Physics of Xray Essay Example
Physics of Xray Essay Example

Physics of Xray Essay Example

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  • Pages: 18 (4814 words)
  • Published: August 9, 2017
  • Type: Essay
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The X-Ray imaging system consists of various sizes and types of imaging systems. However, regardless of the system used, every type will have three main sections: the control console, the high voltage generator, and the X-ray tube. These sections work together to provide a constant flow of electrons strong enough to produce an X-ray beam for imaging.

The x-ray tubing is located in the test room while the control console is adjacent to it, separated by a lead wall for radiation protection. The wall has a window for viewing the patient without entering the radiation area. The high electromotive force subdivision is typically housed in an equipment cabinet next to the x-ray tubing, but in some cases, it may be concealed in false ceilings. The chief x beam circuit consists of two divisions, namely the primary side and the secondary side, which will be discussed in


detail in this paper.

Control Console

The Control Console is the part of the imaging system that most Engineers are familiar with.

The main circuit's primary side consists of the incoming current, the exposure switch, the autotransformer, and the primary winding for the step-up transformer. With the Control Console, the technician can regulate the x-ray tube current and voltage to ensure that the useful x-ray beam reaching the patient has the correct measure or strength (expressed in mR/mAs) and the proper quality (expressed in kVp). The Control Console is responsible for controlling four distinct factors: Line Compensation, kVp, mA, and exposure time.

In addition to being operated by the control console, there are meters for monitoring kVp, ma, and exposure time. Some consoles also have a meter for ma. To minimize the risk of

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electric shock, all electric circuits connecting the meters and controls on the operating console are at low voltage. Modern operating consoles utilize computer technology, with most functions now being selected automatically. The controls and meters are digital, and techniques can be chosen using a touch screen interface or icons representing body parts, size, and shape. However, technicians still need to know how to correctly operate the console.

and understand how to manually adjust the techniques (Line Compensator). Most imaging systems are designed to run on 220V, although some can run on 110V or 440V. However, the power supply from the wall is not always consistent because power companies are unable to constantly provide an accurate 220V. Additionally, as the hospital uses a significant amount of the power being supplied, the voltage provided to the x-ray unit can easily fluctuate by as much as 5%. This variation in voltage can result in a significant fluctuation in the x-ray beam, leading to inconsistent production of high-quality images. To address this issue, a line compensator is used to measure the provided voltage and adjust it to a steady 220V, ensuring constant production of high-quality images. In older units, technicians had to manually adjust the voltage while observing a line voltage meter. However, in modern imaging systems, the line compensator is connected to the autotransformer, enabling automatic line compensation and eliminating the need for a meter.

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The autotransformer, within the kVp Selection, is responsible for supplying power to the x-ray machine. This transformer is designed to provide different magnitudes of electromotive force to various circuits, including the fibril circuit and high electromotive force circuits. Unlike other transformers, the

autotransformer only contains one twist and one iron nucleus. This single twist acts as both the primary and secondary twist. On both sides of the autotransformer, there are a certain number of connections or electric taps where the connections are made. Due to its single twist and nucleus, the autotransformer operates on Self Induction - a magnetic field inducing a counter Electromotive Force.

The ego-induced electromotive force works against the applied current. The autotransformer's purpose is to select the kVp. Many consoles have one or two knobs that adjust the levels of the autotransformer, which determine the major and minor kVp. Modern units have an LED display for the kVp. A kVp meter is placed across the output terminals of the transformer and it provides a pre-reading, measuring the actual electromotive force from the autotransformer rather than kVp. kVp represents the quality of the x-ray beam, or in other words, the amount of penetration in the beam.

(mA Selection) The tubing current, also known as the flow of electrons from the cathode to the anode per second, is measured in mAs (ma). The filament current, a separate circuit, is measured in Amperes (A). The autotransformer's connections provide electrical force for the filament circuit. This electrical force is then delivered to the filament transformer, which is a step-down transformer. This means that the electrical force supplied to the filament is lower than the force applied to the filament transformer. A small change in filament current results in a large change in the tubing current.

The amount of negatrons emitted by the fibril depends on the filament temperature, which is regulated by the filament current measured in Amperes (A).

Increasing the filament current causes more negatrons to be released, resulting in the fibril becoming hotter. This phenomenon is known as Thermionic Emission. The measure of tubing current is monitored using an ma metre, which is connected at the center of the secondary twist of the measure up transformer.

The metre remains safe on the console as it is not in contact with high voltage. The tech determines the number of x beams required for the patient while selecting the ma. Precision resistances are employed to reduce the voltage to the desired value matching the chosen ma. This paper discusses two types of resistances: Saturable reactors and Rheostats. Modern equipment utilizes Saturable Reactors.

