MRI Glossary/Medical Terminology

Active shielding

is a term describing the containment of the static magnetic fringe field through the use of secondary coils attached around the MRI scanner

Active shimming

describe the adjustment of the current within the shim coils on a per/patient or per/sequence basis, for the purposes of achieving uniform fat suppression or optimal prescan calibrations. A well shimmed FID is the desired end result of a good active shim calibration

ADC images

are reconstructed from diffusion weighted images with multiple b-values, and correspond to the spatially distributed diffusion coefficients of the target tissues

Aliasing artifact

occurs when tissue outside the Field of View is undersampled, causing a misregistration of anatomical location, in the phase direction, but on the opposite side of the anatomical location, also known as wrap-around artifact

Amplitude

equates to signal height or strength; the larger the amplitude, the larger the number of protons

Analog

being continuous, or having a continuous range of values

Analog-to-digital converter (ADC)

part of the computer system that converts the analog acquired MR signal into a digital signal

anisotropic voxel

a voxel with uneven measurements, with regards to the phase, frequency and slice thickness dimensions

array processor

comprises a multiprocessor that is switched in sequence and in parallel while simultaneously performing a computing task

Artifacts

signal misrepresentations that do not correspond to the spatial location of the specific tissue imaged

Bandwidth

the range or spectrum of frequencies (minimum to maximum processed frequency) of a pulse sequence acquired by an RF system

body coil

is an integrated part of the magnet design that acts as its own transceiver coil, with large FOV capabilities, but lacking the high SNR of localized coils

b-value

a factor in diffusion weighted imaging; the higher the factor, the stronger the diffusion weighting

B0 field

defined as the static main magnetic field of an MR system

B1 field

alternating magnetic field generated by the RF in a transceiver coil, perpendicular to B0

Cardiac triggering

prevents or reduces motion artifacts in the MR image caused by the heartbeat or pulsating blood flow and enables the images to be acquired synchronized to cardiac movement

Chemical shift artifact

difference in resonant frequencies between fat and water, causing a phase shift in voxels containing fat and water

Cine function

scrolls through the entire sequence's images, giving the appearance of CSF flowing or cardiac movement, from the cycling of the images acquired; a “movie” function of scrolling images

Claustrophobia

the physiological reaction to being confined in a small space or location, the fear of being trapped

Coherent

maintaining a constant state of “in-phase” relationships between protons; located at the same phase cycle simultaneously

Contrast

can be defined as the signal strength differences between two adjacent tissue types

CE MRA (Contrast-enhanced MR angiography

utilizes the reduced T1 relaxation time of blood through the use of an intravenously injected Gadolinium contrast agent

CP coil

Circularly polarized transmission or receiver coil with two orthogonal transmission and/or receiver channels, also known as a quadrature coil. This yields better signal-to-noise than a linear coil

what yields better signal to noise, CP coil or linear coil

CP coil

Cross-talk artifact

occurs when slices are positioned too close together, causing signals from adjacent slices to affect one another. This affects T1 contrast, and is remedied by utilizing an interleaved slice profile or by increasing slice gap

Cryogens

the liquids that are supercooled (4° Kelvin) in order to maintain the superconductivity of the magnet coils

dB/dt

Formula for the temporal change of the magnetic field, especially important with regards to patient safety in relation to gradients. This is due to the electrical field generated in conductive materials, as in human tissue

Dephasing

occurs after initial RF application, causing phase differences to appear between precessing spins, resulting in decay of transverse magnetization

Diamagnetism

an effect resulting in a slightly weakened magnetic field when a substance is introduced into it. This material is considered to have a negative magnetic susceptibility

Diffusion

the movement of atoms or molecules from an area of higher concentration to an area of lower concentration. (Brownian motion)

Diffusion tensor

displays the mobility of water molecules in all three coordinates

Dixon

technique utilizing the differences in resonant frequencies of fat and water separating the two tissues, acquiring each separately, then adding the two to yield a water-only image (fat suppression), then subtracting the two to yield a fat-only image

Double Echo

A spin echo sequence with two echoes, two generated images per slice location

Duty cycle

The time which the gradient system can be run at maximum power

Dynamic Contrast Enhancement

refers to DCE, or Basic T1 perfusion imaging (pituitary or prostate dynamic sequences, longer acquisition times/dynamic compared to DSC)

