4 The lumbar spine

Michelle A. Wessely, Julie-Marthe Grenier, Peter J. Scordilis, David R. Seaman, Martin Young

Introduction

Low back pain is one of the most common presenting complaints to any primary healthcare physician. With the wide variety of structures capable of generating pain and the common inclusion of pathological conditions in the differential diagnosis, it is important for the clinician to be able to reach specific and accurate conclusions as to the precise nature of their patient’s complaint.¹ This involves a detailed clinical history, physical examination and appropriate diagnostic imaging.^2,³ Magnetic resonance (MR) imaging plays a crucial role in determining the cause of the clinical picture when used in the appropriate clinical context.⁴

MR imaging is an invaluable tool for the clinician assessing the lumbar spine. Although previously radiographs were always the mainstay for imaging diagnosis of disorders arising from the low back (and, in some countries, this still holds true), it is now generally accepted that cross-sectional imaging, in particular MR imaging, is the perfect tool to assess the potential pain-generating structures of this region.^5,⁶ MR imaging now has the added benefit of being able to be performed in different positions; therefore, if the patient experiences pain only in a particular position, this posture can be recreated within the MR scanner, in order to better identify the origin of the symptoms. This determination is clearly an important factor in the subsequent management of the patient. Positional and upright MR imaging are not widely available at the time of writing, but the benefits are such that the utilization is likely to increase in the near future.⁷^–⁹

MR imaging’s role in assessment of the lumbar spine is well established. In most clinical situations, it is preferred over computed tomography (CT) and myelography, although the former can still be used in conjunction with MR imaging to more fully assess the multiple components of severe trauma.¹⁰ MR imaging has been shown to be superior to both these modalities in the evaluation of degenerative disc conditions, infections, spinal neoplasms and other intrinsic cord diseases; it allows visualization of both components of the intervertebral disc and direct imaging of the bone marrow.¹¹^–¹⁵ Computed tomography may show an advantage over MRI for the evaluation of postoperative stenosis due to arthrosis, particularly in the case of an instrumented spine, where the artifact produced by the implant affects MR images to a greater extent than CT; however, CT does not allow for distinction between postsurgical fibrosis and scarring and a recurrent disc lesion, even with the use of intrathecal contrast agents (Figure 4.01), a particularly challenging and important clinical determination to make because of the differing management options available.^10,¹⁶

Figure 4.01 • Comparison of structures visualized on a non-contrast computed tomography CT (bone window) axial image [upper images] and MR axial T2-weighted image [lower images] at the lumbosacral junction. The contents of the central canal (filum terminale) are well visualized on the T2-weighted images as well as the relationship between the neural structures and the surrounding borders, which may be involved in neural compromise. MR imaging also allows for closer evaluation of the paraspinal soft tissue including the musculature. The key to the legend is detailed in Box 4.01.

This chapter is designed to give an overview of the basics of MR imaging for the lumbar spine, the indications for imaging and relevant procedures to the ordering clinician. Familiarity with these topics will allow for appropriate patient examination and optimization of this diagnostic tool and help achieve a comprehensive assessment of the patient’s clinical requirements.

History and examination

Low back pain is one of the most common complaints presenting to the offices of any primary care physician – and one of the most poorly understood and badly managed. Part of the problem is that the condition is frequently treated as a disease in itself rather than a symptom that can emanate from dozens of different causes. Family doctors frequently fail to differentiate these causes beyond a basic elimination of ‘red flag’ pathologies; their argument is that such differentiation is not necessary as any musculoskeletal problem will be treated in the same way: non-steroidal anti-inflammatories and analgesia. There is also a belief that back pain is a self-limiting condition that will spontaneously resolve within 6 weeks.^17,¹⁸

Unfortunately, the evidence shows that this treatment regimen does little other than to moderately alleviate symptoms in the short term and, rather than self-resolve, many patients simply stop attending their family practice owing to the lack of effective treatment; this, however, is interpreted as evidence of resolution by the treating physician. Current guidelines emphasize early return to normal activities, acupuncture and spinal manipulative therapy, preferably followed by individually tailored prescriptive exercise.^19,²⁰

An accurate and concise differential diagnosis is important not only to inform management protocols but also to determine what, if any, diagnostic imaging may be required. Orthopaedic textbooks are replete with tests for low back pain, usually eponymic, and usually of unproved or dubious reliability and sensitivity.

The diagnostic dilemma is further compounded by the prevalence of comorbid conditions: sacroiliac syndrome is frequently linked with lumbosacral facet dysfunction and with myofascial syndromes arising from the muscles of the gluteal area; many discal lesions arise following years of low-grade back pain,

again often linked to lumbosacral dysfunction; any acute back pain will cause muscle guarding and spasm that can, in their own right, contribute to and perpetuate the condition.

