MRI Physics (chapter 1)

MRI is explained using both classical theory (mass, spin, and angular momentum) and quantum theory (atomic-scale energy levels of particles).

True or False 

True

Hydrogen is the most abundant atom in the human body.

True or False 

True 

Protons have a negative charge.

True or False 

False

The atomic number is the sum of protons and neutrons in the nucleus.

True or False 

False 

An isotope has the same number of protons and neutrons.

True or False 

False 

MR signal relies on the spinning motion of specific nuclei in tissues.

True or False 

True 

Nuclei with an even mass number are considered MR-active.

True or False 

False

Hydrogen is preferred in MRI because it has two protons, which creates a strong magnetic moment.

True or False 

False 

In a strong magnetic field, more hydrogen protons align parallel than antiparallel.

True or False 

True

Parallel alignment is associated with higher energy and fewer protons.

True or False 

False 

The net magnetic vector increases as magnetic field strength increases.

True or False 

True 

Precession refers to the spin of the entire atom on its axis.

True or False 

False

Precessional frequency is measured in MHz.
True or False 

True 

The RF pulse must be applied at the Larmor frequency to achieve resonance.

True or False 

True

Excitation occurs when protons lose energy.

True or False 

False 

A 90° flip angle means protons are completely moved into the transverse plane.

True or False 

True 

The MR signal is produced when in-phase magnetization cuts across the receiver coil.

True or False 

True 

Relaxation occurs when nuclei gain energy from the RF pulse.

True or False 

False 

TR determines how much T1 relaxation occurs before the next RF pulse.

True or False 

True 

TE determines how much T2 relaxation (dephasing) has occurred when the signal is read.

True or False 

True 

Proton

Positively charged particle

Neutron

No charge

Electron

Negatively charged particle

Hydrogen atom

One proton and one electron

Atomic number

Number of protons in the nucleus

Mass number

Sum of protons and neutrons in the nucleus

Isotope

Atom with different number of neutrons than protons

Angular momentum

Spin of MR active nuclei depending on proton-neutron balance

Magnetic moment

Direction and amplitude of magnetic field of nucleus

MR-active nuclei

Odd mass number; aligns with magnetic field

B₀

Main magnetic field symbol

In phase

Magnetic moments at same position on precessional path

Out of phase

Magnetic moments at different positions

Phase

Position on precessional path at any moment

Frequency

Rate of change of phase over time

Precession

Wobble-like motion around B₀

Precessional path

Circular path of magnetic moments

Gyromagnetic ratio

Constant specific to a nucleus, e.g., 42.57 MHz/T for H

Larmor equation

ω₀ = γ B₀ (precessional frequency = gyromagnetic ratio B₀)

RF excitation pulse

B₁; causes resonance by applying energy

Excitation

Movement of protons to higher energy states via RF pulse

Flip angle

Degree to which NMV is rotated by RF pulse

Faraday’s Law

Changing magnetic field induces current in a coil

Free Induction Decay

Signal loss after RF pulse ends

Relaxation

Spins lose energy and realign with B₀

TR

Repetition time; affects T1 weighting

TE

Echo time; affects T2 weighting

Electrons: particles that spin around the 

nucleus 

According to quantum theory, the position of an electron is not predictable as it depends on the ____ of an individual electron at any moment in time (this is called Heisenberg’s Uncertainty Principle). 

energy 

____ are electrically stable if the number of negatively charged ____ equals the number of positively charged ____. 

atoms, electrons, protons 

The basic principles of magnetic ____ imaging (MRI) form the foundation for further understanding of this complex subject. 

resonance 

MR-active nuclei have a(n) ____ electrical charge (electric field) and are spinning (motion), and, therefore, automatically acquire a magnetic field. 

net 

The total magnetic moment of the nucleus is the vector sum of all the magnetic moments of ____ in the nucleus. 

protons 

Faraday’s law of ______ induction states that a magnetic field is created by a charged moving particle (that creates an electric field).

electromagnetic

Nucleons

Particles in the nucleus

Quantum theory uses the ____ level of the spins (or hydrogen nuclei) to illustrate alignment. 

energy 

A very common misunderstanding is that when a patient is exposed to B0, the hydrogen nucleus itself aligns with the ____ magnetic field. 

external 

What does “NMV” stand for?

net magnetic vector

As the field strength increases, fewer spins possess enough ____ to align their magnetic moments in opposition to the larger B0 field. 

energy 

Phase refers to the position of magnetic moments on their ____ path at any moment in time.

precessional

When the only influence is B0, the magnetic moments of the nuclei are out of phase with each other, and therefore the _____ does not precess. 

NMV

What does “SNR” stand for? 

Signal-to-noise ratio 

Usually, the flip angle is 90,i.e. the NMV is given enough ____ by the RF excitation pulse to move through 90 relative to B0. 

energy 

With a ____ angle of 90, the nuclei are given sufficient energy so that the ____ NMV is completely transferred into a _____ NMV. 

flip, longitudinal, transverse 

If energy is delivered at a different frequency to that of the Larmor frequency, _____ does not occur, and the nucleus does not gain energy.

resonance

Angle of the NMV to B0.

