principle of DWI imaging and tissue suppression

Question 1

define diffusion

Answer 1

random movement of particles due to thermal collisions / Brownian motion

Question 2

what are the 2 types of diffusion and what are they

Answer 2

free diffusion = constant random motion (uniform in all direction)

restricted diffusion

Question 3

what are the 2 forms of restricted diffusion + explain

Answer 3

isotropic restricted diffusion = in all directions e.g seen in tumours

anisotropic restricted diffusion = only restricted in certain directions E.G nerve fibres, white matter tracts (they follow a certain path)

Question 4

note diffusion rate depends on DIRECTION

Question 5

what is the purpose of diffusion in MRI

Answer 5

• identify the value of diffusion of protons within the tissue being sampled using a greyscale known as ADC

Question 6

What is ADC and how does it show rates of diffusion

Answer 6

apparent diffusion coefficient

• unrestricted free diffusion repped with high numerical value
• restricted free diffusion repped with low numerical value

Question 7

what 2 sequences are used to form an ADC map?

Answer 7

B0 and DWI

An ADC MAP is a combination/calculation formed from the 2

Question 8

what sequence is used for the B0 image and why

Answer 8

echo planar imaging (spin echo),

90 RF, FEG, PEG, 180RF, READOUT + multiple re/dephase FEG with blips of PEG

• you need this because its fast and you want to prevent movement within that image esp with pulsating vessels (brain)
• movement artefact will obscure ability to assess diffusion in brain

Question 9

what type of weighted image does a EPI spin echo form and why

Answer 9

t2 (bright fluid) as there is a long TE, B0 = T2*

Question 10

why is a long TE required in the EPI sequence to form imaging B0

Answer 10

We need that additional time (before and after 180RF pulse) to apply the diffusion gradient which helps us get the DWI image

Question 11

explain how the DWI image is formed from the same EPI sequence used to get image B0

Answer 11

• STRONG diffusion gradients (stronger than FEG and PEG) applied before and after 180RF pulse

-

Question 12

Why are areas of greater diffusion darker on DWI than restricted diffusion

Answer 12

• initial 90 RF, this slice selects a excites protons in Mxy and they will eventually lose transverse magnetisation (t2*)
• diffusion gradient is applied, if particles are free moving within this gradient, it causes change in frequency which induces change in phase leading to DEPHASING = SIGNAL LOSS
• if particles are restricted then initial dephasing caused by 1st diffusion gradient will rephase due to 180 RF pulse flipping the second diffusion gradient in an equal opposite direction (causing signal gain)
• there is no rephasing due to equal opposite 2nd diffusion gradient with free diffusion as the initial affected protons in the region will have moved out of that ‘slice/plane’

Question 13

how does b value correlate to signal found in DWI images

Answer 13

the higher the b value, the higher the LOSS of signal due to diffusion in DWI

Question 13

B value is a factor accounted for producing DWI, what 3 things make up a b value

Answer 13

b value accounts for 3 things:
- strength of diffusion gradient
- time between diffusion gradient
- how long DG is on

• the longer and stronger the gradient/ longer time b/w gradient = higher b value

Question 14

NOTE:
longer diffusion gradient is on = increased time for diffusion / signal loss

steeper/stronger gradient = greater difference in precessional freq/phase change

longer time b/w diffusion gradients = more time for spins to move and not experience equal opposite gradient

(but of course if there is restricted diffusion its negigible)

• b value ultimately manipulates the signal seen on DWI (b/w 0-1000)

Question 15

because diffusion occurs anisotropically, what must you do

Answer 15

diffusion gradient must be repeated in all planes (xyz minimum) to see if theres restricted diffusion in all places

• however many places scanned, all those images then get combined to form final DWI image

Question 16

know that in a DWI image, the bright ness seen can either be due to T2 weighting or the ADC

Question 17

why can grey matter be seen in DWI but not in ADC

Answer 17

The visibility of grey matter in DWI was due to the T2 weightings, but the ADC is entirely based on comparison of diffusion therefore you can only see bright areas of high diffusion and dark areas of restricted diffusion

Question 18

compare what diffusion looks like on a DWI and ADC image

Answer 18

DWI = BRIGHT restricted diffusion

ADC = (opposite) DARK restricted diffusion

Question 19

a bright mass seen on a DWI may not necessarily be restricted diffusion, why is this and how is this resolved

Answer 19

because DWI still holds t2* weighting (from B0/ spin echo sequence) therefore that brightness can either be from t2 weighty (bright fluid) or restriction.

