MRI image basics

Question 1

what are pixels

Answer 1

the smallest details of an image (individual numbers) quantifying an amount of physical space

Question 2

how large is a pixel in a typical strutural MRI

Answer 2

1mmx1mm

Question 3

size of pixels=

Answer 3

spatial resolution

Question 4

are pixels bigger or smaller in high resolution image?

Answer 4

smaller

Question 5

what’s the main drawback of high resolution image?

Answer 5

takes a long time e.g. 0.1mmx0.1mm scan took 25 hrs

Question 6

what’s the 3D equivalent of a pixel used in MRI scans

Answer 6

a voxel

Question 7

does MRI= B&W photograph

Answer 7

no

Question 8

outline contrast in regards to MRI

Answer 8

MRI relies on contrast (not the same as that between light and shadow in photography) between different body tissues exhibiting different magnetic properties

Question 9

what are the 3 tissues that structural MRI are primarily looking at?

Answer 9

WM, GM and CSF

Question 10

outline T1-weighted structural MRI

Answer 10

uses T1 relaxation times
CSF = dark
WM = very bright
GM = quite bright
main/average MRI used
- good tissue contrast- gold standard for showing healthy anatomy (modest ability to show pathology)
- quicker to run than T2

Question 11

outline T2-weighted imaging

Answer 11

contrast comes from different tissues T2 relaxation times
CSF = bright (the main distinguishing feature)
WM = darker grey
GM = brighter grey
- good for highlighting WM lesions

Question 12

outline FLAIR imaging

Answer 12

T2 weighted scans where an ‘inversion’ RF pulse is given in the scan which cancels signal from CSF (making it dark)
- good for picking out certain types of brain pathological e.g. WM lesions, vascular dementia, strokes

Question 13

pixel values in structural contrast imaging are usually…

Answer 13

arbitrary - they only make sense relative to eachother (as opposed to numbers quantitatively measuring something

Question 14

what do quantitative scans show

Answer 14

numbers in the image represent real physical properties, where the scans are actually showing the T1/T2 relaxation times. This can help emphasise certain features like the evidence of stroke with specific location

Question 15

name 4 other type of structural scan

Answer 15

T2*
suseptibility weighted imaging (SWI)
proton density
Diffusion weighted imaging (DWI)

Question 16

which scan is good for showing microbleeds?

Answer 16

SWI

Question 17

which scan is good for showing recent scans before damage becomes clear on other scan types?

Answer 17

DWI

Question 18

are 2 scans of the same type going to give the exact same image

Answer 18

no - there will be systematic differences dependent on temp, humidity, scanner sequence, age of machine etc. (important to mitigate this variation as much as possible in cross-centre studies)

Question 19

what is partial volume?

Answer 19

where a pixel/voxel straddles different tissue types e.g. is part in GM and part in WM (cannot define tissue type with a single number)

Question 20

what are the 3 scan orientations?

Answer 20

sagittal, coronal, axial

Question 21

are L+R the same as being viewed of swapped in MRI?

Answer 21

swapped

Question 22

what orientation is coronal scan taken from

Answer 22

as if the person is looking at the scan viewer

Question 23

what orientation is axial scan taken from

Answer 23

as if we are looking up from the person’s feet