MRI Flashcards
Spectroscopic study of the magnetic properties of nucleus of the atom
Nuclear magnetic resonance
Energy coupling that causes the individual nuclei, when placed in a strong external magnetic field, to selectively absorb and later release energy unique to those nuclei and their surrounding environment
Resonance
Fundamental property of matter; it is generated by moving charges, usually electrons
Magnetism
Magnetic properties of materials result from the
Organization and motion of the electrons in either a random or a nonrandom alignment of magnetic “domains” which are the smallest entities of magnetism
Origin of the magnetic field lines
North pole
Return of magnetic field lines
South pole
Can be conceptualized as the number of magnetic lines of force per unit area, which decreases roughly as the inverse square of the distance from the source
Magnetic field strength/magnetic flux density
Earth’s magnetic field is
0.05 mT
magnetic field strength and field density are dependent in the
Amplitude of current and number of coil turns
Magnetic field lines extending beyond the concentrated field are known as
Fringe fields
Performance criteria for magnet type
Field strength, temporal stability, field homogeneity
Characteristic of certain metals that when maintained at extremely low temperatures, exhibit no resistance to electric current
Superconductivity
Replenishment of the liquid helium must occur continuously, because if the temperature rises above a critical value, the loss of superconductivity will occur and resistance heating of wires will boil the helium, resulting in a
Quench
Common superconductive magnets have field strengths of
1.5 to 3 T
Magnetic field strength used for research application
4-7 T
Interact with the main magnetic field to improve homogeneity (minimal variation of the magnetic flux density) over the volume used for patient imaging
Shim coils
Exist within the main bore of the magnet to transmit energy to the patient as well as to receive returning signals
Radiofrequency coils
Contained within the main bore to produce linear variation if the magnetic field strength across the useful magnet volume
Gradient coils
Describes the extent to which a material becomes magnetized when placed in a magnetic field
Magnetic susceptibility
Have slightly negative susceptibility and oppose the applied magnetic field, because of paired electrons in the surrounding electron orbitals
Diamagnetic elements
calcium, water and most organic materials are examples of
Diamagnetic materials
With unpaired electrons, have slightly positive susceptibility and enhance the local magnetic field, but they have no measurable self magnetism
Paramagnetic materials
Molecular oxygen, deoxyhemoglobin, methemoglbin and gadolinium -based contrast agents are examples of
Paramagnetic materials
Superparamagnetic materials that augment the external magnetic field substantially. Exhibits self-magnetism
Ferromagnetic
Iron, cobalt and nickel are examples of
Ferromagnetic materials
Depleting magnetic materials
Diamagnetic
Augmenting magnetic materials
Paramagnetic materials
If there are equal number of protons and neutrons in the nucleus, the nuclear magnetic moment is
Essentially zero
If the number of protons and neutrons are unequal, the nuclear magnetic moment is
Generated
Principal focus for generating MR signals
Nucleus of hydrogen atom, the proton
Under the influence of an applied external magnetic field, the protons assume a nonrandom alignment in 2 possible orientations
Parallel and antiparallel
Between parallel and antiparallel directions, at equilibrium, a slight majority exists in the
Low energy parallel direction
Protons also experience a torque in a perpendicular direction from the applied magnetic field that causes
Precession
Precession occurs at what direction of frequency
Angular frequency
Describes the dependence between the magnetic field and the angular precessional frequency
Larmor equation
Two frames of reference in the applied magnetic field
Laboratory frame and rotating frame
Stationary reference frame from the observer’s point of view
Laboratory frame
A spinning axis system whereby the x-y axes rotate at an angular frequency equal to the Larmor frequency
Rotating frame
Slightly higher precessional frequency is observed as a slow ______ rotation
Clockwise rotation
Slightly lower precessional frequency is observed as a slow _______ rotation
Counterclockwise
Component of the magnetic moment parallel to the applied magnetic field
Longitudinal magnetization
Component of the magnetic moment perpendicular to x-y plane
Transverse magnetization
Corresponds to the energy separation between the protons in the parallel and antiparallel directions
Resonance frequency
Considers the RF energy as photons (quanta) instead of waves
Quantum mechanics model
Result of angular displacement of the longitudinal magnetisation vector from the equilibrium position
Flip angles
Term describing the release of energy back to lattice
Spin-lattice relaxation
T1 time is strongly dependent on the
Physical characteristics of the tissues and their associated hydration layers
Longer T1 relaxation time is taken from
Solid organs, unstructured tissues and fluids in bulk water
Shorter T1 relaxation time is achieved with
Structures and moderately sized proteins and fatty tissues
What influences T1 and T2 relaxation
Molecular motion, size and interactions
T1 values are _______ for higher field strength magnets while T2 values are unaffected
Longer
Period between B1 excitation pulses
Time of repetition (TR)
Time between excitation pulse and the appearance of the peak amplitude of an induced echo, which is determined by applying a 180 degree RF inversion pulse or gradient pplariry reversal at a time equal to TE/2
Time of echo
Time between an initial inversion/excitation (180 degrees) RF pulse that produces maximum tissue saturation and a 90-degree readout pulse
Time of inversion
Produced damped sinusoidal electronic signal from rotating at the Larmor frequency
Free induction decay
State of tissue magnetization from equilibrium conditions
Saturation
At equilibrium, the protons in a material are saturated or unsaturated?
