Tube Vocabulary- Principles of Imaging I

Focal Spot

The portion of the anode where the high-voltage electron stream will impact to produce x-rays

Focal Track

The circular path that will be impacted by the electron stream with a rotating anode to produce x-rays

Anode

The positive side of the x-ray tube; serves as a target surface for high-voltage electrons from filament

Stationary Anode

An anode assembly that is immobile - target is at a 45-degree angle

Rotating Anode

An anode assembly that turns during exposure - most common target angle is 12 degrees

Cathode

The negative side of the tube; produces thermionic cloud and focuses electron stream toward anode

FIlament

A small coil of thin thoriated tungsten wire

Focusing Cup

A shallow depression in the cathode assembly designed to house the filament. A negative charge focuses electron stream into narrower stream.

Actual Focal Spot

The physical area of the focal track that is impacted

Effective Focal Spot

The area of the focal spot that is projected out of the tube toward the patient

Anode Heel Effect

The geometry of the angled anode target causes the radiation intensity to be greater at the cathode side of the tube; photons at the “heel” of the anode are more apt to be attenuated or absorbed

Dual Focus

A two-filament arrangement within the x-ray tube

Tube Envelope

The entire cathode and anode assembly is housed within this glass or metal structure which is sealed tight to maintain the vacuum

Extrafocal/ Off Focus Radiation

Photons that are not produced at the focal point

Grid Biased

Grid or mesh is used in front of filament. Voltage is applied to regulate, pulse, or synchronize x-ray production (quickly regulates flow of electrons). Used where very fast exposures are needed or to pulse x-rays.

E.g. angiography, pulsed fluoro also used to terminate exposure with capacitor discharge mobile units

Line-Focus Principle

When target angle is less than 45-degree, the effective focal spot will be smaller than the actual focal spot. Used to achieve a smaller focal spot size with the enhanced heat dissipation of the larger actual focal spot.

Protective Housing

The tube envelope is housed within this metal structure which may have an additional layer of lead at the cathode end and serves to cushion tube and provide protection from radiation leakage

Rotor

A hollow copper cylinder or cuff that is attached to the anode disk by a molybdenum shaft

Stator

Induction motor electromagnets that turn the anode (located in the protective housing, but outside the glass envelope)

Space Charge Effect

As more and more electrons build up in the area of the filament (during thermionic emission) their negative charges begin to oppose the emission of additional electrons. This is why mA stations cannot be higher than 1200mA.

Target

The portion of the anode where the high voltage electron stream will impact

Thermionic Emission

The ejection of electrons from the surface of the wire is due to increased heat, causing an electron cloud. Also called the space cloud (electron cloud).

Tube Window

A structure where the primary x-ray beam exits the envelope, which allows less absorption or scatter of photons.

Vacuum

A space from which air has been removed