The Application of Physics in Medical Imaging
Student Name: Justin Bunker
Teacher Name: Mr. Ayres
Draft Due Date : 22/10/2018
Final Due Date: 29/10/2018
Table of Contents
1. INTRODUCTION 3
2. IMAGING TECHNIQUES 3
2.1 X-RAY 3
2.1.1 HISTORY: 3
2.1.2 HOW IT WORKS: 4
2.1.3 ABOUT THE MACHINE: 5
2.1.4 TAKING AN X-RAY: 6
2.1.5 RISK FACTORS: 6
2.2 MRI (MAGNETIC RESONANCE IMAGING) 7
2.2.1 HISTORY: 7
2.2.2 HOW IT WORKS: 8
2.2.3 ABOUT THE MACHINE: 9
2.2.3.1 THE MAGNET 9
2.2.3.2 THE GRADIENT COILS 9
2.2.3.3 RADIO FREQUENCY TRANSMISSION AND RECEPTION 9
2.2.3.4 COMPUTER SYSTEM 10
2.2.5 RISK FACTORS: 10
3. EVALUATION 11
3.1 STRESS FRACTURES 11
3.2 MERITS/DEFICIENCIES 11
3.3 DECISION MAKING MATRIX 12
4 CONCLUSION 13
5 REFERENCES 14
1. Introduction
Diagnostic Medical Imaging has been used for many years to help analyse abnormalities that are happening in a person’s body. Doctors rely heavily on the results of such examinations in identifying abnormal conditions which further guides them in deciding upon appropriate treatments. Some of these abnormalities include broken bones or pregnancy. In this text two imaging techniques will be explained and compared in the form of stress fractures in the back. The two imaging devices that are being compared are MRI (Magnetic Resonance imaging) and X-Ray. Each imaging technique will be thoroughly explained before explain the merits and deficiencies of each one and then putting them head to head in a decision making matrix.
2. Imaging techniques
2.1 X-Ray
2.1.1 History:
X-Rays, were discovered in 1895 by Wilhelm Conrad Roentgen a professor at Wuerzburg University in Germany. The discovery of X-rays were an accident when Wilhelm Conrad Roentgen was working with a cathode-ray tube. Roentgen noticed a fluorescent glow of crystals on a nearby work bench. The tube Roentgen was working with was made up of a glass envelope with positive and negative electrodes encapsulated in it. The air inside the tube was taken out and when Roentgen applied a high voltage to the tube there was a fluorescent glow. Roentgen proceeded to shield the tube with heavy black paper, when doing so he discovered a green coloured fluorescent light generated by a material located a small distance from the tube.
From this experiment Roentgen concluded that a new and unknown ray was being emitted from the tube. This ray was capable of passing through the heavy paper covering and reacted with the phosphorescent materials in the room. Roentgen found with further experiments that this ray could pass through most substances casting shadows of solid objects. He also discovered that this new ray could pass through the tissue of humans but not bones. One of Roentgen’s first experiments was of his wife’s hand.
This discovery was received with great interest from scientists. Roentgen’s experiment was recreated everywhere as the cathode tube was very well known during that period. With all of these scientists performing the same experiment many stories about this new ray. The public was most interested in the ability that the new ray could pass through soft tissue. This in conjunction with a photographic plate provided a picture of the bones and interior body parts.
Only one month after Roentgen announced his discovery it was being used in surgery to help guide doctors, six months after the announcement it was being used by battlefield physicians to locate bullets in wounded soldiers. Prior to 1912, X-rays were predominately used in medicine and dentistry. This is due to the X-ray tubes breaking down under higher voltage, this meant that radiographs from industrial purposes could not be taken. More and more powerful versions of the X-ray tube became available in years to come.
2.1.2 How it works:
To understand how an X-ray machine works we first need to understand what an X-ray really is. X-rays are basically the same thing as visible light rays. Both are wavelike forms of electromagnetic energy carried by particles called photons. The difference is between these two rays is the energy level of these individual photons or overwise expressed as the wavelength of the rays.
