oxyhemoglobin), acute (1–3 days, intracellular deoxyhemoglobin), early subacute (3–7 days, intracellular methemoglobin), late subacute (≥14 days, extracellular methemoglobin), and chronic (>4 weeks, intracellular hemosiderin and ferritin) (Table 2).

MR signs of acute or recent bleeding within a hematoma can indicate a potentially unstable abruption, whereas hematomas with late subacute bleeding are stable.

Areas of hemorrhage in asymptomatic women are felt to result from venous bleeding of a remote nature, and treatment would typically be expectant­ management. Evidence of prior hemorrhage is not an uncommon finding in pregnancy, and when identified, it should be described in detail including multidimensional measurements, location, and proximity to the umbilical cord insertion and cervix.

MRI is an extremely accurate exam to identify the origin of secondand third-trimester uterine bleeding with an excellent interobserver agreement. With respect to US, it grants new and additional data that can influence the clinical management of these patients (Masselli et al. 2011c). Obstetricians will use this information to determine proper clinical and sonographic ­follow-up intervals to assess for fetal growth, anemia, and stress.

7\ Solid Placental Masses

Solid placental masses are rare; chorioangiomas are the most common tumor of the placenta and are identified in up to 1% of all placentas evaluated histologically. In up to 1:3500 births, chorioangiomas come to clinical attention (Sakornbut et al. 2007). These masses are typically >5 cm in

size and can be associated with polyhydramnios, hydropic changes in the fetus, intrauterine growth restriction, and congestive heart failure of the fetus due to vascularity of the mass. Chorioangiomas are benign tumors that can show intratumoral hemorrhage. Differentiating a placental hematoma from a solid mass, such as a chorioangioma, can be accomplished using color Doppler during sonographic evaluation (Sinha and Kuruba 2008). However, masses that have undergone hemorrhagic infarction or thrombosis can have limited internal flow and remain difficult to diagnose (Zalel et al. 2002a, b). Chorioangiomas can be homogeneous and nearly isointense to placenta on T1and T2-weighted images (Kawamoto et al. 2000). They are typically round in shape and may protrude from the placental surface. A very subtle thin capsule may be identified on T2-weighted images which does not mimic the regular septae of the placenta (Fig. 11). Peripheral areas of internal hemorrhage, manifesting as T1-shortening, have been described in case reports (Sakornbut et al. 2007; Oyelese and Ananth 2006). Similarly, in those masses with internal infarction or hemorrhage, increased T2-signal and increasing heterogeneity of the signal intensity have been reported. It is important to report the presence of prominent vessels along the fetal surface of the mass given potential for hemodynamic impact on the fetus. If there are early signs concerning for hydrops in the fetus, prompt notification of the ordering provider is indicated.

Similar to teratomas in other tissues, placental teratomas can contain virtually any tissue type including fat, calcification, fluid, and hair. Although teratomas in the placenta are extremely rare, in case reports, pregnancy outcomes are typically good (Elsasser et al. 2010). Although fat and calcification can be readily identified

b

Fig. 11  Coronal (a) and axial (b) T2-weighted half-Fou- rier RARE images shows a well-circumscribed mass (arrows) arising from the fetal part of the placenta immediately adjacent to the insertion of the umbilical cord. This is the classic location for a chorioangioma

sonographically, differentiation from an anomalous additional gestation or fetus acardius amorphous can be difficult, and MRI may be requested if the diagnosis is uncertain. With a large field of view, the entire contents of the uterus can be readily evaluated in a single acquisition which is helpful in the setting of multiple gestations, excessive fetal movement, or unfavorable mater-

8\ MR Functional Imaging

of the Placenta

Despite the fact that MR imaging helps delineate the morphologic alterations of the placenta with appropriate conspicuity during gestation and is of use in the study of placental invasion (as in placenta percreta), few studies have addressed the functional properties of the human placental vasculature.

Magnetic resonance imaging requested for a potentially serious indication provided a unique opportunity to explore the intervillous circulation of placentas from pregnancies complicated by intrauterine growth restriction (IUGR) and to compare them to normal cases. This allowed an innovative characterization of in vivo utero-pla- cental blood flow, correlating a compromised intervillous circulation in IUGR to the deterioration of fetal condition (Moore et al. 2000).

MR images disclosed a homogeneous perfusion overall the placenta in normal cases, while IUGR placentas displayed a low intervillous blood flow, along with many patchy unperfused areas. Intermittent stops worsen the perfusion dynamics of the intervillous mostly in IUGR cases with an elevated ductus venosus pulsatility index. In IUGR placenta maternal placental blood flow is extremely compromised and that superimposed dynamic phenomena concur to worsen the intervillous circulation leading to an end-stage fetal decompensation (Brunelli et al. 2010).

However, MR evaluation of placental perfusion is limited by the inability to administer gadolinium due to concerns for fetal safety, and other forms of evaluation of placental

perfusion, including magnetization transfer, have been described.

This method takes advantage of the ratio of bound protons to total number of protons as a reflection of vascular flow. Owing to the clinical limitations of gadolinium-enhanced perfusion imaging, diffusion-weighted imaging is an alternative technique for studying regional ischemia caused by an insufficient vascular supply.

Among the many causes of diffusion restriction in human pregnancy, hematoma and infarctions are most important because they lead to dysmaturity of the placenta, resulting in smaller diffusive conductance and restricted blood supply owing to tissue degeneration and scarring (Brunelli et al. 2010).

9\ Future Directions

Cell-free fetal DNA is now used frequently in the United States as a screening test for aneuploidy. With the growth of this technology, several investigators have looked at using cell-free placental mRNA in maternal plasma to better identify patients with accreta requiring hysterectomy and also as a tool to combine with ultrasound to improve accuracy (Nyberg et al. 1987; Verswijvel et al. 2002). In the patient with risk factors for placental invasion, the combination of a laboratory serum test with ultrasound and/or MRI might yield the most consistent results.

Fusion of ultrasound images on MR volumes has been feasible for fetal antenatal evaluation in a study conducted by Salomon et al. (Masselli et al. 2011c). Utilizing high-resolution ultrasound images with the capability of real-time color Doppler can help determine vascularity of structures, especially as gadolinium-based contrast agents are not routinely used in the setting of pregnancy. This capability may be of particular interest in placental evaluation. Fetal MRI does not currently have a significant role in the evaluation of Twin-to-twin transfusion syndrome (TTTS); however, there are case reports of using MR-guided high-intensity-focused ultrasound (MR-HIFU) for ablation of the abnormal vessels in TTTS (Sebire et al. 2002).

As elsewhere in the body, functional MR techniques are being applied in the placenta in effort

to better evaluate normal physiology as well as pathologic states. The use of diffusion-weighted imaging has demonstrated restricted diffusion and reduced ADC values in the placentas of fetuses with growth restriction caused by placental insufficiency (Masselli et al. 2016). Diffusionweighted imaging has also been proposed for the evaluation of placental invasion. As gadoliniumbased contrast agents are not routinely used in pregnancy, noncontrast flow-sensitive methods, such as arterial spin labeling (ASL), have been proposed to assess placental perfusion (Masselli and Gualdi 2013).

In conclusion, MRI is an excellent modality in the evaluation of the placenta, and knowledge of the MR finding of various placental pathologies aids radiologists in the appropriate and timely care of the pregnant patients.

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