Unsaturated
Occurs because the repetition time between excitation pulses does not allow for full return to equilibrium, therefore the Mz amplitude for the next RF pulse is reduced
Partial saturation
Describes the excitation of the magnetized protons in a sample with a 90 degree pulse converts Mz into Mzy and creates the largest phase coherent transverse magnetization that immediately begins to decay at a rate described by T2* relaxation
Spin echo
It is proportional to the difference in signal intensity between adjacent pixels in an image, corresponding to different voxels in the patient
Contrast
Sequence designed to produce contrast chiefly based on the T1 characteristic of tissues with de-emphasis of T2 and proton density contributions to the signal
T1-weighted SE sequence
Most intense signal in T1
Fat
Relies mainly on differences in the number of magnetized protons per unit volume of tissue
Proton density contrast weighting
Sequence that achieves the highest overall signal intensity and the largest signal to noise ratio; however, the image contrast is relatively low and therefore the contrast-to-noise ratio is not necessarily larger than achievable with T1 or T2 contrast weighting
Proton density weighting
Generated from the second echo produced by a second 180 degree pulse of a long TR spin echo pulse sequence, where the first echo is proton density weighted with short TE
T2 weighting
Emphasizes T1 relaxation times of the tissues by extending the amplitude of the longitudinal recovery by a factor of two
Inversion recovery
Delay between the excitation pulse and conversion to transverse magnetization of the recovered longitudinal magnetization
Inversion recovery
Pulse sequence that uses a very short T1 and magnitude signal processing, where Mz signal amplitude is always positive
Short tau inversion recovery
Reduces distracting fat signals and chemical shift artifacts
Short tau inversion recovery
Reduces CSF signal and other water-bound anatomy in the MR image by using a T1 selected at or near bounce point of CSF to permit better evaluation of the surrounding anatomy
Fluid attenuating inversion recovery
Downsides of spin echo and inversion recovery SE sequences
Less sensitive to magnetic field inhomogeneities, magnetic susceptibilities and generally gives high SNR and CNR, relatively long TR and corresponding long acquisition times
Uses a magnetic field gradient applied in one direction and then reversed to induce the formation of an echo, instead of 180 degree inverse pulse
Gradient echo
Not a true spin echo but a purposeful dephasing and rephasing of the FID
Gradient echo
Magnetic field inhomogeneities and tissue susceptibilities caused by paramagnetic or diamagnetic tissues or contrast agents are emphasized in
Gradient echo imaging
Magnetic field gradient induces the formation of an _____ Instead of 180-degree RF pulse
Echo
Transverse magnetization spins are ______ with an applied gradient of one polarity
Dephased
Transverse magnetization spins are _____ with the gradient reversed in polarity
Rephased
Transverse magnetization is higher/lower? For small flip angles compared to larger flip angles
Higher
A relatively long TE tends to emphasize the differences between
T2* and T2
Indicates the timing of the RF pulse with the dephasing and rephasing implemented by reversal of gradient polarity to generate an echi at a selectable time TE for the frequency encode gradient, where identification of proton position based upon frequency is performed
Coherent GE
Applied and incrementally changed for each TR to identify proton position in the direction perpendicular to the frequency encode gradient based upon phase changes of the protons after the PEG is turned off
Phase encode gradient
Gradient that generates the echo from the free induction decay plus the stimulated echo from the previous RF pulse
Frequency encode gradient
T2* influence can be reduced by using a long TR, or by
Incoherent, “spoiled” gradient echo techniques
Downsides of spoiled GE techniques
Increased sensitivity to other artifacts such as chemical shift and magnetic field inhomogeneities as well as lower SNR
Sequence that emphasizes acquisition of only the stimulated echo, which arises from the previous RF pulse and appears during the next RF pulse at a time equal to 3 x TR
Steady state free precession
In steady state free precession, there are two TE values
Actual TE and Effective TE
Time between the peak stimulated echo and the next excitation pulse in ssfp
Actual TE
Time from the echo and RF pulse that creates its free induction decay
Effective TE
For GE acquisition in the realm of short TR, persistent transverse magnetization produces two signals
- FID produced from the RV pulse just applied and
- Stimulated echo from the residual transverse magnetization
Uses both gradient and stimulated echo to produce a T2/T1 weighing with symmetrically applied gradients in 3 directions
Balanced SSFP
2 important properties of magnetic gradients are:
