Influences of experimental parameters on chemical exchange saturation transfer (CEST) metrics of brain tumors using animal models at 4.7T

Purpose: To investigate the dependence of magnetization transfer ratio asymmetry at 3.5 ppm (MTR_asym(3.5 ppm)), quantitative amide proton transfer (APT^#), and nuclear Overhauser enhancement (NOE^#) signals or contrasts on experimental imaging parameters. Methods: Modified Bloch equation-based simulations using 2-pool and 5-pool exchange models and in vivo rat brain tumor experiments at 4.7T were performed with varied RF saturation power levels, saturation lengths, and relaxation delays. The MTR_asym(3.5 ppm), APT^#, and NOE^# contrasts between tumor and normal tissues were compared among different experimental parameters. Results: The MTR_asym(3.5 ppm) image contrasts between tumor and normal tissues initially increased with the RF saturation length, and the maxima occurred at 1.6−2 s under relatively high RF saturation powers (>2.1 μT) and at a longer saturation length under relatively low RF saturation powers (<1.3 μT). The APT^# contrasts also increased with the RF saturation length but peaked at longer RF saturation lengths relative to MTR_asym(3.5 ppm). The NOE^# contrasts were either positive or negative, depending on the experimental parameters applied. Conclusion: Tumor MTR_asym(3.5 ppm), APT^#, and NOE^# contrasts can be maximized at different saturation parameters. The maximum MTR_asym(3.5 ppm) contrast can be obtained with a relatively longer RF saturation length (several seconds) at a relatively lower RF saturation power.