5: Pulse Sequence Design Flashcards
Pulse sequence
the process of acquiring patient data by disorienting net magnetization of protons with radio-frequency (RF) pulses, gradient variations, and data collection
Traditional standard pulse sequences are: (4)
- Spin Echo (SE)
- Inversion Recovery (IR)
- Gradient Echo (GE)
- Echo Planar Imaging (EPI)
Pulse Sequence Timing Diagram:
visualizes detailed timing and duration of RF pulses and gradient variations
Echo time (TE)
the time it takes to complete an entire pulse sequence
Events recorded by the Pulse Sequence Timing Diagram: (5)
- RF Transmission (RF-t)
- RF Receive (RF-r)
- Slice select Gradient
- Readout
- Phase
- Each event can initiate or pause another event*
Musical Note Analagy
- imagine each different pulse sequence being its own unique string of musical notes
- in music, there are different sequences of musical notes that have different overall sound and rhythm combinations
- in MRI, there are different sequences of events that create different tissue characteristics and contrast combinations
Step 1: Radio Frequency Transmitter (RF-t)
Excitation event, Applied RT pulse, the beginning of any pulse sequence.
The events that excite or flip net magnetization into the transverse direction.
Begins with a 90 degree initial pulse followed by a 180 degree pulse (Double the strength). 1/2 of a TE period
Step 2: RF-Receive (RF-r)
Dephasing step, begins simultaneously during the 90 degree and 180 degree pulses (RF Transmission)
known as the dephasing event because once the net magnetization is disoriented by the 90 degree pulse, the signal immediately begins to dephase or weaken.
Dephasing
Progressive weakening of the net magnetization
Dephasing is also known as:
Free Induction Decay (FID)
Step 3: Slice Select Gradient:
Also turns on during RF Transmission similar to RF Receive
allows a certain slice to be singled out (spatial localization), allowing us to attain an image of a specific slice.
Step 4: Readout Gradient
Sampling gradient
As RF Transmit, RF Receive, and Slice selection occur, the readout gradient reads or samples the signal through (Frequency encoding)
Turned on during frequency encoding and during the resultant echo. Reads during the echo.
Step 5: Phase Gradient
Turned on between the 90 degree and 190 degree pulses, storing the information.
the phase gradient is solely determined by the number of phase encoding lines in a slice. Ex: 256 (frequency) x 192 (phase) = 192 phase encodes
higher the matrix, longer the time : directly correlated.
Phase encoding: during the echo
Two types of Spin Echo
- Conventional Spin Echo (CSE) (older)
- Fast Spin Echo (FSE) (modern)
Conventional Spin Echo
What images does it produce?
How many echo’s per TE period?
Traditional (not used)
T1, T2, or Proton Density Weighted
Contain ONE echo per TE period
Due to scan time length being long, CSE’s have been innovated and are rarely used.
Fast Spin Echo (FSE)
Aka Turbo Spin Echo (TSE) Siemens
Innovated and faster than CSE due to the added Echo Train Length (ETL)
T1, Td, or Proton Density Weighted
FSE’s ETL is chosen MANUALLY by the technologist during pulse sequence manipulation
More ETL, more echo phases, but progressively gets smaller/weaker as you move away from the initial 90 degree pulse
Shorter ETL, stronger the signal, longer the scan time.
Inversion Recovery (IR)
T1 environment (relaxation) by flipping the net magnetization with a 180 degree pulse
IR is a pulse sequence that contains an additional 180 degree pulse before the 90 degree pulse
Inversion time (TI)
time from the initial 180 pulse to the 90 pulse, First 180 then 90. Take longer
Echo Train Length (ETL)
FSE’s ETL is chosen MANUALLY by the technologist during pulse sequence manipulation, during Fast Spin Echo (FSE)
More ETL, more echo phases, but progressively gets smaller/weaker as you move away from the initial 90 degree pulse
Shorter ETL, stronger the signal, longer the scan time.
Two types of Inversion Recovery (IR)
- STIR (Short Tau (time) Inversion Recovery)
- FLAIR (FLuid Attenuated Inversion Recovery)
TI
the time before the initial 90 degree pulse
180 degree pulse causes…
short T1 tissues to relax (quickly or slowly)
long T1 tissues to relax (quickly or slowly)
Fat (bright or dark)
Water ( bright or dark)
short T1 tissues to relax quickly
long T1 tissues to relax slowly
Fat - dark
water- bright
*Contrast between short and long T1 tissue is created BEFORE the pulse sequence begins
STIR
Short Tau Inversion Recovery
widely used as a fat suppression technique. to suppress fat content and focus on high water content such as fractures and infection
Long TI 10-150 ms
Contrast agents DO NOT work or highlight in STIR imaging due to similar short T1 tissue property of fat and gadolinium
How does STIR suppress fat?
applying 90 degree pulse as Short T1 tissue (fat) returns to net magnetization.
we only image Long T1 tissue (fluid, edema from fracture in bone) and subtract fat tissue.
