APT and CEST Techniques for Clinical MRI

Chemical exchange saturation transfer (CEST) based in vivo detection and quantification of endogenous macro-molecules (i.e., amide proton transfer, APT [1]) or exogenous contrast agents [2] is a highly sensitive molecular MRI technique bearing a substantial clinical potential for example in oncology [3] or for cerebro-vascular applications [1]. Techniques for APT/CEST MRI on clinical MRI scanners are reviewed. Typical limitations arise from hardware specifications of the radio-frequency (RF) source(s) regarding maximum pulse length or RF duty-cycle as well as limits imposed by regulations on specific absorption rates (SAR). Clinical APT/CEST techniques described in the literature [4,5] are compared to two novel approaches: (i) sequences based on operating the scanner in a mode provided for MRS proton-decoupling [6], and (ii), sequences based on the alternated use of the independent RF amplifiers within an MR system equipped with parallel RF transmission [7] (Figure 1). The standard APT/CEST signal detection is based on an asymmetry analysis for saturation offset frequencies around the water resonance and may be strongly biased by local magnetic field inhomogeneity δB0. Efficient CEST acquisition schemes in terms of z-spectral sampling and δB0 correction are presented. This includes CEST-Dixon techniques [6], where the δB0 information is directly obtained from a dual-echo CEST acquisition. At the same time, this allows to separate the fat fraction, which may interfere with the CEST signal evaluation and typically requires fat suppression techniques