x-ray production 2 Flashcards
X-ray tube insert
X-ray tube insert- components inside of the glass, contains anode and cathode.
X-ray tube
Left side is an electric motor- which induces movement to rotate the anode.
X-ray Tube Cathode
Filaments
Fine & broad focus
Thermionic emission
Focussing cup
The cathode is the negative side of the X-ray tube.
The function of the cathode is to:
produce the electrons required for X-ray production
facilitate a large electric field across the gap between the anode and the cathode
focus the stream of electrons on the anode
The cathode contains two primary parts
The filaments
The focusing cup
Circle- focusing cup of the cathode.
Can see two coils/filaments and one is shorter than the other.
Small focus and a large focus- this refers to these filaments.
Filaments
The purpose of the filaments is to produce electrons by a process called thermionic emission (Directed Reading Task) Thermionic emission- heat up a wire and it gets hot, which produces electrons.
Electrons are evaporated from a metal filament (tungsten) coil when it is heated by passing a electrical current through it.
Increasing the filament current, increases its temperature and increases the number of electrons emitted
Thorium added to the tungsten also increases the number of electrons emitted
What are filaments made of and why?
Filaments are made of thin (0.2 mm) tungsten (W) wire because it:
has a high atomic number (Z 74)
is a good thermionic emitter (good at emitting electrons)
can be manufactured into a thin wire
has a very high melting temperature (3422°C)
2 dual focus (filaments)
Most X-ray tubes have two (dual focus) filaments referred to as broad and fine focus
The size of the filament relates to the size of the focal spot.
What will increasing the size of the filaments do?
Increasing the size or surface area of the filaments will also increase the number of electrons released
The surface area of the filaments by double winding the filament
Filament current and tube current
X-ray tube current is the current passing between the anode and cathode of an X-ray tube.
It is related to, but NOT the same as, the filament current.
The filament current is usually between 3-6 amperes resulting from the application of a potential difference in the region of 10 volts
What is a Focussing Cup?
The filaments are mounted within a focusing cup.
The focussing cup is generally made of nickel, stainless steel, or molybdenum because of their rigidity and capacity to withstand heat
What is the purpose of the focusing cup?
The purpose of the focussing cup is to focus the electrons onto the surface of the anode
Without it the electrons would repel each other and the stream of electrons would diverge
The cup has a negative potential applied to it with respect to the cathode which focusses the electron beam onto the anode target
What do negatively charged focussing cups do?
Negatively charged focussing cup creates a coulomb force that pushes the electrons together. And focus the electrons on a specific area onto our anode.
X-ray Tube Anode
Fixed & Rotating
Materials
Anode angle
Focal spots: Line focus principle & focal spot blurring
Field Coverage
Anode heel effect
Off-focus radiation
Fixed Anode X-ray Tubes
Consists of an anode fixed in position with the electron beam focussed onto one small area (the focal spot).
These are generally limited to dental radiology and radiotherapy systems.
Rotating Anode X-ray Tubes
-Used in most radiography
Consists of a disc with a thin rim of tungsten around the circumference
-The anode rotates
Because it rotates it overcomes heating by having different areas exposed to the electron stream over time.
-Rotations speeds: 3,000-10,000 rpm
What will the anode do during exposure?
Anode- Rotates during exposure.
- It will spread thermal energy all the way round the disk not just one spot
The focal spot wont change it’s the disk that’s moving.
Anode structure
Target made of tungsten (~85%) & rhenium (~15%) mix
Rhenium added to tungsten to prevent cracking of anode at high temperatures and usage
Set into an anode disk of molybdenum/graphite
Set at angle to direct x-ray photon beam down towards patient. Usual angle is 5º - 15º
Advantages of Tungsten
High atomic number (Z=74) and is therefore more efficient than other lower Z materials at producing bremsstrahlung
High melting point (>3000°C)
Low rate of evaporation
Very strong at high temperatures which is beneficial given the anode rotations speeds used (approx. 3000-9000 rpm)
Anode Stem
A molybdenum stem is often used as it is a poor conductor of heat to prevent heat transmission to the metal bearings
Focal Spot Blurring
Focal spot blurring (also called geometric blurring or focal spot unsharpness)
Caused by finite size of the X-ray tube focal spot size
- Focal spot has some size associated with it. - Don’t get a sharp edge. The bigger the focal spot, the more blur you will get on the edge.