Gradient field strength
Slew rate
Determined by its peak amplitude and slope (change over distance), and typically range 1 to 50 Tm/m
Gradient field strength
Time to achieve the peak magnetic field amplitude
Slew rate
Typical slew rates of gradient fields are from
5-250 mT/m/ms
Induced in nearby conductors such as adjacent RF coils and the patient, which produce magnetic fields that oppose the gradient field and limit the achievable slew rate
Eddy currents
Middle of the gradient is called _____, where there is no change in the field strength or precessional frequency
Null
It is the range of frequencies over the FOV
Frequency bandwidth
Frequency BW per pixel is BW divided by the
Number of discrete samples
Relationship of gradient strength and frequency BW across the FOV is dependent/independent of the main magnet field strength
Independent
Determines the slice location of protons in the tissues that absorb energy
Slice select gradient
SNR is inversely proportional to the receiver BW, therefore _____ are preferred
Narrow BW and low gradient strength
It is applied simultaneously with s RF pulse of a known BW to create proton excitation in a single plane with a known slice thickness, and to localize signals orthogonal to the gradient
Slice select gradient
Applied in a direction perpendicular to the SSG, along the logical x-axis, during the evolution and decay of the induced echo
Frequency encode gradient
Applied before frequency encode gradient and slice select gradient. It produces a spatially dependent variation in angular frequency of the excited spins for a brief duration, and generates a spatially dependent variation in phase when the spins return to the Larmor frequency
Phase encode gradient
MR Data are initially stored in the _____ matrix, the “frequency domain” repository
K-space
It describes a 2 dimensional matrix of positive and negative spatial frequency values, encoded as complex numbers
K-space
K-space matrix is divided into 4 quadrants, with the origin at the center representing frequency= 0. Frequency domain data are encoded in the kx direction by
Frequency encode gradient
Frequency domain data are encoded in ky direction by _____ in most image sequences
Phase encode gradient
Maximal useful frequency
Nyquist frequency
A 90 degree flip angle produces the largest
Transverse magnetization
Spatial domain representation are produced by
Inverse Fourier Transform decodes
Axial uses what coils
Z-axis coils
Coronal uses what coils
Y-axis coil
Sagittal uses what coils
X-axis coils
For a standard spin echo sequence, the relevant parameters are the
TR, number of excitations (NEX)
used for averaging identical repeat cycles
Tecniques that use multiple PEG steps in conjunction with multiple 180 degree refocusing RF pulses to produce an echo train length with corresponding digital data acquisitions per TR interval
Fast pulse sequences
Determined when the central views in k-space are acquired, which are usually the first echoes and subsequent echoes are usually spaced apart via increased PEG strength with the same echo spacing time
Effective echo time
Optimizes SNR by acquiring rhe low-frequency information with the early echoes (lowest amount of T2 decay) and the high-frequency, peripheral information with late echoes, where the impact on overall image SNR is lower
Phase re-ordering
Technique that has the advantage of spin echo image acquisition, namely immunity from external magnetic field inhomogeneities, with faster acquisition time
Fast pulse sequences
Also known as turbo spin echo or “RARE” (rapid acquisition with refocused echoes)
Fast spin echo
Pulse sequence that is similar to a standard echo sequence with a readout gradient reversal substituting for the 180 degree pulse
Gradient echo acquisition
Technique that provides extremely fast imaging time
Echo planar image aquisition
Sequence that combines the initial spin echo with a series of GEs, followed by an RF rephasing (180 degrees) pulse, and the pattern is repeated until k-space is filled
GRASE (gradient and spin echo)
Another method of k-space filling where the lower strength phase encode gradients are applied first, filling the center of k-space when the echoes have their highest amplitude. This type of filling is also important for fast GE techniques
Centric k-space filling
Method that fill k-space similarly to centric filling, except the central lines are filled when important events occur during the sequence, in situations such as contrast-enhanced angiography
Keyhole filling
An alternate method of filling the k-space radially, which involves the simultaneous oscillation of equivalent encoding gradients to sample data points during echo formation in a spiral, starting at the origin (the center of k-space) and spiraling outward to the periphery in the prescribed acquisition plane
Spiral filling
Technique that fills k-space by using the response of multiple recieve RF coils that are couples together with independent channels, so that data can be acquired simultaneously
