Discussion

In summary, the routine non-invasive determination of regional brain NAA concentrations is possible in the human brain using localized proton magnetic resonance spectroscopy. Since direct biochemical validation is not an option for human studies, verification of MRS measurements of NAA need to be performed in animal models where rapidly frozen biopsy samples can be collected and analyzed by conventional techniques. Generally, studies of this type have found reasonable agreement between MRS and biochemical determinations of NAA concentrations (48).

Table 2. Millimolar metabolite concentrations (mean ± standard deviation) using SENSE-MRSI at 3.0 Tesla in 5 normal adult human subjects.

Region

[Cho]

[Cr]

[NAA]

Thalamus

2.48 ± 0.65

8.44 ± 1.44

11.71 ± 4.83

Corona radiata

2.85 ± 0.73

9.72 ± 1.94

9.72 ± 1.94

Parietal periventricular white matter

2.83 ± 0.74

10.70 ± 1.68

13.69 ± 1.97

Mesial occipital gray matter

2.39 ± 0.28

10.53 ± 2.01

12.06 ± 3.15

Centrum semiovale

2.77 ± 0.41

11.03 ± 1.25

12.31 ± 1.46

Posterior cortical gray matter

2.54 ± 0.48

10.24 ± 1.19

8.05 ± 2.91

The choice of quantitation technique depends somewhat on the localization method used; for single voxel spectroscopy, quantitation of metabolites using the brain internal water signal is a popular method which generally yields reproducible results, particularly when used in combination with the LCModel spectral analysis method. However, it should always be kept in mind with this method that pathological variations in brain water content can occur, and partial volume with CSF should be estimated using segmentation methods. For MRSI acquisitions, water referencing can also be used, although for many protocols there may not be sufficient time available to collect an additional unsuppressed MRSI (in addition to the water suppressed scan). In this case, other quantitation techniques are required, for instance, the phantom replacement method, which does not require any additional patient scan time. Finally, quantitative measurements of brain NAA can be made with this method using either conventional transmit-receive head coils, as well as the newer generation of multi-channel, receive-only phased-array head coils employed in parallel imaging and spectroscopic techniques (49).

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