fAssumes embryo in radiation field for entire examination.
9Bednarek, 1983.
SD = standard deviation.
Angiography and vascular embolization may occasionally be necessary for trauma and for serious maternal disorders, especially renal disease (Wortman, 2013). As before, a greater distance from the embryo or fetus lowers the exposure and risk.
These x-ray images are usually performed by obtaining a spiral of 360-degree images that are postprocessed in multiple planes. Of these, the axial image remains the most commonly obtained
CT (MDCT) images are now standard for common clinical indications. The most recent detectors have 16 or 64 channels, and multidetector protocols may result in increased dosimetry compared with traditional CT imaging. Several imaging parameters have an efect on exposure (Brenner, 2007). These include pitch, kilovoltage, tube current, collimation, number of slices, tube rotation, and total acquisition time. If a study is performed with and without contrast, the dose is doubled because twice as many images are obtained. Fetal exposure is also dependent on factors such as maternal size as well as fetal size and position. And as with plain radiography, the closer the target area is to the fetus, the greater the delivered dose.
Cranial CT scanning is the most commonly performed study in gravidas. It is used for neurological disorders as discussed in Chapter 60 (p. 1156) and with eclampsia as noted in Chapter 40 (p. 724). Nonenhanced CT scanning is commonly used to detect acute hemorrhage within the epidural, subdural, or subarachnoid spaces (Fig. 46-3). Because of the distance from the fetus, radiation dosage is negligible (Goldberg-Stein, 2012).
Abdominal procedures are more problematic. Hurwitz and associates (2006) employed a 16-channel multidetector
FIGURE 46-3 A 37-year-old with intrapartum eclampsia at term.
renchymal hematoma (H) with intraventricular extension (arrow heads). The midline (arrow) is shifted to the right due to mass effect from the hematoma. (Reproduced with permission from Kho KA:
Diagnostic and operative laparoscopy. In Yeomans ER, Hoffman
BL, Gilstrap LC III, et al (eds): Cunningham and Gilstrap's Operative
Obstetrics 3rd ed, New York McGraw-Hili Education, 2017.) scanner to calculate fetal exposure at 0 and 3 months' gestation using a phantom model. Calculations were made for three commonly requested procedures in pregnant women (Table 46-7). he pulmonary embolism protocol has the same dosimetry exposure as the ventilation-perfusion (V/Q) lung scan discussed on page 908. Because of the pitch used, the appendicitis protocol has the highest radiation exposure, however, it is very useful clinically when MR imaging is not available. Using a similar protocol in 67 women with suspected appendicitis, Lazarus and coworkers (2007) reported sensitivity of 92 percent, specificity of 99 percent, and a negative-predictive value of 99 percent. Here, dosimetry was markedly decreased compared with standard appendiceal imaging because of a different pitch. his is discussed further in Chapter 54 (p. 1052). Last, for suspected urolithiasis, the multidetector-scan protocol is used if sonography is nondiagnostic. Using a similar protocol, White and colleagues (2007) identified urolithiasis in 13 of 20 women at an average of 26.5 weeks. Finally, as shown in
TABLE 46-7. Estimated Radiation Dosimetry with , 6-Channel Multidetector Computed
Pulmonary embolism 0.20-0.47 0.61-0.66
Data from Hurwitz, 2006.