MRI
Overview.
MRI Hardware.
Functioning of MRI.
Gradient coils.
Radio frequency coils.
Relaxation.
T1 relaxation.
T2 relaxation.
Computer system.
T2* relaxation.
Typical T1 and T2 relaxation.
Functional MRI.
Overview.
MRI is a advanced imaging technology.
MRI stands for Magnetic Resonance Imaging.
It uses magnetic fields, and radio frequency waves for imaging.
It does not use ionising, like x-rays, and CT-scans.
MRI uses strong magnetic fields.
The strength of a magnetic field is measured in Tesla.
Clinical MRI work in the range of 1.5 to 3 Tesla.
This is a very strong magnetic field.
It is about 50 thousand times stronger, than the magnetic field of the Earth.
MRI is used for imaging parts of the human body, for clinical purposes.
70% of the human body comprises of water.
The magnetic properties of hydrogen, which is ubiquitous in the body,
is used by MRI.
The protons in the hydrogen molecule, have a spinning charge.
This produces a magnetic field, called magnetic moment.
The protons behave like little magnets.
Only certain molecules like hydrogen, have this property.
For example, in carbon atoms the spin of protons and neutrons cancel out.
Only atoms with odd spins, have this property.
In hydrogen atoms, normally the protons will be oriented randomly.
The orientation changes, in the presence of a strong magnetic field.
These changes can be measured, by using controlled radio frequency signals.
Multiple exposures are made, in different orientations.
The composite data collected from all the exposures,
is collated and analysed by a computer, to form an image,
of the relevant organs and tissues of the human body.
MRI hardware.
The subject, typically a human being, lies in the bed, in the centre, of the machine.
The subject is surrounded by a cylinder of magnets.
The outer most magnet, in the cylinder, is the primary magnet.
Below this, there is another magnet, called the gradient magnet.
Radio frequency coils, are located below the gradient coil.
This whole setup surrounds the subject, lying in the bed.
There are also RF detectors, located in the machine.
This send signals to a computer.
Specialised software, translates the signals, into a image.
Functioning of MRI.
The primary magnets, when switched on, generate a magnetic field,
of 1.5 or 3 Tesla.
The protons in the hydrogen atoms, which were originally randomly oriented,
now align themselves to the direction of the magnetic field.
This is called longitudinal alignment.
They align themselves parallel or anti parallel, to the magnetic field.
This is in accordance with quantum mechanic's theory.
We can visualise this as aligned towards, or against the direction of the magnetic field.
The protons are aligned in parallel, have low energy.
The protons align in anti parallel, have high energy.
There are more protons aligned in parallel, then anti parallel.
The net vector, is in the direction of the primary magnetic field.
It is this net vector, that we are concerned with.
Apart from spin, hydrogen protons have another property.
We would have noticed a spinning top, when it starts losing its momentum.
It starts to spin around the axis.
Spinning protons, exhibit a similar property.
It is called precession.
Because of precession, a vector of magnetisation can be visualised,
to be present along the three axes, the x, y and the z axis.
The rate of precession is called as the Larmor frequency.
When protons precess together, they are said to be in phase.
When they precess differently, they are said to be out of phase.
The frequency, are the precession rate changes in proportion to the magnetic field.
When the magnetic field strength, is 1.5 Tesla, the frequency is about 65 mega hertz.
When a magnetic field strength, is 3 Tesla, the frequency is about 130 mega hertz.
The is in the radio frequency range.
Protons can be flipped from a low energy state to a high energy state.
It requires a fixed amount of energy, for a given frequency,
to flip a proton from a low energy state to a high energy state.
Radio frequency waves, have a photon, which has energy specific to its frequency.
When the MRI machine sends a radio frequency pulse, it is able to flip the proton.
The RF pulse, flips them from a low energy state, to a high energy state.
MRI machines are very sensitive to radio frequencies.
It is protected from environmental radio frequencies, with a copper sheath.
Gradient coils.
Gradient coils are present next to the primary magnet.
These coils produce a secondary magnetic field.
These coils are present in the x, y and z axis directions.
They enable the MRI machine to do spatial imaging.
The z gradient, or the long axis, is used for axial imaging.
The y gradient, or the vertical axis, is used for coronal images.
The x gradient, or the horizontal axis, is used for sagittal images.
The MRI machine, takes images in different cross sections,
and composes a 3 dimensional or 3D image, using software.
Radio frequency coils.
The Radio frequency coils transmit and receive radio frequencies.
After the magnetic field is switched on, a radio frequency pulse, is applied.
The RF pulse will have the same frequency, as the precision frequency,
of the hydrogen protons.
This has two effects.
1. Some low energy protons, flip to a high energy state.
Since the protons precess, the magnetisation vectors, can be visualised in 3 planes.
When the protons flip, the longitudinal magnetisation decreases.
2. Protons start to precess, in phase.
The net result of these two effects, is that the net magnetisation,
turns to a right angle, to the primary magnetic field.
This is called as transverse magnetisation.
The receiving RF coils, records this event.
Relaxation.
When the RF pulse is stopped, the protons resume to their normal state,
which existed, before the RF pulse was switched on.
This is called MR relaxation.
Relaxation is measured in two directions.
The longitudinal direction, or along the z axis.
