Anode Types
The more expensive C-arms such as the OEC 9400, 9600, 9800 and the Phillips BV Pulsera have rotating anodes. The energies available for high
dose fluoroscopy are up to 20mA.
Less expensive units have less same image resolution in situations that require either a high mA (thick patients) or continuous high intensity during a
protracted procedure (eg. protracted discography or vertebroplasty) include the OEC7700, OEC-Fluorostar, all Ziehm systems (7000, Vista, Vision,
Vario-some have active cooling), Siemans Siremobile, Phillips BV Endura, Phillips BV Libra. The high dose energies available for fluoroscopy are 8mA.
ACTIVE COOLING is mandatory as those machines without such as the Ziehm 7000, tend to shut down for up to 30 min at a time when hot.
The anode consists of the positive electrode in a vacuum electron tube. In an X ray tube, the electrons are accelerated towards the anode and are
stopped in the anode. When this occurs, primarily heat is created with a small fraction of energy being released as X-rays. The construction of the
anode is therefore highly dependent on different heat removal mechanisms.
There are two basic types of anodes: stationary anodes and rotating anodes. The stationary anode is a much simpler, more reliable and cheaper
construction than the rotating type. However, it cannot be exposed to the very high tube loads that are demanded for most modern X-ray equipment.
Stationary anodes are therefore found mainly in dental equipment and less expensive mobile C-arm units for fluoroscopy.
In the stationary anode the electron target is made of tungsten or an alloy of tungsten and up to 15% rhenium. Tungsten has a very high melting
point, 3370 C. The target is commonly mounted into a large copper block, shaped as a stem, due to the heat conducting capacity of copper. The
copper stem carries the heat away from the target and is sometimes cooled by circulating water or oil. The load limit of a stationary anode is given by
the temperature that is reached in the copper at the boundary between the tungsten target and the copper stem. With a stationary anode, the maximum
tube load for radiography is around 100 W/mm2 and for continuous fluoroscopy around 30 W/mm2.
The rotating anode as we know it today, came into use in 1929. However, the first construction of a rotating anode tube was done by Wood in 1897.
In his construction, the cathode filament was placed inside a rotating glass envelope, which also functioned as the anode. The modern rotating anode
tube uses a disk-shaped anode, normally made of molybdenum, with a 1-2 mm thick surface layer consisting of an alloy of tungsten and rhenium
(5-15%). The rhenium is added because it makes the alloy more elastic, which prevents cracking of the surface and extends the life of the X-ray tube.
Molybdenum is used as the anode base because it has twice the heat capacity of a pure tungsten disk with the same mass. For X-ray tubes where
extremely high heat capacity is needed, a graphite backing is often used behind the molybdenum disk because graphite has a very high heat capacity
per unit mass - twice that of molybdenum. However, the heat is dissipated more slowly. Anodes with large graphite backings are therefore found in
equipment where momentary high tube loads are used, for instance in CT scanners. One drawback with graphite anodes is the risk that the graphite
can come loose, which happens if the temperature exceeds 1, 200-1,300 C. Compared to stationary anodes, tube loads for rotating anodes are much
higher and can exceed 10 000 W/mm2 on the anode surface.
The technique of making the anode disks has changed during the last decade. The disks are now forged instead of sintered. New anode surface
materials are being introduced, such as alloys of tungsten, rhenium, titanium and zirkonium, which will also contribute to anodes with longer life time.
A special problem is the removal of heat from the anode and anode stem. When the temperature difference of the anode and the cooling medium (oil) is
large, most of the energy is given away as radiated heat, the efficiency of which is proportional to [Temperature of the anode - Temperature of oil] to the
fourth power. However, when the oil is heated, most of the anode heat is transported away through the bearings of the anode stem. For common ball
bearings, the contact surface between balls and stem is very small and the heat conduction is inefficient, giving a long cooling time. Some newer
constructions therefore make use of another bearing technique.