MRI Flashcards
What is the Lamor Equation and what does it tell us?
Precession frequency = gyromagnetic ratio x field strength (in Tesla)
Describes the precession frequency of a nuclear magnetic moment and resonant frequency of a nucleus, and relates these aspects to the magnetic field strength.
Basically says the precession frequency gets higher as the field strength increases.
What does an RF pulse do?
- Decreases the longitudinal magnetization
2. Causes the protons to synch up and precess in-phase (which establishes a transverse magnetization).
When can you measure signal in MRI?
When it is NOT in the longitudinal direction.
What is T1?
After you knock the protons down with an RF pulse, they will grow back up to normal size (magnitude will re-orient in the direction of Bo). The time it takes for this to happen is T1 - “longitudinal relaxation”.
Plot of time vs longitudinal magnetization creates the T1 curve - returns to 100% over time. T1 is the time at which longitudinal magnetization is 63% of its final value. Greater field strength = longer T1, b/c net magnetization is greater in a larger field
Sometimes called “spin-lattice relaxation” b/c energy from the RF pulse is handed over to the surrounding lattice.
The “1” looks like a thermometer - T1 relaxation involves the exchange of thermal energy.
Short T1 = bright.
Stronger magnet makes T1 longer - more energy in stronger field - takes longer to hand over to lattice
What is another name for T1?
Sometimes called “spin-lattice relaxation” b/c energy from the RF pulse is handed over to the surrounding lattice.
The “1” looks like a thermometer - T1 relaxation involves the exchange of thermal energy.
What is the definition of T1?
Time at which longitudinal magnetization is 63% of its final value.
Each tissue has different T1 and greater the field strength the longer the T1 (b/c net magnetization is greater in a larger field).
Short T1 = bright.
Is T1 different in a stronger magnet?
Stronger magnet makes T1 longer.
Protons in stronger field have more energy (precess faster), takes longer to hand that over to the lattice.
What is T2?
RF pulse causes the protons to synch up and precess in phase (establishes the transverse magnetization) - will slowly fall out of synch - T2 transverse relaxation.
Time at which the signal has decayed to 37% of its original value of transverse magnetization (63% of its decayed.
Plotting the time vs transverse magnetization creates the T2 curve - downward ski slope - T2 is shorter than T1 - less time to go down a hill than up it.
Also called “spin-spin relaxation”
What is the definition of T2?
Time at which the signal has decayed to 37% of its original value of transverse magnetization - 63% has decayed.
What is the relationship of time of T1 vs T2?
T2 is shorter - faster to go downhill than up it.
What causes protons to lose their transverse sync (T2 relaxation)?
- Inhomogeneities in the external field.
- Inhomogeneities in the local magnetic field - w/in the actual tissues and tissue spin interactions.
Pure things take longer to decay their transverse magnetization and are therefore bright (the opposite is true of impure liquids).
Difference between the T2 of pure vs impure liquids?
Pure things (water) take longer to decay their transverse magnetization - therefore bright (opposite of impure liquids).
What is T2*?
The signal of T2 decays faster than predicted based on tissue spin interactions alone. Math assumes the main external field is absolutely homogeneous, it’s not.
Heterogeneous field creates additional interaction which further speeds decay.
T2* decay is always faster than T2.
What is TR?
Time to repetition - time between initiation of two successive RF pulses.
What is FID?
Free Induction Decay
Give an RF pulse and the protons sync up - start getting a signal. Signal becomes less and less as times goes on - decay via T2* (random + fixed causes).
Will be created with by an RF pulse with any flip angle (90 degree as in spin echo or less than 90 degree as in gradient echo).
Won’t get with an 180 b/c it only inverts the longitudinal magnetization and doesn’t generate a transfer component.
How do you fix T2*?
Wait until half way through T2 decay - hit it with a 180 degree pulse and spin it all the way around - restart the process. This will:
- clear out those inhomogeneities in the field making T2* turn into T2
- Will create an “echo”
What is an “echo”?
The signal tails off, hit it with a 180 pulse and it will come back to refocus = “the echo” - the signal peaks in uniformity at the tip of the echo.
Great time to collect a nice “clean” signal
When do you deliver the 180 pulse?
The 180 is given at the 1/2 T.E. - Time to echo.
What is TE?
Time to echo
Give the 180 pulse at 1/2 TE.
Short TR and Short TE =
T1
Maximize the longitudinal contrast and minimize the transverse contrast - the difference between the T1 and T2 curves
More gap between the lines in the T1 curve early- more closely together early in the T2 curve
Long TR and Long TE =
T2
Maximize the transverse contrast (T2) and minimize the longitudinal contrast (T1).
Longer the TE = greater the T2 effects
Longer TE =
Greater the T2 effects
Increasing ___ increases the T2 effects?
TE
What is a Short TR in Spin Echo?
250-700 ms
What is a Long TR in Spin Echo?
> 2000 ms
What is a Short TE in Spin Echo?
10-25 ms
What is a Long TE in Spin Echo?
> 60 ms
What is a short TR in Gradient Echo?
<50 ms
What is a Long TR in Gradient Echo?
> 100 ms
What is a Short TE in Gradient Echo?
1-5 ms
What is a Long TE in Gradient Echo?
> 10 ms
What is Proton Density?
What you have left when you subtract the bias of T1 or T2 weighting.
With all things equal, the only thing you are measuring is how many protons are present in a thing vs another thing.
Choose a long TR to minimize the longitudinal difference and choose a short TE to minimize transverse differences.
