Lactate detection at 3T: compensating J coupling effects with BASING.
Detection of lactate by in vivo 1H magnetic resonance spectroscopy may provide a means of identifying regions of metabolic stress in brain and other human tissue, potentially identifying regional ischemia in stroke or necrosis in tumors. At higher field strengths (3 and 4 T), which have recently become available for whole-body human studies, the chemical shift difference between the doublet from the methyl protons and the quartet from the methine proton becomes comparable to the available radiofrequency (RF) pulse bandwidth. In this case "anomalous" J modulation occurs in PRESS and STEAM because the coupling partner of the observed resonance may or may not be refocused by the RF pulses depending on the position of the molecule within the voxel and the size of the chemical shift misregistration artifact. These anomalies lead to signal cancellation for echo times near odd multiples of 1/J (often used to highlight the inverted lactate doublet against nearby lipid peaks) in single voxel studies, and spatial variation of the doublet lineshape in chemical shift imaging studies, producing erroneous determination of relative lactate concentrations. While increasing the band-width of the RF pulses can reduce this effect by reducing the signal cancellation, some cancellation will always remain. A means of eliminating this effect using BASING/ MEGA (Mescher M et al. Solvent suppression using selective echo dephasing J Magn Reson A 1996;123:226-229; Star-Lack J et al. Improved water and lipid suppression for 3D PRESS CSI using RF band selective inversion with gradient dephasing (BASING). Magn Reson Med 1997;38: 311-321) water suppression pulses will be described, along with some of its limitations.