9.2 Perfusion and Permeability Imaging

Question 1

What is the main Gadolinium-based MRI contrast agent?
What does it molecular properties have on the strength of the MRI signal?

Answer 1

-gadopentetate dimeglumine
-paramagnetic molecule (like deoxy blood) -> increases magnetism -> CHANGES MRI scan signal (depends on if T1/T2/T2*)

Question 2

What happens if you add a MRI contrast agent?

Answer 2

-contrast agent alters magnetic environment of its locale -> changes MRI signal

Question 3

What effect do Gadolinium based contrast agents have on T1, T2 and T2* weighted scans? why?

Answer 3

-T1 is brighter in Gadolinium locale, slightly darker in T2, darker (more than T2) in T2*

-because T1 and T2 relaxation times are reduced
because in T2* Gadolinium induces local magnetic field inhomogeneities, which lead to faster dephasing of spins ???

Question 4

What does perfusion MRI measure?

Answer 4

measures and describes the local blood flow through a region of brain tissue

Question 5

What do these stand for? CBF, CBV, MTT

Answer 5

CBF = cerebral blood flow
CBV = cerebral blood volume
MTT = mean transit time

Question 6

What is the basic outline of perfusion MRI imaging in three steps?

Answer 6

1) acquire baseline pre-contrast images
2) inject a contrast agent = called a bolus
3) observe signal changes during first passage of bolus through the brain

Question 7

What are Gadolinium perfused T2* images sensitive to?

Answer 7

T2star-weighted sensitive to INTRAvascular contrast (gadolinium in blood vessels) -> causes signal loss -> darker areas on T2* image

Question 8

Is T2* Gadolinium perfused MRI sensitive to changes in Gadolinium inside the blood vessels (intravascular) or outside (extravascular)?
What is this type of perfusion MRI specifically known as?

Answer 8

-sensitive to intravascular contrast -> strong signal loss/darkening
less sensitive extravascular -> less signal loss
-DSC-MRI: Dynamic Susceptibility Contrast MRI (good at imaging intra and weak for extra)

Question 9

What are the requirements for image acquisition of perfusion MRI images?
Why?

Answer 9

need high TEMPORAL resolution
need multi-slice imaging
-> to capture real-time dynamic signal changes as the contrast agent is perfused

Question 10

What measurements can you calculate from perfusion MRI data?
How do you calculate each of them from a perfusion peak (looks like a skilly bell shaped curve/peak)?

Answer 10

CBV = area under curve
MTT = width of curve
CBF = CBV/MTT

Question 11

What does the area under the perfusion curve tell you (bell shaped)?

Answer 11

area under curve = amount of contrast (of contrast agent) = CBV

Question 12

What does MTT mean?
Can you calculate it?

Answer 12

mean transit time = the AVERAGE time taken for blood to flow through a given region of tissue, (from the arterial inflow to the venous outflow)
yes MTT = CBV/CBF

Question 13

What is CBF and CBV?

Answer 13

CBV = how fast blood flows through a given area of brain tissue
CBF = total volume of blood in a given area of brain tissue

Question 14

How do the CBV CBF and MTT change in WM and GM?

Answer 14

WM has less CBV and CBF than GM but MTT is the same as MTT = CBV/CBF

Question 15

What usually happens to CBF CBV and MTT in stroke affected areas?

Answer 15

CBV and CBF decrease dramatically -> MTT increases a bit

Question 16

Can you image the individual CBV CBF and MTT onto MRI images?

Answer 16

yes

Question 17

How do clinicians use DWI and perfusion MRI to help stroke management (reducing further stroke)?

Answer 17

-clinicians use DWI and MTT scans to evaluate if patient need thrombolysis treatment
-if there is a difference in area of signal change/stroke are is larger in MTT is larger than DWI image -> patient need thrombolysis treatment because it is predicted that patient is at risk of FURTHER stroke

Question 18

What is hyperemia?
Which perfusion MRI scan shows this?

Answer 18

-after stroke the body overcompensates for previous flow deficit and oversupplies blood flow to the stroke affected part
-CBF shows faster rate of blood flow to area

Question 19

How can you see from a perfusion MRI scan/data that there is an aggressive tumour?

Answer 19

in scan, tumour NEOvasculature has heighted CBV CBF and MTT than normal tissues -> allows aggressive growth

Question 20

Do the contrast uptake curve values, CBF CBV and MTT, change with tissue types in the brain?

Answer 20

yes they vary depending on tissue type

Question 21

What can you prescribe a tumour patient?
HOw is perfusion MRI helpful in this process?

