Sagittal magnetic resonance scans

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Chapter 19 Sagittal magnetic resonance scans

Chapter contents

Median scan

Paramedian scan

Transpedicular scans

Tangential scans

Peripheral scans

Converse reading

Anyone who has undertaken the exercises described in Chapters 17 and 18, should find it straightforward to read sagittal, magnetic resonance image (MRI) scans of the lumbar spine. Those exercises asked the reader to expect the structures that lie anterior, lateral and posterior to the lumbar vertebral column. In plain radiographs, those structures were invisible, but in MRI scans, they are actually evident. Reading sagittal MRI scans, therefore, amounts to no more than matching expectation with what is actually evident.

The expectations of what should be evident in a sagittal scan can be established by answering four questions:

1 Which parts of the vertebral column will be evident?

2 Which elements of the contents of the vertebral canal will be evident?

3 Which structures anterior to the vertebral column will be evident?

4 Which structures posterior to the vertebral column will be evident?

Sagittal MRI scans are typically taken across five standard planes (Fig. 19.1). Additional scans might be taken between these standard planes, or slightly displaced left or right from them; but the principles of analysis remain the same.

Figure 19.1 An anteroposterior radiograph of the lumbar spine showing the location of standard sagittal magnetic resonance scans. M, median scan; PM, paramedian scan; TP, transpedicular scan; T, tangential scan; P, peripheral scan.

With respect to the vertebral column, a median scan will intersect the vertebral bodies and intervertebral discs anteriorly and the spinous processes posteriorly. Other parts of the vertebral column are not intersected and so will not be seen in the scan. A paramedian scan will intersect the vertebral bodies and intervertebral discs anteriorly, and the laminae posteriorly.

A particular facility of MRI scans is that they can distinguish between the nucleus pulposus of the intervertebral disc and the anulus fibrosus. The nucleus will appear white or grey, reflecting its content of water. The anulus will appear black. Median and paramedian scans will intersect the nucleus pulposus of each disc and the anterior and posterior anulus. Transpedicular scans will do otherwise.

Anteriorly, transpedicular scans will intersect the lateral sectors of the vertebral bodies and the lateral sectors of the intervertebral discs. The latter will be intersected either through the anulus fibrosus or through the most peripheral segments of the nucleus pulposus. Posteriorly, transpedicular scans will intersect the pedicles of the lumbar vertebrae and some portions of the zygapophysial joints. Slightly more lateral transpedicular scans will intersect the superior articular processes. Slightly more medial transpedicular scans will intersect the inferior articular processes.

Tangential scans will intersect the most lateral margins of the vertebral bodies and the anulus fibrosus at each segmental level, or the concavity of the vertebral body. Peripheral scans will intersect only the transverse processes.

With respect to the contents of the vertebral canal, median, paramedian and transpedicular scans will intersect different components of the dural sac and its contents (Fig. 19.2). A median scan will intersect the conus medullaris of the spinal cord at upper segmental levels, and the cauda equina at lower levels. A paramedian scan may miss the spinal cord and show only the cauda equina. A transpedicular scan will intersect the spinal nerves and their dural sleeves, as they pass under the pedicles. Tangential and peripheral scans will not intersect the contents of the vertebral canal.

Figure 19.2 An anteroposterior radiograph of the lumbar spine showing the location of the dural sac, the conus medullaris of the spinal cord, the cauda equina, and the lumbar nerve roots, and how they are various intersected by median (M), paramedian (PM), and transpedicular (TP) sagittal magnetic resonance imaging scans.

In a lateral view, the reader should, therefore, expect a median scan to show the dural sac, the conus medullaris of the spinal cord and the cauda equina (Fig. 19.3A). A paramedian scan would show only the cauda equina (Fig. 19.3B). A transpedicular scan would show only the spinal nerve roots and their sleeves (Fig. 19.3C).

Figure 19.3 A lateral radiograph of the lumbar spine showing the contents of the vertebral canal that would be intersected and shown by various sagittal magnetic resonance imaging scans.

