In clinical magnetic resonance imaging (MRI), gadolinium-based contrast agents are most commonly used for evaluating brain tumors. However, contrast-enhanced MRI can only provide relative signal changes such as mixed information with longitudinal relaxivity (r1) and contrast agent concentration. Herein, we present a new method to evaluate r1 and contrast agent concentration separately in contrast-enhanced lesions using quantitative parameter mapping. We demonstrated that it is possible to evaluate pathophysiological tumor changes owing to therapeutic efficacy. Furthermore, the r1 value can be used as an extracellular pH tumor marker. We believe that our method has an easy clinical application and demonstrates how acidic environments affect the T1 relaxation time of contrast agents. In conclusion, these indices can be useful for brain tumor management.