30 October 2007

Linear Accelearator Construction

Definition: It is a device that uses high frequency electromagnetic waves to accelerate charged particles, such as electrons to high energies through a linear tube.

Types of linear accelerator design:

  1. Standing wave design
  2. Traveling wave design

In the latter design there is a dummy load, which absorbs the excess power and prevents a reflected wave. In the standing wave design, there is reflection of the waves at both the ends of the structure, so that combinations of forward and reverse directed waves give rise to a standing wave design.

Principle:

Whether standing or traveling wave design linear accelerators used in medical purposes, accelerate electrons along electromagnetic waves or frequency in the microwave region that is around 3000 Hz.

Components of a linear accelerator:

1. The Magnetron

2. The Klystron

3. Accelerator tube

4. Gantry & treatment head:

a. Transmission target

b. Flattening filter, and scattering foil.

c. Dose monitoring chambers.

d. Collimators:

i. Photons

1. Primary collimator.

2. Secondary collimators

3. Multileaf collimators.

ii. Electrons

History:

The first medical linear accelerator was installed in the Hammersmith Hospital in United Kingdom, and Henry Kaplan was the first person to use it in Stanford in USA.

Description of each component:

  1. D. C. power source is an independent power source for the machine.
  2. The Moderator generates pulses off high-voltage direct current for both the Electron gun, and the Magnetron at the same time.
  3. The electron gun consists of a metal filament and electrons are emitted by the principle of thermionic emission. The electrons have a energy of 50 KeV.
  4. The Magnetron is a device, which generates microwaves.
    1. Principle: In order to generate these microwaves, electrons are emitted from a cathode filament. A DC electric field is applied between the cathode and the anode while the static magnetic field is applied perpendicular to the plane of cross section of the cavities. Under the influence of the pulsed DC field, electrons are accelerated. Simultaneously due to the magnetic field, they begin moving in complex spirals.
    2. Each Magnetron is a cylindrical structure containing a central cathode and an outer anode, with resonant cavities inside. As the Electron spiral into these cavities, they radiate microwave pulses.
    3. Each microwave pulse has a frequency of 3000Hz.
  5. The Klystron is a microwave amplifier. It needs a microwave oscillator to function.
    1. Principle: An electron stream being accelerated across a potential difference is intercepted by a microwave beam. The electron velocity gets altered and these are separated into bunches. The electron bunches enter a cavity where they induce a charge which slows them down. As the electrons slow down they loose energy by radiation which is radiated as microwaves.
    2. The high powered microwaves are then injected into the accelerator tube via the wave guide where they are used to set up a electromagnetic field.
  6. The Accelerator tube is a long tube made of copper with diaphragms with holes where an electromagnetic wave is setup from the klystron / magnetron and electrons generated from the gun are accelerated. The principle is akin to the surfers of waves in ocean. The electron beam generated is usually of 3mm diameter.
  7. The treatment head is the heavily shielded part of the machine where the electron beam or photon beam exits the machine and it contains:
    1. Bending Magnets: Allow the electron beam to be bent at 90 or 270 degrees prior to striking the transmission target
    2. Transmission target: Made up of a heavy metal where high speed electrons strike and generate an x-ray beam. The transmission target also “hardens” the beam by removing all the low energy photons. It is called a transmission target as all photons are passing through the target before emerging out.
    3. Flattening filter: The beam emitted from the target is forward peaked and a flattening filter is placed in the pathway to ensure that the beam is flat across the field.
    4. Dose monitoring chambers: Are special sealed ion chambers which monitor the:

i. Dose rate

ii. Integrated dose

iii. Field symmetry.

    1. Scattering foils: Allow the thin electron pencil beam to be scattered using thin lead films so that a useful beam can be obtained.
    2. Collimators: 4 types of collimators exist :

i. Primary Collimators: Where the beam is shaped into the useful size and unwanted photons are removed.

ii. Secondary Collimators: Allow customized shaping of the beam by two pairs of movable lead blocs which move in a fashion such that the edge is always parallel to the beam. The maximum field size is 40 x 40 cm when projected at the isocenter.

iii. Multileaf collimators.

iv. Electron Collimators: Are special devices called electron cones used to generate the electron field shape. These are usually attached to the machine externally.

    1. Light localization system: Allows a visual reference to the x ray beam being generated.
    2. Secondary trays: Allow mounting of accessories like shielding blocks and wedges underneath the treatment head.
  1. Gantry: This entire assembly is mounted on a gantry which is usually isocentrically mounted i.e. the axis of rotation of the gantry, couch and collimator pass through the same point.

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