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PHYS4010: Functional Medical Imaging > MRS > Flashcards

MRS Flashcards

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USMLE® Step 1 flashcards
1
Q

What is magnetic resonance spectroscopy (MRS)?

A

A non-invasive measure of tissue chemistry.

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2
Q

How is MRS carried out?

A

A Fourier Transform of FID is acquired in the absence of gradients to generate information about metabolites in a sample and the contributions to the signal at different frequencies.

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3
Q

State the Larmor equation

A

ω₀ = Larmor frequency
γ = Gyromagnetic ratio
B₀ = magnetic field

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4
Q

What is the Larmor frequency dependent on?

A
  1. Chemical shift
  2. Spin-spin or J-coupling
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5
Q

State three properties that are different for different nuclei (relating to MRS)

A
  1. Gyromagnetic ratio
  2. Natural abundance
  3. Chemical shift ranges
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6
Q

_______ _____ is the primary source of variation in resonant frequency between nuclei of the same isotope.

A

Chemical shift

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7
Q

Give the equation for the variations in the magnetic field experienced by an individual nucleus

A

B = magnetic field
σ = shielding constant

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8
Q

Give the equation for the change in frequency experienced by an individual nucleus

A

f = frequency
γ = Gyromagnetic ratio
B = magnetic field
σ = shielding constant

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9
Q

Give the equation for chemical shift

A

δ = chemical shift
f = frequency

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10
Q

In a molecule, shielding is altered by the neighbouring atoms. Why?

A

Because shielding depends on the interaction between electrons and the magnetic field, so when the electrical environment changes the shielding constant does too.

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11
Q

A deshielded nucleus has a ___ shielding constant. Hence, it has a _____ frequency and a _____ chemical shift.

A

Low
High
High

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12
Q

A shielded nucleus has a _____ shielding constant and can have a ____ or ____ frequency and has a ___ chemical shift.

A

High
High
Low
Low

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13
Q

What is spin-spin (or J) coupling?

A

Further perturbation/splitting of spectral lines due to the perturbation of the local magnetic field surrounding a nucleus.

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14
Q

What causes J coupling?

A

Nuclei possess a small magnetic field (and magnetic moment) which influences other non-equivalent groups through electrons. If a non-equivalent nucleus is less than or equal to 3 bonds away then the resonant frequency of the nucleus will change and spectral lines will be split into multiplets.

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15
Q

What makes two nuclei equivalent?

A

If they have the same chemical shift. Nuclei with different chemical shifts are non-equivalent.

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16
Q

Describe be N+1 rule for spin-spin coupling

A

The number of lines of splitting is given by the number of non-equivalent nuclei in the neighbouring functional group. There will be N+1 lines for every N non-equivalent neighbours.

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17
Q

Describe the intensity ratio of spectral lines split by J-coupling

A

The intensity ratio is given by Pascal’s triangle.

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18
Q

State 2 requirements of H-1 MRS

A
  • A high chemical shift resolution
  • Suppression of the intense water peak (55 M) to see metabolites (0.1-2 mM)
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19
Q

How can spectral peak separation be improved for MRS?

A
  • By moving to higher magnetic fields
  • By improving magnetic field homogeneity
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20
Q

Give the equation for the linewidth of emission spectra

A

Linewidth = 1 / T₂*

21
Q

Describe the shape of the H-1 MRS spectrum

A

4 core resonances are: NAA, Creatine, Choline, and Inositol

22
Q

State the 4 core resonances (peaks) for H-1 MRS and what they represent

A
  • 2 ppm NAA: a marker of neuronal viability so is smaller for patients with neuronal loss (e.g. in neurodegenerative disease).
  • 3 ppm Creatine and phosphocreatine: a measure of energy stores so more visible in demyelination and brain tumours.
  • 3.2 ppm Choline: is elevated with membrane turnover so very high within a tumour.
  • 3.6 ppm Inositol: important in osmotic regulation and is a marker of proliferation.
23
Q

What is the frequency of the NAA peak in H-1 MRS?