Short TR + Short TE =
T1
Long TR + Long TE =
T2
Long TR + Short TE =
Proton Density
Short TR + Long TE =
Bullshit
TR and TE of T1?
Short TR and Short TE
TR and TE of T2?
Long TR and Long TE
TR and TE of PD?
Long TR and Short TE
What is Fourier Transform?
Mathematical technique for converting data from the time domain to the data in the frequency domain.
What is K-space?
K-space is a Fourier plane (like an x-y axis coordinate system) in which MR signal is stored.
Turning K-space into an image requires an inverse 2D Fourier Transform.
How is K-space turned into an image?
Inverse 2D Fourier transform.
What are the parts of K-space made up of?
Center = information about gross form and tissue contrast
Periphery = made up of information about spatial resolution.
What is the center of K-space made up of?
Information about gross form and tissue contrast
What is the periphery of K-space made up of?
Information about spatial resolution.
How is localization of signal done?
3 steps:
1. Select the desired slice - slice selection gradient - perpendicular - determines the view (axial, coronal, sagittal…..)
- Encode spatial information along the rows - vertical direction (phase encoding) - gradient applied causing protons in the same row perpendicular to the gradient to have same phase - same frequency.
- Encode spatial information along the columns - perpendicular to phase encoding - modification of Lamor freqs over the duration of its application - end result is column of protons which have identical frequencies- applied at same times are readout.
What are localizing gradients?
Can be turned on and off (vs main magnet is always on).
Have identical properties, just applied at different times and different directions.
Have 3 gradients in 3 planes, can localize anything in the body.
How do you select the desired slice?
Use a slice selection gradient to select area of interest. Perpendicular to the desired slice plane - determines the view (axial, coronal, sagittal, even oblique).
Apply selective pulse on top of this gradient at same frequency as the protons in the slice plane you want to sample - only the protons in this plane will be affected.
What is a selective pulse?
Applied on top of the slice selection gradient - at the same frequency as the protons in the slice plane you want to sample - only the protons in this plane will be affected.
90 pulse.
What is Phase Encoding?
Encodes information in the vertical direction - second step.
Gradient is applied causing protons in the same row perpendicular to the gradient to have the same phase.
All protons at this point will have the same frequency.
Much longer than frequency encoding - done on the thinner portion - contributes to the duration of the study.
In which direction is the phase encoding done?
Thinnest portion of the body part imaged - contributes to duration of study.
What contributes to the duration of a study?
TR = repetition time NPy = number of phase encoding steps Nex = number of excitations
What is frequency encoding?
Encodes spatial information in the horizontal direction.
Gradient applied perpendicular to the phase-encoding direction, which results in modification of Lamor frequencies over the direction of its application.
End result is a column of protons which have identical frequencies.
Applied at the same time as the readout.
How does the difference between slice selection gradient change slice thickeness?
Steep “large” SS gradient slopes = large difference in precessional frequency = thinner slices
Shallow “small” SS gradient slopes = small difference in precessional frequency = thicker slice.
How does the RF transmit pulse applied after the slice selection gradient affect slice thickness?
Deployed at a range of frequencies (or bandwidth) covering the desired area - Transmit bandwidth.
Thicker bandwidth will result in a thicker slice.
Thinner bandwidth will result in a thinner slice.
What slice selection gradient and transmit bandwidth are needed for a thinner slice?
Steep (large) slice selection gradient and thin transmit bandwidth
What slice selection gradient and transmit bandwidth are needed for a thicker slice?
Shallow (small) slice selection gradient and thick transmit bandwidth
What factors into table time?
Applies to 2D imaging:
TR x Phase Matrix x NEX
TR = time between each RF pulse
Phase Matrix = Data the system collects from each phase encoding step
NEX = number of times each set of phase encoding steps is required - “number of excitations”
If 3D imaging:
TR x Phase Matrix x NEX x #Slices
In fast spin echo- the acquisition time is approximately proportional to 1/Echo Train Length
In what situations does the Table Time formula not hold up?
3D imaging
Fast Spin Echo
For 2D:
Time = TR x Phase Matrix x NEX
Fast spin echo: acquisition time is approximately proportional to 1/Echo Train Length
3D imaging:
Time = TR x Phase Matrix x NEX x #slices
How is 3D imaging different from 2D?
In 3D you are acquiring information in blocks instead of slices - improves spatial resolution (can shrink slice thickness) and there is no gap.
Improves SNR plus ability to manipulate the angle of obliquity.
Increased time compared to 2D
Time = TR x Phase Matrix x NEX x #Slices
What factors affect spatial resolution?
Spatial resolution is governed by the size of a voxel. Voxel size is determined by matrix, FOV, and slice thickness.
FOV: smaller is better, but too small will get aliasing or wrap around from signal outside the FOV.
Matrix Size: Image width and height (in pixels) - larger the matrix, the smaller the pixels (pixel = FOV/Matrix).
Gradient: Gradient with higher amplitude (more intense) or one applied for a longer period of time = better spatial resolution.
Slice Thickness = thinner the slice, the better the spatial resolution
What results in better spatial resolution?
Small Voxel
Small FOV
Large Matrix
Thinner Slices
Steep (large) slice selection gradient and thin transmit bandwidth
What factors affect SNR?
