Week 10 - X-Ray Tubes (Part 2) Flashcards
X-Ray Tube Housing
The housing provides:
o Shielding from x-rays
Usually achieved with lead shielding
o Insulation from high voltages
To ensure tube housing does not become live
o Mechanical support for the tube
o Mounting for LBDs, cones & filters
o Access for high voltage cables
o Oil filled to provide additional insulation & cooling
Could also be water
o Bellows allows for thermal expansion of the oil
Principal Parts of a Rotation Anode X-Ray Tube
- Electrons created at filament –> accelerated to the target on the anode
- Window allows for x-rays to exit
The Envelope
Borosilicate glass (like Pyrex) - heat resistant, o Or metal envelope for heavy duty tubes.
Maintains the vacuum necessary for electron flow between cathode and anode.
o Any residual gas would:
Impede projectile electrons moving from cathode to anode
Produce secondary electrons and result in an avalanche of electrons reaching the anode.
o Tube window: approx. 5 cm square thin section to allow maximum transmission of x-rays
The Cathode
- Negative electrode of the x-ray tube
- Source of projectile elections
- Size of the filament determines the size of the electron beam
o Determining the spot size on the target
Small Spot Size = Small filament
Lower resolution / Larger spot size = larger filament
Cathode
Electrons are emitted from a heated filament by thermionic emission.
o The temperature must be >2200oC.
The filament is:
o A coil of wire approx. 2 mm in diameter by 10 to 20 mm long
o Heated by an electric current (Amps)
o Made of tungsten/thorium alloy
Tungsten is used because it:
o Has a high melting point (3410C),
o Does not vaporise easily
o Is mechanically strong.
The thorium (1%) improves thermionic emission
Dual Focus Tubes
Typical sizes:
o Small (fine), short filament focal spot size: 0.1 to 0.5 mm
Can give high resolution
o Large (broad), long filament focal spot size: 1.0 to 2.0 mm Provides low resolution
The two filaments can be side-by side or in-line.
o If in-line then a biangular anode is usually used.
Focussing Cup
Focuses the electron onto a small target area
o If generating a lot of electrons in a narrow region –> they will repel due to charge
o Therefore, some focus of the beam is lost
o Creates an electric field which refocuses the electron
Overcomes the repulsion force between electrons
X-Ray Tube
- Is controlled by the filament temperature which in turn is controlled by the filament current
- Projectile electrons striking the target provide the tube current
- Current applied to the filament is much higher
The filament current must be controlled very precisely
Small changes result in a big change in tube current
• Can reduce image quality
Anode Cooling: Stationary and Rotating
Stationary
o Cooling is by conduction through a large mass of Cu.
Cu is a good thermal conductor and has a high heat capacity.
Rotating
o Cooling is by radiation. The narrow Mo stem minimises heat conduction to the rotor and bearings.
o The rotating anode increases the target area
Hence reduces anode temperatures and allows higher exposure factors.
The Anode: Stationary and Rotating
Stationary
o Low power applications, e.g., dental, mobile, therapy.
Rotating
o General purpose and heavy duty, i.e., high intensity and short exposure times.
o The rotor and stator behave like an electric induction motor.
o Stator: coils of wire carrying an alternating current
o Rotor: Solid Cu and Fe cylinder. Rotation speed is typically 3000 rpm.
Target Design
- As it is rotating, electrons are dissipated across a greater target area
Tungsten Target
- W is alloyed with rhenium to increase strength
- Atomic Number: 74 (high-efficiency x-ray production and in high energy x-rays)
- Thermal Conductivity:
o Thermal conductivity nearly equal to that of copper
o An efficient metal for dissipating the heat produced - High Melting Point
o 3400 degrees celcius
o Cu = 1100 degrees celcius
Line Focus
The line focus principle helps to increase the surface area over which the heat is generated.
Effective Focal Spot Length
Effective focal spot length = actual focal spot length x sin (theta)
o theta = target angle
o Helps to dissipate heat and maintain small focal spot size
- Angle of target = alters size of the focal spot size
o Larger focal spot size –> more heat is dissipated over a larger area
The Focal Spot Size is a Compromise
- It needs to be small to give high spatial resolution (good detail) in the image, i.e. sharp edges to the shadows,
- It needs to be large to spread the heat generated over a large area and keep the target temperature down.