A

2 ppm

24
Q

What is the frequency of the Creatine peak in H-1 MRS?

A

3 ppm

25
Q

What is the frequency of the Choline peak in H-1 MRS?

A

3.2 ppm

26
Q

What is the frequency of the Inositol peak in H-1 MRS?

A

3.6 ppm

27
Q

What is the main use for P-31 MRS? Why?

A

To monitor energy states because the main source of energy is ATP (adenosine triphosphate) which contains P-31.

28
Q

Describe the shape of a P-31 MRS spectrum

A
29
Q

Describe the structure of ATP

A
  • 1 Base: adenine
  • 3 phosphate groups: α, β, γ
  • 1 sugar: ribose
30
Q

How many peaks does an ATP spectrum have?

A

3

31
Q

What process converts ATP to ADP and an inorganic phosphate?

A

Hydrolysis

32
Q

State the reaction for the hydrolysis of ATP

A
33
Q

What is the energy required to hydrolyse the γ anhydride bond in ATP?

A

-30 kJ/mol

34
Q

What is PCr (phosphocreatine)?

A

An energy store that is used to replenish ATP with an inorganic phosphate in cases where there isn’t a constant supply of fuel (oxygen and glucose) to maintain ATP levels.

35
Q

State the reaction for the conversion of PCr to form ATP

A
36
Q

What happens to the P-31 MRS when there isn’t an adequate supply of fuel to replenish ATP?

A

The PCr resonance decreases and the Pi resonance increases.

37
Q

State one use of measuring the shift in Pi (inorganic phosphate) resonance

A

Measuring intracellular pH

38
Q

Why can Pi be used to measure intracellular pH?

A

Pi can exist in 2 forms that are found at different pH levels, and have different chemical shifts relative to phosphocreatine so the chemical shift can be used to determine the intracellular pH.

39
Q

What is the chemical shift of H₂PO⁻₄ relative to phosphocreatine?

A

3.29 ppm

40
Q

What is the chemical shift of HPO²⁻₄ relative to phosphocreatine?

A

5.81 ppm

41
Q

What activity causes pH to decrease?

A

Exercise

42
Q

Give the equation for the intracellular pH based on Pi resonance

A

δ = chemical shift relative to phosphocreatine

43
Q

What is the main use of C-13 MRS? Why?

A

Studying brain glucose levels (such as the rate of the TCA cycle) because C-13 has a low natural abundance (~1%) so can be used as a tracer.

44
Q

How is C-13 used to study brain glucose levels?

A
  1. Glucose is labelled with 1 C-13 molecule.
  2. The labelled glucose is metabolised by the brain and it appears in different brain locations.
  3. Measurements of the rate of appearance of the label in C4 glutamate are made to calculate the rate of the TCA cycle.
45
Q

To be useful, MRS spectra must be localised to a ___-______ _____.

A

Well-defined volume

46
Q

What 3 approaches are used to localise MRS spectra to a well-defined volume?

A
  1. Surface coils
  2. Volume selective localisation
  3. Chemical shift imaging (CSI)
47
Q

How are surface coils used to localise MRS spectra?

A

A circular loop is used to pick up signals from below the loop. It increases the signal strength of the selected tissue but only works in a limited region close to the body’s surface.

48
Q

How is volume selective localisation used to localise MRS spectra?

A

Magnetic field gradients in both the z-direction and the xy-direction are used for localisation (like applying slice select multiple times- as the principle for MRI) but no gradient is applied in the acquisition window so that the frequency of spins doesn’t change (PRESS- Point REsolved SpectroScopy).

49
Q

How is chemical shift imaging used to localise MRS spectra?

A

Images of the distribution of MR spectra are taken, like spin warp imaging but with no gradients used during acquisition. It is time-consuming but gives spectra over the whole area of interest.

PHYS4010: Functional Medical Imaging (8 decks)

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