Rationale: Sensitive and accurate imaging of cancer is essential for early diagnosis and appropriate treatment. For generally employed magnetic resonance imaging (MRI) in clinic, comprehending how to enhance the contrast effect of T 1 imaging is crucial for improving the sensitivity of cancer diagnosis. However, there is no study ever to reveal the clear mechanism of how to enhance the effect of T 1 imaging and accurate relationships of influencing factors. Herein, this study aims to figure out key factors that affect the sensitivity of T 1 contrast-enhanced MRI (CE-MRI), thereby to realize sensitive detection of tumors with low dose of CAs. Methods: Manganese oxide (MnO) nanoparticles (NPs) with various sizes and shapes were prepared by thermal decomposition. Factors impacting T 1 CE-MRI were investigated from geometric volume, surface area, crystal face to r 2/r 1 ratio. T 1 CE-MR imaging of liver, hepatic and subcutaneous tumors were conducted with MnO NPs of different shapes. Results: The surface area and occupancy rate of manganese ions have positive impacts on the sensitivity of T 1 CE-MRI, while volume and r 2/r 1 ratio have negative effects. MnO octahedrons have a high r 1 value of 20.07 mM-1s-1 and exhibit an excellent enhanced effect in liver T 1 imaging. ZDS coating facilitates tumor accumulation and cellular uptake, hepatic and subcutaneous tumors could be detected with MnO octahedrons at an ultralow dose of 0.4 mg [Mn]/kg, about 1/10 of clinical dose. Conclusions: This work is the first quantitative study of key factors affecting the sensitivity of T 1 CE-MRI of MnO nanoparticles, which can serve as a guidance for rational design of high-performance positive MRI contrast agents. Moreover, these MnO octahedrons can detect hepatic and subcutaneous tumors with an ultralow dose, hold great potential for sensitive and accurate diagnosis of cancer with lower cost, less dosages and side effects in clinic.