Magnetism and Physics in MRI

Vocabulary practice flashcards covering fundamental MRI physics concepts, including the Larmor equation, magnetic properties of matter, relaxation mechanisms (T1 and T2), and system hardware components.

Precessional frequency

The resonant frequency of nuclear spins, defined by the Larmor equation as the product of the magnetic field strength ($B_0$) and the gyromagnetic ratio ($\gamma$).

Larmor equation

The formula used to calculate precessional frequency: $\omega_0 = \gamma \times B_0$, where $\omega_0$ is precessional frequency, $\gamma$ is the gyromagnetic ratio, and $B_0$ is magnetic field strength.

Gyromagnetic ratio of Hydrogen ($^1\text{H}$)

The constant of proportionality for hydrogen nuclei, equal to $42.57\,\text{MHz}$ per Tesla ($42.57\,\text{MHz/T}$).

Hydrogen precessional frequency at 0.5 Tesla

$21.28\,\text{MHz}$.

Hydrogen resonance frequency at 1.5 Tesla

$63.86\,\text{MHz}$.

Hydrogen resonance frequency at 3 Tesla

$127.71\,\text{MHz}$.

Tesla ($\text{T}$)

A unit of magnetic field strength, where $1\,\text{Tesla} = 1\,\text{Newton per ampere-meter}$ ($1\,\text{N}/(\text{A}\cdot\text{m})$) and is equivalent to $10{,}000\,\text{Gauss}$.

Coulomb

A standard unit of electric charge equivalent to $6.24 \times 10^{18}\,e^-$.

Ampere

A unit of electric current defined as 1 Coulomb per second ($1\,\text{Coulomb/second}$).

Chemical shift (fat and water)

The difference in resonance frequency between fat and water, approximately $3.5\,\text{ppm}$, which corresponds to ~$147\,\text{Hz}$ at $1\,\text{Tesla}$, ~$220\,\text{Hz}$ at $1.5\,\text{Tesla}$, and ~$440\,\text{Hz}$ at $3\,\text{Tesla}$.

Magnetic field inhomogeneity

The non-uniformity or variance of the static magnetic field across space, expressed in parts per million ($\text{ppm}$).

Gauss

A unit of magnetic field strength, where $10{,}000\,\text{Gauss} = 1\,\text{Tesla}$ (such that a $30{,}000\,\text{Gauss}$ system operates at $3\,\text{Tesla}$).

Faraday's law of induction

A physical law stating that a changing magnetic field will induce a voltage.

Superconducting magnet systems

The most commonly used MRI magnet systems due to high field strength and imaging capabilities, characterized by a main magnetic field that runs parallel to the long axis of $B_0$.

Resistive magnets

A type of MRI magnet system that can be shut off the quickest in case of an emergency.

Permanent magnets

Heavy MRI magnet assemblies constructed from blocks of ferromagnetic plates used to generate a magnetic field.

Ferromagnetic substances

Iron and iron-like materials (such as stainless steel, iron, and nickel) that can generate a relatively strong magnetic field and are strongly attracted to the magnetic field.

Paramagnetic substances

Substances that possess a very weak magnetic field; gadolinium is an example used as an MR contrast material.

Diamagnetic substances

Substances that possess no magnetic field.

Missile effect (Torpedo effect)

The hazardous attraction of ferromagnetic objects into the MR scanner, determined by the ferromagnetic properties of the material, the mass of the projectile, and the field strength of the system.

Non-ferrous substances

Materials that are not attracted to magnetic fields, including wood, plastic, titanium, and copper.

Magnetic susceptibility

The extent to which a material or biological tissue becomes magnetized when placed in an external magnetic field.

Magnetic permeability

The measure of how well a substance attracts the imaginary lines of a magnetic field.

Iron shielding

Iron placed within MRI scan room walls to attract magnetic field lines due to its high permeability, preventing the fringe field from extending into adjacent corridors.

Magnetic field gradients

Static magnetic fields that vary linearly across space to provide the ability to perform spatial encoding.

Parallel proton alignment

Protons that align themselves with the direction of the static magnetic field ($B_0$), existing in a lower energy state and creating a tiny excess that accounts for net longitudinal magnetization.

Anti-parallel proton alignment

Hydrogen protons aligned opposite to the magnetic field direction, referred to as spin down and/or high energy spins.

Longitudinal magnetization (Net Magnetization Vector)

The magnetic vector produced by the excess number of hydrogen protons aligned with the static magnetic field along the longitudinal (Z) axis.

Magnetic vector

A quantity possessing both magnitude (strength) and direction.

Free Induction Decay (FID)

An MR signal waveform produced immediately from an RF pulse that decays over time.

RF Excitation

The process of using a radiofrequency pulse to tip the net magnetization away from the Z axis, stimulating protons to precess in phase and into parallel or anti-parallel alignment.

90° RF pulse

An RF pulse that causes precessing protons to precess in phase and tip entirely into the transverse plane, establishing non-zero transverse magnetization.

Dephasing

The loss of phase coherence that occurs after RF application, causing phase differences between precessing spins and resulting in the decay of transverse magnetization.

T1 relaxation time

The time required for a tissue to recover $63\%$ of its original longitudinal magnetization along the Z axis.

Spin-lattice relaxation

The mechanism of T1 recovery wherein excited nuclei give up their absorbed energy to the surrounding tissue/lattice, allowing longitudinal magnetization regrowth.

T2 relaxation time

The time required for $63\%$ of the transverse magnetization to decay, or the time it takes the spins to de-phase to $37\%$ of their original value.

Spin-spin relaxation

The process (synonymous with T2 decay) in which transverse magnetization returns to its equilibrium value of zero as spins exchange energy and dephase.

Main magnet

The primary hardware component of the MRI system that produces proton spin alignment (magnetization).

Gradient system

The MRI system component responsible for providing the ability to perform spatial encoding.