Researchers seek balance for pregnant patients between sufficient dose and need to scan for PE

November 26, 2007

Pulmonary embolism and other conditions may be detected in pregnant women using lower radiation doses than is currently standard, according to studies presented at the RSNA meeting on Monday. New techniques show promise, but obstacles such as small study populations and the lack of prospective trials must be overcome before a definitive protocol can be devised.

Scans using a reduced-dose protocol provided acceptable image quality to a segmental level, but subsegmental image quality was more limited. (Provided by D. Litmanovich, Beth Israel Deaconess Medical Center)

Pulmonary embolism and other conditions may be detected in pregnant women using lower radiation doses than is currently standard, according to studies presented at the RSNA meeting on Monday. New techniques show promise, but obstacles such as small study populations and the lack of prospective trials must be overcome before a definitive protocol can be devised.

While overall incidence of PE is relatively low, occurring in 0.5 to three patients per 1000, PE is the leading cause of nonobstetric maternal mortality. Between 5% and 17% of maternal mortality cases are due to PE, according to session moderator Dr. Lynne M. Hurwitz Koweek of the Duke University School of Medicine. Ventilation/perfusion lung scans (V/Q) can be an effective triage tool in dealing with PE in pregnant women, despite perceptions that CT pulmonary angiography is more effective, according to a study presented by Dr. Sanjeev Bhalla of the Mallinckrodt Institute of Radiology at Washington University in St. Louis. In a retrospective review of 168 patients, Bhalla found that only 6% of V/Q studies were nondiagnostic. A significant number of those patients with indeterminate studies had an abnormal chest radiograph. But among the 35 CTPA studies, 11 were identified as either limited or moderately limited. These results led Bhalla's institution to adopt a policy of using a V/Q study in the event of a normal chest radiograph and reserving CTPA studies for patients with abnormal chest radiographs. Although CTPA currently tends to involve a higher maternal radiation dose than V/Q, other researchers presented data on lowering radiation dose while maintaining the efficacy of the study. Dr. Diana Litmanovich of the Beth Israel Deaconess Medical Center in Boston presented data on a low-dose protocol tailored for pregnant patients that reduced mAs and kVp and had a shortened z-axis. The study compared 15 pregnant patients scanned with the reduced-dose protocol with 15 nonpregnant women scanned with the standard protocol. The reduced-dose protocol used 200 mAs, 100 kVp, and a z-axis that reached from the aortic arch to the diaphragm. Subjects in the control population were scanned through the entire thorax using the standard dose parameters of 400 mAs and 120 kVp. Litmanovich found that segmental image quality was comparable for all patients. Subsegmental image quality was slightly lower in the reduced-dose group, but that did not affect the diagnostic quality of the images, Litmanovich said. Dose-length product (DLP) and CT dose index (CTDI) were four times lower in the reduced-dose group.Dual-source CT may also be a way to substantially reduce radiation dose, according to a study presented by Dr. Martine Rémy-Jardin of the Hôpital Calmette of the University Center of Lille in France. Rémy-Jardin reported on a study that compared the radiation dose and image quality of nongated CT angiograms of the chest acquired with single- and dual-source CT technology in a population of 42 patients who weighed less than 80 kg. She noted that those scanned using dual-source CT received significantly lower dose regardless of body mass index (BMI).Kilovolt peak with the dual-source CT scanner was 80 or 100, while kVp for the single-source CT was 100 or 120. Mean DLP value was statistically significantly lower in subjects scanned with the dual-source CT. Subjective image quality was similar and no significant difference in objective image quality was noted. In addition, the mean BMI of patients scanned at 100 kVp was significantly higher among those patients imaged with the dual-source CT, again with similar subjective and objective image quality findings. This indicates that dual-source CT allows more frequent use of low kVp than single-source CT without impairing image quality, thus reducing the overall radiation dose delivered during standard CT angiograms of the chest.

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