Role of Imaging Modalities in Obstetric Emergencies
Lama L. Tolaymat
Gwyn Grabner
Imaging has assumed a key role in the management of obstetric patients. Depending upon the patient population and clinical problem, a variety of imaging modalities are available for use, some more appropriate than others. Some women are exposed to x-rays before the diagnosis of pregnancy is known. Occasionally, x-ray procedures may be indicated during the pregnancy. To understand which modality to use for a specific concern, all imaging modalities must be reviewed with emphasis on the method of generating an image, common uses in pregnancy, and the associated risk with use. Table 16.1 summarizes the different modalities and the associated fetal exposure of radiation.
RADIATION EXPOSURE IN PREGNANCY
There has been a growing concern about radiation exposure in the case of pregnant women who undergo radiological examinations. Lack of knowledge may result in unnecessary patient anxiety. Teratogenic effects have developed in animals exposed to large doses of radiation (up to 200 rad). The main effects of radiation on the human embryo and the fetus are prenatal death, growth restriction, congenital malformations, and mental retardation. Based on data from the atomic bomb survivors, it appears that the risk of central nervous system effects is greatest with exposures at 8 to 15 weeks of gestation (1,2). Prenatal doses from properly performed diagnostic procedures present no significant increased risk of prenatal death, malformation, or impairment of mental development over the background incidence of these entities. Rare consequences of prenatal radiation exposure include a slight increase in the incidence of childhood leukemia and, possibly, a very small change in the frequency of genetic mutations. Such exposure is not an indication of pregnancy termination. In 1977, the National Council on Radiation Protection and Measurement issued a report stating that “The risk [of abnormality] is considered to be negligible at 50 mGy [=5 rad] or less when compared to other risks of pregnancy, and the risk of malformation is significantly increased above control levels only at doses above 150 mGy [=15 rad]. Therefore, exposure of the fetus to radiation arising from diagnostic procedures would very rarely be the cause for terminating a pregnancy” (3). The accepted safe cumulative dose of ionizing radiation during pregnancy is 5 rad (0.05 Gy). The American College of Radiology established that “The interruption of pregnancy is rarely justified because of radiation risk to the embryo or fetus from a radiologic examination” (4). The most sensitive time period for central nervous system teratogenesis is between 10 and 17 weeks of gestation. Nonurgent radiologic testing should be avoided during this time.
Risk Related to Gestational Age (5)
1. Fetal exposure prior to 2 weeks post-conception to 10 rad may lead to the death of the embryo.
2. Fetal exposure at 2 to 7 weeks post-conception to 5 to 50 rad leads to an increase in the risk of major malformations and growth restriction. An exposure to >50 rad leads to a substantial risk of malformations, growth restriction, and miscarriages.
TABLE 16.1 Diagnostic Imaging Procedures and the Estimated Average Associated Fetal Exposure | ||||||||||||||||||||||||||||||
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3. Fetal exposure 8 to 15 weeks post-conception to 5 to 50 rad leads to growth restriction, reduction in IQ, and 20% incidence of mental retardation. An exposure to >50 rad may result in a miscarriage.
4. Fetal exposure after 15 weeks post-conception to <50 rad may lead to noncancer health effects at exposures >50 rad.
IMAGING MODALITIES
x-Ray (plain films): Generally during pregnancy, the uterus is shielded for nonpelvic procedures. Plain films are useful in evaluating trauma, fractured bones, pneumonia, and urolithiasis in pregnant women.
Fluoroscopy: Most fluoroscopic examinations result in fetal exposure to millirads. Uses in pregnancy include cholelithiasis related pancreatitis and urolithiasis.
Nuclear Medicine/angiography: The most common nuclear medicine study performed during pregnancy is the ventilation-perfusion scan for suspected pulmonary embolism. Macroaggregated albumin labeled with Technetium Tc99m is used for the perfusion portion, and inhaled xenon gas is used for the ventilation portion. The radiation exposure to the fetus is usually 50 mrad (6). Radioactive iodine readily crosses the placenta and can adversely affect the fetal thyroid as early as 10 weeks, therefore, contraindicated in pregnancy. If a diagnostic scan of the thyroid is necessary during pregnancy, then using123I or Technetium Tc99m should be used (6).
Computed Tomography (CT): The most common uses for CT during pregnancy include head CT to detect acute hemorrhage in eclamptic women or for the
diagnosis of acute neurological catastrophe and spiral CT for evaluation of pulmonary embolism. Pelvimetry is used rarely, but when required, low-dose CT pelvimetry can be performed. Most radiopaque agents used with CT are derivatives of iodine and hence avoided in pregnancy.
diagnosis of acute neurological catastrophe and spiral CT for evaluation of pulmonary embolism. Pelvimetry is used rarely, but when required, low-dose CT pelvimetry can be performed. Most radiopaque agents used with CT are derivatives of iodine and hence avoided in pregnancy.
Ultrasound: Since ultrasound does not involve any radiation, it is the preferred imaging modality in pregnant women. The International Society for Ultrasound in Obstetrics and Gynecology issued a safety statement that “routine clinical scanning of every woman during pregnancy using real-time B-mode imaging is not contraindicated” and “exposure time and acoustic output should be kept to the lowest levels” (7). Ultrasound is used in pregnancy to evaluate the fetus, the placenta, the adnexa, and the cervix. In addition, ultr asound may be used as an initial diagnostic tool to evaluate the kidneys when urolithiasis is suspected or in situations of abdominal trauma. It is the main modality used to evaluate vaginal bleeding any time during pregnancy.
Magnetic Resonance Imaging (MRI): Since MRI does not use any radiation, there is no known risk of using it in pregnant patients. In addition to the nonobstetric indications for MRI, it is used to evaluate fetal malformations specifically CNS (8) and for the evaluation and management of abdominal pregnancy (9). Most MRI machines have a patient weight limit of about 300 lbs, but this limit is usually higher for “open” MRI machines.
General Principles to be Followed in Every Pregnant Patient
Women should be counseled that x-ray exposure from a single diagnostic procedure (<5 rad) does not result in harmful fetal effects (10).
Limit exposures to those that are essential for the diagnosis.
Pelvic shielding should be used whenever possible.
Fluoroscopy should be limited to short bursts as needed.
Ultrasound and MRI should be considered instead of x-rays when appropriate, at least as an initial assessment tool (10).
The use of radioactive isotopes of iodine is contraindicated in pregnancy.
Radiopaque and paramagnetic contrast agents are unlikely to cause harm and may be of diagnostic benefit (10).
OBSTETRIC EMERGENCIES AND IMAGING
The diagnosis of pregnancy requires a multifaceted approach using history and physical examination, hormonal assays, and endovaginal ultrasound. With the combination of early endovaginal ultrasound and beta subunit of human chorionic gonadotrophin (BhCG), we can reassure our patient of a normal progressing pregnancy versus failed early intrauterine pregnancy (miscarriage) or ectopic pregnancy. Readers may also wish to consult Chapter 3.
As early as 4.5 weeks after onset of last menstrual period, the first sign of pregnancy, gestational sac (Fig. 16.1
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