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.

Figure 20.3 The central figure illustrates where a subpedicular scan intersects the lumbar vertebral column, the dural sac and the segmental spinal nerve. 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 subpedicular scan, MRI scan. vb, vertebral body; la, lamina; nr, nerve roots of the cauda equina; sn, spinal nerve.

Transarticular scans will intersect the dural sac and cauda equina centrally, but will also show the spinal nerve or ventral ramus leaving the vertebral column and entering the psoas major muscle (Fig. 20.4). Anteriorly, the scan will show the intervertebral disc. Unlike the vertebral body, which appeared grey, the disc will largely appear black. The centre of the disc may appear grey if the scan is taken close to the vertebral endplate. Posteriorly, the zygapophysial joints will be evident. Centrally, the dural sac will enclose the nerve roots of the cauda equina, as in transpedicular scans. Laterally, near the posterolateral corner of the disc, a dark rounded signal is produced by the spinal nerve becoming the ventral ramus. Because the spinal nerve passes obliquely and caudally through the intervertebral foramen, it will be located further laterally, i.e. peripherally, in transarticular scans than in subpedicular scans, and will be distinctly dissociated from the dural sac.

Figure 20.4 The central figure illustrates where a transarticular 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 transarticular scan, MRI scan. ivd, intervertebral disc; zj, zygapophysial joint; ds, dural sac; nr, nerve roots of the cauda equina; sn, spinal nerve.

Irrespective of which segment is scanned, these three, basic alternatives apply. Consequently, by just looking at the neural relations a reader will have great difficulty ascertaining which segment they are studying – because the neural relations will look basically the same at all segments. Identifying the segment relies on knowing the external relations.

External relations

The external relations that will be seen on an axial scan will differ at different segmental levels. Figure 20.5 summarises what structure the reader should expect at different segments, and what each structure should look like.

Figure 20.5 Anteroposterior radiographs of the lumbar spine onto which its external relations have been superimposed. (A) Anterior relations. (B) Posterior relations. lc, left crus; rc, right crus; ql, quadratus lumborum; pm, psoas major; ivc, inferior vena cava; il, iliocostalis lumborum; lt, longissimus thoracis; m, multifidus. The inferior vena cava, aorta and multifidus have been drawn as if they were transparent in order to reveal the underlying vertebrae.

At upper lumbar levels, the reader should expect to encounter the left crus and right crus of the diaphragm, arising from the front of the vertebral bodies (Fig. 20.5A). In particular, the crura will be large in front of L1 but tapering and dissipating in front of L2. The crura intervene between the vertebral column and the inferior vena cava and the aorta.

The inferior vena cava lies to the right, and the reader should expect to encounter it at all segmental levels as far as L5 (Fig. 20.5A). The aorta lies towards the midline at upper lumbar levels, but deviates somewhat to the left, until it divides at L4 (Fig. 20.5A). At the L1, L2 and L3 levels, the reader should expect to encounter two large vessels: the inferior vena cava and the aorta. At L4, the reader should expect to encounter three vessels: the inferior vena cava and the aorta having divided into the two common iliac arteries. At L5, the reader should expect four vessels: the two common iliac arteries and the two common iliac veins. The number of great vessels in front the vertebra column is the most obvious indication of whether the scan is at L5 or at L4, or at L1, 2 or 3.

Laterally, the reader should expect to encounter quadratus lumborum as a broad, flat mass overlying the transverse processes (Fig. 20.5A); but not at L5, for the quadratus lumborum arises from the iliolumbar ligament and, therefore, above L5. The absence of quadratus lumborum is one of the features that characterise scans at L5.

The psoas major covers the quadratus lumborum, but progressively enlarges from L1 to L5 (Fig. 20.5A). Therefore, in axial scans, the reader should expect psoas to be narrow at upper lumbar levels, and progressively thicker at lower levels. Indeed, compared to other structures, psoas becomes enormous at lower lumbar levels.

Posteriorly, the reader should expect to encounter the three major posterior back muscles. They should expect multifidus to be confined to the region over the lamina at upper lumbar levels, but to expand progressively widely from L3 to the sacrum (Fig. 20.5B). Lateral to multifidus, the reader should expect the longissimus thoracis to be centred over the roots of the transverse processes; and the iliocostalis lumborum to be centred over the tips of the transverse processes.

Axial MRI scans at L1

Using Figure 20.5, the reader should expect to encounter the left crus and right crus in scans at L1, with the inferior vena cava and aorta in front of them. Lateral to the vertebral bodies, the reader should expect a broad, flat quadratus lumborum, but a narrow psoas major, for its fibres will have just arisen from the T12–L1 disc. Posteriorly, the reader should expect a narrow multifidus, flanked by longissimus thoracis and iliocostalis lumborum. This is indeed what scans through L1 show (Fig. 20.6).

Figure 20.6 Axial MRI scans through L1. vb, vertebral body; p, pedicle; rc, right crus; lc, left crus; ivc, inferior vena cava; a, aorta; pm, psoas major; ql, quadratus lumborum; m, multifidus; lt, longissimus thoracis; il, iliocostalis lumborum; sn, spinal nerve; ivd, intervertebral disc; zj, zygapophysial joint.

Axial MRI scans at L2

Using Figure 20.5, the reader should expect modifications at scans through L2. The crura will be attenuated, but the inferior vena cava and aorta will be prominent, anteriorly. The quadratus lumborum should persist, but the psoas major should be larger. Posteriorly, the multifidus, longissimus thoracis, and iliocostalis lumborum should be evident. This is indeed what scans through L2 show (Fig. 20.7).

