Component of Conventional X-ray equipment and their function

Component of convection x-ray equipment and their function

Introduction

X-ray are ionizing electromagnetic radiation from a highly evacuated hight-voltage tube. Inner orbital electrons in the target anode are stimulated to emit radiation via bombardment by a stream of electrons from heated cathode X-ray , like gamma rays, are penetration and carry enough energy to ionize atoms in their path. X-ray require shielding to reduce their intensity and minimize the danger of tissue damage. It can cause severe radiation burs and deep tissue damage and can lead to various cancers

X-ray machine

is the type imaging modality that use x-ray to produce a two-dimensional image of the inside of the body A conventional system uses an intensifying screen to create a latent image on x-ray film. The film is then processed, creating a manifest image that can be interpreted by a physician. It is later stored in the file room.

PARTS OF THE CONVECTIONAL X-RAY MACHINE

• X-ray has three main components;
• Operating console
• High frequency generator
• X-ray tube –internal
• -external
• Other parts
• Collimator
• Patient table
• Grid bucky

X-ray Film

X RAY TUBE

X -ray tube

Within an X -ray tube.

Each component part of the X -ray tube has been designed with the desire to produce electrons, supply them with energy and enable an efficient as possible interaction with a target This tube consists of a cathode and an anode enclosed within an evacuated glass or metal envelope This is all contained within lead – shielded housing

X-ray tubes

X ray tube insert

A vacuumed tube which comprises the cathode and anode.

Cathode

The cathode is the negatively charged electrode, where electrons are released into the X -ray tube by thermionic emission.

• This is the start of the X -ray production process and consists of two parts:
• A filament.

A focusing cup.

The filament is a coil of wire, which is about 2 mm in diameter. It is tightly coiled, similar to the heating element in a bar heater or a toaster, in order to increase the surface area of the metal.

Tungsten is a good material for this purpose

Filament

This means that it can heat and cool quickly, allowing it to be heated rapidly for thermionic emission, and it can withstand high temperatures without becoming damaged.

The cathode has its filament circuit that supplies it with necessary filament current to heat it up The rate at which the electrons are emitted by the cathode is directly related to the tube current.

WITHOUT FOCUSING CUP

WITH FOCUSING CUP

High voltage

The tube current flows from the cathode to the positively charged anode.

A high voltage is supplied across the tube in order to accelerate the electrons and increase their kinetic energy.

This voltage is the kilovolt (kV) setting of the tube and is supplied by a source, which is separate from the X -ray tube.

Anode

• The anode is the positively charged electrode, directly opposing the cathode.
• It consists of a
• high – density metal target, embedded in a copper disc.
• The electrons from the cathode hit the target area of the anode and interact.

Tungsten is usually chosen as a target material.

Anode

• Tungsten is usually chosen as a target material.

This is due to the useful properties of this material.

It has a high density, which increases the number of interactions per projectile electron.

It also has a high melting point, allowing the target to become very hot without becoming damaged, It has a high thermal conductivity. This means the heat generated in the target is quickly dissipated to the surrounding copper, which acts as a heat sink for the anode.

Anode heel effect

This is the variation of the intensity of X-rays emitted by the anode along the anode-cathode axis.

Occurs due to attenuation of produced beam by the anode.

X-rays emitted towards the cathode are in general more intense than those emitted towards the anode.

Factors affecting anode heel effect

• Anode angle
• Target-to-film distance
• Field size

Positioning

Anode angle

By increasing the angle, the amount of target material perpendicular to the anode is decreased resulting in less resorption of x-rays produced.

• Increased anode angle

Decreased anode angle

Target-to-film distance

Increase in distance reduces heel effect by allowing more divergence of the beam which produces a more uniform image.

Field size

The field will be more uniform at the center (i.e. smaller field size) due to the collimator absorbing the peripheral variations.

• SMALL field size

LARGE field size

Positioning

By aligning higher attenuating material towards the cathode and lower attenuating material towards the anode the resulting field is more uniform.

