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9.6 Option – Medical Physics: 4. Magnetic resonance imaging

Syllabus reference (October 2002 version)
4. The magnetic field produced by nuclear particles can be used as a diagnostic tool	Students learn to: identify that the nuclei of certain atoms and molecules behave as small magnets identify that protons and neutrons in the nucleus have properties of spin and describe how net spin is obtained explain that the behaviour of nuclei with a net spin, particularly hydrogen, is related to the magnetic field they produce describe the changes that occur in the orientation of the magnetic axis of nuclei before and after the application of a strong magnetic field define precessing and relate the frequency of the precessing to the composition of the nuclei and the strength of the applied external magnetic field discuss the effect of subjecting precessing nuclei to pulses of radio waves explain that the amplitude of the signal given out when precessing nuclei relax is related to the number of nuclei present explain that large differences would occur in the relaxation time between tissue containing hydrogen bound water molecules and tissues containing other molecules	Students: perform an investigation to observe images from magnetic resonance image (MRI) scans, including a comparison of healthy and damaged tissue identify data sources, gather, process and present information using available evidence to explain why MRI scans can be used to: detect cancerous tissues identify areas of high blood flow distinguish between grey and white matter in the brain gather and process secondary information to identify the function of the electromagnet, radio frequency oscillator, radio receiver and computer in the MRI equipment identify data sources, gather and process information to compare the advantages and disadvantages of X-rays, CAT scans, PET scans and MRI scans gather, analyse information and use available evidence to assess the impact of medical applications of physics on society

Extract from Physics Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit: 18 Aug 08]

perform an investigation to observe images from magnetic resonance image (MRI) scans, including a comparison of healthy and damaged tissue

• To perform your investigation you will need to obtain a number of MRI images. These are available in texts, journals or from an Internet search.
  
  
• Gather observational data to support your comparisons. You need to assess the images for their ability to give you valid and reliable comparisons between healthy and damaged tissues. Having a number of images with the same pulse sequence will assist in this regard.
  
  
• Your observations can be made for a range of image properties. What information is included with the scan? Check that images have the same pulse sequence, indicated by terminology such as ‘T1 weighted, or T2 weighted or proton-density weighted’. The quality of the image includes its resolution and contrast (grey scale). Now look at what is being imaged. What do the various shades or colours imply? Can you make any generalisations about the appearance of various tissues/structures?
  
  
• Your comparison of images of healthy and damaged tissue could be a list of features such as any change in greyscale, contrast and resolution. Your investigation could be concluded with a number of summary statements such as:
  
  ‘On a T2 weighted image bone appears dark whilst different soft tissues appear as pale shades of grey. Numerous different tissues can be identified. Boundaries between tissues appear to be very sharp, indicating a resolution of less than 1mm. Damaged tissue with a high water content, such as a cyst, is imaged as a uniformly bright region surrounded by a much darker boundary.’

identify that the nuclei of certain atoms and molecules behave as small magnets

• You should be able to recognise that:
  • molecules contain more than one atom
  • each atom has a nucleus
  • a nucleus contains protons and neutrons
  • the proton has a net positive charge, whilst the neutron has no charge
  • each proton has an axis upon which it spins
  • the spinning proton acts as a small electric current loop
  • the current loop will develop a magnetic field, analogous to the field around a solenoid, in accordance with Oersted’s Law
  • this magnetic field can be visualised as a bar magnet’s field where the proton’s spin axis becomes the bar magnet.
  • for an atom, the magnetism of the nucleus as a whole is because of the protons it contains.
  • for certain atoms there can be a net nuclear magnetism as a result of the number of protons it contains.
  
  
• You should be able to name:
  • the parts of an atom
  • the charges of each atomic particle
  • the poles of a bar magnet.

identify that protons and neutrons in the nucleus have properties of spin and describe how net spin is obtained

• You should be able to recognise and name:
  • the relative mass and charge of protons and neutrons
  • that a moving object has momentum
  • that a rotating object has angular (rotational) momentum
  • that at the size of the nucleus, properties such as momentum and energy are quantised
  
  
• The nuclear particles are arranged in a shell structure, analogous to the electron shells but much smaller.
  
