Prelim Glossary Exam Flashcards
1
Q
is a continuously changing flow of electrons that alternates its polarity at a periodic rate.
A
AC - Alternating Current
2
Q
- the process of measuring and storing image data
A
ACQUISITION
3
Q
- the total number of independent data samples in the
frequency (f) and phase (f) directions.
A
ACQUISITION MATRIX
4
Q
- 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
A
ACQUISITION TIME
5
Q
- a current that continuously changes in magnitude and
direction. In the US the current changes at a frequency of 60 Hz
A
ALTERNATING CURRENT (AC)
6
Q
- being continuous, or having a continuous range of values.
A
ANALOG
7
Q
- 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 systems.
A
ANALOG-TO-DIGITAL CONVERTER (ADC)
8
Q
- a device that enables the sending and/or receiving of electromagnetic waves. See also Transmitter, Receiver Coils and Surface Coils.
A
ANTENNA
9
Q
- the storage of image and patient data for future retrieval.
A
ARCHIVING
10
Q
- a dedicated computer system used to perform Fourier transformations to accelerate the processing of the received numerical data relative
to the MR imaging process.
A
ARRAY PROCESSOR
11
Q
- 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
A
AXIAL
12
Q
- a conventional symbol for the constant magnetic field produced by the large magnet in the MR scanner
A
B or Bo
13
Q
- the conventional symbol used for identifying
the radio frequency (RF) magnetic field
A
B1
14
Q
- an all-inclusive term referring to the preselected band or range of frequencies which can govern both slice select and signal sampling
A
BANDWIDTH (BW)
15
Q
- 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
A
CHEMICAL SHIFT
16
Q
- 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.
A
CINE
17
Q
- a large network of interconnecting blood vessels at the base of the brain that when visualized resembles a circle
A
CIRCLE OF WILLIS
18
Q
- a psychological reaction to being confined in a relatively small area
A
CLAUSTROPHOBIA
19
Q
- the act of maintaining a constant phase relationship between oscillating waves or rotating objects.
A
COHERENCE
20
Q
- 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
A
CONTRAST
21
Q
- an image phenomenon where the darks become bright, and the brights become dark. This is usually most prevalent in sequences utilizing an
extended TR
A
CONTRAST REVERSAL
22
Q
) - 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 interest.
A
CONTRAST-TO-NOISE RATIO (CNR
23
Q
- 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
A
CORONAL
24
Q
- an artifact introduced into images by interference between adjacent slices of a scan. This artifact can be eliminated by limiting the minimum spacing
between slices.
A
CROSSTALK
25
Q
- 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.
A
CRYOGEN
26
Q
- the fanning out or loss of phase coherence of signals within the transverse plane. See also T2.
A
DEPHASING
27
Q
- a magnetic field characterized by its own north and south magnetic poles separated by a finite distance
A
DIPOLE
28
Q
- a continuous current that flows in only one direction.
A
DIRECT CURRENT (DC)
29
Q
- the total number of pixels in the selected matrix, which is described by the product of its phase and frequency axis
A
DISPLAY MATRIX
30
Q
- a theory of magnetism which assumes that groups of atoms produced by movement of electrons align themselves in groups called”domains” in
magnetic materials.
A
DOMAIN THEORY
31
Q
- Gadolinium chelating (chemical bonding) agent that solves the problem of toxicity
A
DTPA - Diethylenetriaminepentaacetic acid
32
Q
- 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.
A
ECHO PLANAR IMAGING (EPI)
33
Q
- a series of 180° RF rephasing pulses and their corresponding echoes for a Fast Spin Echo (FSE) pulse sequence.
A
ECHO TRAIN
34
Q
ETL -
A
Echo Train Length
35
Q
- 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
A
EDDY CURRENT
36
Q
- 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.
A
ELECTROMAGNET
37
Q
) - the response of electrons to electromagnetic
radiation and magnetic fields at discrete frequencies. EPI - echo planar imaging. See
also Echo Planar Imaging
A
ELECTRON SPIN RESONANCE (ESR
38
Q
- a state of balance that exists between two opposing forces or divergent forms of influence.
A
EQUILIBRIUM
39
Q
- 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
A
EXCITATION
40
Q
(Faraday Cage) - an electrically conductive screen or shield that reduces or eliminates interference between outside radio waves and those from the MRI unit.
A
FARADAY SHIELD
41
Q
- a specialized technique usually associated with short TR, reduced flip angle and repeated 180° rephasing pulses.
