Physics of Xray

The X-Ray imaging system has one function, which is to offer a consistent circulation of electrons that are strong enough to produce and x-ray beam to create an image. There are several sizes and types of imaging systems but no matter what system you use, every type will have 3 main areas. Those sections are the control console, the high voltage generator and the x-ray tube. The x-ray tube is situated in the exam space, the control console lies in an adjacent space that is separated from the test room by a lead wall to secure the radiographer from radiation exposure.

The wall will have a window so that the client can be seen without having to go into the radiation area. The high voltage area will most likely be housed in an equipment cabinet along the wall near to the x-ray tube however, in some cases the exam room will have incorrect ceilings and the generators will be set up there to be hidden.

There are 4 various things that require to be controlled by the control console, the Line Payment, kVp, mA and direct exposure time. Also managed by the control console are meters to keep track of kVp, mA and direct exposure time. Some consoles will also supply a meter for mAs. All electric circuits that link meters and controls on the operating console are at low voltage to lessen the possibility of hazardous shock. (Bushongs) A lot of operating consoles are now based on computer technology and most things now are picked automatically.

The controls and meters are digital and the techniques can be selected with a touch screen as well as the numeric selection is also sometimes replaced by icons indicating body part, size and shape but the Techs must still know how to properly use the console, and know how to adjust the techniques manually (Line Compensator) Most imaging systems are designed to operate on 220V, although there are some that can operate on 110V or 440V. However, the power coming from the wall is not always consistent because the power companies cannot continuously provide 220v accurately.

Because of this and the fact that the hospital is using up a lot of the power being supplied, the voltage that is being provided to the x-ray unit can easily vary as much as 5%. That variation in the voltage can result in a large variation in the x-ray beam which makes achieving a high quality image inconsistent.

A line compensator measures the voltage provided and is designed to adjust the voltage coming in to a steady 220V so that high quality images are constantly produced. The older units required the techs to adjust the voltage while they looked at a line voltage meter but today in modern imaging systems the line compensator is wired to the autotransformer, therefore there is automatic line compensation so a meter is not necessary.

(kVp Selection)The power that is supplied to the x-ray machine has to go through the autotransformer first. The autotransformer is a step up transformer designed to supply voltage of different magnitudes to several different circuits of the x-ray machine including the filament circuit as well as the high voltage circuits. An autotransformer only has one winding and one iron core.

That one single winding acts as both the primary and the secondary winding. It has a certain number of connections or electric taps along its length on both sides where the connections are made. Because the autotransformer has only one winding and one core, it works on Self Induction which is where a magnetic field of a coil induces a counter Electromotive Force. This self induced voltage will oppose the applied current. The function of the autotransformer is to select the kVp.

Most consoles will have one or two knobs that will change the taps of the autotransformer, which will adjust your major and minor kVp and modern units will have an LED readout of the kVp. A kVp meter is placed across the output terminals on the transformer and it is considered a pre-reading because it reads the actual voltage from the autotransformer, not kVp. kVp is considered the quality of the x-ray beam, or in other words, the amount of penetration in the beam.

(mA Selection) The tube current, or the number of electrons crossing from the cathode to the anode per second is measured in milliamperes (mA). A separate circuit called the Filament current, is measured in Amperes (A). Connections on the autotransformer provide voltage for the filament circuit. The voltage is then delivered to the filament transformer, which is a step down transformer meaning that the voltage supplied to the filament is lower than the voltage applied to the filament transformer.

A small change in filament current produces a large change in the tube current. (111 Handout) The number of electrons emitted by the filament is determined by the filament temperature and the filament temperature is controlled by the filament current, which is measured in Amperes (A). More electrons are released when the filament current increases causing the filament to become hotter.

This is called Thermionic Emission. The tube current is monitored with an mA meter and it is connected at the center of the secondary winding of the step up transformer. Because of this, the meter is not in contact with high voltage which allows for the meter to be put on the console safely. When the tech is selecting the mA, they are actually deciding how many x rays are to reach the patient.

Precision resistors are used to reduce the voltage to a value that corresponds to the selected mA.(Bushongs) There are two types of resistors that I’m going to talk about in this paper, they are Saturable reactors and Rheostats. A Saturable Reactor is what is used in modern equipment today. It is a form of inductor where the magnetic core can be saturated by a direct electric current in the control winding. Because of the way that the power windings, the control winding and the core are arranged, the control winding is isolated from the AC power.

