Contents
1-X-Ray beam attenuation
2- X-Ray tube
3-Tube rating and cooling
4- Line focus principle
5-Anode heel effect
6-off focus radiation
7-Questions
1-X-Ray beam attenuation:
· X-Ray is produced from an X-ray tube with different energies (Poly-Energetic).
· The maximum X-ray photon energy is equal to the pre-adjusted Kev parameter.
· The lowest energy of x-ray photons depends on the X-Ray tube filtration.
· When an X-ray incident on any material, the intensity of the x-ray is divided into three parts. The first part is absorbed (attenuated), the second part is scattered, and the 3rd part is transmitted.
· The intensity of transmitted radiation depends on the attenuation property (attenuation coefficient), the thickness of the material, and radiation energy.
· Higher thickness and attenuation coefficient cause higher X-ray attenuation, and less transmission.
· Higher radiation energy causes less attenuation and increases the thickness required for attenuation.
· The thickness required to reduce the intensity of radiation to half is the half-value layer HVL.
· For monoenergetic X-Ray:
· If the X-ray beam passes through a certain material with HVL, the intensity will be reduced to half.
· the transmitted X-ray beam will have the same energy but half the intensity of the primary X-ray.
· the reduction of transmitted X-ray intensity to half requires the same thickness HVL, as no difference in Energy.
· the first HVL is equal to the second HVL, as the mean energy of the primary x-ray is equal to the mean energy of the transmitted X-Ray.
· The reduction in x-ray quantity (beam intensity) not quality (beam energy)
· For poly-energetic X-Ray:
· if the X-ray photons carry a range of energies starting from10 Kev to 100 Kev, so the mean energy is 50 Kev. After the first HVL, the transmitted X-ray photons will carry energies from 50 Kev to 100Kev,so the mean energy will be 75Kev, the mean energy (75 Kev) of the transmitted X-Ray requires a higher thickness of the same material to be reduced to half intensity (higher HVL).
· For poly-energetic X-ray: the second HVL is higher than the first HVL of the same material (Beam Hardening).
· 2- X-Ray tube
1-Glass enclosure
· Necessary to seal the two electrodes of the x-ray tube in a vacuum.
· The vacuum → allows the electrons to move rapidly from cathode to anode without blockage.
· The connecting wires must be sealed into the glass wall of the x-ray tube.
· Special alloys, having approximately the same coefficients of linear expansion as Pyrex
· glass, are generally used in x-ray tubes
2-Cathode Filament
• X-ray tube has a long filament for large focal spots and short filaments for small focal spot size
• Made of thin (0.2 mm) tungsten wire
• has a high atomic number (A 184, Z 74)
• is a good thermionic emitter (good at emitting electrons)
• can be manufactured into a thin wire that has a very high melting temperature (3422°c)
• The size of the filament relates to the size of the focal spot. Some
cathodes have two filaments for broad and fine focusing.
• As current passes through the filament, the atomic and electronic motion in metal is sufficiently violent to enable a fraction of the free electrons to leave the surface despite the net attractive pull of the lattice of positive ions.
• The electrons are then repelled by the negative cathode and attracted by the positive anode. Because of the vacuum, they are not hindered in any way and bombard the target with a velocity around half the speed of light.
• Tungsten is chosen for use in x-ray tubes, because:
1. It can be drawn into a thin wire that is quite strong.
2. Has a high melting point (3370° C).
3. Has little tendency to vaporize.
∴ Tungsten filament has a reasonably long-life expectancy
• When current flows through tungsten wire → heated → its atoms absorb thermal energy→ some of the electrons acquire energy → move a small distance from the metal surface → form a small cloud in the vicinity of the filament “the space charge”.
• The electron cloud, produced by thermionic emission, is also termed “Edison effect”.
•
• Tube potentials (Kv) enable larger x-ray tube current for the same filament current; for instance, for the same filament current of 5 A at 80 kV, a tube current of 800 mA is produced, whereas at 120 kV a tube current of about 1100 mA results.