Therefore:
o dual focus tubes
o angled anode
o rotating anode
Bi-Angular Anode Geometry
- Bi-angular anodes are used when the two filaments are in-line
Effective Focal Spot Varies Across the Image Plane
- Spatial resolution varies across the imaging plane
Heel Effect
- Self-absorption of x-rays in the anode leads to reduced intensity at the anode end of the x-ray field and increased intensity at the cathode end.
o Electrons undergoing Bremsstrahlung deeper into the target –> have more target to escape to
o More x-rays at the cathode side of the x-ray tube than the anode side
o Noise will vary across imaging plane
Off-Focus (Extra Focal) X-Rays
- Off-focus (extra-focal) x-rays degrade the contrast in the image by adding to the background ‘fog’.
- Diaphragm can be used to collimate them out of the field
X-Ray Quantity
Quantity refers to the amount (number) of x-rays produced during an exposure, but it is difficult to count x-rays
o Therefore, quantity is usually measured in terms of:
Radiation exposure unit: roentgen, R (or milli-roentgen, mR) (These are old units and are usually found only in American textbooks.)
Radiation dose SI units: gray (Gy) or sievert (Sv).
X-Ray Quality
Quality refers to how penetrating a beam is and is closely related to the average x-ray photon energy of the beam.
o High quality → high average energy, hard x-rays, highly penetrating.
o Low quality → low average energy, soft x-rays, less penetrating.
Quality is expressed in terms of half-value layer (HVL) in units of mm of aluminium.
o HVL is a measure of how penetrating the beam is.
Tube Current and Exposure Time
- Intensity is proportional tube current
- Intensity is proportional exposure time
- Therefore: I is proportional mAs
Tube Voltage
Ideally: I is proportional (kV)^2
o This is for the x-ray intensity at the anode surface.
o After being filtered through the tube window etc. it is more like:
I is proportional (kV) 2.5 for high kV (60 to 100 kV)
I is proportional (kV) 3.5 for low kV (20 to 40 kV) i.e. the exponent increases as kV decreases.
Distance
X-ray quantity is inversely proportional to the square of the distance from the source
Inverse Square Law
Filtration
- Adding filtration reduces the x-ray intensity
- Inherent filtration = window
Exposure from Changing Filtration
This graph shows X-ray beam intensity dependence on kVp and filtration for a single-phase full-wave rectified x-ray tube.
o Note the strong dependence of intensity on both kV and filtration.
Tube Age
The intensity reduces with tube age due to:
o Grazing of the anode surface (no longer a smooth flat surface)
o Additional filtration due to the W film deposited on the inside of the envelope
Due to vaporisation of the Tungsten
Waveform
For other types of generators the values read from the graph should be multiplied by a constant, i.e.
o For three phase: × 1.3
o For constant potential: × 1.5
Quality
- Quality relates to the energy of the x-rays and therefore how penetrating an x-ray beams is.
- It is measured in terms of half-value layer (HVL) in units of mm of aluminium.
- HVL: the thickness of Al required to reduce the intensity of the beam to ½ of its initial intensity.
(not to be confused with filtration, which is also expressed in mm of Al)
o not to be confused with filtration, which is also expressed in mm of AI
Factors Affecting X-Ray Quality and Quantity
- Increase in mAs
o Quality = none
o Quantity = Increase - Increase in kVp
o Quality = Increase
o Quantity = Increase - Increase in Distance
o Quality = None
o Quantity = Reduce - Increase in Filtration
o Quality = Increase
o Quantity = Reduce
Variation of Quality with kV
- Quality increases with kV
Filtration
- Quality increases with added filtration because low energy x-rays are more likely absorbed than high energy ones, thus increasing the average x-ray photon energy.
- Reduced patient dose by removing soft x-rays that would mainly be absorbed in the patient and not contribute to forming the image
- An ideal quality beam would contain x-rays of only a single energy (mono-energetic beam)
- Not possible to produce a mono-energetic beam with an x-ray tube
o can be approximated with very heavy filtration, however this would reduce the quantity too much
By choosing filtration –> we can modify the penetration of the beam
o By doing so we reduce the number of x-rays but increase the mean energy