Reduce by:
Increasing focal spot to object distance
Decreasing object to image plane distance
Reduce by:
Decreasing focal spot size
Reducing the focal spot size reduces the geometric blurring but reduces the maximum exposure that can be made
Field Coverage
Large anode angles result in larger field coverage but result in larger effective focal spot sizes for a given actual focal spot size
Anode Definitions: Target, focus, focal point, focal spot
where electrons hit the anode
Anode Definitions: Actual focal spot
physical area of the focal track referred to as the line focus (because it looks like a line)
Anode Definitions: Focal track
portion of the anode where the electrons bombard. On a rotating anode this is a circular path
Anode Definitions: Effective focal spot
the area of the focal spot that is projected out of a tube
Anode Heel Effect
X-ray beam intensity is reduced in the direction going from the cathode to the anode
Increasing chance of attenuation leads to reduction in beam intensity
As the X-ray tube anode is repeatedly heated and cooled over time this can result in the surface becoming crazed.
This crazing can increase the impact of the anode heel effect as X-ray production in some parts of the anode will be deeper compared to a new anode – increasing the path length for the photons to exit
X-ray Tube Housing
Outer support shell
Radiation Shielding
Insulating Oil
Tube envelope
Outer shell
Steel/Alloy
Strong & rigid material
Radiation shielding
Lead lining
High Z
Good absorption
Cant use lead because its not rigid and very bendy. That’s why it needs support such as steel or alloy.
Envelope Design
The tube envelope must maintain an adequate vacuum throughout the life of the tube to:
Insulate the anode and cathode
Ensure the filament does not evaporate
Allow electrons to travel between anode and cathode
Not impede electron emission at the cathode
It must also:
Support the anode and cathode structures
have low X-ray absorption
have high temperature cycle resistance
be electrically insulating (if required)
be highly permeable to radiated heat
Modern tubes are mostly constructed of:
evacuated borosilicate glass (Pyrex) envelope
evacuated metal envelope
X-ray Tube Exit Port & Filtration
X-ray tube envelope window
Inherent filtration
X-ray tube house exit port
Added filtration
X-ray Window
The X-ray beam exits the tube through a Window in the envelope
To reduce absorption the thickness of the glass is reduced in this area
Inherent Filtration
Generated X-ray photons pass various attenuating materials before leaving the tube housing
Anode
X-ray envelope exit window (glass/metal)
Insulating oil
Window of tube housing
Generated X-ray photons pass various attenuating materials before leaving the tube housing
Anode
X-ray envelope exit window (glass/metal)
Insulating oil
Window of tube housing
This filtration is referred to as inherent filtration
It is quantified in terms of the thickness of aluminium that would achieve the same filtration (mm Al) – usually specified at 80 kV
Typical ranges 0.5 -1 mm Al
Added X-ray Tube Filtration
Filtration eliminates the low energy component of the X-ray spectrum
This component would be unlikely to contribute to the final image
This component would be absorbed by the subject
This would increase the dose to the subject with no clinical benefit
To achieve effective filtration of the low energy of the X-ray spectrum it is stipulated that the minimum Total filtration must be no less than 2.5 mm Al
Additional Filtration is the material positioned between the tube window and the collimation assembly to achieve the required total filtration
Common materials for additional filtration are aluminium & copper
X-ray Tube Induction Rotor and stator
Rotor
contains solid bars of copper that span the length of the rotor
Stator
situated outside the tube envelope
electromagnet windings that produce a rotating magnetic field inside the rotor
The rotator spins as a result of the rotating magnetic field induced by the stator windings
X-ray HT Cables
High Voltage (High Tension) Cables
Connect the X-ray tube to the high voltage supply
Each cable has three conductors
The conductors of the anode are connected are used in parallel – effectively forming a single conductor
Three conductors are required in the cathode circuit to connect both broad and fine focus circuits
Any visible damage to either the connector or cable
The X-ray system must not be used
The damage should be reported to the appropriate service personnel
X-ray Tube Heating: For a constant potential X-ray system the electrical energy transferred to the X-ray tube is given by the equation….
