Navin Bansal

A novel editing technique for 19F MRI: molecule-specific imaging.

A novel technique is proposed to facilitate the selective imaging of specific molecules from a mixture. The application of the technique presented here demonstrates the ability to selectively produce 19F MR images of either trifluoroacetic acid or the perfluorocarbon emulsion Oxypherol-ET (perfluorotributylamine), when both molecules are present simultaneously. Selective detection is based on the presence of homonuclear J-modulation in one molecule and differential spin-spin relaxation time (T2). Perfluorotributylamine, an A3B2 system, is subject to homonuclear J-modulation, which produces a null signal from the antiphase components of the triplet (A3) when an echo time (TE) = 1/2J is used in a spin-echo image. At this echo time the second molecule, in this example trifluoroacetic acid, a non-coupled spin system, is selectively imaged. At longer echo times, e.g., TE = 1/J there is substantial recovery of the J-modulated signal, which may be solely observed due to T2 decay of the trifluoroacetic acid signal. The method is demonstrated both using phantoms and in vivo.

Multiple quantum filtered 23Na NMR spectroscopy of the isolated, perfused rat liver

Isolated, perfused rat livers were examined by single‐quantum (SQ) and double‐quantum‐filtered (DQ‐filtered) 23Na spectroscopy during prolonged global ischemia and during perfusion with ouabain, low‐buffer potassium, or lithium‐enriched buffer. Baseline separation of the intracellular (Nai+) and extracellular (Nae+) sodium resonances using TmDOTP5‐ allowed a direct comparison of temporal changes in SQ versus DQ‐filtered Nai+. The SQ Nai+ signal increased ∼150% during the first 15 min of global ischemia and then remained relatively constant over the next 45 min, while the DQ‐filtered signal steadily increased ∼400% over the same 60 min period. In similar experiments in which all perfusate sodium was replaced by lithium, the DQ‐filtered Nai+ signal increased ∼180% over a similar period of ischemia. Exposure of livers to ouabain also resulted in larger increases in DQ‐filtered versus SQ signal of Nai+. The ∼290% increase in DQ‐filtered sodium observed during perfusion of livers with a hypokalemic buffer (1.2 mM K+) could be completely reversed by continued perfusion with a buffer containing normal levels of K+ (4.7 mM). These data suggest that the DQ‐filtered Nai+ signal of liver does not simply report an increase in [Nai+], but may be exquisitely sensitive to other intracellular events initiated by altered physiology. Magn Reson Med 41:1127–1135, 1999.

Improved MR imaging in extremely inhomogeneous radio-frequency fields

An improvement to the method for imaging with extremely inhomogeneous radio-frequency fields is presented which uses a pseudo-noise-modulated selective (PNMS) excitation prepulse to randomize the signal from outside the selected slice. This prepulse reduces the problems that arise from imperfect subtraction involved with the one-dimensional ISIS-type slice selection procedures. The results demonstrate two to three times improvement in the signal-to-noise ratio and reduction in the image artifacts when this prepulse is used.