  
• Each of the protons and neutrons has its own angular momentum, termed spin.
  
  
• Spins can be one way or the reverse, termed ‘spin up’ and ‘spin down’.
  
  
• The spins add together to cancel each other out
  
  
• Protons only add with protons and neutrons only add with neutrons
  
  
• If there are any unpaired spins, the nucleus as a whole will have a spin. This is termed ‘net spin’
  
  
• Net spin can be
  1. zero,
  2. a whole number
  3. a half number
  
  
• Net spin can be determined using the following key:
  • Q1: Is the mass number even?
    YES: Go to Q2
    NO: The spin is a half integer
    
    
  • Q2: Is the atomic number even?
    YES: The spin is zero
    NO: The spin is a whole integer.

explain that the behaviour of nuclei with a net spin, particularly hydrogen, is related to the magnetic field they produce

• Your explanation should include:
  • Spins in opposite directions cancel each other out
  • Net nuclear spin is due to the sum of the component spins of its nucleons
  • The proton can be considered to be a small spinning solid sphere with the positive charge attached to its equator
  • A net spin can constitute a current loop
  • A current loop creates a magnetic field aligned along the axis of spin
  • Nuclei with a net spin due to an unpaired proton (ie with an odd atomic number) will have a net magnetic field
  • Nuclei with a net magnetic field will interact with applied external magnetic fields
  • The amount of interaction varies with the nuclear isotope
  • Hydrogen has a single unpaired proton. This can be either spin up or spin down and will interact with an external magnetic field.

identify data sources, gather, process and present information using available evidence to explain why MRI scans can be used to:

• detect cancerous tissues
• identify areas of high blood flow
• distinguish between grey and white matter in the brain

• The data needed could include written explanations, sequences of diagrams, animations or video material.
  
  
• Your data sources could include a variety of texts, journals or an Internet search. You should choose sources that provide sufficient detail to give an explanation that makes sense, yet are not overly detailed. MRI operation is very complex and a full understanding can become a very large task.
  
  
• As you peruse your data sources you will begin to develop an understanding of the explanations required. You will need to check that each new source of data contributes to these explanations. This is a process of evaluation.
  
  
• Your explanations will need to be communicated in an appropriate manner. Complex explanations can often be presented using a combination of short and succinct text statements (bulleted points) and a series of diagrams.
  
  
• Explanations of processes will often become a series of cause and effect statements. You are explaining a complex process that depends on the laws of nature (physics) and the behaviour of a biological system and our ability to exploit this knowledge. We can give a valid and reliable explanation because of our confidence that nature behaves in a repeatable way.
  
  
• MRI scans can be used to detect cancerous tissues because:
  • cancerous tissues are areas of rapidly growing and dividing cells.
  • increased cellular activity is accompanied by an increased level of water around the cells and increased blood flow to the tissue
  • MRI is essentially a way of detecting proton density.
  • the MRI system can be tuned to detect the single protons in Hydrogen.
  • water contains Hydrogen.
  • ‘mobile’ or ‘free’ body water gives a stronger radio frequency signal than Hydrogen protons involved in less mobile molecules.
  • the signal from ‘free’ water can be enhanced by an appropriate pulsing sequence to produce a T2 relaxation weighted image.
  • as a result a T2 weighted image will show cancerous tissue as an area of unusually high brightness if the appropriate scanning parameters are used.
  
  
• the radio frequency signal from the body is detected and processed to provide a two or 3 dimensional image of the cancerous tissue.
  
  
• MRI scans can identify areas of high blood flow because
  • blood is a watery fluid
  • high blood flow means more water can be detected
  • as above, more water can be detected as a brighter image.
  • blood can also be detected using flow analysis
  • flow analysis is a development of MRI that utilises tissue saturation
  • tissue saturation happens when a ‘slice’ of tissue under investigation is repeatedly pulsed with an appropriate radio frequency. This produces a certain level of output radio frequency signal from the tissue.
  • blood flowing into the slice under investigation will not be saturated in this way and so will return a different output radio frequency signal.
  
