Quantitative MRI of lipid content for assessing fetal adipose tissue development and brown-to-white fat conversion

Brown and white adipose tissues in different fetal regions undergo distinct developmental processes. This study aimed to quantitatively evaluate the fetal adipose tissue development of singleton fetuses.

A total of 78 participants were recruited, of whom 42 participants were included in the statistical analyses. Multi-echo water-fat separation (IDEAL-IQ) was performed, and proton density fat fraction (PDFF) and apparent transverse relaxation rate (R2*) values were measured for multiple anatomical regions, including cheeks, occiput, underjaw, neck, back, thorax, abdomen, and thighs. Correlations with gestational age were analyzed, and analysis of covariance (ANCOVA) was conducted by grouping participants into early (28-31 GW), mid (32-34 GW), and late (35-38 GW) gestational stages.

PDFF values in all fetal regions showed significant positive correlations with gestational age (p < 0.05). R2* values demonstrated region-specific patterns: no significant correlation was found for the back, while the cheeks showed a negative correlation; other regions showed significant correlations (p < 0.05). ANCOVA revealed significant differences in PDFF and R2* values across multiple regions among the early, mid, and late gestational groups (p < 0.01), indicating distinct stage- and region-specific characteristics of fetal fat deposition and iron metabolism.

This study demonstrated that fetal PDFF increases globally with GW, while regional R2* patterns may reflect aspects of metabolic maturation. A change in R2* around approximately 32 weeks in the fetal cheek region may be indicative of an early transition from brown- to white-like adipose tissue. These findings indicate that IDEAL-IQ could be a useful tool for exploring fetal fat development.

This study provides quantitative MRI relevant to fetal brown-to-white fat conversion, suggesting a potential role for imaging markers in characterizing metabolic maturation in vivo. These findings may contribute to the development of noninvasive approaches for assessing fetal metabolic status and developmental variability.

PDFF increased with gestational week, indicating progressive fetal fat maturation. Regional R2* patterns varied with gestational age, with changes in the cheek region around ~32 weeks that may be consistent with a transition from brown- to white-like adipose tissue. Fat development followed a "cheek → trunk → limb" sequence, showing spatial heterogeneity.