The power windings cancel out the AC voltages that would be induced into the control winding. They often have multiple taps to allow a small inductance to be used with a large load or a larger inductance to be used with a smaller load. This allows the current to remain constant, no matter what the load may be. A Rheostat is what controls the filament current, or the mA which is quantity or number of x rays produced. It is a variable resistor that is used to vary the amount of current flowing in a filament circuit. It is made by winding a very thinly insulated resistance wire around a barrel.

A metal slider wears away a line of the insulation so it can make electrical contact with the metal underneath. The slider is mounted on a thick metal bar which makes up part of the circuit. As the slider moves along the metal bar, it includes more of the resistance wire in the circuit. The longer the wire, the higher the resistance. As resistance is increased, the current in the filament circuit decreases. Rheostats operate on AC or DC and works on Ohm’s Law which states that the current through a conductor between two points is directly proportional to the potential difference across the two points.

(Timers) The timer circuit consists essentially of a timing device that can be varied. This device allows high tension voltage to be applied to the x ray tube. There is a timing mechanism that automatically cuts off the current after a pre-set time. X-rays are only produced when a current passes through the timer circuit. The timer circuit is separate from the other main circuits. It consists of electronic devices whose purpose is to make or break the high voltage across the tube on the primary side of the high voltage section.

Throughout the history of x-rays, there have been many types of timers but today, all timers are electronic timers of different types. The timers I’m going to talk about are Mechanical, Synchronous, Impulse, mAs, Electronic and Photo timers (AEC). The mechanical timers are an inexpensive and very simple timer that have a clock like mechanism. The operator turns the dial to the desired exposure time and as it unwinds, the exposure is made. The minimum exposure time for a single phase machine is 8ms and the minimum for a three phase machine is 1ms because they are very unreliable.

Therefore, these timers are only used in portable x ray machines or dental x ray machines because they don’t require short, accurate exposures. The Synchronous timers are driven by a synchronous motor which run at about 60rps. The minimum exposure time is 1/60 sec. It is not accurate below 1/20 sec. This timer can only be used for a single exposure because they have to be reset after each exposure.

The Impulse timer also operates on a synchronous motor but at a much higher speed. It provides shorter exposures, as low as 1/120 of a second to as high as 1/5 of a second. This timer is much more accurate than the synchronous timer because it starts and stops the current at the zero point of the AC cycle. The mAs timer is the only timer that is located on the secondary side of the high voltage section because it monitors the actual tube current. It monitors the product of the mA and time and stops exposing when the desired mAs has been reached.

It is designed to provide the shortest exposure and the highest safe tube current. Electronic timers are used in modern equipment today. They are the most sophisticated, the most complicated and the most accurate timer. It consists of complex circuit based on the time required to charge a capacitor through a variable resistor. This timer allows for a wide range of time intervals that can be selected, as small as 1ms.

The reason they are used so much today is because they can take rapid serial exposures. Phototimers, also known as Automatic Exposure Control (AEC), measure the amount of radiation that reach the image receptor and stops exposing automatically when sufficient radiation needed to produce the right amount of density on the IR. With AEC, the Tech can select where to read the radiation, the desired density, the kVp and the backup mAs.

One advantage to the AEC is the backup timer, it is a safety factor that will stop exposure if for some reason it doesn’t stop when it is supposed to . An AEC x-ray machine system must be calibrated when it is installed.

To do this, a phantom is used so that the AEC can be adjusted for the range of intensities required for quality images. The calibration is usually done by the service engineer. Specific anatomy must be positioned above the appropriate chamber, therefore patient positioning is the most important aspect when using AEC. There are two types of photo timers, a photomultiplier and an Ion chamber. Phototimers consist of a fluorescent screen that convert x rays to light which is directed to the photomultiplier.

The photomultiplier converts light into electrons and electrons are then multiplied in the tube. It is located behind the film and the fluorescent screen and the phototube is activated by light. Ionization chambers are the type of AEC that are used the most. The ionization chamber contains a volume of air between two metal electrodes and it is flat and radiolucent so that it will not interfere with the image.

It is located between the patient and the Image Receptor and measures the intensities of the radiation. Radiation is measured at either the center of the film or off to the sides. The center cell is used for most images and the outside cells are used for chest, abdomen and ribs.

(Spin Top Test) Spin top tests are used to check x-ray timers in single phase machines only. It is a flat, heavy metal disk with a hole in the perimeter. The disk is about 5cm-25cm in diameter. A single phase x-ray machine emits x-rays in pulses. A half wave voltage produces 60 pulses/second and full wave voltage produces 120 pulses/second. X-rays are produced by each of these pulsations. The developed film will show a dot for each pulsation that occurred during the exposure.