Stationary and Rotating Anode tubes

• X ray tube can either has a stationary or rotating anode

Most tubes have a rotating anode.

This is to increase the efficiency of removing heat from the target area during the production of X – rays, which makes it possible to produce a higher -intensity beam without damaging the area of the anode struck by the projectile electrons.

Dental x-ray units, some low-output mobile x-ray machines and mobile fluoroscopy systems use fixed anode x-ray tubes A stationary anode tube is cheaper to manufacture and easier to maintain; however, it cannot be used when a high power output is needed

Stationary and Rotating Anode tubes

On a Rotating Anode Tube

Rotating vs Stationary anode tube

• Stationary anode
• It does not rotate during x ray exposure
• The target is located at the center along with the cathode in the x ray tube inert.
• Most of heat lost though copper block by conduction
• It has low rating compared to rotating anode
• Has smaller surface area of the focal spot
• Rotating anode
• It rotates during x ray exposure
• The target and the cathode they are not at the centre of the tube insert.
• Most of heat lost by convection
• It has higher rating compared to fixed anode

Has larger surface area of the focal spot

Tube envelope

The cathode and anode are housed inside a glass or metal envelope collectively called a tube insert The envelope is sealed, and maintained at vacuum pressure so that the electrons can travel from the cathode to the anode without losing any energy during unwanted interactions with air molecules.

A failing vacuum, resulting from leakage causes increased ionization of the gas molecules, which slows down the electrons.

Further, a current of positive ions flowing back could impair or destroy the cathode filament.

Tube envelope

The envelope is commonly made of glass but high performance tubes increasingly have glass–metal or ceramic–metal envelopes 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.

If low energy X rays are used, as in mammography, the exit port is a beryllium window, which has less absorption than glass because of its low atomic number.

Tube housing

• The external casing of the x ray tube

Made of cast steel to help cooling of the x ray tube It is internally lead lined to control leakage and off- focus radiation (scatter radiation) Isolates high voltages

• Finally filters and collimators can be attached to this housing.

TUBE HOUSING

Tube housing

The X ray tube (insert) is installed in a tube housing that provides the structural support required and protection There is a low attenuation window where the radiation beam exits towards the patient The space between the housing and the envelope is filled with transformer oil, serving as electrical insulation and for heat removal from the envelope surface, which is heated by the infrared radiation from the anode

Features of x ray tube Housing

Its inside wall is lined with lead for leaking radiation proof (should not exceed the permitted by code of practice and recommendation protection) Should be well insulated and earthed to prevent electrical shock.

Must have a port though which x rays passes out of the x ray tube.

The oil

• The functions of oil in the x ray tube are:
• Cooling
• Insulation of electricity

Used as an inherent x ray filter

COLLIMATION AND FILTRATION

Collimation

• Collimation is the limitation of the X ray field to the size required for an examination
• The main two benefits are:
• Reduction in patient dose
• Improvement of image contrast due to a reduction in scattered radiation
• It is accomplished with collimators
• A collimator assembly is typically attached to the tube port
• LIGHT
• BEAM

COLLIMATOR

HIGH VOLTAGE

CABLES

X-RAY TUBE HOUSING (ASSEMBLY)

Collimators

A collimator assembly typically has:

Adjustable parallel opposed lead diaphragms or blades (Can be two set) -The second set of lead blades might be installed at some distance from the first blades to improve the effectiveness of collimation.

• The bulb to provide light that mimick the x ray field

A mirror that reflect light from a bulb NOTE: The bulb position is adjusted so that the reflected light appears to have the same origin as the focal spot of the tube.

Collimators

Adjustment of the field size is done manually With a positive beam limitation system, the size of the imaging detector is automatically registered and the field size is adjusted accordingly.

For fluoroscopy, other collimator types are in use, with variable circular and slit diaphragms.

In some applications (dental and head examinations), beam restrictors with a fixed field size are typically used.