Voxel size: bigger voxel size improves SNR (opposite of spatial resolution). Thicker slices (increased transmit RF pulse, decreased slice selection gradient). Large FOV = more SNR Smaller matrix = more SNR
Field Strength = stronger field = more signal
RF coils: smaller surface coils improve your signal (increased SNR) compared to a coil within the scanner
Number of excitations per slice (number of averages): more excitation you perform the more signal you get (increased SNR), but increased imaging time.
Receiver bandwidth: fat bandwidth = rapid sampling, narrow bandwidth = slow sampling. Noise is constant, fatter band will pick up more noise. Fat bandwidth = decrease SNR, narrow bandwidth = increased SNR.
Maximize TR (long) and minimize TE (short) - peaks the signal.
What makes better SNR of signal?
Stronger magnet Long TR Big FOV Large slices - shallow (small) slice selection gradient, and thick transmit bandwidth More NEX Short TE Small Matrix Small Receiver Bandwidth Appropriate Coil Size
How does FOV affect spatial resolution?
Smaller is better, but too small will get aliasing or wrap around from signal outside the FOV.
How does Matrix Size affect spatial resolution?
Larger is better
Image width and height (in pixels) - larger the matrix, the smaller the pixels (pixel = FOV/Matrix).
How does gradient affect spatial resolution?
Gradient with higher amplitude (more intense) or one applied for a longer period of time = better spatial resolution.
How does slice thickness affect spatial resolution?
Thinner the slice, the better the spatial resolution
How does voxel size affect signal to noise?
Anything that makes voxel size bigger improves SNR (opposite of spatial resolution)
Thicker slices (increased transmit RF pulse, decreased slice selection gradient). Large FOV = more SNR Smaller matrix = more SNR
How does field strength affect SNR?
stronger field = more signal
How do RF coils affect SNR?
smaller surface coils improve your signal (increased SNR) compared to a coil within the scanner
How does number of excitations per slice affect SNR?
more excitation you perform the more signal you get (increased SNR), but increased imaging time.
How does receiver bandwidth affect SNR?
fat bandwidth = rapid sampling, narrow bandwidth = slow sampling. Noise is constant, fatter band will pick up more noise. Fat bandwidth = decrease SNR, narrow bandwidth = increased SNR.
How do TR and TE affect SNR?
Maximizing TR (long) and minimizing TE (short)- peaks your signal.
What does a fat receiver bandwidth do to SNR?
Decrease SNR
What does a fat transmit bandwidth do to SNR?
Larger slice = increased SNR
What does a narrow receiver bandwidth do to SNR?
Increased SNR
What does a narrow transmit bandwidth do the SNR?
Thinner slice = decreased SNR
What kind of signal to noise ratio does PD have?
Excellent SNR.
Long TR
Short TE
What is the trade off of using more field strength?
Better SNR, but more field strength increases tissue T1 times (and therefore acquisition time via TR).
What is the trade off of increasing the NEX?
Improved signal, but increasing NEX from two to four doubles the scan time, but increases the signal the signal by only the square root of two.
What is the trade off of shortening TE?
Will improve SNR and doesn’t mess with table time, but increasing the TE or shortening TR decreases the number of slices that can be obtained with one pulse sequence.
What is the trade off of using larger receiver bandwidth?
Decrease SNR but also decreases mismatch artifacts like chemical shift or magnetic susceptibility.
Thinner bandwidths may increase SNR, but they also increase mismatch artifacts like chemical shift or magnetic susceptibility.
What do thicker slices do to SNR, spatial resolution, and duration of exam?
Increased SNR
Decreased spatial resolution
No effect on duration of exam
What do larger FOV do to SNR, spatial resolution, and duration of exam?
Increased SNR
Decreased spatial resolution
No effect on duration of exam
What do larger matrix do to SNR, spatial resolution, and duration of exam?
Decreased SNR
Increased spatial resolution
Increased Duration of exam
What do greater field strength do to SNR, spatial resolution, and duration of exam?
Increased SNR
No effect on spatial resolution
No effect on duration of exam
What do greater receiver bandwidth do to SNR, spatial resolution, and duration of exam?
Decreased SNR
No effect on spatial resolution
Decreased duration of exam
What do greater transmit bandwidth do to SNR, spatial resolution, and duration of exam?
Increased SNR
Decreased spatial resolution
No effect on duration of exam
What do more excitations per slice do to SNR, spatial resolution, and duration of exam?
Increased SNR
No effect on spatial resolution
Increased duration of exam
What do utilizing partial K space sampling do to SNR, spatial resolution, and duration of exam?
Decreased SNR
No effect on spatial resolution
Decreased duration of exam
Flip angle used in gradient echo sequences?
<90
90 in spin echo
What is the 180 pulse used for in spin echo sequences?
Given to try and improve the heterogeneous nature of the field.
Reason why spin echoes give us T2 and not T2*
Sequence of a spin echo sequence
Slice selection gradient is applied with the 90 RF pulse.
Phase encoding and frequency encoding gradients.
The 180 pulse is flanked by self canceling slice selection gradients.
Frequency encoding gradient fires again with the read out echo.
What is the duration of a spin echo sequence determined by?
Duration = TR x Phase Matrix x Nex
What is the basic idea behind a Fast Spin Echo sequence?
Reduce the TR - which is a major reason for the duration of a SE sequence.
Applying multiple 180 RV pulses each resulting in an echo.
What is J Coupling?
Normal phenomenon which occurs between the nucli of lipid molecules, causing intrinsic shortening of T2 signal.
The fast repetition of 180 degree pulses in FSE sequences mess up the J couples and cause the T2 of fat to lengthen.
