Ch.05 X-ray Tube
General tube construction
Housing, x-ray tube
modern x-ray machines have protective circuits built in that prevent the use of unsafe exposure techniques and heat overloads of the x-ray tube
However, even with all of these safety measures, the radiographer must understand anode thermal capacity and keep in mind that the production of x-rays is a very inefficient process, with almost 99% of the energy used being converted to heat.
A technique of 80 kV, 400 mA, 0.8 seconds is to be used on a 3-phase, 12-pulse machine. How many heat units are produced with a single exposure?
36,096
The size of the electron stream depends on the size of the filament.
The smaller this stream, the greater the heat generated in a small area; therefore it is desirable to have a larger actual focal spot area.
With the preparation phase, some power from the autotransformer was diverted to the filament circuit, where it passes through a rheostat (controlled by the mA selector) to a step-down transformer, then to the selected filament (determined on the control panel) within the cathode focusing cup.
This current heats the filament to a point of incandescence (white hot), and electrons are literally boiled off of the filament by thermionic emission. The focusing cup forms them into a cloud.
The focal "spot" becomes a focal "track," with the rotating anode and the heat build-up spread over the focal track circumference rather than on one spot
This greatly increases the heat-load capacity and the exposure techniques that can be used.
Cooling fans
circulate air around the assembly and help dissipate heat
three processes of heat transfer are at play
conduction of heat by heat-tolerant materials, radiation of heat energy from the anode to the oil bath, and convection of heat into the room by the cooling fans.
Which component of the x-ray tube is responsible for concentrating the electron cloud?
focusing cup
three things needed to produce x-rays are now present =Environment for X-ray Production
1. a large potential difference to give kinetic energy to the filament electrons (provided by the kVp setting); 2. a vehicle on which kinetic energy can ride (a quantity of electrons provided by mAs); 3. a place for interaction (the target of the anode).
Two notes of caution about the housing
1. with extended "on" times, the housing can become rather (relatively) hot; most likely to occur with fluoroscopic units that can be easily touched unlike most permanently installed unites with the tube located under the tabletop; use caution when involved in very long cases in which heating is considerable
A technique of 50 kV, 100 mA, 0.1 seconds is to be used on a 3-phase, 6-pulse machine. How many heat units are produced with a single exposure
675
For example, how many heat units are produced if 80 kVp, 200 mA, 0.2 sec is used with a high-frequency generator?
80 × 200 × 0.2 × 1.45 = 4640
Target window (on both enclosure types)
A thinned section of the x-ray tube enclosure that is the desired exit point for the x-rays produced; fashioned to minimally interfere with (absorb) the x-rays; it reduces the amount of absorption by the enclosure
Induction motor
Allows the anode to rotate inside a glass envelope with no mechanical connection to the outside
The target material (coating) is made of tungsten because it has a very high melting point (3400°C, 6152°F), and its thermal conductivity is almost equal to that of copper
Also, it has a high atomic number (74), improving the efficiency of x-ray production. Rhenium may also be added to the tungsten to increase thermal capacity and tensile strength
built-in methods such as oil bath and cooling fan help dissipate heat
Also, rotating anodes spread heat over a larger surface area, helping with the heat-load problem
The housing absorbs most of the photons traveling in directions other than toward the patient
Although x-rays are perceived as being produced and traveling in one direction out through the collimator to the patient and image receptor, this is not the case. X-rays are produced isotropically (in all directions)
sun tanning on x-ray tube
As these tubes age, vaporized tungsten from the filament deposits on the inside of the glass and creates the bronze discoloration of the glass; it causes problems with arcing (electrical bending) and damage
"detour" in this flow of electrons
As they penetrate the target surface, these filament electrons interact with the atoms of tungsten, generating heat and x-rays
Principles of operation
At the operating console the radiographer selects the desired exposure factors (i.e., kVp, mAs, and focal-spot size)
Third, do not routinely use extremes of exposure factors
Consistently using single, very high exposure values results in pitting the anode (small areas of melting), which can then cause irregular outputs; Also, consistently using low but very long exposures also results in uneven heating and wear.; Excessive heating may also cause heat transfer to the bearings of the rotor
2. the high-voltage cables are not "handles.