SATURATION VOLTAGE
• If the potential applied across the tube is insufficient to cause almost all electrons to be
pulled away from the filament when they are emitted → Space Charge Effect: a cloud of
negative charges tend to prevent other electrons from being emitted from the filament
until they have acquired sufficient thermal energy to overcome the force caused by the
space charge → limits the number of electrons → limits X-ray tube current.
• Below the saturation point,
the tube current is limited by the space charge effect (space-charge-limited).
↑↑ kV → significant ↑↑ in x-ray tube current although filament heating is the same.
• Above the saturation voltage,
the space charge effect has no influence on the x-ray tube current.
the tube current is determined by the number of electrons made available by the
heated filament (emission-limited or temperature-limited).
↑↑ kV → very little change in tube current
3-Focusing cup
· Made of molybdenum which has a high melting point
· poor thermionic emitter so electrons aren’t released to interfere with an electron beam from the filament
· Negatively charged to focus the electrons towards the anode and stop spatial spreading
Cathode focusing cup:
· Surrounds the filament & maintained the same negative potential as the filament.
· So, its electrical forces cause the electron stream to converge onto the target anode in the required size and shape → prevent bombardment of a large area on the anode caused by the mutual repulsion of the electrons
· The focusing cup is made of nickel.
·
Modern x-ray tubes may be supplied with a single or, more commonly, a double filament
GRID-CONTROLED X-RAY TUBES
• Conventional x-ray tubes contain two electrodes (cathode and anode).
• The grid-controlled tube has the 3rd electrode → controls the flow of electrons from the
filament to the target.
The third electrode is the focusing cup that surrounds the filament.
• In conventional x-ray tubes a focusing cup is electrically connected to the filament.
• In the grid-controlled tube, the focusing cup is electrically negative relative to the
filament.
the voltage across the filament grid produces an electric field along the path of the
electron beam → pushing the electrons even closer together.
If the voltage is large enough → the tube current may be completely pinched off “act
like a on & off switch for the tube current → used in cinefluorography”.
5-Anode
· It is the positive electrode inside the x-ray tube.
· The place where the electrons hit to produce an x-ray
· 99% of Electron interaction with anode material produces heat, as such anode has to have a high melting point of 3370 0C.
· Tungsten is used as an ideal anode for many reasons:
1- High melting point
2- High Z number (Z-74), which increases X-Ray production.
Anode Types:
1-stationary anode:
· It is used in dental X-rays and portable x-ray where high power (Kv and mAs) is not required.
· It can’t withstand high temperatures produced by filament electron collisions.
· It is composed of tungsten inserted into the copper block
· Copper increases the thermal capacity and the cooling rate of the anode material
· Copper has a low melting point
2-Rotating anode
· The anode is rotating to increase the area of filament electron collisions on the target anode.
· This causes a good heat distribution (increases the heat capacity ) to withstand the tube heat loading.
· The rotating anode is used in radiographic X-ray, CT, and cardiac imaging because of the high parameters KV,mAs,and power.
· Most rotating anodes revolve at 3400 rpm (revolutions per minute). • The anodes of high-capacity x-ray tubes rotate at 10,000 rpm.
· The stem of the anode is the shaft between the anode and the rotor.
· It is narrow so as to reduce its thermal conductivity.
· The stem usually is made of molybdenum because molybdenum is a poor heat conductor.
· Occasionally, the rotor mechanism of a rotating anode tube fails. • When this happens, the anode becomes overheated and pits or cracks, causing tube failure.
A stationary anode tube with a 1-mm focal spot may have a target area of 4 mm2. A 15-cm diameter rotating anode tube can have a target area of approximately 1800 mm2, which increases the heating capacity of the tube by a factor of nearly 500.
3- Line focus principle
• Not all of the anode is involved in x-ray production. The radiation is produced in a very small area on the surface of the anode known as the focal spot.