Flip angle

Is this the correct definition of Free induction decay?

loss of transverse magnetization as a result of spin-spin relaxation. 

Yes or No? 

No 

Is this the correct definition of T2 decay?

loss of transverse magnetization as a result of spin-spin relaxation. 

Yes or No? 

Yes

As relaxation occurs, the ____ returns to realign with B0 because some of the high-energy nuclei return to the low-energy population and there fore align their magnetic moments in the spin-up direction.

NMV

As the received signal amplitude is related to the magnitude of the ____ transverse component, signal in the coil decays as relaxation occurs.

coherent

To do so, the hydrogen nuclei lose ___ given to them by the RF excitation pulse.

energy

Identify the 2 theories used to describe the basic principles of MRI:

Classical theory, Quantum theory

State the most abundant atom in the human body:

Hydrogen

Identify the 2 most common molecules where this atom is found:

Water, Fat

Identify the particles of an atom and their charge:

Neutron (no charge), Proton (positive charge), Electron (negative charge)

Identify the particles of the hydrogen atom:

1 Proton, no neutron

Define atomic number:

The sum of the protons in the nucleus

Define mass number:

The sum of the protons and neutrons in the nucleus

Define isotope:

Atoms of elements with the same number of protons but a different number of neutrons

State what the principles of MRI rely on:

Rely on the spinning motion of specific nuclei present in biological tissues

Define angular momentum:

spin of MR active nuclei, which depends on the balance between the number of protons and neutrons in the nucleus

Describe MR-active nuclei:

Are characterized by their tendency to align their axis of rotation to an applied magnetic field. This occurs because they have angular momentum or spin and, as they contain positively charged protons, they possess an electrical charge

Define magnetic moment

MR-active nuclei have a net electrical charge (electric field) and are
spinning (motion), and, therefore, automatically acquire a magnetic field. In classical theory, this magnetic field is denoted by a magnetic moment.

Discuss why the hydrogen atom is used in clinical MRI:

Hydrogen is very abundant in the human body and because the solitary proton gives it a relatively large magnetic moment. These characteristics mean that the maximum amount of available magnetization in the body is utilized.

Name the symbol used for scanner's magnetic field:

B0

Discuss thermal equilibrium:

Assumes the patient's temperature is constant and therefore does not influence the thermal energy of hydrogen during the MR experiment. The patient's temperature is usually similar inside and outside the magnetic field.

Define NMV:

As there is a larger number aligned parallel, there is always a small excess in this direction that produces a net magnetic moment. reflects the relative balance between spin-up and spin-down nuclei. It is the sum of all magnetic moments of excess spin-up nuclei and is measurable (in the order of micro teslas). It aligns in the same direction as the main magnetic field in the longitudinal plane or z-axis.

Describe what happens to NMV as magnet strength is increased:

As the field strength increases, fewer spins possess enough energy to align their magnetic moments in opposition to the larger B0 field. As a result, the low-energy population increases in size, the high-energy population decreases in size, and therefore the number of excess number of spins also increases. At 1.5 T, the number in excess is about 4.5 for every million protons; at 3 T, this increases to about 10 per million. Consequently, the NMV also increases in size and is one of the reasons why the signal-to-noise ratio (SNR) increases at higher field Strengths.

Define precession:

The influence of B0 produces an additional spin or wobble of the magnetic moments of hydrogen around B0. This secondary spin is called precession and causes the magnetic moments to circle around B0.

Define precessional path:

Circular pathway of magnetic moments as they precess around B0.

Define precessional frequency and its units:

Precessional frequency is often called the Larmor frequency because it is determined by the Larmor equation. The unit of precessional frequency is hertz (Hz) where 1 Hz is one cycle or rotation per second (s), and 1 megahertz (MHz) is one million cycles or rotations per second.

State the Larmor equation and define its components:

ω0 = γB0, ω0 is the precessional or Larmor frequency (MHz), γ is the gyromagnetic ratio (MHz/T) B0 is the strength of the external magnetic field (T)

What is the gyromagnetic ratio of hydrogen?

42.58 MHz/T

What is the Larmor frequency of hydrogen at 1 Tesla?

42.58 MHz

What is the Larmor frequency of hydrogen at 1.5 Tesla?

63.8646 MHz

What is the Larmor frequency of hydrogen at 3 Tesla?

127.74MHz

Which equation relates the gyromagnetic ratio to magnetic field strength?

Larmor frequency = γ × B₀

Define phase:

Refers to the position of a magnetic moment of a spin on its
precessional path at any moment in time

Define frequency:

Refers to how fast magnetic moments of spins are precessing and is measured in MHz in MRI

Define out of phase:

Magnetic moments of hydrogen are at different places on the precessional path at a moment in time.