AC - Alternating Current is a
continuously changing flow of electrons that alternates its
polarity at a periodic rate.
ACQUISITION - the process of
measuring and storing image data.
ACQUISITION MATRIX - the total
number of independent data samples in the frequency (f) and
phase (f) directions.
ACQUISITION TIME - the period
of time required to collect the image data. This time does
not include the time necessary to reconstruct the image. ADC
- analog-to-digital converter
ALIASING (WRAP AROUND ARTIFACT)
- the phenomenon resulting from digitizing fewer than two
samples per period in a periodic function. Aliasing can occur
in MR imaging whenever the area of anatomy extends beyond
the field of view. These areas extending beyond the field
of view boundaries are aliased back into the image to appear
at artifactual locations.
ALTERNATING CURRENT (AC) - a
current that continuously changes in magnitude and direction.
In the US the current changes at a frequency of 60 Hz.
AMPLITUDE - the signal height.
The greater the amplitude of the signal, the larger the number
of protons in the image and the brighter it will appear.
ANALOG - being continuous, or
having a continuous range of values.
ANALOG-TO-DIGITAL CONVERTER (ADC)
- a system that receives analog input data and produces digital
values at its output. Used by the MRI scanner to convert the
received signal into a format more compatible with the computer
ANTENNA - a device that enables
the sending and/or receiving of electromagnetic waves. See
also Transmitter, Receiver Coils and Surface Coils.
ARCHIVING - the storage of image
and patient data for future retrieval.
ARRAY PROCESSOR - a dedicated
computer system used to perform Fourier transformations to
accelerate the processing of the received numerical data relative
to the MR imaging process.
AVERAGING - see Signal Averaging.
AXIAL - a plane,
slice or section made by cutting the body or part of it at
right angles to the long axis. If the body or part is upright,
the cut would be parallel to the horizon. B or Bo - a conventional
symbol for the constant magnetic field produced by the large
magnet in the MR scanner. B1 - the conventional symbol used
for identifying the radio frequency (RF) magnetic field.
BANDWIDTH (BW) - an all-inclusive
term referring to the preselected band or range of frequencies
which can govern both slice select and signal sampling.
CHEMICAL SHIFT - a variation
in the nominal Larmor frequency for a particular isotope within
the imaging volume. The amount of shift introduced is directly
proportional to the strength of the magnetic field, and is
specified in parts per million (ppm) of the resonant frequency.
CINE - a series of rapidly recorded
multiple images taken at sequential cycles of time and displayed
on a monitor in a dynamic movie display format. This technique
can be used to show true range of motion studies of joints
and parts of the spine.
CIRCLE OF WILLIS - a large network
of interconnecting blood vessels at the base of the brain
that when visualized resembles a circle.
CLAUSTROPHOBIA - a psychological
reaction to being confined in a relatively small area.
CNR - contrast-to-noise ratio.
COHERENCE - the act of maintaining
a constant phase relationship between oscillating waves or
CONTRAST - the relative difference
of signal intensities in two adjacent regions of an image.
Image contrast is heavily dependent on the chosen imaging
technique (i.e., TE, TR, TI), and is associated with such
parameters as proton density and T1 or T2 relaxation times.
CONTRAST REVERSAL - an image
phenomenon where the darks become bright, and the brights
become dark. This is usually most prevalent in sequences utilizing
an extended TR.
CONTRAST-TO-NOISE RATIO (CNR)
- the ratio of signal intensity differences between two regions,
scaled to image noise. Improving CNR increases perception
of the distinct differences between two clinical areas of
- a plane, slice or section made by cutting across the body
from side to side and therefore parallel to the coronal suture
of the skull.
CROSSTALK - an artifact introduced
into images by interference between adjacent slices of a scan.
This artifact can be eliminated by limiting the minimum spacing
CRYOGEN - a cooling agent, typically
liquid helium or liquid nitrogen used to reduce the temperature
of the magnet windings in a superconducting magnet. dB/dt
- The rate of change of the magnetic field. This shows the
ratio between the amount of change in amplitude of the magnetic
field (dB) and the time it takes to make that change (dt).
The value of dB/dt is measured in Tesla per second (T/s).
DC - direct current.
DEPHASING - the fanning out or
loss of phase coherence of signals within the transverse plane.
See also T2.
DIPOLE - a magnetic field characterized
by its own north and south magnetic poles separated by a finite
DIRECT CURRENT (DC) - a continuous
current that flows in only one direction.
