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Medical Physics: MRI
Transcript of Medical Physics: MRI
field produced by
can be used as a
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
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
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.
Number of Protons
Number of Protons + Number of Neutrons
To fully explain the spin of the nucleus, quantum physics is required, which is beyond the scope of this course. to simplify the concept of nuclear spin, the nucleus can be visualised to spin on its axis.
A spinning nucleus carries an angular momentum that is related to the spin number (I). The spin number is a quantum mechanical number and is basically determined by the number of protons and neutrons inside the nucleus. There are three groups of value for I: 0, integer values and half-integral values (e.g. / , / , / and so on). The spin number of a nucleus will be 0 if both its atomic number and the atomic mass number are even numbers. When the atomic number is odd and the atomic mass number is even, then the spin number will be an integer. When the atomic mass number is odd, then the spin number is always a multiple of / .
The spin number for some of the common elelments is listed below.
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
a whole number
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.
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.
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 levels)
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
‘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, B 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 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.
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.
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’
‘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.
Using the internet look for MRI images
of healthy and damaged tissue.
MRI images of the brain and brain tumors
are usually quite good.
Find THREE Comparisons
Upload to the MRI comparison Task
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.
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.
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).
Here is a sample answer:
The impact of medical applications of physics on society has been enormous.
This assessment is supported by considering the following aspects of society:
Medical applications of physics provide better and earlier diagnosis and better monitoring of a range of diseases and conditions. This contributes to a healthier society. For example, tuberculosis was a widespread disease in Australia. Chest X-ray screening was instrumental in virtually eradicating this disease. This has led to a healthier society.
Medical applications of physics are expensive to install and to operate. This is an economic burden. Society as a whole has to weigh up the benefits of the technology against these costs. Society has to decide upon how these costs can be met. Issues of equity and provision of service to remote areas are economic issues. For example, MRI machines are over $1 million each to buy and require highly skilled operators. Provision of increased medical physics technology for an aging population is in part an economic issue.
Using medical applications of physics introduces ethical issues. The moral and ethical values we have as a society underpin our legal system. Medical applications of physics give us knowledge and can present us with issues that our value systems have to adjust to.
For example, ultrasound is commonly used to image foetal development. Knowledge of foetal problems presents a dilemma to the prospective parents.
Society as a whole has to confront the ethical issues that are raised. This process can be emotive and challenging and lead to societal unrest.
Medical applications of physics have contributed an enormous amount to our knowledge of the structure, function and development of the human body. Society uses this knowledge to provide for better and more efficient health provision. For example, keyhole surgery is a much cheaper and less invasive procedure that has been made possible by endoscopy.’
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