A
FAST SCANNING
42
Q
- 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
A
FAST SPIN ECHO (FSE)
43
Q
- 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.
A
FAT SATURATION (FAT-SAT)
44
Q
- 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.
A
FAT SUPPRESSION
45
Q
- defined as the size of the two or three dimensional spatial encoding area of the image. Usually defined in units of cm2.
A
FIELD OF VIEW (FOV)
46
Q
- 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.
A
FFT (Fast Fourier Transform)
47
Q
(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.
A
FIELD ECHO (FE)
48
Q
FLAIR
A
FLuid Attenuated Inversion Recovery
49
Q
FLARE
A
Fast Low-Angle Recalled Echoes
50
Q
- 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.
A
FLIP ANGLE (FA)
51
Q
- 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.
A
FLOW COMPENSATION
52
Q
- invisible lines of force that extend around a magnetic material. The greatest density is at the two poles of the magnet.
A
FLUX
53
Q
- the number of lines of force per unit area of a magnetic material
A
FLUX DENSITY
54
Q
- 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
A
FOURIER TRANSFORM (FT)
55
Q
- 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
A
FREE INDUCTION DECAY (FID)
56
Q
- 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.
A
FREQUENCY
57
Q
- 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.
A
FREQUENCY ENCODING
58
Q
- 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.
A
FRINGE FIELD
59
Q
- 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 surroundings.
A
GADOLINIUM (Gd)
60
Q
- timing the acquisition of MR data to physiological motion in order to minimize motion artifacts (e.g., cardiac gating, respiratory gating).
A
GATING
61
Q
- 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
A
GAUSS
62
Q
- 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.
A
GRADIENT COILS
63
Q
- 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.
A
GRADIENT MAGNETIC FIELD
64
Q
- 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).
A
GYROMAGNETIC RATIO (g)
65
Q
- the standard unit of frequency equal to 1 cycle per second. The larger unit
megahertz (MHz) = 1,000,000 Hz.
A
HERTZ
66
Q
- uniformity of the main magnetic field.
A
HOMOGENEITY
67
Q
- 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.
A
HYDROGEN DENSITY (H+)
68
Q
- 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.
A
IMAGE (DATA) ACQUISITION TIME
69
Q
- the mathematical process of converting the composite
signals obtained during the data acquisition phase into an image.
A
IMAGE RECONSTRUCTION
70
Q
- lack of homogeneity or uniformity in the main magnetic field.
A
INHOMOGENEITY
71
Q
- 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).
A
INVERSION RECOVERY (IR)
72
Q
- the time period between the 180° inversion pulse and the 90° excitation pulse in an Inversion Recovery pulse sequence.
A
INVERSION TIME (TI)
73
Q
- 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.
A
ISOTOPE
74
Q
- 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.
A
K-SPACE
75
Q
- 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.
A
LARMOR EQUATION
76
Q
- 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
A
LARMOR FREQUENCY
77
Q
- in MRI, the magnetic and thermal environment through which nuclei exchange energy in longitudinal (T1) relaxation.
A
LATTICE
78
Q
- the component (MZ) of the net magnetization
vector in the direction of the static magnetic field. After RF excitation, this vectoreturns to its equilibrium value at a rate characterized by the time constant T1.
A
LONGITUDINAL MAGNETIZATION
79
Q
- return of longitudinal magnetization to its equilibrium
value after excitation due to the exchange of energy between the nuclear spins and
the lattice.
A
LONGITUDINAL RELAXATION
80
Q
- 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.
A
LONGITUDINAL RELAXATION TIME
81
Q
- 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.
A
MAGNETIC GRADIENT
82
Q
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.
A
MAGNETIC FIELD -
83
Q
- 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).
A
MAGNETIC MOMENT
84
Q
- 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.
A
MAGNETIC RESONANCE
85
Q
- MR image visualization of selected vascular structures, such as the Circle Of Willis or the carotid arteries
A
MAGNETIC RESONANCE ANGIOGRAPHY (MRA)
86
Q
) - 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.
A
MAGNETIC RESONANCE SPECTROSCOPY (MRS
87
Q
- 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.
A
MAGNETIC SUSCEPTIBILITY
88
Q
- the integration of all the individual nuclear magnetic
moments which have a positive magnetization value at equilibrium versus those in a
random state.
A
MAGNETIZATION VECTOR (Mz)
89
Q
- a processing method for MRA images. 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.