DISPLAY MATRIX - the total number
of pixels in the selected matrix, which is described by the
product of its phase and frequency axis.
DOMAIN THEORY - a theory of magnetism
which assumes that groups of atoms produced by movement of
electrons align themselves in groups called"domains" in magnetic
DTPA - Diethylenetriaminepentaacetic
acid - Gadolinium chelating (chemical bonding) agent that
solves the problem of toxicity
ECHO PLANAR IMAGING (EPI) - the
utilization of rapid gradient reversal pulses of the readout
gradient resulting in a series of gradient echo signals to
reduce fast dephasing or signal loss.
ECHO TIME - see TE.
ECHO TRAIN - a series of 180°
RF rephasing pulses and their corresponding echoes for a Fast
Spin Echo (FSE) pulse sequence.
ETL - Echo
EDDY CURRENT - an induced spurious
electrical current produced by time-varying magnetic fields.
Eddy currents can cause artifacts in images and may seriously
degrade overall magnet performance.
ELECTROMAGNET - a type of magnet
that utilizes coils of wire, typically wound on an iron core,
so that as current flows through the coil it becomes magnetized.
See also Resistive Magnet, Superconducting Magnet.
ELECTRON SPIN RESONANCE (ESR)
- the response of electrons to electromagnetic radiation and
magnetic fields at discrete frequencies. EPI - echo planar
imaging. See also Echo Planar Imaging.
EQUILIBRIUM - a state of balance
that exists between two opposing forces or divergent forms
EXCITATION - delivering (inducing,
transferring) energy into the "spinning" nuclei via radio-frequency
pulse(s), which puts the nuclei into a higher energy state.
By producing a net transverse magnetization an MRI system
can observe a response from the excited system.
FARADAY SHIELD (Faraday Cage)
- an electrically conductive screen or shield that reduces
or eliminates interference between outside radio waves and
those from the MRI unit.
FAST SCANNING - a specialized
technique usually associated with short TR, reduced flip angle
and repeated 180° rephasing pulses.
FAST SPIN ECHO (FSE) -
a fast spin echo pulse sequence characterized by a series
of rapidly applied 180° rephasing pulses and multiple echoes,
changing the phase encoding gradient for each echo.
FAT SATURATION (FAT-SAT) - A
specialized technique that selectively saturates fat protons
prior to acquiring data as in standard sequences, so that
they produce negligible signal. The pre-saturation pulse is
applied prior to each slice selection. This technique requires
a very homogeneous magnetic field and very precise frequency
calibration. See also Fat Suppression.
FAT SUPPRESSION - the process
of utilizing specific parameters , commonly with STIR (short
TI inversion recovery) sequences, to remove the deleterious
effects of fat from the resulting images. See also STIR.
FDA - the United States Food
and Drug Administration FID - see Free Induction Decay
FIELD OF VIEW (FOV)
- defined as the size of the two or three dimensional spatial
encoding area of the image. Usually defined in units of cm2.
FFT (Fast Fourier Transform)
- a particularly fast and efficient computational method of
performing a Fourier Transform, which is the mathematical
process by which raw data is processed into a usable image.
FIELD ECHO (FE)
(also known as GRADIENT ECHO) - echo produced by reversing
the direction of the magnetic field gradient to cancel out
the position-dependent phase shifts that have accumulated
due to the gradient.
Attenuated Inversion Recovery
Low-Angle Recalled Echoes
FLIP ANGLE (FA)
- the angle to which the net magnetization is rotated or tipped
relative to the main magnetic field direction via the application
of an RF excitation pulse at the Larmor frequency. The Flip
Angle is used to define the angle of excitation for a Field
Echo pulse sequence.
FLOW COMPENSATION - a function
of specific pulse sequences, i.e., CRISP¿ (Complex Rephasing
Integrated with Surface Probes) spin echo, wherein the application
of strategic gradient pulses can compensate for the objectionable
spin phase effects of flow motion.
FLUX - invisible lines of force
that extend around a magnetic material. The greatest density
is at the two poles of the magnet.
FLUX DENSITY - the number of
lines of force per unit area of a magnetic material.
FOURIER TRANSFORM (FT) - a mathematical procedure used in
MRI scanners to analyze and separate amplitude and phases
of the individual frequency components of the complex time
varying signal. Fourier transform analysis allows spatial
information to be reconstructed from the raw data.
FOV - See Field Of View.
FREE INDUCTION DECAY (FID) -
if transverse magnetization of the spins is produced, e.g.,
by a 90É RF pulse, a transient MR signal at the Larmor frequency
results that decays toward zero with a characteristic time
constant of T2*. This decaying signal is the FID.
