Plain Radiography and Intravenous Urography (IVU)
These studies have limitations in pregnancy. First, the fetal skeleton and the enlarged uterus may obscure ureteric stones, so the imaging study may not be diagnostic. Second, there may be delayed excretion of contrast as a consequence of the physiological dilatation of the kidney. It can be difficult, if not impossible, to differentiate this 'physiological' delay from that due to an obstructing stone. Third, there is also the theoretical risk of fetal toxicity from the contrast material, though none has been reported.
As stated above, ultrasound is an unreliable way of diagnosing the presence of stones in pregnant women. Jets of urine expelled
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by normal peristalsis of the nonobstructed ureter can be seen on ultrasound scanning (Fig. 8.1), and the absence of such ureteric jets is said to have a high sensitivity and specificity for diagnosing obstructing stones (Doyle et al. 1995), though others have reported that ureteric jets may be absent in asymptomatic pregnant women (Burke and Washowich 1998).
Computed Tomography Urography (CTU)
Although CT urography is a very accurate method for detecting ureteric stones and the radiation dose is below 50 mGy, most radiologist and urologists do not recommend this form of imaging in pregnant women. Magnetic resonance urography (see below) provides an alternative form of imaging in this difficult group of patients.
Magnetic Resonance Urography (MRU)
The American College of Obstetricians and Gynecologists and the U.S. National Council on Radiation Protection state, 'Although
FIGURE 8.1. Jets of urine expelled by normal peristalsis of the non-obstructed ureter can be seen on ultrasound scanning or on computed tomography (CT) (as shown here). CT should be avoided if at all possible in pregnancy.
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there is no evidence to suggest that the embryo is sensitive to magnetic and radiofrequency at the intensities encountered in MRI, it might be prudent to exclude pregnant women during the first trimester' (ACOG 1995, NCRP 1997). Given this advice, therefore, MRU can potentially be used during the second and third trimesters, but not during the first trimester.