Glossary


Please click on the term to go to the chapter or section in which the term was introducted.

Anterior
The direction towards the front of the body in an anatomical coordinate system.

Artifact
A feature which appears in an image which is not present in the imaged object.

Angiography
The imaging of veins and arteries.

Axial
A tomographic imaging plane bisecting the body into top and bottom parts.

Birdcage Coil
An transmit and receive RF imaging coil which looks like a birdcage.

Chemical Shift
A variation in the resonance frequency of a nuclear spin due to the chemical environment around the nucleus. Chemical shift is reported in ppm.

Chemical Shift Imaging
The imaging of species in an image with a specific chemical shift and the exclusion of other species with a different chemical shift. Also known as spectroscopic imaging.

Coil
One or more loops of a conductor used to create a magnetic field. In MRI, the term refers to the radiofrequency coil.

Complex Conjugate
Two complex numbers are complex conjugates of each other if their real parts are equal and their imaginary parts are opposite in sign.

Convolution
A mathematical operation between two functions.

Contrast
The difference in signal intensity of two tissues in an image.

Complex Data
Numerical data with a real and an imaginary component.

Continuous Wave (CW)
A form of spectroscopy in which a constant amplitude electromagnetic wave is applied.

Contrast Agent
A chemical substance which is introduced into an organism to change the contrast between two tissues.

Coordinate Transformation
A change in the axes used to represent some spatial quantity.

Coronal
A tomographic imaging plane bisecting the body into front and back parts.

Dephasing Gradient
A magnetic field gradient used to dephase transverse magnetization.

Diffusion Imaging
An imaging technique where image contrast is related to the diffucion coefficient of the molecules being imaged.

Doubly Balanced Mixer
An electrical device, often referred to as a product detector, which is used in MRI to convert signals from the laboratory frame of reference to the rotating frame of reference.

Echo
A form of magnetic resonance signal from the refocusing of transverse magnetization.

Echo-Planar Imaging
An MRI sequence capable of producing images at video rates.

Echo Time (TE)
The time in an imaging sequence between the initial RF pulse and the maximum in the echo.

Electron Spin Resonance (ESR)
A magnetic resonance technique based on the electron spin.

Fast Spin Echo
A multiple echo spin-echo sequence which records different regions of k-space with different echos.

Field of View
The distance across an image, typically in centimeters. The field of view in the frequency and phase encoding directions may be different.

Field of View
A magnetic field gradient coil shaped like the number eight.

Fractional Echo Imaging
An imaging technique that takes advantage of the symmetry in k-space to reconstruct an image using only a fraction of k-space data.

Fractional Nex Imaging
An imaging technique that takes advantage of the symmetry in k-space to reconstruct an image using only a fraction of k-space data.

Free Induction Decay (FID)
A form of magnetic resonance signal from the decay of transverse magnetization.

Fourier Transform (FT)
A mathematical technique capable of converting a time domain signal to a frequency domain signal and vice versa.

Frequency Encoding Gradient (Gf)
A magnetic field gradient applied in an imaging sequence during the acquisition of a signal which encodes spins with different frequencies dependent on their position in the direction of this gradient.

Functional Magnetic Resonance Imaging (fMRI)
An imaging technique based on echo-planar imaging which is used to determine brain function.

Gradient (G)
A variation in some quantity with respect to another. In the context of MRI, a magnetic field gradient is a variation in the magnetic field with respect to distance.

Gradient Echo
A form of magnetic resonance signal from the refocusing of transverse magnetization caused by a the application of a specific magnetic field gradient.

Gradient Recalled Echo Sequence
An MRI sequence producing signals called gradient echoes as a result of the application of a refocusing echo.

Gyromagnetic Ratio (γ)
The ratio of the resonance frequency to the magnetic field strength for a given nucleus.

Hyperpolarized Noble Gas Imaging
Magnetic resonance imaging of noble gasses whose spin distribution is hyperpolarized.

Imaginary
The component of a signal perpendicular to the real signal.

Imaging Sequence
A specific set of RF pulses and magnetic field gradients used to produce an image.

Inferior
The direction towards the feet in an anatomical coordinate system.

Inversion-Recovery Sequence (In NMR) (In MRI)
An MRI pulse sequence producing signals which represent the longitudinal magnetization present after the application of a 180o inversion RF pulse.

Inversion Time (TI)
The time between the inversion pulse and the sampling pulse(s) in an inversion recovery sequence.

Isocenter
A location in an imaging magnet assigned the coordinates (x,y,z)=0,0,0 and having magnetic field strength Bo and resonance frequency no.

Isotope
Two atoms are isotopes if they have the same number of protons and electrons, but a different number of neutrons.

k-Space
That image space reprsented by the time and phase raw data. The Fourier transform of k-space is the magnetic resonance image.

Larmor Frequency
The resonance frequency of a spin in a magnetic field. The rate of precession of a spin packet in a magnetic field. The frequency which will cause a transition between the two spin energy levels of a nucleus.

Longitudinal Magnetization
The Z component of magnetization.

Look-Up Table (LUT)
A table (or function) used to relate intensity of a screen pixel to the data value of that pixel in the image.

