MRI III and IV

Question 1

What is the formula for spin echo contrast?

Answer 1

S is prop to (rhoH)(1-exp(TR/T1))(exp-(TE/T2))

Question 2

What are typical T1 values for fat, CSF, White, and Grey Matter?

Answer 2

fat: 260ms
CSF: 2400
White: 780
Grey: 900

Question 3

What are typical TR and TE values used for T1 weighting?

Answer 3

shorter TR (400-600ms)
short TE (3-10ms) - should be close to T1

Question 4

Why would we use a T1 weighted image?

Answer 4

T1 generally gives more anatomical/Structural info

Question 5

What is Proton Density Weighting? What do we use for Tr, TE?

Answer 5

We wish to minimize contributions from T1 and T2 so use long TR and short TE.

Compared to T1 image, white and grey matter are inverted. So the sulcus in the brain folds will be lighter than other matter

Question 6

What are typical T2 values for fat, CSF, White, and Grey Matter?

Answer 6

fat: 80ms
CSF: 160
White: 90
Grey: 100

Question 7

What values should we choose for TR and TE for T2 weighting?

Answer 7

Long TR (2000-4000ms)
Larger TE (80-120ms)

Question 8

Fill in the table of parameter choices for different weightings:

param………….T1………………..proton……………..T2
TR
TE

Answer 8

param………….T1………………….proton………………….T2
TR…………..400-600……….2000-4000……..2000-4000
TE……………….5-30…………………5-30…………………6-150

Question 9

In which image (T1 weighted, T2 weighted, proton density weighted) is CSF the brightest? Grey matter? White matter? Fat?

Answer 9

CSF - T2, proton, T1
Grey - Proton, T1, T2
White - Proton, T1, T2
Fat - similar (we never want to image fat)

Question 10

What is inversion recovery spin echo? Why would we use it?

Answer 10

180……TI……90readout……TE/2……180……TE/2
Since the first pulse is 180, the Mzs all become -Mz, and we can choose the 90readout pulse when one of the recoveries is at zero, getting NO signal from that! Fat supression!

Question 11

What does STIR stand for? What can we see?

Answer 11

Short Tau Inversion Recovery. Fat.

Question 12

What does FLAIR stand for?

Answer 12

Fluid Attenuation Inversion Recovery. I think it allows us to see fluids going places? Tumors?

Question 13

What does slew rate mean? What is a typical slew rate?

Answer 13

Slew rate is how fast the coil can reach its max strength.

Typically 5-250 mT/m/ms

Question 14

How are SNR and Bandwidth related?

Answer 14

SNR is proportional to (BW)^-1/2

Question 15

What is “RePhasing”?

Answer 15

The equal and opposite area applied by the “rephasing gradient” during the slice selection pulse

Question 16

How do we localize in the x, y, z directions? In what order do we do this?

Answer 16

z: slice selection by only exciting one frequency
y: phase encoding gradient
x: frequency encoding

z, y, x

Question 17

How does phase encoding work?

Answer 17

We apply different phase shifts, very slightly changing the speed of rotation in different y locations. Vary this to get different lines. Strength of the pulse indicated phase shift.

You are always looking for a “special” amount of phase shift so apply different PEG strengths and this “special” amount will be a different y locations.

Question 18

Do we get more info from looking at lower or higher frequencies?

Answer 18

More comes from lower (in the centre of the diagram)

Question 19

How would we calculate image acquisition time?

Answer 19

t = (TR)*(#PEG steps)*(Nexcitations)
t = (600ms)*(192)*(2)

= 3.74 mins per slice!!

Question 20

What is a common pixel size for MRI?

Answer 20

In-plane resolution: 0.5 - 1.0 mm

Slice thickness: 5 - 10 mm

Question 21

Why do we have such a large slice thickness for MRI?

Answer 21

signal is very small so need large slices for decent SNR

Question 22

Would increasing B0 allow us to increase spatial res?

Answer 22

Yes! higher B0 means more signal so we can take smaller slices

Question 23

What are some negatives of increasing B0?

Answer 23

more RF absorption - tissue heating
T1 gets too long
artifact production

Question 24

Would the following increase or Decrease SNR?

Larger flip angle
LongerTR, shorter TE
Smaller pixel size
Thicker Slices
Larger FOV
More averages

Answer 24

Larger flip angle - increase
LongerTR, shorter TE - increase
Smaller pixel size - decrease
Thicker Slices - increase
Larger FOV - increase
More averages - increase

Question 25

What factors is SNR proportional to?

Answer 25

``` intrinsic signal intensity from pulse sequence voxel size (# of repeats in same voxel)^1/2 (BW)^-1/2 f(quality factor of coil) f(magnetic field strength) f(slice gap) f(reconstruction filter) ```

Question 26

Signal average is proportional to what?

Answer 26

proportional to square root of averages

Question 27

if we were to double the number of excitations, we would double the acquisition time, and get a gain of how much?

Answer 27

40%

Question 28

Why do we care about k-space symmetry

Answer 28

we use k-space symmetry to fill in the gaps thus reducing acquisition time

Question 29

If we narrow our bandwidth, do we need a steeper or less steep gradient?

Answer 29

we dont need as steep of gradient because we are more discriminatory between adjacent slice frequencies. But, we would need a longer sampling time.

Question 30

What happens when we put gradient coils closer to the patient? Further away?

Answer 30

Closer we get more patient loading (ie patient-induced eddy currents, meaning the coil can't read as well). Further, we get distortion artifacts

Question 31

How much does SNR increase when we increase B from 0.5T to 1.5T?

Answer 31

SNR increases by 3-5 times!

Question 32

Describe cross-excitation

Answer 32

Because the bandwidth is not perfectly rectangular, so some of the signal comes from adjacent slices. If we go slice by slice, this can lead to proton saturation. To avoid this, we "skip" slices and come back to them after.

Question 33

Name 3 main types of artifacts

Answer 33

Machine dependent Patient Dependent Signal Processing Dependent

Question 34

Discuss Machine-Dependent artifacts

Answer 34

Magnetic Field Inhomogeneities (Global and Focal) Global and be fixed with shims and sheilding Focal (caused by makeup, metal implants etc). This is when you are actually exciting protons in a different location than you think you are. Leads to signal voids, displacement, geometric distortions.

Question 35

What are susceptibility artifacts? Where are they?

Answer 35

susceptibility is ratio of internal to external marnetic fields they occur at tissue interfaces, especially near metal some pulse sequences are more susceptible to this than others

Question 36

What kind of distortion would a Gradient Field Artifact cause?

Answer 36

a stretch (oblong brain)

Question 37

RF artifacts cause cross excitation. How can we get rid of them?

Answer 37

Interleaving | Add a gap

Question 38

What causes "ghost images" and along which direction do we normally see them?

Answer 38

caused by motion artifacts. Often along FEG (x) direction.

Question 39

What is a chemical shift artifact? Are these worse for smaller or large B fields? Are they worse for stronger or weaker gradient fields?

Answer 39

It occurs when there is a difference in precessional frequencies of water and fat. this happens because the rest of a fat molecule shields the hydrogen and produces a change in the magnetic field. Causes a shift in anatomy, sometimes signal voids. Larger B0 makes these worse. Weaker gradients make them worse because of the smaller bandwidth.