MRI

Question 1

T1 vs T2-weighted images

Answer 1

• T2 = very bright image from CSF, grey from brain and not much from fat in skin and bone marrow
• T1 = CSF dark, good contrasts between grey and white matter

Question 2

What parameter does contrast depend on in MRI?

Answer 2

• T1 and T2 relaxation times, but can also depend on other parameters such as water diffusion, blood flow or perfusion

Question 3

What can an MRI tell us?

Answer 3

• anatomy (lesions, abnormal structures, structural changes)
• Function (blood flow, metabolic changes)
• Response to treatment (lesion change, functional changes)

Question 4

Where does the magnetism come from?

Answer 4

Spinning protons within water molecules

- magnetic field doesnt line up completely, so it precesses around the field - allows interference

Question 5

What is the Larmor equation?

Answer 5

• Allows us to work out the exact radiofrequency we have to put in to get the axis to be lined up with the field

Question 6

What is the basic structure of an MR machine?

Answer 6

• patient lies in bore of superconducting magnet
• RF coils surrounding the patient - can transmit a radiofrequency through the patient and pick up the signals they give off from the water molecules
• RF receiver
• computer > MR image
• use magnetic field gradients to be able to take a slice in the body

Question 7

What is T2-relaxation?

Answer 7

The decay of the magnetic field in the XY axis

• more fluid it is, the longer it takes (CSF = bright)
• something rigid will be much quicker (cartilage = dark)

Question 8

What is T1-relaxation?

Answer 8

The magnetisation recovers along the Z axis

- again longer for fluid to recover

Question 9

Pulse sequence

Answer 9

• MR image is built up from a series of signal acquisitions
• repeated several hundred times
• Echo time and Repetition time determine the contrast

Question 10

Pulse sequence for T2

Answer 10

• if you wait for a certain amount of time, you get better contrast due to the T2-relaxation
• CSF dies away slowly, fat dies quickly, grey and white matter are in the middle
• Get contrast due to differential signals

Question 11

Why the differences in T2-relaxation times?

Answer 11

• signals from cell membrane die away very quickly - associated water cannot move very much
• within cell is IC water, and there is EC water - both mobile - signal lasts longer
• Any necrotic damage means you lose rigidity of membrane and get more water - longer t2

Question 12

Cerebellar infarct on T2-weighted MR

Answer 12

• longer echo time, you will get much better contrast from the increased mobile water from the infarct - much stronger signal compared to rest of brain
• lesions in the brain will show up better with t2

Question 13

Pulse sequence for T1

Answer 13

• Recovery of magnetisation along Z axis (at first its along Z - get no signal, give RF pulse, knocks into XY axis)
• CSF has very long T1
• more structured tissue is quicker
• Because you put next RF pulse in before CSF returns to Z axis, it wont give much of a signal

Question 14

Differences in T1-relaxation times

Answer 14

• get much more contrast between grey and white matter
• white matter has lots of myelin, so more water is bound, so quicker than grey matter
• fat shows up bright, CSF dark

Question 15

Contrast agents

Answer 15

• if you chelate Galadium, you can inject it safely
• when water gets in the vicinity, there are strong fluctuations in the magnetic field - decreased relaxation times of both T1 and T2
• If T1 is quick, then with the agent, it will get brighter
• in T2, image will get darker
• difference is bigger with T1 than T2

Question 16

MS lesions on T1 and T2

Answer 16

• T2, CSF is brighter, increase in intensity due to demyelination so more free protons - longer t2 relaxation.
• T1, clear grey and white matter demyelination, darker as there is more water.

Question 17

fMRI

Answer 17

• can use fMRI to interrogate the brain while pt is performing motor or mental tasks
• use BOLD contrast
• DeoxyHb is much more magnetic, so when there is a change in oxygenation of blood, there is a change in MR signal
• When they perform a task, brain becomes active, change in blood flow, changes amount of oxygenated blood in brain, changes MR signal, can detect where in the brain it is being activiated

Question 18

fMRI and T2

Answer 18

• long T2 in parts with oxygenated Hb, but when oxygen is used, the blood becomes paramagnetic
• distortion in magnetism
• shorter T2 relaxation

Question 19

MRS

Answer 19

• spectroscopy doesnt look at protons in water, it uses protons from other molecules such as cholines, creatinines, aspartate, lipids, glutamate and glutamine.
• signal intensity is very small due to low concs so have to use much smaller areas of brain
• protons within molecules precess at different frequencies due to electron cloud shielding

Question 20

MRS and electrons

Answer 20

In water, H atoms are bound to oxygen, electrons more around the oxygen

• in lipids, H atoms are bound to carbon, electrons more around the H atoms
• therefore nucleus of H is much more shielded in lipids, so you get a lower frequency
• can use oscillations to make frequency peaks - can then use these to detect what chemicals are present in other areas

Question 21

Give 3 examples of chemicals observed by MRS

Answer 21

• N-acetyl aspartate - found predominantly in neurons, marker for viable neurons (reduced in pathology)
• Lactate - high in tumours and stroke
• Alanine - marker for meningiomas
• lipids - membrane breakdown product

Question 22

Lupus and MRS

Answer 22

• NAA is reduced by 16% in lesions - decreases because of inflammation

Question 23

Safety issues with MRI

Answer 23

• Machine always turned on - no metal objects allowed in room
• no pacemakers, infusion pumps
• cant do it in 1st trimester