2. Around the neural tube which forms the vertebral arches
3. In the body wall which becomes the ribs.
The mesenchymal cells migrate to the presumptive region of the vertebra. Thechondrification occurs around 6 weeks, which forms the centrum. The cartilageousmass transforms into a cartilaginous model over which the bone forms which is around 10 weeks of gestation.
The ossification of the vertebral body starts in the T12 – L1 level and progresses in cephalic and caudal directions. The vertebral arches ossification progresses from cranial to the caudal end.
The bony ossification (primary ossification) occurs in utero. There is one vertebral body and two vertebral arches (laminae). The secondary ossification centres - tip of the spinous process, two transverse processes, articular processes are cartilaginous inutero. Ossification of typical vertebrae begins during the embryonic period and usually ends by the twenty-fifth year
Ultrasound of the spine
On sonography, the ossification center of the neural arch was visible in the cervical region in all fetuses by 18–19 weeks. The ossification center of the neural arch was first seen in the thoracic region during the 18–19-week period and was consistently demonstrated in the 20–22-week period. In the lumbar and sacral regions ossification was not seen before 19 weeks and was consistently demonstrated in 22–24 weeks (Ref. 1). However with high resolution ultrasound the ossificationcentres are visible from cervical to lumbar level around 12-13 weeks of gestation.
The anterior coronal view helps in identifying the vertebral body. In a study by P. De Biasio et al (Ref.2)they have found that by 17 weeks, S2 ossification is present in all pregnancies. The posterior coronal view shows the normal widening in the cervical and lumbar regions. BudorickN E et al have studied the neural arch ossification centers in the distal fetal spine during the second trimester of pregnancy in 239 fetuses An additional vertebral level became ossified every 2-3 weeks from L-5 through S-5 after 16 weeks gestational age; by 22 weeks, S-2 was ossified in all fetuses studied (Ref.3). In 95% of the fetuses, S-1 was at the top of the iliac wing.
Imaging of the spine is ideally done when the fetus is in prone position and with liquor above it. Physiological cervical and lumbar widening is present which should not be mistaken for an abnormality. The fetal spine has a “c” shaped curvature with sacral tapering.
“c” shaped curvature sacral tapering
The sacral tapering is observed in the parasagital scan in all fetuses beyond 22 weeks of gestation.
The three planes of imaging and the structures seen
1. Parasagital section vertebral body, one lamina and skin line (seen as three parallel lines)
2. Axial section vertebral body and the two laminae seen converging with skin line on top of it.
3. Coronal section – Anterior coronal -Vertebral body, Posterior coronal -Laminae (two in number)
Vertebral body Laminae
· Document the cervical and sacral spine is in prone position with liquor above it.
· When the sacral spine is close to the uterine wall, a repeat examination is essential. However, in low risk patients, if the skin line is seen intact extending beyond the spine towards the gluteal region and if the posteriorfossa is normal, repeat scan is not warranted.
· When the fetus is in supine position with breech close to cervix , transvaginalultrasound to be done to document sacral spine
· Liberal use of 3D in abnormal spine
The abnormal spine
Abnormality is suspected when
· Abnormal curvature/angulationis noted
· Spine persistently in a fixed position( the normal fetal spine is very flexible)
· Sacral tapering not visible
Sacral agenesis / Caudal regression syndrome
Sacral agenesis/ dysgenesis can be suspected around 17 weeks of gestation. A meticulous examination of the fetus in a diabetic mother helps in ruling out isolated sacral agenesis. The other condition is VATER or VACTERL where sacraldysgenesis is present. On ultrasound, the sacral tapering is not visualized and the spine ends abruptly.
Abnormality of the vertebral body
· Failure of formation of part of vertebra – hemivertebra
Hemivertebra involving one or two segments may be overlooked if there is noangulation in the spine. They become evident only when the fetus stretches the spine and the abnormality in the region of hemivertebra is seen. It is important to image the spine in transverse plane also when the spine is anterior. This will help in identifying the abnormality in the vertebral body. Instead of 3 echogenic areas, in the region of the vertebral body two or more echogenic areas are seen. In the coronal view, the alignment of the vertebral body is irregular and we can see part of the vertebral body, which is not formed. Some of them can also be fused.
· Failure of segmentation of vertebra – block or bar vertebrae
Multiple vertebral anomalies are easily diagnosed as the normal “C” curvature is lost and there is kyphoscoliosis.
3D ultrasound and the newer technologies like OMNI VIEW help in bringing out a good reconstructed picture of the spine and the ribs.
In skeletal dysplasia there can be poor mineralization of the vertebral body (achondrogenesis) or there can be platyspondyly - small sized lumbar vertebral bodies with increased intervertebral disc space (thanatophoric dysplasia)
Platyspondyly in thanatophoric dysplasia
Unossified laminae in hypophosphatasia
Abnormalities of the vertebral arch (Laminae)
· Open neural tube defect – Head signs present (abnormal posterior fossa)
· Closed neural tube defect – Normal posterior fossa. Diastematomyeliaotherwise termed as split cord malformation is suspected when there is widening of the laminae with a bony spicule or fibrous spur which splits the cord into two. This commonly occurs in the thoracolumbar region,rarely can affect cervicodorsal region. Associated hemivertebrae can be present.
· Abnormal size, poor mineralization and beaded appearance – Skeletaldysplasia.
· Missing / Fused ribs – Spondylothoracic dysplasia.
Antenatal diagnosis of
Non visualization of sacral tapering, abnormal angulation, widened posterior ossification centres with a bony spicule are certain pointers for suspecting a spinal anomaly.
1. Normal Fetal Lumbar Spine on Postmortem MR Imaging
2. E. Widjajaa, E.H. Whitbya, M.N.J. Paleya and P.D. Griffithsa. American Journal of Neuroradiology 27:553-559, March 2006 © 2006 American Society of Neuroradiology
3. Ossification timing of sacral vertebrae by ultrasound in the mid-second trimester of pregnancy- P. De Biasio1*, G. Ginocchio, G. Aicardi, G. Ravera, P. L.Venturini and M. Vignolo PrenatDiagn2003; 23: 1056–1059.
4. Ossification of the fetal spine E Budorick , D H Pretorius, M R Grafe and LouRadiology 1991 Nov;181(2):561-5Fetal vertebral structure detected by three-dimensional ultrasound Authors: Ritsuko K.Pooha; Kyong-Hon Poohb
5. The Ultrasound Review of Obstetrics & Gynecology, Volume 5, Issue 1 March 2005, pages 29 – 33.