Dynamic Susceptibility Contrast

refers to DSC, or better known as T2* gradient echo Perfusion imaging, utilized in Brain Perfusion studies

Echo

The MR signal generated by an RF or a gradient pulse

Echo Planar Imaging

Very fast MR imaging technique where the complete image is acquired using a single selective excitation pulse, field gradients are periodically switched to generate a series of gradient echoes

Echo Spacing

Distance between two echoes, as in Fast Spin Echo or EPI sequences. The shorter the echo spacing, the more compact the sequence timing and few artifacts result

Echo Time (TE)

The time between the excitation pulse of a sequence and the resulting echo used as the MR signal

Echo Train

Multiple echoes in sequence, each obtaining rows of k-space, shortening overall scan time, also reducing SNR with each additional echo train

Eddy Currents

The electrical currents generated in a conductor by changing magnetic fields or movement of the conductor within the magnetic field

Effective TE

In a pulse sequence, the contrast and overall SNR of an image is determined by the position of the echo when the phase encoding gradient has the smallest amplitude. At this time, the echo signal has the strongest signal and undergoes minimal dephasing. The time between the initial RF excitation pulse and this time is the effective echo time (TEeff)

effective echo time (TEeff)

The time between the initial RF excitation pulse and this time is the

Effective TR

referring to prospective cardiac triggering techniques, the patient’s heart rate controls the TR, or termed the effective or available TR (TReff), according to the beats per minute (BPM) factor

Electromagnetic Induction

The electrical voltage created in a receiver coil as a result of a temporal change in the magnetic field

Emesis

the medical term used to describe vomiting

EPI factor

Number of gradient echoes in an Echo Planar sequence acquired after the initial excitation pulse; equating to the scan time acceleration factor (an EPI factor of 64 is 64 times faster than a routine gradient echo sequence)

Excitation pulse

The radiofrequency (RF) pulse used to disturb the equilibrium of the Net Magnetization Vector; the higher the energy of excitation, the greater the flipped angle of magnetization

Etiology

the cause or origin of a disease

Fast Fourier Transformation(FFT)

Algorithm for fast reconstruction of raw data into an MR image

Fat-only Image

An image yielding only the signals from fat protons in an image and suppressing the signal from water; usually a “phase” of the Dixon technique

Fat Saturation

An MR technique used to suppress signal from fat protons, yielding a “water-only” image, utilizing a chemically selective presaturation pulse

Fat Suppression

the process of utilizing specific sequence parameters to null signal from fat in the MR sequence, as in STIR, for example

Ferromagnetism

the property of being strongly attracted to either pole of a magnet. Ferromagnetic materials, such as iron, contain unpaired electrons, each with a small magnetic field of its own, that align readily with each other in response to an external magnetic field

Field of View

the base size of the slice to be acquired, measured in dimensions of phase (short axis of anatomy) and frequency (long axis); FOV in short

FLAIR – FLuid Attenuated Inversion Recovery

– A technique used to suppress fluids, utilizing a long echo time (TE) coupled with a long Inversion Time (TI)

Flip Angle

the angle to which the longitudinal magnetization is tipped into the transverse plane, following the initial RF excitation pulse

Flow Compensation

an MR technique used to reduce artifacts from spin phase effects through the use of additional gradient pulses. Also known as Gradient Moment Nulling

Gradient Moment Nulling

Flow Compensation

Flux

the invisible lines of force that extend around a magnetic field, with the greatest strength/force being at the two poles/ends of the magnet

Fourier Transform

the mathematical process of converting raw data into an image suitable for display

Free Induction Decay

a time-based electrical signal that is detected in a nuclear magnetic resonance spectrometer, that is produced by induction from the motion of the magnetic moments of nuclei, that decays with time (T2*), that can be converted to a more conventional frequency-based signal using analysis by Fourier transforms; abbreviated FID

Frequency

the number of times that a periodic function or vibration repeats itself in a specified time. It is usually measured in hertz (Hz), where 1 Hz = 1 cycle per second

Frequency Encoding

the process of locating an MR signal in one dimension by applying a magnetic field gradient along that dimension during the period when the signal is being received, utilizing differences in frequencies along the same row in order to spatially differentiate one location from another

Gadolinium

A non-toxic paramagnetic contrast agent used in MR imaging. The infusion of gadolinium based paramagnetic contrast agents (GBCA’s) shortens the T1 (and T2) times of tissue, dependent upon concentration

