Physics of Gray BY Tenths The X-Ray imaging system has one function, and that is to provide a constant flow of electrons that are strong enough to produce and x-ray beam to create an image. There are many different sizes and types of imaging systems but no matter what system you use, every type will have three main sections. Those sections are the control console, the high voltage generator and the x-ray tube. The x-ray tube is located in the exam room, the control console is located in an adjoining room that is separated from the exam room by a lead wall to protect the radiographer from radiation exposure.
The wall will have a window so that the patient can be viewed without having to enter the radiation area. The high voltage section will most likely be housed in an equipment cabinet along the wall close to the x-ray tube however, sometimes the exam room will have false ceilings and the generators will be installed there to be hidden. There are two divisions to the main x ray circuit, those are the primary side and the secondary side and I will talk about both of those throughout this paper. Control Console The Control Console is the section of the imaging system that most Technologists are familiar with.
It is the Primary side of the main circuit and it includes the incoming current, the exposure switch, the autotransformer and the primary winding for the step up transformer. Through the Control Console, the tech is able to control the x-ray tube current and the voltage so that the useful x-ray beam that reaches the patient has the right quantity, or the intensity of the beam and is expressed in Mr./ mass, and the right quality of x-ray beam, which refers to the penetrability and is expressed in WAP .
There are four different things that need to be controlled by the control console, the Line Compensation, kip, ma and exposure time. Also controlled by the control console are meters to monitor kip, ma and exposure time. Some consoles will also provide a meter for mass. All electric circuits that connect meters and controls on the operating console are at low voltage to minimize the possibility of hazardous shock. (Bushings) Most operating consoles are now based on computer technology and most things now are selected 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 happen 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 IV, although there are some that can operate on 1 VIVO or IV.
However, the power coming from the wall is not always consistent because the power companies cannot continuously provide iv 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 oration 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 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. (kip 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 apply 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 kip.
Most consoles will have one or two knobs that will change the taps of the autotransformer, which will adjust your major and minor kip and modern units will have an LED readout of the kip. A kip 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 kip. kip 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 millionaires (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. 1 11 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 Thermion 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 A, 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. Bushings) There are two types of resistors that I’m going to talk about in this paper, they are Saturates reactors and Rheostats. A Saturates Reactor is what is used in modern equipment today. It is a form of 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 ending. 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, mass, Electronic and Photo timers (ACE). 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 ms and the minimum for a three phase machine is 1 ms 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 raps. 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 mass 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 mass has been reached. It is designed to provide the shortest exposure and the highest safe tube 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 1 ms.
The reason they are used so much today is because they can take rapid serial exposures. Photometers, also known as Automatic Exposure Control (ACE), 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 ACE, the Tech can select where to read the radiation, the desired density, the kip and the backup mass. One advantage to the ACE 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 ACE x-ray machine system must be calibrated when it is installed. Dodo this, a phantom is used so that the ACE 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 ACE. There are two types of photo timers, a photomultiplier and an Ion chamber. Photometers 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 photometer is activated by light. Unionization chambers are the type of ACE that are used the most. The unionization chamber contains a volume of air between two metal electrodes and it is flat and radionuclide 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 CM-CACM 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 Rips. 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. 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 iv. It is then applied 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 kip, 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 saturates 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 1 love or iv but that is not enough power to operate an x-ray machine. These machines need much higher voltages of about 30,iv to 1 50,iv or Kiev-1 Kiev 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 kilovolt. The high voltage generator is not usually seen by the radiographer or the patient. They are kept in an electrical cabinet along he 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 kilovolt, 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 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 NEFF. (1 11 handouts). The step up transformer is located in the tube current section of the circuit where the kip’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 mass 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 VSP = Np/ NSA. (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 Hysterics 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 Hysterics 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 ore. (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 Ron 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 ores 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 NEFF 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’s 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. Dodo 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- yep 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 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 UT 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 rent 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 oviform 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 ms.
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 oviform 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 /60 seconds. There is one autotransformer for each phase. With the three phase, three autotransformer (one for each phase) are needed for xv 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 too common neutral point. (Wisped) 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 xv and the exposure time is as low as 1 ms. High frequency generators are increasing application in generating high voltage They are much smaller than the 60 Hz’s 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 off power supply. This is due to insufficient suppression of the alternating waveforms with in the power supplies. Wisped) 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 endings on the primary side than there are on the secondary side, where the voltage is stepped down too lower voltage. From here, it goes to the focal spot then sent to the cathode in the x ray tube.
The X ray Tube In 1895, Wilhelm Roentgen discovered x rays using a Crooked 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 legible 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./hrs at mm 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 CM squared.
When x rays are produced, they are emitted catastrophically 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