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.