The present study investigated the differences of ADC values of brain regions among surviving fetuses, twin controls and single fetus controls. Our study showed lower ADC values of frontal, parietal and occipital lobe in surviving fetuses compared with twin controls and single fetus controls. Furthermore, our findings showed the negative correlations between ADC values in several brain regions and gestational age.
Meta-analysis showed that the risk of neuro developmental morbidity in monochorionic twins was about 5 times higher compared with that in dichorionic twins after a single fetal death . Previous study reported that cerebral hypoxic-ischemic injury might occur in the surviving fetus after demise of one fetus due to the sharing of one placenta. The surviving fetus might present with hypotension and hypoperfusion due to the loss of circulatory equilibrium and the shunting of blood flow. The reduced cerebral blood flow might be accepted as causative factors for cerebral damage in surviving fetus. In this study, compared with twin and single fetus controls, ADC values of bilateral frontal lobes, parietal lobe and occipital lobes were lower in surviving fetuses. The reduction of ADC values might be indicative of parenchymal damage and metabolic compromise while no abnormalities were detected on ultrasound and MR conventional sequences. Decrease of ADC values might reflect the intracellular/extracellular water compartmentalization , especially the decrease in the extracellular water content. ADC value might be more sensitive to detect the subtle anomalies, even changes of signal were not shown on DWI images. Our findings further provided evidence of the possibility of detecting potential brain damage by measuring ADC values in surviving fetuses after demise of one fetus. There were no significant differences of ADC values in all ROIs between left and right hemisphere in surviving fetuses. The characteristics of symmetry indicated that decreases of ADC values might be caused by cerebral hypoperfusion, but not a single vessel supply.
Our present study showed the negative correlations between ADC values of basal ganglia, thalamus and cerebellum and gestational age in control groups which were consistent with previous study. Decreases of ADC values in the majority of brain during fetal development were reported in previous publication . Significant decreases of ADC values were detected in thalamus, basal ganglia, pons and cerebellum with gestational age, but the decrease was not detected in frontal white matter [29–30]. However, the conclusions were controversial. Hoffmann et al  found that a weak trend for regional ADC decline was shown in all regions which didn’t reach statistical significance with brain development. In the present study, ADC values in bilateral frontal, temporal lobes were not correlated with gestational age in two control groups. The above results implied that ADC values in these regions were relatively stable and might be served as developmental indicator. However, ADC values of bilateral thalamus and cerebella, bilateral frontal lobe, parietal lobes were negatively correlated with gestational age in surviving fetuses, which indicated the existed potential damage other than the effects of gestational age in these regions. Our findings suggested the possibility of ADC values within frontal lobes in distinguishing the potential lesions of surviving fetuses.
In this study, we also found that average ADC values of frontal white matter in single fetus group was lower than that in previous study . The possible reason was that the average gestational age in our study was larger than that in other studies. No significant differences of ADC values were found between twin controls and single fetus controls, which suggested that single fetus might serves as control group if there was no suitable twin control group in future study. The follow-up results showed that 3 fetuses subsequently developed brain abnormality by ultrasound or MRI. It indicated the underlying changes of brain might exist even through no abnormal signals on conventional and DWI sequence. The measurement of ADC values in surviving fetuses might help to detect the potential subtle anomalies earlier. Although changes of ADC values were shown in surviving fetuses, making crucial decisions (such as pregnancy termination) on the basis of DWI alone also could be very difficult. It might be more reasonable to discuss the possibility of termination of pregnancy in cases with large cerebral lesion on MR and DWI. For surviving fetuses with ADC values changes alone, the follow-up is necessary. What’s more, as fast sequence, DWI has potential value of clinical application due to limitation of many sequences. It could be effective supplement to conventional fetal MRI examination and may detect the underlying changes earlier. Furthermore, the present study showed the good predictive value of ADC values in single lobe including frontal and parietal lobe in discriminating surviving fetuses and twin controls. The predictive value of combination of ADC value of frontal lobes, parietal lobes and gestational age was stronger than that of single lobe alone. The study demonstrated that ADC values might be effective indicators of subtle anomalies in surviving fetuses in future.
There were several limitations in the current study. Firstly, the sample size of the present study is small, and a larger sample group may contribute to more accurate conclusion. Secondly, there might be manual errors as ROI is selected. And thirdly, well-controlled and long-term studies are needed to reveal the relation between reduction of the ADC values and postnatal outcome.