a protrusion of the positive charge that reaches the surface of the crystal, induced by the lattice defects by silver monosulfide (AgS)
Sensitivity speck
Slight difference in film density depending on the exposure rate is called
Reciprocity law failure
A grain of reduced metallic silver atoms appears as ____ on the film
Black speck
The film developer contains the
Aqueous chemical bath containing reducing agent
After the film has passed thru the developer, it passes thru a bath of an aqueous oxidizing solution called ______ that dissolves the remaining (inactivated) silver halide from the emulsion layer areas that were not exposed (or underexposed) to light
Fixer
After rinsing the film with water and allowing to dry, _______ striking the film emulsion acts locally to cause film darkening
Visible light
Refers to photostimulable phosphor detector system which are historically housed in a cassette similar to screen film cassette
Computed radiography
Traditionally scintilators that emit light promptly (nearly instantaneously) when irradiated by an xray beam
Gadolinium sulfide oxide and cesium iodide
Light that is emitted from the screen that has shorter wavelength and higher energy per photon than the stimulating red light
Blue light
Typical imaging plates are composed of about 85% ____ and 15% _____ activated with a small quantity of europium
85% BaFBr
15% BaFI
The activation procedure called _____, creates defects in the BaFBr crystals that allow electrons to be trapped more efficiently
Doping
Used in commercial grade television cameras and in scientific applications such as astronomy, which form images from visible light
Charge-coupled device detectors
Charge-coupled device chip itself is an integrated circuit made of
Crystalline silicon
Xray photons, visible photons and electrons are all forms of _____
Quanta
Refers to the stage where the number of quanta is the lowest and therefore where the statistical integrity of the signal is the worst
Quantum sink
Alternative to charged-coupled device
Complementary metal-oxide semiconductor
Pixelated discrete detector systems
Flat panel detector system
The photons in this TFT Propagates laterally, compromising resolution
Indirect thin film transistor based xray detectors
In this TFT, the ion pairs liberated by xray interaction follow the electric field likes (electron holes travel upwards, electrons travel downwards) and have negligible lateral spread. Here, the detected electronic signal from one x-ray photons interaction is collected almost entirely in one detector element, and therefore better spatial resolution is achieved
Direct detector TFT based xray detectors
Defined as the amount of energy deposited in a specific location in the detector by scattered photons, divided by the amount of energy deposited by primary (non-scattered) photons in the same location
Scatter-to-primary ratio (SPR) or scatter fraction
Scatter fraction increases as the field size _____ and as the thickness of the patient ____
Field size increases and thickness of patient increases
Most widely used technology for reducing scatter in radiography, fluoroscopy and mammography
Anti-scatter grid
Ratio of the height of the interspace material to its width-the septa dimensions do not affect the grid ratio metric
Grid ratio
Number of grid septa per centimeter
Grid frequency
Grid that moves with a reciprocating motion during the xray exposure, causing the grid bars to be blurred by this motion and not visible in the image. This motion is perpendicular to the long axis of the linear septa in the grid
Bucky grid
Describes the relative increase in xray intensity or equivalently, mAs needed when a grid is used, compared to when a grid is not used
Bucky factor
Typical values for bucky factor for abdominal radiography range from
3 to 8
Refers to the reduction in contrast due to scattered radiation
Contrast degradation factor
Method used in mammography to to reduce scatter
Scanning slit (or “slot-scan”) method
Most ubiquituous tool for scatter reduction
Anti-scatter grid
Refers to the actual anode angle plus the physical tube tilt
Effective anode angle
In mammogram, the anode is placed in the _____, while the cathode is placed in the
Anode is on the Anterior portion (nipple)
Cathode is posterior over the chest wall
Anode is kept at ground potential and cathode is set to highest negative voltage to reduce
Off focal radiation
Focal spot nominal sizes for contact mammography
0.3 to 0.4 mm
Focal spot nominal sizes for magnification imaging
0.10 to 0.15 mm
In order to avoid exposure of patient’s torsos and maximize the amount of breast tissue near the chest wall that is imaged, all dedicated mammography systems utilize a _____ xray beam geometry which is achieved by fixed collimation at the xray tube head
Half field geometry
Tube port window in mammogram is made of
Beryllium
The ideal spectral energy range is at
15-25 keV
The half value layer is highly dependent on ______ and the HVL if incident xray beam
Tissue composition
Useful for calculating the free-in-air incident air kerma or exposure to the breast for a mammography system target and filter combination, kV, mAs and source-to-breast surface distance
Xray tube output values
An xray exposure that is not stopped by the AEC circuit and exceeds a preset time is terminated by a
Back up timer
Most mammographic techniques use phototiming and proper choice of kV is essential for a reasonable exposure time, defined as a range from approximately
0.5 to 2.0 sec
Carries information regarding the attenuation characteristics of the breast and delivers the maximum possible subject contrast to the detector
Primary radiation
An additive, gradually varying radiation distribution that degrades subject contrast and adds random noise
Scattered radiation
Xray scatter increases with
Increasing breast thickness and breast area
Bucky factor in mammography is ____ so the breast dose is doubled or tripled
2-3
Most commonly used screen phosphor in mammogram
Terbium activated gadolinium oxysulfide
Indication of the amount of incident radiation required to achieve a specified OD on the film
Film speed
Rate of change of OD For a known difference in incident radiation
Film contrast
Device that emulates a range of incident radiation exposures by using a constant light source and calibrated optical attenuation steps to expose a mammographic film to known relative light intensities
Sensitometer
Device that measures the light transmission thru a small area kd the film and calculates the OD
Densitometer
Luminance in diagnostic radiology is typically about
1,500 cd/m2
Acceptable illuminance of room lighting for viewing and interpretation of mammograms
Subdued room lighting
Phosphor that converts xray into light
Cesium iodide
Describes absorption of cxrays im the cesium iodide, the production of secondary light photons directed to a photodiode and the generation of the charge, which is stored on the storage capacitor in that detector element
Indirect conversion
Refers to direct generation of charge by xrays within the photoconductor and capture by the electrode within intermediate signals
Direct conversion
Provide the capability to localize breast lesions in three dimensions and physically sample them with a targeted biopsy instrument
Stereotactic breast biopsy
Used to measure the air kerma for a given kV, mAs and beam quality
Air ionization chamber
