RAD 350 Chapter 7 The x-ray tube
The x-ray tube has two primary components - the filament (cathode) and the target (anode). Each can be thought of as a DIODE (“Self rectification” is where the x-ray tube itself acts as the rectifier - half wave rectified).
Tube assembly support system is usually a ceiling mounted
“telescoping” type tube stand assembly so the tube can be turned vertically or horizontally.
Also, floor to ceiling rail systems and “C-arm” assemblies are used.
Surrounding the tube is a ‘PROTECTIVE HOUSING” that consists of the shielding of ‘LEAKAGE RADIATION” as well as oil to help dissipate heat from the anode assembly AND protection against electric shock . The outer portion of the tube itself is made of either glass or metal .
. X-rays are emitted from the tube through a window that is pointing toward the collimator assembly which is located under the tube itself .
LEAKAGE RADIATION IS THE RADIATION THAT IS
EMITTED IN ANY DIRECTION OTHER THAN THE EXIT
WINDOW OF THE TUBE .
Leakage radiation should never EXCEED 100 mR/hr at one meter in any direction from the tube.
The filament of the tube is usually made of”thoriated tungsten” and measure
2mm in diameter and 1 or 2 cm long (most tubes have DUAL
FILAMENTS - a small one for finite anatomical detail and a larger one where finite detail is not as important for radiographic detail - the larger the filament, the less detail due to the large area origination of x-rays produced BUT distributes the electrons over a larger area of the TARGET and therefore distributes the heat over a larger area at the expense of less anatomical detail in the radiograph.
(see figures 7-5 and 7-6 for DUAL filaments and the projectile electron interaction on the surface of the anode)
The “boiling off of electrons” is called thermionic emission. Even

though tungsten has a melting point of 3410 degrees C - the filament
DOES vaporize during use and can sometimes coat the inner part of the tube and actually INCREASE inherent filtration over time.
Surrounding the filament is a NEGATIVELY CHARGED focusing cup.
When the rotor (stator or induction coil) is energized, the system also steps up the current on the filament side to enable an adequate number of electrons to be available to be “boiled off” and form the electron/space- charge cloud. The rotor (stator or induction coil) also increased the high positive charge on the anode (TARGET) side of the x-ray tube resulting in a high EMF (POSITIVE, potential ).
Since 99% of the electrical energy is dissipated in heat (and the majority of that is on the TARGET side, the target materials must withstand very HIGH heat amounts. The target materials consist of
TUNGSTEN - Rhenium, GRAPHITE AND COPPER. The anode is round in shape (circles have the greatest amount of square inch area of any shape), and is mounted on the end of the rotor assembly. This mounting is heavy and the bearings that ease the friction of the anode assembly during it’s rotation and HEAT dissipating action WEAR
OUT often crating a grating sound when the anode
(rotor/stor/induction) motor is energized.
Most anodes rotate at 3400 rpm, however many tubes have a secondary sensor when heat builds up and increases the rotation speed to 10,000 rpm.
Remember the induction/stator/rotor motor is EXTERNAL to the x-ray tube and is a COIL of wire surrounding the OUTSIDE of the x-ray tube (but is within the protective housing). It must operate with A/C current in order to “INDUCE” a current and start the anode assembly to rotate to dissipate the heat and INCREASE the overall area of the target BUT still give a SMALL “ACTUAL” target
“FOCAL SPOT” (the “Line-focus principle” is used to provide a
LARGER ACTUAL focal spot area, BUT since the anode is angled - not truly vertical but angled from 5- 15 degrees from true vertical so when one “looks up” toward the target from the collimator one sees a
SMALLER appearing target) {focal spot }

THE LINE-FOCUS PRINCIPLE ALLOWS FOR A LARGER
“ACTUAL FOCAL SPOT” SIZE BUT RESULTS IN A SMALLER
“EFFECTIVE FOCAL SPOT” SIZE!!!
HEEL EFFECT - since the electrons are suddenly stopped inside the target area of the anode thereby producing X-RAY PHOTONS, the photons must be able to exit the anode in order to exit the tube and reach the patient. Since the anode is made up of ATOMS, some absorption (attenuation) of the photons actually occurs in the anode assembly - the deeper inside the anode and the more the photons have to travel in the anode to get out, the more attenuation and the
LESS INTENSE the exiting photons on the ANODE SIDE of the
TUBE and MORE INTENSE at the FILAMENT side of the tube.
Considerations for the HEEL EFFECT: can be utilized by the technologist to more evenly disperse the x-ray spectrum to the patient for better radiation to be absorbed by the patient’s anatomy. The AP foot is a good example - place the patient’s toe area of the foot toward the ANODE side of the TUBE and the heel of the foot toward the CATHODE side of the tube.
The toes, being less thick than the heel of the foot, require LESS penetrating photons than does the THICKER heel part of the foot that would get HIGHER PENETRATING photons from the CATHODE side of the tube . Lateral L-5/S-1 cone down of the L-spine is another place the HEEL EFFECT can be used to the technologist’s advantage.
THE LESS THE ANODE ANGLE, the GREATER THE HEEL
EFFECT!!!
X-RAY tube failure - HEAT is the MAJOR cause of heat failure.
Tubes should be WARMED UP if no exposures have been made in over one hour.
NEVER apply high kVp and mAs techniques to “warm up” a tube, use low kVp and mAs and gradually build up some heat on the tube prior to use on a patient.

This said, heat is a major draw back on the tube and it’s operation.
Convection, Conduction and Heat Radiation are all heat transferring processes utilized to dissipate heat from the anode assembly.
Tube rating charts.
Radiographic Tube Rating Charts are used by the technologist to assess if proposed x-ray techniques are SAFE to use on the tube to radiograph a patient. EACH x-ray unit/tube with have it’s own unique tube rating chart that shows for a given MA what kVp and time will be safe. (see figure 7-26 on page 135)
An Anode cooling chart is also used and is standard for ALL tubes
BUT are standard (also be aware heat units may also be called joules).
Kvp X mA X time = Heat Units (see figure 7-27 on page 136)
3-phase units create MORE heat than either half wave or full wave rectified and use the same mA X kVp X time PLUS times 1.4 (kVp X mA X time X 1.4)
NOTE: IT TAKES 15 MINUTES FOR A TUBE TO COOL TO
ZERO, BUT ONE NEED NOT WAIT FOR IT TO COOL TO ZERO
TO MAKE MORE EXPOSURES - only wait a few minutes for it to cool down enough to make another exposure (see how one need only wait about one minute for the tube to cool from 350,000 to 250,000 units - just like your iron - if you’ve ever ironed clothes - it drops heat very fast, but takes a long time to cool to ZERO).