  
• the difference in output radio frequency signal levels can be used to determine the rate and amount of blood flow.
  
  
• MRI scans can be used to distinguish between grey and white matter in the brain because:
  • grey and white matter have different biochemistry
  • part of the process of acquiring an MRI image involves allowing the protons to produce a radio frequency signal.
  • whilst producing a signal the protons change their spin orientation. This is termed relaxation and happens in two ways termed T1 and T2.
  • the T1 proton relaxation rate depends upon the surrounding molecules.
  • each tissue has a characteristic T1 rate at a given magnetic field strength. For example, at a field strength of 1.5 Tesla grey matter has a T1 of 920 milliseconds whilst white matter has a T1 of 790 milliseconds.
  • these different values mean that the detected radio frequency signals from each tissue will vary differently over time. This can be analysed to provide image contrast.
  • the imaging process can be made to intensify the effects of T1 relaxation.
  • on a T1 weighted image, white matter appears white and grey matter appears grey. (These shades are not the colour of the tissue).

describe the changes that occur in the orientation of the magnetic axis of nuclei before and after the application of a strong magnetic field

• Each nucleus that has a net magnetism will be affected by an external magnetic field. If the external field is weak, these effects are small and easily disturbed by thermal agitation.
  
  
• Before the application of a strong magnetic field the nuclei will have a random orientation of their magnetic axes (spins). The net result is that there will be no bulk magnetic moment (overall magnetism). All of the spins will cancel each other out. (The Earth’s magnetic field is not strong enough for spin alignment to overcome thermal agitation at normal
  
  
• In the presence of an applied magnetic field the spin vectors (magnetic axes) of the nuclei align themselves with the field lines. This alignment is not perfect, however, and the spin vectors actually rotate around the field lines. This is called precession and has a frequency dependent on the strength of the field and the gyromagnetic ratio of the nucleus concerned.
  
  
• Spin is a quantum process. The spin vector can precess either parallel (up) or antiparallel (down) to the external magnetic field. A small excess in the up state (lower energy) gives rise to a bulk magnetism for the substance.
  
  
• If a strong magnetic field is removed from the nuclei they will gradually lose their alignment. This happens due to thermal agitation and interactions between the nuclei. This decay process eventually leads to a random set of spin orientations

define precessing and relate the frequency of the precessing to the composition of the nuclei and the strength of the applied external magnetic field

• ‘Precessing’ is what happens when a spinning object undergoes precession.
  
  
• An object that undergoes rotational movement about an axis is said to be spinning, eg the Earth spins on its axis through the North and South poles. A spinning object has angular momentum. A spinning top will move so that its axis of spin traces out a cone shape. This movement is called precession. Precession occurs in order to conserve angular momentum.
  
  
• The number of times the axis of spin precesses (traces out a cone shape) per second is the frequency of precession.
  
  
• A nucleus may have a net magnetism and net spin due to its composition. This means that the spinning nucleus will behave like a small spinning bar magnet. If placed in a magnetic field, the ‘nuclear magnet’ will precess around the external field lines. The frequency of precession, ù (Larmor frequency) depends upon the strength of the applied field, B₀ and the gyromagnetic ratio,ã, of that particular isotope.
  
  
• Hydrogen is the most common substance imaged using MRI. Hydrogen has a gyromagnetic ratio of 42.57 MHzT^-1 and so a magnetic field of 1.5 Tesla will produce a precession frequency of 63.9 MHz for it. This frequency lies in the radiofrequency range.

gather and process secondary information to identify the function of the electromagnet, radio frequency oscillator, radio receiver and computer in the MRI equipment

• Information about how MRI works can be found from a range of sources. Texts, popular science and technology magazines and material from an Internet search could all be useful.
  
  
• MRI is a very complex process. Be sure that your sources provide enough depth to give a complete answer. As the process involves a number of cause and effect relationships you can evaluate the data sources by asking if they give enough information at one step to allow the next one to follow.
  
