MRI of the Cervical Spine
6.1
Normal MRI anatomy
of the cervical spine
6.1.1
Spinal Cord
The
cervical spine cord is a cylindrical structure centred in the
subarachnoid space. The
cord’s diameter increases slightly at the C5/C6 levels where the roots
of the brachial plexus arise. It
has a small central canal transmitting CSF and is continuous with the
fourth ventricle. Around the central canal is an H-shaped area of grey matter
possessing cell bodies of motor neurones anteriorly and the posterior
horns have cells in the sensory pathway.
Outside the grey matter is the white matter of the spinal cord
made of long ascending and descending tracts (Ryan and McNicholas,
1994). Some pulse sequences in the axial plane can distinguish areas
of grey and white matter in the cord.
In proton density-weighted images the grey matter has brighter
signal intensity than the surrounding white matter.
In T2-weighted images, the grey matter has a less intense signal
than the surrounding white matter.
In sagittal sections grey matter and white matter can also be
distinguished but the artefacts from CSF pulsation or truncation tend to
obscure the boundaries (Haughton, Daniels, Czervionke, Williams and
Rand, 1999).
6.1.2
Subarachnoid space
About
half of the spinal column consists of the subarachnoid space.
Structures found within the cervical subarachnoid space include
the dentate ligament, the cerebellar tonsils which may extend 1-2mm in
the cranial end, the anterior spinal artery, a pair of dorsally oriented
veins and the dorsal and ventral nerve roots.
T1-weighted images show the cord having higher signal intensity
than the CSF in proton density weighted images.
In T2-weighted images the CSF has a brighter signal than the
spinal cord.
The
dentate ligament and the blood vessels within the subarachnoid space can
be identified rarely (Haughton et al, 1999).
6.1.3 Epidural space
The
epidural space in the cervical spine is narrow containing predominantly
vascular tissue and small amount of fat and connective tissue.
Fat typically gives a bright signal on T1-weighted images.
The spinal canal is lined by the ligamentum flavum in the
posterior aspect having intermediate signal intensity on T1.
The remaining vascular tissue is invisible unless intravenous Gd-DTPA
or GRE imagings are used which give the epidural venous plexus a bright
signal. Using T2-weighted
GRE imaging it is also possible to identify the dura, which is not
usually resolved in conventional SE MRI (Haughton et al, 1999; Ryan and
McNicholas, 1994).
Diagram 6 A typical vertebra
Detailed views of a vertebra and vertebral segment. The drawing to the right represents a top view of a lumbar vertebra. The drawing below is a lateral (side) view of a segment of three lumbar vertebrae.
(Pashman,
2002)
Figure 19 Sagittal T1-weighted image of the cervical spine
|
|
|
6.1.4
Vertebral column
The
cervical spine is made of 7 vertebrae: the ring like atlas, the axis
with its dens and the remaining 5 vertebrae, were C7 is the vertebrae
prominens. The most
distinctive feature of the cervical spine is the foramen transversarium
in the transverse process (except C7) transmitting the vertebral artery,
accompanying veins and sympathetic nerves.
The articulations between the vertebrae are the facet joints
occurring at the articular pillars and between the adjacent vertebrae of
C2 and T1 are the intervertebral discs (Ryan and McNicholas, 1994).
The MR appearance of the bony vertebrae is totally dependent from
the signal of the bone marrow, which is moderately intense on
T1-weighted SE images and less intense on T2-weighted images with
increased noise (Haughton et al, 1999).
The dense cortical bone around the periphery of the vertebral
body produces low signal intensity on all pulse sequences (Brick, 1998).
The
intervertebral disc is composed of the inner gelatinous nucleus pulposus
and the outer annulus fibrosus. The
latter has an outer layer of collagen and an inner layer of a
mixture
of collagen and fibro cartilage. Cartilage
has higher water content than collagen, therefore the nucleus and the
medial portions of the annulus have high signal intensity on T2-weighted
images (Modic and Ross, 1991). Nevertheless,
Haughton et al (1999) state that the greatest contrast between the disc
structures is greatest on T2*-weighted GRE, less on T2-weighted SE and
least on T2-weighted FSE sequences.
In T1-weighted images the nerve-root sheaths can also be
identified as areas of intermediate signal intensity emerging from the
neural foramen. On
T2-weighted images and contrast enhanced MRI, the bright epidural veins
contrast with the negligible signal from the nerve roots.
Diagram 7 The vertebral column
|
|
|
Pashman, (2002)
Figure 20 Axial T2-weighted section through cervical spine
|
|
|