The human body especially the eyes are designed to be sensitive to one particular wavelength of visible light. A ray that has a different size wavelength with not be picked up by the eyes, therefore we will not be able to see it. Visible light photons and X-ray photons are both produced by the movement of electrons in atoms. When an electron drops to a lower orbital energy is needed to be released, this is done through the release of a photon. The energy of this photon depends on how much energy needs to be let off by the electron (how far the electron dropped between orbitals). When a photon collides with an atom the atom can absorb the photons energy. The atom has to boost an electron to a higher level, for this to happen the energy of the photon has to match the difference between the two electron positions.
The atoms inside the human body and body tissue absorb visible light photons easily. The energy level of the photon fits with various differences between electron positions. Radio waves however, don’t have this same quality. Radio waves can pass through most objects as they don’t have enough energy to electrons between orbitals in larger atoms. X-ray photons are the complete opposite of this, they have too much energy so they have the ability to pass through objects. However, it is possible that the photon can knock an electron away from an atom completely. Energy from the X-ray photon works to separate the atom from the electron whilst the rest of the energy sends the electron flying through space. Larger atoms are more likely to absorb the energy of an X-ray photon as they have greater energy differences in-between their orbitals. This means that the energy level fits closer to that of the photon. Smaller atoms are less likely to absorb the energy of an X-ray as the electron orbitals are separated by much lower energy.
The soft tissue of humans is made up of many small atoms, this means that these atoms do not absorb the energy of the X-ray. Calcium atoms however have much larger atoms and absorb X-ray photons very well. These calcium atoms make up our bones. Therefore the X-ray photons pass through the skin and soft tissue of the human body but cannot pass through the bone.
2.1.3 About the Machine:
An X-ray machine is a carefully crafted and precise piece of equipment that is used by doctors around the world. At the centre of the X-ray machine is an electrode pair, a cathode and an anode. These sit inside the glass vacuum tube. The cathode found in these X-ray machines is a heated filament which could be used in an old fluorescent lamp. A current gets passed through this filament heating it up, this heat splits electrons off of the filament surface. These electrons are then drawn across the tube by a positively charged anode which is a flat disc made of tungsten.
At this stage there is an extreme difference in the voltages between the cathode and the anode, this makes the electrons fly through the tube with an incredible force. When an electron with this much force collides with a tungsten atom it knocks loose an electron in one of the lower orbitals. This makes an electron in one of the higher orbitals to fall into the place of the now missing electron. When this happens there is a release of energy in the form of a photon being released and as the electron had to drop so far the photon has a high energy level (an X-ray photon).
The impact of these electron involved in creating X-rays generate a lot of heat. The anode is rotated by a motor to ensure that the electron beam isn’t focused on the same area of the anode and melt it. To absorb some of the heat there is a cool oil bath that surrounds the envelope. This whole mechanism is surrounded by a thick lead shield. This shield only has one small window to ensure that X-rays can’t escape in any direction they want and creates a focused beam of X-ray photons escaping in a controlled direction. On the way to the patient this beam passes through many filters.
On the other side of the patient’s body is a camera that records the pattern of X-ray light that passes all the way through the patient’s body. This camera is similar to a normal camera except that the X-ray lights set of a chemical reaction instead of the visible light. This means that the camera picks up only the X-rays and not standard light.
2.1.4 Taking an X-Ray:
An X-ray can be performed at a hospital, radiology centre, dentist or a clinic that specialises in diagnostic procedures. In most cases there is no preparation needed for the patient getting the X-ray, however, it is recommended that you where loose clothing. At the clinic the operator may ask for you to take off any jewellery or metallic items, and you must always tell your doctor or radiologist about any metal implants in your body. This must be done as these metal implants can cause an unclear image as the X-rays would not be able to pass through the body as explained previously.
Once the patient is prepared the radiologist or X-ray technician will tell you how to position your body for the optimal result. These positions could include you sitting, lying or standing, the technician might also require you to change positions during the test. During the X-ray it is crucial that the patient remains as still as possible again to get the optimal result. The length an X-ray takes depends on the area that is being analysed, but when the radiologist is happy with the images the test will be finished.
