Physics: Mri (Dwi And Dti)

Front 1

What does diffusion MR imaging look at

Back 1

the micromovement of molecules

Front 2

What is it called when a molecule moves in all directions

Back 2

isotropic

Front 3

What is it called when a molecules movement is confined to a given direction

Back 3

anisotropic

Front 4

Why is DWI mostly used to analyze the brain

Back 4

Most applications of the method are restricted to brain since DWI is intrinsically sensitive to movement; making application to the heart, and abdomen extremely challenging.

Front 5

What is a problem with analyzing the spinal cord with DWI

Back 5

pulsation artifact

Front 6

Are water molecules isotropic or anotropic in gray matter and CSF

Back 6

isotropic

Front 7

Are water molecules isotropic or anotropic in white matter

Back 7

anotropic

Front 8

What is diffusion

Back 8

Diffusion is a mass transport process arising in nature. It results in molecular or particle mixing without requiring bulk motion.

Front 9

What is the distance that a molecule will move

Back 9

The diffusion coefficient (DC) is the constant of proportionality between the root-mean-square displacement of a water molecule and the square root of time during which the motion occurred

Front 10

What is the root square mean displacement

Back 10

it quantifies the fraction of particles that will traverse a certain distance within a particular time frame.
x^2 = 2DΔ where x is mean squared displacement.

Front 11

What happens to the DC when there is a boundary surrounding a tissue

Back 11

many tissues show a decrease in the DC when boundaries of varying permeability restrict free diffusion

Front 12

What does the ADC measure

Back 12

the restriction caused by the permeability of certain boundaries

Front 13

What does it mean if the ADC is low

Back 13

there is a high level of restriction

Front 14

What influences ADC

Back 14

The ADC is influenced by the translational movement of water that occurs in the extracellular space via swelling or increased cellularity

Front 15

What are some obstacles that water molecules hit inside the body

Back 15

cell membranes, proteins, macromolecules, fibers…

Front 16

What is Diffusion imaging examining

Back 16

Extracellular fluid. Diffusion data provides indirect information about the structure surrounding these water molecules.

Front 17

What is an example of water molecules in the body that have free motion

Back 17

csf

Front 18

What are examples of water molecules that will have restricted isotropic motion
2

Back 18

abscess, tumor at high cell density

Front 19

What is meant by restricted anisotropic diffusion

Back 19

Certain structured tissues create obstacles that orientate the motion of the water molecules (tendency to displace themselves in one or several particular directions). Diffusion is only restricted in certain spatial directions.
Example: nerve fibers (organization in bundles of axons running in parallel, with concentric layers of myelin restricting transversal diffusion)

Front 20

What does DW MRI aim to look at

Back 20

DW-MRI aims at highlighting the differences in water molecule mobility, irrespective of their direction of displacement.

Front 21

What does DTI aim to look at

Back 21

Diffusion tensor MRI, on the other hand, studies the directions of water molecule motion to determine, for example, whether or not they diffuse in all directions (fractional anisotropy), or attempts to render the direction of a particular diffusion (which can be applied to indirectly reconstituting the nerve fiber trajectory).

Front 22

f

Back 22

w

Front 23

The aim of these diffusion-weighted sequences is to obtain images whose contrast is influenced by the differences in water molecule mobility.

Back 23

yes

Front 24

What does SE-EPI mean

Back 24

Spin Echo ultrafast Echo Planar Imaging

Front 25

How is the diffusion imaging done

Back 25

This is done by adding diffusion gradients during the preparatory phase of an imaging sequence

Front 26

Is SE-EPI T2 weighted

Back 26

yes, usually

Front 27

Is there a special gradient used in DWI

Back 27

yes, diffusion gradients

Front 28

What do diffusion gradients look like

Back 28

Front 29

What does the gradient (on the right) look like during the first during the first gradient before the 180 rephasing

Back 29

Notice that a specific area of water molecules is being analyzed.

Front 30

What does the gradient on the right look like after the second diffusion gradient (meant right on the other one too)

Back 30

the gradient is the same for both

Front 31

What happens to the water moleucles that are stuck in the middle and unable to move

Back 31

the spins of the immobile water molecules between the applications of the two gradients are dephased by the first gradient and rephased by the second.

NOTE THE RESTRICTED SIGNAL IS DARK

Front 32

What is the signal characteristics of when there is restricted diffusion

Back 32

Front 33

restricted diffusion is bright

Back 33

Front 34

What are the 3 types of types of freedom of motion that diffusion imaging looks at

Back 34

free diffusion
restricted isotropic diffusion
restricted anisotropic diffusion

Front 35

When is the diffusion gradient done

Back 35

the preparatory phase of SE-EPI sequence

Front 36

What is the point of having 2 different gradients for diffusion imaging

Back 36

The basic premise behind the addition of these “diffusion-weighting” gradients is that water molecules that are stationary during the application of the pulse sequence will be unaffected by the diffusion gradients. However, water molecules that move will experience a given amount of phase twist from the first gradient, but will have moved to a different location in physical space by the time the second gradient is applied. As a result, the phase twist provided by the first gradient will not be reversed by the second gradient resulting in a signal loss for those water molecules.

Front 37

What are the characteristics of diffusion that we should know about

Back 37

Front 38

Basic elements of a DWI pulse sequence. The diffusion-weighting gradients are of amplitude G, the duration of the gradients is d and the time between the start of the first gradient and the second gradient is Δ.

Back 38

Basic elements of a DWI pulse sequence. The diffusion-weighting gradients are of amplitude G, the duration of the gradients is d and the time between the start of the first gradient and the second gradient is Δ.

