Structural Imaging

Question 1

What is structural imaging?

Answer 1

A structural image shows brain structure

Constrast visually differentiates between types of fluid, tissue and bone whihc have different magnetic properties that the scanner lets us see

Areas of high MR signal are “bright” and low MR signal areas are “dark”

Question 2

What are the major types of strucutral brain imaging?

Answer 2

T1
T2
FLAIR

Question 3

What is a T1 scan?

Answer 3

Shows intensity of T1 signal- the contrast comes from differences in the tissues T1 relaxation time

CSF = dark
White matter = very bright
Grey matter = quite bright

T1 scans are very basic but good at showing anatomy

Question 4

What is a T2 scan?

Answer 4

Shows the intensity of the T2 signal- the contrast comes from differences in the tissues T2 relaxation time

CSF = bright
White matter = darker grey
Grey matter = brighter grey

Question 5

What are FLAIR scans?

Answer 5

FLAIR images are T2 weighted scans where an “inversion” RF pulse in the scan additionally cancels signal from CSF (making it dark not bright)

Useful for picking out properties of brain tissue and for picking out certain types of brain pathology

Question 6

What is the backbone of structural imaging methods?

Answer 6

Exploiting T1 and T2 weighting

Question 7

What determines whether a scan is picking up more T1 or more T2 signal?

Answer 7

It is determined by the sequences “repetition time” (TR) and “echo time” (TE)

Question 8

Heavier T1 weighting is caused by what?

Answer 8

Short TR/TE times

Question 9

Heavier T2 weighting is caused by what?

Answer 9

Long TR/TE times

Question 10

Do scans only show T1 or T2 signal?

Answer 10

No -standard T1 and T2 scans do not show only T1 or T2 signal, they are just weighted more heavily towards one or the other.

The pixel values in these images are arbitrary

Question 11

Can we have scans that only show T1 or T2 signal and not both?

Answer 11

Yes- we can have scans that show actual T1 or T2 times called quantitative T1/T2 mapping

Here the value of each pixel is the actual relaxation time for the tissue present in that location

These scans caln help use emphasise features e.g. evidence of a stroke

Question 12

What are other types of structural imaging?

Answer 12

T2*
Susceptibility-weighted imaging (SWI)
Proton density

Question 13

What are T1 scans the gold standard for?

Answer 13

Gold standard for showing healthy anatomy because they have good tissue contrast

Question 14

What is a benefit of T1 scans?

Answer 14

Short TR/TE times - takes less time to do the scans

Practically means participants spend less time in the scanner which is more ethical

Question 15

What do T1 scans have modest ability for?

Answer 15

Modest ability to show pathology - T1 scans aren’t as good at distinguishing different pathologies in the brain

e.g. can show back dots called perivascular spaces (CSF-filled holes) but these black dots can also represent different things e.g. white matter lesions, but T1 scans can’t differentiate these very well as they are the same colour as CSF in T1 scans so they appear to be perivascular spaces when they aren’t

Question 16

What scan is better than T1 at showing different kinds of pathology?

Answer 16

T2 weighting is better at highlighting white matter lesions as bright tissue

Doing this via a FLAIR also makes the differentiation easy - FLAIR is excellent for investigating brain pathology

Question 17

What are diffusion weighted scans good for in investigating pathology?

Answer 17

DWI not technically a structural image but can show strokes before the damage becomes visible on other scans- this can’t be seen as quickly in T1 or FLAIR

Question 18

What is a 2D structural image?

Answer 18

Each “slice” is an image of a very thin section of tissue

Anatomical information “between” slices is lost

Question 19

What is a 3D structural image?

Answer 19

Enabled by the inclusion of the third MR gradient

All tissue now contributes to the data in the voxels (3D pixels)

Question 20

What is the difference between 2D and 3D images?

Answer 20

When viewing either kind of scan on a computer it will still seem like we move through the brain from one slice to the next

The difference is the amount of anatomy which has contributed to the images we see e.g. 1mm square vs 1mm cubed

Question 21

What is partial volume?

Answer 21

When we zoom in on an image it has a finite resolution, at its most zoomed in state it is made up of squares- each one has a number which sumarises that section of tissue but this has an important limitation
-if there is a section of the brain that is on the border in a pixel e.g. white/grey matter, then partial volume is a way of acknowledging that pixels are not just made up of one thing

Question 22

Why do high resolution scans reduce partial volume?

Answer 22

Give more information, more accuracy, and less blurring so partial volume is less of a problem the finer the pixels get = we can differentiate more cleanly between where one thing ends and another begins

Question 23

What is resolution?

Answer 23

The resolution of a scan is saying the amount of tissue in real terms that contributes to a signal

Question 24

What does it mean if a voxel is isotropic?

Answer 24

Often in a 3D scan the resolution is the same in all directions e.g. 1mm x 1mm x 1mm as a cube and we call this isotropic - the same distance in every direction