Toward nitrogen-14 nuclear quadrupole resonance imaging by nutation experiments performed with a radio-frequency field gradient

Abstract

Until now, NQR imaging has been considered mainly in the case of Chlorine-35. This is a spin 3/2 resonating at relatively high frequency (around 30MHz) thus affording a favorable sensitivity. Conversely, Nitrogen-14 (spin 1) NQR is much less sensitive because its resonances frequencies are below 6MHz. In contrast to already existing methodologies for object localization by 14N NQR, we present here a new, more straightforward, approach and the principles of space dependent 14N Quadrupole Resonance are laid down. The method is based on nutation curves obtained with an inhomogeneous radio-frequency field produced by a dedicated transmit-receive coil. The gradient created that way does not need to be uniform although more accurate results would be obtained with a uniform gradient. Nutation curves have been simulated by a specially designed algorithm which takes into account the 14N Quadrupole Resonance particularities including the so-called powder average. Preliminary experiments were carried out with the highest resonance frequency (4.64MHz) of sodium nitrite (NaNO2). A cylindrical sample of powder sodium nitrite containing a spacer was used. Simulations of the corresponding nutation curves not only demonstrate the existence of this spacer but, in addition, provide its position and its thickness. This clearly ascertains the feasibility of 14N Quadrupole Resonance imaging.