MRI Terms Flashcards
A matrix with a low number of frequency encodings and/or phase encodings and results in a low number of pixels in the FOV.
Course Matrix
Flow in the same direction as slice excitation.
Co-Current flow
in phase
Coherent
The symmetry of data in K space.
Conjugate Symmetry
Flow in the opposite direction to slice excitation.
Counter current flow
energy given to nuclei in adjacent slices by the RF pulse.
Cross excitation
energy given to nuclei in adjacent slices due to spin lattice relaxation.
Cross talk
area around the coils of wire in which cryogens are placed
Cryogen Bath
Point in K space that contains digitized information from encoding.
Data point
loss of transverse magnetization
Decay
the movement of molecules due to random thermal motion.
Diffusion
Sequence in which two 180 degree pulses are used to saturate blood in black blood imaging.
Double IR prep
driven equilibrium - a pulse sequence that achieves a very high signal intensity from water even when using short TR’s.
DRIVE
digital subtraction MR angiography - contrast is selectively produced for moving spins during two acquisitions. These are then subtracted to remove the signal from the stationary spins, leaving behind an image of only the moving spins.
DS-MRA
time in milliseconds from the application of the RF pulse to the peak of the signal induced in the coil - TE determines how much decay of transverse magnetization is allowed to occur.
Echo time (TE)
series of 180 degree rephasing pulse and echoes in a fast spin echo pulse sequence.
Echo train
the time between the echo and the RF pulse that intiated it in SSFP - also the TE used in FSE
Effective TE
particles that spin around the nucleus
Electrons
once a slice is selected, the signal is located or encoded along both axes of the image.
Encoding
contrast difference of flowing nuclei relative to the stationary nuclei because they are fresh
Entry slice phenomena
technique that uses two echoes to reduce flow artefact.
Even echo rephasing
those parameters that can be changed at the operator console.
Extrinsic contrast parameters
causes an increase in the T2 decay time of fat when multiple RF pulses are applied as in fast spin echo
J Coupling
an area in the array processor where data on spatial frequencies are stored.
K space
precessional frequency
Larmor frequency
the axis parallel to Bo
Longitudinal plane
coil that transmits and receives signal over a large volume of the patient.
Volume Coil
Volume of tissue in the patient.
Voxel volume
technique that nulls signal from water by applying an RF pulse at the frequency of water to the imaging volume before slice excitation.
Water saturation
settings that control brightness and contrast in MR images
Window levels and settings
sudden loss of the superconductivity of the magnet coils so that the magnet becomes resistive.
Quenching
Echoes formed when any two 90 degree RF pulses are used in steady state sequences.
Hahn echoes
partial averaging
Half Fourier
increase in time of flight due to an increase in the velocity of flow.
High velocity signal loss
evenness of the magnetic field.
Homogeneity
combination of fast spin echo and EPI sequences where a series of gradient echoes are interspersed with spin echoes - in this way susceptibility artefacts are reduced.
Hybrid sequences
the most abundant atom in the body
Hydrogen
out of phase
Incoherent
another term for entry slice phenomenon.
Inflow effect
magnetic moments that are in the same place on the precessional path around Bo at any given time.
In phase
A method of acquiring data from alternate slices and dividing the sequence into two acquisitions - no slice gap is required.
Interleaving
phase difference between flow and stationary nuclei in a voxel.
Intra-voxel dephasing
those parameters that cannot be changed because they are inherent to the body’s tissues.
Intrinsic contrast parameters
atoms with an excess or deficit of electrons
Ions
atoms with an odd mass number.
Isotopes
voxels that have the samedimensions in all three planes.
Isotropic
technique that nulls signal from fat by applying an RF pulse at the frequency of fat to the imaging volume before slice excitation.