It is an indicator of inductance, whereby a direct electric current can saturate the magnetic nucleus in the control twist. Due to the arrangement of the power twists and the nucleus, the control twist remains isolated from the AC power. The power twists effectively negate the AC electromotive forces that would otherwise be induced into the control weaving. These power twists often have multiple lights-outs, enabling a small induction to be used with a large burden or a larger induction to be used with a smaller burden. This ensures that the current remains constant, regardless of the burden.

A Rheostat is a variable resistance that controls the filament current or the measure of x beams produced. It is used to change the total current flowing in a circuit. It is made by weaving a thinly insulated opposition wire around a barrel. A metal skidder removes a line of the insulation to make electrical contact with the metal underneath. The skidder is mounted on a thick

metal saloon, which is part of the circuit, and moves along it.

The circuit includes more of the opposition wire, resulting in higher opposition and a decrease in current in the fibril circuit. Rheostats can operate on both AC or DC and they adhere to Ohm's Law, which states that the current through a conductor between two points is directly proportional to the potential difference across those two points. The timer circuit is primarily composed of a timing device that can be adjusted.

This device utilizes high voltage to energize the x-ray tube. It features an automated timer that automatically interrupts the current after a predetermined time. X-rays are only produced when the current passes through the timer circuit. The timer circuit is independent from the other main circuits.

It comprises electronic devices designed to create or interrupt the high voltage across the tube on the primary side of the high voltage division. Over time, various types of timers have been used in X-rays. However, currently, all timers are electronic timers of different kinds. The timers I am going to discuss are Mechanical Synchronous Impulse, Electronic, and Photo timers (AEC).

The mechanical timers are inexpensive and very basic timers that use a clock-like mechanism. The operator adjusts the dial to the desired exposure time, and as it unwinds, the exposure is made. The minimum exposure time for a single-stage machine is 8ms, and for a three-stage machine it is 1ms due to their unreliability. As a result, these timers are only used in portable ten beam machines or dental ten beam machines that do not require short, precise exposures. The Synchronous timers are powered by a synchronous

motor running at approximately 60rps.

The minimum exposure clip is 1/60 sec, but it is not accurate below 1/20 sec. Additionally, this timer can only be used for one exposure at a time as it needs to be reset after each one. On the other hand, the Impulse timer operates on a synchronous motor with a faster speed. It allows for shorter exposures ranging from 1/120 to 1/5 of a second. This timer is more precise than the synchronous timer as it begins and ends the current at the zero point of the AC rhythm.

The mAs timer is the solitary timer situated on the secondary side of the high voltage division, tasked with monitoring the actual tube current. It observes the product of mA and time, ceasing exposure when the desired mA has been achieved. Its purpose is to provide the shortest exposure time and the highest safe tube current. In present-day equipment, electronic timers are used, as they are the most sophisticated, intricate, and accurate timers available. These timers consist of complex circuits that rely on the time needed to charge a capacitor through a variable resistance. This type of timer allows for a wide range of selectable time intervals, even as little as 1ms.

The reason why phototimers, also known as Automatic Exposure Control (AEC), are used so extensively today is because they have the capability to take quick consecutive exposures. AEC measures the amount of radiation that reaches the image receptor and automatically stops exposing once the required amount of radiation needed to produce the correct density on the IR is reached. With AEC, the technician has the ability to select where to measure

the radiation, the desired density, the kVp, and the backup ma. One advantage of AEC is the backup timer, which acts as a safety feature and stops exposure if, for some reason, it does not stop when it is supposed to.

An AEC x-ray machine system needs to undergo calibration upon installation. This involves using an apparition to adjust the AEC and ensure it can produce high-quality images across a range of strengths. Typically, the standardization process is carried out by a service applied scientist.

When using AEC, it is crucial to position specific anatomy above the appropriate chamber. There are two types of exposure timers: a photomultiplier and an Ion chamber. Phototimers have a fluorescent screen that converts x beams to visible light, which is then directed to the photomultiplier. The photomultiplier converts light into electrons and these electrons are then multiplied in the tube. The phototube, located behind the film and the fluorescent screen, is activated by light.

The most commonly used type of AEC is the ionization chamber. It consists of a volume of air placed between two metal electrodes. This chamber is level and radiolucent so that it does not interfere with the image. It is positioned between the patient and the Image Receptor to measure radiation strength. The measurement can be taken at the center of the film or at the sides. The center cell is used for most images, while the outer cells are used for thorax, venters, and ribs.

