Technique of spinal ultrasound

•Spinal cord is easily analysable until 3rd month

• Through the posterior arches incompletely ossified.

• Infant in prone, with pillow under the abdomen or lateral decubitus.

• Linear probe of high frequency (at least 7 MHz) with axial and longitudinal cuts from the occiput to the sacrum.

•Analysis of the bulbo-medullary junction: flexion of the neck oor phased-array that follows the physiological cervical lordosis of the infant.

Indications

1. Clinical lumbosacral anomaly :

Cutaneous stigma at high risk of dysraphism:

•  Angioma on the midline, nevus.
•  Subcutaneous mass.
•  Tuft of hair or a pigmented spot.
•  Caudal appendix.

•  Aplasia or cutaneous hypoplasia.
•  Dermal sinus.
•  Sacred agenesis.
•  High-risk coccygeal fossa: atypical fossa,> 5 mm in size and more than 2.5 cm from the anus.

Cutaneous stigma at high risk of dysraphism:

• Simple  coccygeal fossa, <5 mm and <2.5 cm from the anal margin.
• Or bony: hemi vertebrae, dehiscence of the posterior arch.

    2.  Anorectal malformation

   3. Fight Bladder, unexplained bladder  globe , or repetitive urinary tract infections.

    4. Abnormal neurological examination of the lower limbs.

A normal spinal ultrasound eliminates severe dysraphism and provides an MRI.

 Normal radio  anatomy of spinal ultrasound :

1) The spinal cord:

• Marrow: hypoechoic tubular structure,
       thicker in the cone region (about 5.5 mm)
       thinner in the dorsal region (about 4.5 mm).
• Centered by a hyperechoic echo"complex central echo" : at the acoustic interface between the anterior white commissure and the central part of the anterior median fissure.
• Laterally, the marrow is fixed by the serrated ligaments (arachnoidal duplications) which appear as fine linear echoes oriented transversely.

Fig. 1: 1 Spinal cord 2 Central canal of the spinal cord 3 Spinous process 4 Body of vertebrae
References: - MARRAKECH/MA

Fig. 5: normal spinal cord
References: - MARRAKECH/MA

2) The cervico-occipital hinge

Sub-occipital sagittal section:

• Large cistern 
• Cerebellar tonsils to eliminate a chiari malformation. 
• Analyze the pons, bulb and cervical spine
•  Within the subarachnoid spaces

3) terminal conus medullaris :

• Conus  medullaris : ends above L3.
  Its terminal portion gradually tapered  continues with the terminal filum whose thickness varies from 0.5 to 2 mm.
• Filum terminale visualized on the median line, behind the roots. Its thickness is = or less than 2 mm.
  

  Fig. 2: filum terminale
  References: - MARRAKECH/MA

Fig. 4: filum terminale
References: - MARRAKECH/MA

•Lumbar and sacral roots (ponytail): echoic features, arranged around and below the terminal cone.

•The ending of the space dural is in S2.

Fig. 3: Lumbar and sacral roots
References: - MARRAKECH/MA

Simple clinical and ultrasound landmarks to identify vertebral bodies, and locate the terminal cone:

Clinical landmarks :
-  The tip of the last rib is L2.
-  The top of the iliac crest corresponds to L4.

Ultrasound landmarks :
-  Follow the 12th coast until T12
-  Visualization of the renal pedicle located at L2
- Possibly identifying the 1st sacral vertebra (in the absence of transitional anomaly).

The variants of the normal: 

In about 10% of newborns.

1)The dilation of the terminal ventricle (rare):

•  Anechoic formation,
• Ovoid v  Clear  limit
• Hyperechoic in the filum or in the conus medullaris
• Size <5mm  
• Stability over time

2) Filum Terminale cyst:

• Origin discussed
• Arachnoid reflexion or embryonic remnant covered with ependymocytes.
• Less visible on MRI
• Strict criteria: 

                               -  Median line
                               -  In the filum, just below the spinal cone
                               -  Fusiform
                               - Well limited

                               - Anechoic as a simple cyst.

3)Transient dilatation of terminal ependymal canal:

Differential diagnosis of syringomyelia and terminal ventricle.

4) Pseudo dermal sinus

•  Fibrous tissue extended: cutaneous dimple → coccyx.
•  Dermal sinus is rarely located at the tip of the coccyx and often   more cranial.
•  Search mass or liquid well along this fibrous tract.

5) Filum prominent

• More visible compared to nerve roots.
• Thickness> 1mm
• median

6)  Coccyx:

•Many possible variations can be considered as a mass on palpation.