Structural MRI

Question 1

Compared to CT scans - how are MRIs better?

Answer 1

MRI provides better contrast in images of soft tissue (i.e. the brain)

Question 2

How are the different contrasts between tissues/CSF achieved (i.e T1, T2 etc)?

Answer 2

By varying the sequence of the Radiofrequency pulses, different contrasts can be achieved based on the relaxation properties of the hydrogen atoms therein.

Question 3

What is repetition time (TR)?

Answer 3

Repetition time (TR) is the amount of time between successive pulse sequences applied.

Question 4

What is time to echo (TE)?

Answer 4

Time to echo (TE) is the time between delivery of the Radiofrequency pulse and the receipt of the echo signal.

Question 5

What are the features of a T1-weighted image?

Answer 5

Brain = bright
CSF = dark
The optimal contrast is achieved with a short TR

Question 6

What are T1-weighted images mostly used for?

Answer 6

• Great for anatomy (sMRI)
• Obtaining morphological information
• Often used for segmentation:
• Labelling brain tissue (eg. grey matter, white matter, CSF)
• Labelling anatomical structures (eg. hippocampus)

Question 7

What are the features of T2-weighted images?

Answer 7

Brain = Dark
CSF = Bright
Optimal contrast is with a long TE

Question 8

What are the sources of variance when using sMRI?

Answer 8

• Quality and availability of normative data
• Confounding factors - such as daily fluctuations (dehydration)
• Measurement errors.
• Use of different volumetric approaches

Question 9

What is brain morphometry?

Answer 9

It is a measurement of size, volume and shape characteristics of different brain structures and tissue types.

Question 10

What are the two volumetric approaches used in sMRI?

Answer 10

• Manual tracing

* Automated procedures

Question 11

What are the automated morphometry processing steps?

Answer 11

1. Spatial normalisation of brain into standard space
2. Segmentation of tissue classes
3. Smoothing
4. Voxel-wise parametric steps