Scanning Planes and the Normal Appearance of the Fetal Spine
Axial Plane (transverse) Three ossification centers are present at each vertebral level: The body of the vertebra anteriorly. Two laminae posteriorly. Assess the entire spine segment by segment (use ischial ossification centers as a landmark to ensure the caudal end of the spine has been reached). Inward angulation of the laminae (often insufficient ossification of the laminae in the lower lumbar and sacral spine at 18-22 menstrual weeks to confirm normality). Spinous processes - cartilaginous in fetal life and only occasionally seen.
Coronal Plane Sonographically two parallel lines that coincide with the posterior ossification centers or the articular joints formed by adjacent laminae of the vertebrae. Posterior elements are linear bands that flair in the upper C-spine, widen slightly in the lumbar region and converge in the sacrum. Cannot visualize the entire spine in a single longitudinal coronal plane due to the normal fetal kyphosis. Gaps between vertebral bodies represent unossified margins of the vertebral bodies and the discs.
Longitudinal Plane In the sagittal plane through the long axis, the skin and subcutaneous tissues along the posterior aspect of the fetal back are well visualized. The spine appears as two parallel lines that converge at the sacrum. These parallel echogenic lines correspond to the vertebrae and vertebral bodies that encase the spinal canal. S4 is the most caudal ossification center sonographically visible in the second trimester, while S5 is most caudal in the third trimester. Demonstration of the cord and dura may be possible in this plane.
Lumbosacral angle – important to visualize as normality excludes sacral agenesis. · Normal = 35-40 degrees · Hyperlordosis = > 45 degrees · Hypolordosis = < 35 degrees
Skin line and soft tissues overlying the spine: The integrity of the skin line and soft tissues over the spine should be carefully assessed.