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PSY6414 - Neuroimaging 2 > 8.2 Diffusion Tensor Imaging DTI > Flashcards

8.2 Diffusion Tensor Imaging DTI Flashcards

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1
Q

What is diffusion defined as in DTI?

A

diffusion is the random Brownian motion of molecules due to thermal processes

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2
Q

Are fluids ever still?
What is Brownian motion caused by?

A

no! there is always Brownian motion
ambient temperature of the fluid/water molecules

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3
Q

What happens to diffusion properties as you change between fluid molecule types? eg from water to ethanol molecules

A

each fluid molecule has different diffusion property. for example water diffuses more than ethanol (when they are both controlled at the same temperature)

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4
Q

What does diffusion MRI image?

A

the different diffusion properties that different fluid molecules have in the brain (eg water diffuses more than ethanol at 25 degrees)

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5
Q

What type of scan does diffusion MRI use?
What sort of imaging sequence does diffusion MRI use?
What additional technique is added to make the scan a diffusion MRI scan?

A

T2-weighted
EPI
large diffusion weighting gradients (reverse gradient) on both sides of the 180 degree refocussing pulse -> make scan sensitive to the diffusion of water molecules

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6
Q

What is greater attenuation of signal in diffusion MRI/DWI images?

A

greater signal LOSS due to increased diffusion of water molecules

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7
Q

What are the two parts to making a diffusion MRI scan?

A

1) apply gradient -> dephases spins of water protons
DELAY~ water molecules diffuse/or dont
2) after delay, apply reverse gradient -> rephases water proton spins

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8
Q

Why is there a change in signal depending on if the water molecules have diffused or not? (protons)

A

-no diffusion= reverse gradient can rephase the proton spin -> net phase change is zero -> no signal loss
-diffusion present= reverse gradient can’t cancel out the motion -> incomplete rephasing -> signal loss

(more incomplete rephasing -> more signal loss)

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9
Q

As the diffusion of water molecules increases, what happens to the diffusion MRI signal?

A

diffusion increases, signal decreases/more signal loss

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10
Q

How is the diffusion coefficient (D) calculated with images?
What does it mean if D is a higher number?

A

D= can be calculated with the acquired baseline T2 image (S0) and the diffusion weighted image (Sb)

the higher the D number, the greater the diffusion

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11
Q

What is the key marker that the scan is a diffusion MRI scan (DWI)?

A

CSF ventricles are dark due to high diffusion -> greater attenuation of signal

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12
Q

What is b value and what does it show? eg Sb image
What does a high or a low bvalue mean?

A

-diffusion weighting factor = shows how much the applied diffusion gradient weighs the signal based on water motion ie quantifies how sensitive diffusion MRI signal is to diffusion
-high = stronger diffusion weighting -> more signal attenuation
low = less diffusion weighting -> less signal attenuation

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13
Q

DWI, how much signal attenuation happens in WM?

A

~60% attenuation

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14
Q

What is the standard bvalue used?
Units of bvalues?

A

1000 smm2
seconds per mm2

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15
Q

What is the difference in DWI and ADC scans?
What does the intensity mean for diffusion in DWI and ADC scans?

A

-intensity is reversed. for reference DWI is dark in csf and ADc in bright
-DWI: dark = high diffusion/less restricted
ADC: bright = high diffusion

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16
Q

Why is it important that you gauge the bvalue at an effective value?

A

because you must have a statistically significant amount of signal attenuation however you can’t have too much signal attenuation otherwise you are just measuring noise (signal is so low, so youre just measuring noise)
have to be weary of signal-to-noise ratio

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17
Q

What does the b0 image mean in diffusion MRI?

A

images with no diffusion gradients applied which just the T2 EPI image

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18
Q

DWI
How much signal attenuation is in the CSF? -Why?
Why can this be an issue experimentally in DWI images?

A

~95%
-because there is so much movement of water in CSF -> high signal attenuation
-if you wanted to investigate the CSF then you have to change the bvalue so you aren’t just measuring noise

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19
Q

Are ADCs quantitative or qualtitative images?

A

quantitative = each voxel represents a diffusion value in mm2/s

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20
Q

What is the ADC?
How is the ADC created?

