Focal Spot

The Anode

There are two types of anodes; stationary and rotating.  A stationary anode has a static target―electrons from the filament hit one area of the anode.  It is used for low tube current work such as dental units and some portable units.  It is made of tungsten-alloy metal embedded in a copper anode.  A rotating anode has a dynamic target area―electrons hit the target as it spins thus becoming a circular focal tract around the face of the entire rotating disk.  Rotating an anode increases the size of the target 300 times.  It is used in most of our diagnostic equipment allowing the use of higher technical factors. 

The anode functions as an electrical conductor.  It receives electrons from the cathode and conducts them through the tube to connecting cables and back to high-voltage generator.  The anode also functions as a mechanical support for target.  Lastly, the anode functions as a thermal conductor; more than 99% of the energy from electrons is converted to heat. 

Focal Spot

The target is the point from which the SID, SOD, and OID is measured.  The target is also known as the focal spot, the focus, and the focal track.  High-capacity x-ray tubes have molybdenum and/or graphite layered under the tungsten target making it lighter and easier to rotate.  Tungsten is material of choice for target construction because of its high atomic number (74); its thermal conductivity; and its high melting point – 3400° C.

Focal Spot Size

Most equipment has two sizes of focal spots, small and large.  The average sizes range from 0.1 mm to 3.0 mm.  The small focal spot (0.1 mm) is meant for smaller body parts―such as extremities―and used with low mAs technical factor settings.  The large focal spot (3.0 mm) is used for larger body parts―such as the abdomen―and used with high mAs technical factor settings. 

The focal spot can further be categorized as the actual focal spot and the effective focal spot.  The actual focal spot is the area of target actually being bombarded by electrons.  The effective focal spot  is the foreshortened size of the focus as it is projected down onto the patient and the image receptor.  The effective focal spot size varies along the longitudinal axis of the image receptor, being largest at the cathode and smallest at the anode.  The line-focus principle that states that the effective focal spot―what is projected onto the patient and IR―is smaller than the actual focal spot when the target angle is less than 45°, because it is foreshortened.  This design is incorporated into the anode to allow a larger focal area for heating while maintaining a smaller focal spot. 

The effective focal-spot size is controlled by the size of the actual focal spot and the angle of the anode.  The larger the actual focal spot―which is a direct result of the length of the filament―the larger the effective focal spot.  Anode target angles can vary from 5° to 15° with the most common target angle being 12° (this much angle is required to produce a primary x-ray beam large enough to cover 14 x 17 inch field size at 40 inches).  When the anode target angle is decreased, smaller focal spots can be achieved.

The benefits of having a smaller effective focal spot with a larger actual focal spot are that a smaller effective focal spot permits better detail (less focal spot blur) and a larger area of the target is blasted with electrons which allows (1) heat to be dispersed over a larger area; and (2) less problems with pitting, warping, and cracking of the target which prolongs the life of the tube. 

Anode Heel Effect

As electrons bombard the anode, x-rays are produced and emitted at all angles; however, most are between 45° and 90° in the direction of electron travel.   The anode end of the radiographic image has less density than the cathode end because some x-rays are partially absorbed by the anode itself.  This means that the cathode end has the greatest beam intensity and thus the greatest density.  The variation in beam intensity between anode and cathode end is about 45%.  The anode heel effect is most noticeable with:

• Smaller focal spots due to steep target angle (minimum of 12°)
• Large film sizes because you are opening the collimating shutters thus visualizing the edges of the beam
• Short SID (40 inches) because you are not collimating the edges of the beam like you would in a longer SID

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