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 (MTRasym (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 MTRasym (3.5 ppm), APT# , and NOE# contrasts between tumor and normal tissues were compared among different experimental parameters.
RESULTS: The MTRasym (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 MTRasym (3.5 ppm). The NOE# contrasts were either positive or negative, depending on the experimental parameters applied.
CONCLUSION: Tumor MTRasym (3.5 ppm), APT# , and NOE# contrasts can be maximized at different saturation parameters. The maximum MTRasym (3.5 ppm) contrast can be obtained with a relatively longer RF saturation length (several seconds) at a relatively lower RF saturation power.