The most dangerous situation any female could experience during pregnancy is the preeclampsia syndrome whether it presents as an individual symptom or as newly developed on top of chronic hypertension [7].
For many years, ultrasound aided with the Doppler study is the first and sometimes the sole method of evaluation of the placenta. To date, pathologies of the placenta have not been considered a routine indication of MR imaging [8].
Our study included 80 pregnant women, 40 of them presented with gestational hypertension but not diagnosed of preeclampsia yet and another 40 cases were control ones who were free from any medical disorders by history and clinical examination.
The patient’s mean age among the case group was 27.9 + 4.19 and among the control group was 28.9 + 5.65. Both groups were examined between 22 and 34 weeks of gestation by both 3D ultrasound and MRI examinations with a time interval of 7 days between both examinations.
In this study, we evaluated placental function in the second-trimester pregnancy in view of (1) assessment of the uterine and the umbilical arteries by Doppler ultrasound. (2) Evaluation of the signal intensity of the placenta by diffusion MR imaging and measuring the ADC values of the focal areas of abnormal signal. (3) Measuring placental volume by MRI and 3D ultrasound.
During our analysis, we agreed with Derwig et al. [9] who stated that uterine artery resistance index (RI) value was a useful diagnostic tool in assessment of vascular abnormalities in hypertensive pregnancies (Fig. 1) as we found a significant relation (P value 0.014) between the right uterine artery RI (mean) of both the hypertensive cases and the control. We performed Doppler examination of the uterine artery where 7.5% of the included hypertensive patients showed bilateral diastolic notch and elevated RI of both the uterine arteries while the remaining 2.5% had a unilateral notch and elevated RI was noticed in the left uterine artery. The aforementioned findings are important criteria in the assessment of hypertensive patients with suspected placental insufficiency as was justified by the positive correlation (significant P value of 0.012) that was detected between the presence of diastolic notch and RI value.
Our results matched with Coleman et al. [10], they reported that the higher RI cut-off value (≥ 0.7) or the presence of notching at the Doppler test enabled to identify pregnant women at high risk of placental insufficiency.
The presence of a diastolic notch was reported to be a better predictor of pre-eclampsia than an elevated RI or systolic/diastolic ratio, with positive predictive values up to 31% [10].
In the current work, when we performed the qualitative assessment for the MR imaging of the placenta, abnormal heterogeneous bright signal was noted in the T2WIs that was corresponded to areas of restricted diffusion in the diffusion-weighted images in 15% of the included hypertensive pregnant cases. Such areas represented placental ischemic changes. These findings agreed with those of Himoto et al. [11] who reported that placentae with impaired function show decreased signal compared to normal placentas. They also explained that the degree of low signal might depend on the decrease of placental circulation and the degree of compensatory alteration.
Derwig et al. [9] reported that T2 relaxation of the placenta during the second trimester is shorter in hypertensive pregnancies and that it correlates with our findings.
In DWI, areas of necrosis, infarction, or fibrosis that may be seen in the insufficient placenta result in altered diffusion [12]. Also, such sequence can be used to create an apparent diffusion coefficient (ADC) map that identifies and quantifies areas of accelerated and restricted diffusion. Therefore, in the pathologic placenta, DWI and ADC mapping had the potential to quantify early changes in diffusion even in the absence of abnormal findings on US/conventional MRI [12].
Our work showed a significant difference between patients with normal and those with an abnormal placental signal in the diffusion-weighted images (P = 0.047).
In diffusion-weighted images, our study showed that 30% of the hypertensive patients displayed areas of restricted diffusion (in the form of persistent bright signal) in their placenta, compared to only 15% seen on the T2WI sequence as mentioned earlier.
There were a positive correlation and a significant relationship between the T2WI and the DWI among the included hypertensive pregnant females (P = 0.002).
We noticed that measured ADC values of the placental parenchyma were significantly lower in the placentae with ischemic changes in the hypertensive pregnant cases when compared with those of normal pregnancies. The mean ADC value in the control was (1.87 ± 0.26 mm2/s) while in patients with placental ischemic changes, the mean ADC value was (1.36 ± 0.09 mm2/s).
