Chapter 19 Sagittal magnetic resonance scans
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.
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).
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