Nuclear Magnetism and MRI Flashcards
What gives an atom nuclear magnetism?
The imbalance of the number of protons and neutrons in an atom’s nucleus gives the property spin.
A +ve electrical charge + spin = magnetism.
Therefore a Hydrogen isotope containing a single proton exhibits nuclear magnetism. It also exhibits the strongest MR signal when compared to other isotopes.
Very good video giving an overview of the topic:
https://www.youtube.com/watch?v=jLnuPKhKXVM&t=2s
What happens to Hydrogen isotopes in the body when they are placed in a strong magnetic field?
With all these isotopes present in the body, similarly to ferromagnetic materials, they cancel each other’s magnetic field out as are orientated randomly.
When placing a strong magnetic field externally, the majority orientate with the field (they are parallel). There are always some that are aligned against the magnetic field (anti-parallel). This is due to inherent energy within these protons allowing them to oppose the magnetic field.
The net magnetisation (the sum of all the magnetic tissues) will be in alignment with the magnetic field.
What is precession?
The axis of the hydrogen isotopes are at a slight angle to the main magnetic field therefore when they spin they resemble a cone.
This is known as precession.
What is larmor frequency?
Larmor frequency describes the rate of procession.
It can be expressed by the following formula.
Larmor frequency = gyromagnetic ratio* x magnetic field
ω= γ x B0
*this is a constant which varies for all isotopes that exhibit nuclear magnetism
Take home: the lamor frequency is directly proportional to the magnetic field
What is resonance in the context of MRI?
Resonance involves the absorption of energy by the precessing isotopes. As they absorb energy they move away from being aligned with the magnetic field.
For resonance to occur the energy must be delivered at the same rate of oscillation/precession referred to by the Larmor frequency.
The magnetic field is aligned to the Z of an xyz axis.
As energy is absorbed (in the form of radio-frequency pulses) the net magnetism moves towards the y axis.
When there is 90 degree rotation this produces the maximum tissue signal. Therefore a 90 degree radio-frequency (RF) pulse produces the maximum tissue signal.
What is relaxation in the context of MRI?
Refers to the processes after radio-frequency pulses are switched off and the net magnetisation returns to equilibrium.
There are two main components to the relaxation process.
- T1 relaxation aka ‘spin lattice relaxation’ – Longitudinal magnetisation
- T2 relaxation aka ‘spin-spin relaxation’ – Transverse magnetisation
Explain T1 and T2 relaxation and what the T1 & T2 times are.
During a spin echo sequence (an MRI sequence):
Initially there is net longitudinal magnetism (z axis aligned) in line with the nuclear field.
A radio-frequency pulse provides energy to the protons which changes the net magnetism to transverse magnetism (y axis aligned).
As the radio frequency pulse is turned off the protons quickly lose energy and revert back to there original aligned state.
Similarly to the precessing conical movement they will follow this shape back to Z alignment (imaging tracing your finger up and upside down ice cream cone).
T1 relaxation refers to the protons regaining there longitudinal magnetism. T1 time refers to the time at which 63% of the protons have regained their longitudinal magnetism.
T2 relaxation refers to the protons losing there transverse magnetism. As the transverse magnetism is lost the MR signal decreases. T2 relaxation is also known as FREE INDUCTION DELAY.
T2 time refers to the time at which 63% of protons have lost their transverse magnetism.
*Note both T1 and T2 relaxation occur in an exponential manner.
**Different tissues in the body have different T1 and T2 times, comparison in the different relaxation times in both T1 and T2 is what allows contrast and comparison of different structures in MRI.
How are T1 and T2 times affected by the strength of the magnetic field?
The stronger the magnetic field the longer the T1 time.
Magnetic field strength does not really affect T2 time.
What are T2* effects?
T2* effects refer to signal decay which occurs in the MRI images.
Dephasing of protons (free induction decay) occurs at a speed known as the T2* constant.
The exponential decay in synchronisation of proton preccessing (e.g. the loss of transverse magnetism) occurs as each proton are exposed to a slightly different magnetic force. Therefore there is never true precessing uniformity.
Differences in the procession compile and the protons become increasingly asynchronous. This lead to signal drop out and would lead to areas diffuse signal loss in the image.
How does MRI produce high quality images negating the T2* effects?
By using specifically timed and measured RF pulses are known as pulse sequences, to help realign the protons.
Images obtained from the pulse sequences require 100s to 1000s of measurements to allow the data acquired to be converted into recognisable MR images.
The same section of anatomical tissue is repeatedly exposed to the RF pulses. (This is why MRI scans take so long).
What are the main factors that determine the T1 and T2 influence on the contrast of the images?
Repetition Time (TR): The time between each of the RF pulse sequences
Echo Time (TE): The time between the tissue being excited until a signal is detected
Explain how T1 weighted images are obtained?
Repitition time (TR) is a reflexion on the T1 relaxation time: If TR is long i.e. >2000ms, then there is a strong likelihood that a large proportion of most tissues will have returned to equilibrium.
If TR is short i.e. 500ms, then there is a strong likelihood that 2 tissues with different T1 relaxation times will be differentiated as may not have returned to equilibrium.
Therefore to increase T1 weighting you want a short TR.
In addition an Echo time (TE) must be selected to minimise any other effects on the MR signal through T2 relaxation time. The shorter the TE the less T2 influence.
Therefore to achieve the most T1 weighted image you want a short TR and a short TE.
How do you obtain a T2 weighted image?
By Prolonging the TR you minimise the T1 differentiation. T2 differences can be maximised through lengthening of the TE.
How do T1 and T2 images compare?
In T1 images. Only 1 tissue type is bright Fat.
Fluids are dark.
In T2 images 2 tissue types are bright Fat and Fluid.
