Chemical‐shift encoding–based water–fat separation with multifrequency fat spectrum modeling in spin‐lock MRI

Purpose Chemical exchange saturation transfer is used commonly to generate MRI contrast based on the chemical exchange effect. The spin‐lock techniques can also be used to probe the chemical exchange and other molecular motion processes in tissues. The presence of fat can cause errors in spin‐lock MRI. Signals from fat are typically suppressed based on spectral selectivity or T 1 nulling approaches in spin‐lock imaging. However, these methods cannot be used to suppress fat signals from multiple fat peaks. To address this problem, we report chemical‐shift encoding–based water–fat separation approaches with multifrequency fat spectrum modeling. Methods Both the conventional spin‐lock and the adiabatic continuous‐wave constant‐amplitude spin lock (ACCSL) with multi‐echo acquisitions are investigated for chemical‐shift encoding–based water–fat separation in spin‐lock imaging. A comparison is made of reconstructions based on 3 models: a single‐peak fat spectrum model, a standard precalibrated proton density 6‐peak fat spectrum model, and the self‐calibrated relaxation‐dependent 3‐peak fat spectrum model. Comparisons were performed using Bloch simulations, phantom, and in vivo experiments at 3 T. Results Conventional spin‐lock acquisitions cannot be used for reliable water–fat separation with a multipeak fat spectrum model. Water–fat separation based on ACCSL acquisitions achieves superior performance compared with the use of conventional spin‐lock acquisitions. The best result is achieved from ACCSL acquisition with self‐calibrated relaxation‐dependent multipeak fat spectrum modeling. Conclusion The ACCSL acquisition can be used for chemical‐shift encoding–based water–fat separation with multipeak fat spectrum modeling. This approach has the potential to improve quantitative analysis using spin‐lock MRI for assessing the biochemical properties of tissues.