Objective

The purpose of this article is to evaluate the utility of whole-placenta volumetric IVIM analysis for evaluation of the placenta.

Materials and methods

This prospective study included 29 patients with normal pregnancy. Only single pregnancy with a living fetus and a gestation length between 28+0 and 39+6 weeks were included. All women underwent MRI examination including an IVIM sequence at 1.5T scanner. Evaluation of the IVIM sequence was performed with research software (vusion tech). Two independent blinded observers, with 3 and 10 years of experience in obstetric imaging, respectively, carried out the measurement of IVIM. The volume of the placenta, ADC, eADC, perfusion fraction(f), pseudodiffusion coefficient (D*) and standard diffusion coefficient(D) were calculated using all slices of the IVIM images. Intraclass correlation coefficient was calculated for measurements of the parameters provided by two radiologists. Pearson correlation was used between the parameters from volumetric IVIM analysis and the week of gestation.

Results

The interobserver agreement was excellent between two radiologists for the parameters. The average value of ADC was 1.52±0.08×10-3mm2/sec, eADC was 0.10±0.02,f was 42.73±2.81 %(mean±SD), D was 1.60±0.11×10-3mm2/sec, and D* was 37.90±8.21×10-3mm2/sec in the 29 patients. A moderately negative correlation was found between ADC, D and the gestational week(R=-0.449, p=0.015 and R=-0.394, p=0.034). The average volume of the placenta was 724.48±308.99ml,the volume of the placenta showed a moderately positive correlation with the week of gestation(R=0.411, p=0.027).

Discussion

The human placenta is a rapid developing organ that undergoes structural and functional changes throughout the pregnancy. It is a highly vascularized organ containing a high blood fraction and a large perfusion component, so it is appropriate for evaluation with IVIM. However, traditionally, most of the prior investigations1-4 measured IVIM parameters from manually placed ROIs on a representative section of the placenta, which might have led to interobserver variability in ROI selection. In addition, inappropriate ROI selection may not accurately reflect the physiological features of the placenta. Recently, from Jakab et al3’s research, the test-retest variability was in the range of 14-20% for f, 12-14% for D, and 17-25% for D% for the placenta as well as lungs and liver of the fetus, f and D* showed inferior reproducibility compared to corresponding measures of the above organs. Whole-placenta volumetric IVIM analysis is used to evaluate the parameters from IVIM of the entire lesion and avoids the subjectivity of ROI placement to ensure calculation accuracy and repeatability. This method captures the parameters of the entire placenta and thus can potentially eliminate sampling bias during data processing. So we tried to use whole-placenta volumetric IVIM analysis to assess the entire placenta.
The interobserver agreement was excellent between two radiologists for all the parameters using this method. So the VOI-based estimation of IVIM parameters are highly reproducible and repeatable. From our previous research, the average value of f was 33.20 ± 5.99%, D was 1.71 ± 0.21×10-3mm2/sec, and D* was 20.37 ± 6.20×10-3mm2/sec in patients without PAS disorders when we put ROI in the middle slice of the placenta4. Using whole-placenta volumetric analysis, we drew ROI on every slice of the placenta, the average value of f was 42.73±28.10 %(mean±SD), D was 1.6±0.11×10-3mm2/sec, and D* was 37.90±8.21×10-3mm2/sec in patients with normal pregnancies. The different methods may lead to the different results of the parameters.
We reported a mean placental perfusion fraction of 42.73% indicating a highly perfused microvascular compartment of the placenta in the third trimester. This value is higher than prior reports, this may be due to the different method of measurement2,5,6. We found a negative correlation between ADC and the gestational week in the patients. The decrease of ADC with the GA increasing may highlight the parenchymal changes characterized by a more fibrotic environment during last gestational weeks5. We also found a negative correlation between D and the gestational week in the patients. The diffusion coefficient reflects cellular and interstitial characteristics of the tissue7. The decrease of the D with the increase of the GA would indicate stable mircrovascular perfusion during late gestation2. We found a positive correlation between the placental volume and the week of gestation, which denoted continuous increase of placental volume in the third trimester of pregnancy.

Conclusion

Whole-placenta volumetric IVIM analysis provides a novel method for examining perfusion and diffusion in the developing human placenta. ADC and D decreased with the GA increase. The volume of the placenta also increased with the GA increase. Whole-placenta volumetric IVIM analysis may help provide information about placenta changes during its development.

Acknowledgements

n/a

References

1. Capuani S, Guerreri M, Antonelli A, et al. Diffusion and perfusion quantified by magnetic resonance imaging are markers of hunman placenta development in normal pregnancy. Placenta,2017,58:33-39 2.Jakab A, Tuura RL, Kottke R, et al. Microvascular perfusion of the placenta, developing fetal liver, and lungs. Assessed with intravoxel incoherent motion imaging. J magn reson imaging,2018,48:214-225 3. Jakab A, Tuura RL, Kottke R, et al. Intra-voxel incoherent motion MRI of the living human feotus: technique and tes-retest repeatability. European Radiology experimental,2017,1:26 4. Lu T, Pu H, Cui W, et al. Use of intravoxel incoherent motion MR imaging to assess placental perfusion in patients with placental adhesion disorder on their third trimester. Clinical imaging,2019,56:135-139 5. Moore RJ, Issa B, Tokarczuk R, et al. In vivo intravoxel incoherent motion measurements in the human placenta using echo-planar imaging at 0.5T. Magn Reson Med,2000,43:295-302 6. Sohlberg S, Mulic-Lutvica A, Lindgren P, et al. Placenta perfusion in normal pregnancy and early and late preeclampsia: a magnetic resonance imaging study. Placenta,2014,35：202-206 7.I Broses, R Pijnenborg, L Vercruysse, R Romero. The great obstetrical syndromes are associated with disorders to deep placentation. Am J Obstet Gynecol, 2011,24:193-201