Lorentzian Lineshape
A function obtained from the Fourier transform of an exponential signal.

Magnitudee
The length of a magnetization vector. In MRI the square root of the sum of the squares of Mx and My components of transverse magnetization.

Magnetic Resonance Angiography (MRA)
An imaging technique which images flowing blood.

Magnetic Resonance Elastography (MRE)
An imaging technique which images the shere modulus of tissues.

Magnetic Resonance Imaging (MRI)
An imaging technique based on the principles of NMR.

Magnetization Transfer
The transfer of net magnetization from one species to another due to overlaping spectral positions for the two.

Net Magnetization Vector
A vector representing the sum of the magnetization from a spin system.

Nuclear Magnetic Resonance (NMR)
A spectroscopic technique used by scientists to elucidate chemical structure and molecular dynamics.

Oblique Imaging
An imaging technique which produces images along oblique planes between the conventional X, Y, and Z axes.

Parallel Imaging
An imaging technique which utilizes signals phased array coils to reconstruct an image in less time than conventional imaging.

Phantom
An anthropogenic object that can be imaged to test the performance of a magnetic resonance imaging system.

Pixel
Picture element.

Posterior
The direction towards the back in an anatomical coordinate system.

Precess
A rotational motion about an axis of a vector whose origin is fixed at the origin of the coordinate system.

Proportionality Constant
A constant used to convert one set of units to another.

Pulse Sequence
A series of RF pulses and/or magnetic field gradients applied to a spin system to produce a signal representative of some property of the spin system.

Quadrature Detection
Detection of Mx and My simultaneously as a function of time.

ρ-Weighted Image
A magnetic resonance image where the contrast is predominantly dependent on spin density.

Radio Frequency (RF)
A frequency band in the electromagnetic spectrum with frequencies in the millons of cycles per second.

Raw data
The Mx and My data as a function of phase and time from an imaging sequence. This is also called k-space data.

Real
The component of a signal perpendicular to the imaginary signal.

Relaxometry
The measurement and study of spin-lattice and spin-spin relaxation times.

Repetition Time (TR)
The time between repetitions of the basic sequence in an imaging sequence.

Resonance
An exchange of energy between two systems at a specific frequency.

RF Pulse
A short burst of RF energy which has a specific shape.

Rotation Matrix
A matrix used to describe the rotation of a vector.

Saddle Coil (RF Coils) (Gradient Coils)
A coil geometry which has two loops of a conductor wrapped around opposite sides of a cylinder.

Sagittal
A tomographic imaging plane bisecting the body into left and right parts.

Single-Turn Solenoid
An transmit and receive RF imaging coil which, in general, has a cylindrical shape.

Slew Rate
The rate at which a gradient may be turned on or off. The faster the slew rate the more possible it is for an MRI system to do EPI or fMRI, and the shorter the TE value that can be achieved in a spin echo sequence.

Specific Absorption Rate (SAR)
The number of Watts of RF energy per kilogram of body weight in an imaging sequence.

Spin
A fundamental property of matter responsible for MRI and NMR.

Spin Density
The concentration of spins.

Spin Density Weighted Image
A magnetic resonance image where the contrast is predominantly dependent on spin density.

Spin-Echo Sequence (In NMR) (In MRI)
An MRI pulse sequence whose signal is an echo resulting from the refocusing of magnetization after the application of a 90o and 180o RF pulses.

Spin-Lattice Relaxation
The return of the longitudinal magnitization to its equilibrium value along the +Z axis.

Spin-Lattice Relaxation Time (T1)
The time to reduce the difference between the longitudinal magnitization and its equilibrium magnetization by a factor of e.

Spin Packet
A group of spins experiencing the same magnetic field.

Spin-Spin Relaxation
The return of the transverse magnitization to its equilibrium value, zero.

Spin-Spin Relaxation Time (T2)
The time to reduce the transverse magnetization by a factor of e.

Sinc Pulse
An RF pulse shaped like Sin(x)/x.

Spectroscopy
The study of the electromagnetic radiation frequencies absorbed by matter.

Superconduct
To have no resistance. A perfect superconductor can carry an electrical current without losses.

Superior
The direction towards the head in an anatomical coordinate system.

Surface Coil
An receive only RF imaging coil which, in general, fits against the surface of the object being imaged.

T1-Weighted Image
A magnetic resonance image where the contrast is predominantly dependent on T1.

T2-Weighted Image
A magnetic resonance image where the contrast is predominantly dependent on T2.

T2*
Pronounced T-2-star, is the spin-spin relaxation time composed of contributions from molecular interactions and inhomogeneities in the magnetic field.

Timing Diagram
A multiaxis plot of some aspects of a pulse sequence as a function of time.

Tissue Classification
The classification of tissues in a magnetic resonance image using computer algorithms and some property of the imaged tissue.

Tomographic
A slice with some thickness.

Thk
Slice thickness.

Transverse Magnetization
The XY component of the net magnetization.

Variable Bandwidth Imaging
Imaging with different bandwidths (digitization rates) in order to controll the noise and chemical shift artifact.

Volume Imaging
Imaging which produces a three-dimensional image of an object.

Voxel
Volume element.


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