Figure 20.7 Axial MRI scans through L2. vb, vertebral body; p, pedicle; rc, right crus; ivc, inferior vena cava; a, aorta; pm, psoas major; ql, quadratus lumborum; m, multifidus; lt, longissimus thoracis; il, iliocostalis lumborum; sn, spinal nerve; ivd, intervertebral disc; zj, zygapophysial joint. In this scan, the right renal vein (rrv) can be seen draining into the inferior vena cava.

Scans through L1 and L2 may be quite similar, and may be difficult to distinguish. The immediate differences are only relative: the crura are small, or absent, at L2; and the psoas is only relatively larger. On these grounds alone, readers could be excused for not being able to distinguish absolutely scans through L1 and L2. Further clues appear in the visceral and vascular structures lying anterior to the lumbar spine. For example, the right renal vessels tend to be more caudal than those on the left. Therefore, left renal vessels are likely to be encountered at L1, whereas right renal vessels will be seen at L2 (Fig. 20.7). The anatomy of visceral and vascular structures, however, is outside the scope of this text.

Axial MRI scans at L3

Using Figure 20.5, the reader should expect no crura in scans at L3, but the inferior vena cava and aorta will be prominent, anterior to the vertebral column. The quadratus lumborum should be evident, and the psoas major should appear as a substantial mass. Posteriorly, the multifidus should be expanding lateral to the limits of the laminae; but otherwise, it will be flanked by the longissimus thoracis and the iliocostalis lumborum. This is indeed what scans through L3 show (Fig. 20.8). The absence of crura, the prominence of psoas and the expansion of multifidus all characterise scans through L3.

Figure 20.8 Axial MRI scans through L3. vb, vertebral body; p, pedicle; ivc, inferior vena cava; a, aorta; pm, psoas major; ql, quadratus lumborum; m, multifidus; lt, longissimus thoracis; il, iliocostalis lumborum; sn, spinal nerve; ivd, intervertebral disc; zj, zygapophysial joint.

Axial MRI scans at L4

Using Figure 20.5, the reader should expect to see three great vessels in front of the vertebral column in scans through L4. Laterally, the psoas should be large. Posteriorly, the multifidus should be wide, and the erector spinae should be correspondingly small. This is indeed what scans through L4 show (Fig. 20.9).

Figure 20.9 Axial MRI scans through L4. vb, vertebral body; p, pedicle; pm, psoas major; ql, quadratus lumborum; m, multifidus; lt, longissimus thoracis; il, iliocostalis lumborum; sn, spinal nerve; ivd, intervertebral disc; zj, zygapophysial joint; ivc, inferior vena cava; rca, right common artery; lca, left common artery.

Axial MRI scans at L5

Using Figure 20.5, the reader should expect to see four great vessels in front of the vertebral column in scans through L5. Laterally, the psoas should be huge, but the quadratus lumborum should be absent. If the scan passes through the appropriate level, the iliolumbar ligament might be evident. Posteriorly, the multifidus will be very wide. The iliocostalis will be lacking, because its insertion – into the iliac crest – lies above L5. Only a small portion of longissimus thoracis should be evident. This is indeed what scans through L5 show (Fig. 20.10).

Figure 20.10 Axial MRI scans through L5. vb, vertebral body; p, pedicle; pm, psoas major; m, multifidus; lt, longissimus thoracis; sn, spinal nerve; ivd, intervertebral disc; zj, zygapophysial joint; rcia, right common iliac artery; lcia, left common iliac artery; rciv, right common iliac vein; lciv, left common iliac vein; reia, right external iliac artery; riia, right internal iliac artery; leia, left external iliac artery; liia, left internal iliac artery.

Converse applications

Rarely would practitioners be required to predict the appearance of an axial MRI scan. Instead, they are more likely to be confronted with a particular scan, and would be required to interpret across which segmental level that scan was taken. For this problem, the principles outlined in this chapter, nevertheless apply, with few adjustments.

The first step pertains to the internal relations. Determine if the scan is transpedicular, subpedicular or transarticular. Upon doing so, the reader should be able to identify the dural sac, the nerve roots of the cauda equina and the spinal nerve (if evident).

The second step is to estimate the segmental level. This is achieved by examining the external relations in each of the surrounding sectors.

In the anterior sector, two great vessels together with distinctive crura, place the scan at L1. This should be corroborated by finding a narrow psoas major in the lateral sector, and a narrow multifidus in the posterior sector.

Two great vessels in the anterior sector but with dissipating crura, place the scan at L2. This should be corroborated by a less than narrow psoas major in the lateral sector; and confirmed by the presence of right renal vessels.

The presence of two great vessels in the anterior sector but with no crura suggests that the scan is through L3. This should be corroborated by finding a substantial psoas major in the lateral sector, and a multifidus that is starting to expand beyond the lateral margins of the laminae in the posterior sector.

Three great vessels in the anterior sector place the scan at L4. This should be corroborated by a large psoas major in the lateral sector, and a wide multifidus in the posterior sector.

Four great vessels in the anterior sector indicate that the scan is at L5. This should be corroborated by the presence of a large psoas major in the lateral sector, together with an absence of quadratus lumborum. In the posterior sector, the multifidus will be large.