MRI TERMINOLOGIES Flashcards
1
Q
is a continuously changing flow of electrons that alternates its polarity at a periodic rate.
A
AC
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
the phenomenon resulting from digitizing fewer than two samples per period in a periodic function. __ 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.
A
Aliasing (wrap around artifact)
6
Q
current that continuously changes in magnitude and direction. In the US the current changes at a frequency of 60 Hz.
A
Alternating current
7
Q
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.
A
Amplitude
8
Q
being continuous, or having a continuous range of values.
A
Analog
9
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)
10
Q
a device that enables the sending and/or receiving of electromagnetic waves.
A
Antenna
11
Q
the storage of image and patient data for future retrieval.
A
Archiving
12
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
13
Q
See signal averaging
A
Averaging
14
Q
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.
A
Axial
15
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)
16
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
17
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
18
Q
a large network of interconnecting blood vessels at the base of the brain that when visualized resembles a circle.
A
Circle of willis
19
Q
a psychological reaction to being confined in a relatively small area.
A
Claustrophobia
20
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
21
Q
the act of maintaining a constant phase relationship between oscillating waves or rotating objects.
A
Coherence
22
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
23
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
24
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
25
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.
Crosstalk
26
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.
Cryogen
27
the fanning out or loss of phase coherence of signals within the transverse plane. See also T2.
Dephasing
28
a magnetic field characterized by its own north and south magnetic poles separated by a finite distance.
Dipole
29
a continuous current that flows in only one direction.
Direct current
30
the total number of pixels in the selected matrix, which is described by the product of its phase and frequency axis.
Display matrix
31
a theory of magnetism which assumes that groups of atoms produced by movement of electrons align themselves in groups called"domains" in magnetic materials.
Domain theory
32
Diethylenetriaminepentaacetic acid - Gadolinium chelating (chemical bonding) agent that solves the problem of toxicity
Dtpa
33
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 planar imaging (EPI)
34
See TE
Echo time
35
a series of 180° RF rephasing pulses and their corresponding echoes for a Fast Spin Echo (FSE) pulse sequence.
Echo train
36
Meaning of ETL?
Echo train length
37
an induced spurious electrical current produced by time-varying magnetic fields.__ can cause artifacts in images and may seriously degrade overall magnet performance.
Eddy current
38
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.
Electromagnet
39
the response of electrons to electromagnetic radiation and magnetic fields at discrete frequencies.
Electron spin resonance
40
a state of balance that exists between two opposing forces or divergent forms of influence.
Equilibrium
41
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.
Excitation
42
an electrically conductive screen or shield that reduces or eliminates interference between outside radio waves and those from the MRI unit.
Faraday shield
43
a specialized technique usually associated with short TR, reduced flip angle and repeated 180° rephasing pulses.
Fast scanning
44
a __ sequence characterized by a series of rapidly applied 180° rephasing pulses and multiple echoes, changing the phase encoding gradient for each echo.
Fast spin echo
45
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.
Fat saturation
46
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.
Fat suppression
47
the United States Food and Drug Administration
FDA
48
defined as the size of the two or three dimensional spatial encoding area of the image. Usually defined in units of cm2.
Field of view
49
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.
Fast fourier transform
50
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.
Field echo ( gradient echo)
51
FLuid Attenuated Inversion Recovery
Flair
52
Fast Low-Angle Recalled Echoes
Flare
53
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 __ is used to define the angle of excitation for a Field Echo pulse sequence.
Flip angle
54
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.
Flow compenstation
55
invisible lines of force that extend around a magnetic material. The greatest density is at the two poles of the magnet.
Flux
56
the number of lines of force per unit area of a magnetic material.
Flux density
57
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. __ analysis allows spatial information to be reconstructed from the raw data.
Fourier transform
58
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.
Free induction decay
59
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
60
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.
Frequency encoding
61
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. Gx, Gy, Gz - the conventional symbols for the three orthogonal magnetic gradients. The subscripts designate the conventional spatial direction of the gradient.
Fringe field
62
is a non-toxic paramagnetic contrast enhancement agent utilized in MR imaging. When injected during the scan, __ will tend to change signal intensities by shortening T1 in its surroundings.
gadolinium
63
timing the acquisition of MR data to physiological motion in order to minimize motionartifacts (e.g., cardiac gating, respiratory gating).
