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Flashcards in Module 5 Deck (36)
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1

What is signal to noise ratio (SNR)?

amplitude of signal vs amplitude of noise

2

What factors cause variations in noise ?

different patient's height, weight, anatomy being scanned and inherent noise of the system

3

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

4

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

5

True or false: high proton density results in high SNR

True

6

What parameters make up voxel volume?

1. Slice thickness
2. phase
3. frequency

7

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?

8

Which has smaller pixels a fine or coarse matrix?

Fine matrix has smaller pixels

9

Complete the following statement :
Large pixels have more (1)___________ resulting in a (2)_________ SNR

1. protons
2. higher

10

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 !

11

Why does increasing TR increase SNR?

Increased TR allows for more time for recovery so more is available to be flipped.

12

If TE is long more decay occurs how does this affect the signal?

Less signal which means less SNR

13

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.

14

What is Nex?

Nex is the number of times data is collected

15

Halving bandwidth increases SNR by:
a. 20%
b.60%
c. 50%
d. 40%

d. 40%

16

Why do small pixels have better spatial resolution than large pixels?

Small structure can be differentiated easier when the pixels are smaller.

17

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?

Partial voluming

18

What happens to spatial resolution and SNR when slice thickness is decreased?

SNR decreases but spatial resolution increases

19

Increasing FOV results in ____(1)____ pixels and ____(2)___ spatial resolution.

(1) larger
(2) decreased

20

How does increasing matrix size effect SNR? What happens to resolution ?

Results in lower SNR but improves resolution

21

How does an operator of a MRI machine alter pixel geometry ?

Selecting FOV dimensions, slice thickness and image matrix.

22

Which pixel shape is better for spatial resolution : square or rectangle?

Square

23

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

24

When TR, matrix size and NEX are increased what happens to scan time ?

Scan time is increased.

25

True or false : SNR is the most important factor in image quality.

True

26

What adjustments can be made when scanning an anatomical area of low signal?

More NEX and larger voxel size

27

What 2 variables increase when receive bandwidth is decreased ?

SNR and chemical shift artefact

28

Which type of coil increases SNR more: Quadrature or Phased array?

Phased array

29

What is the optimal coil position?

Transverse plane perpendicular to B0

30

What is contrast to noise ratio

The difference in SNR between 2 adjacent points