DWI

Question 1

What pulse sequence is used for DWI?

Answer 1

PGSE: Pulsed Gradient Spin Echo
90-G-180-G-ADC
Single direction G provides DW
EPI readout

Question 1

Equation for average distance travelled under free diffusion in time t

Answer 1

<r>=sqrt(6 Dt)
</r>

Question 2

Equation for b value and what does it mean?

Answer 2

b = gamma^2 G^2 delta^2 (Delta-delta/3)
gamma: gyromagnetic ratio
G: gradient strength
delta: gradient duration
Delta: inter-gradient period

b value a measure of diffusion weighting

Question 3

Equation for signal as a function of ADC

Answer 3

S=S0exp(-bADC)

Question 4

Two clinical example use-cases for DWI

Answer 4

1) Imaging following ischaemic stroke:
cell swelling occurs
=> extracellular water more restricted + more water is contained in cells
=> ADC reduced

2) Imaging of tumours
cell density can be high
=> ADC reduced

Question 5

Equation for directionally averaged Mean Diffusivity in DTI?

Answer 5

Mean of principle eigenvalues of diffusion tensor (= Tr(D))

Question 6

Equation for fractional anisotropy in DTI?

Answer 6

FA=sqrt(3/2) * [sqrt ((l1-MD)^2 - (l2-MD)^2 - (l3-MD)^2)] / [sqrt(l1^2 +l2^2 + l3^2)]

Question 7

What is tractography?

Answer 7

Assess connectivity through matching up pixels with similar ppl diffusion ellipsoids.

Question 8

Three clinical applications of DTI

Answer 8

1) Assess brain connectivity
2) Neurosurgery planning
3) visualise WM tracts in brain for dementia assessment

Question 9

How is a principle diffusion direction map created?

Answer 9

Use FA to control intensity

Use principle eigenvector components in x,y,z = R,B,G to control hue