X-ray Tube and X-ray Production Flashcards

Question

How Does an X-Ray Tube Work?

Answer 1

Current flows along a wire = electrons moving A potential difference is applied at both ends of the conductor As current flows through there is an increase mechanical vibrational energy Raises temperature of the conductor = increase in heat energy Energy conversion: from electrical to heat energy

Answer 2

Emission of electrons from heated metal Current increases temperature of tungsten filament in the cathode to about 2000oC Increased energy overcomes binding energy of atom Electrons are freed forming a ‘cloud’

Answer 3

The current (mA) applied to the cathode filament and the duration (seconds) will determine the total number of electrons released (mAs) The released electrons are charged with high velocity towards the positive anode by the tube voltage (kVp) applied across the tube At the anode, 99% of the energy from the electrons is converted to heat, and only 1% is converted to x-ray photons The x-ray photons are targeted perpendicularly out of the x-ray tube through a window and towards the patient producing the x-ray beam

Answer 4

Rotating anode Tungsten Focal spot Glass Envelope Insulating oil Steel housing / lead casing

Answer 5

The parameters (kVp, mAs) that can be safely used during its operation without causing damage to the x-ray tube itself Unique to each individual x-ray tube model The amount of heat produced depends on: Voltage (kV) Current (mA) Length of exposure (s) Type of voltage waveform Number of exposure taken in rapid sequence Energy and heat are usually measured in Joules (J) but this is not normally used to measure x-ray tube heat The heat unit (HU) was introduced when single-phase equipment was common to make it easy to calculate heat

Answer 6

When the accelerated electrons hit the anode they interact with the atoms of the anode and produce x-ray photons via two mechanisms: Characteristic Radiation (10%-20%) Bremsstrahlung Radiation (80%-90%)

Answer 7

A high energy electron collides with an inner shell electron and both end up ejected from the tungsten atom leaving a ‘vacant’ electron space An outer shell electron moves to fill this inner space emitting energy in the form of an x-ray photon in the process. The energy of the emitted x-ray photon is equivalent to the energy level difference between the outer and inner shell electron involved in the transition This is called characteristic radiation because the energy of the emitted electrons is dependent upon the anode material, not on the tube voltage Energy is released in specific values corresponding to the binding energies of different shells

Answer 8

When high energy electrons penetrate the anode target, some electrons travel close to the nucleus due to the attraction of its positive charge and are subsequently influenced by its electric field These electrons are slowed down or ‘braked’ and their path is deflected whilst loosing a portion of their kinetic energy The energy lost is emitted as Bremsstrahlung radiation and this ranges over a spectrum of energies

Answer 9

Characteristic radiation Only accounts for small percentage of x-ray photons produced Bombarding electron interacts with inner shell electron Radiation released due to electron dropping down into lower energy state Radiation released is of a specific energy X-ray beam energy depends on element of target atoms not tube voltage Bremasstrahlung Accounts for 80% of photons in x-ray beam Bombarding electron interacts with whole atom Radiation released due to diversion of bombarding electron as a result of the atomic pull Radiation released is of a large range of energies X-ray beam energy depends on tube voltage

Answer 10

As a result of characteristic and bremsstrahlung radiation generation, a spectrum of X-ray energy is produced within the X-ray beam

Answer 11

There are different ways of altering the range of energies in an x-ray spectrum: Changing the intensity of the x-ray beam – this can be achieved by altering the kilovoltage peak (kVp) which is the peak voltage applied to the x-ray tube and determines the acceleration of the electrons from the cathode to the anode A low accelerating voltage will produce lower intensity x-ray photons across the spectrum whilst a high accelerating voltage will produce higher intensity x-rays including larger characteristic peaks

Answer 12

Changing the amount of x-ray photons present – this can be achieved by altering the current over a set period of time at the cathode and across the x-ray tube An increase in current (mA) results in a higher production of electrons that are inside the x-ray tube which will increase the quantity of x-ray photons produced The amount of time (seconds) used for a set current will also determine the number of electrons produced A combination of current and time known as milliampere-seconds (mAs) is a measure of radiation produced over a set amount of time via an x-ray tube The number of x-ray photons increase but the size of the characteristic peaks stays the same

Answer 13

Superficial X-rays: 35 to 60 kV (Mammography) Diagnostic X-rays: 20 to 150 kV( General X-rays) Orthovoltage X-rays: 200 to 500 kV (Superficial Therapy) Supervoltage X-rays: 500 to 1000 kV (Superficial Therapy) Megavoltage X-rays: 1 to 25 MV (Linac)

Answer 14

In radiotherapy, the required energy of the produced photons is much greater (usually 1-25MV) There is still a negative cathode, positive anode and vacuum The photons are produced by a linear accelerator A linear accelerator is a device that uses high Radio‐Frequency (RF) electromagnetic waves to accelerate charged particles (electrons) to high energies in a linear path, inside a tube like structure called the accelerator waveguide.

Answer 15

In radiotherapy, the required energy of the produced photons is much greater (usually 1-25MV) There is still a negative cathode, positive anode and vacuum The photons are produced by a linear accelerator A linear accelerator is a device that uses high Radio‐Frequency (RF) electromagnetic waves to accelerate charged particles (electrons) to high energies in a linear path, inside a tube like structure called the accelerator waveguide.

Answer 16

Electrons are emitted from an electron gun and accelerated in a straight line (to about 40% of the speed of light) Radiofrequency waves generated by a magnetron or klystron pass through a waveguide, establishing an electric field The radiofrequency waves ‘carry’ the electrons and give them their energy Electrons strike the target (at approximately 98% of the speed of light)