MR Imaging of the Placenta

In this chapter, we review the appearances and the role of MRI in diagnosis and management of these conditions.

supine and still for a prolonged period in advanced gravid state (Bardo and Oto 2008).

2\ Imaging of the Placenta

The placenta is responsible for the nutritive, respiratory, and excretory functions of the fetus (Bernirschke and Kaufmann 2000). The placenta is often overlooked in the routine evaluation of a normal gestation, receiving attention only when an abnormality is detected. Although uncommon, abnormalities of the placenta are important to recognize owing to the potential for maternal and fetal morbidity and mortality (Elsayes et al. 2009). Pathologic conditions of the placenta include placental causes of hemorrhage, gestational trophoblastic disease, retained products of conception, nontrophoblastic placental tumors, and cystic lesions (Linduska et al. 2009).

Ultrasound is widely used as the initial diagnostic imaging technique during pregnancy because of its availability, portability, and lack of ionizing radiation (Elsayes et al. 2009).

Magnetic resonance (MR) imaging provides superior soft tissue contrast resolution, multiplanar imaging capabilities, and image quality independent of the mother’s size or fetus’ positioning, and it lacks ionizing radiation (Masselli et al. 2011a). MRI can be of added diagnostic value when further characterization is required, particularly in the setting of invasive placental ­processes such as placenta accreta, placental abruption, and gestational trophoblastic disease (Masselli et al. 2011a, b; Baughman et al. 2008). In particular, MR imaging might well have a pivotal role in the diagnosis of intrauterine bleeding thanks to its high spatial resolution and to the known high sensitivity and specificity in distinguishing blood from other fluid collections (Masselli et al. 2011b).

Moreover, in advanced gestational age, obese women, and posterior placental location, MRI is advantageous due to the larger field of view and its multiplanar capabilities.

Its drawbacks, however, include prolonged imaging time, cost, lack of skilled experience, claustrophobia, and the challenge of remaining

3\ MRI Protocol

Most patients in the second trimester of pregnancy can tolerate supine positioning within the MR system bore.

For patients in the third trimester, left lateral decubitus positioning may be better tolerated and decrease the risk of impaired venous return from caval compression by the uterus. To maximize signal, a multichannel surface coil is used whenever possible. Ideally, the bladder should be only moderately full, both for patient comfort and to avoid overor underdistension that could complicate assessment for potential bladder invasion.

All examinations should be performed on a 1.5Tesla (T) system in the supine position with a phased-array body coil. The safety of MR at 3 T has not yet been proven; however, to our knowledge, no published human studies documented any adverse effect on children exposed at higher magnetic fields, such as 3 T (Baughman et al. 2008).

A phased-array coil is preferred to use because it provides a superior signal-to-noise ratio, but in larger patients and towards the end of pregnancy, a body coil may be necessary.

If patients feel uncomfortable lying supine in the scanner (especially in the third trimester), imaging can be obtained with the patient in the lateral decubitus position, decreasing the pressure on the inferior vena cava.

Steady-state free-precession sequences (fast imaging employing steady-state acquisition [FIESTA], true fast imaging with steady-state precession [FISP], balanced fast field echo [FFE]) can provide motion-free images of the abdomen. The protocol includes multiplanar T2-weighted singleshot echo train spin-echo imaging (half-Fourier rapid acquisition with relaxation enhancement [RARE], single-shot turbo spin-echo, or singleshot fast spin-echo), in addition to sagittal T1-weighted gradient-echo imaging with fat suppression (Table 1) (Nagayama et al. 2002).

We recommend that MR examinations performed for placental abnormalities be monitored by a physician who can prescribe additional