Three phase x-ray machines do not use pulsed radiation because the output is constant. To check the timers on these machines, a physicist will use a powered synchronous spin top that rotates at 1rps. This test is measured in degrees. A half second equals a 180 degree image, a quarter of a second equals a 90 degree image and a 1 second equals a 360 degree image.

Flow of current through the console
The flow of current through the control console of an x-ray machine starts with the power coming in from the wall outlet which receives its electricity from a source outside the building. The current goes to primary side of the control console and then to the line compensator which maintains the voltage at a steady 220v. It is then supplied to the primary side of the autotransformer.

The autotransformer steps up the voltage by electrostatic self induction which makes the input twice the voltage value. The taps on the secondary side of the autotransformer are what are used to select the major and minor kVp, they are pre-read by the kilovolt meter on the secondary side. After the electricity leaves the autotransformer, it is divided into two separate currents, the tube current and the filament current.

The voltage that is carried through the tube current then goes to the step up transformer on the primary side of the high voltage section. The voltage that is carried through the filament current is carried to the mA selector, which is a rheostat that has a series of resistor coils although today a saturable reactor is preferred. It has an iron core that is saturated with magnetic flux.

The current is then passed to the step
down transformer which is also in the primary side of the high voltage section. When the exposure switch is pressed, the current is passed to the high voltage section. The exposure timer is what regulates how much of an exposure is made. Its purpose is to make or break the high voltage across the x ray tube.

High Voltage Generators
When power is supplied to a building, it is usually supplied at 110v or 220v but that is not enough power to operate an x-ray machine. These machines need much higher voltages of about 30,000v to 150,000v or 30kv-150kv in order to throw the electrons across the tube at the proper speed. That is why a high voltage generator is a major component of the x ray machine, its main purpose is to convert the low supply voltage into the desired kilovoltage.

The high voltage generator is not usually seen by the radiographer or the patient. They are kept in an electrical cabinet along the wall or if false ceilings are available, they are sometimes put there so they are out of sight. The high voltage section of the x ray machine has three main parts. Those are the high voltage step up transformer, the filament transformer (step down transformer) and rectifiers. All three of these components are immersed in oil for electrical insulation.

(Step up transformer) The high voltage transformer is a step up transformer which means that the secondary side, which is measured in kilovoltage, is higher than the primary side, which is measured in voltage, because there are more windings on the secondary coil than on the primary coil so the function of the step up transformer is to convert the incoming volts on the primary side to kilovolts on the secondary side. The ratio of windings on the primary side and on the secondary side is called turns ratio.

The turns ratio for most x ray high voltage transformers is between 500 and 1000 Transformers only operate on alternating current (AC) and the wavelengths on both sides, primary and secondary, is sinusoidal and the only difference between the two is their amplitude which is from the peak to the valley. There are three parts to the high voltage step up transformer: the primary coil, the secondary coil and the iron core.

In transformers, the primary coil and the secondary coil are wrapped around an iron core and unlike the autotransformer that operates on self induction, step up/step down transformers operate on mutual induction. Mutual induction is the varying alternating current flow in the electromagnet crates a varying magnetic field, so when it passes through the primary coil an induced current will flow through the secondary coil.

Alternating coil flows through the primary coil and sets up a magnetic field around the coil, the changing magnetic flux cuts or links with the secondary coil, inducing in it an alternate EMF. (111 handouts). The step up transformer is located in the tube current section of the circuit where the kVp’s are selected and the intensity or penetration of the beam is determined.

(Step Down Transformer) In the step down transformer, the primary coil will have more windings than the secondary coil which will result in lower volts but more amps. The step down transformer is located in the filament circuit section of the circuit after the mAs has been selected which determines the number of x rays to be emitted. The transformer law describes how electric current and voltage change from the primary coil to the secondary coil. The formula for this law is Vs/Vp = Np/Ns.

(Energy Losses) In a perfect world, transformers would be 100% effective but in reality, they are only 90%-95% effective due to energy losses. In most cases, the power lost is usually in the form of heat. There are three types of losses of power in transformers: Copper losses, Eddy current losses and Hysteresis losses. Copper losses are due to resistance in the coils. Heat is produced by the electrical currents in the conductors of the windings. This type of loss can be reduced by using copper wire of adequate diameter. A thicker wire creates less energy waste.

Eddy current losses are swirling currents in the core that are caused by alternating magnetic flux set up in the core by alternating current which produces heat. Eddy current losses can be reduced by making the core of a stack of laminated silicon steel plates. These plates are electrically insulated from each other and increase electrical resistance of core which reduces the size of the eddy currents. The third type of loss is Hysteresis losses. This loss is caused by constant rearrangement of the magnetic fields which produce heat in the core. This can be reduced by using a laminated silicon steel core.

