MRI Pulse sequences
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A gradient echo sequence lacks a _______, making it ____________.
- 180* refocusing RF pulse
- more susceptible to magnetic field inhomogeneities
Gradient echo sequences tend to be ______, due to the lack of 180* refocusing pulses, the TR______, and scan time______.
In a gradient echo sequence what is used to refocus the protons and create the echo?
What kind of characteristics do gradient echo sequences yield?
T1 or T2*
When does the condition of steady state occur? Results in?
- When the TR in a gradient echo is less than the T2
- residual transverse magnetization at the time of the next excitation pulse
What do T2* gradient echo require?
high signal from fluid
What do balanced steady state sequences result in?
high SNR image with bright fluids
What are balanced steady state sequences useful for evaluating?
What does a spoiled gradient echo sequence do?
Removes any residual transverse magnetization prior to next excitation pulse
Why is "spoiling" used?
- to reduce stead state (T2*) effect
- allows for fast scans with T1 contrast
What is RF spoiling?
When additional RF pulses spoil away the steady state effect (residual transverse magnetization) before the next excitation
What is a spoiled gradient echo also called?
Incoherent gradient echo imaging
Conventional spin echo sequences begin with?
90* RF excitation pulse
What refocuses a decaying spin echo?
180* RF pulse
How many lines of Kspace are acquired during a conventional spin echo?
How many lines of Kspace are filled per TR in fast spin echo sequences?
multiple (2-50+) lines
In a spin echo sequence, what is the time between the 90* RF pulse and the 180* RF pulse?
1/2 TE time (Tau)
How is an echo produced in a spin echo pulse sequence?
from a combination of 2 or more RF pulses
A dual echo spin echo sequence generates ______, both with _______, but the ______
- two images for every slice
- varying TE's
- same TR
In dual contrast spin echo sequences, the second echo image has ______, but ______
than the first echo image
- more T2 contrast
- Lower SNR
In Fast spin echo sequences if the ETL is increased, the scan time is _____, and SNR is _____
In a fast spin echo the effective TE are what?
The echoes that are encoded with a low amplitude phase encoding gradient
If ETL is increased what can occur?
Increase in image blurring
In FSE the number of shots is calculated by what formula?
# phase encodings ÷ ETL
What is an inversion recovery sequence?
Sequence begins with 180* RF pulse followed by a 90* RF excitation pulse
In and IR sequence what does the 180* RF pulse do?
Nulls signal from fat or fluid
What is the TI (inversion time)?
The time interval between the 180* inverting RF pulse and the 90* RF excitation pulse
If the desire is to null the signal fro a specific tissue using an inversion recovery, what should the inversion time be?
69% of the T1 relaxation time of that tissue
What are STIR sequences typically used for?
Evaluation of compression fracture, lesions within retro-orbital fat, musculoskeletal contusions and fat suppression
To null the signal from fluid in FLAIR sequence what TI should be used?
What are T2 FLAIR sequences typically used to evaluate?
Periventricular white matter
What is the TR?
time between two 90* RF excitation pulses
What is the TE?
The time between the 90* RF pulse and the peak of the signal in the receiver coil
What will increase proton density weighting?
Lengthening TR with a short TE in a spin echo sequence
What affect does TE have on number of slices?
TE increases = # of slices decreases
What affect does TR have on # of slices?
TR increases= # of slices increases
Scan time Formula
TR x Matrix x NEX ÷ ETL
If ETL increases, scan time _____
What sequence is the most INSENSITIVE to inhomogeneity?
Spin echo sequences
What gradient is on during production of the echo?
Frequency encoding gradient (readout)
What is the fastest sequence commonly available?
Echo planar imaging
90* RF followed by 180* RF
Fast spin echo
90*RF followed by train of 180* RF pulses
180* RF followed by 90* RF
variable degree RF followed by gradient to produce echo
EPI (echo planar imaging)
a train of gradient echoes
_____ the flip angle yields images with ____T1 information
______ the TE yields images with _____ T2 information
______the flip angle _____ SNR up to the _____
- Ernst angle
What factors will lengthen scan time?
- Increasing TR
- Decreasing the # ETL
- Increasing NEX
- Decreasing parallel imaging factor
What factors will shorten scan time?
- Decreasing TR
- Increasing the # ETL
- Decreasing NEX
- Enabling half scan
- Increasing parallel imaging factor
What factors will improve SNR?
- Increasing TR
- Reducing phase matrix
- Increasing FOV
- Reducing (narrowing) receive bandwidth
- Increasing NEX/NSA
- Reducing # ETL
What will decrease the SNR?
- Decreasing TR
- Reducing Pixel size
- Decrease FOV
- Increasing Receiver bandwidth
- Decrease NEX
- Increase # ETL
What would you like to do?
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