· The dimensions of the focal spot are determined by the dimensions of the electron beam arriving from the cathode.
· In most x-ray tubes, the focal spot is approximately rectangular. The dimensions of focal spots usually range from 0.1 mm to 2 mm.
· X-ray tubes are designed to have specific focal spot sizes; small focal spots produce less blurring and better visibility of detail, and large focal spots have a greater heat-dissipating capacity.
· Focal spot size is one factor that must be considered when selecting an x-ray tube for a specific application. Tubes with small focal spots are used when high image visibility of detail is essential and the amount of radiation needed is relatively low because of small and thin body regions as in mammography.
Most x-ray tubes have two focal spot sizes (small and large), which can be selected by the operator according to the imaging procedure.
· Radiology requires small focal spots because the smaller the focal spot, the better the spatial resolution of the image.
• Unfortunately, as the size of the focal spot decreases, the heating of the target is concentrated onto a smaller area.
· Before the rotating anode was developed, another design was incorporated into x-ray tube targets to allow a large area of heating while maintaining a small focal spot. This design is known as the line-focus principle.
· By angling the target(Angle θ → the angle between the central ray and the target face)
, one makes the effective area of the target much smaller than the actual area of electron interaction.
· The line-focus principle allows high anode heating with small effective focal spots. As the target angle decreases, so too does the effective focal spot size.
· The effective target area, or effective focal spot size, is the area projected onto the patient and the image receptor.
· When the target angle is made smaller, the effective focal spot size also is made smaller.
· Diagnostic x-ray tubes have target angles that vary from approximately 5 to 20 degrees
· Field coverage and effective focal spot length vary with the anode angle. A. A large anode angle provides good field coverage at a given distance; however, to achieve a small effective focal spot, a small actual focal area limits power loading. B. A large anode angle provides good field coverage, and achievement of high-power loading requires a large focal area; however, geometric blurring and image degradation occur. C. A small anode angle limits field coverage at a given distance; however, a small effective focal spot is achieved with a large focal area for high power loading.
· Effective focal spot size is 0.3mm for mammography while reaching to 1.2mm for radiography.
· Smaller focal spot size is mandatory for better resolution, and small field coverage, but increases the tube loading (heat loading).
· Effective focal spot size for different imaging modalities:
· Within the design of an x-ray tube there is a balance between the dual aims of increasing the focal spot size as far as possible and yet maximizing heat dissipation. There are two features that work towards optimizing these two antagonists.
· Using a rotating disc anode (instead of a stationary one) to spread the heat out over the circumference of the disc. Rotating discs are usually not possible for dental radiography, mobile units, and intra-operative radiography owing to the smaller size of the set-up used. Stationary anodes are still used for most of these.
· Changing the target angle – the larger the angle the larger the effective spot size but the worse the heat loading due to a smaller actual spot size.
Large focal spot size is useful because it means a larger area of the patient can be captured in the resultant field of view (remembering that the photons diverge from their point of origin although diagrammatically represented in parallel in this diagram), but the usefulness of a large focal spot also needs to be balanced against the degree of geometrical unsharpness it produces.
The design of the anode also helps reduced heat loading further:
A larger disc circumference means a greater area for the heat to spread over.
· Disc sizes of 20-30cm are usual
The disc itself (into which sits a ring of target material) can be made from molybdenum which has a high melting point and low density but is a poor conductor of heat, with a thin molybdenum stem transmitting the rotation from the motorized part of the assembly.
· The target material itself can be made from an alloy of tungsten and rhenium which does not coarsen as fast as tungsten on its own.
4-Anode heel effect
· The x-ray photons are produced at the focal spot area to travel in a circular path.
· The only photons moving towards the window of the X-ray tube can reach the patient.
· The photons move a certain distance through the target anode before leaving it.
· The angulation of the target anode causes unequal distance traveled by photons from the interaction site to the surface of the target material.
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· The distance traveled is long for photons moving toward the anode, while short for photons moving toward the cathode.