Don't use them as handles to avoid a risk to the radiographer and potential for damage to the equipment.
a lead-lined metal structure
Electrical insulation because of the large current and voltage needed to produce x-rays
To extend tube life
First, the warm-up steps specific to the unit should be followed completely and routinely; , the x-ray tube should be warmed before normal operation; newer units may have automatic warm-up protocols
In the case of the anode heel effect, the lower intensities on the anode side will still be detected and accurately displayed on the final image.
However, it is still useful for the radiographer to be mindful of and understand the anode heel effect
Single-phase = 1.0 Three-phase, 6-pulse = 1.35 Three-phase, 12-pulse = 1.41 High-frequency = 1.45
If multiple exposures are made using a given technique, the answer from this formula is multiplied by the number of exposures.
How Induction Motor works?
Induced magnetic field opposes that of the stator pair, and the rotor turns to correct that orientation. Just as the two fields align, the next pair of stators is energized and again a new electric current and magnetic field is induced, causing the rotor to turn again. This process continues with the energizing of each pair of stators in sequence. The response of the rotor is to continuously turn as the induced magnetic fields try to orient with the ever-changing external fields.
Rotor
L: in the center of the stator (don't touch stator) but within the enclosure; made of an iron core (iron bars embedded in the copper shaft) surrounded by coils; No electric current supply; operated through mutual induction
Stators
L: outside the tube enclosure; energized in opposing pairs and induce an electric current in the rotor with an associated magnetic field.
Quality control and extending tube life
Several factors can shorten the life of an x-ray tube or damage it; Most have to do with the thermal characteristics of x-ray production and are within the radiographer's control.
Copper is used as part of the shaft, because it has excellent thermal and electrical conductive properties.
Molybdenum is used as the disc base and core because it has a low thermal conductivity, which slows migration of heat into the rotor bearings (minimizing heat damage), and it is a light but strong alloy, making it easier to rotate the anode.
dual-focus tubes
Most general-purpose tubes have two filaments; filaments are represented by the large and small focal-spot options on the operating console
At the same time that the rotor is spinning up, the selected filament is energized until the desired degree of thermionic emission is achieved.
Prepping the rotor is the first phase of a two-phase switch.
The use of heat-tolerant materials in the construction of the tube also helps deal with heat load.
Radiational cooling of the anode is also used; the anode "radiates" heat within the tube away from itself
Two parts of the induction motor
Stator, rotor
Using an induction motor allows for the rotation of the anode in a vacuum without engineering a motor into the vacuum.
Such motors are capable of rotating the anode at speeds of 3400 revolutions per minute (rpm) for general-purpose tubes and 10,000 rpm for specialty tubes.
The size, shape, charge of the focusing cup, and how the filaments are designed and placed within it, affect how well it "focuses" the electrons on the target. All of these things are taken into consideration in the design for optimum performance.
The focusing cup serves its function through electrostatic repulsion.
The second phase actually initiates the x-ray production process.
The process from rotor preparation to exposure lasts only a few seconds, with the actual exposure generally measured in milliseconds.
The Line-Focus Principle and Anode Heel Effect
The rotating anode design uses the line-focus principle, which means that the target face is angled to create a large actual focal spot for heat dissipation and a small effective focal spot for improved image quality. But by angling the face, the "heel" of the target is partially placed in the path of the x-ray beam produced, causing absorption and reduced intensity of the beam on the anode side.
The Rotating Anode sum
The rotating anode is turned using an induction motor that operates through electromagnetic mutual induction. The rotation of the anode spreads the heat produced during x-ray production over a larger surface area, greatly increasing the thermal and exposure technique capacity of the tube.
