2.
The principles of the rotating anode (C:
14-17; H: 24)
·
The ability of the x-ray tube
to achieve high x-ray outputs is limited by the heat generated at the anode
·
The purpose of the rotating
anode is to spread the heat produced during exposure over a large area of the
anode.
·
The rotating anode consists of
a large disc of tungsten or an alloy with a beveled edge (6o – 20o)
which rotates at a very fast speed (e.g. 3600 rpm)
·
Because of the rotation, the
electrons accelerated from the cathode will bombard a constantly changing area
of the target.
·
For 3600 rpm any one area is
only exposed to the beam every 1/60 seconds, and the remainder of the time heat
generated during the exposure is dissipated.
·
The faster the anode rotates
the greater is its ability to withstand heat.
·
The diameter of the disc determines
the total length of the target track and affects the maximum permissible
loading of the anode.
·
The stem of the rotating tube
is made from Molybdenum (a poor heat conductor) to prevent the anode assembly
and bearings from overheating and binding/ sticking.
·
The inertia of the heavier
anode causes a delay between the application of force to the anode assembly and
the time at which maximum angular velocity is reached – therefore a circuit is
incorporated to prevent the exposure until the rotor has reached its full
speed.
·
Also the stem is made shorter
to decrease the inertia as much as possible.
·
A rotating tube greatly increases the effective target area used
during an exposure and therefore raises the heat capacity
The line focus principle (C: 13; BB:
108-109; H: 25)
·
Beveling of the anode edge (6o
– 20o) takes advantage of the line focus principle
·
Most of the energy of the
electrons in the tube current is converted to heat
·
A large focal spot allows the
accumulation of larger amount of heat before damage (i.e. a larger focal spot
will allow greater heat loading)
·
But for good radiographic
detail a small focal spot is required
·
The size and shape of the focal
spot is determined by the size and shape of the electron beam which is
determined by:
o
The dimensions of the filament
o
Construction of the focusing
cup
o
Position of the filament in the
cup
·
But when the target is beveled,
the focal spot, when viewed from the direction in which the x-rays emerge, is foreshortened
and appears small
·
The EFFECTIVE/ APPARENT focal
spot is smaller than the ACTUAL focal spot on the target – the size being
related to the sine of the angle of the target (Effective focal length = Actual
focal length x sinQ)
·
As the angle of the anode is
made smaller the apparent focal spot becomes smaller
·
But for practical purposes
there is limit to which the anode angle can be decreased as dictated by the
heel effect (the point of anode cut-off)
·
There are three trade-offs to
consider for the choice of anode angle:
o
Heat/ power loading
o
Effective focal spot size
(small = good resolution)
o
Field coverage (small angle =
small field coverage)
·
The line focus principle is used to permit larger heat loading while
minimizing the size of the focal spot by
orientating the anode at a small angle to the direction of the x-ray beam
irradiating the patient
Cathodes produce the electrons necessary for x-ray generation. Cathodes typically consist of a tungsten filament that is heated >2200°C. The filament is surrounded at its cathode end by a negatively charged focusing cup to direct the electrons in a small beam toward the anode.
ReplyDeleteThe anode has 2 functions: to convert electron energy into x-rays and to dissipate heat. Heat dissipation is achieved by rotating the angulated target (at approximately 3600 rpm, or 60 cycles/sec). The amount of x-rays produced depends on the atomic number (Z) of the anode material and the energy of electrons. Common anode materials include tungsten (W, Z = 74) and W/rhenium (Re) alloys (90%/10%). These materials are used because of their high melting point and the high yield of x-rays. Mammography units frequently use different anode materials (e.g., molybdenum [Mo]).
ReplyDeleteThe focal spot is the small area on the anode in which x-rays are produced. The size of the focal spot is determined by the dimensions of the electron beam and the anode angulation. Typical angles are 12° to 20°; however, smaller angles (6°) are used for neuroangiography. Tubes with smaller focal spot size are used when high image quality is essential.
Small Focal Spot is required for magnification views and mammography to reduce geometric blurring