In chemical exchange saturation transfer MRI, low-concentration metabolites (millimolar level) are detected with greatly enhanced sensitivity by using radiofrequency (RF) pulses to magnetically label exchangeable protons (NH, NH2, OH, etc) and look at the accumulative effect of this continuous labeling transferring to water protons via proton exchange. Below is an example for the detection of phosphocreatine (PCr), which plays a vital role in neuron and myocyte energy homeostasis. Currently, there are no routine diagnostic tests to non-invasively map PCr distribution with clinically relevant spatial resolution and scan time. Below is an example where we developed an artificial neural network-based chemical exchange saturation transfer (ANNCEST) experiment (1) to rapidly quantify PCr concentration with robust immunity to commonly seen MRI artefacts. High-quality PCr mapping of human skeletal muscle, as well as spatially localized quantification of exchange rate, magnetic field (B0) inhomogeneity and radiofrequency transmission field (B1) inhomogeneity, was obtained within 1.5 minutes on a 3T MRI scanner. This fast PCr ANNCEST has tremendous potential to bring non-contrast metabolic imaging to the clinic whereby identifying, quantifying and mapping metabolic changes at rest and exercise may guide the diagnosis of many muscle-related, neurologic and other diseases.

(1) Chen L, Schär M, Chan KWY, Huang J, Wei Z, Lu H, Qin Q, Weiss RG, van Zijl PCM, Xu J. In vivo imaging of phosphocreatine with artificial neural networks. Nat Commun. 2020 Feb 26;11(1):1072.

PCr mapping on human skeletal muscle using ANNCEST
PCr mapping on human skeletal muscle using ANNCEST.
Left: T2 weighted anatomical image with segmentation of gastrocnemius medial (GM), soleus (SOL), tibialis anterior (TA), and peroneus (P). 2nd column: PCr concentration and exchange rate maps. 3rd column: B0 and B1 maps. 4th column: Reference B0 and B1 maps obtained by the established methods compare favorably. From ref. 1, with permission.