(Types of Transformers) A transformer is a device that changes an alternating current from low voltage to high voltage or from high to low. They transfer electrical energy from one circuit to another without using any moving parts or any electrical contact between the two circuits. They operate only on an alternating current and work off of mutual induction.

There are a variety of different transformers that are made for different purposes and even though the designs are different, they all are similar in their purpose. The transformers that I am going to talk about in this paper are: Closed core, Shell type and autotransformers. Closed core transformers have an iron core that is not a single piece but made up of layers of laminated iron. Layering helps reduce energy loss which results in greater efficiency. It is a closed ring with which two heavily insulated coils are wrapped around it.

This provides a continuous path for magnetic flux so only a small fraction of power is lost by leakage. Shell type transformers are the most advanced and the most used type of transformer. This type of transformer confines more of the magnetic field lines of the primary winding because the secondary winding is wrapped around it so it is technically two closed cores which makes this type more efficient than the closed core.

The autotransformer has one iron core and only one winding of wire around it. This single winding serves as both the primary and the secondary winding. The autotransformer is the only one that operates on self induction which is where the magnetic field of a coil induces a counter EMF in the coil itself. This self induced voltage will oppose the applied current.

(Rectifiers) When there is a current coming in from the wall outlet, it is coming in at 60 Hz of alternating current. That current will change directions 120 times per second. The x ray tube, however, requires a direct current which means that electrons only flow in one direction. X rays are produced by the acceleration of electrons from the cathode to the anode and cannot be produced in the opposite direction. Because the cathode assembly is constructed so that it cannot withstand a lot of heat, reversal of the flow of electrons would be bad for the x ray tube.

Electron flow should only be in the cathode to anode direction, therefore, the secondary voltage of the high voltage transformer has to be rectified which means that the incoming alternating current must be converted to direct current. To do this, a device called a rectifier is needed. Rectification is accomplished with devices called diodes, which is an electronic device that contains two electrodes.

These electrodes are located between the secondary coil of the transformer and the x ray tube and they only allow the flow of electrons in one direction. Currently, rectifiers are made of silicon but they used to be vacuum tubes called valve tubes that were similar to the x ray tube. The advantage of silicon rectifiers over the valve tubes are its compact size, there is no filament, it lasts longer, it has low reverse current and a low forward voltage drop. Conductors such as metal or water allow the free flow of electrons and insulators, such as plastic or rubber, inhibit the flow of electrons.

Semiconductors such as silicon, are between the two in their ability to conduct electricity. Semiconductors are classed into two types: N-type and P-type. N-type semiconductors have loosely bound electrons that are free to move and P-type semiconductors have spaces called holes, where there are no electrons. These holes are just a space between two objects and can move as easily as electrons. A P-N junction is formed when a tiny crystal of N-type material is placed in contact with P-type material.

If a higher potential is placed on the p side of the junction, the electrons and holes will move towards the junction and eventually move across it causing an electrical current. If a positive potential is placed on the n side of the junction, the electrons and the holes will be swept away from the junction which will result in no electrical current passing through the p-n junction. Because a solid state p-n junction will only conduct electricity in one direction, this is called a solid state diode.

(Waveforms) There are three types of rectifications: self, half wave and full wave. In Self rectification, there are no diodes and the x ray tube itself will work as the rectifier but when one or two diodes are placed in the circuit that stops the negative flow of electrons it is called Half wave rectification. During half wave rectification, the inverse voltage is removed from the supply to the tube. The voltage is not allowed to swing negatively during the negative half of its cycle resulting in no electrical current.

However, during the positive cycle, there is a current being passed through the x ray tube. As a result of the half wave cycle, there are a series of positive pulses separated by gaps when the negative current is not conducted. This is a rectified current because the electrons are only flowing in one direction. Half wave rectification produces 60 pulses per second.

Because half wave rectification only uses half of the power being supplied and also requires twice the exposure, it is not ideal. Therefore, it is possible to have a circuit that will rectify the entire alternating waveform. This is called Full wave rectification. Full wave rectification is used in almost all stationary x ray machines and contain at least 4 diodes. During this rectification, the negative half cycle is reversed so that the anode is always positive.

There are no gaps in the output waveform and the input waveform is rectified into usable output. This results in pulsating direct current. The advantage of using full wave rectification over half wave is the exposure time is cut in half which increases the tube rating or heat load capacity. Full wave rectification produces 120 pulses per second and the minimum exposure time is 8ms.

