|
X-ray Tube
The x-ray tube consists of a glass
tube which has been evacuated to produce a high vacuum and into which are sealed
to electrodes, the cathode (-) and the anode (+).
The
Cathode
-
The negative side of the X-ray tube is called
cathode. The cathode assembly consists of a filament and its supporting wires,
and a focusing cup. Terms cathode and filament are often used interchangeably.
-
The cathode serves as a source of electrons for
the X-ray tube and direct their flow towards the anode.
-
The X-ray tube current measured in milliamperage (mA) refers to the number of electrons flowing per second from the
filament to the target.
The Anode
-
The anode is the positive side of the X-ray
tube. It is of two types; stationary and rotating.
-
A stationary anode is used in dental X-ray
machines, portable X-ray units, and special purpose units, where high tube
current and power are not required.
-
Tubes with rotating anodes are used in X-ray
units of larger capacity capable of producing high intensity X-ray beam in a
short time. The anode serves two main functions in an X-ray tube:
-
provides mechanical
support for the target.
-
acts as a good thermal
conductor for heat dissipation.
The effective Focus
-
In diagnostic X-ray tubes the
actual size of the effective focus is likely to be within the range 0.6-105 mm2.
Some tubes provide a choice of two sizes, the smaller or fine focus being
employed when fine detail is required in the radiograph and when the output of
the machine can be restricted, while the larger or coarse focus is used when it
necessary to subject the tube to heavier current.
-
A further way of extending the area of the
actual focus without affecting the effective focus is by means of a rotating
anode which will be found in the larger and more powerful type of X-ray tube and
apparatus.
-
Here the target is situated on a flange at the
edge of a wheel-like structure which rotates at high speed during the exposure
and so expands the area subjected to bombardment by several hundred times.
|