FREQUENCY - the number of cycles
or repetitions of any periodic wave or process per unit time.
In electromagnetic radiation, it is usually expressed in units
of 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.
FRINGE FIELD - a term usually
relating to the extents of the magnetic field surrounding
the magnet. Safety requirements dictate that the distances
of particular field strengths from the magnet must be known,
and that potentially unsafe areas must be indicated with appropriate
warning signs. Access to areas with field strengths of 5 gauss
and higher must be strictly controlled. FSE - See Fast Spin
Echo. Gx, Gy, Gz - the conventional symbols for the three
orthogonal magnetic gradients. The subscripts designate the
conventional spatial direction of the gradient.
GADOLINIUM (Gd) - gadolinium
is a non-toxic paramagnetic contrast enhancement agent utilized
in MR imaging. When injected during the scan, gadolinium will
tend to change signal intensities by shortening T1 in its
timing the acquisition of MR data to physiological motion
in order to minimize motion artifacts (e.g., cardiac gating,
GAUSS - a unit of magnetic field
strength that is approximately the strength of the earth's
magnetic field at its surface (the earth's field is about
0.5 to 1G). The value of 1 gauss is defined as 1 line of flux
per cm2. As larger magnetic fields have become commonplace,
the unit gauss (G) has been largely replaced by the more practical
unit tesla (T), where 1 T = 10,000 G. GHOSTING - an image
artifact primarily associated with the phase direction.
GRADIENT COILS - three paired
orthogonal current-carrying coils located within the magnet
which are designed to produce desired gradient magnetic fields
which collectively and sequentially are superimposed on the
main magnetic field (Bo) so that selective spatial excitation
of the imaging volume can occur. Gradients are also used to
apply reversal pulses in some fast imaging techniques.
GRADIENT ECHO (GE) - see Field
GRADIENT MAGNETIC FIELD - A small
linear magnetic field applied in addition to (superimposed
on) the large static magnetic field in an MRI scanner. The
strength (amplitude) and direction of the gradient fields
change during the scan, which allows each small volume element
(voxel) within the imaging volume to resonate at a different
frequency. In this way, spatial encoding may be performed.
GYROMAGNETIC RATIO (g) - a constant
for any given nucleus that relates the nuclear MR frequency
and the strength of the external magnetic field. It represents
the ratio of the magnetic moment (field strength) to the angular
momentum (frequency) of a particle. The value of the gyromagnetic
ratio for hydrogen (1H) is 4,258 Hz/Gauss (42.58 MHz/Tesla).
HERTZ - the standard unit of
frequency equal to 1 cycle per second. The larger unit megahertz
(MHz) = 1,000,000 Hz.
HOMOGENEITY - uniformity of the
main magnetic field.
HYDROGEN DENSITY (H+) - the concentration
of Hydrogen atoms in water molecules or in some groups of
fat molecules within tissue. Initial MR signal amplitudes
are directly related to H+ density in the tissue being imaged.
IMAGE (DATA) ACQUISITION TIME
- the time required to gather a complete set of image data.
The total time for performing a scan must take into consideration
the additional image reconstruction time when determining
how quickly the image(s) may be viewed.
IMAGE RECONSTRUCTION - the mathematical
process of converting the composite signals obtained during
the data acquisition phase into an image.
INHOMOGENEITY - lack of homogeneity
or uniformity in the main magnetic field.
INVERSION RECOVERY (IR) - an
imaging sequence that involves successive 180É and 90É pulses,
after which a heavily T1-weighted signal is obtained. The
inversion recovery sequence is specified in terms of three
parameters, inversion time (TI), repetition time (TR) and
echo time (TE).
INVERSION TIME (TI)
- the time period between the 180° inversion pulse and the
90° excitation pulse in an Inversion Recovery pulse sequence.
ISOTOPE - Atomic nuclei that contain the same number of protons,
but differ in the number of neutrons in the nucleus of the
atom for the element concerned. K-SPACE - a data acquisition
matrix containing raw image data prior to image processing.
In 2DFT, a line of data corresponds to the digitized NMR signal
at a particular phase-encoding level.
LARMOR EQUATION - an equation
that states that the frequency of precession of the nuclear
magnetic moment is directly proportional to the product of
the magnetic field strength (Bo) and the gyromagnetic ratio
(g). This is stated mathematically as å = g Bo.