(Spin Top Test) Spin top trials are used to investigate x-ray timers in individual stage machines only. It is a flat heavy metal disc with a hole in the margin. The disc is

approximately 5cm-25cm in diameter. An individual stage x-ray machine emits X-rays in pulses. A half wave electromotive force generates 60 pulses/second and full wave electromotive force generates 120 pulses/second. X-rays are generated by each of these pulsations.

The movie produced will demonstrate each pulsation that occurred during the exposure. Unlike three-phase x-ray machines, pulsed radiation is not used because the output remains constant. To investigate the timers on these machines, a physicist will employ a powered synchronous spin top that spins at 1 rotation per second. This test is measured in degrees.

The time it takes for a half-second to pass allows for a 180-degree image to be seen, while a quarter-second allows for a 90-degree image and a full second allows for a 360-degree image.

Flow of current through the console

The flow of current through the control console of an x-ray machine begins with the power coming in from the wall outlet, which receives electricity from an external source. The current then travels to the primary side of the control console and then to the line compensator, which maintains a steady voltage of 220v. It is then supplied to the primary side of the autotransformer.

The autotransformer increases the electromotive force by using electrostatic initiation, resulting in twice the input electromotive force value. The lights-outs on the secondary side of the autotransformer are used to select the major and minor kVp, which are pre-read by the kV meter on the secondary side. Once the electricity exits the autotransformer, it splits into two separate currents: the tubing current and the filament current. The tubing current carries the electromotive force to the step-up transformer on the primary side of

the high electromotive force subdivision. The filament current carries the electromotive force to the mA picker, a variable resistor with a series of resistance spirals (although a saturable reactor is preferred nowadays). The mA picker contains a saturated Fe nucleus with magnetic flux.

The current passes through the measure down transformer, located in the primary side of the high electromotive force subdivision. When the exposure switch is pressed, the current is transferred to the high electromotive force subdivision. The exposure timer controls the duration of the exposure. Its purpose is to create or interrupt high electromotive force across the ten beam tubing.

High Voltage Generators

When a building is supplied with power, it is typically supplied at either 110v or 220v. However, this power is not sufficient to run an x-ray machine. X-ray machines require much higher voltages, ranging from approximately 30,000v to 150,000v or 30kv-150kv. This is necessary in order to propel the electrons across the tube at the correct velocity. It is for this reason that a high voltage generator is a crucial component of the x-ray machine. Its main purpose is to convert the low supply voltage into the desired kilovoltage.

The high electromotive force generator, which consists of the high electromotive force measure up transformer, filament transformer (step down transformer), and rectifiers, is typically concealed from both the radiographer and the patient. It is usually placed in an electrical cabinet along the wall or hidden in false ceilings when available, ensuring it remains out of sight.

All three constituents - primary spiral, secondary spiral, and Fe nucleus - are immersed in oil for electrical insularity. The high electromotive force transformer, known

as a step-up transformer, has more twists on the secondary spiral than on the primary spiral. As a result, the secondary side, measured in kilovoltage, has a higher electromotive force than the primary side. The main function of the step-up transformer is to convert the entrance Vs on the primary side into kilovolts on the secondary side. The ratio of twists on the primary and secondary sides is referred to as bends ratio. For most ten beam high electromotive force transformers, the bends ratio ranges between 500 and 1000. Transformers exclusively operate on alternating current (AC), with sinusoidal wavelengths on both primary and secondary sides. The only difference between them is their amplitude, ranging from peak to valley.

In transformers, the primary and secondary spirals are wound around an Fe nucleus. Unlike autotransformers, which operate on self-initiation, step-up/step-down transformers operate on common initiation. Common initiation refers to the changing alternating current flow in the electromagnet creating a variable magnetic field. When this current passes through the primary spiral, it induces a current that flows through the secondary spiral. The alternating current flows through the primary spiral and generates a magnetic field around it. This changing magnetic flux then interacts with the secondary spiral, inducing an alternate electromotive force (EMF).

(111 press releases). The measuring up transformer is situated in the tubing current subdivision of the circuit, where the kVp's are chosen and the intensity or penetration of the beam is determined. (Step Down Transformer) In the measuring down transformer, the primary coil will have more turns than the secondary coil, resulting in lower voltage (Vs) but higher current (As). The measuring down transformer is located in the

fibril circuit subdivision of the circuit after the ma has already been chosen, which determines the number of x beams 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 transformers are a result of their efficiency being less than 100%. In reality, transformers are only 90%-95% efficient, with the majority of power loss occurring as heat. These power losses in transformers can be categorized into three types: copper losses.