T2 fat signal is longer with FSE (interferes with J coupling)
What does fat signal look like on FSE?
T2 signal is longer due to J coupling.
Normal phenomenon which occurs between the nucli of lipid molecules, causing intrinsic shortening of T2 signal.
The fast repetition of 180 degree pulses in FSE sequences mess up the J couples and cause the T2 of fat to lengthen.
What is T2 Blurring?
With each progressive echo train the transverse signal gradually decreases.
What is acquisition time in FSE sequences proportional to?
1/ETL
What is Inversion Recovery?
Start with a 180 “preparation” pulse instead of a 90 RF pulse. Wait for the relaxation of the thing you want to saturate (water, fat, myocardium) to hit its null point then you hit it with the 90 pulse - that particular tissue gives you no signal.
TI - time between 180 and 90 pulse.
What is STIR?
Short TI Inversion Recovery
Uses short TI (120-160 ms) to suppress fat, which is based on the TI for fat.
What is FLAIR?
Fluid Attenuated Inversion Recovery
Uses a TI designed to suppress water signal (approximately 2000 ms)
How is STIR different from other fat suppression techniques?
STIR is much less “susceptible” to magnetic susceptibility (metal) and field homogeneity.
Can STIR be used with Gadolinium?
No.
Gad enhanced tissues have a similar TI to fat- get nulled out.
What is the trade off of STIR sequences?
Longer study as the extra pulse and subsequent “time to inversion (TI)” increases the time to repetition (TR).
How are GRE sequences different than SE sequences?
Flip angle less than 90
Do not have a 180 pulse- so dealing with T2* (not T2)- more susceptible to artifacts.
Lower specific absorption rate (less heating)
What is the advantage of the <90 flip angle in GRE?
Faster recovery, shorter TR/TE times and a faster scan.
How are GRE sequences different with patient heating?
Lower specific absorption rate (less heating).
How are echoes created in GRE sequences without a 180 pulse?
A bipolar readout gradient (basically a frequency encoding gradient) is used.
Trade off of GRE sequences?
Fast and low SAR.
Signal to noise isn’t that great and get more susceptibility artifacts.
What is steady state?
B/c TR is shortened in GRE- get stuck with permanent residual transverse magnetization - never completely goes away. The TR is shorter than the T1 and T2 of the tissues, so T2* dephasing dominates.
Two types of GRE imaging classified depending on how this residual transverse magnetization is handled.\
Spoiled (incoherent) GRE: Gradients and/or RF pulses are used to get rid of the transverse magnetization (T2) that is persisting in the steady state = basically T1
Refocused (coherent) GRE: Steady state is preserved by using a “rewind gradient”= the sequences are T2* weighted. The refocused sequences can be partial or full = basically T2
SSFP - fast with high signal to noise.
What is Spoiled (incoherent) GRE?
B/c TR is shortened in GRE- get stuck with permanent residual transverse magnetization - never completely goes away. The TR is shorter than the T1 and T2 of the tissues, so T2* dephasing dominates.
Gradients and/or RF pulses are used to get rid of the transverse magnetization (T2) that is persisting in the steady state = basically T1
What is Refocused (coherent) GRE?
B/c TR is shortened in GRE- get stuck with permanent residual transverse magnetization - never completely goes away. The TR is shorter than the T1 and T2 of the tissues, so T2* dephasing dominates.
Steady state is preserved by using a “rewind gradient”= the sequences are T2* weighted. The refocused sequences can be partial or full = basically T2
What is MRIs fastest acquisition method?
Echo-planar imaging (EPI)
What is Echo-Planar Imaging (EPI)?
Can be done with a spin echo (90 + 180) or gradient echo (90 + bunch of gradients).
One RF pulse can be used to acquire date for an image (aka single shot).
Works by turning the phase and frequency encoding gradients on and off very rapidly - causing very fast filling of k-space.
More vulnerable to magnetic susceptibility, but gives you better tissue contrast and is faster- compared to regular GRE.
Difference between GRE and EPI?
EPI is more vulnerable to magnetic susceptibility, but gives you better tissue contrast and is faster.
What are the artifacts liked to EPI?
Magnetic Susceptibility - can be improved with segmented sequences (instead of single shots)
Ghosting - Gradient imperfections mess with spatial encoding.
Chemical Shift - Narrow readout bandwidth is used.
What is the technique of choice for Diffuse Weighted Imaging?
Echo-planar
What is diffusion imaging?
Base sequence is a fast GRE or echo planar.
Uses two very strong and symmetric MR gradients.
Acquisition is repeated in each of the 3 dimensions in space with b-factors of 0 and b-factors of 1000. The signal differences are based on mobility and direction of water.
Scenario 1 (no net movement): The first gradient fires dephasing the spins - molecules don’t move. Second gradient fires rephasing the same molecules - giving you high signal.
Scenario 2 (net movement): First gradient fires dephasing spins- the molecules move out of the way. Second gradient fires missing the original protons - gives you low signal.
What is Isotropic vs Anisotropic?
Isotropic - movement in all spatial directions
Anisotropic - movement in a single direction
What is B Zero?
Set the contribution of diffusion to 0, assuming your TR and TE are long - which they usually are - basically have a T2 = “poor mans T2”
What is the basis of fMRI?
Increased blood flow to local vasculature that accompanies neural activity resulting in local reduction in deoxyhemoglobin.
Deoxyhemoglobin acts as a contrast agent b/c it is paramagnetic (thus alters T2* MR signal).
fMRI depends on T2* effects
What is 2D TOF MRA?
