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๐˜พ๐™š๐™ก๐™ก๐™ช๐™ก๐™–๐™ง ๐™‹๐™–๐™ฉ๐™๐™ค๐™ก๐™ค๐™œ๐™ฎ ๐Ÿงซ COPY COPY > MRI & MRS > Flashcards

MRI & MRS Flashcards

1
Q

Colour Cryo Section:

A

Transverse Photograph through the body.

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2
Q

CT (Computer Tomography):

A

Best images for Bones.

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3
Q

MRI (Magnetic Resonance Imaging):

A

โ€œBest for Soft Tissue because you pick the signals up from the protons in fat and water.
The protons have to have a certain mobility to generate a signal.
You need different tissues to give different signals.
As this is what provides the contrast for the images.โ€

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4
Q

Describe the Imaging Parameters for CT Scanning:

A

โ€œ1. CT contrast is due to Tissue Density Dependent Attenuation of X-rays.

  1. The Hounsfield Number shows how much the X-rays are attenuated in passing through any material.
    It goes from -1000 in air and +1000 in cortical bone.
  2. There is a small range in the middle which covers water and the Brain is 75% water.
    Haemorrhage shows up very brightly.
    Congealed Blood attenuates the X-rays much more than normal tissue. โ€œ
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5
Q

What advantage does CT have over MRI?

A

โ€œCT is the first port of call because you can put anybody in a CT and it is quick.
MRI is a big magnet so you need to be aware of things like pacemakers. โ€œ

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6
Q

How does MRI generate an image?

A

โ€œThe relative signal intensities between different tissue types and pathologies.
Depends on physical properties of the tissue such as Water, Fat Content and the Cellular Structures.โ€

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7
Q

What is shown in a T2 Weighted MRI?

A

โ€œ1. Ventricular signals are very bright because it is coming from Water.

  1. The signal from Fat in the scalp is much darker.
  2. The Water Content changes the signal intensity.
  3. T2 is more sensitive to the structural qualities of the Brain.โ€
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8
Q

What is shown in a T1 Weighted MRI?

A

โ€œ1. The CSF in the middle is dark.

  1. The Grey and White Matter have different signals.
  2. The Fat has a different signal.
  3. T1 is more sensitive to Water.โ€
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9
Q

Describe how Meningioma would show up on different MRI sensitivities?

A

โ€œT2 Map:
- You can see the inflammation and oedema caused by the tumour.

Diffusion Map:

  • Red represents the most Free Diffusion (CSF).
  • Most of the brain has restricted diffusion but the meningioma has some diffusion (blue/green).

Perfusion Map:

  • Grey Matter has higher perfusion than White Matter (dark blue is the low perfusion in white matter).
  • The Meningioma is Red which means it is very highly vascularised

White Matter Map:

  • Only shows White Matter as being bright.
  • The Tumour shows up as being surrounded by White Matter.โ€
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10
Q

How is an signal generated in an MRI?

A

โ€œ1. The signal is coming from the Hydrogen in Water.

  1. Hydrogen Atoms have a Proton at their core.
  2. The Proton is +ve and it spins around on its axis which generates a Magnetic Moment
  3. The MRI signal generated is Nuclear Magnetic Resonance.
  4. Every Proton in the body aligns with the Magnetic Field of an MRI.โ€
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11
Q

What affect does the Magnet have on the Hydrogen Atom?

A

โ€œThe Hydrogen Axis precesses round the Magnet:

  • It precesses at a specific frequency called the Larmor Frequency.
  • The Frequency of Precession is 138 MHz which causes the Protons to be unaligned.
  • The MRI scanners used have a magnetic field strength of 3 Wb/m^2 or 3 Teslas.โ€
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12
Q

How is an Image generated from MR signals?

A

โ€œ1. When the patient is in the scanner all their Protons Align with the Magnetic Field.

  1. A Radiofrequency Pulse is emitted which knocks the Protons of the Brain by 90 degrees.
  2. This is what generates a signal.
  3. Several Hundred Pulses are needed to create an image.
  4. Instead of having a Flat Magnetic Field that is the same everywhere you have a Gradient. โ€œ
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13
Q

Why is a Gradient of Magnetic Fields needed to generate an image?

A

โ€œMR imaging is formed using a Radiofrequency Pulse to generate an MR signal from a slice of tissue.
Magnetic Field Gradients are used to encode the signal in space.
So that the computer can generate an image. โ€œ

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14
Q

What safety precautions need to be taken during an MRI Scan?

A

No Ferromagnetic Objects in the exam room.

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15
Q

List the Contraindications for an MRI Scan?

A

โ€- Pacemakers.

  • Infusion Pumps.
  • 1st Trimester Pregnancy.
  • Aneurysm Clips.
  • Metallic Foreign Bodies (orbit X-ray, shrapnel). โ€œ
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16
Q

What is the Signal Source in an MRI?

A

โ€œ1. The Strong Magnetic Field creates Magnetisation in ALL the Tissue.

