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P2 PHYSICS > MRS I > Flashcards

MRS I Flashcards

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1
Q

What are compounds normally present?

A
  1. NAAG, Aspartate
  2. Taurine, Scylla-inositol
  3. Betaine, Ethanolaminr
  4. Purine nucleotides
  5. Histidine
  6. Glucose
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2
Q

What are compounds observed using “spectral editing”?

A
  1. GABA
  2. Ascorbic acid
  3. Glutathione
  4. Macromolecules
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3
Q

What are compounds which may be detectable under abnormal/pathological conditions?

A
  1. B-hydroxy-butyrate, acetone
  2. Phenylalanine (PKU)
  3. Galactitol, Ribitol, Arabitol
  4. Succinate, pyruvate
  5. Alanine
  6. Glycine
  7. Valine,leucine, isoleucine
  8. threonine
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4
Q

What are exogenous compounds?

A
  1. Propan-1,2-diol
  2. Mannitol
  3. Ethanol
  4. MSM
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5
Q

What are single voxel sequences?

A
  1. PRESS

2. STEAM

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6
Q

What is PRESS?

A

Only protons in lying at the intersection of 3 crossing planes experience all 3 RF pulses and generate PRESS echo

The selected planes are usually orthogonal

The resultant voxel has a cuboid shape

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7
Q

What is STEAM?

A

Here all 3 RF pulses are applied simultaneously with slice select gradient along x y z axes

This produced a STR from the voxel at the intersection of 3 planes

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8
Q

What is the relationship between MRI and MRS?

A

In MRS, the temporal information of FID is not used to encode space

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9
Q

Why do we see different protons at different frequencies?

A

Because of their different electronic shielding, leading to different perceived magnetic field

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10
Q

What happens when two spins are resonanting very close to each other?

A

They interact with each other. This interaction can be modelled by an energy constant J

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11
Q

What is the frequency range (in ppm) of MRS spectra if most compounds in the brain at 3.0T?

A

0.8->4.0ppm; or 102->512Hz

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12
Q

What are the most important metabolites detectable by 1H-MRS?

A 
NAA
Choline
Creatine
Lactate 
MI, GLX
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13
Q

What is J coupling?

A

Splitting of spectral peaks into doublets, triplets or multiplet by electron-mediated interaction of two nuclear spins residing on the same molecule

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14
Q

What are two requirements for J-Coupling?

A
  1. The nuclei must lie in close proximity to one another , typically less than 3-4 bonds away
  2. The nuclei must be chemically distinguishable
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15
Q

What is determined by a coupling constant (J)?

A

The spacings between the sub-peaks

J is independent of field strength and is reported on Hz

Predicted by n+1 rule

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16
Q

What is the 3 largest peaks in brain MRS?

A
  1. NAA
  2. Choline
  3. Creatine

Do not have nearest neighbours protons
Do not experience J coupling and manifest as singlets

17
Q

What does the principal peak of lactate near ppm=1.32 split into?

A

Doublet that is easily visualised

18
Q

What do single voxel techniques used?

A

Sequentially applied RF-pulse coupled with gradients in 3 orthogonal planes

The intersection of these planes defines a cube shaped voxel as the source of MR signal

19
Q

What are the three principal SVS ?

A
  1. PRESS
  2. STEAM
  3. ISIS
20
Q

What is PRESS?

A

It uses 3 RF pulses (90-180-180) and generates a spin-echo signal

21
Q

What is STEAM?

A

Uses three 90 pulses and generates a stimulated echo

22
Q

What is ISIS?

A

Used primarily for 31-P spectroscopy
FID signals are generated from 8 separate RF pulses cycles
Then added and subtracted in a certain order to define volume of interest

23
Q

What is MRSIv

A

The entire volume can be excited with a non-selective RF pulse
With sampling of FID signal after each phase-encoding step

24
Q

What is the advantage of MRSI?

A
  1. A larger total coverage area

2. High spatial resolution

25
Q

What is the disadvantage of PRESS?

A

Limitation of its minimum achievable TE

26
Q

What is PRESS?

A

Core sequence consists of 3 slice-selective RF pulses (90-180-180) applied concurrently with three orthogonal gradients (x,y,z)

The PRESS signal at time T/E is a spin echo derived only from protons that have experienced all 3 RF pulses

These protons are located in a cuboidal-shaped box where the three imaging planes overlap

27
Q

What is the process of fat suppreseion?

A

Short-duration RF pulses turned to the resonance frequency of fat

Apples immediately before start of an MR imaging sequence

These chemically selective pulses cause signal from fat to be nulled while the water signal is relatively unaffected

28
Q

How do you saturated fat peak?

A

Apply a narrow bandwidth RF pulse tuned to centre of lipid resonance

29
Q

What is the concentration of water ?

A

In tissue at concentration 10,000 higher than metabolites of interest

An unprepared MR spectrum would this be dominated by a giant water peak while small organic molecules would be virtually undetectable above background noise

30
Q

What is outer volume suppression?

A

Eliminate unwanted fat signal is to place multiple saturation band over lipid containing regions

The bands are spatially but not frequency specific, reducing or eliminating signals from tissues

They are commonly placed in planes above or below volume of interest

31
Q

What happens if shimming is not good?

A

End up hitting water

Shimming is the process by which the main magnetic field B0 is made more homogenous

32
Q

CHESS pulse - water suppression

A

Tune in CHESS pulse to resonant frequency of water

Chess pulse selectively rotates water magnetisation into transverse plane where it is immediately de phased by strong spoiler gradient

3 chess pulses are required and they are long

33
Q

What are the common nuclei in in-vivo MRS?

A

1: proton
2: phosphorus
3: sodium
4: carbon
5: deuterium

34
Q

What are criteria for good detectability ?

A
  1. High gyromagnetic ratio
  2. High natural abundance
  3. Spin 1/2
  4. High concentration
35
Q

Why is it difficult to use PRESS/STEAM?

A

Short T2

36
Q

What is CSI?

A

Uses phase-encoding (in whole or in part) for localisation

ADC: larger spatial coverage and smaller voxel sizes

Dis: imaging time, spectral contamination

37
Q

Why do we need larger voxels in non -1H MRS?

A

Signal is scalable with resonance frequency and their gyro magnetic ratio is much smaller than that of 1H

38
Q

Why does the chemical shift displacement cause localisation problems?

A

Each metabolite has it’s own resonant frequency defined by environment of spins within metabolite through electron shields effects

Localisation depends on frequency so two peaks at different frequencies can be interpreted as having the same frequency but coming from different locations so voxels will appear displaced

39
Q

Why is water suppression important?

A

If water suppression was not used, the water signal will dominate and obscure the metabolite signal because the inherent water signal is larger in amplitude than the metabolite signal

P2 PHYSICS (16 decks)

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