A

-Apparent Diffusion Coefficient
-combining the effects of the b0 (T2) and the b1000 (the diffusion-weighted) images -> ADC image

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21
Q

In an ADC image, what does each voxel represent? units

A

represents a diffusion value in mm2/s

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22
Q

Why is ADC called ‘apparent’ diffusion coefficient?

A

this naming acknowledges that we are measuring diffusion at the voxel level and so looking at diffusion across a complex substrate, rather than accurately measuring the diffusion coefficient of a specific substance

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23
Q

What is the limitation of diffusion MRI in regards to direction?

A

diffusion is measured only in one direction in space at a time (either x y or z) -> diffusion varies depending on direction of the applied diffusion gradient (whether its x y or z) -> images look different depending on direction

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24
Q

Which direction is diffusion measured in?

A

diffusion is measured along the gradient direction which can be applied in any direction (x, y and z)

so any!

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25
Why does the amount of diffusion change within the brain?
amount of diffusion changes with different anatomy
26
What is the difference between ADC and MD?
ADC is the diffusion from a single-direction acquisition whereas MD = mean diffusivity is the average diffusion across multiple directions
27
What is directional resolution? High directional resolution = X? -> X?
-the number of directions that the diffusion images were acquired from -high directional resolution = more directions -> greater accuracy of the water diffusion
28
What is the minimum number of directions to create a MD diffusion weighted image? What is a MD image commonly called in called in clinical practice
at least 3 (x y and z) DWI = image taken in clinical settings refers to the 3-directional MD image acquired and is broadly named a DWI image
29
What is WM made of?
nerve fibre tracts which are parallel to eachother (wiring of the brain)
30
What is diffusion restricted by in human tissue?
restricted by the complex microstructural environment (eg myelin sheath doesnt allow diffusion perpendicular to the direction of the nerve fibre)
31
What is isotropic diffusion? How isotropic diffusion modelled/what shape? ditto anisotropic
-when molecules diffuse equally in all directions -average diffusion modelled by a sphere -when molecules diffuse more easily along some directions than others -average diffusion modelled by a ellipsoid
32
How does diffusion tensor imaging model diffusion/MD?
each voxel is modelled as a sphere/ellipsoid to show diffusion properties
33
The moment you dont have a perfect sphere, what is the diffusion like?
not perfect sphere -> anisotropic diffusion
34
How many direction must the diffusion measurements be made in to make DTIs? How many directions are used in research today?
-at least six direction -in reality, many more directions than six are used (30-32)
35
Which values define the shape and the magnitude of the diffusion ellipsoid?
shape/direction determined by eigen vectors magnitude determined by eigen values
36
What is the symmetric diffusion tensor and what is it estimated statistically by?
-diffusion tensor is a 3×3 matrix that describes the rate, direction and shape of water diffusion in 3D space -by using a process eg multivariate linear regression
37
What do the eigen vectors describe? HOw are eigen vectors assigned
direction of diffusion eigen vector 1 = along the principle length, direction where the most diffusion is happening eigen vectors 2 and 3 = the other two directions
38
What do eigenvectors/values look like graphically (as arrows) when there is isotropic and anisotropic diffusion?
isotropic = arrows are all the same length (magnitude) anisotropic = arrows are all different lengths
39
In diffusion MRI/DTI, what values can you show on your diffusion MRI images which are useful for research/clinical use? Why are they useful?
you can create images showing MD or FA (fractional anisotropy) -many diseases show changes in FA and MD. they are a sensitive variable
40
How is image acquisition of DTI different to MD images (DWI clinical name)?
more directions needed: at least 6 directions required for DTI whereas MD uses/needs 3 directions
41
What is FA? What is FA in laymans? What do the FA values represent?
-a number between 0 and 1 which represents the amount of anisotropic diffusion -measure how water moves in one direction or not -closer to 0 = more isotropic closer to 1 = more anisotropics
42
What tissue is FA images good at showing?
good for imaging WM tracts as WM is highly anisotropic
43
Summary How do you get an ADC image? How do you get the MD image?
-combine effects of T2 image and direction-specific DWI images -> ADC image - average of all the ADC in (at least 3) different directions -> MD
44
How does disease usually change the values of MD and FA?
MD INcreases and FA decreases