· The target anode material cause attenuation to the x-ray photons which reduces the intensity of x-ray photons on the anode side more than on the cathode side.
·
· The x-ray beam becomes not uniform, where the x-ray photons have a higher intensity at the cathode side than photons at the anode side.
· Smaller anode angle causes an increase in the distance traveled by photons moving towards the anode side and subsequently reduces the X-Ray intensity at the anode side.
· Factors affecting the healing effect:
· Anode Angle: reducing the anode angle causes an increase in the heel effect
· Small anode angle
· Large anode angle
· Field size:
· Using less field size(small coverage area ) by tight collimation cause fewer variations in X-Ray photon intensities
· Large field size causes high variation between X-ray photons intensities at the cathode and anode sides as shown below:
·
· Less field size cause less variation in X Ray photon intensities
· Source to image receptor distance SID
· The distance from the X-ray origin to the image receptor at the anode side is shorter than the distance towards the cathode side.
· If SID increases the variation in X-ray intensities decreases at the anode direction against the central line.
· If SID increases the variation in X-ray intensities decreases at the cathode direction against the central line.
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· the variation in X-ray intensities decrease at the anode direction against the central line
·
· the variation in X-ray intensities decrease at the cathode direction against the central line
·
· If SID increases, the loss of X-ray relative intensity at the cathode side is higher than at the anode side
Uses of heel effect :
Chest X Ray imaging :
Make the cathode side towards the abdomen region, as the abdomen is high attenuating organ than the lung
Abdomen X-ray imaging :
• Cathode side is directed toward the upper abdomen where high attenuation happens to the X-Ray beam.
• Anode side is directed towards the lower abdomen where lower X-Ray attenuation happen
• Mammography imaging
• Positioning the cathode toward the thickest part (Chest wall) for obtaining uniform image .
•
5-off focus radiation:
· Off-focus radiation results from rebounding electrons from the focal spot striking other areas of the anode, thereby producing a large low-intensity x-ray source.
· This extra-focal radiation increases the patient dose and image blurring from shadowing and decreases image contrast.
· Patient anatomy appearing outside of the exposure field (e.g., ears on a skull examination) is attributed to off-focus radiation.
· The upper fixed shutters of a collimator (beam-limiting device) help reduce the amount of off-focus radiation reaching the film.
6-Tube rating and cooling
· The heat loading (rating) depends on acquisition parameters Kv., mA,…..
· When Kv or mA increase, the number of filament electrons increases which leads to higher collisions and heat dissipation.
· Although the joule is the basic unit for energy and heat, it is not always used to express x-ray tube heat. The special heat unit (HU) was introduced when single-phase equipment was common to make it easy to calculate heat.
·
The relationship between a quantity of heat expressed in heat units and in joules is given by
· Heat (HU) = 1.4 x heat (J).
· Note: A heating unit is a smaller quantity of heat than a joule since one joule is equal to 1.4 heat units.
· Since the product of the joules-to-heat unit conversion factor (1.4) and the waveform factor for single-phase (0.71 ) is equal to 1, the following relationship is obtained:
Heat (HU) = KVp x MAS.
· Here it is seen that for single-phase operation, the heat produced in heat units is the product of the KVp and MAS. In fact, this is why the heat unit is used. In the earlier days of radiology, when most equipment was single-phase, it was desirable to calculate heat quantities without having to use a waveform factor. This was achieved by introducing a new unit, the heat unit. For three-phase, six-pulse equipment, the heat in heat units is given by
Heat (HU) = 1.35 x KVp x MAS.
· The factor of 1.35 is the ratio of the waveform factors, 0.96/0.71.
· The rate at which heat is produced in a tube is equivalent to the electrical power and is given by
Power (watts) = w x KVp x MA.
· The total heat delivered during exposure, in joules or watt-seconds, is the product of the power and the exposure time.
· Heat is normally removed from the anode through radiation through the vacuum and into the conducting oil outside the glass envelope.