LARMOR FREQUENCY - the frequency
at which magnetic resonance in a nucleus can be excited and
detected. The frequency varies directly with magnetic field
strength, and is normally in the radio frequency (RF) range.
LATTICE - in MRI, the magnetic
and thermal environment through which nuclei exchange energy
in longitudinal (T1) relaxation.
LONGITUDINAL MAGNETIZATION -
the component (MZ) of the net magnetization vector in the
direction of the static magnetic field. After 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
excitation due to the exchange of energy between the nuclear
spins and the lattice.
LONGITUDINAL RELAXATION TIME
- the time constant, T1, which determines the rate at which
excited protons return to equilibrium within the lattice.
A measure of the time taken for spinning protons to re-align
with the external magnetic field. The magnetization will grow
after excitation from zero to a value of about 63% of its
final value in a time of T1.
MAGNETIC GRADIENT - one of three
linear magnetization waveforms superimposed on the main magnetic
field at specific times within a pulse sequence to select
the imaging region or provide necessary spatial localization
information. A magnetic gradient is defined as the amount
and direction of the linear rate of change of the magnetic
field in space. MAGNETIC FIELD - magnetic lines of force which
extend from a north polarity and enter a south polarity to
form a closed loop around the outside of a magnetic material.
MAGNETIC MOMENT - a measure of
the net magnetic properties of an object or particle. A nucleus
with an intrinsic spin will have an associated magnetic dipole
moment so that it will interact with a magnetic field (as
if it were a tiny bar magnet).
MAGNETIC RESONANCE - the absorption
or emission of energy by atomic nuclei in an external magnetic
field after the application of RF excitation pulses using
frequencies which satisfy the conditions of the Larmor equation.
MAGNETIC RESONANCE ANGIOGRAPHY (MRA)
- MR image visualization of selected vascular structures,
such as the Circle Of Willis or the carotid arteries.
MAGNETIC RESONANCE SPECTROSCOPY (MRS)
- an MR technique wherein a sample is placed in a strong,
very uniform, magnetic field, and stimulated with RF electromagnetic
energy. If the field is uniform over the volume of the sample,
"similar" nuclei will contribute a particular frequency component
to the detected response signal irrespective of their individual
positions in the sample. Since nuclei of different elements
resonate at different frequencies, each element in the sample
contributes a different frequency component. A chemical analysis
can then be conducted by analyzing the MR response signal
into its frequency components.
MAGNETIC SUSCEPTIBILITY - the
extent to which a material becomes magnetized when placed
within a magnetic field. Differences in magnetic susceptibilities
at tissue borders are a frequent cause of MRI artifacts.
MAGNETIZATION VECTOR (Mz) - the
integration of all the individual nuclear magnetic moments
which have a positive magnetization value at equilibrium versus
those in a random state.
MAXIMUM INTENSITY PROJECTION (MIP)
- a processing method for MRA images. A MIP is a record of
a maximum intensity ray (generated through a mathematical
algorithm) as it passes through an angiographic volume. Each
point in an MIP represents the highest intensity experienced
in that location on any partition within the imaging volume.
MR IMAGING - the use of magnetic
resonance principles in the production of diagnostic views
of the human body where the resulting image is based upon
three basic tissue parameters (proton density, T1 relaxation
time, T2 relaxation time) and flow characteristics. MRA -
See Magnetic Resonance Angiography. MRS - See Magnetic Resonance
- the ability to display anatomical structures in a variety
of planes from the data acquired in just one scan.
MULTI-ECHO IMAGING - imaging
using a series of echoes acquired as a train following a single
excitation pulse. In spin-echo imaging, each echo is formed
by a 180É pulse. Typically, a separate image is produced from
each echo of the train.
MULTI-SLICE IMAGING - an imaging
technique in which the repetition period (TR) is utilized
for acquiring additional slices in other layers or planes.
NET MAGNETIZATION VECTOR - a
vector which represents the sum of all of the contributions
of the magnetic moments within the magnetic field; the magnitude
and direction of the magnetization resulting from this collection
of atomic nuclei.
NEUTRON - an uncharged neutral
particle located in the nucleus of most atoms which serves
as a stabilizer.
NEX - number
of excitations. See also Number of Excitations, Signal Averaging.
NMR SIGNAL - the electromagnetic
signal in the radio-frequency range produced by the precession
of the transverse magnetization of the spins. The rotation
of the transverse magnetization induces a voltage in a receiving
antenna (coil) which is amplified and demodulated by the receiver
NOISE - an undesirable background
interference or disturbance that affects image quality. NSA
- the number of signal averages performed during the scan.
