Aliasing: Sometimes used to refer to wraparound artifact, which results when portions of the body outside of the field of view are spatially mismapped to the contralateral side of the image. In phase-contrast magnetic resonance angiography (MRA), aliasing refers to the representation of rapid flow as slow flow in the opposite direction due to setting an inappropriately low velocity encoding parameter.

Acquisition: An acquired MR data set sufficient to create an image.

Background projection: An artifact of maximum intensity projection. Background projection occurs when high signal intensity voxels resulting from random noise are projected as pixels on the image. The probability of these high signal intensity noise voxels being encountered by a projection ray increases with reconstructed volume thickness. As a result, thick reconstructions tend to have a higher signal intensity background.

Bandwidth: See sampling bandwidth.

Body coil: The permanent transmit and receive coil built into the scanner was originally referred to as the body coil. The term is also used to refer to a surface coil used for body imaging applications (e.g., phased-array body coil, quadrature body coil).

Centric ordered: A sequence designed to acquire the central lines of k-space before the peripheral lines. This type of acquisition is necessary for contrast-enhanced sequences triggered automatically or manually using real-time bolus tracking.

Chemical shift: The difference in resonant frequency between protons in different substances or environments. This difference increases with field strength. Chemical shift may be expressed independent of field strength as parts per million.

Coil: Refers to any scanner element designed to generate a magnetic field gradient (e.g., gradient coil), detect an MR signal (e.g., receive coil), or generate a radiofrequency (RF) pulse (e.g., transmit coil).

Crosstalk: When an imaging slice is excited with an RF pulse, some tissue on either side of the slice is excited due to the imperfect slice profile of the RF pulse. When adjacent slices are acquired sequentially without an intervening gap, this phenomenon results in signal loss in the final image.

Decay: Loss of net magnetization.

Dephasing: Loss of phase coherence of precessing protons.

Diffusion: Random movement of molecules or small particles as a result of Brownian (thermal) motion.

Diffusion imaging: An MR imaging (MRI) technique that creates image contrast from differences in the diffusion characteristics of tissues.

Echo: The signal measured as a result of transverse magnetization rephasing.

Echo train: Multiple echoes produced as a result of multiple refocusing steps after a single excitation.

Effective TE: The time from the excitation pulse to the echo created during application of the zero phase-encoding step of an echo train MR acquisition.

Fat suppression: Selective elimination of signal from fat through one of a variety of methods.

Fat saturation: A method of fat suppression. A common method of fat saturation uses a fat-selective RF pulse to tip the net magnetization vector of fat into the transverse plane. This transverse magnetization is then rapidly spoiled (dephased).

Ferromagnetic: Substances with a large positive magnetic susceptibility are referred to as ferromagnetic. These substances become permanently magnetized when exposed to an external magnetic field.

Flip angle: The degree (often displayed using a rotating frame of reference) to which the net magnetization vector is tipped by an RF pulse from its initial longitudinal orientation. The flip angle is controlled by the amplitude and duration of the RF pulse.

Flow compensation: See gradient moment nulling.

Fold-over suppression: See phase oversampling.

Fourier transform: The mathematic operation performed on a set of digitized MRI data to create an image. The Fourier transform extracts spatial information from the phase and frequency data obtained during an MR acquisition.

Fractional echo: A method of reducing scan time by sampling only a portion of the echo during readout. The inherent symmetry of k-space allows interpolation of the remaining data necessary to create an image. Similar to partial echo, half-echo, and asymmetric echo.

Free induction decay: The rapidly decaying signal (net transverse magnetization) produced immediately after an RF excitation.

Frequency encoding: The process of spatially encoding the MR signal along one axis through the application of a magnetic field gradient. The frequency-encoding gradient is typically applied during creation of the echo and, unlike the phase encoding gradient, remains constant for each echo.

Ghost: A faint duplicate image of a structure resulting from spatial misregistration of some of the signal from that structure. Ghosts typically occur along the phase-encoding axis and are most often the result of phase errors induced by periodic motion.

Gradient: A variation, typically linear, in magnetic field strength over distance. The hardware producing this variation is also commonly referred to as a gradient or gradient coil.

Gradient echo: Signal or echo produced after an excitation by using gradient reversal to reestablish phase coherence.

Gradient moment nulling: Also known as flow compensation, gradient moment nulling refers to modifications of the imaging gradient profiles to correct for phase shifts created by motion. Theoretically, this correction can be performed for higher orders of motion, such as acceleration, but it is typically performed only for constant velocity flow.

Half-Fourier: A technique for reducing image acquisition time by sampling just over half the total number of phase-encoding steps necessary to create an image. The inherent symmetry of k-space allows for extrapolation of the remaining data. The use of half-Fourier reduces scan time by slightly less than 50%. Similar to halfscan and ½ NEX.

Halfscan: See half-Fourier.

In-phase image: Image produced when the net magnetization vectors of water and fat are in-phase. This requires use of a specific echo time (TE) that differs depending on magnetic field strength.

Intermediate weighted: Image contrast that is neither T1-weighted nor T2-weighted, but mainly reflects differences in tissue proton densities. This is accomplished with a sufficiently long TR to allow longitudinal magnetization of tissues to recover nearly completely (minimizing T1-weighted contrast) and a sufficiently short TE to reduce image contrast based on differences in T2-relaxation between tissues (minimizing T2-weighted contrast). These images are sometimes referred to as proton density weighted.

Inversion pulse: A 180-degree RF pre-pulse given before the excitation pulse of an imaging sequence. Varying the time between the inversion pulse and the excitation pulse controls image contrast.

Inversion time (TI): The time between an inversion pulse and an excitation pulse.

Isotropic: A voxel with equal dimensions along each axis is said to be isotropic. Motion that is uniform in all three dimensions is likewise isotropic.

k-space: The single most reviled term in MRI. K-space is a digital representation of the phase and frequency data inherent in the analog signal, or echo, collected during each phase-encoding step. (The frequency-encoding gradient is applied during the echo sampling. Therefore, frequency encoding also occurs during each phase-encoding step.) The raw MR data points are sampled in k-space and converted into image data via the Fourier transform.