Sodium (²³Na) In Vivo Magnetic Resonance Imaging of the Human Eye at 7.0 Tesla: a New Tool to Investigate Ocular Disorders

Abstract

Purpose : Sodium ions (Na⁺) play a central role in the energy-consuming processes of cell membrane transport which are carried out by sodium/potassium pumps (Na⁺/K⁺-ATPase). This fact is critical for many physiological processes which occur in the human eye and its compartments. An opportunity to image sodium in vivo using magnetic resonance imaging (MRI) might add a new dimension to our understanding of various eye diseases like melanoma, glaucoma and cataract. The goal of this study is to achieve exceptional spatial resolution (1.0x1.0x1.0) mm³in vivo²³Na MRI of the human eye at 7.0 Tesla.

Methods : We developed and used a lightweight six-channel local transmit/receive array which conforms very well to an average human head (Figure 1). Human imaging studies (n=7 volunteers; 4 male: mean age=33.5 years, mean BMI=23.9 kg/m²; 3 female: mean age=28.5 years, mean BMI=22.0 kg/m²) were conducted on a 7.0 Tesla whole-body system (Magnetom,Siemens,Erlangen,Germany) using 3D-DAPR imaging technique for ²³Na imaging and T₂-weighted 2D RARE imaging technique for standard proton imaging to obtain an anatomical reference image.

Results : The spatial resolutions we obtained: (1.4x1.4x1.4) mm³ and (1.0x1.0x1.0) mm³ using the developed methodology clearly outperforms the spatial resolution (3.0x3.0x3.0) mm³, which is currently used for ²³Na MRI of the human brain, in terms of revealed detail (Figure 2). We achieved a remarkable level of fidelity (1 mm isotropic) within about 10 minutes scan time using our customized transmit/receive radio frequency array.

Conclusions : Sodium in vivo MRI of the human eye using our methodology provides millimeter isotropic spatial resolution images of excellent quality obtained within clinically acceptable scan times. The quality of the images permits clear distinctions between the vitreous, aqueous humor and crystalline lens. Now we will explore the efficacy of the method in a feasibility study involving patients. The broad roles of sodium in processes related to eye physiology suggest a range of questions for ophthalmological investigations.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

Figure 1: The six-channel transmit/receive array: the electronic circuit (top); the array placed on the face of a volunteer (bottom).

 

Figure 2: In vivo²³Na image of the eyes of a healthy male volunteer obtained with enhanced fidelity which enables clear distinctions between the vitreous and the aqueous humor and the lens.