The self, half wave and full wave rectification waveforms that were previously discussed are all produced by single phase which result in a pulsating x ray beam. Single phase power uses just one autotransformer and has one single phase on the waveform that goes from zero to the maximum positive potential back to zero then to a maximum negative potential and back to zero again. Because the x rays produced during single phase waveforms have low energy and little penetrability due to their near zero values, they are of little diagnostic value.

One way they have figured out how to get better results is to use three phase power. Three phase power generates three simultaneous voltage waveforms that are out of step with one another, this causes nearly constant high voltage. Compared to the 2 pulses of the single phase power, the three phase power has six pulses per 1/60 seconds. There is one autotransformer for each phase. With the three phase, three autotransformers (one for each phase) are needed for kV selection.

They are arranged in either star or delta configuration. A delta transformer winding is connected between phases of a three phase system. A star transformer connects each winding from a phase wire to a common neutral point. (Wikipedia) Three phase circuits have all delta wound primary coils but differ in form of secondary. The ratings of three phase power are 1600 mA, 150 kV and the exposure time is as low as 1ms.

High frequency generators are increasing application in generating high voltage for may imaging systems. One advantage to the high frequency generator is its size. They are much smaller than the 60 Hz generators and they produce a near constant voltage waveform which improves the image quality and lowers patient dose.

High frequency generators were first used in portable x ray machines but now they are used in most modern equipment today. High frequency voltage generators us inverter circuits, which are high speed switches, also known as choppers. These convert direct current into a series of square pulses.

(Voltage Ripple) Another way to characterize voltage waveforms is by Voltage Ripple. Voltage ripple is the small unwanted residual periodic variation of the direct current at the output stage of a power supply. This is due to insufficient suppression of the alternating waveforms with in the power supplies. (Wikipedia) A larger ripple means less effective filtering and a smaller ripple means more effective filtering. Single phase power has 100% voltage ripple meaning that the voltage varies from zero to its maximum value.

The three phase six pulse power, which has 6 diodes and 1 star and 2 delta, has a 14% ripple so the voltage that is supplied never falls below 86% of the max value. An improvement was made in the three phase power using 12 pulses instead of 6. The three phase twelve pulse, which has 12 diodes, 1 star and 2 delta, has only a 4% voltage ripple and so the voltage does not fall below 96% of its max value.

High frequency generators only have a 1% voltage ripple resulting in better x ray quality and quantity which is the biggest advantage in the voltage with the least amount of ripple. When the voltage ripple is low, it increases radiation quality because fewer electrons are passing from the cathode to the anode, producing low energy x rays.

Flow of current through the High Voltage Generator
Once the electricity leaves the control console, in the tube current part, the current leaves the secondary side of the autotransformer and goes to the primary side of the high voltage transformer. It goes through the step up transformer where the where the voltage is stepped up from volts to kilovolts due to the fact that there are more windings on the secondary side than there are on the primary side. After the current leaves the step up transformer, it passes through the secondary side of the high voltage transformer to the rectifiers, which change the alternating current to direct current that is needed in the tube.

There are two types of solid state diodes, P-type and N-type semiconductors that make the current flow in one direction. On the secondary side of the rectifier is the mA meter which measures the amperage. After the current has been changed to direct current, it goes to the cathode in the x ray tube. In the filament current part, the alternating current goes through the mA selector in the control console and is then carried to the primary side of the filament transformer, which also works by electromagnetic mutual induction.

In this circuit, the voltage goes through the step down transformer, meaning that there are more windings on the primary side than there are on the secondary side, where the voltage is stepped down to a lower voltage. From here, it goes to the focal spot selector which picks the filament to be used to boil off electrons and the current is then sent to the cathode in the x ray tube.
The X ray Tube
In 1895, Wilhelm Roentgen discovered x rays using a Crookes Tube. However, in 1913, William Coolidge made improvements to the tube so today we use a Coolidge tube to produce x rays. The x ray tube is a part of the imaging system that is not seen by the technologists. That is because it is contained within a protective housing, making it inaccessible.

There are two main parts to the x ray tube. Those are the Cathode and the Anode. Each one of these is considered an electrode and because there are two electrodes in the Tube, it makes it a diode. The outside of the x ray tube has three parts: the support structure, the protective housing and the glass
or metal enclosure. The inside is where the Cathode and Anode are contained.

(Support System) The first part of the external components I want to discuss is the support system. There are three types of support systems for x ray tubes: Ceiling support, Floor to Ceiling support and C-arm support. Ceiling support is the most used support system out there. It consists of two perpendicular sets of ceiling mounted rails, allowing a longitudinal and transverse movement of the x ray tube. The floor to ceiling support system has just a single column with rollers at the end.

