Pulsed Z‐spectroscopic imaging of cross‐relaxation parameters in tissues for human MRI: Theory and clinical applications

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Abstract
A new method of pulsed Z-spectroscopic imaging is proposed for in vivo visualization and quantification of the parameters describing cross-relaxation between protons with liquid-like and solid-like relaxation properties in tissues. The method is based on analysis of the magnetization transfer (MT) effect as a function of the offset frequency and amplitude of a pulsed off- resonance saturation incorporated in a spoiled gradient-echo MRI pulse sequence. The theoretical concept of the method relies on an approximated analytical model of pulsed MT that provides a simple three-parameter equation for a pulsed steady-state Z-spectrum taken far from resonance. Using this model, the parametric images of cross-relaxation rate constant, content, and T(2) of the semisolid proton fraction can be reconstructed from a series of MT-weighted images and a coregistered T(1) map. The method was implemented on a 0.5 T clinical MRI scanner, and it provided high-quality 3D parametric maps within an acceptable scanning time. The estimates of cross-relaxation parameters in brain tissues were shown to be quantitatively consistent with the literature data. Clinical examples of the parametric images of human brain pathologies (multiple sclerosis and glioma) demonstrated high tissue contrast and clear visualization of the lesions.