Gating

– timing the collection of MR data to physiological motion in order to minimize motion artifacts, as in cardiac gating or respiratory gating

Gauss

unit of magnetic induction in the centimeter-gram-second system of physical units. Old unit for magnetic field strength. Today, the unit Tesla (T) is used (1 Tesla = 10000 Gauss). The 5 Gauss line is currently used as a safety standard for the exclusion of implanted metallic devices

Gradients

the linear increased or decreased change in the magnetic field of a certain orientation. A gradient defines the strength and orientation of change of a magnitude in space

Gradient Coils

Coils used to generate magnetic gradient fields. Gradient coils are operated in pairs in the magnet, at the same current, however, of opposite polarities

Gradient Echo

Echo generated by switching a pair of dephasing and rephasing gradients, without the use of a 180° refocusing pulse (seen in spin echo sequences)

Gyromagnetic Ratio

the ratio of the magnetic moment (field strength) of a rotating charged particle, such as an electron, to its angular momentum (frequency). The value of the gyromagnetic ratio of hydrogen is 42.57 MHz/Tesla

Homogeneity

Uniformity of the main magnetic field; a magnetic field is considered homogenous when it has the same field strength across the entire field

Half-Fourier

Due to the symmetry of the matrix to be sampled, a little more than half of the raw data is necessary to be acquired, reconstructing the remaining data to complete the image, greatly reducing scan time

Hypoxia

decrease in the amount of oxygen in the body as a whole (generalized hypoxia) or a specific region (tissue hypoxia)

Image Reconstruction

Mathematical computation converting the collected raw data signals into images using Fourier Transform

Image Resolution

The ability to differentiate adjacent tissue structures. Higher image resolution increases detection of smaller pathologies. Resolution increases with a smaller FOV, higher matrix, and thinner slices

Inhomogeneity

Lack of homogeneity or uniformity in the main magnetic field

Inflow technique

Also referred to as Time of Flight (TOF) MR angiography

Inversion Recovery

MR sequence that begins with 180º RF inverting pulse followed by 90º RF excitation pulse, then 180º RF refocusing pulse

Inversion time (TI)

The time between the initial 180º RF and the 90º RF excitation pulse in an Inversion Recovery sequence

Isotope

Atomic nuclei that contain the same number of protons but differ in the number of neutrons in the nucleus of the atom

Isocenter

A location within the magnet bore assigned coordinates X-0, Y-0, Z-0, and having magnetic field strength B0 and center frequency relative to field strength

k-space

data acquisition matrix containing raw image data prior to image processing. The Fourier transform of k-space is the magnetic resonance image

Larmor equation

an equation stating that the frequency of precession of nuclear magnetic moment is directly proportional to the product of the magnetic field strength(B0 ) and the gyromagnetic ratio

Larmor frequency

the specific frequency at which magnetic resonance in a nucleus can be excited and detected, and varies directly with magnetic field strength

Lattice

the magnetic and thermal environment through which nuclei exchange energy in longitudinal (T1) relaxation

Longitudinal magnetization

The Z component of the net magnetization vector in the direction of the static magnetic field. Following RF excitation, this vector returns to its equilibrium value at a rate characterized by the time constant T1

Longitudinal relaxation

Return of longitudinal magnetization to its equilibrium value after RF excitation due to the exchange of energy between the nuclear spins and the lattice

Magnetic field

Magnetic lines of force which extend from a north and south pole to form a closed loop around the outside of a magnetic material. A magnetic field has properties of both magnitude (strength) and direction

Magnetic resonance angiography (MRA)

MR technique utilized to image flowing blood and specific vasculature structures

Magnetic resonance elastography (MRE)

MR technique which quantitatively assesses the mechanical properties of tissues, based on the propagation of shear waves

Magnetic resonance spectroscopy (MRS)

MR technique generating a spectrum of metabolites in a specific volume of interest

Multi-echo imaging

an MR technique using a series of echoes following a single excitation pulse, usually in the form of 180º RF pulses that generate images

Net magnetization vector (NMV)

a vector which represents the sum of the magnetization from a spin system; the magnitude and direction of the magnetization resulting from the collection of atomic nuclei

Neutron

an uncharged neutral particle located in the nucleus of most atoms which serves as a stabilizer