Transtympanic iontophoresis with a biocompatible paramagnetic solution at MR imaging: experimental feasibility study in rabbits

PURPOSE: To determine the feasibility of transtympanic iontophoresis in experimental animals with a paramagnetic contrast agent at magnetic resonance (MR) imaging. MATERIALS AND METHODS: Optimal MR sequence parameters and appropriate paramagnetic ion concentrations of a water and gadopentetate dimeglumine solution were initially assessed with phantoms. Iontophoresis was performed in left ears of five rabbits after the external auditory canals were filled with a solution of water and gadopentetate dimeglumine of optimal concentration, and right ears were used as controls. Signal-to-structural noise ratio (SSNR) and contrast-to-structural noise ratio (CSNR) were measured by using regions of interest, and the overall image quality was assessed subjectively. RESULTS: Spin-echo (SE) MR sequences were superior to gradient-echo (GRE) MR sequences in terms of SSNR, CSNR, and overall image quality. Highest SSNR and CSNR values were achieved with 2 mmol/L (2 mM) of gadopentetate dimeglumine solution with both SE (repetition time msec/echo time msec, 500/12; flip angle, 90 degrees ) and GRE (300/10; flip angle, 90 degrees ) sequences in both phantoms and animals. The high signal intensity of gadopentetate dimeglumine solution was recognized in middle ears, vestibules, and semicircular canals of all rabbit ears that had undergone iontophoresis and in none of the control ears. CONCLUSION: With the solution of water and gadopentetate dimeglumine, the maximum SSNR and CSNR with both SE and GRE MR imaging sequences were achieved. The solution can be transferred to the middle and inner ear cavities across an intact tympanic membrane by using transtympanic iontophoresis.