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
What are the technical difficulties?
- Voxel position
- Very big voxel size < 4cm3 poor SNR
- Relatively long acquisition time
- Time intensive post-processing techniques
What is multi-voxel spectroscopy?
Rather than getting spectra from one single voxels -> get spectrum
From number of different voxels - fives spatial efficiency and takes much longer
Better quality data - better shim - better water suppression
What are the clinical applications?
- Tumour identification
- Tumour grading
- Important clinical Applications
- identifying abscesses
- oedema vs tumour infiltration
- recurrent tumour vs radiation necrosis
What is the tumour evaluation for the spectrum from the gliomas?
- A high increase in choline and a decrease in NAA
- this is because of an increase in choline membrane turnover - choline increase
- there is a lot of axonal death in the process - NAA peaks decreases into the house
What is the tumour evaluation for metastasis?
- When the tumour gets so big that the vascularise can’t feed it anymore - anaerobic glycosis - seen in areas where you have a necrotic tumour
- Decrease NAA, increase in choline, there is a lactate doublet
- Lactate doublet is an indication that there is anaerobic glycolysis
- J-coupling between lactate at certain times is 144ms and 288ms you see doublet -inverted relative to 144ms
What is observed in high grade glioma?
- NAA is within the noise
2. Lactate peak seen due to necrotic tissue observed
What is observed in grade IV gliomas?
- Increase in lactate
2. More variability in Cr and Cho detected
What do childhood tumours have?
18 Medulloblastomas:
Increase Tau, Pcho, Glu
Decrease Glen
12 Astrocytomas
High NAA (relative to other tumours)
Low avR
5 Ependymomas
High ml and GPC
Validated by NMR of ex-vivo samples
What is the classic signatures of tumours?
- Decrease in NAA (neuro-axonal loss in the area)
2. Increase in choline (high proliferation of the tumour cells)
What is observed in radiation necrosis?
In necrosis
Decrease all metabolites
Peak intensities
And presence of lactate
With tumour recurrence we would see included in tachometer and decrease in NAA
What is MRS?
- Feasible, reproducible, diagnostic and accurate
- Improved patient outcome and cost
- prospective studies
- increased sample size > 50
- multiple blinded analysis
- quantitative analyses
- confirmation of MRS results (biopsy)
- added value over routine MRI
- impact in diagnostic thinking
What is required in the future for MRS?
- High field systems
- 31P MRS - choline kinase is a target of anti-cancer based treatment
- Multi-modality imaging: MRS, perfusion, DTI