This is the direction of the applied magnetic field, by the MRI machine.
The transverse direction, or along the x, y plane.
T1 relaxation, is the longitudinal relaxation, which happens along the z axis.
T2 relaxation, is the transverse relaxation, which happens along the x, y plane.
Each pixel in a MRI image corresponds to millions of protons.
The collective effort of these protons, can be only explained statistically.
This drop from high energy, to MR relaxation energy is measured.
The protons flip back at a certain rate.
The effect of this drop is measured as T1, T2 and T2*.
The relaxation happens over a period of time, measured in milliseconds.
They are equivalent to measurement of half life.
The time of relaxation varies with type of molecule.
Hydrogen bonded to oxygen, differs from hydrogen bonded to carbon.
This helps to differentiate tissues.
Water for example, relaxes at a different rate, than fat tissue.
T1 relaxation.
When the RF pulse is applied, some protons do a 180 degree flip.
The RF pulse induces the larmor frequency spins, to a higher level.
After the RF pulse is removed, several protons slip back to their low energy state.
They do this over a small period of time, measured in milli seconds.
In the process, they give up energy, to the lattice or surroundings.
The magnetisation vector gets smaller, becomes positive and reaches a steady state.
T1 is the time constant.
T1 relaxation is called as spin lattice relaxation.
T1 gives us the longitudinal contrast.
T1 weighted images are good for anatomy.
They are not good for pathology.
T2 relaxation.
A different RF pulse is applied, typically a weaker pulse, to measure T2 relaxation.
This causes a 90 degree flip in the precession.
After the RF pulse is removed, another type of relaxation takes place.
When the RF pulse is removed, the protons begin to phaseout.
They phaseout of the larmor frequency, in the transverse or x,y plane.
The magnetic vector, in the x,y plane starts to reduce.
This is known as transverse spin relaxation, or T2 relaxation.
T2 relaxation is much faster than T1 relaxation.
The T2 relaxation signals are picked up by RF coils.
The net magnetic vector, is the sum of all the proton magnetisation.
This is same as the sum of the longitudinal and transverse magnetisation.
The net magnetic vector, spirals around the z axis.
The changing magnetic momentum, of the net magnetic vector,
induces an electrical signal.
This signal is received by the RF coil, in the transverse plane.
We can plot the transverse magnetisation, against time.
We will find that this reduces over time.
Eventually the transverse magnetisation becomes zero.
The T2 relaxation varies with the tissue.
For example, T2 relaxation of water takes longer.
T2 relaxation of fat, is faster.
This property is useful in identifying tissues.
Computer system.
The computer system, receives the RF signals, from the receiving RF coils.
The signals are received in analog form, and are translated into digital form.
Specialised image processing software, convert these signals, into a visible image.
This image is displayed in the MRI screen.
Skilled personnel can interpret this image.
This can be used for various clinical purposes.
T2* relaxation.
Different tissues have slightly different magnetic environment.
This is due to local random interactions.
This is an internal inhomogeneity.
This property is specific to each substance, or tissue.
The external magnetic field, due to the MRI apparatus, causes external homogeneity.
When we combine both the internal and external properties,
we arrive at a revised parameter called T2*.
This also is used to interpret images.
Typical T1 and T2 relaxation.
Different tissues had different T1 and T2 relaxation properties.
T1 and T2 are measured in milliseconds.
We can discuss a few examples of T1 and T2 relaxation, in different tissues.
Water has a T1 of 3000 milliseconds, and T2 relaxation of 3000 milliseconds.
Neurons or grey matter in the brain, has a T1 of 950, and a T2 of 100.
White matter in the brain, has a T1 of 600, and T2 of 80.
Fat has a T1 of 250, and T2 of 50.
In this manner a wide variety of tissues, can be identified using MRI imaging.
MRI imaging is much more sophisticated, than other clinical methods.
This has many useful clinical applications.
Functional MRI.
Functional MRI is a specialised use of MRI techniques.
It is used by scientists to research how the brain works.
We now have a very good idea, of the anatomy of the brain.
However, our understanding of the functioning of the brain, is still under research.
Functional MRI promises to give us a better understanding of brain functioning.
When the brain is involved in a specific activity,
different regions of neurons clusters, become active.
For example, when picking up a cup of tea,
certain groups of neurons in the motor cortex, become active.
Some other associated region can also become active.
This pattern can be co-related to the function that the brain is performed.
Similarly, there are patterns of brain functioning while viewing an image,
speaking, listening, recalling memory, etc..
By making the subject perform different tasks, while inside the MRI machine,
scientists can observe the pattern of neuron activation.
In this way they can research the functional aspect of the brain.
When a neuron becomes active, it requires more oxygen to work.
The brain has many blood vessels supplying oxygen and nutrition to the neurons,
and groups of neurons.
The blood carrying oxygen, has oxygenated haemoglobin.
Oxygenated haemoglobin, and de-oxygenated haemoglobin exhibit different MRI properties.
They can be easily identified using functional MRI techniques.
By measuring the ratio of oxygenated and deoxygenated MRI,
we can indirectly derive which parts of the brain are active.
By measuring this for different tasks performed by the brain,
we can get a better understanding of the functioning of the brain.
Scientists are extensively using this for this purpose.
Many exciting discoveries have been made, and we can expect many more in the near future.