Gadolinium-based MR contrast agents are gadolinium (Gd3+) chelates. These contrast agents readily enter the fetal circulation and are excreted via fetal urination into amnionic luid, where they may remain for an indeterminate period before being ingested and reabsorbed. he longer the gadolinium-chelate molecule remains in a protected space such as the amnionic sac, the greater the potential for dissociation of the toxic Gd3+ ion. Accordingly, gadolinium contrast should be avoided during pregnancy because of this potential for dissociation. Routine use of gadolinium is not recommended unless there are overwhelming potential benefits (American College of Radiology, 2013). In adults with renal disease, this contrast agent has been associated with development of nephrogenic systemic fibrosis, a potentially severe complication.
Before MR examination, all women complete a written safety questionnaire that includes information about metallic implants, pacemakers, or other metal-or iron-containing devices that may alter the study (American College of Radiology, 2013).
Iron supplementation may cause artifact in the colon but does not usually afect the resolution of fetal images. In more than nancy during the past 15 years, maternal anxiety secondary to claustrophobia and/or fear of MR equipment has developed in less than 1 percent of our patients. To reduce maternal anxiety in this small group, a single oral dose of diazepam, 5 to 10 mg, or lorazepam, 1 to 2 mg, may be given.
To begin an MR examination, women are placed in a supine or left lateral decubitus position. A torso coil is used in most circumstances to send and receive the radiofrequency pulses, but a body coil can be used alone to accommodate large mater nal habitus. A series of three-plane localizers, or scout views, are obtained relative to the maternal coronal, sagittal, and axial planes. he gravid uterus is imaged in the maternal axial plane (7-mm slices, 0 gap) with a T2-weighted fast acquisition.
Typically, these may be a single-shot fast spin echo sequence (SSFSE), half-Fourier acquisition single-shot turbo spin echo (HASTE), or rapid acquisition with relaxation enhancement (AE), depending on the brand of machine. Next, a fast
T1-weighted acquisition such as spoiled gradient echo (SPGR) is performed (7-mm thickness, 0 gap). hese large-field-of-view acquisitions through the maternal abdomen and pelvis are par ticularly good for identiying fetal and maternal anatomy.
Orthogonal images of targeted fetal or maternal structures are then obtained. In these cases, 3-to 5-mm slice thickness, o gap T2-weighted acquisitions are performed in the coronal, sagittal, and axial planes. Depending on the anatomy and underlying suspected abnormality, T1-weighted images can be performed to evaluate for subacute hemorrhage, fat, or location of normal structures that appear bright on these sequences, such as liver and meconium in the colon (Brugger, 2006; Zaretsky, 2003b).
Short Tl inversion recovey (STIR) and frequency-selective fat-saturated T2-weighted images may provide diferentiation in cases in which the water content of the abnormality is similar to that of the normal structure. An example is a thoracic mass compared with normal lung. Difusion-weighted imaging may be employed to evaluate for restricted difusion, which can be seen in ischemia, cellular tumors, or clotted blood (Brugger, 2006; Zaretsky, 2003b). Our series also includes an axial brain 3-to 5-mm T2-weighted sequence to obtain head biometry for gestational age estimation using the biparietal diameter and head circumference (Reichel, 2003).