So, what is the clinician to do? As with any presentation, the key is a thorough history and examination; with experience, this will become honed both with practice and with the rejection of tests that prove regularly unhelpful or invariably duplicate the findings from other tests; remember, many orthopaedic tests are aggravational … too much aggravation and the patient may never return! The detail of this clinical encounter will depend on the discipline of the clinician; the specificity of diagnosis required prior to spinal manipulation is likely to differ from that needed in general family practice.

Differential diagnosis

Of the many causes of low back pain, sacroiliac joint syndrome and facet joint syndrome are perhaps the most commonly presenting conditions, contributing to approximately 50% of cases. The accurate diagnosis of these entities, which are best identified by orthopaedic testing and by response to conservative care, either manipulation or injection, is complicated by comorbidity: 30% of low back patients have more than one anatomical source of pain.²¹ MR imaging is, however, considered the gold standard for several other common low back disorders, including disc lesions and central canal stenosis.^22,²³

The common differential diagnoses for the lumbar spine are detailed in Table 4.01.^24,²⁵

Table 4.01 Differential diagnosis for low back pain

Clinical indications for diagnostic imaging

The symptoms and presentations associated with back pain that are indications for spine imaging are detailed in Box 4.02.^21,²⁶ It is important that the clinician orders MR imaging only with valid clinical suspicion and appropriate history and examination findings because, without these, the results of the study may be misleading; in the early years of MR imaging, there were frequent examples of inappropriate back surgery, with its attendant risks and complications, on disc bulges that were not only asymptomatic but simply failed to correlate to the radicular level of the patient’s complaint.

After determining the indications for MR imaging, it is important to clearly communicate them to the facility or attending radiologist; although few, the risks and contraindications need to be evaluated on a case-by-case basis. Additionally, depending on the clinical question or differential diagnoses, intravenous contrast use may be recommended and the need for this has to be assessed by the imaging team. Findings from any plain film imaging or the absence thereof should also be included in the referral details.

Contraindications

Whilst MR imaging is generally considered safe, the procedure does have a few cautions and contraindications. The following situations are common but do not comprise an inclusive list and should merely serve as a guide for patient screening and consideration. Before undergoing an MR imaging examination, the facility performing the procedure should screen the patient, but it is important that referring doctors are familiar with common contraindications and cautions to better inform the patient and avoid referring inappropriately.²⁷

Caution needs to be taken when referring patients experiencing claustrophobia, anxiety or panic attacks for MR imaging examination. It is important that the process is thoroughly explained to the patient in order to decrease stress and anxiety prior to the procedure. Also, there are several options offered to these patients to minimize the discomfort. Sedatives, open tube architecture, headphones and prisms that allow the patient to see outside the unit all may help to calm these individuals and allow the acquisition of higher-quality images. The first trimester of pregnancy also requires caution with regard to MR imaging.²⁷ The necessity for the imaging procedure should be made on an individual clinical basis if:

• the information cannot be obtained by non-ionizing means

• the emergent data could affect the care of the patient and/or fetus

• the referring physician feels the test is necessary and cannot be postponed.

It is important that the patient is aware of the benefits and risks of the examination and any alternatives if applicable.²⁷

Absolute contraindications to MR imaging generally involve implants or foreign bodies. The most important concern is whether or not the object is ferromagnetic; objects with ferromagnetic properties need to be assessed for size, shape and anatomical location. The magnetic field strength of the unit also needs to be considered. Common foreign bodies that represent contraindications to MR imaging are detailed in Box 4.03.

Most surgical clips or orthopaedic devices will not contraindicate MR imaging. Electrically, magnetically or mechanically activated devices that cannot be removed during an examination represent another category of contraindications to MR examination and are also included in Box 4.03.

Contrast

Gadolinium is the most frequently used contrast agent in MR imaging. It is a paramagnetic ion that is attached to a chelating agent in the form of gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA), which decreases the toxicity of the gadolinium, restricts its activity and alters its pharmacokinetics. The contrast agent is intravenously administered.

