Magnetic Resonance Spectroscopy Of Tumours Flashcards
What is in-vivo spectroscopy?
- The main peak we see in-vivo human brain is NAA
- Each of the peaks is unique to a metabolite in the brain
- Each of the metabolite have different chemical environment and therefore resonate at different frequencies
- The most abundant is NAA in healthy brain tissue
- In the proton spectra we can also measure other nuclei such as: 1H, 31P, 13C which give different information
What is MRS physics?
- Standard imaging is from water molecules
- There are other proton containing metabolites in our body other than water - however they have very different chemical environment, and this gives them very unique resonances because each of the nuclei are shielded by an electron cloud
- The higher the shielding effect, the lower the resonating frequencies
- There are different resonances for different chemical structures and therefore metabolites
What is the chemical shift scale?
- If we measure things in parts per million - all the metabolites have a set ppm where they occur
- These are measured relative to 0ppm which is where you get a resonance for tetramethylsilane which is not found in the body - this is very highly shielded
What is the spectral analysis?
- Analyse the spectrum
- LC model has a basis set of different metabolites that e.g. you are likely to find in a brain, in a tumour and it is fit to the data so it knows what the ppm difference should be for e.g. NAS and choline etc
- It can determine what each of the peaks are from that and measure the area underneath the peak and that can give us a concentration
What are the practical points for magnetic resonance?
- Identify peaks from their positions
- Narrow line widths - area well shimmed
- Area under the peak proportional to metabolite concentration
- Spectrum is scaled to the highest peaks
What is PRESS?
- 90-degree pulse
- Selective in one plane
- Followed by two 180-degree pulses that are selective in the other two planes
- Get a signal that is the intersection of those three planes
- The other signals are de phased by these grades
- Achieve echo time of about 30ms
What is STEAM?
- 90-90-90 to select voxels
- The next pulse is flipped onto the longitudinal axis therefore it is spending less time in that transverse plane and has less opportunity to dephase - this is how we achieve short echo times
Disadvantage:
- loose half of the signal
What are the 3 metabolites that are observed at longer echo times?
- NAA
- Cr
- Cho
What are the metabolites observed at shorter echo times?
See GLX, Myoinositol, more lipids and lactate
What is NAA?
- Neuronal marker
2. It is the main peak we see in healthy tissue
What is tCH?
Marker if cell membranes turnover
What metabolites are important to identify a tumour?
- NAA
2. tCh
What is tCr?
Marker of overall brain energy metabolism
What is GLX?
Combination of glutamate and glutamine - excitatory and inhibitory neurotransmission
What is lactate?
Marker of anaerobic glycosis