𝑬=𝒌𝑽 ×𝒎𝑨𝒔
kV is the X-ray tube voltage expressed in kilovolts
mAs is the product of the X-ray tube current mA, expressed in milliamperes and the time expressed in seconds s.
Thermal (Heat) Energy Transfer
∆𝑻=𝑬/𝒌𝒎
∆𝑻 temperature change (°C)
𝑬 energy (J)
𝒎 mass (kg)
𝒌 specific heat (J/kg°C)
The bigger the mass the smaller the temperature change.
Energy that is deposited can reduce the temperature.
Anode Heating
Heat (thermal energy) is normally removed from the anode byradiationthrough the vacuum and into the oil outside the glass envelope.
The molybdenum stem conducts very little heat to prevent damage to the metal bearings.
Advantages of a Large Anodes
Greater mass results in smaller temperature rises per unit of absorbed energy—especially when using materials with good thermal properties
Greater area to distribute the thermal energy when rotating the anode with out melting the focal track
Will allow longer exposure times
Disadvantages of a Large Anodes
More difficult to accelerate and brake the anode
More difficult to manufacture
More expensive
X-ray Tube Rating
X-ray tubes have a different capacity for dissipating heat before damage is caused according to their rating.
The capacity for each focal spot on a machine is given in tube rating graphs provided by the manufacturer.
These display the maximum power (kV and mA) that can be used for a given exposure time before the system overloads.
X-ray Tube Rating: The maximum allowable power for a given exposure time decreases with:
The maximum allowable power for a given exposure time decreases with:
Larger target angles for a given effective focal spot size.
This is because for a given effective focal spot size the actual focal spot track is smaller with larger anode angles. This means the heat is spread over a smaller area and the rate of heat dissipation is reduced
X-ray Tube Rating: The maximum allowable power for a given exposure time decreases with:
The maximum allowable power for a given exposure time decreases with:
Decreasing anode diameter
Because heat is spread over smaller circumference and area
Decreasing speed of disk rotation
X-ray Tube Rating: factors to take into consideration are:
Other factors to take into consideration are:
By using a higher mA the maximum kV is reduced and vice versa.
A very short exposure may require a higher power to produce an adequate image. This must be taken into consideration as the tube may not be able to cope with that amount of heat production over such a short period of time.
Summary: X-ray Production Overview
Filament currentapplied through tungsten filament at cathode.
Heats up filament to produce enough energy to overcome binding energy of electrons (thermionic emission).
Electrons released from filament.
Tube voltage is applied across the X-ray tube.
Electrons, therefore, are accelerated towards positively charged anode, which gives them a certainenergy.
The electrons strike the anode and the energy released via interaction with the anode atoms producesX-ray photons.
These X-ray photons leave the X-ray tube through the window in an X-ray beam towards the patient.
They pass through the patient to the detector to produce the X-ray image (this section is covered in the next chapter “Interaction with matter”).
