Triple-quantum-filtered imaging of sodium in presence of B 0 inhomogeneities Costin Tanase a,b, * , Fernando E. Boada b a Department of Physics and Astronomy, University of Pittsburgh, 35905 OÕHara Street, Pittsburgh, PA 15213, USA b MR Research Center, University of Pittsburgh Medical Center, 200 Lothrop Street, Pittsburgh, PA 15213, USA Received 21 November 2004; revised 8 February 2005 Available online 14 March 2005 Abstract Triple quantum filtered sodium MRI techniques have been recently demonstrated in vivo. These techniques have been previously advocated as a means to separate the sodium NMR signal from different physiological compartments based on the differences between their relaxation rates. Among the different triple quantum coherence transfer filters, the three-pulse coherence transfer filter has been demonstrated to be better suited for human imaging than the traditional four-pulse implementation. While the three-pulse structure has distinct advantages in terms of RF efficiency, the lack of a refocusing pulse in the filter introduces an increased depen- dence on the main magnetic field inhomogeneities, which can sometimes lead to significant signal loss. In this paper, we characterize these dependencies and introduce a method for their compensation through the acquisition of a B 0 map and the use of a modified phase cycling scheme. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Multiple quantum filter; Imaging; Coherence; Inhomogeneity 1. Introduction Multiple quantum filtered (MQF) techniques have been used extensively in NMR to separate the NMR sig- nal from environments where non-negligible, second- order, contributions to the Zeeman Hamiltonian allow for non-trivial nuclear transitions. In the case of sodium, a very important ion in cell physiology, these techniques have been further explored for the separation of the NMR signal between the intra- and extra-cellular com- partments in the context of animal models of disease [1]. Triple quantum sodium NMR techniques, in partic- ular, have been used for the observation of sodium ion shifts during ischemia [2] and for the identification of neoplastic changes in human and/or animal tissues [1,3]. While well known in the NMR literature, the con- ventional implementation of TQ sodium NMR relies on the use of a four RF pulse coherence transfer filter. In this four pulse structure, the first pulse creates coherences that evolve freely and are refocused by the second RF pulse before being converted into triple quantum coher- ences and observable magnetization by the third and four pulses, respectively. As shown by Hancu et al. [4], the four pulse structure, while being well suited for NMR experiments over small samples, introduces a strong dependence on the RF field leading to strong, and difficult to compensate, signal modulation across the field of view when imaging applications are consid- ered. By eliminating the second RF pulse from this struc- ture, the signal dependence on the RF field becomes less severe [4,5] and, moreover, this dependence factors out from that of all other experimental parameters. This fea- ture of the three-pulse coherence transfer filter makes it better suited for imaging experiments because the afore- mentioned factorization allows for compensation of the www.elsevier.com/locate/jmr Journal of Magnetic Resonance 174 (2005) 270–278 1090-7807/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2005.02.012 * Corresponding author. Fax: +1 4126479800. E-mail address: tanc@phyast.pitt.edu (C. Tanase).