• by combining b0 and DWI, you can cancel out the t2 weighting aspect to form a ADC map which compares ONLY diffusion differences
• also known as T2 SHINE THROUGH

Question 20

it is important to always compare b0, DWI and ADC as it helps to identify artifact, lesions etc

FOR EXAMPLE: check images

Question 21

what is T2 Blackout

Answer 21

b0 has region of darkness (meaning the area has short T2)

DWI also has same region of darkness but we dont know if its again due to short T2 or free diffusion

• both b0 and DWI combined/mathematical calculations for ADC MAP cannot provide determination for what that region of darkness is , this is T2 BLACKOUT

Question 22

what 3 things determined rate of diffusion

Answer 22

particle size
temp
viscosity of liquid

Question 23

note: measureD diffusion isnt true diffusion coefficient in tissue, its APPARENT DIFFUSION COEFFICIENT

Question 24

A higher b-value typically results in a reduced SNR not only because of increased signal attenuation due to diffusion but also increased TE (hence additional signal loss due to T2 decay) when the maximal gradient strength is used, as is often the case for DWI.

Question 25

compare diffusion coefficient of water, csf, grey and white matter

Answer 25

highest to lowest: water, csf, grey matter, white matter

Question 26

DWI is highly susceptible to magnetic field inhomogeneity, what must be done

Answer 26

shimming

Question 27

what is ghosting artefact in mri

Answer 27

- structured noise appearing as repeated versions of the main object in the image. - occur because of signal instability between pulse cycle repetitions. Ghosts are usually blurred, smeared, and shifted and are most commonly seen along the phase encode direction. (motion artefacts are more pronounced in PE direction)

Question 28

how can you fix susceptibility artefact, what does it look like

Answer 28

use a multi shot segmented EPI (RESOLVE) / (filling in k-space with multiple lines (not just one)) (circles/patches of darkness in image)

Question 29

what is DWI protocol mainly used for

Answer 29

brain scan e.g stoke, dementia, tumors, abcess

Question 30

what does STIR stand for and what does it suppress

Answer 30

short tau inversion recovery - suppress FAT

Question 31

what does FLAIR stand for and what does it suppress

Answer 31

fluid attenuated inversion recovery - suppress FLUID

Question 32

how does stir and flair work

Answer 32

- 180 RF applied to flip protons in anti parallel longitudinal direction - there is no t2 decay (as theres no transverse magnetisation), there is only t1 recovery - we know the time it takes for each tissue to recover in the parallel longitudinal direction, thus we also know the point when each tissue recovers to a null point of longitudinal magnetisation (equal parallel/anti-parallel magnetisation) - if we apply a 90RF at the chosen null point of the tissue you wish to suppress, you will received signal from the desired tissue (as you the tissue NOT at null point will have longitudinal magnetisation that can be excited into transverse plane) - following the name fat suppression has short TI (TIME INVERSION) (as fat recovers faster) - fluid suppression has LONG TI (as fluid recovers slower)

Question 33

why is STIR or FLAIR good to use when there are artefacts such as inhomogeneities or metal artefacts

Answer 33

- because spin echo sequence is used (after initial 180RF) which already accounts for those issues

Question 34

Pros and cons of STIR

Answer 34

PRO: - independent of inhomogeneity and field distortion - only option for low-field systems CONS: - long acquisition (due to long TR from additional 180rf) - only makes fat suppressed t2 w imaged

Question 35

explain how 'double inversion recover' + spin washout forms black blood

Answer 35

- 2 180 RF used consecutive - initial is non-slice selective and inverts everything into anti-parallel longitudinal direction - 2nd 180 is slice-selective and inverts everything in a specific slice back in +ve z direction. - blood flowing in that slice has inverted magnetisation whilst all the other tissue has their signal preserved - this nulls the signal of blood from that slice and makes it ' black blood' (used in cardiac mri)

Question 36

what is a triple IR + spin washout forming black blood and no fat

Answer 36

- can be thought of as a double IR + STIR - same process of double IR to form black blood but additional 180 RF (to flip -z direction) and 90RF (at point of fat null in z axis) to get signal from only from fluid (also cardiac mri)

Question 37

what is chemical shift and compare it b/w fat and water

Answer 37

- resonant freq due to the interaction of protons with the magnetic field (b0) - fat precess at lower lamour freq than water as it has larger electron clouds (as its a larger molecule) partially shield the fat protons from the full effects of an externally applied magnetic field

