P.exam 1 Flashcards
1
Q
- the process of measuring and storing image data
A
ACQUISITION
2
Q
• the total number of independent data samples in the frequency (f) and phase (f) directions
A
ACQUISITION MATRIX -
3
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 -
4
Q
- being continuous, or having a continuous range of values.
A
ANALOG
5
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)
6
Q
- a device that enables the sending and/or receiving of electromagnetic waves. See also Transmitter, Receiver Coils and Surface Coils.
A
ANTENNA
7
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)-
8
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
9
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
10
Q
- a large network of interconnecting blood vessels at the base of the brain that when visualized resembles a circle.
A
CIRCLE OF WILLIS
11
Q
- a psychological reaction to being confined in a relatively small area.
A
CLAUSTROPHOBIA
12
Q
- the act of maintaining a constant phase relationship between oscillating waves or rotating objects
A
COHERENCE
13
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
14
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
15
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
16
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
17
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
18
Q
- a cooling agent, typically liquid helium or liquid nitrogen used to reduce the temperature of the magnet windings in a superconducting magnet. .
A
CRYOGEN
19
Q
- 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
dB/dt
20
Q
- the fanning out or loss of phase coherence of signals within the transverse plane.
A
DEPHASING
21
Q
- a magnetic field characterized by its own north and south magnetic poles separated by a finite distance.
A
DIPOLE
22
Q
- a continuous current that flows in only one direction
A
DIRECT CURRENT (DC)
23
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
24
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
25
- Gadolinium chelating (chemical bonding) agent that solves the problem of toxicity
DTPA - Diethylenetriaminepentaacetic acid
26
- 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)
27
- a series of 180° RF rephasing pulses and their corresponding echoes for a Fast Spin Echo (FSE) pulse sequence.
ECHO TRAIN
28
- an induced spurious electrical current produced by timevarying magnetic fields. Eddy currents can cause artifacts in images and may
seriously degrade overall magnet performance.
EDDY CURRENT
29
- 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.
ELECTROMAGNET
30
- a state of balance that exists between two opposing forces or divergent forms of influence
EQUILIBRIUM
31
- 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
32
(Faraday Cage) - an electrically conductive screen or shield that reduces or eliminates interference between outside radio
waves and those from the MRI unit.
FARADAY SHIELD
33
- a specialized technique usually associated with short TR, reduced flip angle and repeated 180° rephasing pulses.
FAST SCANNING
34
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.
FAST SPIN ECHO (FSE) -
35
) - A specialized technique that selectively saturates fat protons prior to acquiring data as in standard sequences, so that they produce negligible signal. The pre-saturation pulse is applied
prior to each slice selection. This technique requires a very homogeneous
magnetic field and very precise frequency calibration. See also Fat
Suppression.
FAT SATURATION (FAT-SAT
36
- 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.
FAT SUPPRESSION
37
- 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 (FOV)
38
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.
FFT (Fast Fourier Transform) -
39
(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
FIELD ECHO (FE)
40
FLAIR
FLuid Attenuated Inversion Recovery
41
FLARE
Fast Low-Angle Recalled Echoes
42
- 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.
FLIP ANGLE (FA)
43
- 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 COMPENSATION
44
- invisible lines of force that extend around a magnetic material. The greatest density is at the two poles of the magnet.
FLUX
45
- the number of lines of force per unit area of a magnetic material
FLUX DENSITY
46
- 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.
FOURIER TRANSFORM (FT)
47
- 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 (FID)
48
- timing the acquisition of MR data to physiological motion in order to minimize motion artifacts (e.g., cardiac gating, respiratory gating).
GATING
49
- 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
GAUSS
50
- an image artifact primarily associated with the phase direction.
GHOSTING
51
- three paired orthogonal current-carrying coils located
within the magnet which are designed to produce desired gradient magnetic fields which collectively and sequentially are superimposed on the main magnetic field (Bo) so that selective spatial excitation of the imaging volume can occur. Gradients are also used to apply reversal pulses
in some fast imaging techniques
GRADIENT COILS
52
- 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)
GYROMAGNETIC RATIO (g)
53
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 (H+) -
54
- an imaging sequence
that involves successive 180É and 90É pulses, after which a heavily T1-weighted signal is obtained. The inversion recovery sequence is specified in terms of three parameters, inversion time (TI), repetition time (TR) and echo time (TE)
INVERSION RECOVERY (IR)
55
- the time period between the 180°inversion pulse and the 90° excitation pulse in an Inversion Recovery pulse sequence
INVERSION TIME (TI)
56
- 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.
ISOTOPE
57
- an equation that states that
the frequency of precession of the nuclear magnetic
moment is directly proportional to the product of the
magnetic field strength (Bo) and the gyromagnetic
ratio (g). This is stated mathematically as å = g Bo.
LARMOR EQUATION
58
- in MRI, the magnetic and thermal environment through which nuclei exchange energy in longitudinal (T1) relaxation.
LATTICE
59
- 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
60
- 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
61
- 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
62
- one of three linear magnetization waveforms superimposed on the main magnetic field at specific times within a pulse sequence to select the imaging region or provide necessary spatial
localization information. A magnetic gradient is defined as the amount and direction of the linear rate of change of the magnetic field in space.
MAGNETIC GRADIENT
63
magnetic lines of force which extend from a north polarity and enter a south polarity to form a closed loop around the outside of a magnetic material.
MAGNETIC FIELD -
64
- 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
65
- 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
66
- 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.
MAGNETIC SUSCEPTIBILITY
67
- 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 (Mz)
68
- a processing method for MRA images. A MIP is a record of a maximum intensity ray (generated through a
mathematical algorithm) as it passes through an angiographic volume.
Each point in an MIP represents the highest intensity experienced in that location on any partition within the imaging volume.
MAXIMUM INTENSITY PROJECTION (MIP)
69
- 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.
K-SPACE
