"Three-D is very important for analyzing the presence and severity of spina bifida. The multiplanar view is more accurate than rendered views for localizing the bony defect of the fetal spine," said lead researcher Dr. Wesley Lee, an attending physician of fetal imaging at William Beaumont Hospital in Royal Oak, MI.

Using the thoracic vertebra with the most caudal rib as the reference point, Lee and colleagues compared prenatal diagnostic impressions from 2D and 3D ultrasound with a postnatal analysis of the bony spinal defect level by radiography or MRI. A single reviewer who analyzed 3D volume data was blinded to postnatal imaging results. When feasible, researchers displayed spinal abnormalities using 3D surface rendering techniques.

Researchers evaluated the spinal defect level of nine fetuses by both 2D ultrasound (21.8+/-3.6 weeks) and 3D ultrasound (22.9+/-4.7 weeks). The study found that 2D ultrasound agreed with postnatal analysis to within one vertebral segment in six of nine infants. In comparison, 3D ultrasound agreed to within one vertebral segment in eight of nine infants. Three fetuses had bony defects within 1.5 to two segments away from the postnatal findings identified using 2D imaging. In the same three patients, 3D ultrasound was accurate to within one vertebral segment.

"Two-D and 3D ultrasound provides a more detailed view of the level of the anatomic defect, which correlates with the neurological deficit of the fetus. This is important for counseling parents," Lee said.

If ultrasound had discovered lower lesions, they would involve numbness of the perineal area and varying levels of bowel and bladder incontinence. Higher lumbar lesions might prevent an infant from walking.

Lee and colleagues displayed a detailed surface rendering of the vertebral defect in relation to the spine and ribs in one case. They attempted to create a surface reconstruction of the meningeal sac or external skin lesion in all patients, but half of these attempts were unsuccessful due to fetal position.

"Whether we're using 2D or 3D, we still have the limitations of acoustic shadowing of fetal position and movement," he said.

The researchers concluded that both imaging modalities are likely to provide similar results for localizing spinal defects. Lee described 3D ultrasound as complementary to 2D ultrasound, rather than replacing it, and said 3D should be used to clarify regions of interest highlighted by 2D.

Although the cohort was small, this study was important in that the researchers emphasized correlating prenatal findings with postnatal findings and that 3D imaging corresponded well to the postnatal findings, according to Lee.

"Three-D ultrasound is not just a face," he said. "It is a set of digitalization analysis tools that can be used to look more in-depth at regions of interest. Three-D multiplanar allows you to see the volume of data in three different ways, and surface and volume rendering allow you to see what the defects actually look like. Important qualitative and quantitative tools with 3D ultrasound technology allow us to better characterize spina bifida."

-- By Merlina Trevino