Eddy current losses and hysteresis losses are both types of copper losses caused by resistance in the coils. The flow of electrical currents in the conductor of the coils generates heat. These types of losses can be minimized by using copper wire with the same diameter. A thicker wire results in less energy waste.

Eddy current losses refer to the circulating currents within the core, induced by fluctuating magnetic flux caused by alternating current. These currents generate heat. To minimize these losses, the core is composed of stacked laminated Si steel plates, which are insulated from each other. This arrangement increases the electrical resistance of the core, reducing the magnitude of eddy currents. The third type of loss is known as hysteresis losses.

The loss of energy in a transformer is caused by the consistent rearrangement of magnetic fields, resulting in heat generation in the nucleus. To minimize this loss, a laminated Si steel nucleus can be used. (Types of Transformers) A transformer is a device that converts an alternating current from low voltage to high voltage or vice versa. It transfers electrical energy between two

circuits without the need for moving parts or electrical contact between the circuits. Transformers operate solely on an alternating current and function independently of common initiation. There are various types of transformers designed for different purposes, but they all share the same objective.

The transformers I will discuss in this paper are the following:

  • Closed core transformers
  • Shell type and autotransformers.

Closed core transformers have a laminated iron core made up of layers, reducing energy loss and increasing efficiency. It has a closed ring shape with two insulated coils wrapped around it, allowing for continuous flow of magnetic flux and minimizing power loss. Shell type transformers are the most advanced and commonly used type. They have a primary coil enclosed within a secondary coil, creating two closed cores, making them more efficient than closed core transformers.

The autotransformer consists of a single Fe nucleus with only one wire wrapped around it. This single wire serves as both the primary and secondary winding. What sets the autotransformer apart is its ability to work through self-induction, where the magnetic field of the spiral generates a counter electromotive force (EMF) within itself. This self-induced EMF opposes the applied current.

(Rectifiers) The current supplied from the wall outlet is an alternating current with a frequency of 60 Hz. This means that the direction of the current changes 120 times per second. However, X-rays can only be generated when electrons accelerate from the cathode to the anode in one direction, and not in the opposite direction.

Because the cathode assembly is constructed in a way that it cannot withstand

excessive heat, it would be detrimental for the X-ray tube if the flow of electrons were to reverse. Therefore, electron flow should only occur from the cathode to the anode. Consequently, the high voltage transformer's secondary voltage needs to be rectified. This involves converting the incoming alternating current into direct current using a device known as a rectifier, which contains two electrodes and is classified as an electronic device.

The electrodes permit the flow of negatrons in a singular direction and are positioned amidst the transformer's secondary spiral and the x-ray beam tubing. Originally, rectifiers were constructed from vacuum tubings known as valve tubings, similar to the x-ray beam tubing, but they are now typically made of Si. Si rectifiers possess various advantages in comparison to valve tubings, including their smaller size, absence of fibrils, longer lifespan, low contrary current, and low forward electromotive force bead. Negatrons flow freely through conductors like metal or water, while dielectrics like plastic or rubber hinder their movement.

Semiconductors, like Si, possess intermediate conductivity between conductors and insulators. They are divided into two categories: N-type and P-type. N-type semiconductors have loosely bound electrons that are able to move freely, while P-type semiconductors have empty spaces referred to as holes, where electrons are absent. These holes act as spaces between two objects and can move as easily as electrons. When a small crystal of N-type material is in contact with P-type material, a P-N junction is created.

If a higher potential is applied to the p side of the junction, electrons and holes will move towards the junction and create an electric current. Conversely, if a positive potential is applied to the n

side of the junction, electrons and holes will be expelled from the junction, resulting in no electric current passing through the p-n junction. This one-way conduction of electricity in a solid state p-n junction is known as a solid state rectifying diode. Three types of rectification exist: self-rectification, half-wave rectification, and full-wave rectification. Self-rectification occurs when there are no rectifying diodes, and the X-ray tube itself acts as the rectifier. In half-wave rectification, one or two rectifying diodes are placed in the circuit to prevent the negative flow of electrons.

In half wave rectification, the reverse electromotive force is eliminated from the supply to the tubing. This prevents the electromotive force from turning negative during the negative half of its cycle, resulting in no electric current. However, during the positive half of the cycle, an electric current flows through the x-ray tubing.

The half wave rhythm results in a sequence of positive pulsations with gaps when the negative current is not conducted. This creates a rectified current where the electrons flow in only one direction. Half wave rectification results in 60 pulsations per second. However, it is not ideal because it uses only half of the supplied power and requires twice the exposure. Therefore, it is possible to design a circuit that rectifies the full alternating waveform.