See also NEX and Signal Averaging.
NUCLEAR SPIN - also known as
inherent spin, this defines the intrinsic property of certain
nuclei (those with odd numbers of protons and/or neutrons
in their nucleus) to exhibit angular momentum and a magnetic
moment. Nuclei that do not exhibit this characteristic will
not produce an NMR signal.
NUCLEUS - the core or center
part of an atom, which contains protons having a positive
charge and neutrons having no electrical charge, except in
the common isotope of hydrogen, where the nucleus is a single
NUMBER OF EXCITATIONS - an indicator
of how many times each line of k-space data is acquired during
- a plane or section not perpendicular to the xyz coordinate
system, such as long and short axis views of the heart.
ORTHOGONAL - a plane or section
perpendicular to the xyz coordinate system.
OSCILLATION - rhythmic periodic
PARAMAGNETIC SUBSTANCE - a substance
with weak magnetic properties due to its unpaired electrons.
Researchers are developing certain paramagnetic materials,
such as gadolinium, as MRI invasive contrast media.
PARTIAL VOLUMING - a loss of
resolution due to excessively large voxels, typically caused
by slices that are too thick.
- a magnet design that utilizes blocks of ferromagnetic materials
(permanent magnets) to generate a magnetic field between the
two poles of the magnet. There is no requirement for additional
electrical power or cooling, and the iron-core structure of
the magnet leads to a limited fringe field and no missile
effect. Due to weight considerations, permanent magnets are
usually limited to maximum field strengths of 0.3T.
PHANTOM - an artificial object
of known dimensions and properties that is used to test or
monitor an MRI systems homogeneity, imaging performance and
PHASE - an angular relationship
describing the degree of synchronism between two sinusoidal
waveforms of the same frequency.
PHASE COHERENCE - a term describing
the degree to which precessing nuclear spins are synchronous.
PHASE CONTRAST - an MRA technique
utilizing the change in the phase shifts of the flowing protons
in the region of interest to create an image.
PHASE ENCODING - the process
of locating an MR signal by altering the phase of spins in
one dimension with a pulsed magnetic field gradient along
that dimension prior to the acquisition of the signal. As
each signal component has experienced a different phase encoding
gradient pulse, its exact spatial reconstruction can be specifically
and precisely located by the Fourier transformation analysis.
Spatial resolution is directly related to the number of phase
encoding levels (gradients) used.
PIXEL - acronym for a picture
element, the smallest discrete two-dimensional part of a digital
PLANAR IMAGING - a method of
scanning in which the data is collected simultaneously from
an entire layer.
PRECESSION - comparatively slow
gyration of the axis of a spinning body so as to trace out
a cone. Caused by the application of a torque tending to change
the direction of the rotation axis and continuously directed
at right angles to the plane of the torque. The magnetic moment
of a nucleus with spin will experience such a torque when
inclined at an angle to the magnetic field, resulting in precession
at the Larmor frequency.
PRESATURATION (PRE-SAT) - a specialized
technique employing repeated RF excitation of structures adjacent
to the ROI for the purpose of reducing or eliminating their
phase effect artifacts.
PROTON - a positively charged
particle located in the nucleus of an atom. The number of
protons in the nucleus governs the chemical properties of
PROTON DENSITY - the concentration
of mobile Hydrogen atoms within a sample of tissue. See also
PROTON DENSITY WEIGHTED IMAGE
- an image produced by controlling the selection of scan parameters
to minimize the effects of T1 and T2, resulting in an image
dependent primarily on the density of protons in the imaging
PULSE PROGRAMMER - the computer-controlled
component of the MRI scanner that determines the timing of
the pulse sequence parameters of the scan, such as echo time,
pulse amplitude, phase and frequency.
PULSE SEQUENCE - a preselected
set of defined RF and gradient pulses, usually repeated many
times during a scan, wherein the time interval between pulses
and the amplitude and shape of the gradient waveforms will
control NMR signal reception and affect the characteristics
of the MR images.
QUENCH - an event which can only
occur in superconducting magnets, it is caused by a loss of
superconductivity; a rapid increase in the resistivity of
the magnet, which generates heat that results in the rapid
evaporation of the magnet coolant (liquid helium). This evaporated
coolant is a hazard that requires emergency venting systems
to protect patients and operators. A quench can cause total
RADIO FREQUENCY - an electromagnetic
wave with a frequency that is in the same general range as
that used for the transmission of radio and television signals.
