Indian Journal of Radiology and Imaging Indian Journal of Radiology and Imaging

OBS/GYNEC
Year
: 2016  |  Volume : 26  |  Issue : 1  |  Page : 44--51

Magnetic resonance imaging - A troubleshooter in obstetric emergencies: A pictorial review


Rohini Gupta, Sunil Kumar Bajaj, Nishith Kumar, Ranjan Chandra, Ritu Nair Misra, Amita Malik, Brij Bhushan Thukral 
 Department of Radiodiagnosis, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi, India

Correspondence Address:
Dr. Rohini Gupta
Department of Radiodiagnosis, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi - 110 029
India

Abstract

The application of magnetic resonance imaging (MRI) in pregnancy faced initial skepticism of physicians because of fetal safety concerns. The perceived fetal risk has been found to be unwarranted and of late, the modality has attained acceptability. Its role in diagnosing fetal anomalies is well recognized and following its safety certification in pregnancy, it is finding increasing utilization during pregnancy and puerperium. However, the use of MRI in maternal emergency obstetric conditions is relatively limited as it is still evolving. In early gestation, ectopic implantation is one of the major life-threatening conditions that are frequently encountered. Although ultrasound (USG) is the accepted mainstay modality, the diagnostic predicament persists in many cases. MRI has a role where USG is indeterminate, particularly in the extratubal ectopic pregnancy. Later in gestation, MRI can be a useful adjunct in placental disorders like previa, abruption, and adhesion. It is a good problem-solving tool in adnexal masses such as ovarian torsion and degenerated fibroid, which have a higher incidence during pregnancy. Catastrophic conditions like uterine rupture can also be preoperatively and timely diagnosed. MRI has a definite role to play in postpartum and post-abortion life-threatening conditions, e.g., retained products of conception, and gestational trophoblastic disease, especially when USG is inconclusive or inadequate.



How to cite this article:
Gupta R, Bajaj SK, Kumar N, Chandra R, Misra RN, Malik A, Thukral BB. Magnetic resonance imaging - A troubleshooter in obstetric emergencies: A pictorial review.Indian J Radiol Imaging 2016;26:44-51


How to cite this URL:
Gupta R, Bajaj SK, Kumar N, Chandra R, Misra RN, Malik A, Thukral BB. Magnetic resonance imaging - A troubleshooter in obstetric emergencies: A pictorial review. Indian J Radiol Imaging [serial online] 2016 [cited 2020 Apr 5 ];26:44-51
Available from: http://www.ijri.org/text.asp?2016/26/1/44/178292


Full Text

 Introduction



USG is the mainstay modality for obstetric imaging. It is safe, accurate, fast, and inexpensive. However, it is operator dependent and limited by factors like bowel gas, small field of view, and obesity. USG suffers limitation in differentiating hemorrhage from other fluids which is important in ectopic pregnancy (EP) and antepartum hemorrhage (APH). Poor acoustic window in advanced pregnancy hampers proper visualization of posterior placenta and adnexal regions, which is important in placenta accreta and adnexal torsion. Small field of view (FOV) makes assessment of congenital uterine anomalies difficult. If USG findings are equivocal, the imaging work-up of a pregnant patient meets a dead end. Therefore, the need for a complimentary obstetric imaging modality has long been felt. Computed tomography (CT) does not fill this void because of the risk of radiation-induced teratogenesis. [1],[2]

Magnetic resonance imaging (MRI), apart from being non-ionizing, offers other advantages like larger FOV, multi-planar imaging, excellent soft tissue contrast, and ability to differentiate blood from other fluids. [1],[2],[3] To date, no adverse effect to the fetus has been documented at the magnetic strengths used in diagnostic imaging and earlier fears concerning teratogenicity of MRI have been allayed. The 1.5 T MRI is safe in 2 nd and 3 rd trimesters as per the current US Food and Drug Administration (FDA) guidelines. MRI can even be performed in the 1 st trimester when used judiciously. Gadolinium (Gd) is known to cross the placenta into the fetal circulation from where it is excreted by the fetal kidneys into the amniotic fluid, swallowed by the fetus, and can remain in this cycle for an indeterminate period. The potential risk is unknown, so routine use of Gd in pregnancy is not recommended as yet. [4],[5]