45
Why are changes MD and FA good markers for disease?
changes in FA and MD are sensitive variables to show disease progression and are highly correlated to cognitive impairment other variables such as atrophy are not as sensitive and dont show a correlation between cognitive performance
46
How is DTI more effective than other scans? give an example
conventional structural imaging is not sensitive enough to show changes due to mild TBI
47
What does colour represent on FA mapped images?
colours represent the direction of diffusion
48
Why is DWI (the 3 direction scan) is used in stroke clinics? What brightness are areas which show recent stroke on DWI scans? Do these stoke areas have increased diffusion or decreased?
-sensitive to showing that a patient has had a stoke very recently -really bright -decreased diffusion = bright
49
What causes a bright signal in stroke areas on DWI images?
cells swell as water moves from extra- to intracellular space -> cytotoxic oedema -> restricted diffusion -> reduced diffusion -> bright signal on DWI
50
What is vasogenic oedema? How does it show up on a DWI scan?
-post stroke (days) Breakdown of the BBB → fluid leaks from blood vessels into the extracellular space of the brain. (not after healed from stroke) -greater diffusion of water than normal -> abnormally dark signal in stroke affected area
51
Why is a DWI image is better than common structural images?
showing signs of a very recent stroke. signs of stroke shown within hours on DWI but takes days-weeks for structual
52
Is motion correction used in DWI?
yes it is also important here
53
Why are diffusion MRI images susceptible to susceptibility induced distortions? What do these distortions look like? What is the direction of distortion dependent upon?
-because diffusion MRI uses EPI sequence which has a very fast acquisition of an image within one TR -> susceptible to distortions -look like brain is extended (positive blip) or crushed an some points (negative blip) -distortion direction is dependent on the polarity of the phase encode gradient blips
54
How do you correct for susceptibility induced distortion?
use FSL tool topup which fills in the distorted area by averaging the different changes in the positive and negative blips
55
What does tractography maps show in each voxel?
principal eigen vector overlaid onto a FA image to show the main direction of diffusion in the nerve tracts/fibres
56
What does streamline tractography show? How does it do this?
-represents nerve fibre tracts by connecting all the voxel eigen vectors to form long tracts using a tractography algorithm
57
With streamline tractography, when is the tractography algorithm programmed to terminate the tract?
tract stops either when: -the angle BETWEEN the principal eigenvectors/voxels exceeds a certain threshold -FA drops below a certain value (ie when its isotropic, low anisotropy)
58
What are the issues with streamline tractography? What effect does this have on the data?
-does not compensate for branching fibres (fibres that are perpendicular to the main tract). -different directions (branching fibres) might cancel ether other out -> poor diffusion detected when actually there is a lot of diffusion but in different directions -gives no indication/calculation of how likely the streamline tracts are compare to the actual anatomical tracts -streamline is not a good representation and rarely used in research
59
How does ODF sampling (many times) improve tractography?
more holistic view as each iteration of ODF gives another vector field on which the tractography can run
60
What is wrong with logic in tractography? What is the ODF? How can you alleviate the above issue?
-that water diffuses in that direction only via a majority diffusion/ primary eigen vector -Orientation Distribution Function where the function defines the relative likelihood of travelling in any given direction -use ODF sampling (many iterations of ODF
61
What does ODF sampling show? What does ODF sampling form the basis for?
multiple likelihoods show, based on how spread out each eigen vector is, if the diffusion is likely to be anisotropic or isotropic -probabilistic tractography
62
How does streamline tractography work?
used to estimate the most likely paths of WM tracts by accounting for uncertainty in fiber orientation at each voxel.
63
How do you perform probabilistic tractography? in four steps
1) ODF sampling done many times 2) streamlines created from a starting seed point using the sampled directions 3) number of streamlines passing through a voxel is counted -> used to give a connection probability 4) permutation testing is used to determine whether the observed connectivity is significantly greater than chance
64
What is the advantage of using probabilistic tractography over streamline tractography?
higher chance of giving anatomically correct information than just streamline
65
How can probabilistic tractography and ROI analysis be used to together to help calculate diffusion parameters (FA, MD) of a specific tract?