Fat saturation
gradient moment nulling
First order motion compensation
axes along which bipolar gradients act in order to sensitize flow along the axis of the gradient used in phase contrast MRA
Flow encoding axes
artefacts produced by flowing nuclei
Flow phenomena
decrease in time of flight due to a decrease in velocity of flow
Flow related enhancement
partial averaging
Fractional averaging
partial echo
Fractional echo
loss of signal due to relaxation
Free induction decay (FID)
Aliasing along the frequency encoding axis
Frequency wrap
nuclei that have not been beaten down by repeated RF pulses
Fresh spins
When the NMV is pushed to a full 180 degrees
Fully saturated
these allow MRI to be used to assess fuction and physiology
Functional imaging technique
basic law of electricity - voltage= current x resistance
Ohms law
area of K space filled with the steepest phase encoding gradient slopes
Outer lines
when magnetic moments are not in the same place on the precessional path
Out of phase
motion artefact in the phase axis
Ghosting
line of low signal in the cervical cord image due to truncation
Gibbs artefact
supplies power to the gradient coils
Gradient amplifier
gradient echo sequence with EPI readout
Gradient echo - EPI
a system of gradients that compensates for intra-voxel dephasing
Gradient moment nulling (rephasing)
coils of wire that alter the magnetic field strength in a linear fashion when a current is passed through them
Gradients
the use of gradients to dephase magnetic moments - the opposite of rewinding
Gradient spoiling
Gradient echo and spin echo
GRASE
the precessional frequency of an element at 1.0 T
Gyro-magnetic ratio
field created by passing current through a gradient coil
Magnetic Field Gradients
effect that causes elevation of the T-wave of the ECG of the patient when placed in a magnetic field- this is due to the conductivity of blood
Magneto-hemodynamic effect
the center of the bore of the magnet in all planes
Magnetic isocentre
denotes the direction of the north/south axis of the magnet and the amplitude of the magnetic field
Magnetic moment
ability of a substance to become magnetized
Magnetic susceptability
technique used to suppress background tissue and increase CNR
Magnetization transfer contrast/coherence (MTC)
a property of all matter that depends on the magnetic susceptability of the atom
Magnetism
un-subtracted image combination of flow sensitized data
Magnitude image
sum of neutrons and protons in the nucleus
Mass number
where two or more atoms are arranged together
Molecules
nuclei that possess an odd mass number
MR active nuclei
nuclei by producing a contrast between them and the stationary nuclei
MR angiography (MRA)
the voltage induced in the receiver coil
MR signal
method combining a number of high resolution 3D acquisitions to produce an image that has good resolution and a large area of coverage
Multiple overlapping thin section angiography (MOTSA)
where K space is divided into segments and one segment is acquired per TR
Multi-shot
Technique that acquires multiple voxels by encoding in K space in MR signal
Multi-voxel
the magnetic vector produced as a result of the alignment of excess hydrogen nuclei with Bo
Net magnetization vector (NMV)
neutrally charged element in an atomic nucleus
Neutron
number of excitations (also known as number of signals averages or acquisitions depending on manufacturers) the number of times an echo is encoded with the same slope of phase encoding gradient
NEX
Frequencies that exist randomly in time and space
Noise
particles in the nucleus
Nucleons
the point at which there is no longitudinal magnetization in a tissue in an inversion recovery sequence
Null point
states that a frequency must be sampled at least twice in order to reproduce it reliably.
Nyquist theorem
a technique that uses multiple coils to fill segments of K space
Parallel imaging
filling only a proportion of K space with data and putting zeros in the remainder
Partial averaging
sampling only part of the echo and extrapolating the remainder in K space
Partial echo imaging
occurs when the NMV is flipped beyond 90 degrees (91-179)
Partially saturated
loss of spatial resolution when large voxels are used
Partial voluming
acheived in IR pulse sequence with a long TE-pathology appears bright even though the image is T1 weighted
Pathology weighting
the position of a magnetic moment on its precessional path at any given time
Phase
technique that degenerates vascular contrast using the phase difference between stationary and flowing spins
Phase contrast angiography (PCMRA)
subtracted image comination of flow sensitized data
Phase image
aliasing along the phase encoding axis
Phase wrap
the direction of a gradient, ie which end is greater than Bo and which end is lower than Bo. Depends on the direction of the current through the gradient coil
Polarity
single voxel technique in MRS
Point resolved spectroscopy spin echo(PRESS)
the speed of precession
Precessional (larmor) frequency
the circular pathway of magnetic moments as they precess around Bo
Precessional path
positively charged element of an atomic nucleus
Proton
number of protons per unit volume of that tissue
Proton density
image that demonstrates the differences in the proton densities of the tissue
Proton densty (PD) weighting
frequency that is indirectly derived from a change of phase
Pseudo-frequency
co-ordinates switching on and off the gradient and RF transmitter coils at appropiate times during the pulse sequence
Pulse control unit
emission of energy caused by adeficit in the number of electrons compared with protons
Radioactivity
low energy, low frequency electromagnetic radiation. Used to excite hydrogen nuclei in MRI
Radio frequency (RF)
where sampling data points are collected when the gradient rise time is almost complete- sampling occurs while the gradient is still reaching maximum amplitude, while the gradient is at maximum amplitude and as it begins to decline
Ramp sampling
the frequency encoding gradient
Readout gradient
range of frequencies that are sampled during readout
Receive bandwidth
growth of longitudinal magnetization
Recovery
also known as asymmetric FOV - uses a FOV in the phase direction that is different to that in the frequency direction of the image
Rectangular FOV
the factor by which the scan time is reduced using parallel imaging. Equals the number of coils used
Reduction factor
process by which the NMV loses energy
Relaxation
the effect of a substance on relaxation rate
Relaxivity
time between each excitation pulses
Repitition time (TR)
transverse magnetization left over from previous RF pulses in steady state conditions
Residual transverse magnetization
another term for solenoid magnet
Resistive magnet
uses mechanical motion of air in bellows to order K space filling and reduce respiratory motion artefacts
Respiratory compensation
gates the sequences to chest wall movements to reduce respiratory motion artefacts
Respiratory gating/triggering
gradients that rephase
Rewinders
Supplies power to the RF transmitter coil
RF amplifier
short burst of RF energy that excites nuclei into a high-energy stage
RF pulse
the use of digitized RF to transmit and receive at a certain phase
RF spoiling
coil that transmits RF at the resonant frequency of hydrogen to excite nuclei and move them into a high-energy state
RF transmitter coil
the time it takes a gradient to switch on, acheive the required gradient slope, and switch off again
Rise time
time between each R wave in gated studies
R to R interval
rate at which samples are taken during readout
Sampling rate or frequency
the time that the readout gradient is switched on for
Sampling time
standardized absorption rate - a way of measuring the USA Food and Drug Administration limit for RF exposure
SAR
time between each pre-saturation pulse
SAT TR
acquisition where all the data from each slice is acquired before going on to the next
Sequential acquisition
parallel imaging
Sensitivity encoding
extra coils used to make the magnetic field as homogeneous as possible
Shim coil
voltage induced in the receiver coil
Signal
ratio of signal relative to noise
Signal to noise ratio (SNR)
a fast spin echo sequence where all the lines of K space are acquired during a single TR period
Single shot FSE (SS-FSE)
technique that uses three intersecting slices to locate a single voxel in MRS.
