MRI

Question 1

What does MRI use for signal detection?

Answer 1

The signal from H+ atoms nuclei (protons)

Question 2

What does external torque do to the H+ particles?

Answer 2

Makes them ‘precess’ with rotational frequency

Question 3

What is ‘precess’?

Answer 3

Move in a gyrating fashion like the wobble of a spinning top

Question 4

What are the basic principles of MRI?

Answer 4

A strong magnetic field is used to align the proton spins, a radio frequency (RF) pulse briefly rotates the alignment by 90 degrees into the xy plane - the magnetisation in this xy plane is detected as a signal in the recieving coils.

Question 5

How does MRI enable us to measure signal variations between diff regions in an MRI scan and hence generate an image?

Answer 5

• reciever coils measure only the unstable magnetization in the xy plane. After RF applied, in-plane magnetisation diminishes (detected signal fades). This is ‘relaxation’ and is dependent on 2 independent processes, T1 and T2. By utilizing variations in the relaxational processes of protons in different tissues and altering the time at which we measure the signal, we attain contrast and ‘weight’ the contrast to highlight specific tissue types.

Question 6

What is magnetisation in the xy plane known as?

Answer 6

transverse magnetisation

Question 7

What is transverse magnetisation?

Answer 7

magnetisation in the xy plane

Question 8

What is T1?

Answer 8

longitudinal relaxation - z plane

Question 9

What is T1?

Answer 9

Transverse relaxation - xy plane. Rapid and related to de-phasing of the spins bc the spins interact and can be affected by local field inhomogeneities (contrast agents)

Question 10

Fancy name for contrast agents?

Answer 10

local field inhomogeneities

Question 11

What are T1 and T2 features of?

Answer 11

Intrinsic features of biological tissues, and vary widely for diff tissue types.

Question 12

What is the diff between relaxation times in T2 magnetic tagged particles and untagged?

Answer 12

Protons in cells tagged by magnetic particles have a shorter T2 relaxation time than in untagged cells

Question 13

What do superparamagnetic contrast agents do to the contrast and how?

Answer 13

They change the contrast by distorting the mag field around ferromagnetic material in the contrast agent - changing the T2 of water molecules around the ferromag agent.

Question 14

What do paramag contrast agents do to the contrast and how?

Answer 14

Paramagnetic agents change the contrast by creating time varying mag fields which promote T1 and T2 relaxation of the water molecules.

Question 15

What are MRI contrast agents useful for, biomedically speaking?

Answer 15

Detecting tumours, infection, inflammation, infarction and lesions

Question 16

What contrast media is used to enhance contrast in x rays?

Answer 16

Barium and iodine compounds - their presence appears directly in images

Question 17

Why are most MRI contrast agents only about 10nm in diameter?

Answer 17

Because large particles are quickly scavenged by the RES (reticuloendothelial system)

Question 18

What is nanomagnetic actuation?

Answer 18

Using nans to switch on cell channls/trigger receptors/intercellular signalling by mechanical stimulation (aggregation in the plane of the membrane)

Question 19

Whatis a method we use to bind nanoparticles to the cell membrane (to apply mechanical stimulation)

Answer 19

Using a magnetic field gradient to stretch cell membranes via force on the magnetic particles attached to receptors (in this example, integrin receptors)