Click on image to view larger version.
Fig 3. Top: A, Schematic representation of the 2-step WBNAA sequence. It comprises an alternating inversion pulse (applied every odd acquisition) followed by a TI designed to null the NAA signal. It ends with a chemical-shift-selective (CHESS) and a 1
3
water-suppression pulse (4-ms interpulse delay at 1.5T). The latter also serves as the 90° readout pulse. Acquisition commences immediately (TE = 0), and every even acquisition is subtracted from every odd one. Because the TR is long, 10 seconds, no T1- or T2-weighting is incurred. Note that all the radio-frequency (RF) pulses are (spatially) nonselective and the gradient "blips" are just crushers. Center: B, The result of a 90° 13 on a human head, demonstrating the problem of the immense metabolite-obscuring lipid signal from the bone marrow and adipose tissue. Bottom: C, The resulting spectrum from the WBNAA sequence on the same head as in B, demonstrating almost complete lipid suppression. Note that the NAA is implicitly localized to the brain, whereas with the other metabolites whose peaks are distinct and nonlocalized, it is impossible to ascertain where their signals come from. Also note the excellent SNR from this 2.5-minute acquisition.