Abbreviated RF. The RF pulses used in MR are commonly in the
1-100 megahertz range, and their principle effect upon a body
is potential tissue heating caused by absorption of the applied
pulses of RF energy.
READOUT GRADIENT - magnetic field
gradient applied during the period when the receiver components
are on. The application of this gradient, which is active
during the period when the echo is being formed, results in
the frequency encoding of the object being imaged. See also
RECEIVER - the portion of the
MRI equipment that detects and amplifies the RF signals picked
up by the receiver coil. Includes a preamplifier, NMR signal
amplifier, and demodulator.
RECEIVER COIL - a coil , or antenna,
positioned within the imaging volume and connected to the
receiver circuitry that is used to detect the NMR signal.
In certain applications, the same coil can be used for both
transmission and reception. Receiver coils types include solenoidal,
planar, volume, quadrature and phased array coils.
RECONSTRUCTION - the mathematical
process by which the displayed image is produced from the
raw k-space data obtained from the receiver circuitry, typically
utilizing Fourier transformation and selective filtering.
REGION OF INTEREST (ROI) - the
area of anatomy being scanned that is of particular importance
in the image.
RELAXATION TIME - after excitation
the spins will tend to return to their equilibrium distribution
in which there is no transverse magnetization and the longitudinal
magnetization is at its maximum value and oriented in the
direction of the static magnetic field. After excitation the
transverse magnetization decays toward zero with a characteristic
time constant T2, and the longitudinal magnetization returns
toward equilibrium with a characteristic time constant T1.
REPETITION TIME (TR) - the amount
of time that exists between successive pulse sequences applied
to the same slice. It is delineated by initiating the first
RF pulse of the sequence then repeating the same RF pulse
at a time t. Variations in the value of TR have an important
effect on the control of image contrast characteristics. Short
values of TR (< 1000 ms) are common in images exhibiting T1
contrast, and long values of TR (> 1500 ms) are common in
images exhibiting T2 contrast. TR is also a major factor in
total scan time. See also TR.
REPHASING - the process of returning
out-of-phase magnetic moments back into phase coherence. Caused
either by rapidly reversing a magnetic gradient (Field Echo)
or by applying a 180É RF pulse (Spin Echo). In the spin-echo
pulse sequence this action effectively cancels out the spurious
T2* information from the signal.
RESISTIVE MAGNET - a common type
of magnet that utilizes the principles of electromagnetism
to generate the magnetic field. Typically large current values
and significant cooling of the magnet coils is required. Resistive
magnets fall into two general categories - iron-core and air-core.
Iron-core electromagnets provide the advantages of a vertically-oriented
magnetic field, and a limited fringe field with little, if-any,
missile effects due to the closed iron-flux return path. Air-core
electromagnets exhibit horizontally oriented fields, which
have large fringe fields (unless magnetically shielded) and
are prone to missile effects. Resistive magnets are typically
limited to maximum field strengths of approximately 0.6T.
RESONANCE - a large amplitude
vibration in a mechanical or electrical system caused by a
relatively small periodic stimulus with a frequency at or
close to a natural frequency of the system. The exchange of
energy at a particular frequency between two systems. ROI
- see Region Of Interest.
- a plane, slice or section of the body cutting from front
to back through the saggital suture of the skull, and continued
down through the body in the same direction, dividing it
two parts, then turning one half to view it from its cut
SAMPLING - the conversion of
analog signals to discreet digital values through a preselected
measurement process. SAR - see Specific Absorption Rate.
SATURATION RECOVERY - a little-used
pulse sequence that generates a predominately proton density
dependent signal, basically employing a 90° RF excitation
pulse, with a very long repetition time. This procedure allows
the saturated spins to return to equilibrium before the next
pulse is activated.
SELECTIVE EXCITATION - controlling
the frequency spectrum (bandwidth) of an RF excitation pulse
while imposing a gradient magnetic field on spins so that
only a desired region will have a suitable resonant frequency
to be excited. SCAN TIME - a description of the total time
required to acquire all the data needed to produce the programmed
image. See also Acquisition Time, Image (Data) Acquisition
SHIM COILS - coils positioned
near the main magnetic field that carry a relatively small
current that is used to provide localized auxiliary magnetic
fields in order to improve field homogeniety. See also Shimming
SHIMMING - The process of improving
field homogeniety by compensating for imbalances in the main
magnetic field of an MRI system. This can be accomplished
through a combination of passive (mechanical) shimming (e.g.,
adding or removing steel from the magnets poles) and active
shimming (the use of shim coils) to fine-tune the magnetic
SIGNAL AVERAGING - a signal-to-noise
improvement method that is accomplished by taking the average
of several FIDs made under similar conditions. This is also
referred to as the number of excitations (NEX) or the number
of acquisitions. The approximate amount of improvement in
signal-to-noise (S/N) ratio is calculated as the square root
of the number of excitations ( ).