10-150 ms
How will fat look on a STIR? what anatomy?
saturated black
bone, bone marrow, tendon, muscle
How will water look on a STIR? what anatomy?
bright
Blood, edema, infection
FLAIR
Used to show/highlight:
Fluid Attenuated Inversion Recovery
suppress CSF and highlight diseased periventricular tissue such as infarction (stroke), brain/cord lesions (MS), subdural hemorrhage, and meningitis. Past Blood brain barrier (BBB)
Long TI time ( ~ 2000 ms)
Long TR time ( ~ 8000 - 9000ms )
How will CSF look on FLAIR?
saturated black or “null”
How will diseased tissue look on FLAIR?
Bright
Stroke, MS, etc.
If you have a patient MRI Brain who may not cooperate, what sequence should be first?
FLAIR followed by Diffusion Weighted Imaging Sequence
Difference between FLAIR and STIR?
Much longer TI in FLAIR, longer pulse sequence
4 Types of Gradient Echo
Gradient Recall Eco (GRE)
Spoiled Gradient Echo (Spoiled GE)
Steady State Gradient Echo (SS-GE)
Fast Gradient Recall Echo (Fast GRE)
Conventional Gradient Echo (GE) used to show 3 things…
Blood in brain, ligaments, cartilage
GRE imaging is completely different from FSE’s and IR’s for three reasons…
- RF Flip Angles are applied first, NOT initial 90 degree pulses
- Gradient pulses are used to refocus decay, NOT 180 degree pulses
- Slice and Readout gradients are used to balance the phase gradient
Spoiled Gradient Echo’s (Spoiled GE) differ from Gradient Recall Echo (GRE) mainly due to…
the addition of a spoiler pulse after the readout of a gradient echo
A spoiler pulse can be either…. or …
A - strong RF pulse
B - strong gradient pulse
** What is the reason for a spoiler pulse?
To cancel or terminate any remaining magnetization still in the transverse plane
Steady State Gradient Echo (SS-GE) or Coherent differ from Spoiled Gradient Echo’s (Spoiled GE) because
Steady state gradients want to preserve remaining transverse magnetization
Steady State Gradient Echo’s (SS-GE) preserve remaining transverse magnetization by _____, which allows…..
rephrasing, allows both relaxed and remaining net magnetization to both be present for the next initial RF flip angle.
What is the opposite of Steady State Gradient Echo? (SS-GE)
Spoiled Gradient Echo (Spoiled GE)
Another name for Coherent Echo
Steady State Gradient Echo (SS-GE)
What is the result of Steady State Gradient Echo (SS-GE)?
T2* Weighted Images
Fast Gradient Echo (Fast GE) requires 3 things…
- Short TR
- Short TE
- Smaller flip angle
Fast GRE gains contrast properties by…
establishing the contrast before the initial flip angle by applying RF pulses
T1 weighting Fast GRE: What degree of pulse is used?
180 degree pulse is used
T2 weighting Fast GRE: what pulse degree?
90 degree, 180 degree, 90 degree pulse combination is used
What is used at the end of a Fast GRE to accelerate the relaxation process?
A Spoiler
Echo Planar Imaging (EPI)
- The fastest and loudest MRI technique
GE - Fiesta
Echo Planar Imaging is good for…
IAC
The readout portion of the pulse sequence reads or ________ the signal data through ____________ encoding
Samples, Phase
Which pulse sequence has the largest TVMF (Time Varying Magnetic Field) effects?
Echo Planar Imaging (EPI)
The general public is limited to a ________ gauss magnetic field.
5 Gauss
- In some MR rooms, they will have a “5 Gauss Line” which shows how far you can proceed before significant pull from the fringe field. *
If a fast spin echo sequence has parameters set as: 224 x 256, 3 NEX, with a FOV of 24. How many phase encoding steps have to be filled in order to correctly produce a FSE?
224
- Remember * some vendors read matrix (frequency/phase) or (phase/frequency). Remember that the phase encoding steps will ALWAYS be the lower value and it can never be higher than frequency encoding. *
MRI Techs should consistently employ ______ asepsis cleaning techniques after each patient and at the end of every night.
Medical Asepsis
When positioning patient’s it is important not to loop coil cables thus reducing the chances of ________.
Decoupling
When scanning EKG leads and RF coils should be placed:
straight throughout the MR scanner to reduce decoupling.
Which pulse sequences employ an initial 180 RF pulse followed by a 90 RF pulse and another 180 RF pulse?
STIR and FLAIR
