CT reveals pelvic disease in late-stage pregnancy

Exposing a fetus to ionizing radiation is never a good idea, but it may be necessary to diagnose potentially life-threatening gastrointestinal and genitourinary conditions during a pregnancy.

Pain that persists after a negative ultrasound exam indicates that CT may be required, especially when the symptoms appear to have GI or GU origins, said Dr. Elizabeth Lazarus, an assistant professor of radiology at Brown University. Her study of women in various stages of pregnancy found that CT contributed relevant diagnostic information about the source of abdominal pain in 13 of 52 cases after a negative ultrasound exam. It was presented at the 2004 RSNA meeting.

CT successfully diagnosed 11 of 12 cases of surgically proven appendicitis, for a sensitivity of 92%. Ultrasound was performed in six cases and rendered a positive reading in one instance for a sensitivity rate of 17%.

CT also outperformed ultrasound for detecting urinary tract calculi. Of four positive cases examined with both modalities, three involved nonobstructing inter-renal stones that were identified only with CT, Lazarus said. The patient's complaint of lingering pain in the fourth case originated from a left-side urinary tract blockage and lack of a left ureteral jet. Ultrasound indicated that the origin of the pain was a mechanical obstruction. When the patient's condition did not improve, a subsequent CT demonstrated asymmetric hypronephrosis and hydroureter.

Normal anatomic changes associated with a growing fetus is the real culprit in this game of lower abdominal hide-and-seek, according to Lazarus.

"Because of changes in organ location, particularly the appendix, during pregnancy, ultrasound imaging is difficult," she said.

But fetal radiation exposure is the trade-off for the higher anatomic conspicuity CT provides. A wide variation in radiation exposure was observed among the different scanners used in the study. The exposure was lowest with a single-slice scanner and highest with 16-slice equipment. Lazarus's group attempts to use a single-slice system whenever possible.

Elsewhere, multiplanar MR is showing potential to play the same diagnostic role as CT in these situations without exposing the mother and fetus to ionizing radiation. A study of 29 patients conducted by Dr. Katherine R. Birchard and colleagues at the University of North Carolina found that unenhanced MR identified various abdominal and pelvic disorders that escaped detection with ultrasound. Their results were published in the February issue of the American Journal of Roentgenology (2005;184:452-458).