  1. This Magnetisation is from the Protons in Water and Fat in the Tissue.
  2. The Magnetisation can be manipulated by Radiofrequency Pulses to produce an MRI signal to create an image.
  3. The intensity in the image depends on:
    • Water Content
    • Tissue Structure
    • Blood Flow
    • Perfusion
    • Diffusion
    • Paramagnetics โ€œ
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17
Q

How is the Signal Generating Magnestisation Generated?

A

โ€œ1. The Magnetization is along the magnetic field which doesnโ€™t give a signal.

  1. You put a Radiofrequency Pulse in that knocks the magnetization at 90 degrees.
  2. This is the Signal Generating Magnetisation.โ€
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18
Q

What times are there in an MRI?

A

โ€œPulse Sequence:
The time of putting the Radio Frequency Pulse.

Echo Signal:
The time between the Radiofrequency Pulse and picking up the signal.

Repetition Time:
The time between each of the Radiofrequency Pulses.โ€

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19
Q

What is the T2 Relaxation Time?

A

โ€œThe time it takes for the 90 degree Magnetisation to Decay away.
Different Tissues have this loss of Magnetisation at different rates.โ€

20
Q

What factor determines the T2 Relaxation Time?

A

โ€œDepends on the properties of the Tissue:

  • Fluid like CSF the signal dies away in a few seconds.
  • Rigid like Fat Tissue the signal dies away in hundreds of milliseconds.โ€
21
Q

What are the two Radiofrequency Pulses for T2?

A

โ€œ1. Knocks the protons through 90 degrees.

2. Knocks the protons through 180 degrees.โ€

22
Q

From what areas can Signals be picked up from?

A

โ€œWithin the tissue you are looking at there is Extracellular and Intracellular Water.
Signals will be generated from both these areas.
Signals from Cell Membranes are NOT picked up as they drop away very quickly.โ€

23
Q

What does the T2 determine?

A

The T2 of tissue determines how quickly the MRI Signal decays away after a Radiofrequency Pulse.

24
Q

What can Increase T2?

A

โ€œT2 is very dependent on how mobile the water is in the Tissue.
The T2 increases with:
- Oedema: an increase in Water Content.
- Demyelination: a loss of Brain Tissue Structure.โ€

25
Q

What can Decrease T2?

A

โ€œT2 is Reduced by the presence of Paramagnetic Ions:

  • Fe from Blood Breakdown Products.
  • Gd (Gadolinium) from Contrast Agents.โ€
26
Q

How does changing the Echo Time affect what is seen?

A

โ€œ1. A short Echo Time shows mostly Proton Density.

  1. As you increase the Echo Time the CSF remains bright because it has a Long T2.
  2. The Brain Tissue gets darker because it has a Short T2.โ€
27
Q

What is the T1 Relaxation Time?

A

โ€œWhen the Magnetisation comes back along the Z axis.
This depends on Tissue Structure.
Tissues with Long T1 produce a Smaller Signal than Tissue with Short T1. โ€œ

28
Q

Why does there have to be a pause between each Pulse?

A

โ€œAn MR image is built up from a series of Signal Acquisitions:

  1. The Magnetisation has to realign with the Magnetic Field.
    This is the T1 Relaxation.
  2. T2 Relaxation and T1 Relaxation occur independently.
  3. After a Radiofrequency Pulse is generated there is Magnetisation along the XY axis that is generating the signal.
    But there is NO Magnetisation along the Z axis
  4. So adding another Pulse would NOT generate another Signal.
    You have to wait for the Magnetisation to realign along the Z axis.
  5. You have to wait for the T1 Relaxation to occur,
    Before the Signal Strength/Intensity can recover.โ€
29
Q

What differences are there in T1 & T2 in the Body?

A

โ€œCSF and other Fluids have Long T1 and T2.

Tissue has a Shorter T1 so the Signal Strength (Mz) recovers more quickly.โ€

30
Q

How does a short Repetition Time (TR) affect the MRI?

A

โ€œIf you have a short Repetition Time (TR):

  • You are not allowing for recovery along the Z axis.
  • As the the time between each Radiofrequency Pulse is too short.
  • When the repetition time (TR) between Pulses is much shorter than T1.
  • The Magnetisation that can produce the MRI Signal is reduced
  • As it has been Saturated.
  • As a result of this saturation Tissues with Long T1 produce a Smaller Signal than Tissues with Short T1. โ€œ
31
Q

What are the two types of Contrast Agents?

A

โ€œ1. Paramagnetic (Unpaired Electrons).

2. Superparamagnetic (Ferrites).โ€

32
Q

How do the Contrast Agents work?

A

โ€œThe contrasts are Chelated to reduce toxicity.
Wherever this molecule encounters Water it has a very Large Reduction in T1 Relaxation Time.
So the Signal Intensity increases.โ€

33
Q

Why do Tumours show a change in Signal Intensity?