Filters and Filtration

X rays generated in the anode pass various attenuating materials before leaving the tube housing (Inherent filtration) .

These materials include the anode, tube envelope exit port (glass or metal), insulating oil and the window of the tube housing Additional filter material is positioned between the tube window and collimation assembly also attenuates the x ray primary beam (Added filtration)

Filters

The x ray tube has two types of x ray filters:- Inherent filtration: The in born filtration of x ray tube caused by the glass envelop, oil and the tube housing window .( It is equal to O.5 mm Al equivalent) Added filtration: thin sheet of aluminum metal inserted between metal head seal and the beam collimates. (It is equal to 1.5mm Al equivalent) Compensation filters

Compensation filters

Used to equlize the intensity of the x ray beam.

Reduce the high intensities resulting from thinner body parts or regions of low attenuation.

Such filters are usually inserted in the collimator assembly or close to the tube port.

Examples of compensation filters include wedge filters for lateral projections of the cervical spine, or bowtie filters in CT.

METHODS FOR COOLING THE

X RAY TUBE

X ray tube cooling methods

Less than 1% of the energy is converted into x ray.

99% is converted into heat energy. This large percent of heat may melt the target and therefore should be removed very quickly from the target.

The different methods of cooling are employed to ensure the removal of heat from the tube is mantained.

Tube cooling methods

Target are made by inserting a layer of tungsten in a copper block, which removes the heat from the target very quickly.

In a rotating tube this process is enhanced more by rotation of a copper disc.

In stationary anode tube, most heat removed from the target by conduction, while it is thermal radiation in rotating anode tube.

Tube cooling methods

X ray tube are enclosed in a metal cases which is filled with oil for insulation purpose.

This oil surround the glass envelope and the copper block. The oil transfers heat from the envelope and cu block to the tube housing by convection. Air convection removes the heat from the metal case.

Tube cooling methods

Sometime a small fan is arranged to blow air from a metal case.

In some modern tubes, the anode is earthed and water is allowed to circulate through the anode.

Tube cooling methods

X Ray Tube Support System

It is part of the x ray set which support the x ray tube so that it can be applied for radiological examinations Types of an X Ray Tube Support System Floor mounted x ray tube stand

• Ceiling mounted system

Floor – celing x ray tube stand

Floor mounted x ray tube stand

Ceiling mounted support system

Floor – celing x ray tube stand

X ray Tube Stand

The x ray tube stand consists of a column of heavy gauge steel which is amounted on a carriage for movements between tracks on the floor and ceiling On the vertical column a cross- arm supports the x ray tube.

The cross- arm can be moved up and down the column, at right angle to the column, and in the rotational motion about the vertical axis of the column.

Cross-arm

Features of an Xray tube stand

The x ray tube support should be adequately rigid so that vibration of x ray tube is avoided.

All movements of the support and x ray tube about it should be smooth, unrestricted and easy to perform.

It must be possible to make certain precise angulations of the x ray tube It must be possible to direct the x ray tube parallel to the floor as well as in a perpendicular direction The controls providing for the tube movements should be easily readily accessible.

X ray tube Movements

The x ray tube can be rotated up on the cross-arm and can be tilted about an axis parallel to itself.

• These movements made for the following excursion of x ray tube
• Longitudinal travel (Parallel to the x ray table)
• Horizontal travel (At right angle to the x ray table)
• Vertical travel along the column
• Rotation travel about the vertical column
• Rotation on an axis parallel to the cross- arm of 180 degree to 180 degree

Rotation round the tube’s own long axis.

Intro. to the x ray circuit

• X ray circuit divided into:
• The main circuit
• Filament circuit
• The main x-ray circuit also divided into:
• Primary or control console section
• Incoming current
• Autotransformer
• Exposure switch
• Primary winding of the step-up transformer
• Secondary or high voltage section
• Secondary step-up transformer
• Full-wave rectification circuits

Wiring leading to & from the x-ray tube