Structural Imaging Flashcards
What is structural imaging?
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”
What are the major types of strucutral brain imaging?
T1
T2
FLAIR
What is a T1 scan?
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
What is a T2 scan?
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
What are FLAIR scans?
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
What is the backbone of structural imaging methods?
Exploiting T1 and T2 weighting
What determines whether a scan is picking up more T1 or more T2 signal?
It is determined by the sequences “repetition time” (TR) and “echo time” (TE)
Heavier T1 weighting is caused by what?
Short TR/TE times
Heavier T2 weighting is caused by what?
Long TR/TE times
Do scans only show T1 or T2 signal?
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
Can we have scans that only show T1 or T2 signal and not both?
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
What are other types of structural imaging?
T2*
Susceptibility-weighted imaging (SWI)
Proton density
What are T1 scans the gold standard for?
Gold standard for showing healthy anatomy because they have good tissue contrast
What is a benefit of T1 scans?
Short TR/TE times - takes less time to do the scans
Practically means participants spend less time in the scanner which is more ethical
What do T1 scans have modest ability for?
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
What scan is better than T1 at showing different kinds of pathology?
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
What are diffusion weighted scans good for in investigating pathology?
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
What is a 2D structural image?
Each “slice” is an image of a very thin section of tissue
Anatomical information “between” slices is lost
What is a 3D structural image?
Enabled by the inclusion of the third MR gradient
All tissue now contributes to the data in the voxels (3D pixels)
What is the difference between 2D and 3D images?
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
What is partial volume?
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
Why do high resolution scans reduce partial volume?
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
What is resolution?
The resolution of a scan is saying the amount of tissue in real terms that contributes to a signal
What does it mean if a voxel is isotropic?
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
