MRI Spectroscopy
Basics
- 3 peaks (Cho, Cr, NAA) and lipi-lactate peak
Cho & Cr
- Always together
- Energy marker
- Around 3 (b/c Cr rhyms w/ 3)
NAA
- Neuronal health (N for 'N'euron)
- Double (2x) Cho (NAA has double b/c 2 As)
Rules
❶Hunters angle
- UP is good, DOWN is bad
❷Ratios
-NAA/Cr(abN <1.6)→ viability of the tissue
-Cho/Cr (abN >1.5)→ indicates cellular proliferation, used to grade neoplasms
-Cho/NAA(N=0.6) ⬆️SN&SP than NAA/Cr & Cho/Cr ratio to differentiate gliomas
❸TE (echo time)
- Short TE→ long spectrum, lots of extra peaks (but variable baseline)
- Long TE→ short spectrum (baseline more stable)
>Higher field (1.5 vs 3T)→ better baseline
❹Signature
- Each region has a unique signature
>GM: ⬆️ Cho and Cr, ⬇️NAA
>WHM: ⬆️NAA
- Select control appropriately
❺Lactate
- Lactate peak is a sine wave, TE-dependent
>Short TE→ peak 🔼
>Middle TE→ peak 🔽
>Long TE→ peak 🔼
❻Peak valuable table
- Except by Canavan, all other conditions are nonspecific
Considerations
History
- 1H (Proton)-MRS available since 1980
>Add 5-10 min extra; but, limited clinical value
>1.5T and above
XY axis
X axis=resonance frequency of metabolite (ppm)
Y Axis=Height of molecule peak depends upon concentration and available 1H
AUC=concentration of metabolite
-3RF pulses 90, 180, and 180 (≠ angle)
-Free induction decay ➡ Fourier transform ➡ Spectrogram
>Metabolites waves are compared to water
>Remember to suppress water; otherwise, huge peak and no metabolites
MRS types
-Single-voxel (PRESS) vs multi-voxel (MRSI/CSI)
>Better multi-voxel, but takes longer time
>MC in neuro is for tumor (board and interior)→ use multi-voxel