Erythrocyte magnetics: timescale of alignment in NMR spectrometers

Discocyte erythrocytes (red blood cells; RBCs) undergo spontaneous alignment in a uniform magnetic field, <b>B</b>₀ – a phenomenon initially observed via optical dispersion and light microscopy and later confirmed with nuclear magnetic resonance (NMR) spectroscopy. The phenomenon arises due to the diamagnetic anisotropy of membrane lipids and proteins, which have different energies when oriented parallel or perpendicular to the magnetic field. This behaviour is directly relevant to modern NMR experiments performed on cells, in particular multiple-quantum-filtering of quadrupolar nuclei with nuclear spin quantum number I &gt; ½, which are sensitive to the degree of alignment in intact cells and tissues, forming the basis of our present investigations.<br/>Using a quartic equation to represent the surface of a discocyte cell and the boundary element method to integrate over a triangular mesh, we compute the minimum-energy orientation of an RBC in <b>B</b>₀. At a field strength of <b>B</b>₀ = 9.4 T, the energy difference between parallel and orthogonal alignments was calculated to be ~1,200 k_BT, the latter being Boltzmann’s constant k_B times the absolute temperature T, which is the thermal energy of a single particle in solution. Using hydrodynamic theory, we estimate a characteristic realignment time (which has a theoretical 1/<b>B</b>₀² dependence) of ~300 ms at this field strength and extend this to consider the effects of collective behaviour in densely packed RBC suspensions. Our findings agree with experimental data showing sub second reorientational dynamics in strong magnetic fields and provide a physical framework for interpreting magnetic-field-induced structural order in biological tissues, which inform the development of new NMR and magnetic resonance imaging (MRI) methodologies for probing anisotropy, membrane architecture and cellular organization in vivo.<br/>