Single voxel
the strength of the gradient over distance
Slew rate
the seperation of individual slice locations by phase in volume acquisitions
Slice encoding
selecting a slice using a gradient
Slice selection
magnet that uses current passed through coils of wire to generate a magnetic field
Solenoid electromagnet
encoding or locating signal in spatial three dimensions of the imaging volume
Spatial encoding
creates a saturation effect which produces a cross hatching of stripes on the image. these can be compared with moving anatomy to determine its function
Spatial modulation of magnetization (SPAMM)
the ability to distinguish two points as seperate
Spatial resolution
the population of high energy hydrogen nuclei that align their magnetic moments anti-parallel to Bo
Spin-down
echo produced as a result of a 180 degree rephasing pulse
Spin echo
Spin echo sequence with EPI readout
Spin echo-EPI (SE-EPI)
one that uses a 180 degree rephasing pulse to generate an echo
Spin echo pulse sequence
process by which energy is given up to the surrounding lattice
Spin lattice relaxation
process by which interactions between the magnetic fields of adjacent nuclei causes dephasing
Spin-spin relaxation
the population of low energy hydrogen nuclei that align their magnetic moments parallel to Bo
Spin-up
gradients that dephase
Spoilers
condition where the TR is less than T1 and T2 relaxation times of the tissue
Steady state
echoes formed when any two RF pulses are used in steady state sequences
Stimulated echoes
single voxel technique in MRS
Stimulated echoes acquisition mode (STEAM)
solenoid electromagnet that uses super cooled coils of wire so that there is no inherent resistance in the system through which the current flows, and therefore the magnetism is generated without driving voltage
Superconducting magnet
a contrast agent that shortens T1 relaxation in tissue that take up the agent
T1 enhancement agent
growth of longitudinal magnetization as a result of spin lattice relaxation
T1 recovery
time taken for 63% of the longitudinal magnetization to recover
T1 relaxation time
image that demonstrates the differences in the T1 times of the tissues
T1 weighted image
dephasing due to magnetic field inhomogeneities
T2*
agents that shorten T2 relaxation times in tissue that take up the agent
T2 enhancement agents
loss of transverse magnetization as a result of spin-spin relaxation
T2 decay
time taken for 63% of the transverse magnetization to decay
T2 relaxation time
when lesions remain bright on a trace image in DWI
T2 shine through
image that demonstrates the differences in the T2 times of the tissues
T2 weighted image
acquisition where the whole imaging volume is excited so that the images can be viewed in any plane
3D volumetric acquisition
the time between the excitation pulse and the 180 degree rephasing pulse and the time between this and the echo. Sometimes used in STIR sequences as an alternative to the TI
TAU
assumes patient temperature is constant and therefore does not influence the thermal energy of hydrogen during the MR experiment
Thermal equalibrium
curve produced in perfusion imaging to show perfusion kinetics of a tissue
Time intensity curve
rate of flow in a given time - causes some flowing nuclei to receive one RF pulse only and therefore produce a signal void
Time of flight
technique that generates vascular contrast by using the inflow effect
Time of flight MR angiography (TOF-MRA)
echo time
Time to echo
time from 180 degree inverting pulse to 90 degree excitation pulse in inversion recovery pulse sequences
Time from inversion (TI)
repetition time
TR
image in DWI where abnormal tissue is brighter than normal tissue
Trace image
coil that both transmits RF and receives the MR signal
Transceiver
range of frequencies transmitted in an RF pulse
Transmit bandwidth
the axis perpendicular to Bo
Transverse plane
waiting period after each R wave - the time between the R wave and the beginning of data acquisition
trigger delay
waiting period before each R wave in gated studies
Trigger window
artefact caused by under-sampling so that edges of high and low signal are not properly mapped into the image
Truncation artefact
echo train length
Turbo factor
acquisition where a small amount of data is acquired from each slice before repeating the TR
2D volumetric acquisition