Anteriorly, different scans will variously intersect the crura of the diaphragm and the great vessels, or the psoas major and quadratus lumborum (Fig. 19.4). Posteriorly, the scans will pass through multifidus, longissimus thoracis, or iliocostalis lumborum (Fig. 19.5).

Figure 19.4 An anteroposterior radiograph of the lumbar spine on which the anterior relations of the lumbar vertebral column have been superimposed, and showing how these structures are intersected by standard sagittal magnetic resonance scans. M, median scan; PM, paramedian scan; TP, transpedicular scan; T, tangential scan; P, peripheral scan.

Figure 19.5 An anteroposterior radiograph of the lumbar spine on which the posterior relations of the lumbar vertebral column have been superimposed, and showing how these structures are intersected by standard sagittal magnetic resonance scans. M, median scan; PM, paramedian scan; TP, transpedicular scan; T, tangential scan; P, peripheral scan.

The actual appearance of individual structures: their longitudinal extent and their shape that should be expected on sagittal scans, can be derived from what the reader would expect to see on lateral radiographs. Across the concavities of the vertebral bodies, the reader would expect to encounter the lumbar arteries and lumbar veins, covered by psoas. (Fig. 19.6A). In front of the vertebral bodies, but to the right-hand side, they would expect the right crus and the inferior vena cava (Fig. 19.6A). Posteriorly, but behind the laminae, they would expect the multifidus (Fig. 19.6A). In front of the vertebral bodies, but more to the centre and to the left, the reader would expect the left crus and the aorta (Fig. 19.6B). Laterally, in front of the transverse processes they would expect the psoas major and the quadratus lumborum (Fig. 19.6B). Posteriorly, behind the transverse processes they would expect the various elements of the erector spinae (Fig. 19.6B).

Figure 19.6 Lateral radiographs of the lumbar spine on which the external relations of the lumbar vertebral column have been superimposed. (A) The right crus (rc), inferior vena cava (ivc), lumbar arteries (la), lumbar veins (lv) psoas major (psoas) and multifidus (m). (B) The left crus (lc), aorta, quadratus lumborum (ql), longissimus thoracis pars lumborum (ltpl), iliocostalis lumborum pars lumborum, (ilpl) and the lower thoracic fibres of erector spinae (tes).

Median scan

From Figure 19.1, the reader would expect that a median scan should intersect the vertebral bodies and intervertebral discs anteriorly, and the spinous processes posteriorly. Since it is a median scan, it should show the nucleus pulposus at the centre of each disc, and the anulus fibrosus anterior and posteriorly. From Figures 19.2 and 19.3, the reader would expect a median scan to intersect the conus medullaris and the cauda equina. From Figure 19.4, the reader would expect that a median scan should intersect the aorta. From Figure 19.6, the reader would expect the aorta to terminate opposite L4. It thus transpires that this is what a median MRI scan shows (Fig. 19.7).

Figure 19.7 A pair of median MRI scans of the lumbar spine. Because the patient had a slight scoliosis, scan A is median at L4 but slightly to the right of midline at upper lumbar levels. Scan B is median at upper lumbar levels, but slightly to the left of midline at lower lumbar levels. Where the scan is in the median plane spinous processes (sp) are clearly evident. The roots of the cauda equina appear as black strings against a background of intensely white signal that is generated by the cerebrospinal fluid in the dural sac. The nucleus pulposus (np) appears white or grey, consistent with its fluid-content. The surrounding anulus fibrosus (af) is black.