Still, most of the obstetric MRI requests are limited to evaluation of fetal anomalies [3],[6] and there is general reluctance in the obstetrician family to use MRI in the emergency situations. This is mainly because of the practical difficulties in conducting emergent MRI study in an unstable gravid patient with acute hemorrhage or pain and obtaining optimal image quality in such conditions. [2],[7] Although these factors greatly affected image quality in the past, the current-day scanners have reduced the acquisition time considerably and most of the motion artifacts due fetal movement, maternal peristalsis, and breathing have been overcome. [1],[6] Diagnostic quality images can now be obtained even in challenging situations. This pictorial review highlights the above message.

The role of MRI in obstetric emergencies is discussed herein as it evolves and becomes one of the more important and safe tools in the hands of radiologist, particularly in situations where other modalities are found wanting.

 MRI Protocol



A robust MRI protocol is a prerequisite. Informed consent is needed, whether or not Gd is used. No specific patient preparation or premedication is required. In early pregnancy and postpartum cases, the FOV is usually limited to pelvis, so Turbo Spin Echo images can be used [Table 1]. [8],[9] In the 2 nd and 3 rd trimesters, abdominopelvic imaging is required. MRI technique should be such as to overcome the limitations caused by fetal motion and maternal breathing [Table 2]. Optimal use of parallel imaging can produce quality images. Specific absorption rate (SAR) is the mass normalized rate at which Radiofrequency energy is coupled to biologic tissue and is dependent on multiple variables in the MRI protocol. [10] MRI protocol should be tailored to minimize the SAR in a pregnant patient. [11]{Table 1}{Table 2}

Gd is best avoided during pregnancy, but is safe when used selectively for postpartum indications. [11],[12] Available data suggests that the percentage of Gd excreted in breast milk is too small to cause any adverse effect in the infant, so, it is safe to continue breast feeding, although ultimately the informed decision may be left to the mother to discontinue breast feeding for 12-24 h after Gd injection. [4]

 MRI in Early Pregnancy



Ectopic pregnancy

EP is the most common life-threatening emergency in early gestation, presenting usually around 6 weeks gestation. Incidence is rising due to infertility treatments and tubal surgeries. [8],[13]

The most definite diagnostic finding of EP on transvaginal sonography (TVS) is the presence of an extrauterine gestational sac (GS) with yolk sac and embryo within. The diagnosis is, however, difficult in the presence of hematoma and hematosalpinx, or in cases of extratubal implantation, where findings may mimic other adnexal masses because of inability of USG to identify the blood products [Figure 1]. [13] MRI can serve as a troubleshooting modality in such cases. On MRI, GS is seen as a cystic structure with thick T2-hyperintense wall. Hallmark of GS is presence of fresh blood within the wall, seen as distinct low signal intensity areas on T2-weighted images [Figure 2]. GS mimickers like corpus luteal cyst, on the other hand, rarely contain acute hematoma. In the presence of hemoperitoneum, GS may not be clearly discernible within the heterogeneous pelvic mass. Contrast-enhanced MRI is then useful to identify GS as a discrete structure with thick, brightly enhanced wall and papillary solid placental tissues. Therefore, when TVS is inconclusive, MRI helps to diagnose EP by detection of fresh blood and is highly specific in rare extratubal types of EP [Figure 3]. [8],[14],[15] As per the literature, if both direct and indirect signs are included, the overall sensitivity and specificity of USG and MRI in EP are 63% and 69%, and 91% and 100%, respectively. [16],[17]{Figure 1}{Figure 2}{Figure 3}

 MRI in 2 nd and 3 rd Trimester



Antepartum hemorrhage

APH is one of the major conditions where accurate diagnosis and timely intervention can be life saving. TVS has a high accuracy for diagnosis of placenta previa, but does not always rule out or allow confident diagnosis of placental abruption.