Our results agreed with other studies as Siauve et al. [13] who reported that ADC value was significantly lower in placentae of fetuses with growth retardation when compared with normal ones. They stated that the most promising parameters appear to be the mean ADC value in the placenta which was in normal pregnancy—which according to them—1.77 ± 0.19 × 10−3 mm2/s, while in placentas with insufficient function, the mean ADC value was decreased, 1.46 ± 0.1 × 10–3 mm2/s.
Also, Manganaro et al. [14] studied fetal MRIs of 145 pregnant women with gestational age range 19–40 weeks. ADC values calculated on the DWI images obtained had a range from 1 to 2.4 × 10−3 mm2/s.
In our analysis, the area under the curve (AUC) of the hypertensive patients was significantly high (1.00 = 100%). The maximum sensitivity and specificity was achieved (sensitivity 91% and specificity 100%) with ADC value of 1.46 × 10−3 mm2/s. A higher sensitivity of 100% was noted with a mean ADC value of 1.59 × 10−3 mm2/s.
A retrospective study of DW imaging of the placenta in growth-restricted and non-growth restricted fetuses showed low measurements of ADC values of the placenta in the growth-restricted fetuses [12]. In our work, the addition of DWI to the 3D ultrasound assessment of the placenta showed increased sensitivity to detect placental insufficiency from 73 to 100%, increased accuracy of diagnosis from 91 to 99%, and sustained specificity of 99%.
We found significant agreement between the results of both 3D US with Doppler application and DW MR imaging in hypertensive pregnant cases as P value was significant (0.006) and kappa value was 0.318.
Our results agreed with Bonel et al. [15] which stated that Doppler US may be insensitive to detect early placental dysfunction. Also, Messerschmidt et al. [8] mentioned that a significant placental vascular disease that remains below the threshold of detection by Doppler assessment of the uterine and umbilical arteries can be predicted by MR imaging.
It is impossible to image the whole placenta by ultrasound which interferes in the accurate measurements of placental size. The measurement by ultrasound is likely to be affected by maternal habitus, amniotic fluid volume, fetal position, multiple gestation, and gestational age. In contrast, MRI has a wide field of view and is not affected by variables as amniotic fluid volume or maternal habitus [16].
Several investigators were concerned about measuring the placental volume using 3D ultrasound to predict infants who could be small for gestational age [17,18,19]. Also, for the same purpose, the placental volume was assessed by MRI [16]. These studies concluded that pregnancies complicated by fetal growth restriction are associated with placental insufficiency and reduced placental volume.
To our knowledge, the measurement of the placental volume by 3D US and MR has not been used before to assess the risk of placental insufficiency.
In our study, MR imaging presented more accurate measurements of the placental volume than ultrasound in 70 patients (87.5%). We measured the surface area of the placenta in each taken slice, and with the aid of advanced workstation, we calculated the total placental volume. We found a positive correlation between the measurements of placental volume by both 3D US and MRI in the control and hypertensive cases, respectively. The MRI values were the ones closer to the post delivery real placental volume (Figs. 1 and 3). There was a significant difference between the measurements of the placental volume by both modalities as P values were 0.001 and 0.017, respectively.
In our study, diffusion MR imaging revealed that 30% had placental ischemic changes (n = 12/40) and out of them, 17.5% developed preeclampsia (n = 7) and preterm labor. This could be attributed to the different clinical condition of each patient, the medical treatment received, and the capability of the fetoplacental unit to compensate.
Again, here, we agreed with Bonel et al. [15] who reported that DWI showed signs of placental ischemia not obtained with US. However, one must be aware that not all abnormalities detected at diffusion-weighted imaging indicate growth restriction; it is possible that the fetoplacental unit will partially compensate for placental insufficiency.
The strength of the study was the inclusion of pregnant patients with a high level of adverse outcome, especially that gestational hypertension is uncommon representative of the obstetric population.
