Alternate ascending/descending directional navigation approach for imaging magnetization transfer asymmetry.

Magnetization transfer (MT) effects (1) have been observed in MR imaging. MT effects that arise from interaction between solid-like macromolecules and bulk water exist in a wide range of frequency offsets from the water resonance frequency. The center of the spectral distribution of solid-like macromolecules in tissue is slightly lower than the center of the water resonance spectrum (2), hence these MT effects are not symmetric around the water resonance frequency (2–4). These conventional MT asymmetry effects were recently quantified using a modified two-pool MT model (3) and were demonstrated to reside in cervical spinal cords (5). Distinct from the conventional MT effects, asymmetry in MT effects have been observed in frequency offsets <8 ppm from the water resonance frequency, associated with amide protons of endogenous mobile proteins and peptides (4,6,7). The MT asymmetry imaging associated with amide proton transfer (APT) is one kind of chemical exchange saturation transfer techniques (8,9) and is sensitive to cellular pH (4,6,7). It has been shown to be useful for diagnosis of tumor (7,10–12) and stroke (4,13–15). In addition to APT, asymmetry in the conventional MT effects associated with solid-like macromolecules is also potentially useful for diagnosis of tumor (16). These factors emphasize necessity of reliable techniques for high-resolution multislice MT asymmetry imaging for clinical studies. Imaging of MT asymmetry for clinical studies has not been easy because of its sensitivity to magnetic field inhomogeneity and difficulty in multislice acquisition. A few techniques have been proposed to correct magnetic field inhomogeneity in MT asymmetry imaging (4,7,12,17,18). These techniques require pixel by pixel correction of the offset frequency (4,7,17), establishment of a theoretical model in in vivo brains (18), or selection of an appropriate offset interval between three offset frequency points (12). Methods for multislice imaging of MT asymmetry also have been proposed in a few recent studies (13,19). In one of these techniques, the long MT saturation pulse was broken down into a series of short preparation pulses in multislice gradient echo sequence (19). One concern in the multislice imaging method is interslice MT effects, which have been commonly observed in general two-dimensional (2D) multislice imaging (20–23). These MT effects associated with multiple slices have been generally a confounding factor, but may be used as an alternative approach for imaging MT asymmetry. In this article, a new method for imaging MT asymmetry with no saturation pulse is proposed. MT effects were generated from sequential 2D balanced steady-state free precession (bSSFP) imaging, where high interslice MT effects exist because of high flip angle and short pulse repetition time. For MT asymmetry, sequential 2D bSSFP slices were acquired by turns between acquisitions with MT effects at positive and negative frequency offsets. For the suppression of interslice blood flow, magnetic field inhomogeneity, and gradient imperfections, the positive and negative frequency offsets were generated by alternating ascending and descending slice orders and by alternating positive and negative slice-select gradients. This new MT asymmetry imaging technique, alternate ascending/descending directional navigation (ALADDIN), was tested at various scan conditions. ALADDIN MT asymmetry signals were measured in gray matter (GM) and white matter (WM) regions, and the results were compared with previously published values from the conventional methods.