Overview video of an X-ray tube insert
- Which one of the following is NOT contained inside the envelope of an X-ray insert
a. anode
b. filaments
c. stator
d. rotor
e. focusing cup
c. stator
- The cathode in an X-ray tube is made up of which two component?
a. filaments & tungsten target
b. rotor & focussing cup
c. filaments & focusing cup
d. stator & rotor
e. filaments & exit port
filaments & focusing cup
- Molybdenum is often used as a material for the stem the X-ray tube anode is attached. This is because it is
a. a good conductor of heat
b. a good absorber of X-ray photons
c. a poor conductor of heat
d. a good emitter of electrons
a poor conductor of heat
- The purpose of the filaments inside and X-ray tube is to
a. light the inside of the X-ray tube
b. accelerate electrons towards the anode
c. produce electrons
d. heat the anode up
produce electrons
- Thermionic emission is a process that produces
a. protons by applying a magnetic field to an insulator
b. electrons by heating a conductor
c. electrons by cooling a conductor
d. neutrons by applying an electric field to a semiconductor
electrons by heating a conductor
- Increasing the filament current in an X-ray tube
a. decreases its temperature and decreases the number of electrons produced by thermionic emission
b. increases its temperature & increases the number of electrons produced by thermionic emission
c. increase the energy of the X-ray photons produced
d. decreases the energy of the X-ray photons produced
increases its temperature & increases the number of electrons produced by thermionic emission
- Filaments in X-ray tubes are made of
a. copper
b. silver
c. tungsten
d. gold
tungsten
- The reduction of the intensity of the X-ray beam in the anode to cathode direction of the X-ray field is called
a. Anode heal effect
b. Cathode heel effect
c. Anode heel effect
d. Cathode heal effect
Anode heel effect
- The filament current is produced using a ________ voltage when compared to the X-ray tube current which is produced by applying a _______ voltage.
a. alternating, high
b. low, alternating
c. high, low
d. low, high
low, high
- The purpose of the focussing cup is to
a. keep the filaments in a fixed position
b. accelerate the electrons toward the anode
c. hold the electrons in a fixed position
d. reduce heat from the cathode
e. focus the electrons onto a small area of the anode
focus the electrons onto a small area of the anode
- Which one of the following materials is commonly used for the focussing cup in an X-ray tube?
a. gold
b. silver
c. stainless steel
d. lead
stainless steel
- The focusing cup is kept at an electrical potential that is
a. negative compared to the filaments
b. positive compared to the filaments
c. neutral compared to the filaments
d. positive compared to the anode
negative compared to the filaments
- Fixed anode X-ray tubes are commonly found in
a. general radiography
b. angiography
c. dental radiography
d. mammography
e. cardiac imaging
dental radiography
- In rotating anode X-ray tube the range of rotation speeds commonly used are in the region of
a. 300-900 rpm
b. 3000-10,000 rpm
c. 30,000-50,000 rpm
d. 500,000- 1,000,000 rpm
3000-10,000 rpm
- In an X-ray tube a rotating anode is used to
a. to stabilize the X-ray tube using gyroscopic forces
b. spread thermal energy over the surface of the anode
c. attract the electrons coming from the cathode
d. to increase the specific heat capacity of tungsten in the anode
spread thermal energy over the surface of the anode
- X-ray tube anodes are commonly manufactured using tungsten and
a. copper
b. rhenium
c. thorium
d. gadolinium
e. gold
rhenium
- X-ray tube anode angles are commonly in the range
a. 1-4 degrees
b. 5-15 degrees
c. 15-30 degrees
d. 150-300 degrees
5-15 degrees
- Focal spot blurring can be reduced by
a. decreasing the focal spot size
b. increasing the focal spot size
c. increasing the filament current
d. decreasing the filament current
a. decreasing the focal spot size
- For a fixed actual focal spot size, the apparent focal spots size _______ with increasing anode angle.
a. decreases
b. increases
c. stays the same
d. accelerates
increases
- Extra-focal radiation is produced
a. by electrons from the envelope of the X-ray tube striking the anode
b. by rebounding electrons from the X-ray tube anode striking other areas of the anode
c. by poor focussing of the electron beam
d. by air leaking into the X-ray tube
by rebounding electrons from the X-ray tube anode striking other areas of the anode
