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

Abstract

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.

Keywords: APT; CEST; RF saturation length; RF saturation power; brain tumor.