· The molybdenum stem conducts very little heat to prevent damage to the metal bearings.
· In x-ray tube operation, the goal is never to exceed specific critical temperatures that produce damage.
· This is achieved by keeping the heat content below specified critical values related to the tube’s heat capacity.
· In most x-ray tubes there are three distinct areas with critical heat capacities, as shown below.
· The area with the smallest capacity is the focal spot area, or track, and is the point at which heat is produced within the tube. From this area, the heat moves by conduction throughout the anode body and by radiation to the tube housing; heat is also transferred, by radiation, from the anode body to the tube housing.
· Heat is removed from the tube housing by transfer to the surrounding atmosphere. When the tube is in operation, heat generally flows into and out of the three areas shown. Damage can occur if the heat content of any area exceeds its maximum heat capacity.
· it is seen that the safe power limit of a tube is inversely related to the exposure time. This is not surprising, since the total heat developed during exposure is the product of power and exposure time. It is not only the total amount of heat delivered to the tube that is crucial, but also the time in which it is delivered.
X-ray tube rating:
Thermal rating
• X-ray tubes are inherently inefficient; the vast majority (>99%) of energy used in the process becomes unwanted heat with barely 1% being converted to x-rays. The unwanted heat can damage the x-ray tube so x-ray machines have a rating, above which they should not be operated, in order to reduce the chance of damaging the machine.
The parameters which can be changed to preserve the tube rating are:
• kV
• mA
• exposure time
• Taking all these three into consideration, each machine has a limit measured in kW per 0.1 second within which the damage risk to the x-ray tube during use is acceptable.
• There are ways of mitigating some of the heat production in order to try and improve the length of time that the tube can be used during each examination before it must be switched off and allowed to cool. These include:
• Larger anode disc to spread the heat over a larger area
• Larger effective focal spot size
• Larger target angle (with the same effective focal spot size)
• Faster disc rotation
• Since these factors cannot be changed once the machine has been produced, in practice the thermal rating is used as a time limitation for a given kV/mA setting. While setting up an image, the kVs and mAs can be adjusted (or are set to protocols) to give a time length of how long the Xray exposure can be without causing heat stress on the machine.
The maximum allowable power decreases while Tube loading , Heat loading increase with:
• Lengthening exposure time
• Decreasing effective focal spot size (heat spread over a smaller area)
This means the heat is spread over a smaller area and the
rate of heat dissipation is reduced)
• Decreasing disk diameter (heat spread over smaller circumference and area)
• Decreasing the speed of disk rotation
• There is more efficient heat loss from a rotating anode, so the rating is higher.
• A larger focal spot causes less heating than if the beam were focused onto a
smaller area.
• A smaller anode angle has a higher heat rating.
• Rating is increased with full wave rectification.
Other factors to take into consideration are:
• By using a higher mA, the maximum kV is reduced, and vice versa.
• A very short examination may require a higher power to produce an adequate image. This must be taken into consideration as the tube may not be able to cope with that amount of heat production over such a short period of time.
Anode cooling chart
• As well as withstanding high temperatures an anode must be able to release the heat quickly too. This ability is represented in the anode cooling chart. It shows how long it takes for the anode to cool down from its maximum level of heat and is used to prevent damage to the anode by giving sufficient time to cool between exposures.
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Others
• Window: made of beryllium with aluminum or copper to filter out the soft x-rays. Softer (lower energy) x-ray photons contribute to patient dose but not to image production as they do not have enough energy to pass through the patient to the detector. Therefore, to reduce this redundant radiation dose
• To the patient, these x-ray photons are removed.
• Insulating oil: carries heat produced by the anode away via conduction.
• Filter: Total filtration must be >2.5 mm aluminum equivalent (meaning that
the material provides the same amount of filtration as a >2.5 mm thickness of
aluminum) for a >110 kV generator
Total filtration = inherent filtration + additional filtration
(removable filter)