-probabilistic tractography to extract major white matter fasciculi (bundle of nerve fibres eg corpus callosum) -3D ROI masks applied on top -ROI masks placed on patient's native scan and summary diffusion parameters calculated for the tract (FA, MD) -correlate these diffusion parameters with clinical/cognitive investigations
66
What is the biggest limitation of the DT model (DTI)? What is a solution to this issue? why? What is the disadvantage of this solution?
-doesnt account for crossing fibres (in the single tensor/ellipsoid model) -the dual tensor model -> models restricted diffusion in the plane of the crossing fibre with no preference being given to the two fibres orientations -more time consuming acquisition of images (more direction/bvalues required)
67
How is tractography used for cortical connectivity? What is the issue with this technique?
-tractography finds connections between cortical regions or areas that exhibit abnormal cortical thickness -the current streamline-based algorithms perform poorly
68
How does global tractography differ from streamline or probabilistic?
tractography which constructs the entire brain’s fiber architecture simultaneously, rather than tracking individual streamlines one at a time like in local (streamline or probabilistic) tractography.
69
What does Global Tractography show? What algorithm does it use? what does this algorithm do?
-produces a measure of connectivity from a seed point to every other point in the brain -a front propagation algorithm which identifies geodesic pathways through the brain and scores each pathway to give a connectivity value
70
What is a geodesic pathway? what is it not?
the most efficient or plausible pathway a white matter fiber would follow through the brain. (not necessarily the shortest geometric distance, but the path that best follows the white matter fiber orientations)
71
Which technique performs better when trying to map cortical connectivity, streamline tractography or global tractography? Which is better at penetrating deeper into the cortex? Is the above an advantage or a disadvantage? why?
-global performs better than streamline -global can penetrate much further into the cortex than local streamline methods -both advantage and disadvantage because you can capture as much of the tract as possible but you also introduce more measurement error
72
What is TBSS? What does it add to remove the issues of early FA signal VBM experiments?
tract based spatial statistics TBSS uses a skeletonised average FA image
73
Which DTI experimental was popular in the early parts of the new millennium? How was TBSS born of an issue from this experiment?
-using VBM to identify voxel-wise difference between groups based on their FA signal -experiment is highly dependent on registration but there was no existing non-linear registration algorithm capable of accurately aligning WM tracts between subjects -> many false positives due to misregistration. experiments were also dependent on spatial smoothing and no consensus about the correct degree of spatial smoothing. -> TBSS design to overcome both these issues: registration + smoothing
74
When is skeletonisaton implemented in TBSS? What does the FA skeleton represent? How does skeltonisation help FA VBM experiments?
-following registration to a standard space, an FA skeleton is made. Individual subject data are projected onto FA skeleton by taking the nearest maximum FA value -FA skeleton represents the core of the average tract shape across the group -refines misregistrations, ensures that data is anatomically comparable for voxelwise analysis
75
What is NODDI? Why was it designed? What does NODDI model? how does it differ from the traditional DTI model?
Neurite Orientation Dispersion and Density Imaging -to overcome the limitations of the tensor model (DTI) -models intra-neurite, extra-neurite and CSF compartments in each voxel whereas the DTI model assigns a single tensor model to each voxel
76
What are the image acquisition requirements for NODDI?
needs a multi-shell diffusion acquisition (images which require two or more different bvalues)
77
FOr NODDI, why are multi-shell image acquisitions required?
-different tissues have different diffusion properties
78
What are the three compartments that NODDI models for?
Intra-neurite space (inside axons/dendrites): restricted diffusion Extra-neurite space (outside neurites): hindered diffusion Isotropic compartment (e.g., CSF): free diffusion
79
Whys is NODDI better than traditional DTI?
provides more reliable measurements as it separates the CSF from neuronal tissue. Also separates intra and extra neural
80
What is a future diffusion imaging technique which is even better than NODDI? What does this technique measure? Why is it important?
-Double Diffusion Encoding DDE -measures microscopic diffusion anisotropy, even when tissue orientation is disorganized (e.g., gray matter or tumors). -it can differentiate tissue types based on microscopic structure. thus, detect early stage pathology before larger scale damages

PSY6414 - Neuroimaging 2 (16 decks)

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