Parallel imaging
Requires the use of a broadband, non-selective or slab-selective RF pulse to excite a large volume of protons simultaneously
Three-dimensional image acquisition (volume imaging)
Also known as the number of excitations
Signal averaging
Defines the range of frequencies to which the detector is tuned during the application of the readout gradient
Receiver bandwidth
Often a result of high-velocity signal loss, in which protons in the flowing blood move out of the slice during echo reformation, causing a lower signal
Low signal intensities (flow voids)
This causes flow voids by causing a dephasing of protons in the blood with a resulting loss of tissue magnetization in area turbulence
Flow turbulence
Pulse sequences to produce “black blood” can be very useful in
Cardiac and vascular imaging
A typical black blood pulse sequence uses a ______ method whereby a non-selective 180 degree RF pulse is initially applied, inverting all protons in the body, and is followed by a selective 180-degree RF pulse that restores the magnetization in the selected slice
Double inversion recovery method
A process that causes increased signal intensity due to flowing protons; it occurs during imaging of a volume of tissues
Flow-related enhancement
A phenomenon that causes flow to exhibit increased signal on even echoes in a multiple-echo image acquisition
Even-echo rephasing
Unsaturated blood exhibits the greatest signal if the blood velocity is increased or decreased?
Increased
Undesirable Flow-related enhancement and motion artifacts are eliminated with the use of
Presaturation pulses
MRA technique that relies on tagging of blood in one region of the body and detecting it in another
Time-of-flight technique
MR angiography techniques to create images of vascular anatomy include what 2 techniques
Time-of-flight and phase contrast angiography
Often used for two-dimensional image acquisition is ______ that produces relatively poor anatomic contrast, yet provides a high-contrast “bright blood” signal
GRASS or FISP
Detects the largest signal along a given ray thru the volume and places this value in the image
Maximum intensity projection algorithm
Relies on the phase change that occurs in moving protons such as blood
Phase contrast angiography
Degree of phase shift in phase contrast angiography is directly related to the
Velocity encoding time
Moving protons are subjected to gradients, the amount of phase dispersion is not compensated. This phase dispersal can cause ghosting in images. Rephasing of photons is done by
Gradient moment nulling
Produces multiple T2*- weighted images of the head before the application of the stimulus
Blood oxygen level dependent (BOLD)
Relates to the random motion of water molecules in tissues
Diffusion
Use strong MR gradients applied symmetrically about the refocusing pulse to produce signal differences based on the mobility and directionality of water diffusion
Diffusion weighted imaging sequences
Sensitive indicator for early detection of ischemic injury
Diffusion weighted imaging
Areas of stroke in DWI show
Drastic reduction in the diffusion coefficient compared with nonischemic tissues
Challenges in DWI include
Extreme sensitivity to motion
Eddy currents
Result of selective observation of the interaction between protons in free water molecules and protons contained in the macromolecules of a protein
Magnetization transfer contrast
This technique is used for anatomic MRI of the heart, eye, multiple sclerosis, knee cartilage and general MRA
Magnetization transfer contrast
Artifacts caused by nonferromagnetic conducting materials produce field distortions that disturb the local magnetic field, this pulse sequence reduces these artifacts
Partial compensation by spin echo
This echo sequence accentuates the artifacts caused by nonferromagnetic materials
Gradient refocused echo sequence
Ratio of the induced internal magnetization in a tissue to the external magnetic field
Susceptibility artifacts
Most common susceptibility changes occur at
Tissue-air-interfaces
Causes image distortions by mis-mapping anatomy
Gradient field artifacts
Produces variations in uniformity across the image caused by RF excitation variability, attenuation, mismatching and sensitivity falloff with distance
RF surface coils
Technique to mitigate cross-excitation by reordering slices into two groups with gaps
Slice interleaving
This artifact mostly occurs along the phase encode direction, as adjacent phase encoding measurement in k-space are separated by a TR Interval that can last 3000ms or longer
Motion artifacts
Motion compensation method that signal acquisition at a particular cyclic location synchronizes the phase changes applied across the anatomy
Cardiac and respiratory gating
Motion compensation method that reduce artifacts of random motion by making displaced signals less conspicuous relative to stationary anatomy
Signal averaging
What is more susceptible to motion, short or long TE spin echo sequence?
