MR Evaluation of Acute Abdominal Pain in Pregnant Patients

http://www.appliedradiology.com/Article.aspx?id=13583

Gregory Wolfe, MD ; Aytekin Oto, MD

The evaluation of abdominal pain in the pregnant patient is complicated by the physical changes in pregnancy as well as by the need to avoid fetal radiation exposure. While ultrasound is the preferred initial imaging study, some conditions require alternative imaging. This article presents a review of the use of MRI in the evaluation of abdominal pain in the pregnant patient.

The evaluation of abdominal pain in the pregnant patient can be a difficult task that is magnified by related sociologic issues and complicated by the anatomic and physiologic adaptations of the pregnant state. Evaluation is also limited by the need to avoid ionizing radiation. Pathologies directly related to pregnancy are generally evaluated by ultrasound, which usually provides for accurate assessment of the uterus, fetus, ovaries, kidneys, and gallbladder. 1,2 However, ultrasound provides limited and inconsistent imaging of the bowel, pancreas, and other deep structures and may be rendered ineffective by overlying bowel gas or anatomic alteration or variation. In addition, fetal exposure to ionizing radiation limits the use of computed tomography (CT) during pregnancy.

Serious fetal risk has not been shown to occur with radiation doses below 10 rad, 3 which is less than any single commonly ordered diagnostic study. However, the International Commission on Radiological Protection has recommended that one should question whether a diagnosis can be obtained without using ionizing radiation if the expected fetal dose is high. 4 The Safety Committee of the Society for Magnetic Resonance Imaging has issued a report stating that “MR may be used in pregnant women if other nonionizing forms of diagnostic imaging are inadequate or if the examination provides important information that would otherwise require exposure to ionizing radiation.” 5 While fetal harm has not been reported as a result of magnetic resonance imaging (MRI), the effects of fetal exposure have not yet been fully determined, which suggests cautious use, especially during the first trimester. Particular care should be taken with the use of intravenous contrast, as gadolinium-based agents cross the placenta. Although adequate controlled studies of gadolinium use during pregnancy have not been performed, animal studies have shown an increase in skeletal malformations. 6 It is suggested that these contrast agents be used only in the second and third trimester and when potential benefit outweighs the risk.

At our institution, images are obtained with a Signa EXCITE LX 1.5T system (GE Healthcare, Milwaukee, WI) using a phased-array surface coil when possible. Toward the end of pregnancy, a body coil may be required. Informed consent is obtained from each pregnant patient prior to imaging. The backbone of our protocol is T2-weighted (T2W) imaging in 3 orthogonal planes with single-shot fast spin-echo (SSFSE) or half-Fourier acquisition single-shot spin-echo (HASTE) imaging (effective repetition time [TR] = infinite; echo time [TE] = 80 msec; slice thickness = 6 mm). Typically, a 35-cm field of view is used, with a 160-192 × 256 matrix. A fat-saturated T2W sequence (SSFSE/ HASTE or fast spin-echo [FSE]) improves detection of inflammation and characterization of pelvic masses. An axial T1-weighted (T1W) sequence is also included, which may be either a breath-hold spoiled gradient-echo or a respiratory-gated fast spin-echo (FSE) sequence; TR = 700 msec; TE = minimum; number of excitation (NEX) = 3; echo-train length (ETL) = 2; matrix = 256 × 192, slice thickness = 6 mm with 1 mm spacing). The FSE sequence provides better resolution than gradient echo and may be useful for detecting small structures, such as a normal appendix. After review of the noncontrast images, the radiologist may request contrast-enhanced imaging in selected situations. If required, gadolinium may be administered at a dose of 0.1 mmol/kg, and 2-dimensional or 3-dimensional gradient-refocused echo (GRE) T1W images may subsequently be acquired.

The evaluation of abdominal pain in the pregnant patient is complicated by the body’s adaptation to the pregnant state. Physiologic changes include mild leukocytosis, “physiologic” anemia, mildly elevated alkaline phosphatase, dilatation of the renal pelvis and ureter, and a relative hypercoagulable state. In addition, the gravid uterus compresses and displaces abdominal contents, and abdominal wall laxity during late pregnancy may diminish signs of peritonitis. 7,8 Diagnosis may also be confounded by other conditions common in pregnancy, such as round ligament pain, constipation, and Braxton Hicks contractions. This article presents a brief summary of the use of MRI in the evaluation of abdominal pain in the pregnant patient.