Early detection of increased marrow adiposity with age in rats using Z-spectral MRI at ultra-high field (7 T)

Nowadays, the drive towards high-field MRI is fueled by the pursuit of higher signal-to-noise ratio, spatial resolution, and imaging speed. However, high field strength is associated with field inhomogeneity, acceleration of T₂ * decay, and increased chemical shift, which may pose challenges to conventional MRI for fat quantification in complex tissues such as bone marrow. With proton MRI spectroscopy (¹ H-MRS), on the other hand, it is difficult to produce high resolution. As a novel alternative fat quantification method, high-resolution Z-spectral MRI (ZS-MRI) can achieve fat quantification by acquiring direct saturated images of both fat and water under the same T_E , which may be less affected by T₂ * decay and field inhomogeneity.

To demonstrate ZS-MRI for marrow adipose tissue (MAT) quantification in rat's lumbar spine and the early detection of MAT changes with age.

The accuracy of ZS-MRI for fat quantification at ultra-high-field MRI (7 T) was verified with MRS and conventional Dixon MRI in water-oil mixed phantoms with varying fat fraction (FF). Dixon MRI data were processed with iterative decomposition of water and fat with echo asymmetry and least-squares estimation. ZS-MRI was then used to longitudinally monitor the adiposity in the lumbar spine of young healthy rats at 13, 17, and 21 weeks to detect the early changes of FF with age in MAT. Hematoxylin-eosin staining of lumbar spines from separated rat groups was performed for verification.

In ex vivo phantom experiments, both Dixon MRI and ZS-MRI were well correlated with ¹ H-MRS for the quantification of FF at 7 T (R > 0.99). Compared with Dixon MRI, ZS-MRI showed reduced image artifacts due to field inhomogeneity and presented better agreement with ¹ H-MRS for the early detection of increased MAT due to age at 7 T (ZS-MRI R = 0.78 versus Dixon MRI R = 0.34). The increased MAT FF due to age was confirmed by histology.

ZS-MRI proves itself as an alternative fat quantification method for bone marrow in rats at 7 T.