Measuring large lipid droplet sizes by probing restricted lipid diffusion effects with diffusion‐weighted MRS at 3T

Abstract

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.

Reduction of vibration‐induced signal loss by matching mechanical vibrational states: Application in high b‐value diffusion‐weighted MRS

“…In the 11,000 rpm WF phantom, the ADC measurement seems to not be affected a lot by vibrations; therefore, the 3 measurement scenarios give very similar lipid ADC values. The differences in both phantoms can be explained by different oil droplet sizes within the 2 phantoms, resulting in different viscoelastic properties of the phantoms: the 11,000 rpm phantom is more viscous than the 6000 rpm phantom. Therefore, the effect of the vibrations on the overall signal loss is highly dependent on the tissue properties and can vary a lot between different tissue type.…”

“…The coefficient of variation for the estimation of in vivo lipid ADC decreased significantly when the VMG was employed in all 3 volunteers. A correct estimation of the diffusion properties of lipids is particularly important when high order effects such as diffusion restriction effects are investigated . The ADC value changes in an 80‐µm diameter fat cell in vivo by approximately 1% per 100 ms diffusion time increase.…”

“…In the 11,000 rpm WF phantom, the ADC measurement seems to not be affected a lot by vibrations; therefore, the 3 measurement scenarios give very similar lipid ADC values. The differences in both phantoms can be explained by different oil droplet sizes within the 2 phantoms, 12 resulting in different viscoelastic properties of the phantoms: the 11,000 rpm phantom is more viscous than the 6000 rpm F I G U R E 9 In vivo lipid ADC measurement in the tibia bone marrow for different loadings (0-10-20 kg additional weighting) on the scanner table in 3 healthy volunteers. Without the VMG, the measured ADC is dependent on the extra loading of the scanner table, and a larger ADC SD is observed.…”

“…Multiple research applications require particularly high b ‐values diffusion measurements, including high‐resolution diffusion tensor imaging of the brain or the assessment of fat microstructure with diffusion MRI . Specifically, measuring the diffusion properties of lipids has been proven to be challenging because fat has a diffusion coefficient approximately 2 orders of magnitude lower than water due to the large molecular size .…”

“…4 Multiple research applications require particularly high b-values diffusion measurements, including high-resolution diffusion tensor imaging of the brain 9 or the assessment of fat microstructure with diffusion MRI. [10][11][12] Specifically, measuring the diffusion properties of lipids has been proven to be challenging because fat has a diffusion coefficient approximately 2 orders of magnitude lower than water due to the large molecular size. 4,13 The assessment of fat microstructure is important in the study of adipose tissue and ectopic lipids in both healthy subjects and subjects affected by metabolic dysfunction across organs and tissues.…”