fMRI

Question 1

What is functional MRI (fMRI)?

Answer 1

fMRI measures brain activity by detecting changes associated with blood flow. The primary form of fMRI uses the blood-oxygen-level dependent (BOLD) contrast, used to map neural activity in the brain or spinal cord.

Question 2

Name differences of fMRI and standard MRI.

Answer 2

MRI:
- anatomy
- high resolution (~1mm)
- beautiful
- slow (~100s)

fMRI:
- function
- low resolution (~4 mm)
- ugly
- fast (~0.1s)

Question 3

For what can fMRI be used for?

Answer 3

For pre-surgical planning. E.g. to identify the criticial motor brain regions that must not be damaged during surgery.

Question 4

Blood-oxygen-level-dependent (BOLD) imaging is a method used in fMRI to observe different areas of the brain. Explain this method.

Answer 4

• Neurons do not have the capacity to store any form of energy (e.g. sugar or oxygen) and thus rely on quick energy production and delivery.
• For this, there is the haemodynamic response, a response where blood releases oxygen to active neurons at a greater rate than to inactive neurons. This causes a change of the relative levels of oxyhemoglobin and deoxyhemoglobin that can be detected on the bases of their differential magnetic susceptibility.
• This variation in magnetic properties of both hemoglobins leads to a magnetic signal variation which can be detected using an MRI scanner.

Question 5

fMRI uses T2* instead of T2. What is the meaning of T2*?

Answer 5

• T2 is defined as the time constant for the decay of transverse magnetization arising from natural interactions at the atomic or molecular levels.
• However, this transverse magnetization decays much faster than would be predicted by natural atomic and molecular mechanisms. Therefore, T2* is used. Here, T2* is seen as the observed or effective T2, whereas the first T2 can be considered the natural or true T2 of the tissue.
• T2* is always less than or equal to T2.

Question 6

Describe how MRI-BOLD is used to visualize active neurons.

Answer 6

• Active neurons → have excess supply of oxygenated blood → increase of oxyhemoglobin and lower concentration of deoxyhemoglobin
• Less pertubation of magnetic field
• Increase in T2*
• Increase in signal in T2*-weighted image

Question 7

Echo-planar imaging (EPI) is a very fast MRI technique that is capable of acquiring an entire MR image in only a fraction of a second. What is EPI and how is EPI so fast?

Answer 7

• (Single-shot) EPI uses a single radio-frequency excitation to obtain all the spatial-encoding data of a(n 2D) image. It does this by quickly switching the magnetic gradients back and forth after a single excitation pulse.

Question 8

Name the benefits, drawbacks and applications of echo-planar imaging (EPI).

Answer 8

Benefits:
- reduced imaging time
- decreases motion artifact
- ability to image rapid physiologic processes of the human body

Drawbacks:
- sensitive to artefacts
- sensitive to susceptibility effect
- sensitive to main magenetic field inhomogeneity
- long echo train length causes greater T2*-weighting
- requires high performance gradients

Applications:
- cardiac imaging
- abdominal imaging (e.g. breath-hold sequences)
- diffusion imaging
- perfusion imaging
- functional imaging

Question 9

For what is fMRI important in regard to pre-surgical planning?

Answer 9

It is important for the localization of crucial brain regions such as Broca’s and Wernicke’s area.