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What are the two theories used to describe the basic principles of MRI?

Quantum physics as well as Classical physics.


Define Atoms and molecules.

All things are made of atoms.
An atom is a unit of matter that consists of a dense central nucleus and is surrounded by a cloud of negatively charged electrons.

Atoms are arranged in molecules.

“A molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong covalent chemical bonds.


Define covalent bond.

A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms and other covalent bonds.”


Define nucleon.

A collective name for two particles subdivided into protons and neutrons. The mass number comes from nucleons.


Define Proton, Neutron and Electron.

Proton: It is a subatomic particle that has a positive charge. It is found in the nucleus of each atom along with neutrons it is also known as the hydrogen ion.

Neutron: It is a subatomic particle that has a no net charge. It is found in the nucleus of each atom along with protons.

Electron: It is a subatomic particle that carries a negative electrical charge and spins around the nucleus.


Isotopes are:

Mass number is:

Isotopes: Atoms in the same element having different mass numbers.

Mass number: The sum of the number of neutrons and protons in an atomic nucleus.



Ion: “Is an atom or molecule in which the total number of electrons it not equal to the total number of protons, giving it a net positive or negative electrical charge” Atoms that are electrically unstable and leads to an emission of energy (radioactivity).


Why is the mass number important in MRI?

MRI active nuclei are nuclei with an odd mass number, where the neutrons are slightly more or less than the number of protons and the spin directions are not equal and opposite.

The nucleus in those cases has a net spin or angular momentum.

They are characterized by their tendency to align their axis to the applied magnetic field.


What conditions are necessary for resonance in MRI?

Radiofrequency of exactly the Larmor frequency of hydrogen must be applied.


What is the difference between phase and frequency?

Frequency and Phase the watch analogy.

Frequency is: The time it takes the little hand to make one revolution of the watch face i.e. 12 hours. It moves around the face of the watch every 12 hours.

Phase is: The degrees of the little hand of the watch or the radians e.g. 1 o’clock, 2 o’clock, which corresponds to its position on the face of the watch.

Phase and frequency is connected. The phase of the little hand depends on the frequency. It will tell the correct time if the watch is not fast or too slow.

Another way to say it is that frequency is the change of phase over distance.


Why is hydrogen used in MRI and not the other MRI active nuclei?

It is abundant in the human body.
It contains one proton that gives it a relatively large magnetic moment.


Describe the three motions present within the atom.

1) Electrons spinning on their own axis.
2) Electrons orbiting around the nucleus.
3) Nucleus spins on its own axis.


What is the Larmor equation and what do the symbol’s represent?

The Lamor equation governs the precessional frequency, it states:

ῳ = Bo x γ

ῳ = Precessional frequency
Bo = Magnetic field strength of the magnet
γ = Gyro-magnetic ratio

Precessional frequency is therefore often called the Larmor frequency.


What does the gyro-magnetic ratio expresses and what is the gyro-magnetic ratio of hydrogen?

Gyro-magnetic ratio is constant and expresses the relationship between the angular momentum and the magnetic moment of each MR active nucleus. It is expressed as the precessional frequency of a specific MR active nucleus at 1T and therefore the unit of gyro-magnetic ratio is MHz/T.

The gyro-magnetic ration of hydrogen is 42.57 MHz/T.

Other MR active nuclei have different Gyro-magnetic ratios and therefore different precessional frequencies at the same field strength.


What is resonance?

“Resonance is a phenomenon that occurs when an object is exposed to an oscillating perturbation that has a frequency close to its own natural frequency of oscillation.”

When this process happens to a nucleus, it gains energy from the external force. The frequency delivered must be exactly the same as the as the precessional frequency of the nucleus otherwise resonance does not occur.

The energy (precessional frequency of hydrogen) at all field strengths in clinical MRI corresponds to (RF) radio frequency band of the electromagnetic spectrum.

The results of resonance?
Magnetism in the transverse plane that is in phase and Presecces at the Larmor frequency.