.The target angles are 7 to 18 degrees for a general-purpose tube, with 12 degrees being the most common.
The smaller the anode angle, the smaller the effective focal spot will be while maintaining a large actual focal spot area (small anode angle, small effective focal spot for improved image quality, large actual focal spot for heat dissipation)
When the exposure switch is pressed, the voltage from the autotransformer (controlled by the kVp selector) passes to the step-up transformer (or in the case of high-frequency generators, to capacitor banks, then inverter circuit, to the step-up transformer).
This voltage (and current) then passes through a rectifier bank before passing to the anode and cathode of the x-ray tube so that the anode is always positive and the cathode is always negative; This voltage creates a huge potential difference between the electrodes
Tungsten is used because it has a very high melting point and does not vaporize easily
Thorium is a radioactive metallic element that is added to increase thermionic emission (boiling off of electrons) and extend filament life
hitting the rotor
When the exposure switch is first pressed, some of the electricity is diverted to the induction motor of the x-ray tube to bring the rotor up to speed.
the line-focus principle achieves the balance between heat area and projected focal spot, BUT it has angle limitation
When the target angle becomes too small, the x-ray beam area may not be large enough to fully expose a 14-by-17-inch image receptor at a 40-inch source-to-image receptor distance (SID)
High voltage receptacles (connectors)
Where two large electrical cables enter the housing and are securely attached to the x-ray tube
Each filament
a coil of wire usually 7 to 15 mm long and 1 to 2 mm wide; usually made of tungsten with 1% to 2% thorium added; situated parallel to each other in the focusing cup and shares a common ground wire.
heat units
a measure of the amount of heat stored in a particular device
(two anode types) 1. Stationary anode
a tungsten button embedded in a copper rod; called stationary because the target does not move; used in old tube designs and may still be found in dental x-ray units or those requiring very small exposure techniques; The disadvantage of this design is that, because the electrons always hit the same small target area, heat builds up rapidly and can damage the tube; This problem limits the exposure technique factors that can be used.
X-ray tube
an electronic vacuum tube that consists of an anode, a cathode, and an induction motor all encased in a glass or metal enclosure (envelope)
the line-focus principle
by angling the face of the anode target, a large actual focal spot size (area actually bombarded with filament electrons) can be maintained and a small effective focal spot size (the x-ray beam area as seen from the perspective of the patient) can be created
Glass envelope variety (enclosure type)
generally made of borosilicate glass which is very heat resistant
Major Causes of X-ray Tube Failure
he frequent use of very high or maximum exposure factors, the use of lower but very long exposure factors (maintaining the tube at high temperatures), and overload of the filament (prolonged excessive heating or arcing from filament).
How to be aware of how much heat may be produced during an exposure?
heat units (HUs)
The electron cloud is attracted to the anode target because of the huge potential difference.
these filament electrons will reach speeds of about half the speed of light in the short 1 to 3 cm between the focusing cup and anode target.
HUs are calculated using the following formula:
kVp × mA × s × c (kVp is the kilovoltage selected, mA is the milliamperage station selected, and s is the exposure time in seconds; The c represents a correction factor and depends on the generator type)
The purpose of the line focus principle is to create which of the following?
large actual and small effective focal spot size
Anode heel effect issue with digital technology
less issue because these systems can record and display many shades of gray, a characteristic called dynamic range; Digital systems have a wide dynamic range, meaning that they can accurately detect, record, and display very high and very low x-ray photon intensities.
The focusing cup
made of nickel and surrounds each filament on its back and sides, leaving the front open and facing the anode target; receives a strong negative charge from the secondary circuit that forces the electrons together into a cloud as they are boiled off of the filament.
X-ray production is a rather inefficient process because
much of the electrical energy that goes into the housing is converted to heat (99% heat, 1% x-rays)
The intensity of the x-ray beam is less:
on the anode side.
