Last Session

Question 1

What is the limitation of reducing TR?

Answer 1

It limits the amount of T1 relaxation that occurs

Question 2

What is FLASH?

Answer 2

Fast Low Angle Single Shot imaging

Question 3

Why do we use FLASH?

Answer 3

You are not tipping gradient as far, therefore it speeds up the process.
Majority of net magnetisation remains in longitudinal, therefore don’t have to wait as long before take the next measurements.

Question 4

What is the Ernst flip angle?

Answer 4

cos alpha = e^(-TR/T1)
It refers to For a fixed TR, the flip angle which gives the maximum signal for particular tissue types.

Question 5

What molecules have most efficient recovery time for T1?

Answer 5

Large molecules for example fat.

Question 6

How does 3D MRI imaging differ?

Answer 6

You get rid of slice selection.
Do phase encoding in two directions, and instead of exciting a slice you can excite a cube in one go. This gives you a 3D data set.

Question 7

What is a disadvantage of 3D imaging?
How can you get around this?

Answer 7

Phase-encoding in two directions means that you have TR in two directions, which increases scan time.

Parallel imaging techniques. Uses information from individual receiver coils, then in post-processing assembles the information.

Question 8

How do you obtain T1 contrast?>

Answer 8

Use spine echo or gradient echo with a short TE and a short TR. Short T1s appear bright (fat), longer T1s appear dark (CSF).

Question 9

How do you obtain T2 contrast?

Answer 9

Obtained using a spin echo sequence with a long TE and TR. Short T2s appear dark, long T2s appear bright

T2* is the same as above, but uses gradient echo

Question 10

What is the inversion recovery method?

Answer 10

Tissue can be nulled out/made dark

Question 11

What is the best sequence to look for blood products?

Answer 11

T2* gradient echo

Question 12

What is FLAIR?

Answer 12

Basically like a T2 sequence (in that grey/white matter not anatomical), inversion pulse gets rid of CSF/water and allows us to therefore see peri-ventricular structures.

Fluid is nulled

Fluid-attenuated Inversion Recovery

Question 13

What is STIR?

Answer 13

Fat is nulled with an inversion pulse. Only water is bright

Short Tau Inversion Recovery

Question 14

How do MRI contrast agents work?

Answer 14

They cause relaxation changes (T1, T2 or T2*) in the surrounding tissue water.
Most common are paramagentic agents, which use metals with unpaired electrons that have a high magentic moment.
Eg Gadolinium, Manganese, Dysprosium

Question 15

What are the two effects you can have from contrast agents?

Answer 15

1. Positive contrast effect - tissue is brighter
2. Negative contrast effect

Question 16

How do you make gadolinium safe for use?

Answer 16

Chelate it.
E.g gadolinium-dota (covalently bonded)

Question 17

What type of contrast agent is gadolinium and how does it work?

Answer 17

Positive - induces
Acts like an enzyme, has an entry point for water to enter, and it is excited and longitudinally recovers (this happens very efficiently as it is at the lamor frequency).
The chelate is also very magentic, so can exert secondary effects due to this at a distance

Question 18

What type of contrast MRI imaging is used in cardiology?

Answer 18

Late gadolinium imaging, looks at infarcted area/fibrotic change

Question 19

What are negative contrast agents?

Answer 19

Iron oxide nanoparticles, which are packaged in stabilising colloid.
E.g dextran

Magnetic effects of iron increases when in crystal form.
Manifests as predominantly T2 effects.