Answer 21

take a steroid to remove tumour
you can image the effect of the steroid -> see if tumour is reduced

Question 21

How does ASL work to measure perfusion in the brain?

Answer 21

blood is magnetically labelled in the neck as it flows to the brain -> this is the ‘contrast agent’ but its endogenous

-> difference between labelled and non-labelled blood is proportional to CBF

Question 22

What is the benefit of using arterial spin labelling (ASL) instead of perfusion MRI?

Answer 22

ASL measures perfusion without having to have inject exogenous contrast agent in patient
-> less invasive

Question 23

How do you measure CBF in ASL?

Answer 23

-> difference between labelled and non-labelled blood is proportional to CBF

Question 24

Why must you add a mask to raw ASL images?

Answer 24

-because its hard to see with naked eye the differences between labelled and non-labelled blood

Question 25

What is they key limitation to using ASL?

Answer 25

ASL is good for showing contrast in blood flow in GM but not great in WM. there is weak ASL signal in WM and strong in GM

Question 26

Why is there weak ASL signal in WM?

Answer 26

weak signal because in WM there is lower baseline perfusion and longer arterial transit times -> lower signal-to-noise ratio

Question 27

In ASL, why does WM appear hypoperfused even when its healthy normal WM?

Answer 27

appear as really low perfusion/an artefact because of the weak ASL signal WM has due to low baseline perfusion in WM

Question 28

What is the basic outline of permeability MRI in steps?

Answer 28

1) acquire baseline pre-contrast images 2) inject contrast agent 'bolus' 3) observe signal change for a period up to 30 minutes 4) convert signal change into contrast agent concentration

Question 28

What is DCE-MRI? What type of MRI is it?

Answer 28

dynamic contrast enhanced MRI Permeability MRI

Question 29

What does permeability imaging investigate? Why?

Answer 29

attempt to quantify the permeability of the blood vessel endothelial wall many disease have imperfections in the blood vessel wall -> causing changes in blood vessel endothelial wall permeability

Question 30

Which structural image does permeability MRI use? why?

Answer 30

T1 because T1 is sensitive to leakage if contrast agent into the EXTRAcellular space (EES)

Question 31

What information does single DCE-MRI image supply? -what does many of these images give^? What does it stand for? What type of MRI imaging is it?

Answer 31

-information on the amount of leakage at a specific time -DYNAMIC info on leakage -dynamic contrast enhanced MRI -permeability MRI

Question 32

Permeability MRI: What do the contrast agent uptake curve look like in healthy tissue and in benign and malignant tumours? What are the x and y axis labels

Answer 32

healthy = no change = flat line on x-axis benign = large increase but less steep than malignant malignant = larger and faster than benign = larger increase and steeper x = time y = signal enhancement

Question 33

What is the issue with signal in DCE-MRI? What must you use instead?

Answer 33

-you can't directly infer concentration of contrast agent from signal intensity -pre-contrast tissue (T10) must be measured to enable contrast agent concentration to be calculated from the signal changes

Question 34

What is T10? Why is it important

Answer 34

-T10 = pre-contrast tissue T1 relaxation time -the signal intensity depends on the T1 relaxation time of tissue -> therefore you can't infer the contrast agent concentration directly from the signal intensity

Question 35

Why is T10 important?

Answer 35

In T1-weighted MRI, signal intensity depends on the T1 relaxation time of tissue. When a contrast agent like gadolinium is injected, it shortens the T1 of tissues it enters. To determine how much T1 has changed, and therefore how much contrast agent is present, you must know the tissue’s original T1 — that is, T10

Question 36

Give an example of modelling in permeability MRI? Why is it effective?

Answer 36

-pharmacokinetic model describing the interaction between blood plasma and extravascular space -the contrast agent uptake curves produced from the model are very similar to the curves observed in human tissues

Question 37

When does angiogenesis occur? What does angiogenesis do to the BBB?

Answer 37

-when tumours want to grow more they create their own blood vessels -new blood vessels are formed imperfectly without tight endothelial junctions -> BBB is impaired and more permeable/leaky

Question 38

What is the endothelial pore size in healthy blood vessels and tumour blood vessels?

Answer 38

tumour is 2000nm whereas healthy is 4nm

Question 39

What is the function of permeability MRI with steroids and tumour?

Answer 39

-response to steroid tumour treatment can be vary variable -> permeability MRI is used to see progress/no response to treatment by looking at BBB integrity

Question 40

How is initial relative CBF MRI imaging used in steroid treatment for tumour patients?

Answer 40

initial CBF is a good predictor if patient will respond to steroid treatment or not