INTERNAL STRUCTURE (Cathode) Flashcards
Primary Parts of Cathode
Filament and focusing cup
➢ A coil of wire that emits electrons when it is heated
➢ Size and Length: approximately 2 mm in diameter and 1 or 2 cm long.
Made of thoriated tungsten:
Filament
Tungsten melting capacity
3410 C
Most common cause of tube failure
Tungsten Vaporization
➢ Metal shroud that surrounds the filament
➢is negatively charged so that it electrostatically confines the electron beam to a small area of the anode
Focusing cup
A tube which contains its own switch which allows it to turned on and off very rapidly.
Grid controlled tubes
Element in the tube that acts as a switch
Grid
Acts as a grid (exposure switch) in grid controlled x-ray tube
Focusing cup
A phenomenon of the space charge that makes it difficult for subsequent electrons to be emitted by filament because of electrostatic repulsion
-the tendency of the space charge to limit emission of electrons from the focusing cup
Space charge effect
Cloud of electrons. As electrons carry negative charges they repel one another and tend to form a cloud around the filament.
Space charge
Filament current that has risen to its maximum value because all available electrons have been used.
X-ray tube at Saturation Current: emission limited
Saturation current
➢A region of the anode target in which electrons interact to produce x-rays
➢the area of the target from which x-rays are emitted.
➢ It is the actual x-ray source.
➢Sizes: small (0.1-1 mm)/large (0.3-2 mm)
➢Shapes: circular (preferred)/double banana
Focal spot
Important for high resolution magnification radiography & mammograph
Round Focal Spot
➢It is used when small body parts are image & when better spatial resolution is required
Small Focal Spot
It is used when large body parts are imaged
Large Focal Spot
