Magnetic Resonance Spectroscopy (MRS)
by Denis Hoa
After reading this chapter, you should be able:
- To set out the physical mechanisms used to differentiate metabolites according to resonance frequency
- State the material conditions and optimization required to measure a spectrum
- List the quality criteria of a spectrum
- Present the different metabolites explored in brain MRS, their position on the spectrum and their interest
- Explain the different stages to obtain a monovoxel spectrum
- Describe PRESS and STEAM sequences and the influence of TE on the spectrum
- Specify the adaptations required for spectroscopic imaging: spatial encoding, reduced acquisition time
- The resonance spectrum identifies metabolites by:
- Locating the peak(s), determined by chemical shift (ppm) resulting from the shield formed by the electronic cloud of hydrogen nuclei in the molecules
- The same compound being characterized by several peaks (doublet, triplet) due to spin-spin coupling (or J coupling) phenomena: Lac (1.7 and 1.33 ppm)
- Magnetic resonance spectrometry requires a very homogeneous magnetic field (shimming), a volume voxel and a sufficient number of measurements
- Principal metabolites studied in MRS of the 1H nucleus: see tab. 15.1
- Any method of spectroscopy, calls for suppression of the water signal (CHESS), and possibly of the fat signal: present in large quantities in the body, these have a masking effect on the metabolites close to their resonance peaks.
- The basic spectroscopy sequences are PRESS and STEAM. PRESS records a spin echo whereas STEAM only records a stimulated echo, of weaker intensity. Both have an excitation pattern comprised of 3 RF pulses.
- In single voxel spectroscopy the 3 RF pulses select the voxel of interest, located at the intersection of the 3 orthogonal planes. The recorded echo comes from the voxel submitted to the 3 RF pulses only.
- In chemical shift imaging (CSI), the 3 RF pulses select a slice or volume that is spatially encoded by phase gradients. There are different methods of accelerating CSI data acquisition. Chemical shift imaging yields multiple spectra of the slice or volume of interest. These are represented as a parametric image or studied separately.
- Quantification tends to be relative, although it is possible under certain conditions, after calibration, to produce absolute quantification of the metabolite concentration.
- The fields of application of magnetic resonance spectrometry are essentially those of: tumors, inflammatory and infectious pathologies and metabolic pathologies, mainly of the brain. Numerous developments are taking place in regard to other organs (prostate, breast, bones and joints…)
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