MRI Pulse Sequences Flashcards
What is a pulse sequence?
A series of RF pulses and gradient events, fired to excite the spins and induce a signal in the reeive coil, in a short time (TR)
What are the goals of imaging sequences?
- To generate an RF signal perpendicular to static magnetic field
- To generate good tissue contrast
- To minimize artifacts
- To maximize the SNR
- To acheive all the above, in the shortest possible scan time
What is a Spin Echo?
The basic sequence (with good contrast)
-90-180 echo
The function of a spin echo is to:
- generate T1, PD (proton density), and T2 weighted images
- minimizes artifacts
List some advantages of spin echo imaging.
- High signal to noise (SNR)
- Least artifact prone sequence due to 180 refocusing pulse
- Contrast mechanisms accurate and easy to understand
List some disadvantages of spin echo imaging.
- High SAR (how much RF you put into the patient) because of 90 degree and 180 degree RF pulses
- Long scan time due to single encoding per TR
Scan time Equation
Scan time = TR x phase resolution x averages (nex)
T1 weighted has a short/long TR & short/long TE.
short TR & short TE
T2 weighted was a short/long TR and short/long TE.
long TR and long TE
A PD weighted image has a short/long TR and short/long TE.
long TR and short TE
In a T2 weighted image, fluid appears _.
bright
What occurs during Fast/Tubo Spin Echo?
- Multiple 180 pulse is used
- For one excitation (90) there are many 180 pulses -> echoes
- Multiple phase encodes per TR
- Amplitude of echoes decrease as the time from 90 increases due to T2 decay
List some advantages of fast/turbo spin echo.
- Much faster sequence than SE (spien echo)
- Has nearly the same contrast as that of SE
- Magnetic susceptibility effects are minimized
- Artifact due to metallic implants are reduced (inhomogeneity reduced)
What is the scan time equation for fast/turbo spin echo?
Scan time = TR x phase resolution turbo factor x average (nex)
List some disadvantages of fast/turbo spin echo.
- The weighting is not perfect due to multiple TEs -> it works better for T2 than T1 images
- Multiple 180 pulses make fat look bright on T2 weighted images
- Magnetic susceptibility is reduced so the sequences is not good for Hemorrhages or head trauma
- Image blurring might occur (SNR of the last echoes that are filling the k-space border could be very low)
- Very high SAR (due to multiple 180 pulses (high chance of burning)
What is Single Shot Fast Spin Echo (SS FSE)?
Fills half of k-space in one TR! Large echo trains, uses half Fourier to predict the rest of the k-space; very fast but has low SNR
What is a gradient echo?
the simplest sequence; also known as gradient recalled echo (GRE)
What are the three main parameters of a gradient echo and what are its effects?
3 main parameters: TR, TE, & flip angle
* low SAR
* Artifact prone
* weak signal (as compared to SE)
What is a partial flip angle?
flip/rotate a small amount of magnetization (no more than 90) = fast recovery
T2 decay occurs between the _ and the _ gradients.
dephasing; rephasing
Rephasing incompletely recovers the _.
signal
Signal loss is greater with longer/shorter TEs
longer
Gradient Echo imaging cannot recover signal losses from:
- magnetic field inhomogeneity
- magnetic susceptibility
- water-fat incoherence
List some advantages of Gradient Echo.
- Faster imaging-can use shorter TR and short TEs than SE
- Low flip angle -> deposits less energy (more slices per TR than SE, decreases SAR)
- Compatible with 3D acquisitions
List some disadvantages of gradient echo?
- Difficult to generate good T2 weighting
- T2* relaxation influences contrast
- Magnetic field inhomogeneity cause signal loss
- worse with increasing TE times
- -susceptibility effects
- dephasing of water and fat photons
What is an Inversion Pulse?
Initial 180 RF pulse which is followed by the excitation (90 pulse)
What is Inversion T1 time?
time from inital 180 pulse to 90 pulse
What are the advantages and disadvantages of inversion recovery (IR)?
Disadvantages: longer imaging time, fewer slices
Advantages: better T1 contrast
How is suppression achieved?
by sampling, as the longitudinal magnetization of the tissue (a) is at the zero crossing
What does STIR stand for?
Short Tau Inversion Recovery
What does STIR do?
Short Tau Inversion Recovery:
nullifies fat signal; short T1; very useful in musculoskeletal and body imaging
What does FLAIR stand for?
Fluid Attenuated Inversion Recovery
What is FLAIR and what is it used for?
Fluid Attenuated Inversion Recovery:
A turbo-spin echo sequence with IR; routinely used for brain imaging; long TR and T1, dark CSF; bright tumors, fat and bleeds; very sensitive to parameter change
True or false. Water and fat resonate at slightly different frequencies.
True
A gradient echo image used in-phase looks _ and out-of-phase looks _.
bright; black outline around organs
Ernst Angle
A defined flip angle less than 90 degrees used to optimize contrast of a given tissue or fluid
What is SPGR?
Spoiled (incoherent) Gradient Echo- spoiling destroys the accumulated transverse magnetization; it is done by an additional de-phasing gradient; it can also be done by RF (phase detection)
What is the advantage of Spoiled Gradient Echo (SPGR)?
it maximizes T1 contrast
What is FLASH?
Fast Low Angle Shot (RF or Gradient Spoiled)
FLASH
T1 Contrast has a short/long TE, short/long TR, large/small flip angle.
short TE, short TR, large flip angle
Spoiled Gradient Echo T2* Contrast has short/long TE, short/long TR, and small/large flip angle.
long TE, long TR, small flip angle
What is EPI?
Echo Planar Imaging- uses gradient echo
starts with 90 -> 180 -> gradient echo
What are the advantages of EPI?
very fast sequence, all k-space in one TR, and 3D image in below a minute
What are disadvantages of EPI?
low SNR, very noisy, and very susceptible to artifacts, specifically susceptibility
Why are gradient echo images used?
due to short TR times
In 3D imaging, an additional _ is added perpendicular to the imaging plane.
phase encoding gradient
Partitions
planes of the 3D volume
What is MPRAGE?
Magnetization Prepared Rapid Acquisition Gradient Echo- Fast 3D Gradient Echo with good T1 contrast that has an initial 180 inversion pulse followed by GRE sequence with rewinding gradient
MPRAGE
-In order to produce 3D images, a _ occurs in the slice selection direction
secondary phase encoding
What are the different k-space fillings?
linear, centric, spiral, and propellar/blade
Which k-space filling is less motion sensitive?
spiral
What is parallel imaging?
using multi-channel coils
Which k-space filling corrects for motion?
propeller/blade