These are attached to the ceiling mounted rails and the floor mounted rails allowing the tube to slide up and down as the column rotates. A different type of this support system has the column placed on a single floor supported system with one or two floor mounted rails. The C-arm support system gets there name because they are shaped like a “C”. These support systems are mounted to the ceiling and provide flexible tube positioning.

(Protective Housing) The second part of the external components is the protective housing. This protective housing is what is around the x ray tube. It is lead lined to prevent excessive radiation leakage. Leakage radiation is considered the x rays that escape from the protective housing resulting in unnecessary exposure to the patient but plays no part in achieving diagnostic information. If the housing is designed properly, it will reduce the leakage radiation to less than 100 mR/hr at 1m when it is operated at maximum conditions. The x rays that do achieve diagnostic information are called the useful beam. These are the x rays that are emitted through a window that is 5cm squared.

When x rays are produced, they are emitted isotropically which means that they are emitted with equal intensity in all directions. Another function of the housing is to prevent electric shock to the patient and to the radiographer by being designed with high voltage receptacles. The protective housing also provides mechanical support for the x ray tube, protecting it from damage due to rough handling. Some contain oil around the tube that acts as an insulator to prevent electric shock and also as a thermal cushion to dissipate the heat. Some also contain a cooling fan to cool the machine or the tube.

When the tube
is heated the oil expands but if there is too much expansion, it activates a microswitch which will prevent the tube from being used again until it is cooled.

(Envelope) The x ray tube is an electronic vacuum that creates efficient x ray production and longer tube life. The vacuum is an empty space that is air tight, allowing electrons to move freely with in the tube. There are two types of envelopes: a glass envelope and a metal envelope.

The glass envelope is made of Pyrex glass which allows it to be able to with stand the large amount of heat that is produced. This enclosure maintains a vacuum inside the tube. If just a little bit of gas gets in the enclosure, the electron flow from the cathode to the anode is reduced which produces more heat and fewer x rays. As the glass enclosure ages, some of the tungsten vaporizes and coats the inside of the glass enclosure. When this happens, it can change the properties of the tube which will allow the tube current to stray and interact with the glass enclosure resulting in arcing and tube failure.

There has been an improvement in the tube that is a metal envelope instead of glass. Almost all high capacity x ray tubes now use metal enclosure instead of glass. This is because the metal enclosure maintains a constant electric potential between the electrons of the tube current and the enclosure which results in longer life of the tube. (Cathode) The x ray tube contains two electrodes: the cathode and the anode. The cathode is on the right side and it is the negative side of the x ray tube. It contains two main parts, the filament and a focusing cup.

The filament is a coil of wire that is about 2mm in diameter and 1 or 2 cm long. When the tube filament is heated, it emits electrons. When the current through the filament is high, the outer shell electrons are boiled off and removed from the filament. This “boiling off” is called Thermionic Emission. The filament is made of thoriated tungsten, has an atomic number of 74 which results in high efficiency production and high energy x rays and provides for a higher thermionic emission than other metals. It has a melting point of 3410 degrees Celsius so it will not burn out quickly and it doesn’t vaporize quickly.

The 1% to 2% of thorium that is added to the tungsten filament enhances the efficiency of thermionic emission and gives it a longer life. Electrons carry a negative charge and which causes them to repel each other. Therefore, when they are emitted from the filament, the electrons are in the vicinity of the filament before they are accelerated to the anode causing a cloud of electrons to form around the filament. This cloud is called a space charge. This space charge makes it difficult for electrons to be emitted by the filament because of electrostatic repulsion, this is called space charge effect.

The focusing cup is a metal cup in which the filament is imbedded. The electrons that are thrown from the cathode to the anode are negatively charged and because of electrostatic repulsion, the electron beam tends to spread out, some even missing the anode completely. The focusing cup is also negatively charged and it electrostatically confines the electron beam to a small area of the anode. How effective the focusing cup is determined by its size and shape, by its charge, the filament size and shape and the filament in the focusing cup. (Anode) The anode is on the left side and it is the positive side of the tube.

This is the part of the tube where the accelerated electrons move to after the kV has been applied to the tube. It has three main functions in an x ray tube: it is an electrical conductor, a thermal dissipater and mechanical support.

