Lecture 17 & 18 - Neuroimaging

Question 1

How does MRI produce images?

Answer 1

• contrast between isotopes spins in the horizontal and vertical planes (difference in orientation depends on type of tissue, hence abnormalities will be depicted) using magnetisation and enducing radio frequencies.

Question 2

what is T1 decay?

Answer 2

the time it takes for isotopes spinning in the transverse plane (due to excitation) to return back to the Z-plane (relaxation).

• relaxation time depends on spins environment
• more mobile/fluid tissue takes LONGER to relax to z-plane (e.g., blood, CSF)

Question 3

What is T2 decay?

Answer 3

the time it takes for isotopes spinning in the transverse plane, to become DEPHASED (fan out, decreasing net moment)
- dephasing time depends on spin-spin interaction (constricted tissue decays faster)

Question 4

what is T2* decay?

Answer 4

same as T2 but taking into account the heterogeneity of the brain environment (additional fields besides neural tissue)

• presence of bone, sinuses, air, leads to additional fields -> more obstruction -> more spin interaction between isotopes -> faster dephasing.

Question 5

what will mobile substances appear as on the T2 contrast images?

Answer 5

brighter (longer time to decay)

Question 6

what are the pros and cons of each MRI type?

Answer 6

T1 - good for gray/white matter contrast
- poor CSF contrast

T2 - poor gray/white matter contrast
- good CSF/fluid contrast

T2 * - can detect fluctuations in bloodstream caused by iron

Question 7

how does fMRI work?

Answer 7

uses changes in oxygenation/deoxygenation of blood in local areas to facilitate neuronal activity in response to stimulus using BOLD (blood oxygen level dependency)

Question 8

what are three types of PERFUSION images?

Answer 8

• Mean transit time (MTT; time blood/substance stays in capillary bed of target brain region)
• Cerebral blood volume (CBV; amount of blood present in target tissue)
• cerebral blood flow (CBF; amount of blood delivered to target tissue per time constant)

Question 9

What does perfusion MRI allow us to detect? (x4)

Answer 9

• stroke
• ischaemia (low blood flow)
• tumour malignancy
• cortical atrophy (dementia)

Question 10

What is diffusion?

Answer 10

Measure of random thermal motion in brain, based on Brownian-motion.

Question 11

what is Isotropic and Anisotropic diffusion?

Answer 11

ID - particles diffuse in random free directions (spherical; no boundaries; e.g., water, CSF)

AD - diffusion is contained within boundaries (Ellipsoid; white matter, muscle)

Question 12

Describe the contrast that would be seen on DWI and ADC images of Diffusion scans (in terms of diffusion)

Answer 12

• on DWI images, the more spinning particles move (due to lack of boundary), the higher the diffusion gradient (b), resulting in LOSS OF SIGNAL leading to a darker appearance.
• on ADC images, it is the INVERSE (more diffusion/free space = brighter)

Question 13

what does D stand for in Diffusion?

Answer 13

membrane permeability (greater permeability = more diffusion)

Question 14

early detection of stroke is only possible for

Answer 14

DWI and ADC (not T2 or CT)

Question 15

What is a DIFFUSION TENSOR

Answer 15

• similar to ADC, but robust to head rotation.

- FA value close to 1 signals more directional/anisotropic structures (axons)