Front 39

What is the duration of a gradient

Back 39

d

Front 40

What determines the amount of diffusion weighting that is applied during a pulse sequence

Back 40

the amplitude

Front 41

What happens to the spins of the water molecules from the first gradient

Back 41

dephased

Front 42

What happens to the spins of the water molecules from the second gradient

Back 42

rephased

Front 43

What happens to the signal if the water molecule stays in the same location b

Back 43

bright

Front 44

If the water molecules migrates what happens to the signal of the water molecule from a particular voxel

Back 44

it is dark

Front 45

How do you determine if there is restricted anisotropic diffusion

Back 45

by applying gradients at 90 degrees to eachother

Front 46

What happens if there is a lot of fast moving water molecules

Back 46

Taking the entire population of water molecules in a voxel, the faster the water molecules diffuse, the more dephased they will be and the weaker the recorded signa

Front 47

What amplitude is chosen for slow diffusion

Back 47

lower amplitude.

Front 48

What is the b-factor

Back 48

This is the degree of diffusion weighting of the sequence

Front 49

What are 3 characteristics that determine the diffusion weighting (b-factor)

Back 49

gradient amplitude
application time
time between 2 gradients

Front 50

What does a low ADC value indicate

Back 50

high degree of restriction

Front 51

How many directions must diffusion gradients be applied

Back 51

in atleast 3 directions

Front 52

How is ADC determined

Back 52

Two diffusion sequences with different b-factors can be used to quantitatively measure the degree of molecular mobility, by calculating the apparent diffusion coefficient (ADC).

Front 53

Once ADC values for numerous voxels are obtained what is done

Back 53

they are put in to an ADC map/image (like the one that we see at our PACS station)

Front 54

So if the ADC value is low what is the signal that we will see

Back 54

An ADC hyposignal thus corresponds to a restriction in diffusion.

Front 55

When we do imaging how many DWI do we obtain

Back 55

3 (i think)

Front 56

From the information derived from the DWI images what do are we able to calculate

Back 56

ADC

Front 57

Within each DWI we obtain how many spatial directions are the gradients applied

Back 57

3 (X, Y and Z)

Front 58

What signal intensity corresponds to restricted diffusion on diffusion weighted images

Back 58

bright

Front 59

What is a common B-values used in clinical practice (B-values are the degree of diffusion weighting)

Back 59

B- 1000

Front 60

What if wanted an imaging that is not diffusion weighted

Back 60

B=0

Front 61

What is a problem with DWI

Back 61

there is going to be a T2 component so this means that that there may be brightness from restriction or brightness from T2 effect

Front 62

How does an ADC help the T2 shine through effect

Back 62

this does not and will be dark if there is restricted diffusion

Front 63

The stronger the b factor the greater the diffusion. What does that mean exactly

Back 63

The stronger the gradients, the longer the gradients are applied and the more time between gradients, the greater the b-factor.

Front 64

How is the ADC calculated

Back 64

twin acquisitions with different b-factors, the ADC can be calculated.

Front 65

This is what a diffusion weighted sequence really looks like

Back 65

Front 66

What are signal characteristics that are seen in DWI

Back 66

Front 67

What tissue is most commonly analyzed with DTI

Back 67

The microarchitecture specific to nerve tissues causes diffusion anisotropy in the white matter of the brain: water molecule diffusion preferably follows the direction of the fibers and is restricted perpendicularly to the fibers.

Front 68

What is the most common application of DWI

Back 68

stroke

Front 69

How does an acute stroke restrict fluid

Back 69

In acute cerebral ischemia, if the cerebral blood flow is lowered, the cell membrane ion pump fails and excess sodium enters the cell, which is followed by a net movement of water from the extracellular to intracellular compartment resulting in cytotoxic edema. Relative to the extracellular space, diffusion of the intracellular water molecules is restricted by intracellular organelles that comprise parts of the cytoarchitecture and the cell membranes

Front 70

What are the signal characteristics of DWI and ADC in stroke

Back 70

The restricted diffusion results in a decreased ADC and increased signal intensity on diffusion-weighted images

Front 71

What is a major difference between DWI and DTI

Back 71

the amounts of angles that diffusion gradient is is aquired

Front 72

How many spatial directions is the diffusion gradient applied in DWI

Back 72

usually 3

Front 73

How many direction is the diffusion gradient applied in DTI

Back 73

It is done by performing diffusion-weighted acquisitions in at least 6 directions (and far more in angular high resolution imaging), it is possible to extract the diffusion tensor which synthesizes all the data

Front 74

Visual representation of how DTI is acquired in many directions

Back 74

Front 75

What is parameters are measured in DTI
3

Back 75

anisotropic coefficient
preferred direction
restriction in space

Front 76

What are the signal characteristics of fractional anisotropy

Back 76

null if there is isotropic movement
increased signal if there is anisotropic movement

Front 77

What are 3 post processing images that can be obtained from DTI

Back 77

Fraction anisotropy map
Main diffusion direction
Fiber Tracking

Front 78

What is a limitation of DTI

Back 78

precision of direction of movement

Front 79

What are new models for DTI that are coming down the pike

Back 79

Multitensor model (several diffusion tensors coexisting in a voxel)
Q-ball (S-space): requires a large number of acquisitions in different directions, with a constant b-factor
Q-space (Diffusion spectrum imaging) (figure 13.11): the ultimate technique, able to describe fiber crossings, but requiring a high number of acquisitions (129 to 515 !) in different directions and with different b-factors to sample the diffusion equivalent of a 3D k-space