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

Abstract

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. 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. 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. 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.