  
• You are asked to identify a number of MRI components. You can recognise them from a description of their role, in the context of their technical terms or form a diagram or photograph. Common symbols and diagrams are regularly used for a magnet and magnetic field, a radio pulse and a computer.
  
  
• A number of magnets and magnetic fields are used in MRI. The large field magnet may be permanent, or a resistive or superconducting electromagnet. Whichever type is used, its function is to provide a uniformly high magnetic field in which the patient is placed. A series of smaller electromagnets are used to add a changing component (gradient) to the large field. These gradient magnets introduce a variation into the Larmor frequency of the protons and so allow the receiver to be tuned to a small section (slice) of the patient. Gradient electromagnets also allow spatial positioning within the slice to be determined by using Fourier transform calculations.
  
  
• The radio frequency oscillator produces radio wave pulses. These can be given a certain frequency, amplitude, bandwidth, phase and pulse duration. Recall that the nuclei will be precessing around the magnetic field lines. They will be doing this at their Larmor frequency. However, they will not be precessing ‘in time’ (in phase). The radio wave pulse is directed at the nuclei and gets all of the precessing nuclei to ‘go into phase’. (An analogy is a crowd of people walking down the street being made to start marching in step.) The nuclei ‘go into phase’ only if the radio pulse is at their Larmor frequency, as they absorb energy from the pulse as they resonate with it.
  
  
• Once in phase, the nuclei will stay in phase until the external radio pulse is turned off. The nuclei now start to lose energy. They go out of phase and release a radio frequency signal at their Larmor frequency. The radio receiver detects these. The signal is typically very weak and must be amplified considerably before being further analysed
  
  
• The computer plays a central role in the operation of MRI. Part of the computer is involved in image acquisition and control. It provides:
  • sequencing and duration of radio frequency pulses
  • control of the gradient magnetic fields
  • control of the sampling by the receiving coils
  • conversion of analog signals to digital data
  
  Another part of the computer is involved in making the image. It provides:
  • Fourier transform analysis to give spatial signal data
  • image construction and manipulation
  
  Another part of the computer involves the operator. It provides:
  • overall control of the imaging process and patient parameters
  • storage of image data
  • image viewing and display

discuss the effect of subjecting precessing nuclei to pulses of radio waves

• Nuclei will be precessing if they are in the presence of an external magnetic field and the nuclei have a magnetic moment of their own. The precession rate will be at the Larmor frequency which depends on the gyromagnetic ratio of that particular isotope and the external field strength.
  
  
• Although the nuclei are precessing they will not be doing so in phase (in time). As a result there will be no net transverse (across the external field) magnetism.
  
  
• A pulse of radio waves at the Larmor frequency will force the precessing nuclei to do so in phase, ie they all precess together. This means that they will have a net transverse magnetism component which rotates.
  
  
• The precessing transverse magnetic field is in effect a changing field. It will induce a small AC emf in a receiving coil. This emf will have the same frequency as the precessing nuclei ie the Larmor frequency.
  
  
• Radio frequency pulses can also be used to change the spin direction of the nucleus from parallel to antiparallel with the field (180° pulse) or to precess at the maximum angle from the field (90° pulse). This is done by varying the pulse size and strength.

explain that the amplitude of the signal given out when precessing nuclei relax is related to the number of nuclei present

• Precessing nuclei will not give a signal unless they are doing so in phase. This situation happens when the nuclei have been subjected to a pulse of radio frequency electromagnetic radiation at their Larmor Frequency (frequency of precession). They resonate with this pulse and so develop a net transverse magnetic field.
  
  
• The rotating transverse magnetic field will be the sum of all of the ‘in time’ transverse field components from each nucleus.
  
  
• Therefore, more nuclei will mean a stronger transverse field
  
  
• When the stimulating (resonance inducing) RF pulse is turned off the nuclei will continue to precess.
  
  
• This precessing (rotating) transverse magnetic moment induces an AC emf in nearby receiver coils.
  
