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Ultrasound can be useful for determining:

  • the midline of the spinal column

  • the correct space for insertion

  • the depth of the dura.

Use the curvilinear probe because it gives a wider image, deeper penetration and clearer anatomy.

Transverse view

  1. When the probe is placed over a spinous process, a shape that looks like a fan is seen with the spinous process as the handle (Figure U1).

  2. The fan spreads at the base of the handle. These structures are the vertebral laminae.

  3. When the probe is placed between spinous processes, a central blur is visible with a concave white line inferiorly between two small humps.

  4. The two small humps are the articular processes and the concave white line is the dura (Figure U2).

  5. Measure the depth to the dura.

Figure U1

Transverse view through spine with the probe over a lumbar spinous process. Note the fan-like appearance of the vertebral lamina/transverse processes.

Figure U2

Probe placed transversely over a lumbar interspinous space. The dura can be seen at 5 cm depth in the midline. The dark areas on either side of the midline are the articular processes

Sagittal and para-sagittal view

If the probe is placed in the midline of the spinal column, a series of bumps is seen—the spinous processes. Again, this will confirm where the midline is. Move the probe slightly to one side. This will show a row of triangular smaller humps—the articular processes. Next, tilt the probe inwards (like a paramedian approach to an epidural). This will show the intralaminar view. In this view there is a deeper white line (posterior dura), and a still deeper white line (anterior dura), with saw-tooth humps on either side—the vertebral laminae.

How to find the L3–4 interspace

Place the probe longitudinally at the top of the natal cleft in a para-sagittal position and tilted slightly towards the midline. The sacrum looks like a more or less straight line. See Figure U3.

  1. Just cranial to the sacrum triangular humps are seen. These are the articular processes. See Figure U4.

  2. The space between the first hump and the sacrum is L5–S1. The space between the second and third humps (counting up from the sacrum) will be the L3–4 interspace.

Figure U3

Para-sagittal longitudinal view of sacrum with probe tilted inwards. Note the almost straight-line appearance of the sacrum

Figure U4

Moving the probe cranially from the sacrum (para-sagittal longitudinal view) the articular processes are clearly ...

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