Week 10 - X-Ray Tubes (Part 2) Flashcards Preview

Radiation Physics > Week 10 - X-Ray Tubes (Part 2) > Flashcards

Flashcards in Week 10 - X-Ray Tubes (Part 2) Deck (32)
Loading 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



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 (3410C),
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

o Cooling is by conduction through a large mass of Cu.
 Cu is a good thermal conductor and has a high heat capacity.

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

o Low power applications, e.g., dental, mobile, therapy.

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.

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.



X-ray quantity is inversely proportional to the square of the distance from the source

Inverse Square Law



- 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



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 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