Review on parameters Flashcards
What are the parameters that scan time is directly affected by?
TR
Phase matrix
NEX
ETL
Rectangular FOV (created by reducing the number phase encodings) ____ scan time while maintaining spatial resolution (but ____ SNR).
a) reduces, reduces
b) reduces, increases
c) increases, reduces
d) increases, increases
a) reduces, reduces
___ FOV = ↑ SNR
↑ FOV = ↑ SNR
___ FOV = ↓ spatial resolution
↑ FOV = ↓ spatial resolution
___ matrix = ↓ SNR
↑ matrix = ↓ SNR
___ matrix = ↑ spatial resolution
↑ matrix = ↑ spatial resolution
___ slice thickness = ↓ SNR
↓ slice thickness = ↓ SNR
___ slice thickness = ↑ spatial resolution
↓ slice thickness = ↑ spatial resolution
How does changing receive bandwidth affect SNR?
Recall: receive bandwidth is the range of frequencies sampled during application of the readout gradient.
Decreasing bandwidth results in less noise being sampled relative to the signal.
↑receive bandwidth = ↓SNR
↓ receive bandwidth = ↑SNR
Halving bandwidth increases SNR by about 40%.
Reducing bandwidth also increases chemical shift artefact, and increases the minimum TE available.
↓ receive bandwidth = ↑chemical shift artefact
How does changing NEX affect SNR?
Recall: NEX is the number of times data is collected (i.e. number of times each line of K space is filled).
Data contains both signal and random noise – doubling NEX therefore doubles the amount of signal collected, but also the random noise.
↑NEX = ↑SNR
Doubling NEX only creates a √2 = 1.4 (or 40%) increase in SNR
2x NEX = 1.4x SNR
4x NEX = 2x SNR
How does changing TE affect SNR?
↑TE = ↓SNR
-Therefore, sequences with short TE’s (T1- and PD-weighted) have greater SNR than those with long TE’s (T2-weighted).
A coarse matrix has a:
i) low number of frequency encodings and/or phase encodings
ii) high number of frequency encodings and/or phase encodings
iii) No frequency encodings and/or phase encodings
i) low number of frequency encodings
A fine matrix has a:
i) low number of frequency encodings and/or phase encodings
ii) high number of frequency encodings and/or phase encodings
iii) No frequency encodings and/or phase encodings
ii) high number of frequency encodings and/or phase encodings
A range of freq. that are sampled during readout is known as___
receive bandwidth
A range of freq. transmitted in an RF excitation pulse is known as___
transmit bandwidth
The outer lines of K-space are filled with…?
high-resolution data
The central lines of K-space are filled with…?
signal & contrast data
What are the 3 factors that control contrast for an inversion recovery sequence?
TR
TE
TI
A spin echo pulse sequence preceded by a 180 RF pulse is…..
an inversion recovery pulse sequence
An inversion recovery sequence with a short TI is…
a STIR
An inversion recovery sequence with a long TI is…
a FLAIR
The time between the 180° preparation pulse and the 90° excitation pulse during an inversion recovery sequence is known as…
TI (inversion time)
What type of pulse sequence does a 90° RF followed by 180° RF produce?
spin echo
A 90° RF followed by a train of 180° RF pulses produces what type of pulse sequence?
fast spin echo (FSE)
What type of pulse sequence does a 180° RF followed by 90°RF produce?
inversion recovery
What type of pulse sequence is produced by using a variable RF pulse followed by a gradient?
gradient echo
Which of the following is a benefit of increasing NEX?
a. Decreases T1 weighting
b. Increases SNR
c. Decreases slices
d. Increases scan time
increases SNR
What increases the image resolution at the expense of reduction of SNR?
thinner slices
increasing matrix size
decreasing FOV
What is the difference between high and low bandwidths?
High bandwidth: reduce the time of sample = faster scans and shorter TEs, but decreases the SNR
Low bandwidth: improves the SNR but increases the min TE
What are the consequences of decreasing Phase Matrix?
reduce scan time
reduce reso
high SNR
increase truncation artifact
How do you improve low SNR?
increase FOV
decrease matrix
increase slice thickness
decrease bandwidth
How do you improve poor resolution?
increase matrix
decrease slice thickness
decrease FOV
How do you improve chemical shifts?
increase bandwidth
How many lines of k-space are filled per echo?
1 line of k-space
Where are a series of 0s and 1s series are stored in k-space?
all over the MRI image
How do frequency and phase fill in k-space?
frequency = row
phase = column
What are the 2 benefits of Parallel Imaging?
decrease scan time
lower SAR
What are the 2 disadvantages of Parallel Imaging?
low SNR
reduce artifacts
Does Frequency Matrix have any direct effect on scan time?
No, only Phase Matrix has
What are the multi-coil benefits without Parallel Imaging?
Better SNR
What are the advantages of FSE?
more NEX
more matrices
improve scan time
What are the disadvantages of FSE?
image blurring
increased SAR
In FSE, what drives longitudinal recovery?
driven equilibrium in shorter TR
What makes fat brighter in a T2?
FAT SAT
STIR
What happens when you increase ETL per TR?
less slices available per TR
Does k-space fill in order?
No. It’s filled out of order
Which technique fills all k-space in one TR?
single shot spin echo
Where is Effective TE stored in FSE?
in the center of k-space (due to shallow gradient slope)
What does the FLAIR sequence suppress?
CSF
What does the STIR sequence suppress?
fat