Uses gradient echo sequence where a saturation pulse is employed to null venous or arterial blood flow.
Has SMALL VOXEL SIZE.
What is 3D TOF MRA?
Collected as a 3D volume as opposed to slices and allows for smaller voxels than 2D.
Well suited for high flow arterial systems like COW.
Higher SNR than 2D, shorter imaging time, more smooth vessel contours, and better saturation (which limits venous circulation).
What is phase contrast MRA?
Uses bipolar gradients to create contrast from flow.
High velocity encoding time (VENC) is needed for arterial imaging and low VENC for veins and sinuses.
Phase contrast MRA is a quantitative image and can measure mean blood flow velocity and direction.
What are the two types of fat saturation?
Inversion Sequences (STIR) that are based on the inversion time “T.I.” of fat.
Those that exploit the resonance difference between fat and water protons.
Which fat saturation technique can be used with Gadolinium?
Selective Pulse.
Can’t use Gd with STIR.
Which fat saturation technique does great with metal artifact?
STIR
How does selective saturation work for fat saturation?
Selective saturation works by capitalizing on the resonance difference between protons w/in their respective microenvironment
Fat precesses faster than water - this difference gets more exaggerated with a stronger magnet
3 Steps:
- Prepatory Pulse - Dropping a very narrow RF pulse just at the resonance of fat.
- Spoiler Gradient - Results in dephasing of the protons primed by the prepatory pulse - eliminating their ability to produce a signal. Remaining protons will be only ones left able to give signal.
- Scan as normal - sending the first RF pulse immediately after the spoiler gradient.
Requires excellent field homogeneity.
NOT typically done with very low field magnets (<1T) b/c can’t accurately separate the peaks between fat and water.
Steps involved in selective saturation for fat saturation?
3 Steps:
- Prepatory Pulse - Dropping a very narrow RF pulse just at the resonance of fat.
- Spoiler Gradient - Results in dephasing of the protons primed by the prepatory pulse - eliminating their ability to produce a signal. Remaining protons will be only ones left able to give signal.
- Scan as normal - sending the first RF pulse immediately after the spoiler gradient.
What is in an out of phase imaging?
The chemical environments of fat and water are different for protons - causes these protons to precess at different rates - 2.2 msec at 1.5 T.
Spoiled GRE is performed when the protons are spinning with each other and directly out of phase of each other.
Stronger magnet = changes occur twice as fast.
For in and out of phase imaging- does timing of the out of phase sequence matter?
No - if looking for hepatic steatosis. Liver is fatty you will see signal loss not matter when you get it.
Yes - In hemochromatosis iron acts as a “magnet within a magnet” - which messes up the signal in the local area - have T2* effects that are more pronounced - why you get signal loss with iron.
Signal is lost as a function of time - longer you wait, the darker it gets.
How does out of phase imaging with hemochromatosis work?
In hemochromatosis iron acts as a “magnet within a magnet” - which messes up the signal in the local area - have T2* effects that are more pronounced - why you get signal loss with iron.
Signal is lost as a function of time - longer you wait, the darker it gets.
In what direction does chemical shift artifact occur?
Read out (frequency encoding) direction
What are the two types of chemical shift artifact?
Type I: Bright rim on one side and a dark rim on the other side. Can be on either spin echo or GE sequences.
Type II: India Ink (black boundary) - black line all the way around the fat-water interface. On GE sequences, if a voxel has 50% fat and 50% water the signals will cancel out - leaving black line - on GE sequences, SE sequences will get rid of India Ink
What is Type I chemical shift artifact?
Bright rim on one side and a dark rim on the other side. Can be on either spin echo or GE sequences.
In which sequences does type I chemical shift artifact occur?
spin echo or GE sequences
What is Type II chemical shift?
India Ink (black boundary) - black line all the way around the fat-water interface. On GE sequences, if a voxel has 50% fat and 50% water the signals will cancel out - leaving black line - on GE sequences, SE sequences will get rid of India Ink
What sequence will get rid of india ink artifact?
SE sequences.
It occurs with GE sequences.
What factors affect chemical shift?
Increases with field strength (not seen below 1T)
Decreases with increased gradient strength
Decrease with wider readout bandwidth
What are the two types of MRI contrast?
Positive agents - cause bright T1 signal
Negative agents - produce magnetic inhomogeneity from susceptibility leading to T2 shortening.
What causes the toxicity, clearance routes, bio-distribution, and relaxation properties of Gadolinium?
The chelating agent (DTPA)
How does gadolinium cause enhancement?
T1 shortening.
Gd has 7 unpaired electrons and interaction of these ELECTRONS causes augmentation of the external magnetic field
At high concentration, T2 effects dominate - pseudolayer in the bladder.
What is aliasing?
When the area is under sampled.
Get wrapping of anatomy from under sampled portions.
Occurs in the phase encoding direction.
Correct by increasing FOV or changing the phase encoding direction.
If in a 3D sequence- can add slices or increase coverage to cover your FOV.
What is chemical shift artifact?
Protons of different molecules precess at different frequencies - fat and water.
This shift in the Lamor frequency is the so called “chemical shift”.
Type 1 occurs in the frequency encoding direction.
In which direction does chemical shift artifact occur?
Frequency encoding direction.
What is Truncation/Gibbs artifact?
Get transformation of K space through inverse Fourier transform resulting in a block of data.