(S/N, SNR) - The ratio between the amplitude of the
received signal and background noise, which tends to obscure
that signal. SNR, and hence image quality, can be improved
by such factors as increasing the number of excitations, increasing
the field of view, increasing slice thickness, etc. SNR also
depends on the electrical properties of the patient being
studied and the type of receiving coil used.
SLICE - the term describing the
planar region or the image slice selection region.
SLICE ENCODING - relates to the
addition of phase encoding steps for 3D volumetric imaging.
SLICE SELECTION - exclusive excitation
of spins in one slice performed by the coincident combination
of a gradient magnetic field and a narrow bandwidth or slice
selective RF pulse at a specific Larmor frequency.
- the thickness of an imaging slice. Since the slice profile
is not sharply edged, the distance between the points at half
the sensitivity of the maximum (full width at half maximum)
is used to determine thickness.
SMART - the
acronym for Slice-specific, Multi-Angle, multi-Resolution,
multi-Thickness scanning. This function allows the operator
to individually customize the thickness, field-of-view and
position of each slice in a multi-angle study. SNR - see Signal-To-Noise
SPATIAL RESOLUTION - the ability
to define minute adjacent objects/points in an image, generally
measured in line pairs per mm (lp/mm).
SPECIFIC ABSORPTION RATE - an
RF exposure concern that describes the potential for heating
of the patient's tissue due to the application of the RF energy
necessary to produce the NMR signal. The RF induced heat load
can be directly related to the
SAR (Specific Absorption Rate),
which is defined as the RF power absorbed per unit of mass
of an object, and is measured in watts per kilogram (W/kg).
SPIN - the property exhibited
by atomic nuclei that contain either an odd number of protons
or neutrons, or both.
- re-appearance of the NMR signal after the FID has apparently
died away, as a result of the effective reversal (rephasing)
of the dephasing spins by techniques such as specific RF pulse
sequences or pairs of field gradient pulses, applied in time
shorter than or on the order of T2. Proper selection of the
TE time of the pulse sequence can help control the amount
of T1 or T2 contrast present in the image. Also a pulse sequence
type that usually employs a 90° pulse, followed by one or
more 180° pulses.
SPIN-LATTICE RELAXATION TIME -
see T1 and Longitudinal Relaxation Time.
SPIN-SPIN RELAXATION TIME - see
T2 and Transverse Relaxation Time.
STEADY-STATE FREE PRECESSION
- the name for any field echo or gradient echo sequence in
which a non-zero steady state develops for both transverse
and longitudinal components of magnetization. If the RF pulses
are close enough together, the MR signal will never completely
decay, implying that the spins in the transverse (x-z) plane
never completely dephase. STIR - the acronym for Short TI
Inversion Recovery. A specialized application of the Inversion
Recovery pulse sequence that sets the inversion time (TI)
of the sequence at 0.69 times the T1 of fat, thereby suppressing
the fat in the image. See also Fat Suppression.
- a magnet whose field is generated by current in wires made
of a superconducting material such as niobium-titanium, that
has no resistance when operated at temperatures near absolute
zero(-273°C, -459°F). Such magnets must be cooled by, for
example, liquid helium. Superconducting magnets typically
exhibit field strengths of >0.5T and have a horizontal field
orientation, which makes them prone to missile effects without
significant magnetic shielding. See also Quenching.
a type of receiver coil which is placed directly on or over
the region of interest for increased magnetic sensitivity.
These coils are specifically designed for localized body regions,
and provide improved signal-to-noise ratios by limiting the
spatial extent of the excitation or reception. T - tesla T1
- spin-lattice longitudinal relaxation time. The characteristic
time constant for spins to realign themselves with the external
magnetic field after excitation.
T1 WEIGHTED -
an image created typically by using short TE and TR times
whose contrast and brightness are predominately determined
by T1 signals.
- see Longitudinal Relaxation Time.
T2 - spin-spin
or transverse relaxation time. The time constant for loss
of phase coherence among spins oriented at an angle to the
static magnetic field due to interactions between the spins.
