Chapter 6: Xray Production, Xray Tubes and Xray Generators Flashcards
Contains the electron source and target within an evacuated glass or metal envelope
xray tube insert
Provide protective radiation shielding and cools the xray tube insert
Tube Housing
Supplies the voltage to accelate the electrons
xray generator
tube port shape the xray energy spectrum
xray beam filters
Define the size and shape of the xray field incident on the patient
Collimators
Xrays are created from the conversion of KE of electrons into electromagnetic radiation when they are decelerated by interaction with a target material
Bremsstrahlung Spectrum
source of electrons
cathode
target of electrons
anode
TRUE or FALSE. The amt of energy lost by the electron and thus the energy of the resulting xray are determined by the distance between the incident electron and the target nucleus.
TRUE
Probability distribution of xray photons as a function of photon energy
Bremsstrahlung spectrum
What are the major factors that affect xray production efficiency
atomic number of the target material, and
KE of the incident electrons
Discrete xray energy peaks, which are dependent on the elemental composition of the target electrode and the applied xray tube voltage.
Characteristic xray
xray tubes voltages must be greater than \_\_\_\_\_ kV for W targets \_\_\_\_\_ kV for Mo targets \_\_\_\_\_ kV for Rh targets to produce k xrays
69.5 kV for W targets
20 kV for Mo targets
23 kV for Rh targets
TRUE or FALSE. In mammography characteristic x rays from Mo and Rh target xray tubes are particularly useful in optimizing image contrast and radiation dose.
TRUE
The xray generator supplies the power and permits selection of ___, ___ and ____.
Tube voltage
Tube current
Exposure time
Tube voltage is set to
____ kV for diagnostic imaging
____ kV for mammography
40 to 150 kV for diagnostic imaging
25 to 40 kV for mammography
xray Tube current is proportional to the NUMBER of electrons per second, measured as mA, and is set to
____ mA continuous fluoroscopy
____ mA projection radiography
____ mA pulsed fluoroscopy
1-5 mA continuous fluoroscopy
50 to 1,200 mA projection radiography
10-50 mA pulsed fluoroscopy
Cathode is teh negative electrode in the xray tube, comprised of a ____ and a _____.
Filament (Tungsten wire) and Focusing Cup
This process releases electrons from the tungsten filament through electrical resistance.
Thermionic emission
NOTE: The RATE of electron release is determine by mA. However, it ONLY occurs when kV is applied.
The xray is _____ if the filament current determine the xray tube current; at any kV, xray flux is proportional to the mA.
emission limited
A ______ xray tube has a foxusing cup totally insulted from filament wires so that the volatge is INDEPENDENT of the filament
Biased xray tube
part of the xray tube cathode which shapes the electron distribution when it is at the same voltage as the filament.
Focusing cup
This is a metal target electrode that is maintained at a large positive potential difference relative to the cathode.
Anode
Most widely used anode material
Tungsten.
This is bec of its high melting pt, high atomic #, and greater bremsstrahlung production.
This type of anode configurationis used by dental xrays, mobile xray machines and mobile fluoroscopy systems
Stationary or fixed anode
This anode configuration are used for most diagnostic xray applications. This design spreads the heat over a much larger area than does the stationary anode design, permitting much larger tube currents and exposure duration.
Rotating Anodes
The angle of the target surface with respect to the central ray inthe xray field.
What is the anode angle for diagnostic xrays?
anode angle
7 to 20 degrees, with 12 to 15 degrees being the most common
this is the area in the anode that is struck by electrons
actual focal spot size
Foreshortening of the focal spot length at the central xray
Line focus principle
This is the ability tohave a smaller effective focal spot size for a large actual focal spot increases the power loading for smaller effective focal spot sizes.
TRUE or FALSE. As the anode angle increases, the effective focal spot becomes smaller for the same actual focal area, providing bettre spatial resolution of the object when there is geometric image magnification.
FALSE
As the anode decreases…
Enumerate the common tools for measuring focal spot size
Pin hole camera
slit camera
star pattern
resolution bar pattern
measures the focal spot size by using a very small circular aperture in a thin, highly attenuating metal disk to project a magnified image of the focal spot onto the image receptor
Pinhole camera
Measures the focal spot size by a highly attenuating metal plate with a thin slit
Slit camera
measures the focal spot size using a radial pattern of lead spokes of diminishing width and spacing on a thin plastic disk
Star pattern
measures the focal spot by demonstrating the effective resolution parallel and perpendicular to the A-C axis for a given magnification geometry
Resolution Bar pattern
Refers to an increase in teh size of the focal spot resulting from high tube current and is caused by electron repulsion in the electron beam between the cathode and the anode. Seen in low kV.
Focal spot blooming
Refers to a slight decrease in the measured size with increasing kV
Therefore, electron repulsion and spreading in the xray tube is reduced.
Focal spot thinning
NEMA require measurement at ___ kV using 50% of the maximal rated mA for each focal spot.
75 kV
This refers to a reduction in the xray beam intensity toward the anode side of the xray field
Heel effect
NOTE: Decrease heel effect, Increase SID
Results from electrons that scatter from teh anode and are reaccelerated back to the anode, outside the focal spot area.
Off focal radiation
Contains the cathode, anode, rotor assembly and support structures sealed in a glass or metal enclosure under a high vacuum.
xray tube insert
Supports, insulates and protects the xray tube insert from the environment
xray tube housing
between the xray tube insert and housing, ____ provides heat conduction and electrical insulation.
Oil
This is found inside the housing attenuates nearly all xrays that are not directed to the tube port
Lead shielding.
NOTE: lead sheet thickness is 2 mm.
Adjust the size and shape of the xray field emerging from the tube port.
Collimators
Automatically limit the field size to the useful area of the detector
Positive beam limitation (PBL) collimators
What are the main differences between a dedicated mammography tube and a conventional xray tube
Target materials: Mo, Rh and W
Output port: Be vs glass or metal insert
Smaller effective focal spot sizes
Use of grounded anodes
Recommendations to Maximize Xray Tube Life
- Minimize filament boost “prep” time
- Use lower tube current with longer exposure times to arrive at the desired mA if possible
- Avoid extended or repeated operation of the xray tube with high technique factors
- Always follow the manufacturer’s recommended warm up procedure
- Limit rotor start and stop operations
The removal of xrays as the beam passes therough a layer of material
Filtration
Inherent filtration materials (1 to 2 mm) comprises of
Glass (SiO2) and Almuninum (Diagnostic xrays)
Be (Mammo)
most commonly used added filter material
Aluminum
Used to change the spatial pattern of the xray intensity incident on the patient
Compensation/Equalization filters
Used for chest radiography; Has a centrally located vertical band of reduced thickness and consequantly produces greater xray fluence in the middle of the field.
Trough filter
Useful for lateral projections in cervical-thoracic spine imaging, where the incident fluene is increased to match the increased tissue thickness encountered
Wedge filters
Used in CT to reduce dose to the periphery of the patient where xray paths are shorter and fewer xrays are required.
Bow tie filters
