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Purpose The in vivo probing of restricted diffusion effects in large lipid droplets on a clinical MR scanner remains a major challenge due to the need for high b‐values and long diffusion times. This work proposes a methodology to probe mean lipid droplet sizes using diffusion‐weighted MRS (DW‐MRS) at 3T. Methods An analytical expression for restricted diffusion was used. Simulations were performed to evaluate the noise performance and the influence of particle size distribution. To validate the method, oil‐in‐water emulsions were prepared and examined using DW‐MRS, laser deflection and light microscopy. The tibia bone marrow was scanned in volunteers to test the method repeatability and characterize microstructural differences at different locations. Results The simulations showed accurate and precise droplet size estimation when a sufficient SNR is reached with minor dependence on the size distribution. In phantoms, a good correlation between the measured droplet sizes by DW‐MRS and by laser deflection (R 2 = 0.98; P = 0.01) and microscopy (R 2 = 0.99; P < 0.01) measurements was obtained. A mean coefficient of variation of 11.5 % was found for the lipid droplet diameter in vivo. The average diameter was smaller at a proximal (50.1 ± 7.3 µm) compared with a distal tibia location (61.1 ± 6.8 µm) ( P < 0.01). Conclusion The presented methods were able to probe restricted diffusion effects in lipid droplets using DW‐MRS and to estimate lipid droplet size. The methodology was validated using phantoms and the in vivo feasibility in bone marrow was shown based on a good repeatability and findings in agreement with literature.

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