Results in a loss of transverse magnetization and the MRI
T2* ("T-two-star") - the time
constant for loss of phase coherence among spins oriented
at an angle to the static magnetic field due to a combination
of magnetic field inhomogeneities and the spin-spin relaxation.
Results in a rapid loss of transverse magnetization and the
MRI signal.T2* < T2.
T2 WEIGHTED - an image created
typically by using longer TE and TR times whose contrast and
brightness are predominately determined by T2 signals. TAU
(t) - the interpulse times (time between the 90° and 180°
pulse, and between the 180° pulse and the echo) used in a
spin echo pulse sequence. TE (Echo Time) - represents the
time in milliseconds between the application of the 90°
pulse and the peak of the echo signal in Spin Echo and Inversion
Recovery pulse sequences.
TE (Echo Time)
- represents the time in milliseconds between the application
of the 90° pulse and the peak of the echo signal in Spin
Echo and Inversion Recovery pulse sequences.
- the preferred unit of magnetic flux density. One tesla is
equal to 10,000 gauss. The Tesla unit value is defined as
a field strength of 1 Weber per meter 2, where 1 Weber represents
1 x 108 (100,000,000) flux lines.
THREE DIMENSIONAL IMAGING (3DFT)
- a specialized imaging technique that uses computer processing
to combine individual slice acquisitions together to produce
an image that represents length, width and height. TI (Inversion
Time) - the time between the initial (inverting) 180°
pulse and the 90° pulse used in inversion recovery pulse
TIME OF FLIGHT (TOF) - and MRA
technique relying solely on the flow of unsaturated blood
into a magnetized presaturated slice. The difference between
the unsaturated and presaturated spins creates a bright vascular
image without the invasive use of contrast media.
TIP ANGLE - angle between the
net magnetization vector before and after an RF excitation
pulse. Small tip angles allow a decrease in TR, which is used
to decrease scan time in Field Echo pulse sequences. See Flip
TR (Repetition Time)
- the amount of time that exists between successive pulse
sequences applied to the same slice. See also Repetition Time.
TRANSAXIAL - a plane perpendicular
(rotated 90°) to the long axis of the human body. See also
TRANSCEIVER COIL - an MRI surface
coil that acts as both transmitter and receiver.
TRANSMITTER - the portion of
the MR scanner that produces the RF current and delivers it
to the transmitting coil (antenna). The RF signal produced
by the transmitter is used to excite the protons in the imaging
TRANSVERSE MAGNETIZATION - component
of the net magnetization vector at right angles to the main
magnetic field. Precession of the transverse magnetization
at the Larmor frequency is responsible for the detectable
NMR signal. In the absence of externally applied RF energy,
the transverse magnetization will decay to zero with a characteristic
time constant of T2, or more strictly T2*.
TRANSVERSE RELAXATION TIME -
the time constant, T2, which determines the rate at which
excited protons reach equilibrium, or go out of phase with
each other. A measure of the time taken for spinning protons
to lose phase coherence among the nuclei spinning perpendicular
to the main field due to interaction between spins, resulting
in a reduction in the transverse magnetization. The transverse
magnetization value will drop from maximum to a value of about
37% of its original value in a time of T2.
TUNING - the process of adjusting
the transmitter and receiver circuitry so that it provides
optimal signal performance at the Larmor frequency. A properly
tuned scanner will produce images with a higher signal- to-noise
ratio, and therefore improved diagnostic versatility.
TWO-DIMENSIONAL IMAGING (2DFT) -
the Fourier transformation process reconstructs the detected
frequency and phase encoded image information (which are rotated
90° from each other) into a usable image.
VECTOR - a quantity that has
both magnitude and direction and that is commonly represented
by an arrow. The length of the line segment represents the
magnitude, and its orientation in space represents its direction.
Vector quantities can be added to or subtracted from one another.
VELOCITY - speed in a particular
VELOCITY ENCODING (VENC) - a
specialized technique used fro encoding flow velocities.
VISCOSITY - a property of a fluid
or semi-fluid that affects its mobility, and therefore its
intensity in an image.
VOLUMETRIC IMAGING - a specialized
technique where all the MR signals are collected from the
entire tissue sample and imaged as a whole entity. Compare
with slice select.
VORTEX FLOW - area within a
blood vessel where the blood is suddenly accelerated, then
rapidly decelerated. This would be commonly seen in blood
passing through a vascular stenosis (narrowing), and becomes
a factor in MRA.
VOXEL - volume element; the element
of the three-dimensional space corresponding to a pixel, for
a given slice thickness.