Gating
64
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 to1G).
Gauss
65
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.
Gradient coils
66
A small linear magnetic field applied in addition to (superimposed on) the large static magnetic field in an MRI scanner.
Gradient magnetic field
67
a constant for any given nucleus that relates the nuclear MR frequency and the strength of the external magnetic field.
Gyromagnetic ratio
68
the standard unit of frequency equal to 1
cycle per second.
Hertz
69
uniformity of the main magnetic field.
Homogeneity
70
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.
Hydrogen density
71
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 data acquisition
72
the mathematical process of converting the composite signals obtained during the data acquisition phase into an image.
Image reconstruction
73
lack of homogeneity or uniformity in the main magnetic field.
Inhomogeneity
74
an imaging sequence that involves successive 180É and 90É pulses, after which a heavily T1-weighted signal is obtained.
Inversion recovery
75
the time period between the 180° inversion pulse and the 90° excitation pulse in an Inversion Recovery pulse sequence.
Inversion time
76
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).
Larmor equation
77
the frequency at which magnetic resonance in a nucleus can be excited and detected.
Larmor frequency
78
in MRI, the magnetic and thermal environment through which nuclei exchange energy in longitudinal (T1) relaxation.
Lattice
79
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 magnetization
80
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
81
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.
Longitudinal relaxation time
82
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. is defined as the amount and direction of the linear rate of change of the magnetic field in space.
Magnetic gradient
83
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 moment
84
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
85
MR image visualization of selected vascular structures, such as the Circle Of Willis or the carotid arteries.
Magnetic resonance angiography
86
an MR technique wherein a sample is placed in a strong,cvery 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.
Magnetic resonance spectroscopy
87
the extent to which a material becomes magnetized when placed within a magnetic field. Are a frequent cause of MRI artifacts
Magnetic susceptibility
88
the integration of all the individual nuclear magnetic moments which have a positive magnetization value at equilibrium versus those in a random state.
Magnetization vector
89
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.
Maximum intesity projection
90
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.
MR IMAGING
91
the ability to display anatomical structures in a variety of planes from the data acquired in just one scan.
MULTI-ANGLE OBLIQUE
92
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-ECHO IMAGING
93
an imaging technique in which the repetition period (TR) is utilized for acquiring additional slices in other layers or planes.
Multi-slice imaging
94
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.
Net magnetization vector
95
an uncharged neutral particle located in the nucleus of most atoms which serves as a stabilizer.
Neutron
96
number of excitations.
Nex
97
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.
Nmr signal
98
an undesirable background interference or disturbance that affects image quality.
Noise
99
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.
Nuclear spin
100
an indicator of how many times each line of k-space data is acquired during the scan.
Numbers of excitation
101
a plane or section not perpendicular to the xyz coordinate system, such as long and short axis views of the heart.
Oblique
102
a plane or section perpendicular to the xyz coordinate system.
Orthogonal
103
rhythmic periodic motion.
OSCILLATION
104
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
PARAMAGNETIC SUBSTANCE
105
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.
PERMANENT MAGNET
106
an artificial object of known dimensions and properties that is used to test or monitor an MRI systems homogeneity, imaging performance and orientation aspects.
PHANTOM
107
an angular relationship describing the degree of synchronism between two sinusoidal waveforms of the same frequency.
PHASE
108
a term describing the degree to which precessing nuclear spins are synchronous.
PHASE COHERENCE
109
an MRA technique utilizing the change in the phase shifts of the flowing protons in the region of interest to create an image.
PHASE CONTRAST
110
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.
PHASE ENCODING
111
acronym for a picture element, the smallest discrete two-dimensional part of a digital image display.
PIXEL
112
a method of scanning in which the data is collected simultaneously from an entire layer.
PLANAR IMAGING
113
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.
PRECESSION
114
a specialized technique employing repeated RF excitation of structures adjacent to the ROI for the purpose of reducing or eliminating their phase effect artifacts.
PRESATURATION (PRE-SAT)
115
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.
PROTON
116
the concentration of mobile Hydrogen atoms within a sample of tissue. See also Hydrogen Density.
PROTON DENSITY
117
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.
PROTON DENSITY WEIGHTED IMAGE
118
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 PROGRAMMER
119
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.
