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

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1

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

2

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

3

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

4

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

5

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 (3410C),
o Does not vaporise easily
o Is mechanically strong.

The thorium (1%) improves thermionic emission

6

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.

7

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

8

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

9

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.

10

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.

11

Target Design

- As it is rotating, electrons are dissipated across a greater target area

12

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

13

Line Focus

The line focus principle helps to increase the surface area over which the heat is generated.

14

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

15

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

16

Bi-Angular Anode Geometry

- Bi-angular anodes are used when the two filaments are in-line

17

Effective Focal Spot Varies Across the Image Plane

- Spatial resolution varies across the imaging plane

18

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

19

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

20

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).

21

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.

22

Tube Current and Exposure Time

- Intensity is proportional tube current
- Intensity is proportional exposure time
- Therefore: I is proportional mAs

23

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.

24

Distance

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

Inverse Square Law

25

Filtration

- Adding filtration reduces the x-ray intensity

- Inherent filtration = window

26

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.

27

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

28

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

29

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

30

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