XR production 2

Question 1

What is an XR tube insert?

Answer 1

components inside of the glass.
Contains anode and cathode

Question 2

What does the XR tube cathode consist of?

Answer 2

-Filaments - fine & broad focus
-thermionic emission
-focusing cup

Question 3

Cathode

Answer 3

-negative side of XR tube
function:
-produce electrons required for XR production
-facilitate large electric field across the gap between anode and cathode.
-focus the stream of electrons on the anode.

Question 4

Filaments

Answer 4

Produce electrons by a process called thermionic emission.

-Electrons evaporated from a metal filament (tungsten) coil when it is heated by passing through an electrical current.

increasing filament current, increases its temp and increases the no. of electrons emitted.
Thorium added to the tungsten also increases the no, of electrons emitted.

Question 5

What are filaments made from?

Answer 5

made of thin tungsten because it:
-has a high atomic no.
-good thermionic emitter (good at emitting electrons).
-can be manufactured into a thin wire.
-has a very high melting temperature (3422 degrees)

Question 6

How many filaments do most Xr tubes have?

Answer 6

two dual focus filaments- broad and fine focus.
increasing the size or surface area of the filaments will also increase the no. of electrons released.
-can increase the surface area of filaments by double winding the filament.

Question 7

what’s the difference between filament current and tube current

Answer 7

Xr tube current is the current passing between node and cathode of an XR tube.
It is related to but NOT the same as filament current.
Filament current is usually between 3-6 amperes resulting from the application of a potential difference in the region of 10 volts.

Question 8

What is the focussing cup?

Answer 8

mounted within a focusing cup.
-purpose is to focus the electrons onto the surface of the anode,
-without it, electrons would repel each other and stream of electrons would diverge (because of coulomb force).

-Negatively charged focusing cup creates a coulomb force that pushes the electrons together. And focus the electrons onto a specific area onto our anode.
-Focal spot- where the electrons strike the anode.

Question 9

Xr tube anode

Answer 9

-fixed and rotating
-materials
-anode angle:
-focal spots: line focus principle & focus spot blurring.
-Field coverage
-Anode heel effect
-Off- focus radiation.

Question 10

Rotating anode

Answer 10

-Used in most radiography
-consist of a disc with a thin rim of tungsten around the circumference
-Because anode rotates, it overcomes heating by having different areas exposed to the electron stream over time.
-Need to get heat away from the focal spot as it can melt the tungsten
-Rotating the anode spreads the thermal energy.

Question 11

Fixed Anode

Answer 11

-Target made of tungsten and rhenium
-Rhenium added to tungsten to prevent cracking of anode at high temperatures and usage.
-set into an anode disk
-set at an angle to direct XR photon beam down towards patient, usually 5-15 degrees.

Question 12

Advantages of tungsten

Answer 12

-high atomic number, making it more efficient.
-high melting point.
-low rate of evaporation
-Very strong at high temperatures which is beneficial given the aloe rotations speeds used.

Question 13

Anode stem

Answer 13

A molybdenum stem is often used as it is a poor conductor of heat to prevent heat transmission to the metal bearings.

Question 14

Focal spot blurring

Answer 14

Due to the finite size of the focal spot, when the object is further away from the detector, the edges in the image of the object get blurred

Question 15

what is focal track

Answer 15

portion of the anode where the electrons bombard. On a rotating anode this is a circular path

Question 16

Anode Heel effect summary

Answer 16

-XR Photons produced at a greater depth are more likely to be attenuated.
XR beam intensity is reduced in the direction going from the anode to the cathode.

Question 17

What is the anode heel effect?

Answer 17

The anode heel effect is the variation in X-ray beam intensity along the anode-cathode axis, with higher intensity on the cathode side due to the angled anode target in X-ray tubes.

-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

Question 18

what are the components of the XR tube housing

Answer 18

Outer support shell
Radiation Shielding
Insulating Oil
Tube envelope

Question 19

why do we use insulating oil in the XR tube housing

Answer 19

electrical insulation between anode and cathode
cooling of X-ray tube insert by convection

Question 20

why must the tube envelope design maintain an adequate vacuum throughout the life of the tube?

Answer 20

-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

Question 21

apart from a vacuum, what else must the envelope design have?

Answer 21

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

Question 22

X-ray Tube Exit Port & Filtration- what does it consist of?

Answer 22

X-ray tube envelope window
Inherent filtration
X-ray tube house exit port
Added filtration

Question 23

what’s an XR window?

Answer 23

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

Question 24

What are the 2 types of filtration?

Answer 24

Inherent XR tube filtration
Added XR tube filtration

Question 25

what is inherent filtration?

Answer 25

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

Question 26

What is added XR tube filtration?

Answer 26

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

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.

Question 27

What is the XR tube induction motor

Answer 27

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

Question 28

XR tube high voltage cables

Answer 28

-HT cables and connectors
-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

Question 29

XR tube heating

Answer 29

Heat (thermal energy) is a major limiting factor in X-ray production
For a constant potential X-ray system the electrical energy transferred to the X-ray tube is given by:

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.

Question 30

anode heating

Answer 30

-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.

Question 31

Advantages of a large anode

Answer 31

-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

Question 32

Disadvantages of large anodes

Answer 32

-More difficult to accelerate and brake the anode
-More difficult to manufacture
-More expensive

Question 33

What is XR tube rating

Answer 33

-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.

Question 34

The maximum allowable power of the XR tube rating decreases with:

Answer 34

-Increasing exposure time
-Reducing the actual focal spot size because heat is spread over a smaller area

Question 35

The maximum allowable power of the XR tube rating for a given exposure time decreases with:

Answer 35

-Larger target angles for a given effective focal spot size.
-Decreasing anode diameter
Because heat is spread over smaller circumference and area
-Decreasing speed of disk rotation

Question 36

what is thermionic emission

Answer 36

Heats up filament to produce enough energy to overcome binding energy of electrons