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Home
Research
Research Topics
Imaging and Radiooncology
Medical Physics in Radiology

Research

23Na Magnetic Resonance Imaging (23Na-MRI)

Fig. 1: Scheme of the sodium-potassium pump (Na+-K+-ATPase). In healthy tissue, cells contain relatively high concentrations of potassium ions but low concentrations of sodium ions. This concentration gradient is maintained by the sodium-potassium pump.
© dkfz.de

Sodium (23Na) ions are important for cellular homeostasis and cell viability. In healthy tissue, the extracellular sodium concentration ([Na+]ex = 145 mM) is about 10 times higher than the intracellular concentration ([Na-]in = 10-15 mM) (Fig. 1). Using 23Na-MRI, volume- and relaxation-weighted signal of these compartments can be measured. Thus, 23Na MRI is a promising diagnostic tool to detect pathologic processes that alter the 23Na ion gradient.

However, 23Na MRI is challenging because the 23Na nucleus posses a spin of 3/2 and an electrical quadrupole moment which lead to a fast bi-exponential transverse relaxation in tissue. Thus, pulse sequences with very short echo times (TE) are a necessary pre-requisite for 23Na MRI.

As the total 23Na concentration is only about 50 mM in brain tissue and about 30 mM in the muscle, and the 23Na NMR sensitivity is 10-fold lower compared to 1H MRI, 23Na MRI suffers from an approximately 20,000 times lower in vivo signal. This sensitivity limitation can partly be overcome by shorter repetitions (TR) times, since the longitudinal relaxation times T1 are much shorter than for 1H. Nevertheless, the overall sensitivity remains more than 2000-fold lower when compared to 1H MRI.

Fig. 2: 23Na-images of the healthy human brain obtained at three different field strengths (1.5 / 3 / 7 T).
© dkfz.de

As the signal to noise ratio (SNR) increases approximately linearly with the magnetic field, 23Na MRI profits from an increase in the magnetic field strength (Fig. 2).

To further increase the SNR, an optimized pulse sequence was developed, that utilizes a density-adapted k-space acquisition scheme. In Fig. 3 23Na MR images that were acquired with this new density-adapted 3D Radial imaging technique (DA-3D-RAD) are compared with a conventional 3D radial sampling scheme. For equivalent scan times, the SNR is up to 80% higher, artifacts due to B0-inhomogeneities are less pronounced and anatomic details are better resolved [1].

Fig. 3: Selected images of 3D 23Na in vivo brain data sets. The images in the upper row were acquired with a conventional 3D-RAD sequence and in the lower row, with a density-adapted sampling scheme (DA-3D-RAD). The DA-3DPR sequence shows a significantly increased SNR and less images artifacts. Parameters: TE = 0.2 ms; TR = 50 ms; B0 = 3 T; resolution: 4 x 4 x 4 mm3; acquisition time: 10 min 50 s.
© dkfz.de

With this newly developed pulse sequence and the high magnetic field strength (7 T) 23Na-images with an isotropic resolution of 2.5 x 2.5 x 2.5 mm3 can be acquired




Reference:
[1] Nagel AM, Laun FB, Weber MA, Matthies C, Semmler W, Schad LR. Sodium-MRI Using a Density-Adapted 3D Radial Acquisition Technique. Magn Reson Med 2009 Dec; 62(6):1565-73.

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