A
MAXIMUM INTENSITY PROJECTION (MIP)
90
Q
- 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 Spectroscopy
A
MR IMAGING
91
Q
- the ability to display anatomical structures in a variety of planes from the data acquired in just one scan
A
MULTI-ANGLE OBLIQUE
92
Q
- 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.
A
MULTI-ECHO IMAGING
93
Q
- an imaging technique in which the repetition period (TR) is utilized for acquiring additional slices in other layers or planes.
A
MULTI-SLICE IMAGING
94
Q
- 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.
A
NET MAGNETIZATION VECTOR
95
Q
- an uncharged neutral particle located in the nucleus of most atoms which
serves as a stabilizer
A
NEUTRON
96
Q
- number of excitations. See also Number of Excitations, Signal Averaging.
A
NEX
97
Q
- 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 circuits.
A
NMR SIGNAL
98
Q
- an undesirable background interference or disturbance that affects image
quality.
A
NOISE
99
Q
- the number of signal averages performed during the scan. See also NEX
and Signal Averaging.
A
NSA
100
Q
- 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.
A
NUCLEAR SPIN
101
Q
- 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 proton.
A
NUCLEUS
102
Q
- an indicator of how many times each line of k-space data
is acquired during the scan.
A
NUMBER OF EXCITATIONS
103
Q
- a plane or section not perpendicular to the xyz coordinate system, such as
long and short axis views of the heart
A
OBLIQUE
104
Q
- a plane or section perpendicular to the xyz coordinate system
A
ORTHOGONAL
105
Q
- rhythmic periodic motion
A
OSCILLATION
106
Q
- 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
A
PARAMAGNETIC SUBSTANCE
107
Q
- a loss of resolution due to excessively large voxels, typically
caused by slices that are too thick
A
PARTIAL VOLUMING
108
Q
- 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
A
PERMANENT MAGNET
109
Q
- an artificial object of known dimensions and properties that is used to test or monitor an MRI systems homogeneity, imaging performance and orientation
aspects.
A
PHANTOM
110
Q
- an angular relationship describing the degree of synchronism between two sinusoidal waveforms of the same frequency
A
PHASE
111
Q
- a term describing the degree to which precessing nuclear spins
are synchronous
A
PHASE COHERENCE
112
Q
- an MRA technique utilizing the change in the phase shifts of the flowing protons in the region of interest to create an image.
A
PHASE CONTRAST
113
Q
- 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.
A
PHASE ENCODING
114
Q
- acronym for a picture element, the smallest discrete two-dimensional part of
a digital image display
A
PIXEL
115
Q
- a method of scanning in which the data is collected
simultaneously from an entire layer.
A
PLANAR IMAGING
116
Q
- 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
A
PRECESSION
117
Q
- a specialized technique employing repeated RF
excitation of structures adjacent to the ROI for the purpose of reducing or eliminating their phase effect artifacts.
A
PRESATURATION (PRE-SAT)
118
Q
- a positively charged particle located in the nucleus of an atom. The
number of protons in the nucleus governs the chemical properties of that element
A
PROTON
119
Q
- the concentration of mobile Hydrogen atoms within a sample of tissue. See also Hydrogen Density.
A
PROTON DENSITY
120
Q
- 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 volume.
A
PROTON DENSITY WEIGHTED IMAGE
121
Q
- 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
A
PULSE PROGRAMMER
122
Q
- 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
A
PULSE SEQUENCE
123
Q
- 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 magnet failure.
A
QUENCH
124
Q
- 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.
A
RADIO FREQUENCY
125
Q
- 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 Frequency Encoding.
A
READOUT GRADIENT
126
Q
- 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.
A
RECEIVER
127
Q
- 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.
A
RECEIVER COIL
128
Q
- 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
A
RECONSTRUCTION
129
Q
- the area of anatomy being scanned that is of particular
importance in the image.
A
REGION OF INTEREST (ROI)
130
Q
- 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.
A
RELAXATION TIME
131
Q
- 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.
A
REPETITION TIME (TR)
132
Q
- 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.
A
REPHASING
133
Q
- 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.
A
RESISTIVE MAGNET
134
Q
- 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
RESONANCE
135
Q
- 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 into two parts, then turning one half to view it from its cut
surface.
A
SAGITTAL
136
Q
- the conversion of analog signals to discreet digital values through a preselected measurement process.
