Advanced MRI- 1 Flashcards
(33 cards)
What is the Brownian motion of water?
- Water molecules are constantly diffusing if they have thermal energy
- Is a random, translational motion
What does t represent?
A given diffusion time
What is the formula for root mean squared displacement?
= 6Dt
What time of process if diffusion?
-time dependent
What does root mean squared displacement of molecules means? What is its role in diffusion?
-Molecules diffuse further from their starting point as time increases
What does D represent and what is this value dependent upon?
- Diffusion coefficient
- Temperature dependent
What is D for water at 37*C?
3.0 x 10-3mm2 s-1
What is isotropic diffusion?
What is the shape of the root mean squared displacement?
- Diffusion that is same in all directions
- Sphere
What is anisotropic diffusion?
Diffusion is hindered along some directions more than others
Describe diffusion in cerebrospinal fluid
- Diffusion is free
- Same in all directions
- Isotropic
- Essentially free water
- So D is 3.0 x 10-3mm2 s-1
Describe diffusion in grey matter
- Hindered diffusion in all directions due to the presence of neuronal structure
- Isotropic
- So D is smaller, 0.8x10-3mm2s-1
Describe diffusion in white matter
- Hindered
- Anisotropic
- Greater diffusion along the axonal structure than across
- Cylindrical Shape (ellipsoid)
- D is 0.8x10-3mm2s-1
What is Diffusion MRI and how is it useful?
- Stands for diffusion magnetic resonance
- Used to measure the brain structure
- Infers brain structure by measuring diffusion of hydrogen ions
- essentially quantifies water diffusion in mm2s-1
How is D MRI used to view white matter?
- Used to infer health of white matter microstructure
- Reveals white matter damage in disease
What are 2 advantages of D MRI?
- Non-invasive (involves no radiation, i.e. safe for subject)
- Good spatial resolution (~2mm3)
What occurs during DWI?
- Pulse sequence is sensitised so diffusion can be detected
- The diffusion between these pulse sequences causes signal loss
- From this D can be calculated
Can diffusion be quantified in only one direction?
-No can be quantified in all directions
What is the purpose of sensitising diffusion in different directions and what is thus known as?
- Geometrical models can be applied to determine isotropic and anisotropic diffusion characteristics
- This is known as Diffusion Tensor Imaging (DTI)
Describe diffusion weighted sequence
- First 90 degree pulse applied
- Flips magnetisation at right angles
- Pulsed gradient that has given duration and magnitude is applied.
- Then defazing of magnetisation occurs
- Pulsed gradient causes additional defazing of magnetisation
- Then 180 degree pulse is applied
- Rephrasing pulse gradient applied on the other side of 180 degree pulse to ensure that so that same gradient applied either side of pulse
- This rephrases net magnetisation
- So that we obtain an echo signal of the original net frequency
What is purpose of 2 additional pulse gradients?
- If there is no random motion due to diffusion that we will see standard t2 weighted echo signal
- If diffusion has taken place, there would have been some molecules that have been excited by the initial RF pulse and defazed that have not been refazed by the second pulse gradient
- This will cause a drop in echo signal intensity due to diffusion effects
What is the b value?
-Increasing diffusion sensitisation
What is b-value related to?
-B value is related to duration of pulse gradient (known as little delta), magnitude of pulse gradient and separation between the loads of these pulse gradients
In which fluids is a larger drop in echo signal observed?
- CSF
- Free water
What is signal loss proportional to?
-b and D
