Filament: Thoriated tungsten, rhenium, or molybdenum.
Thermionic emission at 2,200oC forms space charge
Vaporization leads to arcing and tube failure, grounded
metal jacket elevates arcing. Duel focus is common. Ave.
tube life is 10,000 to 20,000 exposures.
Appendix D1: Components of the x-ray tube
Focusing cup: see appendix D2
Anode: Stationary and rotating at 3400 rpms (low speed)
and 10,000 rpms (high speed rotors). (See composition
of anode illustration appendix D2)
Target, focus, focal point, focal spot, and focal track are
terms used to describe area of electron bombardment
on anode.
7-15 mm L
1-2 mm W
.1-.2 mm T
Tube rating charts and anode cooling curve
Actual and effective focus: See Line focus principle
appendix D2
Off focus radiation: Photons produced on tube parts
other than the focal spot. Lead shielding protects patients
from off focus and leakage radiation.
Heat Units (HU) = kVp x mA x mAs x S x generator constant
Heat unit constants for: 1 = 1
(Correction factors)
3 = 1.35
HF = 1.4
Appendix D2: Components of the x-ray tube
Focusing cup: Nickel housing around
filament repels electrons into narrow
beam.
Space charge effect: 1,200 mA is upper
limit due to the repulsion of electrons in
the space charge.
Focusing cup with space charge
Composition of anode
Grid biased (pulsed or controlled) tubes: A 2000
volt negative potential on the focusing cup attracts
the space charge for precision pulse control
Saturation current: When kVp is sufficently high
all electrons are drawn from the space charge during
the exposure. Space charge compensator ups mA
Line focus principle: The effective focus is
controlled by the actual focus and the angle
of the anode. The actual focus is controlled
by the size of the filament and the charge
on the focusing cup. When size of focal spot
is referred to, it is the effective that’s cited.
Anode angles must be less than 45 degrees
for the line focus principle to work. The
result is improved thermal loading and better
spatial resolution. Angles range from 7 to 17
degrees, 12 being average.
To cover a 17” field at a 40” SID a 12 degree angle or greater is required. Anything
less results in anode cutoff.
Pin hole camera
photograph of
a focal spot.
“Double banana”
shape
Anode heel effect: Due to
the angle of the anode amd
the absorbtion of photons
in the anode, distribution
of photons is greater on cathode
side of tube. Effect is most
pronounced
on a 17”
field size.
AP thoracic
spine is
classic
example.
Envelope: Pyrex
glass vacuum
tube. Photon
energy exits
window.
Focal spot size ranges between .1 and 3 mm
Fractional focal spots: < 1 mm
Blooming: Increase in focal spot size due to increases in mA.
Rotors: 3400 rpms (low speed)
and 10,000-12,000 rpms (high speed rotors).