Measurement of the cerebral metabolite y-aminobutyric acid (GABA) has been performed on
clinical MRI scanners using a variety of magnetic resonance spectroscopy (MRS)
techniques. MRS studies of GABA are difficult, especially at 1.5T due to low in-vivo
concentrations and overlapping of higher concentration metabolites. Unlike spectral editing
methods, two-dimensional (2D) MRS allows the simultaneous measurement of GABA and
other, more traditional metabolites. This work evaluates three implementations of 2D MRS
for both in-vitro and in-vivo GABA measurement on a clinical MRI scanner.Existing spectroscopy sequences were used to develop a protocol for performing 2D Jresolved MRS without a dedicated sequence. GABA was measured in-vitro at
concentrations approaching normal physiological levels and volunteer results allowed
assignment of the 3.01ppm GABA resonance at its J-coupling frequency (7.4Hz). However,
the prolonged scan time of over two hours prevented practical application of this approach.A far more efficient method of acquiring 2D J-resolved spectra is achieved with a dedicated
2D J-resolved sequence. An optimised set of acquisition parameters was produced to allow
GABA measurement with maximum SNR, and without macromolecule contamination, in 35
minutes. Since the reproducibility of the sequence must be sufficient to detect physiological
changes, a formal reproducibility study was performed acquiring three measures of
reproducibility at six concentrations of GABA, using a standard volume head coil, 3"- and
5"- surface coils. To our knowledge, this is the first such reproducibility study dedicated to
2D J-resolved GABA measurement, and as such, could have significant implications on the
interpretation of in-vivo results. In-vivo 2D J-resolved spectra were acquired and compared
well to the published results, allowing assignment of the 3.0Ippm GABA (plus
macromolecule) peak (J = 7.4Hz). In the first reported 2D J-resolved spectra specifically
designed to reduce the macromolecule contribution by optimising the echo time range,
assignment of the in-vivo 3.01 ppm GABA peak was less convincing.As an alternative to 2D J-resolved spectroscopy, preliminary testing of 2D correlation
spectroscopy (COSY) showed that it was not as sensitive or robust for either in-vitro or invivo GABA measurement. Although provisional assignment of the 3.01 ppm GABA peak
was made, in their current form, neither technique is suitable for pure GABA measurement
at 1.5T
