MRI Physics Overview: Key Principles and System
Components for Imaging
Basic MRI Principles:
Nuclear Magnetic Resonance ( NMR)
Spin ( non-zero spin of hydrogen atoms)
Magnetic Moment
Alignment with the main magnetic field (B₀)
Precession
Larmor Frequency ( processional frequency)
Resonance ( nuclear magnetic resonance)
Radio Frequency (RF) Pulse (perpendicular to B₀)
Excitation ( flipping of spins into the transverse plane)
Flip Angle ( e.g., 90 degrees, 180 degrees)
Nuclear Magnetic Resonance The phenomenon that occurs when nuclei with a non-
( NMR) zero spin, like H protons, are placed in an external
magnetic field and absorb energy from RF pulses that
match their precessional frequency ( Larmor Frequency) ,
causing them to resonate
Spin A quantum property of subatomic particles, like
( non-zero spin of hydrogen atoms) protons, that describes how they will react to an external
magnetic field.
Hydrogen atoms have a non-zero spin value of one-half.
Atoms with non-zero spin have a magnetic moment.
Magnetic Moment A vector-like property of atoms with non-zero spin, such
as hydrogen protons, describing the magnetic field
around that proton.
It has both direction and magnitude.
The net magnetization vector is the sum of all these
magnetic moments.
Alignment with the main magnetic field When hydrogen atoms with magnetic moments are
(B₀) placed in an external magnetic field (B₀), they will align
with this field.
Precession The secondary rotation of an atom’s magnetic moment
around the axis of an external magnetic field (B₀),
similar to how a spinning top wobbles around its axis
due to gravity. This occurs at a specific frequency
known as the Larmor Frequency.
, Larmor Frequency Precessional Frequency of a nucleus in an external
( precessional frequency) magnetic field.
It is dependent on the gyromagnetic ratio of that
element ( ex. Hydrogen) and the strength of the
external magnetic field.
Resonance NMR occurs when an RF pulse is applied at the Larmor
( nuclear magnetic resonance) frequency of the spins, causing them to absorb energy,
resonate in phase, and flip into the transverse plane.
Radio Frequency ( RF) Pulse Electromagnetic pulse in the radio frequency range that
(perpendicular to B₀) is applied perpendicular to the main magnetic field
(B₀).
When its frequency matches the Larmor Frequency, it
can cause NMR.
Excitation Process of using an RF pulse at the Larmor Frequency to
( flipping of spins into the transverse plane) transfer energy to the spins, causing them to flip from
the longitudinal plane into the transverse plane.
This results in a gain of transverse magnetization and a
loss of longitudinal magnetization.
Flip Angle The angle through which the net magnetization vector
( e.g., 90 degrees, 180 degrees) is rotated by the application of an RF pulse.
90 degrees - maximum transverse magnetization
180 degrees - spin echo sequences
MRI System Components:
Main Magnet/Coil (generates B₀) B₀ (Main
Magnetic Field)
Gradient Coils ( for spatial encoding)
Slice Selection Gradient ( along the z-axis) Frequency Encoding
Gradient ( along the x-axis) Phase Encoding Gradient ( along the
y-axis)
Radio Frequency ( RF) Coils ( transmit RF pulse and receive signal) Receiver Coil
Shims (for improving B₀ homogeneity)
Passive Shims Active Shims
Superconductivity ( in main coils) Quenching ( loss
of superconductivity) Bore ( of the MRI machine)
Main Magnet/Coil Outermost layer of the MRI machine. Generates the strong, static
(generates B₀) main magnetic field (B₀), along the longitudinal ( z) axis.
Typically uses superconducting wires to carry high currents
Components for Imaging
Basic MRI Principles:
Nuclear Magnetic Resonance ( NMR)
Spin ( non-zero spin of hydrogen atoms)
Magnetic Moment
Alignment with the main magnetic field (B₀)
Precession
Larmor Frequency ( processional frequency)
Resonance ( nuclear magnetic resonance)
Radio Frequency (RF) Pulse (perpendicular to B₀)
Excitation ( flipping of spins into the transverse plane)
Flip Angle ( e.g., 90 degrees, 180 degrees)
Nuclear Magnetic Resonance The phenomenon that occurs when nuclei with a non-
( NMR) zero spin, like H protons, are placed in an external
magnetic field and absorb energy from RF pulses that
match their precessional frequency ( Larmor Frequency) ,
causing them to resonate
Spin A quantum property of subatomic particles, like
( non-zero spin of hydrogen atoms) protons, that describes how they will react to an external
magnetic field.
Hydrogen atoms have a non-zero spin value of one-half.
Atoms with non-zero spin have a magnetic moment.
Magnetic Moment A vector-like property of atoms with non-zero spin, such
as hydrogen protons, describing the magnetic field
around that proton.
It has both direction and magnitude.
The net magnetization vector is the sum of all these
magnetic moments.
Alignment with the main magnetic field When hydrogen atoms with magnetic moments are
(B₀) placed in an external magnetic field (B₀), they will align
with this field.
Precession The secondary rotation of an atom’s magnetic moment
around the axis of an external magnetic field (B₀),
similar to how a spinning top wobbles around its axis
due to gravity. This occurs at a specific frequency
known as the Larmor Frequency.
, Larmor Frequency Precessional Frequency of a nucleus in an external
( precessional frequency) magnetic field.
It is dependent on the gyromagnetic ratio of that
element ( ex. Hydrogen) and the strength of the
external magnetic field.
Resonance NMR occurs when an RF pulse is applied at the Larmor
( nuclear magnetic resonance) frequency of the spins, causing them to absorb energy,
resonate in phase, and flip into the transverse plane.
Radio Frequency ( RF) Pulse Electromagnetic pulse in the radio frequency range that
(perpendicular to B₀) is applied perpendicular to the main magnetic field
(B₀).
When its frequency matches the Larmor Frequency, it
can cause NMR.
Excitation Process of using an RF pulse at the Larmor Frequency to
( flipping of spins into the transverse plane) transfer energy to the spins, causing them to flip from
the longitudinal plane into the transverse plane.
This results in a gain of transverse magnetization and a
loss of longitudinal magnetization.
Flip Angle The angle through which the net magnetization vector
( e.g., 90 degrees, 180 degrees) is rotated by the application of an RF pulse.
90 degrees - maximum transverse magnetization
180 degrees - spin echo sequences
MRI System Components:
Main Magnet/Coil (generates B₀) B₀ (Main
Magnetic Field)
Gradient Coils ( for spatial encoding)
Slice Selection Gradient ( along the z-axis) Frequency Encoding
Gradient ( along the x-axis) Phase Encoding Gradient ( along the
y-axis)
Radio Frequency ( RF) Coils ( transmit RF pulse and receive signal) Receiver Coil
Shims (for improving B₀ homogeneity)
Passive Shims Active Shims
Superconductivity ( in main coils) Quenching ( loss
of superconductivity) Bore ( of the MRI machine)
Main Magnet/Coil Outermost layer of the MRI machine. Generates the strong, static
(generates B₀) main magnetic field (B₀), along the longitudinal ( z) axis.
Typically uses superconducting wires to carry high currents
