MR Glossary-Copy-From Quizlet

Gauss

Unit of magnetic induction in the centimeter-gram-second system of physical units.

Tesla

SI unit of magnetic field strength.

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.

TI

See Inversion Time.

Flux

The invisible lines of force that extend around a magnetic field.

Permanent magnet

A magnet designed with blocks of ferromagnetic materials to generate a magnetic field.

Resistive magnet

A magnet that can be turned on and off, and uses the principles of electromagnetism to generate a magnetic field.

Superconducting magnet

A magnet whose field is generated by current in wires made of a superconducting material.

Homogeneity

Uniformity of the main magnetic field.

Inhomogeneity

Lack of homogeneity or uniformity in the main magnetic field.

Passive shimming

Shimming the magnetic field with metal pieces for a more homogenous magnetic field.

Active shimming

Adjustment of the current within the shim coils for uniform fat suppression or optimal prescan calibrations.

Passive shielding

Lining the MR room with steel or other ferromagnetic plating.

Isocenter

A location within the magnet bore assigned coordinates X-0, Y-0, Z-0.

Neutron

An uncharged neutral particle located in the nucleus of most atoms.

TR

See Repetition Time.

Proton

A positively charged particle located in the nucleus of an atom.

Isotope

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

Spin

A fundamental property of matter in MR imaging.

Vector

Quantity that has both magnitude and direction.

Precess

A rotational motion about an axis of a vector.

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 and the gyromagnetic ratio.

Gyromagnetic Ratio

The ratio of the magnetic moment of a rotating charged particle to its angular momentum.

Larmor frequency

The specific frequency at which magnetic resonance in a nucleus can be excited and detected.

Frequency

The number of times that a periodic function or vibration repeats itself in a specified time.

Ferromagnetism

The property of being strongly attracted to either pole of a magnet.

Paramagnetic

Property of magnetism signifying weak magnetic properties due to its unpaired electrons.

Gadolinium

A non-toxic paramagnetic contrast agent used in MR imaging.

Diamagnetism

An effect resulting in a slightly weakened magnetic field when a substance is introduced into it.

Radiofrequency

A frequency band in the electromagnetic spectrum with frequencies in the millions of cycles per second.

Excitation pulse

The radio frequency pulse used to disturb the equilibrium of the Net Magnetization Vector.

Longitudinal magnetization

The Z component of the net magnetization vector in the direction of the static magnetic field.

Transverse magnetization

The X-Y component of the net magnetization, at right angles to the main magnetic field.

Flip Angle

The angle to which the longitudinal magnetization is tipped into the transverse plane.

Resonance

The exchange of energy between two systems at a specific frequency.

Phase

An angular relationship describing the degree of synchronism between sinusoidal waveforms.

Phase coherence

The degree to which precessing nuclear spins are synchronous.

Coherent

Maintaining a constant state of 'in-phase' relationships between protons.

Transmitter

The segment of the MR scanner that produces the RF current.

Receiver

MRI equipment device that detects and amplifies the RF signals.

Receiver coil

A coil used to detect the NMR signal.

CP coil

Circularly polarized transmission or receiver coil with two orthogonal channels.

Electromagnetic Induction

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

Free Induction Decay

A time-based electrical signal detected in a nuclear magnetic resonance spectrometer.

Amplitude

Equates to signal height or strength.

Gradients

The linear increased or decreased change in the magnetic field of a certain orientation.

Gradient Coils

Coils used to generate magnetic gradient fields.

Slice thickness

The thickness of an imaging slice.

Field of View

The base size of the slice to be acquired.

Orthogonal

A plane or section that is perpendicular to the X,Y, or Z axes.

Oblique

An imaging technique which produces images along planes that are not perpendicular to the X,Y, or Z axes.

Phase encoding

The process of locating an MR signal along the phase axis, directly correlating spatial resolution and scan time with the number of phase encodings in an MR image.

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.

Readout gradient

Magnetic field gradient applied during the moment when the echo is formed, and is responsible for frequency encoding in the image.

Slew rate

Is the speed rate of ascent or descent of a gradient from zero to its maximum amplitude, either positive or negative.

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.

k-space

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

Analog

Being continuous, or having a continuous range of values.

Sampling

The conversion of analog signals to digital values through a specific measurement process.

Fourier Transform

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

Pixel

Picture element, two-dimensional.

Voxel

Volume element with 3-Dimensional characteristics.

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.

T1

Spin-lattice or longitudinal relaxation time.

Lattice

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

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.

T1 Weighting

Image contrast created with short TR's and short TE's whose contrast is determined by the T1 times of tissues.

T2

Spin-spin or transverse relaxation time.

T2 Weighting

Image contrast created with long TR's and long TE's whose contrast is determined by the T2 times of tissues.

Repetition time

The time between successive pulse sequences applied to the same slice, the time between consecutive 90º RF excitation pulses.

Pulse Sequence

A set of specifically timed pulses and/or magnetic field gradients producing an image with specific characteristics based on the timing and duration of the pulses programmed.

Quench

An event in a superconducting MR system causing a loss of superconductivity, a rapid loss of the magnet cryogens, potentially creating a hazard for patients / operators in the magnet room.

Spin echo

An MRI pulse sequence whose signal is an echo resulting from the refocusing of magnetization after the application of 90º and 180º RF pulses.

Tau

The time between the pulses (btwn 90ºRF - 180ºRF and 180ºRF - echo) in spin echo (also known as 1/2 TE).

Echo Train

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

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).

Inversion Recovery

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

STIR

Short TI Inversion Recovery: a version of the Inversion Recovery pulse sequence where the inversion time (TI) is set to 0.69 times the T1 of fat, nulling the signal from fat at the specific field strength in which it's being acquired.

FLAIR

FLuid Attenuated Inversion Recovery - A technique used to suppress fluids, utilizing a long echo time (TE) coupled with a long Inversion Time (TI).

EPI factor

Number of gradient echoes in an Echo Planar sequence acquired after the initial excitation pulse; equating to the scan time acceleration factor.

Phase contrast

An MRA technique based on the speed of the vessels of interest, utilizing velocity encoding (VENC).

Velocity

Speed in a particular direction.

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.

Fat Saturation

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

Dixon

A 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.

Image Resolution

The ability to differentiate adjacent tissue structures. Higher image resolution increases detection of smaller pathologies.

NEX

Number of Excitations: how many times each line of k-space data is acquired during the scan, or how many times each slice is repeated to gain signal and reduce noise, improving SNR.

NSA

Number of Signals Averaged: see NEX.

Parallel Imaging

A scan time acceleration technique which requires the use of a phased array coil to reconstruct an image in less time than conventional imaging.

Chemical shift artifact

Due to the difference in resonant frequencies between fat and water, causing a phase shift in voxels containing fat and water.

Eddy Currents

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

Partial volume averaging

A loss of resolution due to excessively large voxels, typically caused by slices that are too thick.

Flow Compensation

An MR technique used to reduce artifacts from spin phase effects through the use of additional gradient pulses.

dB/dt

Formula for the temporal change of the magnetic field, especially important with regards to patient safety in relation to gradients.

Gating

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

Effective TR

Referring to prospective cardiac triggering techniques, the patient's heart rate controls the TR, or termed the effective or available TR (TR eff), according to the beats per minute (BPM) factor.