Long TE sequence
Motion artifact compensation that help rephase protons that are dephased due to motion
Gradient moment nulling
Helps reduce flow artifacts from inflowing protons, as well as other patient motions that occur in the periphery
Presaturation pulses applied outside the imaging region
To identify and remove those sequences contributing to motion, without deleteriously affecting the image
Multiple redundant sampling in the center of k-space
Refers to the resonance frequency variations resulting from intrinsic magnetic shielding of anatomic structures
Chemical shift artifacts
Chemical shift artifact is more severe for higher or lower field strength magnets?
Higher field strength magnets
Chemical shift occurrence is more severe for lower or higher gradient strengths?
Lower gradient strengths
Occur when data is acquired without consideration of physiologic periodicity
Motion artifact
Artifacts that occur with the GE images, resulting from rephasing and dephasing of the echo in the same direction relative to the main magnetic field
Chemical shift artifacts of the second kind
Also known as Gibbs phenomenon, occurs near sharp boundaries and high-contrast transitions in the image, and appears as multiple, regularly spaced parallel bands of alternating bright and dark signal that slowly fades with distance
Ringing artifacts
Artifacts that are more likely for smaller digital matrix sizes. Commonly occurs at skull/brain interfaces, where there is a large transition in signal amplitude
Ring artifact
Artifact that results from the mismapping of anatomy that lies outside of the FOV but within the slice volume
Wraparound artifacts
Artifact that arise from the finite size of the voxel over which the signal is averaged. It results in a loss of detail and spatial resolution
Partial volume artifacts
Method to measure tissue chemistry by recording and evaluating signals from metabolites by identifying metabolic peaks caused by frequency shifts (in parts per million) relative to a frequency standard
Magnetic resonance spectroscopy
Used to evaluate serial evaluation of biochemical changes in tumors, analyzing metabolic disorders, infections and diseases, as well as evaluation of therapeutic oncology treatments for tumor recurrence versus radiation damage
MR spectroscopy
Single voxel MRS sampling areas, covering a volume of about 1cm3 are delineated by
Stimulated echo acquisiton mode (STEAM) or
Point Resolved Spectroscopy (PRESS)
Method that uses a 90 degree excitation pulse and 90 degree refocusing pulse to collect the signal in conjunction with gradients to define each dimension of voxel
STEAM (stimulated echo acquisition mode)
Sequence that uses a 90-degree excitation and 180-degree refocusing pulse in each direction
Point resolved spectroscopy (PRESS)
Active or passive magnetic field devices that are used to adjust the main magnetic field and to improve the homogeneity in the sensitive central volume of the scanner
Shim coils
Wire conductors that produce a linear superimposed gradient magnetic field on the main magnetic target
Gradient coils
Creates an oscillating secondary magnetic field formed by passing an alternating current thru a loop of wire
RF transmitter coils
Encompass the total area of the anatomy of interest and yield uniform excitation and SNR over the entire imaging volume
volume coils
Enhanced performance is obtained with a process known as _______, which enables the energy to be transmitted with the signals to be received by two pairs of coils oriented at right angles, either electronically or physically
Quadrature excitation and detection
Used to achieve high SNR and high resolution when imaging anatomy near the surface of the patient, such as the spine
Surface coils
Consisting of multiple coils and receivers are made of several overlapping loops, which extend the imaging FOV in one direction
Phased array coils
With as many as N=32 elements allow for detection and encoding based upon the detection of a sensitivity map of the signals near the coil. There are used in parallel imaging
Multi-channel encoding coils
Patients with pacemakers or ferromagnetic aneurysm clips must avoid fringe fields above ____ mT
0.5 mT
Room containing the MRI system is typically lined by
Copper sheet walls and mesh windows
Achieved by manipulating the main field peripherally with passive and active “shim” coils which exist in proximity to the main magnetic field
Field uniformity
Area freely accessible to the general public, in essence everywhere outside the MR magnet area and building
Zone 1
Represents the interface between zone 1 and zone 3— typically the reception area, where the patients are registered and MR screening questions take place
Zone 2
Typically the reception area, comprised of the MR control room and computer room that only specific personnel can access, namely those specifically trained as MR personnel
Zone 3
Represents the MR magnet room, and is always located within the confines of zone 3
Zone 4
Have passed minimal safety and education training on MR safety issues and have a basic understanding of the effects of MR magnets, dangers of projectiles, effects of strong magnetic fields, etc
Level 1 MR personnel
More extensively trained in the broader aspects of MR safety issues, for example, understanding the potential for thermal loading, burns, neuromuscular excitation and induced currents from gradients
Level 2 MR personnel