Ripples in this data - especially at abrupt intense tissue changes result in the appearance of lines - at high contrast interfaces.
CSF-Cord interface is the most classic - mimicking a syrinx.
Cause is limited sampling of the free induction decay.
Can be seen in both the frequency and phase encoding directions, but is more commonly seen in the phase encoding direction b/c phase encoding matrix is smaller than the readout matrix selected to reduce time.
Fix by increasing matrix or decrease bandwidth or decreasing pixel size.(more PE steps, less FOV, more matrix) but get increased acquisition time and reduced per-pixel signal to noise.
What causes Truncation/Gibbs artifact?
Ripples in Fourier transform data - especially at abrupt intense tissue changes result in the appearance of lines - at high contrast interfaces.
CSF-Cord interface is the most classic - mimicking a syrinx.
Cause is limited sampling of the free induction decay.
In what direction is Truncation/Gibbs artifact seen?
Can be seen in both the frequency and phase encoding directions, but is more commonly seen in the phase encoding direction b/c phase encoding matrix is smaller than the readout matrix selected to reduce time.
How do you fix Truncation/Gibbs artifact?
Fix by increasing matrix or decrease bandwidth or decreasing pixel size.(more PE steps, less FOV, more matrix) but get increased acquisition time and reduced per-pixel signal to noise.
How do you fix partial volume?
Make pixels smaller - more slices in the z-direction.
How are motion artifacts created?
Motion creates differences between the frequency encoding (which is fast) and phase encoding (which is slow) - see ghosting or smearing primarily in the phase encoding direction.
How can you fix motion artfacts?
Breath holding/hold still.
Respiratory gating - increases acquisition time
ROPE (respiratory ordered phase encoding) - phase encoding steps are ordered with respiration.
Breathing Navigator - Echo from the diaphragm determines its position, then timing and acquisition are based off this.
Apply fat sat band across the abdomen
Switch the phase encoding direction.
What is flow artifact?
Type of motion artifact related to blood flow - causes ghosting in the phase encoding direction.
GRE sequences are more susceptible than SE sequences.
SE: flow looks dark - moving blood got hit with the 90 pulse, moves out of the way prior to getting the 180 pulse - doesn’t have any signal.
GRE: In flowing blood looks bright.
Apply a sat band adjacent to the imaging section - 90 pulse followed by crusher gradient.
What does flow artifact look like on SE sequences?
SE: flow looks dark - moving blood got hit with the 90 pulse, moves out of the way prior to getting the 180 pulse - doesn’t have any signal.
To make an echo, the proton must be exposed to both a 90 pulse and a 180 pulse. If blood is flowing fast it will get hit with the 90 pulse but then miss the 180 pulse. No echo = no signal.
No flow makes an echo. Slow flow gets some echo = some signal.
GRE: In flowing blood looks bright.
What does flow artifact look like on a GRE sequence?
SE: flow looks dark - moving blood got hit with the 90 pulse, moves out of the way prior to getting the 180 pulse - doesn’t have any signal.
GRE: In flowing blood looks bright.
How do you fix flow artifact?
Apply a sat band adjacent to the imaging section - 90 pulse followed by crusher gradient.
What is Magic Angle?
MSK artifact seen in tendons.
See with short echo time (TE) sequences where the focus form an angle of 55 degrees with the main magnetic field (magic angle phenomenon).
Will not be seen in T2 sequences (with long TE).
This phenomenon is reduced at higher field strengths due to greater shortening of T2 relaxation times.
In what sequences do you see Magic angle artifact?
With short echo time (TE) seuences - T1, PD, and GRE. Will not see on T2 sequences.
What are the RF related artifacts?
Cross talk and Zipper artifacts
What is cross talk artifact?
RF and FT pulses are not perfectly rectangular. If placed close enough together you can get excitation of neighboring section more than once in a single repetition.
Can lead to partial saturation and lower signal.
All sections (except the ones on the ends) will be subjected to this.
Improve by increasing the gap between sections. Interleave slices (all odds, then all evens)
3D images are not susceptible to cross talk artifact b/c the entire volume undergoes excitation with sections within the volume acquired with gradients.
How do you fix cross talk artifact?
Improve by increasing the gap between sections. Interleave slices (all odds, then all evens)
3D images are not susceptible to cross talk artifact b/c the entire volume undergoes excitation with sections within the volume acquired with gradients.
Are 3D images susceptible to cross talk artifact?
3D images are not susceptible to cross talk artifact b/c the entire volume undergoes excitation with sections within the volume acquired with gradients.
What is Zipper Artifact?
Defective or inadequate shielding can get stray RF signals (radio, tv, etc..).
Get “zipper” of high signal - 1-2 pixels in width running across the image. Typically in the phase encoding direction.
Close door. Remove electronics. Repair RF shielding.
What causes inhomogeneous fat suppression?
Local field inhomogeneities cause fat protons to precess at different frequencies which allow certain areas of fat to resist suppression - which can mimic edema.
Use an inversion recovery - STIR, especially in the setting of metal.
What fat suppression technique should be used in the setting of metal?
STIR - less susceptible to inhomogeneous fat suppression.
What is susceptibility artifact?
Refers to the ability of a substance to become magnetized by the external field.
Metals have high susceptibility. Calcium hydroxyapatite and accumulations of gadolinium chelate can do the same thing.
Generally susceptibility affects all pulse sequences, but is most severe with GRE images and least severe with SE (180 refocusing pulse lost the T2* effects)
Less prominent version occurs at tissue interfaces (bone and muscle, or air and bone)- transition from paranasal sinus to skull base.
