BP5910 MRI Hardware Vocabulary

A set of vocabulary flashcards covering the basic hardware components of an MRI system, including magnets, field gradients, and radiofrequency chains.

Main Magnetic Field ($B_0$)

The primary magnetic field produced by the main magnet in an MRI system, measured in Tesla ($T$) or Gauss ($G$), which must be stable in time and spatially homogeneous.

Field Homogeneity

A measure of how uniform the main magnetic field $B_0$ is, commonly quoted as a fractional variation or in parts per million (ppm) over a $40$–$50\,cm$ diameter spherical volume (DSV) at the isocentre.

Isocentre

The central physical origin of the MRI magnet where $x = 0$, $y = 0$, and $z = 0$.

Resistive Magnet

A simple solenoid magnet design made from a conductor (such as copper) that produces a magnetic field proportional to applied current, practically limited to very low field research devices ($0.02$–$0.5\,T$) due to substantial resistive heating.

Aberdeen Mk1 Scanner (1978)

An early MRI scanner featuring a $0.04\,T$ air core resistive magnet with a vertical field, driven by a current of about $40\,A$ across hundreds of turns of water-cooled copper wire.

Aberdeen Field-Cycling Scanner (2025)

An MRI scanner featuring a $0.2\,T$ air core resistive magnet with a horizontal field, a drive current of about $2000\,A$, and a water cooling facility dissipating approximately $100\,kW$ of heating.

Permanent Magnet

A magnet design utilizing materials (such as neodymium) that produce an intrinsic magnetic field providing $B_0$ without needing a current supply, achieving up to $1\,T$ field strength.

Fujifilm Aperto Lucent

A $0.4\,T$ permanent "horseshoe" design scanner weighing $14,800\,kg$ with a $9.5\,kVA$ power supply.

Hyperfine Swoop

A portable, head-only scanner featuring a $64\,mT$ "Halbach" permanent magnet design with virtually no fringe field, weighing about $600\,kg$ and running on a $13\,A$ wall power supply.

Aberdeen PEDRI Scanner (1996)

A scanner featuring a $59\,mT$ "Halbach" permanent magnet design with a resistive offset $B_0$ coil to allow combined proton and electron spin resonance experiments.

Superconductive Magnet

A magnet design using superconductive materials (such as Nb-Ti) cooled below a critical temperature ($T_c \approx 4.2\,K$ using liquid helium) where resistance drops to zero ($R \rightarrow 0$), enabling high magnetic fields without resistive heating.

Quench

An event in a superconductive magnet where the magnet windings become resistive ($T > T_c$), causing liquid helium to rapidly boil off and expand into gas.

Field Gradients (Gradient Coils)

Specialized coils housed within the main magnet structure designed to impose a very small linear variation in $B_0$ along physical axes ($x$, $y$, $z$) of the scanner to enable spatial encoding.

Maxwell Pair

A specific coil design principle used to produce linearly shaped magnetic field gradients along the $z$-axis.

Golay Coil

A specific coil design principle used to produce linearly shaped magnetic field gradients along the $x$ and $y$ axes.

Gradient Slew Rate

The rate of change of the gradient field over time, measured in $\text{mT/m/ms}$, which is limited by the available voltage supply $V$.

B1 Field

The rotating magnetic field generated by passing an oscillating current ($I_{\text{rf}}$) through an RF coil, used to tip the magnetization vector $\mathbf{M}$ during excitation.

RF Coil Efficiency

A measure of how much $B_1$ field per unit current an RF coil generates, defined as $\eta = \frac{B_1}{I}$ and usually on the order of $10$–$20\,\mu\text{T/A}$.

Impedance Matching

The process of using a matching network to match coil impedance with the characteristic impedance of the RF chain (typically $50\,\Omega$) to maximize power transfer and minimize power lost to reflection.

Body Coil

A large single volume RF transmit coil built into the bore of the magnet, designed to accommodate any part of the patient.

Birdcage Coil

The most common body and head transmit coil design, which produces a horizontal $B_1$ field relative to its axis of symmetry.

Linear Polarisation Mode

A birdcage coil transmit driving mode that generates an oscillating magnetic field consisting of clockwise and anti-clockwise components, wasting half the delivered power as patient tissue heating.

Circular Polarisation (CP) Mode

A transmit and receive driving mode that eliminates the wasted counterclockwise component, offering up to twice the power efficiency of a linearly driven coil and improving reception SNR by about $\sqrt{2}$.

Principle of Reciprocity

The principle derived from Faraday's law of induction stating that nuclear magnetic resonance signal voltage depends on a coil's transmit efficiency, meaning an efficient transmit coil is also an efficient reception coil.

Surface Coils

RF receiver coils placed close to the patient that offer superior signal-to-noise ratio (SNR) by receiving noise only from tissues within about one coil radius.

Quadrature Demodulation

A signal reception stage that shifts the signal frequency from Larmor frequency $\omega_0$ down to $0\,\text{Hz}$ (DC) and resolves it into real ($x'$) and imaginary ($y'$) complex components to resolve trajectory sign ambiguity.