CHICAGO - Although kidney stones are infrequent during pregnancy, management of renal colic in pregnant women can be difficult because two common diagnostic methods, the limited excretory urogram and pelvic examination with CT both expose the patient and fetus to radiation. The question is, which dose is higher?
According to Cynthia McCollough, Ph.D. of the Department of Diagnostic Radiology at the Mayo Clinic in Rochester, MN, CT’s dose can be far lower than that of urography, especially in the later stages of pregnancy when abdominal-pelvic thickness is greater. The need to choose between the two methods may arise when ultrasound is nondiagnostic and MR urography isn’t available, she said.
“The clinical indication that spawned this study was the event of an after-hours emergency room [visit], and a pregnant patient presenting with possible renal colic. Ultrasound would be the first step, and then triage, and then CT. MR may not be available in all cases in that situation,” McCollough said.
To comprehensively gauge the difference in radiation dose between the procedures, the researchers looked at three measures of radiation exposure: the entrance skin exposure (ESE), effective doses (ED), and fetal doses (FD) for both KUB radiographs and spiral CT.
Limited excretory radiographs were prepared using an average of four KUB films for each patient using Kodak TML film, Lanex medium screens, 40” sid, and hvl of 3.5 mm at 80 kVp, with and without contrast.
CT was performed on either a single-detector (HiSpeed Advantage, GE Medical Systems, Waukesha, WI) or multislice scanner, (GE LightSpeed QX/i) at a kvp of 120, mA of 240, 1 second duration, pitch of 1.6 mm (single-slice) or 1 mm (multislice) and collimation of 5 mm.
The radiation exposure was compared in four patient sizes at abdominal-pelvic thicknesses of 21, 25, 30 and 33 cm, corresponding to the various stages of pregnancy, she said.
“Entrance skin exposure does not tell the whole story,” McCollough, said. “So we went on to take those numbers and compute organ dosage. Specifically we care about the organ dose to the uterus, which is going to be our marker for the fetal dose.”
The investigators used the Monte Carlo method with the ESE figure as a basis for determining the specific organ doses. The last step uses both the ESE and the FD as the basis for determining the effective dose.
“Effective dose is a single dose parameter – one number – which is the best reflector of risk when you have a non-uniform exposure such as abdominal CT or abdominal radiograph,” she said.
For the 21, 25, 30 and 33-cm patients respectively, a limited excretory urogram yielded total ESE of 1.7, 3.7, 10 and 14 r, the researchers found. The KUB radiograph yielded a total ED of 0.28, 0.60, 1.7 and 2.3 r, and a total FD of 0.57, 1.2, 3.6 and 4.8 r.
“What we find here for one KUB is that you get an eight-fold increase in entrance skin exposure dependent on patient size…. What you find with CT is that your technique factors remain relatively constant independent of patient size,” McCollough said.
The ESE averaged 1.7 R for the single-slice scanner, and 2.5 R for the multislice scanner, she said. The ED ranged from 0.6 to 1 r, and the FD was between 1 and 2 r. However, that difference doesn’t reflect a need for higher radiation doses with the multislice scanner as compared with single-slice, she said.
“This data is reflective of the fact that we used an extended pitch of 1.6 mm” with the single-slice scanner to adequately image the thicker volume, she said. “If we had used a pitch of 1 mm on the single slice, the exposures would have been the same.”
The urographic exposure data are comparable to figures published annually by the FDA’s Center for Devices and Radiological Health (CDRH) known as the Nationwide Evaluation of X-Ray Trends, McCollough said. CT was also comparable to published figures, she said.
The ESE for pelvic CT exam was comparable to excretory urography for 21 and 25 cm-thick patients, but four to eight times lower for 30 and 33-cm patients. The ED and FD for CT were 2 to 3.5 times higher for the 21-cm patients, roughly equal for the 25-cm patients, and 2 to 5 times lower than excretory urography for 30 and 33-cm patients.
“We conclude that spiral dose is comparable to limited excretory urography in small patients, but when it gets to larger patients, the CT dose is less, so radiation dose should not at all preclude the use of CT in pregnancy, but rather may be the modality of choice.”
By Eric Barnes
Auntminnie.com staff writer
November 26, 2000
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