Lumbosacral Spine MRI

Alignment

Assessment of alignment should include the presence of normal lumbar lordosis and proper positioning of each vertebra on top of the other. MRI of the lumbar spine is normally obtained with the patient in supine position. Previous studies have shown that MR images taken with the patient in supine position and legs straight out were comparable with standing MRI; however, a supine position with bent hips was found to significantly reduce lumbar lordosis by approximately 6.3° (14%). Spinal alignment should be judged on sagittal T1- or T2-weighted images ( Fig. 1 ). It should be emphasized that instability cannot be assessed on a supine MRI. Deviation of vertebrae from the midsagittal plane may indicate scoliosis; however, as previously stated, MRI is not the imaging modality of choice to categorize the scoliosis, to measure angles and to make a treatment plan.

The most common number of lumbar vertebrae is five. In up to 20% of the general population lumbosacral transitional vertebrae (LSTV) are seen, of which the sacralization of L5 is the most common (~ 17%) and the lumbarization of S1 is rarer (~ 2%). Recognition of these anomalies is important as it can influence clinical decision making and help to avoid wrong-level surgery. The Castellvi classification ( Table 2 ) can be used to classify LSTV, but does not replace an accurate description of the anomaly. LSTV may be best visualized in a coronal view, because of the depiction of possible enlarged transverse processes of the lowest lumbar vertebra and the connection with the superior sacrum. However, coronal images are not routinely included in a standard lumbosacral MRI protocol. Ways to recognize LSTV on sagittal images are the recognition of the iliolumbar ligament as this is found to arise from L5 in more than 95% of people ( Fig. 2 ) and a complete S1–2 intervertebral disc. If a LSTV is seen or suspected, aforementioned characteristics should be noted, including where the lowest well-formed intervertebral disc can be found, as this can be recognized at fluoroscopy during invasive treatments.

Bone

Bony and bone-related structures that need to be assessed are vertebral body, arch, spinous processes, endplates, fractures, lesions, and ligaments. The vertebral body is the largest part of the vertebral column and in its upper part arise two pedicles, which are connected to two laminae. These, together, form the vertebral arch, connected to a spinous process in a normal situation. The vertebral body is oval-shaped in the axial plain and square in the sagittal plane. Its upper boundary is formed by the upper endplate and the lower boundary is formed by the lower endplate, which are normally symmetrically concave-shaped and without disruptions in young people. With age and degenerative processes, endplates can become thinner and even completely absent. Whether the endplate belongs to the disc or the vertebral body is still debated, since it encompasses both an osseous component as well as a hyaline cartilage component. In young patients, mainly under the age of 25, areas of focal fat deposition are common in the posterior elements of the vertebral body, as well as areas of high vascularity, due to conversion and reconversion processes. These areas include the subendplates and subcortical zones and also the area surrounding the basivertebral vein in the middle of the vertebral body ( Fig. 1 ). This is a normal aging process of conversion and reconversion, expressed by the replacement of red marrow by yellow marrow, and should not raise suspicions when encountered. Other frequent and usually not pathological lesions in vertebral bodies are hemangiomas and vertebral enostosis.

The most important ligaments of the lumbar spine are the anterior longitudinal ligament (ALL) running in front of the vertebral bodies and intervertebral discs from the pelvic surface of the sacrum inferiorly to the anterior tubercle of C1 and basilar portion of the occipital bone superiorly, the posterior longitudinal ligament (PLL) running posteriorly over the vertebral bodies and discs from the sacrum to C2. It then extends as the tectorial membrane at the level of the odontoid. The ligamentum flavum connects the inner lamina, interspinous ligaments, and supraspinous ligaments. Ligaments and also cortical bone appear hypointense on both T1 and T2 and exhibit a slim and regular appearance.

Spinal Canal

The vertebral arches surround the spinal canal. This canal contains the thecal sac with the spinal cord above the L1-L2 level, cauda equina below this level and the conus at the interface of spinal cord and cauda equina floating in cerebrospinal fluid. Cerebrospinal fluid is hyperintense on T2 and hypointense on T1. However, flow voids can mimic a hypointense signal on T2. The neural structures are normally iso- to hypointense on T1- and T2-weighted images. Nerves of the cauda equina can be followed along their course running from the thecal sac through the lateral recess and foramen intervertebralis to the neural plexus. The nerve is normally running just below the pedicle. This is above the level of the discus in the foramen intervertebralis. The nerve is surrounded by fat, which makes it assume the shape of an oval fried egg on sagittal images. This is best visible on a T1-weighted sequence ( Fig. 3 ), when no compression is present. Normal presence of epidural fat is also seen on the dorsal side of the thecal sac anterior to the ligamentum flavum. Furthermore, a venous plexus is present in the canal and normally shows enhancement on a T1 with gadolinium.

There are various ways to measure spinal canal diameter, without consensus in literature nor uniform appliance in practice. As a result, various quantitative and qualitative measurements exist for the measurement of spinal canal diameter. In an international consensus meeting between radiologists, consensus was reached about the use of five radiological criteria for lumbar spinal stenosis (LSS), which are all qualitative measurements related to LSS and therefore further described later in this chapter. Canal diameter can be quantitatively measured on axial T2, sagittal T1, and sagittal T2 images, as the mid-diameter of the thecal sac at the level of the intervertebral disc and midvertebral level ( Fig. 3 ).

Intervertebral Discs

Intervertebral discs connect the vertebral bodies and together with ligaments buffer and support the spine from the distribution of forces that arise from axial compression and movement. The discs consist of a soft gelatinous core, which is named nucleus pulposus and firm rim, composed of concentric lamellae of fibrocartilage named annulus fibrosus. These can be seen on an MRI T2 as a relatively hyperintense nucleus pulposus and hypointense annulus fibrosus. Degeneration of discs is a common finding in symptomatic and asymptomatic subjects, which leads to dehydration of discs, visible as lower intensities on T2-weighted images. Herniated discs will be discussed further on in this chapter. When looking at discs, one should also judge the ligamentum flavum, neuroforamina, lateral recesses, and facet joints with their synovium. Figs. 1 and 2 show the normal anatomy of these structures, in relation to adjacent nerve roots. Knowledge of this anatomy is compulsory, as it determines the different levels of neural compression and related clinical symptoms. For example, the L4 root exits just below the pedicle of L4 and therefore will be only compressed by a far lateral disc herniation of the L4-L5 disc.

Enhancement and Extraspinal Structures

As a lumbar spine MRI also shows the abdominal area, visible extraspinal structures should also be assessed. These include organs as bladder, kidneys, uterus, bowel, aorta, muscles, subcutaneous tissue, and skin. Furthermore, enhancement after gadolinium administration should be assessed. Normal areas of contrast enhancement are epidural venous plexus, basivertebral veins, dorsal nerve root ganglion, and veins over the surface of the conus and cord. This enhancement can be differentiated from pathological enhancement by its expected anatomical location and flat and regular appearance in contrast to, for example, leptomeningeal metastasis, which are more nodular ( Fig. 4 ).