2.1.5 Risk Factors:
X-rays use small amounts of radiation to create the images of your body. This level of radiation exposure is safe for most adults, but not for developing babies. When X-rays where first getting developed doctors and patients would spend long periods of time exposed to the X-rays. As a result of this many doctors and patients developed radiation sickness. X-rays can be harmful as they are a form of ionizing radiation. This means that when they hit and atom they can knock electrons off the atom and create an ion, an electrically charged atom. These free electrons collide with other atoms and create more ions.
Ions have an electrical charge that can lead to the unnatural chemical reactions inside cells. This charge can among others can break DNA chains. A cell with a broken strand of DNA will either die or develop mutations. If a number of cells die, various diseases can be developed by the body. If DNA mutates, a cell has the possibility of becoming cancerous with the chance of it spreading through the body. This mutation can also lead to birth defects. Due to these risks X-rays are used much less often than when they were first developed.
2.2 MRI (Magnetic resonance imaging)
2.2.1 History:
MRI imaging along with all medical imaging is a very modern technique used. The foundation of MRI images date only date back to 1946 when founders Felix Bloch and Edward Purcell discovered the phenomena of magnetic resonance. Felix and Edward were later awarded the Nobel Prize in 1952. Until the 1970’s the MRI imaging machine was being used for a much different purpose than it is currently. The MRI imaging machine was being used for chemical and physical analysis. The first Human MRI did not occur until 1977, this was due to the advancement in technology that allowed super conductors to be used safely in the machine.
The creation of MR imaging for humans would not have been possible without the work of Nikola Tesla. Tesla made a breakthrough in rotating magnetic fields in 1882, this breakthrough allowed physicists in the future to create MR imaging. Before any MRI machines were made a different method called Nuclear Magnetic Resonance (NMR) had been developed by Isidor Rabi in 1937. This method was only used to analyse the structure of chemical substances. In the 1960’s a doctor Raymond Damadian had the thought to use the same methods on living organisms. In 1971, Damadian concluded that since cancerous tissue contained less healthy tissue than water. Therefore, scanners could detect it by bathing the body in radio waves and measuring the emissions of the hydrogen atoms nearby.
The findings of Damadian’s research were heard by many chemists and physicist around the world who thought they could use their own work to help. Paul Lauterbur a chemist was one of the people who saw Damadian’s findings. At the time Lauterbur was working on using the NMR to create images. He started off just imaging water but then moved onto objects with more substance. When he read Damadian’s 1971 findings, he realized that his work could have biomedical applications. Lauterbur was the first person to realise that a gradient magnetic field would two-dimensional images to be seen by observers. If these images were stacked right they could be used to create a three-dimensional view.
At the same time in England a Physicist by the name of Peter Mansfield was working on shortening the time required to complete a scan. He stated using a new method called the “line scan imaging” which allowed him to perform a scan on a colleagues finger in only 15-23 minutes per section. This was the first time NMR technology was successfully used to complete a scan of a human body part. After completing his target of lower times to complete a scan he then started working on a full-body scanner. On May 11, 1977 Mansfield finished his creation by putting the cardboard backed antenna coil onto the moving platform and lay inside while his assistants started up the machine. No resulting image was showing even after hours of adjusting the device. There was a suggestion that Mansfield’s body-fat content was too high. A grad student Larry Minkoff volunteered to be the next trial as he was much thinner. After watching Mansfield for seven weeks to see if there were any side effects he decided to climb into the machine. After nearly five hours, they finally got their first two-dimensional image of Minkoff’s chest.
From these test to the modern day MRI there have been some big changes. One of these is the change of name from Nuclear Magnetic Resonance to Magnetic Resonance imaging. This change was made as people had fears about the radiation when in fact MRI’s don’t use anything of the sort.
2.2.2 How it works:
An MRI uses molecules within the human body to complete the process of taking an image. The human body is made up of 72% water, an MRI uses these water molecules but in particular the hydrogen nuclei (protons). These Hydrogen molecules can be thought of much like the planet earth, spinning on its axis with both a north and south pole. This means that hydrogen acts like a small bar magnet. Majority of the time these hydrogen protons float around in the body randomly spinning their axis.
