Magnetic Resonance Imaging MRI. Magnetic Resonance Imaging MRI uses the interaction between the magnetic properties of hydrogen nuclei, external magnetic

Magnetic

Resonance Imaging

MRI

Magnetic Resonance Imaging MRI uses the interaction between the magnetic properties of hydrogen nuclei, external magnetic fields and electromagnetic radiation to obtain data used produce the image.

To produce the strong magnetic field required for MRI, the scanners require the use of a superconducting magnet that needing a liquid helium coolant.

Magnetic Resonance ImagingMRI machines look similar to CT and PET scanners

…but they operate on totally different principles

Magnetic Resonance ImagingMRI was originally called nuclear magnetic resonance imaging (NMRI). The word nuclear was dropped for two reasons …. Why do you think this was?• The term “nuclear” suggests that ionising radiation

is involved - this is not the case. Since the term is potentially misleading, it was dropped.

• There are negative associations for many people with the word “nuclear”. The term was dropped as a marketing strategy to make MRI more acceptable to the public (and to save doctors the time of having to explain to patients that the process is perfectly safe.

•

Magnetic Resonance Imaging

The strong magnetic fields produced by MRI machines results in unusual

occupational risks!


Question

Identify the three interacting factors essential to the principle used in MRI.

• The magnetic property of the protons• The strong external magnetic field• An electromagetic wave

Magnetic Resonance Imaging Advantages

MR does not involve the use of ionising radiation with its associated risks to the patient and the medical staff

It is non-invasive MRI provides excellent soft tissue imaging, providing better

contrast than CT or conventional x–rays and much better resolution than ultrasound

MRI data can be processed to produce a tomographic image or a 3-D image

Except in the case of a few patients who experience anxiety in the MR tube because of the confined space, there is no discomfort to the patient




• Both protons and neutrons in the nuclei of atoms have a property called spin

• This spin property can have one of two possible directions for any given nucleon and alignment of the axis

• If there are even numbers of protons or neutrons, then their spins of each pair align in opposite directions so that the net spin of the pair is zero

Magnetic Resonance Imaging• If there is an odd number of protons or neutrons,

then the nucleus must possess a net spin since pairing cannot occur with one of the nucleons

• Nuclei having a net spin include hydrogen, phosphorus 31, fluorine 19, nitrogen 15 and carbon 13


• In addition to the spin of the nucleons, electrons also have the property of spin

• The electron has a spin associated with both its orbital motion and its axial rotation

• The spin of the electron is small in magnitude compared with that of the much larger and more massive nucleons



A current loop creates a magnetic field

I


A spinning charge behaves like a current loop,creating a northand a southmagnetic pole

Protons spin on their axis, creating a pair of magnetic poles

The proton’s behaviour in an external magnetic field is determined by its magnetic moment


In atomic nuclei, paired protons having opposite spins result in a zero net magnetic effect.

Hydrogen having one proton must have a non-zero magnetic moment.


In materials containing many protons, such as hydrogen rich compounds including water, the protons have randomly oriented magnetic fields.


A strong external magnetic field causes the magnetic moments of the protons to become aligned

Magnetic field

Magnetic Resonance ImagingThe alignment of the magnetic moment of the protons is not exactly in the same direction as the external magnetic field

Magnetic Resonance ImagingProton spins usually become aligned parallel or the external magnetic field

A few proton spins become aligned antiparallel to the external field


The angle between the magnetic moment of the proton and the external magnetic field produces a torque on the proton.


The effect of the torque is to cause the axis of rotation of the proton to precess.

This is the same effect that occurs with a spinning top if the axis of rotation is not parallel to the Earth’s gravitational field.


Precession is the motion that results in the axis of rotation of a body sweeping out a conical motion when a torque acts to affect a change in the axis of rotation of that body.

Precession is a consequence of the law of conservation of angular momentum.

The frequency of precession is called the Larmor frequency.

Magnetic Resonance ImagingThe Larmor frequency depends on• composition of the nucleus• magnitude of the external field

For a proton in a 2 T magnetic field, the Larmor frequency is 85.2 MHz.