Make it better using SE and FSE instead of GRE.
Swap the phase and frequency, use wider receiver bandwidth, or align the longitudinal axis of a metal implant with the axis of the main field. STIR does way better than frequency selective fat suppression.
Worse on in phase imaging relative to out of phase. Has to do with the in phase being done later.
The longer the TE, the more susceptibility (T2*).
Air will do the same thing.
Is susceptibility artifact worse on in phase or out of phase imaging?
In phase b/c its done later.
Longer TE = more susceptibility (T2*).
Difference between active and passive shimming?
Passive - shim plates are adjusted until field becomes homogeneous - done at time of installation.
Active- Using the electromagnetic coil- can be done after each patient (or sequence). Gives you the chance to have a homogeneous field (or nearly) regardless of size of the patient.
What are the gradient related artifacts?
Eddy Currents
What are Eddy Currents?
Generated when gradients are rapidly turned on and off- actual location of the currents can be in the magnet, the cables, the wires, or even in the patient.
Looks like distortion (contraction or dilation of the image) or shift/shear.
Most severe with DWI pulses
Improve by optimizing the sequence of gradient pulses.
Which sequences are most affected by eddy currents?
DWI pulses
How do you improve eddy currents?
Optimizing the sequence of gradient pulses.
What artifacts are caused by errors in data?
Dielectric effects and cirsscross
What is the Dielectric effect/Standing wave effect artifact?
Biologic tissues have a dielectric constant that results in reduction of wavelength by the inverse of some constant.
Interactions can cause local eddy currents in the imaged tissues.
Since RF waves are shorter at 3T - the effects are worse with a stronger magnet.
Worsen with large bellies, especially if they have ascites.
Larger body parts (the abdomen) are primarily affected.
Classic look/location: dark signal in the central abdomen over the left lobe of the liver.
Make it better by application of dielectric pads - placed between patient and anterior body array coil.
Parallel RF transmission (SENSE) - RF pulses from a set of coils; each coil sends an independent RF pulse. Gives a longer pulse.
What is the classic location of Dielectric effect/Standing wave effect artifact?
Dark signal in the central abdomen over the left lobe of the liver.
How to improve Dielectric effect/Standing wave effect artifact?
Make it better by application of dielectric pads - placed between patient and anterior body array coil.
Parallel RF transmission (SENSE) - RF pulses from a set of coils; each coil sends an independent RF pulse. Gives a longer pulse.
What is Crisscross or Herringbone artifact?
Obliquely oriented stripes throughout the image.
Data processing and/or reconstruction errors.
Reconstruct the image again.
Aliasing artifact
Cause, Direction, Makes it better, Makes it worse
Caused by small FOV
In the phase encoding direction
Better: increase FOV, change the phase encoding direction
Worse: Smaller FOV
Chemical Shift artifact
Cause, Direction, Makes it better, Makes it worse
Caused by differences in resonance frequencies
In the frequency encoding direction
Better: bigger pixels, fat suppression, increase receiver bandwidth
Worse: stronger magnetic fields, lower receiver bandwidth
Gibbs/Truncation artifact
Cause, Direction, Makes it better
Caused by limited sample of FID. Classically seen in spinal cord
Occurs in both phase and frequency encoding direction (more in phase)
Better: Bigger matrix, decrease bandwidth, decrease pixel size (increase PE steps, decrease FOV).
Partial Volume artifact
Makes it better, makes it worse
Better: decrease pixel size (increase PE steps, decrease FOV)
Worse: thicker slices
Motion artifact
Direction and makes it better
Occurs in the phase encoding direction
Better: saturation pulses, respiratory gating, faster sequences (BLADE, PROPELLER)
Cross talk artifact
Cause and makes it better
Caused by overlap of slices
Better: increase slice gap and interleave slices
Zipper artifact
Cause and direction
Caused by poor shielding
Occurs in phase encoding direction
Field homogeneity artifact
Cause, makes it better, and makes it worse
Caused by geometric distortion
Better with shimming
Worse with GRE sequences
Susceptibility artifact
Cause and makes it worse
Caused by augmentation of magnetic field
Very bad in EPI
Worse with GRE sequences
Eddy Current artifact
Cause, makes it better, and makes it worse
Caused by geometric distortion or non-uniformity
Better: Optimize sequence of gradient sequences
Worse: DWI - large gradients
Dielectric effects artifact
Cause, makes it better, and makes it worse
Caused by standing waves created as radiowave approaches length of body part
Better with parallel transmit and use 1.5T
Worse with 3T
Magic angle artifact
Cause, makes it better, and makes it worse
Occurs at 55 degrees
Better with T2
Worse with T1 and PD
What are bright blood cardiac sequences?
Gradient sequences - majority used.
Steady state free precession - SSFP - CINE clips used for wall motion and volume analysis.
What are dark blood cardiac sequences?
Spin echo sequence - double inversion recovery.
Two consecutive 180 pulses - one that is “non selective” and the next is “selective” to the blood and heart.
End up with with the tissue left alone - blood is nulled.
Good for anatomy.
Is a Spin echo sequence (not GRE)- less susceptibility artifacts.
What is double inversion recovery?
Dark blood
Spin echo sequence - double inversion recovery.
Two consecutive 180 pulses - one that is “non selective” and the next is “selective” to the blood and heart.
End up with with the tissue left alone - blood is nulled.