PULSE SEQUENCE
120
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.
Quench
121
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.
RADIO FREQUENCY
122
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.
READOUT GRADIENT
123
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
124
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
RECEIVER COIL
125
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.
RECONSTRUCTION
126
the area of anatomy being scanned that is of particular importance in the image.
REGION OF INTEREST (ROI)
127
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 magneticfield. 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
RELAXATION TIME
128
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.
REPETITION TIME (TR)
129
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.
REPHASING
130
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. __ 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. __ are typically limited to maximum field strengths of approximately 0.6T.
RESISTIVE MAGNET
131
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.
RESONANCE
132
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.
SAGITTAL
133
the conversion of analog signals to discreet digital values through a preselected measurement process.
SAMPLING
134
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.
SATURATION RECOVERY
135
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.
SELECTIVE EXCITATION
136
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.
SHIM COILS
137
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.
SHIMMING
138
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
SIGNAL AVERAGING
139
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.
SIGNAL-TO-NOISE RATIO (S/N, SNR)
140
the term describing the planar region or the image slice selection region.
SLICE
141
relates to the addition of phase encoding steps for 3D volumetric imaging.
SLICE ENCODING
142
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.
SLICE SELECTION
143
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.
SLICE THICKNESS
144
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
SMART
145
the ability to define minute adjacent objects/points in an image, generally measured in line pairs per mm (lp/mm)
SPATIAL RESOLUTION
146
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.
SPECIFIC ABSORPTION RATE
147
which is defined as the RF power absorbed per unit of mass of an object, and is measured in watts per kilogram (W/kg).
SAR (Specific Absorption Rate)
148
the property exhibited by atomic nuclei that contain either an odd number of protons or
neutrons, or both.
SPIN
149
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-ECHO (SE)
150
see T1 and Longitudinal Relaxation Time.
SPIN-LATTICE RELAXATION TIME
151
see T2 and Transverse Relaxation Time.
SPIN-SPIN RELAXATION TIME
152
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.
STEADY-STATE FREE PRECESSION
153
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.
SUPERCONDUCTIVE MAGNET
154
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.
SURFACE COIL
155
an image created typically by using short TE and TR times whose contrast and brightness are predominately determined by T1 signals.
T1 WEIGHTED
156
see Longitudinal Relaxation Time.
T1 RELAXATION
157
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.
T2
158
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 transversemagnetization and the MRI signal.
T2* ("T-two-star")
159
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.
T2 WEIGHTED
160
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)
161
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.
TESLA (T)
162
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 sequences.
THREE DIMENSIONAL IMAGING (3DFT)
163
an 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.
TIME OF FLIGHT (TOF)
164
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.
TIP ANGLE
165
the amount of time that exists between successive pulse sequences applied to the same slice.
TR (Repetition Time)
166
a plane perpendicular (rotated 90°) to the long axis of the human body.
TRANSAXIAL
167
an MRI surface coil that acts as both transmitter and receiver.
TRANSCEIVER COIL
168
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.
TRANSMITTER
169
component of the net magnetization vector at right angles to the main magnetic field. Precession of the __ at the Larmor frequency is responsible for the detectable NMR signal. In the absence of externally applied RF energy, the ___ will decay to zero with a characteristic time constant of T2, or more strictly T2*.
TRANSVERSE MAGNETIZATION
170
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.
TRANSVERSE RELAXATION TIME
171
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.
TUNING
172
the Fourier transformation process reconstructs the detected frequency and phase encoded image information (which are rotated 90° from each other) into a usable image.
TWO-DIMENSIONAL IMAGING (2DFT)
173
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. __ quantities can be added to or subtracted from one another.
VECTOR
174
speed in a particular direction.
VELOCITY
175
a specialized technique used for encoding flow velocities.
VELOCITY ENCODING (VENC)
176
a property of a fluid or semi-fluid that affects its mobility, and therefore its intensity in an image.
VISCOSITY
177
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.
VOLUMETRIC IMAGING
178
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.
VORTEX FLOW
179
volume element; the element of the three- dimensional space corresponding to a pixel, for a given slice thickness.
VOXEL
180
loss of resolution due to excessively large voxels, typically caused by slices that are too thick.
Partial voluming
181
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.
NUCLEUS