An MRI scanner applies a magnetic field around the body, this force typically ranges from 0.5-1.5 teslas. The protons within the water molecules are attracted to this strong magnetic field and therefore align each protons spin. This uniform alignment produces a magnetic vector orientated along the axis of the MRI scanner.
After this additional energy is then added in the form of radio waves. These radio waves are a the specific frequency (pulse) that they only apply to hydrogen atoms. These radio waves are then directed to the part of the body the scan is being performed on. When the pulse is applied the protons that are unmatched absorb the energy and begin to spin in a different direction.
At the same time three gradient magnets are arranged in a specific manner. They are arranged inside the main magnets so that that when they are turned on and off quickly in a certain way it will alter the main magnetic field on a local level. This mean that an exact area can be picked and targeted in the body and pictured. These pictures are referred to as a “slice”, slices can be as thin as a few millimetres. Slices can be taken at any part of the body in any direction, this gives doctors a huge advantage over every other imaging technique.
When the radio frequency pulse is turned off the hydrogen protons slowly return to their natural alignment within the magnetic field and release the energy absorbed from the radio frequency pulses. When this is done they give off a signal that the coils in the MRI scanner pick up and send to a computer system. The computer system in the MRI scanner can go through a patient’s body point by point and almost ask the body what kind of tissue it is. The system goes through the body point by point doing this and building a map of the tissue types. The system then integrates this data to create 2-D or 3-D models with a mathematical formula known as the Fourier transform. The computer system receives the signals from the spinning protons in the form of mathematical data, this data is then converted into a picture.
2.2.3 About the Machine:
An MRI machine is made up of four different components: the magnet, the gradient coils, radio frequency transmitter and receiver and the computer.
2.2.3.1 The Magnet
The magnet is the most important but also the most expensive part of the MRI machine, modern versions of the scanner use superconducting magnets which are able to generate stronger and larger magnetic fields.
2.2.3.2 The Gradient Coils
There are two main requirements that the gradient coils have to achieve. First they are required to produce a linear variation in field along one direction. Secondly they have to have a high efficiency, low inductance and low resistance. The gradient coils are required to have these features in order to reduce the current requirements and heat deposition.
The first requirement, producing a linear variation is usually produced by a Maxwell coil. This Maxwell coil includes a pair of coils that are separated by a distance that is 1.73 times the coils radius. In these Maxwell coils the current flows in the opposite sense in the two coils, and produces a very linear gradient. However, an MRI requires a linear gradient in all three axes.
To produce linear gradient in the remaining two axes the magnet requires wires running along the bore. This can be done using a saddle-coil, an example of this is the Golay coil. This coil consists of four saddles running along the bore of the magnet which produce a linear variation along either the x or y axis. This depends on the axial orientation
2.2.3.3 Radio Frequency Transmission and Reception
2.2.3.4 Computer System
The image to the right shows the complex
computer system of the MRI machine. The
scanning operation is controlled from the
central computer. This central computer
specifies all of the timings, the shape of the
gradient coils, the radio frequency waveforms
and passes all of this information to the
waveform generator. The waveform generator
outputs the signals and passes them to be
amplified and sent to the coils. All data is
stored to allow for post processing
corrections.
2.2.5 Risk Factors:
There are no known side effects of an MRI, this is assuming that you don’t have any implants or objects that cannot go inside the scanner. The only danger from an MRI is not following procedure and taking objects in that will interact with the magnets inside the machine. If metal objects are taken in they will move around, heat up and electrical currents can be created which leads to a malfunction of a device.
3. Evaluation
In this evaluation it will be discussed the advantages and disadvantages of each scanning method. The two methods will then be compared to each other in a decision making matrix which will then lead into the final result. However to start the medical condition in question must be explained. The condition in question is stress fractures in the back, this is a very common injury in athletes and many different sports.
3.1 Stress Fractures
There are many different types of stress fractures in the back however the one that we will be focusing on is the stress fracture of the lumbar pedicle. There are two main types of stress fracture, stress fracture caused by fatigue and the other being caused by structure insufficient. Stress fracture of lumbar pedicle occur mainly with repetitive and large activities of spine. The main stresses causing stress fractures of the lumbar pedicle are shear stress and twisting stress, followed by sudden hyperflexion or hyperextension of the spine. Stress fracture of lumbar pedicle was easily missed by conventional X-ray examination. To identify a Stress fracture of the lumbar pedicle usually a XCT, MRI, or bone scan is needed. The results of the scans are divided into 4 types or 4 periods according to examination findings: stress reaction, incomplete fracture, complete fracture, and pseudarthrosis.