The anode serves as an electrical conductor because it receives electrons emitted by the cathode and conducts them through the tube to the connecting cables and back to the high voltage generator. It must be a good thermal dissipater because when the electrons that are being thrown from the cathode to the anode, 99% of them are converted to heat which needs to be dissipated quickly. The anode also provides support for the target. There are two types of Anodes: Rotating and Stationary

(Stationary and Rotating Anodes) Stationary anodes were introduced in 1936. They have a lower heat capacity and are used when high tube current and power are not required such as in dental x ray imaging systems and some portable machines. All other x ray machines use a rotating anode because they must be capable of producing high intensity x ray beams in a short amount of time.

Rotating anodes allow the electron beam to interact with a larger target area so all of the heating of the anode is not just in one small spot like it is in the stationary anode. A rotating anode is a disk with a diameter of about 3-5 inches, the larger the disk, the more of a workload there will be. Most rotating anodes rotate 3,600 rpms and the high capacity tubes rotate at speeds up to 10,000 rpms. The heat capacity can be increased with higher rotation speeds.

The stem of the anode is usually made of molybdenum because it is a poor heat conductor and it is narrow to reduce the thermal conductivity. It is located between the anode and the rotor. When the rotor mechanism of a rotating anode fails, it becomes over heated and pits or cracks causing tube failure. A rotating anode is driven by an electromagnetic induction motor. This motor consists of two main parts: the stator and the rotor.

These two parts are separated from each other by the glass or metal enclosure. The part that is outside the enclosure is called the stator which consists of a series of electromagnets that are equally spaced around the neck of the tube. The part inside the enclosure is the rotor. It is made up of bars of copper and soft iron fabricated into one mass. When the tech pushes the exposure button on the console, there will be a short delay before the exposure is made. During this time, the rotor is accelerated to the appropriate rpm while the filament is being heated.

After the exposure has been made, the rotor slows down and stops with in one minute because the induction motor is put into reverse. (Target) The target is area of the anode where the electrons strike after leaving the cathode. In the stationary anode the target is embedded in the copper anode and does not move, therefore the electrons will strike the target in the same place over and over causing it to wear down quickly. In rotating anode the entire disk is the target which gives the beam a larger target area to interact with.

The material used in targets is Tungsten because of its high atomic number of 74 and its high melting point of 3,400 degrees Celsius. Adding Rhenium to the tungsten gives it added strength so that it can with stand the stresses of high speed rotation and the effects of repetitive expansion and contraction. (Line Focus Principal) When x rays are emitted, they strike the focal spot on the target.

The focal spot is
the actual x ray source. Smaller focal spots have better spatial resolution of the image but when the size of the focal spot decreases the heat of the target is put onto a smaller area. A design to allow a large area for heating while maintaining a small focus spot is called Line Focus Principal. By angling the target it makes the effective area of the target much smaller than the actual area of electron interaction. Actual focal spot size is the area of the target that is being hit with electrons from the filament.

The effective focal spot size is the imaginary line that can be drawn based on the actual focal spot size versus the angle of the anode. This is the part that is projected onto the patient and the IR creating an image. One advantage to angling the target is that it helps dissipate heat, while creating better detail. Diagnostic x ray tubes have target angles ranging from 5 degrees to 20 degrees. When the angle is made smaller, the effective focal spot size will also be smaller.

Flow of current through the x ray tube
When the radiologic technologist pushes the exposure button on the control console there is a short delay before the exposure is made. This delay is to allow the rotor to accelerated to the desired RPM which will cause the anode to spin. In the tube circuit, the rotor is a shaft made of bars made of copper and soft iron and it is located on the inside of the enclosure of the tube.

On the outside of the exposure is the stator, which consists of a series of electromagnets that are equally spaced around the neck of the tube. The stator is what causes the rotor to spin by the principle of electromagnetic induction. In the filament circuit, during the spinning of the rotor and anode, the filaments on the cathode are heating up.

Once the filaments are heated sufficiently, the electrons are boiled off by a process called Thermionic emission. This causes a small rise in the filament current results in a large rise in the tube current. After the electrons have been boiled off of the cathode, they continue to hang around. This cloud of electrons is called a space charge. Those electrons are negatively charged and because of this, electrostatic repulsion takes place, meaning they are repelled by the cathode.

This process is known as a space charge effect. When the kVp in the tube circuit is applied to the tube, the electrons in the space charge will be thrown across to the anode. The focusing cup, which is what contains the filament, is also negatively charged so it electrostatically confines the electron beam to a small area of the anode. The anode is positively charged which attracts the electrons to it. When the electrons are thrown from the cathode to the anode, they hit the target.