This process, known as Full Wave Rectification, is commonly employed in nearly all stationary x-ray machines using at least four rectifying tubes. In Full Wave Rectification, the negative half cycle is reversed so that the anode always remains positive. There are no gaps in the output waveform, and the input waveform is rectified into a usable output. This results in a

pulsating direct current.

The benefit of using full wave rectification instead of half wave is that it cuts the exposure time in half, increasing the tube rating or heat load capacity. Full wave rectification generates 120 pulses per second with a minimum exposure time of 8ms. The self half wave and full wave rectification waveforms previously discussed are both produced by a single stage, resulting in a pulsating ten beam beam. Single stage power only uses one autotransformer and has one single stage on the waveform that goes from zero to maximum positive potential, then back to zero, to maximum negative potential, and back to zero again. The x beams produced during single stage waveforms have low energy and low permeability due to their close-to-zero values.

The diagnostic value of is limited. However, one way to improve results is by using three-stage power. Three-stage power involves generating three concurrent electromotive force waveforms that are out of sync with each other. This results in a nearly constant high electromotive force. In comparison, single-stage power only has two pulsations per 1/60 seconds, while three-stage power has six pulsations. Each stage requires its own autotransformer.

The kV choice requires three stage three autotransformers, one for each stage. These transformers can be arranged in either a star or delta constellation. A delta transformer is connected between the stages of a three-stage system, while a star transformer connects each weaving from a stage wire to a common impersonal point.

According to Wikipedia, three-stage circuits have primary coils wound in a delta configuration, but differ in the design of their secondary coils. The power ratings for three-stage power are 1600 mA and 150 kilovolts,

with an exposure time as low as 1ms. High-frequency generators are being increasingly used to produce high voltage for many imaging systems.

One advantage of high frequency generators is their smaller size. Compared to 60 Hz generators, they are much smaller and produce a consistent electromagnetic wave form, which enhances image quality and reduces patient dosage. Initially used in portable X-ray machines, high frequency generators are now used in most modern equipment. These generators utilize inverter circuits, also known as choppers, which are high-speed switches.

These devices transform direct current into a sequence of square pulses, known as Voltage Ripple. Voltage rippling refers to the small residual periodic fluctuations of direct current at the output stage of a power supply. This occurs when the alternating waveforms within the power supplies are not sufficiently suppressed. A higher level of rippling indicates less effective filtering, while a lower level signifies more efficient filtering.

A single stage power system has 100% electromotive force rippling, meaning that the electromotive force fluctuates from zero to its maximum value. In contrast, the three-stage six-pulse power system consists of 6 rectifying tubes and 1 star and 2 delta configurations, resulting in a 14% rippling. Therefore, the electromotive force supplied never drops below 86% of its peak value. However, an improvement was made by using 12 pulses instead of 6 in the three-stage power system. The three-stage twelve-pulse system includes 12 rectifying tubes, 1 star, and 2 delta configurations, resulting in only a 4% electromotive force rippling. As a result, the electromotive force does not fall below 96% of its peak value. On the other hand, high-frequency generators have a mere 1% electromotive force rippling, leading

to improved X-ray beam quality and quantity. This low amount of rippling in the electromotive force provides the biggest advantage in terms of electromagnetic radiation quality, as fewer electrons pass from the cathode to the anode, producing low-energy X-rays.

Flow of current through the High Voltage Generator

The current through the high voltage generator starts when it leaves the control console and enters the tubing current portion. It then exits the secondary side of the autotransformer and enters the primary side of the high electromotive force transformer. Next, it passes through the measure up transformer, where the electromotive force is stepped up from Vs to kVs. This is achieved by having more twists on the secondary side than on the primary side.
After leaving the measure up transformer, the current continues through the secondary side of the high electromotive force transformer and reaches the rectifiers. These rectifiers convert the alternating current (jumping current) into direct current, which is required in the tubing. The rectifiers consist of two types of solid province rectifying tubes: P-type and N-type semiconducting materials that allow current flow in one direction.

On the rectifier's secondary side, there is a ma metre that measures the amperage. Once the current is converted to direct current, it flows to the cathode in the x beam tubing. In the filament current part, the jumping current passes through the ma picker in the control console and is then carried to the filament transformer's primary side. This transformer also operates using electromagnetic common initiation. Within this circuit, the electromotive force flows through the step-down transformer, which means there are more twists on the primary

side than on the secondary side. This results in a lower electromotive force on the secondary side.

From here, the text describes the process in which the focal topographic point picker selects the fibril for boiling off electrons, and then sends the current 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. As a result, we now use a Coolidge tube to generate x-rays.

The ten beam tubing is a hidden part of the imagination system, concealed within a protective housing, making it inaccessible to engineers. There are two main components:

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