Axial magnetic resonance imaging

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Chapter 20 Axial magnetic resonance imaging

Some practitioners find axial scans of the body difficult to understand and, therefore, to be oppressive. For them, it is as if axial anatomy constitutes yet another subject in anatomy that needs to be separately and additionally studied and mastered. This need not be so.

It can be shown that the appearance of axial scans can be derived from prior knowledge of anatomy. All that is required is that that prior knowledge be rotated 90°, and systematically reassembled. This chapter describes that process.

Principles

The prediction of the appearance of an axial scan of the lumbar spine can be reduced to two major steps. The first involves considering what the internal, i.e. neural, relations will be. The second step involves considering what the external relations will be.

Internal neural relations

Axial magnetic resonance imaging (MRI) scans of the lumbar spine are typical taken through three standard locations at any given segment. The scans can be through the pedicles (transpedicular), below the pedicles (subpedicular) or through the intervertebral disc and zygapophysial joints (transarticular) (Fig. 20.1).

Figure 20.1 An anteroposterior radiograph of the lumbar spine showing the location of typical, axial MRI scans. TP, transpedicular; SP, subpedicular; TA, transarticular.

Each of these scans intersects slightly different elements contained within the vertebral canal. Transpedicular scans will intersect the dural sac and the nerve roots of the cauda equina (Fig. 20.2). In these scans, the pedicles will be evident projecting from the posterior surface of the vertebral body. These bones present as a grey signal. Posteriorly, various parts of the laminae and spinous process will complete the vertebral canal. Within the vertebral canal, the dural sac will appear as a black circle or oval, enclosing a white signal generated by the cerebrospinal fluid within the sac. Inside the sac, the nerve roots of the cauda equina appear as block dots, because the nerves appear as if they have been transected and looked at end-on. The nerve roots typically appear towards the posterior surface of the dural sac because patients typically lie supine when the scan is taken, and the nerve roots ‘fall’ to the rear.

Figure 20.2 The central figure illustrates where a transpedicular scan intersects the lumbar vertebral column, the dural sac and the cauda equina. The figure on the left is a diagram of the vertebral elements and neural elements that would be seen in the scan. The figure on the right depicts an actual transpedicular scan, MRI scan. vb, vertebral body; p, pedicle; ds, dural sac; nr, nerve roots of the cauda equina.

Subpedicular scans will intersect the nerve roots of the cauda equina but also the spinal nerves of the segment, as they run through the intervertebral foramen of the segment (Fig. 20.3). The scan will show the vertebral body anteriorly, and the lamina on each side posteriorly. Behind the centre of the vertebral body, the dural sac will appear as a dark ring, with the nerve roots of the cauda equina appearing as black dots lying in a sea of white signal formed by the cerebrospinal fluid. Lateral to the dural sac a dark signal will appear, behind the vertebral body, created by the spinal nerve. Depending on exactly where the scan is taken, the spinal nerve may be accompanied by a sleeve of dura mater, which in turn may contain a sliver of cerebrospinal fluid surrounding the nerve.