Flashcards in Module 5 Deck (36)
What is signal to noise ratio (SNR)?
amplitude of signal vs amplitude of noise
What factors cause variations in noise ?
different patient's height, weight, anatomy being scanned and inherent noise of the system
which of the following does not affect SNR?
a. Voxel volume
b. Sending bandwidth
c. Coil type
d. Flip angle
B. Sending bandwidth does not affect SNR it is receiving bandwidth
Why does higher magnetic field strength cause higher SNR?
Higher energy results in more of a gap between high and low energy nuclei which allows for more magnetization to be available
True or false: high proton density results in high SNR
What parameters make up voxel volume?
1. Slice thickness
What is the difference between a coarse matrix and a fine matrix?
Coarse matrix has a low number of frequency encodings and a fine matrix has a large number of frequency encodings?
Which has smaller pixels a fine or coarse matrix?
Fine matrix has smaller pixels
Complete the following statement :
Large pixels have more (1)___________ resulting in a (2)_________ SNR
If you decrease flip angle what will happen to SNR?
SNR will decrease if you decrease the flip angle
However scan time will also decrease !
Why does increasing TR increase SNR?
Increased TR allows for more time for recovery so more is available to be flipped.
If TE is long more decay occurs how does this affect the signal?
Less signal which means less SNR
Why is increasing Nex not the best method of increasing SNR?
It is not the best method of increasing SNR because it increases the scan time significantly without enough of an increase in SNR to make it worth it.
What is Nex?
Nex is the number of times data is collected
Halving bandwidth increases SNR by:
Why do small pixels have better spatial resolution than large pixels?
Small structure can be differentiated easier when the pixels are smaller.
What is it called when the increase in pixel size causes areas of light and dark to be averaged together resulting in a gray area?
What happens to spatial resolution and SNR when slice thickness is decreased?
SNR decreases but spatial resolution increases
Increasing FOV results in ____(1)____ pixels and ____(2)___ spatial resolution.
How does increasing matrix size effect SNR? What happens to resolution ?
Results in lower SNR but improves resolution
How does an operator of a MRI machine alter pixel geometry ?
Selecting FOV dimensions, slice thickness and image matrix.
Which pixel shape is better for spatial resolution : square or rectangle?
What situation would a rectangular FOV be beneficial? Why?
If the anatomy being scanned is smaller in the phase axis than in the frequency axis?
It reduces scan time but also maintains spatial resolution
When TR, matrix size and NEX are increased what happens to scan time ?
Scan time is increased.
True or false : SNR is the most important factor in image quality.
What adjustments can be made when scanning an anatomical area of low signal?
More NEX and larger voxel size
What 2 variables increase when receive bandwidth is decreased ?
SNR and chemical shift artefact
Which type of coil increases SNR more: Quadrature or Phased array?
What is the optimal coil position?
Transverse plane perpendicular to B0