What is FID?

Free Induction Decay

When we switch the RF pulse off the NMV is again influenced by Bo and tries to realign with it.

In order to do so the hydrogen nuclei must lose the energy given to them by the RF pulse.

This process in which the hydrogen loses this energy is called relaxation.

Some of the high-energy nuclei returns to the low-energy population and align their magnetic moments to the magnetic field.

Recovery is when the amount of magnetization in the longitudinal plane gradually increases.

Decay is when at the same time, but independently the amount of magnetization in the transverse plane gradually decreases.

The magnetization in the transverse plane decreases as well as the voltage induced in the receiver coil.

This induction of reduced signal is called the free induction decay signal (FID).


What is Relaxation?

During this process the hydrogen give up their absorbed RF energy and their NMV returns to Bo. Independently, but at the same time the magnetic moments of hydrogen lose coherency due to dephasing.

What happens in relaxation? It results in:
Recovery of magnetization in the longitudinal plane (by a process termed T1 recovery).
Decay of magnetization in the transverse plane (by a process called T2 decay).


What is T1 Recovery?

When nuclei give up their energy to the surrounding environment (lattice) it is called: spin lattice relaxation.

The energy releases to the surrounding environment causes the magnetic moments of the nuclei to recover in the longitudinal plane.

The rate of recovery is an exponential process and has a recovery time constant called T1 relaxation time.

It is the time it takes for 63% of the longitudinal magnetization to recover in the tissue.

(MRI Intro sides - draw the diagram and graph on slide 17)

T1 Recovery
Longitudinal relaxation causes recovery of the
longitudinal (z) component of
magnetization (from blue to red)

T1 Recovery curve
Higher magnetic fields are
associated with
longer T1 times.


What is T2 decay?

When nuclei exchange their energy with their
neighbouring nuclei it is termed spin-spin relaxation
and results in the loss of coherent transverse
magnetization in the transverse plane.

This rate of decay is also an exponential process.

It is the time it takes 63% of the transverse magnetization to be lost.

(Draw the diagram and graph from MRI Intro - slide 19)

T2 Decay
Transverse relaxation causes shortening
of the transverse component of magnetization
(from blue to red), at an exponential rate
with time-constant T2.

T2 Decay curve
The T2 and T1 times are
approximately the same, 2-3


Describe TR and TE.

TR (Repetition time):
is the time from application of one RF pulse to the application of the next RF pulse for each slice and is measured in milliseconds(ms).

TE (echo time):
is the time from the application of the RF pulse to the peak of the signal induced in the coil and is also measured in milliseconds (ms).

By altering the TE or TR, i.e. the time between
successive 90° pulses, the signal contrast can be altered or weighted.

(Draw graph MRI intro - slide 21)


Law of electromagnetic induction, what are the three forces?

Charge, magnetism, motion
If there are two, the third one will occur.


Define the magnetic moment.

each nucleus has size and direction (vector properties)


The north/south axis of each nucleus is represented by:

This north/south axis of each nucleus is represented by a magnetic moment and is used in the principles of classic theory of MRI.


A magnetic field is created when:

A magnetic field is created when a charged particle moves.


What is the quantum theory?

Describes discrete quantities of energy.
Hydrogen nuclei possess discrete quantities of energy of populations termed high and low.


What are the only two possible energy states of hydrogen in a magnetic field?

Parallel (spin down) and Anti-parallel (spin up)


What two factors determine how the hydrogen nucleus will align?

The strength of the magnetic field applied and the thermal energy of the nuclei determines how they will align.


Why is there an improved signal when there is a higher field strength?

The net magnetization vector is high with a lower energy nuclei because the higher field strength magnets will be too strong, so that nuclei can't align antiparallel.



The secondary spin of a nuclei is precession. Precession is a simple rotation of the magnetization vector about the longitudinal (z) axis.


Precession rate is governed by:

Lamor frequency