In our study, in both lung and renal ROIs, there is an increasing perfusion fraction with gestational ageing, depicting the physiological increase of perfusion occurring in those tissue in fetal life.
According to literature, there is a correlation between diffusion properties and GE.
However, the possibility that a significant correlation exists between diffusion properties and gestational age in renal and lung parenchymal maturation is still under question. Moore et al reported a correlation between diffusion coefficient and GE in fetal lung maturation, while Balassy et al have reported that the average ADC of the fetal lungs remains constant during gestation.In the study of fetal kidney development using DWI sequences with ADC maps, Manganaro et al found a negative correlation between ADC values and gestational age.
Our study, however, showed no significant correlation between diffusion coefficient(D) and GE. This can be explained by the fact that blood flow in fetal lung and kidney significantly influences diffusion and ADC properties.
Therefore, similar to the hypothesis of some studies (Jakab et al), we suppose that an increase in pulmonary and renal vascularization might be the cause for an increase in overall ADC, but this increase is better reflected by the IVIM model.
IVIM model, using more low b-values, quantifies separately diffusion coefficient and perfusion fraction and shows a selective increase of the microvascular perfusion fraction, but not of the diffusion coefficient.
In conclusion IVIM model is able to detect microstructural physiological tissue changes occurring in fetal lung and kidneys during prenatal life.
In particular Perfusion Fraction could be proposed as a marker of pulmonary and renal maturation.
Our results highlight that DW imaging with IVIM model is useful as an additional prenatal diagnostic tool to study the lung and renal maturation during prenatal life. These normal values may have clinical relevance as benchmark values to compare to IVIM values of restricted lungs and kidneys maturation, especially in fetal growth restriction and oligohydramnios.