CEST MRI is a relatively complex contrast mechanism, varying with multiple factors including the type and concentration of in vivo compounds and regional properties such as pH and temperate. Importantly, the contrast depends on experimental conditions such as field strength, saturation waveform, power and duration. Therefore, quantitative/semi-quantitative description of tumor CEST effect is crucial to provide new insights of the underlying multi-factorial pathophysiology.

The typical CEST effect can be generally described by CESTR=f*k*alpha*(1-sigma)/R1w, where f and k are labile proton concentration and exchange rate, respectively, which confers CEST imaging sensitivity to in vivo compound concentration and regional pH and temperature. Notably, CEST effect also depends on relaxation (T1 and T2), water content, and experimental conditions via the experimental factors (i.e. alpha and sigma). Recent development of quantitative CEST (qCEST) has elucidated the CEST contrast mechanism. However, CEST imaging of biological tissue is complicated by concomitant magnetization transfer (MT) and nuclear overhauser effects (NOE), and our work herein describes semi-quantitative CEST MRI and its applications in tumor imaging.

Amide proton CEST-weighted imaging captures composite changes in amide proton content, NOE and relaxation changes. Whereas the contribution from each source is not fully resolved, it is simplified and easy to use. For example, it has been shown that the APT-weighted MRI is capable of resolving recurrent tumor from necrosis and edema following treatment (Fig. 1), augmenting routinely used FLAIR and Gd-enhanced MRI.Although the simplistic MTR asymmetry analysis is commonly used, emerging qCEST techniques aim to isolate multiple sources of apparent in vivo CEST effects. Because the CEST spectral imaging is of substantially higher sensitivity than MR spectroscopy, multi-pool analysis at high magnetic field allows resolving contributions from amides (proteins), amines (creatine, glutamate), NOE (proteins, structure), and semi-solid MT at relatively higher spatiotemporal resolution (Fig. 2).Using normal adult Wistar rats, we evaluated the correlation of commonly used CEST (MTRasym) map with relaxation and MT contrast. There was significant correlation between R1w-scaled MTRasym (R1*MTRasym) and R1w (Fig. 3a), R2w (Fig. 3b) and mean MTR (Fig. 3c). With multiple regression test, Fig. 3 shows that the majority of cerebral heterogeneity in normal brain can be corrected based on relaxation and MT (R2=0.83±0.05, N=10). By suppressing intrinsic heterogeneity in normal brain, multi-parametric analysis of in vivo CEST MRI greatly enhances the conspicuity of subtle tissue changes such as pH.Exogenous dynamic CEST imaging captures CEST signal change following administration of pre-selected CEST agents to specifically probe certain tissue characteristics. Most notably, glucose has a unique CEST signature, which is associated with tissue metabolism and cell division cycle. Indeed, glucose CEST imaging has been demonstrated in resolving heterogeneous glucose consumption in tumor, even in cases without Gd enhancement (Fig. 4). In addition, reporter gene CEST MRI has been demonstrated to monitor oncolytic viral activity using lysine rich protein (LRP) production (Fig. 5). Other emerging exogenous CEST MRI applications include glutamate, lactate and pH imaging.