## 1 Introduction  
Magnetic resonance imaging (MRI) was developed in 1973 for application to medical diagnosis; it since has become an indispensable technology [1, 2, 3, 4, 5, 6, 7]. According to the basis of imaging, the magnetic momentum generated by the spinning of hydrogen protons in organisms, when placed in an applied external static magnetic field, causes them to move longitudinally towards the direction of the static magnetic field and be arranged in a consistent direction with the magnetic field. At this time, when a radio frequency (RF) with appropriate energy is applied, hydrogen protons will absorb energy from RF pulse, and those that originally moved in the longitudinal direction will reduce the longitudinal moving magnetic momentum value in the direction of the static magnetic field and increase the 90-degree transverse moving magnetic momentum value. When the RF pulse stops, the hydrogen protons will release energy, and the magnetic moment direction will gradually return to the original. The transverse moving magnetic momentum value of the hydrogen protons will also gradually decrease. This action is called transverse or spin-spin relaxation time. The magnetic moment of protons will slowly return to the static magnetic field or longitudinal motion direction in the balanced state. This action is called longitudinal or spin lattice relaxation time. After the released energy is received by the receiver coil, and the signals are converted by the terminal, they will present different grayscale images [8]. Through the relaxation time, T1-weighted (T1W) and T2-weighted (T2W) pulse sequences enhance the difference in proton density between tissues, which is the most important physical basis for MRI in clinical diagnosis [9]. When hydrogen protons are placed in an applied external magnetic field, T1 relaxation time is defined as the time required for 63% of the longitudinal hydrogen protons to return to the direction of the static magnetic field. T2 relaxation time is defined as the time required for the transverse static magnetic moment to decay by 63%, i.e., the time required for the transverse magnetic moment to decay to 37% of the original value, which is also called transversal or spin-spin relaxation time [10]. MRI uses different important parameters in imaging. Taking water as an example, it is characterized by long T1 and long T2. Long T1 means that most of the longitudinal magnetization cannot be recovered during the interval of TR (repetition time), so weak magnetic resonance signals are generated, and the image is dark; long T2 means that after the radio frequency pulse is applied, a relatively long echo time (TE) is generated, so relatively strong MR signals are received and thus the image is bright. These parameters form an important basis for magnetic resonance imaging [11]. In order to strengthen the diagnosis of tissue recognition, by virtue of the development of magnetic nanoparticles, the application fields of MRI technology have also extended to the field of biomedical materials, from animals and human bodies at the cellular and molecular levels [12]. Magnetic resonance contrast agents are used to enhance the imaging recognition of soft tissues and body structures in medical images; contrast agents are also divided into two structures with different attributes, i.e., paramagnetism and superparamagnetism [13; 14]. Through the paramagnetic substance of unpaired electrons in the contrast agent in tissues, the relaxation time value of spin-lattice T1 or T2 is reduced to accelerate the changes in signal intensity generated through the proton relaxation of water molecules in tissues in the magnetic environment, thereby presenting different signal intensities generated among tissues and organs and improving the sensitivity of contrast presentation accordingly [15]. Gadolinium-based paramagnetic contrast agent is a gadolinium-diethylDetermamine penta-acetic acid. Gd\({}^{3+}\) has a large magnetic moment.