Module 5: Parameters and Trade-Offs Flashcards
Define signal to noise ratio (SNR), contrast to noise ratio (CNR), and spatial resolution (SR). Understand which parameters influence SNR, CNR, resolution, and scan time. Define voxel, saturation band, chemical pre-saturation, partial voluming, rectangular field of view, isotropic, and anisotropic. Understand that for every parameter manipulation there are trade-offs, and be able to explain what the trade-offs are for each parameter as well as its effects (advantages and disadvantages).
What is the signal to noise ratio?
ratio of the amplitude of the signal received to the average amplitude of the noise
What is signal?
Voltage induced in the receiver coil
What is noise?
the random background frequencies that are always present (due to the magnet, patient, and electrical systems).
is noise the same for each patient?
Noise is constant for each patient, but varies between patients depending on their age, height, weight, anatomy being scanned, and the inherent noise of the system.
What are 7 factors that affect SNR?
- Magnetic field strength of the scanner (e.g. 1.5T, 3T)
- Proton density of the area being examined
- Voxel volume
- TR, TE, flip angle
- NEX (or number of average)
- Receive bandwidth
- Coil type
How does increasing magnetic field strength affect SNR?
↑Magnetic Field Strength = ↑SNR
True or False:
You can increase magnetic field strength when performing scans
False. Magnetic field strength is set for the machine.
Low field systems therefore are inherently low SNR
How does decreasing proton density affect SNR?
↑Proton Density = ↑SNR
True or False:
Areas of low proton density (e.g. lungs) have low signal and therefore low SNR; areas of high proton density (e.g. abdomen) have high signal and therefore high SNR.
True
How does increasing voxel volume affect SNR?
↑Voxel Volume = ↑SNR
Pixel area = _____ ÷ matrix size
Pixel area = FOV dimensions ÷ matrix size
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
Which voxels contain more spins? Large voxels or small voxels?
Large voxels contain more spins (protons) and therefore have higher SNR (smaller voxels that contain fewer spins and therefore have lower SNR)
Voxel volume can be changed by changing what?
Changing slice thickness
- ↑slice thickness = ↑voxel size = ↑SNR
Changing theimage matrix
- Assuming same FOV: ↑image matrix = ↓voxel size = ↓SNR
Changing the FOV
- Assuming same matrix: ↑FOV matrix = ↑voxel size = ↑SNR
Note: 2x FOV = 4x SNR
Doubling the FOV doubles the pixel dimension along both the phase and frequency axes (but not slice thickness), thereby increasing the SNR by a factor of 4.
Which generally have more signal, spin echo (SE) or gradient echo (GRE) sequences.
Spin echo (SE) sequences generally have more signal than gradient echo (GRE) sequences.
How does changing Flip angle affect SNR?
↓flip angle = ↓SNR
How does changing TR affect SNR?
↑TR = ↑SNR
-Controls amount of longitudinal magnetization that is allowed to recover before the next excitation pulse. Long TR allows full recovery, so more is available to be flipped with the next repetition.
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).
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 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
What are the three types of coils we can place?
- Quadrature coils:
Two coils are used to receive signal, slightly increasing SNR. - Phased Array coils:
Multiple coils are used to receive signal, greatly increasing SNR. - Surface coils:
Coil is placed very close to the surface of the area being scanned, thereby increasing SNR.
Do Quadrature coils, phased array coils and surface coils increase or decrease SNR?
increase
If using a coil which size coil should you use?
Size of the coil should be the same size (or slightly larger) than the area being imaged (so that it optimally fills the sensitive volume of the coil).
Do large or small coils increase the risk of aliasing (wrap-around)? because tissues outside the FOV are more likely to also produce signal.
Large coils, because tissues outside the FOV are more likely to also produce signal.
For maximum signal how should coils be placed?
coils should be positioned in the transverse plane perpendicular to B0.
(Tilting the coil reduces SNR.)
What is CNR?
Contrast to noise ratio, the difference in SNR between 2 adjacent areas
What are 4 ways CNR can be increased?
1) Using a T2 weighted scan
2) Using contrast agents (Gadolinium)
3) Using chemical pre-saturation technique:
4) Using magnetization transfer contrast (MTC)
What are three components that control voxel size?
1) FOV
2) Matrix Size
3) Slice Thickness
Do small or large voxels lead to partial voluming?
Large voxels result in lower spatial resolution because the individual signal intensities are averaged together rather than being distinctly represented in the image. This results in partial voluming.
___ slice thickness = ↑ spatial resolution
↓ slice thickness = ↑ spatial resolution
___ slice thickness = ↓ SNR
↓ slice thickness = ↓ SNR
___ matrix = ↑ spatial resolution
↑ matrix = ↑ spatial resolution
___ matrix = ↓ SNR
↑ matrix = ↓ SNR
___ FOV = ↓ spatial resolution
↑ FOV = ↓ spatial resolution
___ FOV = ↑ SNR
↑ FOV = ↑ SNR
Which provides better spatial resolution, square or rectangular pixels?
Square pixels always provide better spatial resolution than rectangular pixels
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
How is scan time calculated for conventional MRI acquisitions?
TA = TR x NEX x Ny
Where TA is acquisition time, Ny is the number of phase encoding steps, and NEX is number of signal averages.
What are 3 things that scan time is affected by?
TR: ↑ TR = ↑ scan time
(2x TR = 2x scan time)
Phase matrix: ↑ matrix = ↑ scan time
(2x matrix = 2x scan time)
NEX: ↑ NEX = ↑ scan time
(2x NEX = 2x scan time)
Name one advantage and one disadvantage of volume imaging?
Advantages:
-No slice gaps (slice thickness does not affect SNR, no gaps in anatomy)
Disadvantage:
-Log scan time
How is scan time calculated for volume imaging?
TA = TR x NEX x Ny x Nz
T/F For volume imaging voxels must be isotropic?
True
What are some common uses for volume imaging?
- Useful for imaging joints (especially the knee)
- Where anatomy can be confusing and runs across multiple planes.
- Used for following ligaments, arteries, etc, that cross over the imaging plane
- Viewing very small lesions (voxel sizes of 1mm or less can be achieved).
