Introduction

Sodium ions (Na⁺) play a key role in the physiology of living cells. MRI is of proven value for probing Na⁺ content in vivo ^1,2. Sodium (²³Na) holds the second place in terms of NMR sensitivity among all nuclei present in biological tissues. Its accessible signal-to-noise ratio (SNR) is still around 3000 to 20.000 times lower than SNR of proton MRI. This limitation induces severe spatial resolution constraints and/or long acquisition times. To overcome this challenge, ultrahigh magnetic fields and ultrashort echo time imaging techniques are of substantial benefit. Compressed sensing (CS) based reconstruction methods provide an alternative for enhancing spatial resolution and for shortening scan times ³. This work examines the applicability 3D dictionary learning compressed sensing reconstruction (3D-DLCS) for isotropic (sub)millimeter spatial resolution ²³Na in vivo MRI of the human eye at 7.0 Tesla.

Methods

To acquire high-spatial resolution ²³Na images of the human eye, we used a six-channel transceiver array that conforms very well to an average human head and a power divider which splits RF signal into six channels supporting equal amplitude and phase at all of the outputs. The multipurpose interface box (MRI.TOOLS GmbH, Berlin, Germany) 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). 3D-density-adapted projection reconstruction was used for ²³Na MRI data acquisition (TR = 17 ms, TE = 0.55 ms, FA = 41°). Proton MRI was conducted with a birdcage coil (Siemens, Erlangen, Germany) (Figure 1). Human imaging study was performed on a 7.0 Tesla whole-body system (Magnetom, Siemens Healthcare, Erlangen, Germany). ²³Na MRI of the human eye was performed in one healthy, adult volunteer (sex: male; age = 26 years; BMI = 19.9 kg/m²). The data acquired with N_projections = 50000, N_averages = 1, TA = 14.2 min were reconstructed with conventional gridding and applying Hamming filter (Figure 2). The data with N_projections = 10000, N_averages = 5, TA = 14.2 min were additionally reconstructed with the 3D-DLCS algorithm (block size B = 6, dictionary size D = 500, weighting parameter for dictionary representation λ = 0.5, sample number N_samp = 500000). Furthermore, one reconstruction was performed for a reduced dataset (N_averages = 4, TA = 11.3 min).

Results

For sodium MRI of the eye two protocols were applied to achieve a nominal isotropic spatial resolution of 1 mm³ within an identical total acquisition time (TA) of 14 min 10 s: (A) N_averages = 1, N_projections = 50000; (B) N_averages = 5, N_projections = 10000. Both datasets (A) and (B) were reconstructed with a standard reconstruction method and were used as baseline images ⁴ (Figure 2). Dataset (B) was reconstructed with 3D dictionary learning compressed sensing (3D-DLCS) algorithm using 4 and 5 averages (Figure 2). Using only 4 averages instead of 5 does not lead to any drop in the performance of the 3D-DLCS reconstruction algorithm and the quality of the final image. This gain can be used to reduce total acquisition time for protocol (B) by almost 3 minutes. It is demonstrated that applying 3D ²³Na dictionary learning compressed sensing reconstruction for ²³Na in vivo MRI of the human eye at 7.0 Tesla preserves all of the details and leads to markedly reduced noise.

Discussion and Conclusion

To summarize, 3D ²³Na dictionary learning compressed sensing reconstruction is conceptually appealing for enhancing SNR in ²³Na in vivo MRI of the human eye at 7.0 Tesla. This gain can be invested in reduction of total acquisition time or in spatial resolution enhancements. This improvement makes submillimeter ²³Na MRI of the human eye at 7.0 Tesla feasible within acceptable scan times. This feasibility of high definition ocular ²³Na MRI is of high relevance for precise diagnostics of various diseases of the eye including radiation planning and therapy monitoring of small ocular masses. Eye compartments, which are crucial in the context of sodium physiology, are very subtle (e.g. aqueous humor, lens). Resolving these subtle eye compartments requires high definition sodium MRI of the eye with submillimeter spatial resolution to obtain meaningful results from patient studies including ocular melanoma, glaucoma and cataract.

Acknowledgements

No acknowledgement found.