The anode is angled which causes the target to have a larger area, therefore resulting in a smaller effective focal spot size. When the electrons that are thrown to the anode hit the target, they are redirected down to the patient, forming an image on the IR. The area that reaches the patient called line focus principle. After the current goes to the cathode, because it is now direct current, it is redirected back to the high voltage section.

Summary of the flow of current through the X-ray Imaging System The flow of current through the control console of an x-ray machine starts with the power coming in from the wall outlet which receives its electricity from a source outside the building. The current goes to primary side of the control console and then to the line compensator which maintains the voltage at a steady 220v. It is then supplied to the primary side of the autotransformer.

The autotransformer steps up the voltage by electrostatic self induction which makes the input twice the voltage value. The taps on the secondary side of the autotransformer are what are used to select the major and minor kVp, they are pre-read by the kilovolt meter on the secondary side. After the electricity leaves the autotransformer, it is divided into two separate currents, the tube current and the filament current. The voltage that is carried through the tube current then goes to the step up transformer on the primary side of the high voltage section.

The voltage that is carried through the filament current is carried to the mA selector, which is a rheostat that has a series of resistor coils although today a saturable reactor is preferred. It has an iron core that is saturated with magnetic flux. The current is then passed to the step down transformer which is also in the primary side of the high voltage section. When the exposure switch is pressed, the current is passed to the high voltage section. The exposure timer is what regulates how much of an exposure is made. Its purpose is to make or break the high voltage across the x ray tube.

Once the electricity leaves the control console, in the tube current part,
the current leaves the secondary side of the autotransformer and goes to the primary side of the high voltage transformer.

It goes through the step up transformer where the where the voltage is stepped up from volts to kilovolts due to the fact that there are more windings on the secondary side than there are on the primary side. After the current leaves the step up transformer, it passes through the secondary side of the high voltage transformer to the rectifiers, which change the alternating current to direct current that is needed in the tube.

There are two types of solid state diodes, P-type and N-type semiconductors that make the current flow in one direction. On the secondary side of the rectifier is the mA meter which measures the amperage. After the current has been changed to direct current, it goes to the cathode in the x ray tube. In the filament current part, the alternating current goes through the mA selector in the control console and is then carried to the primary side of the filament transformer, which also works by electromagnetic mutual induction.

In this circuit, the voltage goes through the step down transformer, meaning that there are more windings on the primary side than there are on the secondary side, where the voltage is stepped down to a lower voltage. From here, it goes to the focal spot selector which picks the filament to be used to boil off electrons and the current is then sent to the cathode in the x ray tube section.

When the radiologic technologist pushes the exposure button on the control console there is a short delay before the exposure is made. This delay is to allow the rotor to accelerated to the desired RPM which will cause the anode to spin. In the tube circuit, the rotor is a shaft made of bars made of copper and soft iron and it is located on the inside of the enclosure of the tube.

On the outside of the exposure is the stator, which consists of a series of electromagnets that are equally spaced around the neck of the tube. The stator is what causes the rotor to spin by the principle of electromagnetic induction. In the filament circuit, during the spinning of the rotor and anode, the filaments on the cathode are heating up. Once the filaments are heated sufficiently, the electrons are boiled off by a process called Thermionic emission.

This causes a small rise in the filament current results in a large rise in the tube current. After the electrons have been boiled off of the cathode, they continue to hang around. This cloud of electrons is called a space charge. Those electrons are negatively charged and because of this, electrostatic repulsion takes place, meaning they are repelled by the cathode.

This process is known as a space charge effect. When the kVp in the tube circuit is applied to the tube, the electrons in the space charge will be thrown across to the anode. The focusing cup, which is what contains the filament, is also negatively charged so it electrostatically confines the electron beam to a small area of the anode.

The anode is positively charged which attracts the electrons to it. When the electrons are thrown from the cathode to the anode, they hit the target. The anode is angled which causes the target to have a larger area, therefore resulting in a smaller effective focal spot size. When the electrons that are thrown to the anode hit the target, they are redirected down to the patient, forming an image on the IR.

The area that reaches the patient called line focus principle. After the current goes to the cathode, because it is now direct current and can only go one direction, it is redirected back to the high voltage section.

References

Website Article

http://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/EquipmentMaterials/xrayGenerators.htm

http://faculty.mwsu.edu/radsci/gary.morrison/RADS_1513/Chapters_5&39/X-ray_Circuits_and_Equipment.pdf

http://www.wikiradiography.com/page/Physics+of+the+X-Ray+Tube

http://en.wikipedia.org/wiki/

Books
Sinclair Tousey, Medical electricity and Rontgen rays (3rd Edition) W. B. Saunders Co. 1921