MRI Flashcards
1
Q
T1 vs T2-weighted images
A
- 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
2
Q
What parameter does contrast depend on in MRI?
A
- T1 and T2 relaxation times, but can also depend on other parameters such as water diffusion, blood flow or perfusion
3
Q
What can an MRI tell us?
A
- anatomy (lesions, abnormal structures, structural changes)
- Function (blood flow, metabolic changes)
- Response to treatment (lesion change, functional changes)
4
Q
Where does the magnetism come from?
A
Spinning protons within water molecules
- magnetic field doesnt line up completely, so it precesses around the field - allows interference
5
Q
What is the Larmor equation?
A
- Allows us to work out the exact radiofrequency we have to put in to get the axis to be lined up with the field
6
Q
What is the basic structure of an MR machine?
A
- 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
7
Q
What is T2-relaxation?
A
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)
8
Q
What is T1-relaxation?
A
The magnetisation recovers along the Z axis
- again longer for fluid to recover
9
Q
Pulse sequence
A
- MR image is built up from a series of signal acquisitions
- repeated several hundred times
- Echo time and Repetition time determine the contrast
10
Q
Pulse sequence for T2
A
- 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
11
Q
Why the differences in T2-relaxation times?
A
- 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
12
Q
Cerebellar infarct on T2-weighted MR
A
- 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
13
Q
Pulse sequence for T1
A
- 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
14
Q
Differences in T1-relaxation times
A
- 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
15
Q
Contrast agents
A
- 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
16
Q
MS lesions on T1 and T2
A
- 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.
17
Q
fMRI
A
- 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
18
Q
fMRI and T2
A
- long T2 in parts with oxygenated Hb, but when oxygen is used, the blood becomes paramagnetic
- distortion in magnetism
- shorter T2 relaxation
19
Q
MRS
A
- 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
20
Q
MRS and electrons
A
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
21
Q
Give 3 examples of chemicals observed by MRS
A
- 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
22
Q
Lupus and MRS
A
- NAA is reduced by 16% in lesions - decreases because of inflammation
23
Q
Safety issues with MRI
A
- Machine always turned on - no metal objects allowed in room
- no pacemakers, infusion pumps
- cant do it in 1st trimester
