MRI unit 2 - fundamentals

Weber

flux density

The __________ is the precessional frequency of the magnetic moments of a specific nucleus at 1T and has units of MHz/T

gyromagnetic ratio

Nuclei with odd mass number

have a net spin

Regardless of the direction of the main magnetic field, the z-axis is along the

long axis of the patients body

As all nuclei contain at least one positively charged proton, those that also spin have a ______________ induced around them

magnetic field

An arrow called a _________________ denotes the magnetic fi eld of a nucleus.

magnetic moment

At rest (when the patient is simply placed inside the magnetic fi eld and exposed to B0), the magnetic moments of the hydrogen spins are out of phase with each other and therefore the _____________ does not precess.

net magnetization vector (NMV)

At thermal equilibrium, there are more spin‐up, low‐energy than spin‐down, high‐energy spins, so the _______________ of the patient is orientated in the same direction as B0

net magnetization vector (NMV)

Once the net magnetization begins to move out of alignment with B0, it becomes the __________________

net magnetization vector (NMV)

The gyromagnetic ratio for hydrogen protons is approximately _________

42.6 MHz/T

alignment of magnetic moments in the opposite direction to the main field

Anti-parallel alignment

The difference in energy between these populations (spin-up or spin-down) is determined by the strength of ________

B0

As ________ increases the energy difference between the two populations also increases, as the number of spin‐up, low‐energy spins ____________ relative to the number of spin‐down, high‐energy spins

B0, increases

Requirements for MRI: • Strong magnetic field-collectively known as ____________ • External __________ energy source • Odd number of ____________ • For MRI purposes, the proton within the _______ atom is the particle of interest

B0, radio frequency (RF), nuclear protons, Hydrogen

Nuclei with angular momentum are called _________

MR active nuclei

What is protium?

MR active nucleus that is an isotope of hydrogen

___________ is the most abundant element in the human body.

Hydrogen

__________ means that the magnetic moments of hydrogen spins are at the same place on the precessional path at a moment in time

In phase or coherent

The ______________ is used to calculate the frequency or speed of precession for the magnetic moments of a specific nucleus in a specific magnetic field strength

Larmor equation

The magnetic moments of all the spins precess around B 0 at the ______________ that is proportional to B0 for a given MR active nucleus. _________ therefore refers to how fast the magnetic moments of spins are precessing and is measured in MHz in MRI.

Larmor frequency, Frequency

_________ means that the magnetic moments of hydrogen spins are at different places on the precessional path at a moment in time

Out of phase or incoherent.

alignment of magnetic moments in the same direction as the main field.

Parallel alignment

__________ refers to the position of the magnetic moments of spins on their precessional path at any moment in time.

Phase

Size of flip angle depends on what?

RF amplitude and duration

The angle that is made by the net magnetization tilting away from B0 is known as the ____________

RF flip angle (How far the NMV moves away from B0)

___________ nuclei have high energy and have enough energy to oppose the main field. These are nuclei that align their magnetic moments ______________ to the main field

Spin‐down, anti‐parallel

When placed in an external magnetic field, the magnetic moments of hydrogen either align in a _____________ or ___________ orientation.

Spin‐up, low‐energy or spin‐down, high‐energy

___________ nuclei have low energy and do not have enough energy to oppose the main field. These are nuclei that align their magnetic moments __________ to the main field

Spin‐up, parallel

When spins are placed in an external magnetic field, their magnetic moments line up with the magnetic field flux lines. This is called _____________

alignment

What is net spin

angular momentum

The y-axis extends ___________ through the patient

anterior to posterior (coronal)

Nuclei with even mass number

have no net spin

Due to the low radio frequency waves used, RF energy is not sufficiently energetic to cause __________.

ionization

the process of removing electrons from the atom is called ____________

ionization

Atoms with a deficit or excess number of electrons are called ______________

ions

The anatomy we are imaging must be placed at ___________

isocenter

Atoms of the same element having a different mass number are called ______________

isotopes

The x-axis extends from the patient's ___________

left to right side (sagital)

The _____________ is composed of the vectors of all the magnetic moments of all the hydrogen protons in all the different tissues of the patient, combined together

net magnetization vector (NMV)

The nuclei that are available for MRI are those that exhibit a ______________________?

net spin (because their mass number is an odd number)

At one tesla, only ___________ unpaired H protons are required to achieve equilibrium.

one in a million

At rest the magnetic moments of the spins are ___________ with each other.

out of phase

B0 will cause an additional movement of the magnetic moment of the H proton. Instead of just spinning, they now will spin and wobble. This additional wobble is called ___________

precession

Every MR active nucleus is spinning on its own axis. The magnetic field exerts a torque on the magnetic moments of all MR active nuclei, causing a secondary spin. This spin is called ______ and causes the magnetic moments of all MR active nuclei (spin up and spin down) to describe a circular path around B0

precession

The frequency of rotation, speed at which they wobble around B0 , is known as the ___________

precessional frequency

The speed at which the magnetic moments spin about the external magnetic field is called the ____________

precessional frequency .

For fi eld strengths used in clinical imaging, the Larmor frequency of hydrogen is in the _____________ of the electromagnetic spectrum (why MRI is considered a "safe" modality)

radiofrequency band

Th e precessional frequency corresponds to the range of frequencies in the electromagnetic spectrum of ____________.

radiowaves

The _____________ increases at higher values of B0

signal to noise ratio (SNR)

Magnetic moments are __________

vectors (has size and direction?

What is the Larmor equation?

ω0 = γB0. The precessional frequency is denoted by ω0 and expressed in megahertz The strength of the external field is expressed in tesla (T) and denoted by the symbol B0 The gyromagnetic ratio is the precessional frequency of the magnetic moments of a specific nucleus at 1T and has units of MHz/T. It is denoted by γ