Introduction

Sodium ions (Na⁺) play a major role in the physiology of human organism and sodium/potassium pumps (Na⁺/K⁺-ATPase) are essential for the maintenance of homeostasis ¹. Primary active transport, which is carried out by Na⁺/K⁺-ATPase, is undoubtedly a crucial link in processes which occur in the human eye and its compartments: the formation of the aqueous humor, the maintenance of sodium/potassium gradient between the lens and the vitreous humor, the removal of water and lactic acid from the retina ². Once the function of sodium/potassium pump is impaired it might lead to disturbances in concentration/distribution of these ions what might be indicative of early pathological changes ³. ¹H MRI demonstrated that imaging of subtle ocular structures requires a (sub)millimeter spatial resolution over a small field of view (FOV) ⁴. The goal of this study is to show why superior nominal spatial resolution (1.0 x 1.0 x 1.0) mm³ is absolutely essential for ²³Na in vivo MRI of the human eye. Here we show in vivo sodium images of the human eye in order to benchmark the fidelity we obtained (1.0 x 1.0 x 1.0) mm³ with the proposed approach against nominal spatial resolution (3.0 x 3.0 x 3.0) mm³ which is typically used for ²³Na MRI of the human brain ⁵.

Methods

We used a six-channel transceiver array (Figure 1) which conforms very well to an average human head (Figure 1) along with a power divider which splits RF signal into six channels supporting equal amplitude and phase at all of the outputs (Figure 2). The multipurpose interface box (MRI.TOOLS GmbH, Berlin, Germany) (Figure 2) we used consists of 16 transmit/receive switches (Stark Contrasts, Erlangen, Germany): 8 for ¹H and 8 for ²³Na resonant frequency at 7.0 T along with integrated low-noise preamplifiers (Stark Contrasts, Erlangen, Germany). Human imaging studies 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 RARE imaging technique for proton imaging. Proton MRI was feasible by incorporating a single-tuned volume coil (Siemens, Erlangen, Germany).

Results

We obtained sodium images from the eyes of two healthy, adult volunteers: one male (age = 53 years, BMI = 23.5 kg/m²) and one female (age = 28 years, BMI = 25.1 kg/m²) with nominal isotropic spatial resolution of (3.0 x 3.0 x 3.0) mm³, (1.4 x 1.4 x 1.4) mm³ and (1.0 x 1.0 x 1.0) mm³ (Figure 3a-c and Figure 4a-c). We also achieved an isotropic spatial resolution of (1.0 x 1.0 x 1.0) mm³ within 10:50 min scan time by reducing TR (Figure 3d). Images acquired with an isotropic spatial resolution of 3 mm are depicted in Fig. 3a (or Fig. 4a) and show a SNR which is superior to the high resolution datasets (Figure 3b-d and Figure 4b,c). Yet, all of the most important ocular compartments in the context of sodium physiology (vitreous humor, aqueous humor and lens) cannot be clearly delineated from the low resolution data. This shortcoming is resolved by using high spatial resolution ²³Na MRI of the eye as highlighted in Figure 3c,d and Figure 4c. This finding underscores the need and value of high definition (isotropic spatial resolutions: ≤1mm) for ²³Na in vivo MRI of the human eye.

Discussion and Conclusion

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. Our data demonstrate that applying higher spatial resolution (1.0 x 1.0 x 1.0) mm³ for ²³Na eye imaging at 7.0 Tesla, what is made feasible by using a 6-channel transceiver array, clearly outperforms spatial resolutions currently used for brain imaging (3.0 x 3.0 x 3.0) mm³ in terms of revealed detail. The size of an average eye and – particularly - its substructures has major impact on the imaging protocols which will be used in future patient studies. For instance, mean axial and equator thickness of an average healthy human lens is about 4 mm. Mean anterior chamber depth, which is filled with aqueous humor, is around 3 mm. Typical intraocular tumors are between 1 mm and 3 mm in height and between 5 mm and 16 mm on the basis. Unlike a spatial resolution of 3 mm isotropic, a spatial resolution of 1 mm isotropic enables the delineation of subtle structures of the eye as well as potential pathologies.

Acknowledgements

No acknowledgement found.