How is inversion recovery used in cardiac?
Used to null myocardium (like fat in STIR and CSF in FLAIR).
Look for delayed enhancement (scar)
A PSIR - phase sensitive inversion recovery - A TI scout series is done to help choose the correct time to use for inversion. Will produce a series - pick the one with the darkest myocardium - will give you the correct inversion recovery time. Highly variable between people.
What is PSIR?
Phase sensitive inversion recovery - Used in cardiac.
A TI scout series is done to help choose the correct time to use for inversion. Will produce a series - pick the one with the darkest myocardium - will give you the correct inversion recovery time. Highly variable between people.
How do you correct chemical shift artifact in breast?
Increase the bandwidth to capture both fat and water in the same phase.
How is motion corrected in breast imaging?
Run the phase encoding direction side to side instead of front to back.
Axial: left to right
Sagittal: top to bottom
What is signal flair artifact in breast imaging?
Breast is too close to the coil element - will not fat sat out correctly (look bright)
Reposition the patient.
What is the 5G line?
Line drawn around the magnet with “5G” written on it
The 5-Gauss exclusion zone - outside of this line, magnetism from the field won’t mess up a pacemaker, insulin pump, or vagus nerve stimulator.
The risk to implanted devices line.
Will still have pulling force.
What is the main source of noise in MRI?
Gradient coils due to rapid changes in current.
FDA max is 140 dB for MR (99 dB for patients with hearing protection).
Gradient intensive sequences - such as EPI are the loudest - diffusion is an EPI sequence.
What causes neurostimulation in MR?
High-bandwidth readouts and rapid gradient switching (echo-planar imaging) are the usual culprits.
Reduce by reducing the readout bandwidth and increasing the TR.
What contributes to SAR?
Specific Absorption Rate
Strength of the magnet x 2
Flip angle x 2
Duty cycle - the “cool down” - the longer your TR the more cool down you get - will decrease SAR. Double the TR = 1/2 the duty cycle. Half the duty cycle will half your SAR.
What happens to SAR if you double the strength of the magnet?
Quadruples
What happens to SAR if you double the flip angle?
Quadruples
What happens to SAR if you double the duty cycle (make TR 1/2)?
Doubles
Which has higher SAR, SE or GRE?
Spin echo - has higher flip angles - especially inversions.
What is the SAR limit?
No tissue shall endure a temperature increase of greater than 1 degree C.
FDA limits are 4 W/kg over 15 min and 3 W/kg over 10 minutes.
What is Antenna Effect?
Occurs when an electric field resonant coupling with a wire- heating
Abandoned intracardiac pacemaker lead are at risk for this and therefore contraindicated.
Temporary epicardial leads are supposedly safe.
What are abandoned intracardiac pacemaker leads at risk for?
Antenna effect - heating
How often is QC done on MR scanners?
Weekly and yearly
Weekly (by technologist): Center frequency, table positioning, setup and scanning, geometric accuracy, high contrast resolution (verified by phantom), low contrast resolution, artifact analysis, film quality control, and visual checklist
Yearly (by medical physicist or MR scientist): Magnetic field homogeneity, slice position accuracy, slice thickness accuracy, radiofrequency coil check, display monitor check
Who does the weekly QC?
Technologist
Center frequency, table positioning, setup and scanning, geometric accuracy, high contrast resolution (verified by phantom), low contrast resolution, artifact analysis, film quality control, and visual checklist
Who does the yearly QC?
Medical physicist or MR scientist
Magnetic field homogeneity, slice position accuracy, slice thickness accuracy, radiofrequency coil check, display monitor check
What are the weekly QC items?
Center frequency, table positioning, setup and scanning, geometric accuracy, high contrast resolution (verified by phantom), low contrast resolution, artifact analysis, film quality control, and visual checklist
Done by technologist
What are the yearly QC items?
Magnetic field homogeneity, slice position accuracy, slice thickness accuracy, radiofrequency coil check, display monitor check
Done by Medical physicist or MR scientist
What are the 4 MRI zones?
Zone I: No restriction
Zone II: No restriction - waiting room and dressing room - screen patients and control access to Zones 3 and 4.
Zone III: Restricted Room - control room - locked door between 2 and 3.
Zone IV: Restricted room - actual MRI scanner
What is MRI Zone I?
No restriction
What is MRI Zone II?
No restriction - waiting room and dressing room - screen patients and control access to Zones 3 and 4 - locked door to zone 3
What is MRI Zone III?
Restricted Room - control room - door locked between 2 and 3.
What is Zone IV?
Restricted room - actual MRI Scanner
What MRI zones are restricted?
Zones 3 and 4
What are the guidelines restricting entry to Zones 3 and 4?
Must complete screening form, etc.
No exceptions - if code - MRI techs stabalize and get them out of zone 4 (preferrably to zone 2) for the code team.
What does MRI scanning start with?
Focused history to identify patients who may have metal in them
What happens if a person is unable to be screened and answer questionaire?
Ask family members, consult the medical record, and possibly do screening x-rays as needed.
What MRI zones are restricted?
Zones 3 and 4
What are the guidelines restricting entry to Zones 3 and 4?
Must complete screening form, etc.
No exceptions - if code - MRI techs stabalize and get them out of zone 4 (preferrably to zone 2) for the code team.
What does MRI scanning start with?
Focused history to identify patients who may have metal in them
What happens if a person is unable to be screened and answer questionaire?
Ask family members, consult the medical record, and possibly do screening x-rays as needed.