USG features of placental hemorrhage are known since decades, but there has been no significant improvement in the sensitivity for detecting placental hematoma despite technological advances in USG over the years. [11]

Acute and subacute hematoma may be isoechoic and appears only as a thickened placenta [Figure 4]A. Blood generally passes through the os and USG fails to detect the thin sleeve of hemorrhage left behind. This can be picked up on MRI due to its ability to detect blood products [Figure 4]B. [18] The reported sensitivity and specificity of USG and MRI in diagnosis of APH are 53-82% and 71-85%, and 100% and 100%, respectively. [11],[19] T1 gradient echo and diffusion weighted imaging are reportedly most accurate, but SSh T2W and SSFP sequences are also very efficacious. MRI can also accurately localize the hemorrhage as marginal subchorionic or retroplacental [Figure 5]. [11],[18] This affects the management because retroplacental hematoma has much higher risk of fetal demise. [18] The signal intensity based age estimation criteria of intracranial hematoma have been extrapolated to intrauterine hematoma by some authors and have been found to be helpful, as patients with hyperacute and acute hematoma rapidly worsened clinically. Hyperaute blood appears iso- to hypointense on T1-weighted, hyperintense on T2-weighted, and hyper- to hypointense on diffusion-weighted images. The presence of active bleeding is an indicator of instable abruption and, hence, such information is invaluable. [19]{Figure 4}{Figure 5}

Morbidly adherent placenta

The incidence of morbidly adherent placenta (MAP) has increased from 1 in 7000 in 70s to 1 in 533 live births now due to increasing number of cesarean sections combined with increasing maternal age and repeated cesarean sections. Failure to diagnose the condition prenatally can result in catastrophic intrapartum or postpartum hemorrhage and various surgical complications.

In case of suspicious MAP, imaging should be performed at 32 weeks gestation to confirm the diagnosis and allow time to plan 3 rd trimester management and surgical intervention.

The most predictive sonographic sign is presence of placental lacunae which show turbulent flow. A gray-scale ultrasound evaluation along with Doppler enhances the accuracy of detecting adherent placentae, although these are by no means very sensitive, particularly when the placenta is posterior. [20],[21] The sensitivity, specificity, and positive predictive value of USG including 3D and Doppler have been reported as 97%, 92%, and 77%, respectively. [22] The use of MRI as an adjunct modality in troublesome cases increases the detection of true positive cases.

The indications for MRI evaluation include inconclusive or questionable USG findings, inadequate visualization due to posterior placenta, low-lying placenta with other risk factors, and anterior placenta with poor visualization of cesarean scar region or with focal loss of retroplacental myometrial zone. [22]

The cogent role of MRI in placenta accreta is well established in literature. Abnormal features include placental heterogeneity, dark intraplacental bands on T2-weighted images due to fibrin deposition, disorganized intraplacental vascularity with hypertrophied vessels, loss of retroplacental T2 dark zone, focal uteroplacental defect, uterine bulge, and direct invasion of adjacent structures. Of these, intraplacental bands and flow voids are very important for diagnosing placenta accreta [Figure 6]A and B. [20] The pitfall is that thin bands and some flow voids can be seen in normal near-term placentae and in placental insufficiency. Keeping the criteria of band thickness >1 cm and deep location of flow voids >6 mm in diameter increases the specificity. Preserved retroplacental clear zone in on USG in doubtful areas on MRI can reduce the rate of false-positive diagnosis. Color Doppler and MRI are, hence, complementary in diagnosis of MAP. [20],[22],[23],[24]{Figure 6}

Uterine rupture

Uterine rupture in pregnancy is defined as a full-thickness disruption of the uterine wall that also involves the overlying visceral peritoneum. Diagnosis is often made intraoperatively.