How to improve image quality by understanding the anode heel effect?
placing the thinner or less dense portion of the patient's anatomy under the anode end
Second, do not prep the rotor excessively.
preexposure phase maintains the filament in an energized state and thus shortens its useful life; It is usually preferable to press both the rotor and exposure buttons almost simultaneously, so that the filament is heated for the minimum time necessary. The machine will not apply the high voltage until the rotor reaches full operating speed
Which of the following reduces leakage radiation to required standards?
protective housing
metal envelope variety (enclosure type)
provides a constant electric potential between the electron stream from the cathode and the enclosure, thereby avoiding the arcing problem and extending tube life
(protective) Housing
provides solid, stable mechanical support; a lead-lined metal structure that also serves as an electrical insulator and thermal cushion for the tube itself; incorporates an oil bath and cooling fans to protect it from thermal damage
A small anode target angle:
results in an increase in anode heel effect.
space charge (a cloud)
self-limiting; . Once the space charge reaches a size commensurate with the current used, it becomes difficult for additional electrons to be emitted
Inside the x-ray tube, the induction motor turns the anode at approximately 3400 rpm (or faster depending on the tube type and purpose)
so as to spread the generated heat over a larger total surface area.
The actual focal spot
the area being bombarded by the filament electron
When the angle of the target face is less than 45 degrees,
the effective focal spot will be smaller than the actual focal spot
anode target angle is determined based on
the intended use of the tube and is not something the radiographer "selects" at the operating console; e.g: the angles are optimized for mammography units, angiographic units, general radiography units, etc.
electrostatic repulsion
the negative charge of the focusing cup is greater than the negative charges of the electrons and thus forces them together. Otherwise, the individual electron-negative charges would cause them to repel each other and scatter as they are boiled off of the filament.
Cathode
the negative end of the tube; provides the source of electrons needed for x-ray production; made up of the filaments and the focusing cup and is connected to two different parts of the x-ray circuit (the filament is connected to the filament circuit and the focusing cup is connected to the secondary circuit)
The effective focal spot
the origin of the x-ray beam and is the area as seen from the patient's perspective; The smaller this area of origin, the sharper the image will be. It is desirable to keep this as small as practical to improve image quality.
the anode heel effect (particularly true of film/screen technology but less of an issue with digital technology)
the percentage difference in x-ray beam intensity from cathode side to anode side; This lowering of intensity on the anode side of the beam can cause the image to be "lighter" on that end; This is because there are fewer high-energy x-ray photons on the anode side, and not enough penetrate the patient to expose the image receptor
Anode of x-ray tube
the positive end of the tube; provides the target for electron interaction to produce x-rays and is an electrical and thermal conductor; designed to dissipate this heat.
Additionally the angle causes the intensity of the x-ray beam to be less on the anode side because the "heel" of the target is in the path of the beam
the x-rays on the anode side must first penetrate a portion of the target before exiting the tube.
Causes of tube failure are most often related to which of the following
thermal characteristics
What metal is added to the filament to increase thermionic emission and extend tube life?
thorium
Why the x-ray tube is enclosed
to maintain a vacuum within the tube; Because the production of x-rays involves the interaction between filament electrons and the anode target, if any air were present, the electrons from the air would contribute to the electron stream, causing arcing and damage to the tube.
the face of the target is angled. Why?
to make use of the line-focus principle
The purpose of rotating the anode
to spread the tremendous heat produced during x-ray production over a larger surface area; Instead of electrons always striking the same small surface area (as with stationary anodes), the electrons strike only a small part of the total anode surface area at any one time and that area changes.
A dual focus tube refers to a tube with:
two filaments.
2. the rotating anode
used in general-purpose tubes today; consists of a rotating disc made of molybdenum as a core material coated with tungsten and mounted on a copper shaft with a molybdenum core
Oil bath
where the tube is immersed to draw heat away from the tube
The x-ray tube is a part of the:
x-ray circuit secondary