Paramedian scan

From Figure 19.1, the reader would expect that a paramedian scan should intersect the vertebral bodies and intervertebral discs anteriorly, and the laminae posteriorly (or the more medial elements of the zygapophysial joints). It should show the nucleus pulposus at the centre of each disc, and the anulus fibrosus anteriorly and posteriorly. From Figures 19.2 and 19.3, the reader would expect a paramedian scan to intersect the cauda equina. Paramedian scans that are to the right of the midline should pass near or through the right-side edge of the aorta, and in front of L5 they should pass through the right common iliac artery passing to the right, and covering the left common iliac vein passing to the left (Fig. 19.4). Paramedian scans to the left of the midline should substantially intersect the aorta. Posteriorly, the scan should pass through the multifidus. It thus transpires that this is what paramedian MRI scans show (Fig. 19.8).

Figure 19.8 A pair of paramedian MRI scans of the lumbar spine. Scan A is to the right of the midline. At most segmental levels, scan (A) passes through the laminae, but at lower levels it intersects the medial edge of the inferior articular processes (iap). Anteriorly, the lower end of the aorta is evident, and the right common iliac artery (rcia) crossing the left common iliac vein (lciv) in front of L5. Scan (B) is to the left of the midline. Posteriorly, it shows the same structures as does scan (A). Anteriorly, the aorta is evident along its entire length, as far as its bifurcation. The left common iliac vein continues across the front of L5. Posteriorly, the multifidus (m) is evident in both scans.

Transpedicular scans

From Figure 19.1, the reader would expect that a transpedicular scan should intersect the vertebral bodies and intervertebral discs anteriorly, and the pedicles and zygapophysial joints posteriorly. From Figures 19.2 and 19.3, the reader would expect a transpedicular scan to intersect the lumbar spinal nerve in their intervertebral foramina. Transpedicular scans on the left most likely will pass lateral to the aorta, although they might intersect the more lateral segments of the aorta. On the right, transpedicular scans should intersect the inferior vena cava (Fig. 19.4). In front of L5 they should pass through the right common iliac artery (Fig. 19.4). Posteriorly, the scan should pass through the multifidus. It thus transpires that this is what paramedian MRI scans show (Fig. 19.9).

Figure 19.9 A right transpedicular MRI scan of the lumbar spine. The spinal nerves are evident below the pedicles. Posteriorly, the pedicles expand into the superior articular processes (sap), behind which the fibres of multifidus (m) pass dorsally and cephalad. Anteriorly, the scan passes through the left-hand edge of the inferior vena cava (ivc). The right common iliac artery (rcia) and the right internal iliac artery (riia) are evident.

Tangential scans

From Figure 19.1, the reader would expect that a tangential scan should pass through the edges of the vertebral bodies and intervertebral discs, and across the concavities of the vertebral bodies. From Figure 19.6, the reader would expect a transpedicular scan to intersect the lumbar arteries and veins. Transpedicular scans on the left most likely will pass lateral to the aorta, although they might intersect the more lateral segments of the aorta (Fig. 19.4). On the right, transpedicular scans should intersect the inferior vena cava (Fig. 19.4). In front of L5 they should pass through the right common iliac artery (Figs. 19.4, 19.6). Posteriorly, from Figure 19.5, the reader would expect tangential scans to intersect the erector spinae at most levels, and the multifidus at lumbosacral levels. It thus transpires that this is what paramedian MRI scans show (Fig. 19.10).

Figure 19.10 A right tangential MRI scan of the lumbar spine. The scan passes through the vertebral bodies and outer margins of the intervertebral discs (ivd). On the lateral surface of the vertebral bodies, the scan reveals the lumbar arteries and veins. Anteriorly, the inferior vena cava (ivc) is seen along its entire length, continuing caudally (in front of L5) as the right common iliac vein (rciv). The scan intersects the roots of the transverse processes (tp). Posteriorly, the fibres of the erector spinae (es) are seen running ventrally and cephalad; behind the sacrum lies multifidus (m).

Peripheral scans

From Figure 19.1, the reader would expect that a peripheral scan should pass through the transverse process. From Figures 19.4, 19.5 and 19.6, the reader would expect a peripheral scan to intersect the psoas major and quadratus lumborum anteriorly, and the erector spinae posteriorly. It thus transpires that this is what peripheral MRI scans do (Fig. 19.11).