3.2 Merits/Deficiencies
MRI
Merits X-Ray
Merits
MRI can easily create hundreds of images from almost any direction and in any orientation Ready Availability
MRI gives extremely clear, detailed images of soft-tissue structures that other imaging techniques cannot achieve
Low cost
No radiation exposure and is non-invasive
Deficiencies Deficiencies
Higher cost than other Stress Fracture imaging
Unable to clearly identify a stress Pars stress fracture until it has been present for some time.
Expensive and time consuming. Emits Radiation that can cause skin cancer and has the potential to change DNA.
3.3 Decision making matrix
MRI Score X-Ray Score
Cost and availability $50-300
AS this is one of the more expensive and unavailable scans around it gets a 4 for this category
4 $10-70
Majority of the cost of a X-ray can be bulk billed, however even when it isn’t X-ray is one of the cheaper and most available scan. For this it receives an 8 for this category. 8
Risk To date there are no known risks or side effects of an MRI. However on the odd case the MRI could affect a person’s pacemaker or shift implanted metal screws or pins.
As all MRI scans have no known risk factors to everyone who completes the MRI safety quiz it gets a 10 for this section 10 There is a risk factor in an X-ray scan as the patient is exposed to a high level of X-rays. Patients are also exposed to low levels of radiation which can cause forms of skin cancer and can changes to DNA.
There is much risk and chance for damage during an X-ray. Even though all of the risk listed above are very rare they are still risks so it gets a 3.5 for this category. 3.5
Speed MRI of the Lumbar Spine … 20-35 minute scan time.
The speed of an MRI is pretty standard no matter what scan you get, however it is still time consuming. Therefore it gets a 6 for this section 6 An X-ray of the spine takes 10-15 minutes of scan time.
The speed of an X-ray is one of its best assets along with the cost and availability. An X-ray is a very quick scan and therefore receives a 9 for this section. 9
Image quality MRI scans provide the highest quality image, clearest and most detailed images compared to every other imaging techniques.
MRI’s are as said above the best scan available for the stress fracture issue. Therefore is receives a 10 for this category 10 X-rays can’t identify a pars stress fracture until it has been present for some time.
X-rays as said above are unclear when it comes to determining stress fractures early. For this reason it receives a 1 for this category. 1
We are able to use the data found in the above matrix to establish which is the best scan to identify stress fractures. Looking at the cost and availability section of the matrix the X-ray is much better as it is cheaper and much more accessible. Moving to the risk factor MRI scans are much better as they have no known side effects compared to X-rays. Both of these scans take similar times to complete however, the X-ray is quicker. Finally looking at the Image quality of these scans finding a stress fracture the MRI is much better than the X-ray.
If we add up the scores given in the matrix we will be able to identify the best imaging technique to diagnose stress fracture. When the scores from the X-ray scan was added up it came to a total of 21.5 points. When the scores of the MRI were added up they came to a total of 30 points. Therefore, it can be seen that MRI is better than X-ray to identify stress fractures in the lower back. This is due to the MRI having no risks, a reasonable speed and the best image quality of any imaging technique.
4 Conclusion
In conclusion, this text has identified and explained two imaging techniques that are both used to identify an issue in the body. The chosen abnormality was stress fractures in the lower back comparing X-ray and Magnetic Resonance imaging (MRI) to see which one is best. Both imaging techniques were broken down and explained thoroughly before finding the merits and deficiencies of each. The two imaging techniques were then compared head to head in a decision making matrix where the best imaging technique was found.
It came out that an MRI is the best imaging technique to identify stress fractures in the lower back as it has no side effects, is relatively quick and has the best image quality of any scanning technique. X-rays on the other hand were very available with a small cost and little time to complete. However, an X-ray scan has many risk and very poor image quality when it coming to identifying stress fractures