MRS I Flashcards
What are compounds normally present?
- NAAG, Aspartate
- Taurine, Scylla-inositol
- Betaine, Ethanolaminr
- Purine nucleotides
- Histidine
- Glucose
What are compounds observed using “spectral editing”?
- GABA
- Ascorbic acid
- Glutathione
- Macromolecules
What are compounds which may be detectable under abnormal/pathological conditions?
- B-hydroxy-butyrate, acetone
- Phenylalanine (PKU)
- Galactitol, Ribitol, Arabitol
- Succinate, pyruvate
- Alanine
- Glycine
- Valine,leucine, isoleucine
- threonine
What are exogenous compounds?
- Propan-1,2-diol
- Mannitol
- Ethanol
- MSM
What are single voxel sequences?
- PRESS
2. STEAM
What is PRESS?
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
What is STEAM?
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
What is the relationship between MRI and MRS?
In MRS, the temporal information of FID is not used to encode space
Why do we see different protons at different frequencies?
Because of their different electronic shielding, leading to different perceived magnetic field
What happens when two spins are resonanting very close to each other?
They interact with each other. This interaction can be modelled by an energy constant J
What is the frequency range (in ppm) of MRS spectra if most compounds in the brain at 3.0T?
0.8->4.0ppm; or 102->512Hz
What are the most important metabolites detectable by 1H-MRS?
NAA Choline Creatine Lactate MI, GLX
What is J coupling?
Splitting of spectral peaks into doublets, triplets or multiplet by electron-mediated interaction of two nuclear spins residing on the same molecule
What are two requirements for J-Coupling?
- The nuclei must lie in close proximity to one another , typically less than 3-4 bonds away
- The nuclei must be chemically distinguishable
What is determined by a coupling constant (J)?
The spacings between the sub-peaks
J is independent of field strength and is reported on Hz
Predicted by n+1 rule
What is the 3 largest peaks in brain MRS?
- NAA
- Choline
- Creatine
Do not have nearest neighbours protons
Do not experience J coupling and manifest as singlets
What does the principal peak of lactate near ppm=1.32 split into?
Doublet that is easily visualised
What do single voxel techniques used?
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
What are the three principal SVS ?
- PRESS
- STEAM
- ISIS
What is PRESS?
It uses 3 RF pulses (90-180-180) and generates a spin-echo signal
What is STEAM?
Uses three 90 pulses and generates a stimulated echo
What is ISIS?
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
What is MRSIv
The entire volume can be excited with a non-selective RF pulse
With sampling of FID signal after each phase-encoding step
What is the advantage of MRSI?
- A larger total coverage area
2. High spatial resolution
What is the disadvantage of PRESS?
Limitation of its minimum achievable TE
What is PRESS?
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
What is the process of fat suppreseion?
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
How do you saturated fat peak?
Apply a narrow bandwidth RF pulse tuned to centre of lipid resonance
What is the concentration of water ?
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
What is outer volume suppression?
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
What happens if shimming is not good?
End up hitting water
Shimming is the process by which the main magnetic field B0 is made more homogenous
CHESS pulse - water suppression
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
What are the common nuclei in in-vivo MRS?
1: proton
2: phosphorus
3: sodium
4: carbon
5: deuterium
What are criteria for good detectability ?
- High gyromagnetic ratio
- High natural abundance
- Spin 1/2
- High concentration
Why is it difficult to use PRESS/STEAM?
Short T2
What is CSI?
Uses phase-encoding (in whole or in part) for localisation
ADC: larger spatial coverage and smaller voxel sizes
Dis: imaging time, spectral contamination
Why do we need larger voxels in non -1H MRS?
Signal is scalable with resonance frequency and their gyro magnetic ratio is much smaller than that of 1H
Why does the chemical shift displacement cause localisation problems?
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
Why is water suppression important?
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
