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.