A

โ€œ1. The Blood Vessels created by Tumours are very poor quality,

  1. Unlike in the normal Blood Brain Barrier where the Contrast Molecules canโ€™t get out,
  2. In Tumours the Contrast Molecules can get across.
  3. The majority of the Brain does not show a change in signal intensity because the Blood Brain Barrier is intact.โ€
34
Q

What areas show up in an MRI of a Brain Tumour?

A

โ€œThe Sagittal Sinus at the back of the brain shows up brightly because the T1 in the Large Vein is High.
Due to the large amount of Fluid (Venous Blood) it holds.

The larger area to the left shows the Contrast Agent leaking out of the Blood Brain Barrier.โ€

35
Q

What is Multiple Sclerosis?

A

โ€œThere is inflammation so there is an increase in water content.
There is also Demyelination and White Matter is lost which leads to less bound water and more free water.โ€

36
Q

What can be seen in an MRI on Multiple Sclerosis?

A

โ€œWith the T2 weighted you can see ALL the Lesions.

With the T1 weighted you can see the Active Lesion and where there might have been a Blood Brain Barrier leak.โ€

37
Q

What is Magnetic Resonance Spectroscopy (MRS)?

A

โ€œInstead of getting an image you can specify a Small Region of Brain where you get a Small Spectrum
The columns correspond to a different chemical like in NMR.
This is less sensitive than MRI because the regions of the Brain that are being investigated are Smaller. โ€œ

38
Q

How does MRS work?

A

โ€œ1. The Signal Intensity from different Chemicals is different because each substance is surrounded by an electron cloud
Which shields the proton slightly form the magnetic field.
2. Depending on the Chemical structure the Shielding changes.
3. A stronger Shielded Nucleus has a lower Resonant Frequency.โ€

39
Q

Which chemicals are found in which types of cells?

A

โ€œ1. N-Acetylaspartic acid (NAA) comes from Neurons.
NAA goes down as you Lose Neurons.

  1. Myo-inositol (MI) is found in Glial Cells.
    MI can go up during inflammation or if there is an increase in Glial Cells.
  2. Choline (Cho) & Creatinine (Cr) are found in All Cells. โ€œ
40
Q

Evaluate the usefulness of an MRS?

A

โ€œA 15mm section takes 5 minutes which is less efficient than an MRI.
But it is the only way to get a Chemical Analysis of the brain non-invasively,
And relate it to Pathological changes in the brain.
The MRI can look normal but MRS is used to detect chemical imbalances.โ€

41
Q

What is observed normally in the 1H MRS of the brain?

A

โ€œNAA (N-acetyl aspartate):

  • Found predominantly in neurons.
  • Marker for viable neurons, reduced in pathology.

tCR (Cr and PCr):

  • Cell energy metabolism.
  • Marker of viable cell density in some tissue.

tCho (Cho, GPC, PC):

  • Cell membrane metabolism (growth and degradation).
  • Elevated in tumours and gliosis.

Glx (glutamate & glutamine):
- Amino Acids.

MI (myo-Inositol)
- Osmolyte found in glial cells, marker for gliosis inflammation, elevated cell membrane synthesis.โ€

42
Q

What is not normally observed in the 1H MRS of the brain?

A

โ€œLac (lactate):

  • End product of anaerobic metabolism.
  • High in tumours & stroke.

Ala (alanine)
- Amino acid, marker for meningiomas.

Lipids:
- Membrane breakdown products.

Macrophages.โ€

43
Q

What is Neuropsychiatric Systemic Lupus?

A

Inflammatory Disease that can spread to the brain.

44
Q

What two spectra are generated in Neuropsychiatric Systemic Lupus?

A

โ€œ1. Comes from the Parietal White Matter where there are no lesions.
2. Comes from a region where there is a small Stroke.

The NAA peak is reduced in both spectra which shows Neuronal Degeneration.
MI is increased in both regions which could show Gliosis Processes or Inflammation. โ€œ

45
Q

What MRS is seen in a Low Grade Tumour?

A

โ€œLow Grade Tumour:

  • There is no NAA peak because there are no neuronal structures within the tumour.
  • Which means it would be safe to remove it Neurosurgically.
  • tCho is elevated compared to tCr which is a marker for tumours.
  • MI is elevated which is a marker for the presence of Glial Cells.โ€
46
Q

What MRS is seen in a High Grade Tumour?

A

โ€œHigh Grade Tumour:

  • There is no NAA which means that it is Pure Tumour.
  • tCho is elevated compared to tCr.
  • There is a large peak of Lipids which shows that it is Necrotic.
  • One of the markers of High Grade Tumours is Necrosis,
  • Because they are growing so big that they cannot generate their own blood supply in time.โ€

๐˜พ๐™š๐™ก๐™ก๐™ช๐™ก๐™–๐™ง ๐™‹๐™–๐™ฉ๐™๐™ค๐™ก๐™ค๐™œ๐™ฎ ๐Ÿงซ COPY COPY (13 decks)

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