Question 38

chemical shift = 3.5 parts per million at 1.5T, water has chemical shift of 64MHZ CHEMICAL SHIFT IS INDEPENDANT OF B0

Question 39

what is the chemical shift different b/w water and fat

Answer 39

220Hz

Question 40

FAT saturation is a technique used to suppress the signal from fat tissues in order to enhance the visibility of other tissues or structures of interest. By applying a radiofrequency pulse at the resonance frequency of fat, the fat signal can be selectively suppressed while leaving the signals from other tissues relatively unaffected. what are the 3 steps to the principle of fat sat?

Answer 40

1. radiofrequency pulse is applied to selectively excite the fat tissue. This causes the fat protons to precess, and the signal is phase-shifted. 2. delay known as the “fat saturation time.” During this delay, the fat signal decays while other tissue signals remain relatively constant. 3. images are acquired after the fat saturation time, during which the fat signal has decayed. Since the fat signal is suppressed, the resulting images have reduced or eliminated fat signal, thereby improving the visibility of other tissues or structures.

Question 41

what are 4 the fat SUPRESSING techniques (besides STIR)

Answer 41

- Chemical Shift Selective (CHESS) fat saturation / pulse - Spectral Adiabatic Inversion Recovery (SPAIR) fat saturation - Water excitation fat saturation - Dixon

Question 42

how does CHESS work

Answer 42

selective RF (Radiofrequency) pulse is applied that specifically targets the fat resonance frequency This pulse effectively saturates the fat spins by flipping their magnetization, reducing their signal intensity. Meanwhile, the water protons, having a different resonant frequency, are minimally affected by the RF pulse and retain their magnetization.

Question 43

how does SPIR work

Answer 43

- applying a tailored RF (Radiofrequency) pulse that selectively inverts the magnetization of fat protons - the fat protons’ magnetization is flipped, effectively suppressing their signal during the subsequent image acquisition.

Question 44

how does Water Excitation Fat Saturation

Answer 44

- specific RF (Radiofrequency) pulses or pulse sequences are employed to target the resonant frequency of water protons. - These RF pulses are designed to selectively excite the water signal, (doesnt excite lipid protons so no need to suppress) - As a result, the water signal becomes prominent in the acquired images, while the contribution from fat is reduced or eliminated.

Question 45

how does Dixon work

Answer 45

- phase shifts caused by the chemical shift difference between fat and water protons are captured in these images. By analyzing these phase shifts, the Dixon algorithm can separate the fat and water components from the acquired data. - uses mathematical calculations to estimate the fat and water signal contributions for each pixel or voxel in the image. This separation process results in the generation of separate fat-only and water-only images, where the fat signal is suppressed in the water-only images. - essentially, utilises the different precession of fat and water molecules. As these separate precessions can go in and out of phase with each other over time, calculations are done to create 4 images: water only, fat only, in phase and out of phase (refer to image to see equation)

Question 46

in silicon only sequences, why is frequency selective fat saturation of limited use when dealing with tissue containing silicon

Answer 46

there is a small frequency shift between fat and silicon (frequency range to suppress fat is too near that of silicon)

Question 47

how would you go about highlighting silicon only in a slice

Answer 47

- use STIR to supress fat signal and use frequency selective saturation pulse to suppress water signal

Question 48

fat and water have different resonant frequencies but eventually they can move IN phase and back OUT of phase with each other, e.g at 1.5T= IN PHASE AT 4.6ms, 9.2ms etc 1.5T= OUT PHASE AT 2.3ms, 6.9ms etc (DIXON )

Question 49

what is subtraction in mri and why's it used

Answer 49

pre contrast image is subtracted from post contrast image - removes tissue not affected by contrast medium (DIS = decreased SNR, affected by patient motion)

Question 50

what is a spatial saturation slab/sat band and why's it used

Answer 50

- band excited with spatially selective 90 RF pulse - large dephasing/spoiler gradient applied to ensure no signal is contributed - applied to each TR period, used to remove areas of signal that may cause artefact and remove signal from flowing blood

Question 51

what is aliasing/wrap around artefact in MRI

Answer 51

- occurs when the field of view (FOV) is smaller than the body part being imaged. The part of the body that lies beyond the edge of the FOV is projected onto the other side of the image - (sat band can be used for this as well as prevent motion artefacts)