Navigator‐free metabolite‐cycled proton spectroscopy of the heart

Purpose Respiratory gating in cardiac water‐suppressed (WS) proton spectroscopy leads to long and unpredictable scan times. Metabolite cycling allows to perform frequency and phase correction on the water signal and, hence, offers an approach to navigator‐free cardiac spectroscopy with fixed scan time. The objective of the present study was to develop and implement navigator‐free metabolite‐cycled cardiac proton spectroscopy (MC nonav) and compare it with standard navigator‐gated WS (WS nav) and navigator‐free WS (WS nonav) measurements for the assessment of triglyceride‐to‐water ratios (TG/W) and creatine‐to‐water ratios (CR/W) in the intraventricular septum of the in vivo heart. Methods Navigator‐free metabolite‐cycled spectroscopy was implemented on a clinical 1.5T system. In vivo measurements were performed on 10 young and 5 older healthy volunteers to assess signal‐to‐noise ratio efficiency as well as TG/W and CR/W and the relative Cramér‐Rao lower bounds for CR. The performance of the metabolite‐cycled sequence was verified using simulations. Results On average, scan times of MC nonav were 3.4 times shorter compared with WS nav, while no significant bias for TG/W was observed (coefficient of variation = 14.0%). signal‐to‐noise ratio efficiency of both TG and CR increased for MC nonav compared with WS nav. Relative Cramér‐Rao lower bounds of CR decreased for MC nonav. Overall spectral quality was found comparable between MC nonav and WS nav, while it was inferior for WS nonav. Conclusion Navigator‐free metabolite‐cycled cardiac proton spectroscopy offers 3.4‐fold accelerated assessment of TG/W and CR/W in the heart with preserved spectral quality when compared with navigator‐gated WS scans