This corresponds to a radio frequency (RF) electromagnetic wave.

(only hydrogen is used in MRI)


To utilise the magnetic properties of the proton to produce medical images…

The patient is first placed in a very strong magnetic field

The magnetic field, although very strong, is non-ionising and low risk



Superconducting electromagnets

Superconducting electromagnetsRF Oscillatorand Receiver


The supercooled magnets are electromagnets using superconductors requiring liquid helium to reach the superconducting transition temperature.Only superconducting electromagnets are capable of producing the strong magnetic fields required to align the proton magnetic moments.

The RF oscillator and receiver are usually a single unit, capable of emitting and receiving pulses of radio frequency electromagnetic radiation


Magnetic Resonance ImagingWhen protons in a strong magnetic field radio are exposed to radio waves with a frequency equal to the Larmor frequency, their energy is absorbed by the protons in a process called resonance.This causes the proton’s magnetic alignment to flip from the parallel state, a phenomenon referred to as spin flip.


It is energetically more favourable for hydrogen nuclei to return to their original state in the external magnetic field after the RF pulse. As they do so, they re-emit the energy absorbed from the radio wave in about 0.01 to 0.1 seconds.

The emitted energy is a radio wave that is detected with the same coil that emitted the RF waves to flip the protons.

The signals emitted by the proton are used to create the MR image.

No external field

Random orientation

External field

Alignment

Precession

RF pulse

Spin flip

Relaxation

RF emission

Magnetic Resonance ImagingThe amplitude of the signal produced as the nuclei relax increases with the number of nuclei present

Magnetic resonance imaging results in the production of a map of the hydrogen density throughout a volume of the patient.

The signal strength is greater from tissues having a greater density of hydrogen nuclei.

Magnetic Resonance ImagingThe use of hydrogen in MRI

MRI uses hydrogen because

• Hydrogen has a magnetic moment because the protons are unpaired

• A strong signal from the nuclear relaxation is possible because hydrogen is abundant in human tissues in

• Water• Proteins• Fats• Carbohydrates

Magnetic Resonance ImagingGradient coils produce small variations in the magnetic field across the patient’s body so that the magnetic field intensity has a unique value at every point in the patient’s body.Thus, the hydrogen atoms at each point have a known, and unique Larmor frequency.Both the exact position and the corresponding Larmor frequency are accurately known.

Magnetic Resonance ImagingThe pulse of radio waves are transmitted through the patient’s body from the RF coils.This flips the magnetic axes of the hydrogen nuclei.As they flip back (relaxation) the atoms within each volume element (voxel) emit radio waves.The intensity is proportional to the number of hydrogen atoms in the voxel.


• Data is thus gathered relating the location of the voxel to the hydrogen atom concentration.

• From this data the MR image is computed.



Heart and associated blood vessels

Blood vessels in the brain



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

The difference in relaxation times is significantly greater for tissues containing relatively large amounts of water because of the hydrogen atoms present.MRI is very sensitive to variations in water content of tissues and this is a significant factor in its being able to produce high resolution high contrast images.Because tumours are characterised by rapid cell division and high growth rates, they typically have a higher percentage of water than similar non-cancerous tissue, and can thus be clearly imaged using MR.


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

Haemoglobin molecules in red blood cells provides a strong resonance signal, and so MRI can be used to compare the blood content of different tissues. This is often greater in cancerous tissue, because of the high growth rates, and so MRI is an effective diagnostic tool for cancerous tissue.MRI scans of the brain show more contrast and detail than conventional x-ray or CT scans because of the differences in water content of the grey matter and white matter of the brain. Excellent soft tissue resolution can also be achieved using MRI. Colour enhancement provides clearer analysis.





PET SCAN

A word from the creator

This Powerpoint presentation was prepared by Greg Pitt of Hurlstone Agricultural High School.

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Magnetic Resonance Imaging MRI. Magnetic Resonance Imaging MRI uses the interaction between the magnetic properties of hydrogen nuclei, external magnetic