The inner ear is an important organ for the production of hearing and the maintenance of homeostasis, and its cavity is filled with two types of fluid, the endolymphatic and the exolymphatic fluid. The ionic and pressure stability of the endolymphatic fluid is the guarantee of the normal physiological activity of the inner ear, and its abnormalities are the direct cause of sensorineural deafness and the induction of Ménière’s disease. However, endolymphatic fluid has not been directly visualized in the past due to its small size and the fact that it is encapsulated by the hardest bone in the body (rock bone). Magnetic resonance imaging (MRI) can show the fluid, however, it cannot distinguish between endolymphatic fluid and exolymphatic fluid. We have used novel superparamagnetic iron oxide nanoparticles (SPION) to suppress the signal of exolymphatic fluid and display endolymphatic fluid in vivo for the first time by MRI in rats (see figure below), providing new clues for imaging studies of inner ear pathology and accurate clinical diagnosis. Figure 1. Magnetic resonance imaging of the inner ear in rats. A. Inner ear visualization without SPION contrast, i.e., mixed signal of internal and external lymphatic fluid; B. Vestibular visualization without SPION contrast, i.e., mixed signal of internal and external lymphatic fluid; C. In vivo display of inner ear lymphatic fluid directly after suppression of external lymphatic signal with SPION; D. SPION diffusion into the vestibule and suppression of its external lymphatic signal, showing internal vestibular lymphatic fluid. Am: ampulla of the lateral semicircular canal; Am_LSCC: ampulla of the posterior semicircular canal; Am_PSCC: ampulla of the posterior semicircular canal; Am_SSCC: ampulla of the superior semicircular canal; Coch: cochlea; LSCC: lateral semicircular canal; PSCC: posterior semicircular canal; SSCC: superior semicircular canal; Sa: saccule; SM: scala media; Ut: utricle.