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MRI PRELIM > Prelim Glossary Exam > Flashcards

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

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2
Q
  • the process of measuring and storing image data
A

ACQUISITION

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3
Q
  • the total number of independent data samples in the
    frequency (f) and phase (f) directions.
A

ACQUISITION MATRIX

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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

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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)

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6
Q
  • being continuous, or having a continuous range of values.
A

ANALOG

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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)

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8
Q
  • a device that enables the sending and/or receiving of electromagnetic waves. See also Transmitter, Receiver Coils and Surface Coils.
A

ANTENNA

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9
Q
  • the storage of image and patient data for future retrieval.
A

ARCHIVING

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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

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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

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12
Q
  • a conventional symbol for the constant magnetic field produced by the large magnet in the MR scanner
A

B or Bo

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13
Q
  • the conventional symbol used for identifying
    the radio frequency (RF) magnetic field
A

B1

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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)

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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

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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

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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

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18
Q
  • a psychological reaction to being confined in a relatively small area
A

CLAUSTROPHOBIA

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19
Q
  • the act of maintaining a constant phase relationship between oscillating waves or rotating objects.
A

COHERENCE

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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

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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

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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

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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

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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

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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

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26
Q
  • the fanning out or loss of phase coherence of signals within the transverse plane. See also T2.
A

DEPHASING

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27
Q
  • a magnetic field characterized by its own north and south magnetic poles separated by a finite distance
A

DIPOLE

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28
Q
  • a continuous current that flows in only one direction.
A

DIRECT CURRENT (DC)

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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

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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

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31
Q
  • Gadolinium chelating (chemical bonding) agent that solves the problem of toxicity
A

DTPA - Diethylenetriaminepentaacetic acid

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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)

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33
Q
  • a series of 180° RF rephasing pulses and their corresponding echoes for a Fast Spin Echo (FSE) pulse sequence.
A

ECHO TRAIN

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34
Q

ETL -

A

Echo Train Length

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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

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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

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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

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38
Q
  • a state of balance that exists between two opposing forces or divergent forms of influence.
A

EQUILIBRIUM

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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

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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

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41
Q
  • a specialized technique usually associated with short TR, reduced flip angle and repeated 180° rephasing pulses.
A

FAST SCANNING

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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)

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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)

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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

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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)

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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)

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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)

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48
Q

FLAIR

A

FLuid Attenuated Inversion Recovery

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49
Q

FLARE

A

Fast Low-Angle Recalled Echoes

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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)

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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

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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

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53
Q
  • the number of lines of force per unit area of a magnetic material
A

FLUX DENSITY

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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)

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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)

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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

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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

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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

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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)

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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

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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

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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

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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

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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)

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65
Q
  • the standard unit of frequency equal to 1 cycle per second. The larger unit
    megahertz (MHz) = 1,000,000 Hz.
A

HERTZ

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66
Q
  • uniformity of the main magnetic field.
A

HOMOGENEITY

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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+)

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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

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69
Q
  • the mathematical process of converting the composite
    signals obtained during the data acquisition phase into an image.
A

IMAGE RECONSTRUCTION

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70
Q
  • lack of homogeneity or uniformity in the main magnetic field.
A

INHOMOGENEITY

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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)

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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)

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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

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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

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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

MRI PRELIM (5 decks)

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