A
SAMPLING
137
Q
- a little-used pulse sequence that generates a
predominately proton density dependent signal, basically employing a 90° RFexcitation pulse, with a very long repetition time. This procedure allows the saturated
spins to return to equilibrium before the next pulse is activated.
A
SATURATION RECOVERY
138
Q
- 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.
A
SELECTIVE EXCITATION
139
Q
- 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 Time.
A
SCAN TIME
140
Q
- 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
A
SHIM COILS
141
Q
- 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 field
A
SHIMMING
142
Q
- 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 ( ).
A
SIGNAL AVERAGING
143
Q
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.
A
SIGNAL-TO-NOISE RATIO (S/N, SNR) -
144
Q
- the term describing the planar region or the image slice selection region
A
SLICE
145
Q
- relates to the addition of phase encoding steps for 3D volumetric imaging.
A
SLICE ENCODING
146
Q
- 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.
A
SLICE SELECTION
147
Q
- 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.
A
SLICE THICKNESS
148
Q
- 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 Ratio
A
SMART
149
Q
- the ability to define minute adjacent objects/points in an image, generally measured in line pairs per mm (lp/mm).
A
SPATIAL RESOLUTION
150
Q
SAR - 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
A
SPECIFIC ABSORPTION RATE
151
Q
which is defined as the RF power absorbed per unit
of mass of an object, and is measured in watts per kilogram (W/kg).
A
SAR (Specific Absorption Rate),
152
Q
- the property exhibited by atomic nuclei that contain either an odd number of protons or neutrons, or both.
A
SPIN
153
Q
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
A
SPIN-ECHO (SE) -
154
Q
- see T1 and Longitudinal Relaxation Time.
A
SPIN-LATTICE RELAXATION TIME
155
Q
- see T2 and Transverse Relaxation Time
A
SPIN-SPIN RELAXATION TIME
156
Q
SSFP- 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
STEADY-STATE FREE PRECESSION
157
Q
- 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
Quenchin
A
SUPERCONDUCTIVE MAGNET
158
Q
- 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.
A
SURFACE COIL
159
Q
- an image created typically by using short TE and TR times whose contrast and brightness are predominately determined by T1 signals
A
T1 WEIGHTED
160
Q
- see Longitudinal Relaxation Time.
A
T1 RELAXATION
161
Q
- 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 signal.
A
T2
162
Q
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.
A
T2* (“T-two-star”) -
163
Q
an image created typically by using longer TE and TR times whose
contrast and brightness are predominately determined by T2 signals.
A
T2 WEIGHTED -
164
Q
- 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
A
TAU (t)
165
Q
- 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
A
TE (Echo Time)
166
Q
- 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.
A
TE (Echo Time)
167
Q
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.
A
TESLA (T) -
168
Q
) - a specialized imaging technique that uses
computer processing to combine individual slice acquisitions together to produce an
image that represents length, width and height.
A
THREE DIMENSIONAL IMAGING (3DFT
169
Q
- the time between the initial (inverting) 180° pulse and the 90° pulse used in inversion recovery
pulse sequences
A
TI (Inversion Time)
170
Q
) - 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.
A
TIME OF FLIGHT (TOF
171
Q
- 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 Angle.
A
TIP ANGLE
172
Q
- the amount of time that exists between successive pulse sequences applied to the same slice. See also Repetition Time.
A
TR (Repetition Time)
173
Q
- a plane perpendicular (rotated 90°) to the long axis of the human body. See also Axial.
A
TRANSAXIAL
174
Q
- an MRI surface coil that acts as both transmitter and receiver.
A
TRANSCEIVER COIL
175
Q
- 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 volume
A
TRANSMITTER
176
Q
- 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
A
TRANSVERSE MAGNETIZATION
177
Q
- 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.
A
TRANSVERSE RELAXATION TIME
178
Q
- 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.
A
TUNING
179
Q
(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
A
TWO-DIMENSIONAL IMAGING
180
Q
- 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.
A
VECTOR
181
Q
- speed in a particular direction
A
VELOCITY
182
Q
(VENC) - a specialized technique used fro encoding flow velocities
A
VELOCITY ENCODING
183
Q
- a property of a fluid or semi-fluid that affects its mobility, and therefore its intensity in an image.
A
VISCOSITY
184
Q
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.
A
VOLUMETRIC IMAGING -
185
Q
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.
A
VORTEX FLOW -
186
Q
- volume element; the element of the three-dimensional space corresponding
to a pixel, for a given slice thickness.
A
VOXEL