  
• The strength of the induced emf depends on the strength of the transverse magnetic field and hence on the number of nuclei present.
  
  
• After the stimulating RF pulse stops the nuclei will start to go out of phase or relax. As they do so they cause the net transverse magnetic field to weaken. This means that the amplitude of the signal given out gradually decreases. This process is sometimes termed ‘free induction decay’

explain that large differences would occur in the relaxation time between tissue containing hydrogen bound water molecules and tissues containing other molecules

• ‘Relaxation’ is the name of the processes whereby the nuclei return to random, out of phase, precession. There are two relaxation processes, termed T1 and T2.
  
  
• Spin-lattice relaxation, T1, happens as the nuclei transfer energy quanta to the nearby molecular lattice.
  
  
• Spin-spin relaxation, T2, happens as nuclei transfer energy quanta between each other.
  
  
• In biological tissues only spins in the ‘free water pool’ contribute to the measurable MR signal whereas spins in ‘bound water’ do not.

identify data sources, gather and process information to compare the advantages and disadvantages of X-rays, CAT scans, PET scans and MRI scans

• Your can choose from a wide variety of data sources. Texts, popular and scientific journals and medical reference books can be used. An Internet search will reveal many other data sources
  
  
• The information you need may be widespread and so efficient summarising and collating will be required.
  
  
• Information about medical technology is of great interest to the general public. You will need to check that your data sources are valid. Critically consider the data source for its unbiased and complete presentation of facts and issues.
  
  
• The most efficient way to present this comparison is probably to prepare a table. Comparisons show how things are similar or different so a table of advantages and disadvantages may be accompanied by a number of statements that highlight these.
  
  
• An example is shown below. This is not an exhaustive comparison and is quite succinct in its listings. You may wish to expand upon its contents.

Imaging	Advantages	Disadvantages
X-Ray	Cheap Simple to use Readily available Rapid imaging Good bone resolution	Uses ionising radiation Radiation dose is cumulative Does not show soft tissue well Does not show functioning
CAT scan	Resolution better than X-ray Can show three dimensions Can ‘remove’ unwanted layers	Uses ionising radiation Radiation dose is cumulative Does not show functioning More expensive than X-ray
PET scan	Shows organ functionality Gives nervous system detail	Uses ionising radiation Expensive and uncommon Poor resolution Requires care with radionuclides
MRI scan	Can show three dimensions Can ‘remove’ unwanted layers Gives high resolution Excellent soft tissue contrast Can show functionality Gives nervous system detail Safe to use for most patients	Very expensive Scanning takes a long time Hazards with implants Claustrophobia when imaging High skill in using it is needed

• Some examples of comparison statements are:
  • X-ray, CAT and PET scans all use ionising radiation whereas MRI does not
  • CAT, PET and MRI scan technologies are all quite expensive and fixed in position, whereas X-Ray technology is cheap and relatively mobile.

gather, analyse information and use available evidence to assess the impact of medical applications of physics on society

• The information you gather could come from a wide variety of sources. Apart from scientific texts and journals you could search a wider variety of current ‘social issues’ media such as newspaper articles and popular magazines. There is a range of medical industry journals as well.
  
  
• Your data will be quite wide-ranging and it would be appropriate to classify the impacts into a number of categories about which you could make generalisations. You could choose to categorise on the basis of a particular medical imaging technology or on the basis of a list of ‘impacts’. Any generalisations you make should contribute towards your assessment.
  
  
• Your information will come from a variety of sources. You will need to apply critical thinking to ensure that the assessment you make accurately reflects the information you obtained
  
  
• Your assessment should make a judgement and then providing supporting evidence. The supporting evidence could be presented in many ways:
  • A number of succinct paragraphs, each addressing a different impact
  • A set of bulleted statements under separate impact headings
  • A table of medical imaging technologies, each with a list of impacts
  
  
• The typical answer below is not intended to be exhaustive. A complete answer could be very large and could draw upon a huge number of concepts and examples. Your answer should show that you have a good understanding of the physics as well as its impact on society. A good answer would be supported by a number of statements.