In addition to superior depiction of hemoperitoneum, the excellent soft tissue resolution of MRI permits visualization of the actual uterine disruption, allowing a preoperative diagnosis [Figure 7], [Figure 8]A and B. [25],[26] However, as this condition can be catastrophic, only those patients who are hemodynamically stable should be taken up for MRI.{Figure 7}{Figure 8}

Adnexal masses

Both ovarian torsion and leiomyoma degeneration have a higher incidence during pregnancy and can present with acute abdominal pain. By virtue of its multiplanar capability, MRI can be a problem-solving tool as it can determine uterine versus adnexal nature of masses. Leiomyoma has a claw-shaped interface with the uterus and degenerated leiomyoma may show classic diffuse or peripheral high signal intensity on T1-weighted images, while its USG features are nonspecific [Figure 9]A-C. [27]{Figure 9}

USG features of torsion ovary include unilateral enlarged ovary, uniform peripheral cystic structures, a co-existent ovarian mass, pelvic free fluid, lack of arterial or venous flow, and a twisted vascular pedicle. There are several mimics of these findings, including hemorrhagic cyst, polycystic ovary syndrome (PCOS), and ovarian hyperstimulation syndrome. Also, a co-existent mass may confound the diagnosis. Color Doppler may be normal due to dual arterial supply of ovary. [27],[28] MRI features of ovarian torsion initially are ovarian enlargement with diffuse stromal edema seen as hyperintensity on T2-weighted images. Later, the ovary undergoes hemorrhagic infarction; hemorrhage within pedicle, ovary, or peritoneum is noted as dark signal on T2-weighted images and bright signal on fat-saturated T1-weighted images. Tubal thickening >10 mm (mass/target or beak like) may be present [Figure 10]A-C. Lack of post Gd enhancement in the non-viable ovary is best appreciated on subtraction images. [29],[30]{Figure 10}

Uterine anomalies like bicornuate uterus or rudimentary horn seen during pregnancy can also appear like adnexal masses on USG. Large FOV and tissue signal characterization of MRI help in making proper diagnosis [Figure 11]A and B.{Figure 11}

 MRI in Postpartum/Post-abortion Period



Retained products of conception (RPOCs) appear on MRI as intracavitary uterine soft tissue mass with variable T1- and T2-weighted signal intensity, variable contrast enhancement, variable degree of myometrial thinning, and loss of junctional zone [Figure 12]A and B. MRI can also show associated uterine anomalies, recognition of which can be of profound assistance in surgical planning.{Figure 12}

MRI findings of gestational trophoblastic disease (GTD) overlap with RPOCs. Myometrial thinning, loss of junctional zone, and post Gd enhancement are not reliable signs for differentiation. However, presence of abnormal uterine vasculature and theca lutein cysts favor GTD [Figure 13]. MRI is also beneficial in disease staging by showing the uterine invasion and extrauterine disease in GTD. [31]{Figure 13}

 Conclusion



To summarize, MRI has an excellent troubleshooting role in various obstetric emergency situations. If TVS findings are inconclusive for ectopic gestation or when rare extratubal sites are suspected, MRI has a definite role. It has an edge over USG in APH by detection of small hematomas and active bleeding and should be done when USG is negative or inconclusive. The indications of MRI as an adjunct modality in MAP are now well defined. MRI can avert potential catastrophic event in uterine rupture by showing actual uterine defect as a cause for hemoperitoneum and should be done in hemodynamically stable patients. Multiplanar imaging and large FOV help solve many mysteries of torsed adnexae, uterine masses, and uterine anomalies complicated during gestation. In postpartum patients, RPOCs and GTN have many overlapping findings. Still, MRI can help in diagnosing gestational trophoblastic neoplasia with confidence by showing flow voids within the uterine mass in association with theca lutein cysts.

In conclusion, MRI is a safe imaging modality in pregnancy and fast enough to be done in emergency situations. The authors' experience is that it fills the lacuna for a problem-solving tool in potentially catastrophic obstetric conditions where USG is inadequate. Gd should be used judiciously, mainly in EP and postpartum or post-abortion conditions. Close coordination between the obstetric, USG, and MRI departments is crucial for pragmatic use of this much underutilized modality in emergency obstetric conditions.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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