Total incidence of adverse reactions for all types of MR contrast ranges from 2% to 4%. Gadolinium is safer than the iodinated contrast utilized in plain film and CT imaging, with fewer side effects.²⁸^–³⁰ When side effects do occur, the most common reactions are nausea, emesis, hives, headaches and local injection site symptoms.^29,³¹ Adverse events following gadolinium injection are more common in patients who have had previous reactions to MR imaging or iodinated contrast; therefore, it is necessary to inquire about past reactions and take these into account before requesting studies with contrast. MR contrast agents have been shown to cross the placenta and are therefore not routinely given to pregnant patients, who should only receive contrast medium if the potential benefits outweigh the risk to the fetus; this should be determined by the attending radiologist.²⁷

The traditional recommendation for breastfeeding is a 24-hour suspension following gadolinium injection; however, it has now been shown that less than 0.04% of the administered dose is absorbed by the nursing infant (100 times less than the permitted intravenous dose for infants) and the length of the suspension needs to be reviewed based on the potential risks against the stress that may be placed on a nursing infant due to the suspension.³²

An additional, though rare, reaction to gadolinium contrast has also now been recognized: nephrogenic fibrosis. This tends to occur in those patients with kidney failure or where a patient has a dramatically lowered glomerular filtration rate; therefore, if gadolinium is to be used, the patient’s renal status must be assessed. Where clinical suspicion exists, the glomerular filtration rate must be evaluated prior to the imaging study, and, if lowered, gadolinium cannot be used – nephrogenic fibrosis is an irreversible condition that is both debilitating and extremely severe.^29,³³

Techniques and protocols

The most common sequences used for imaging the lumbar spine are T1-weighted and T2-weighted; fat suppression techniques are added on occasion. T2-weighted sequences will highlight fluid and, to some extent, fatty tissues as well (Figure 4.02). Sequence combinations are determined by the clinical question being addressed; in most centres, routine orders include T1-weighted and T2-weighted sagittal and continuous axial images from L1 to S1. Slice thickness usually varies from 3 to 4 mm with a 0.5–1.0 mm gap.^10,³⁴

Figure 4.02 • Sagittal, T1-weighted (A) and T2-weighted (B) MR imaging of the lumbar spine demonstrating a Tarlov (perineural) cyst at the level of the second sacral segment. Bright structures on T1-weighted images have a high fat content; fluid produces the brightest signal on T2-weighted sequences.

When evaluating degenerative disc conditions or patients with radiculopathy, contiguous axial slices through the plane of the disc are obtained in addition to the regular continuous axial images (Figure 4.03). Axial, T1-weighted slices will nicely demonstrate the outline of the intraspinal fat against disc material, bone and nerves. On the T2-weighted sequences, the cerebrospinal fluid (CSF) will act as a contrast agent (the myelographic effect) and allow for good distinction between the disc and adjacent structures (Figure 4.04).

Figure 4.03 • MR imaging of the lumbar spine, T2-weighted sagittal planning scans (A and B) and corresponding axial images (C and D) through the disc obtained on a 25-year-old patient with radicular symptoms. Note on the plan scans (A and B) that the slices are at the level of the disc only in (A), whilst in (B) they are continuous through both the intervertebral discs and vertebral bodies. It is important to evaluate the whole of the spine since it is possible to miss diagnoses using (A) alone, for example in cases of sequestered discs and vertebral metastatic deposits that might be located between the slice levels.

Figure 4.04 • MR imaging of the lumbar spine in the axial plane using T1-weighted (A) and T2 -weighted (B) sequences. The key to the identification of structures is given in Box 4.01; note the different structures that appear with different signal on each type of imaging sequence.

Axial slices of the lumbar spine are very useful in determining the morphology of the disc, particularly posteriorly and posterolaterally. They can also be used to accurately assess the dimensions of the central canal, subarticular recesses and foramen, providing information regarding their patency, which is important to determine not only from the clinical explanation of the patient’s complaint but also for the interventional clinician, where injections are being considered.

Also in this plane, the clinician can get a good feel for the volume and state of the paraspinal and psoas muscles, both of which have been identified among the multifactorial contributors to low back pain. Individual nerve roots can also be followed to their point of exit, establishing evidence of impingement by surrounding structures. As the nerve exits, the sagittal images become more useful for evaluating the intervertebral foramen and determining whether there is evidence of impingement, most commonly from degenerative facet hypertrophy or a far lateral, extraforaminal disc lesion.

The sagittal sequences are also useful in assessing the status of both discal and ligamentous structures. Parasagittal slices depict the intervertebral foramen and the position of the dorsal root ganglion (Figures 4.05, 4.06). Locating levels on a sagittal slice can be difficult; rudimentary disc spaces and transitional lumbosacral segments are fairly common, making a precise count effectively impossible in certain circumstances. Although identification of the twelfth rib can help to suggest the levels distally, even this is prone to error since transitional lumbosacral anomalies are commonly associated with thoracolumbar abnormalities such as lumbar ribs; or hypoplasticity or agenesis of the twelfth rib.³⁵ Correlation with plain film radiography can be helpful in this regard, along with identification of the conus medullaris and right renal artery, both usually located at the level of L1 or L2. On the axial images, assessment of the orientation of the lumbosacral facet can be used to clarify the findings. Lumbar facets are oriented in the sagittal plane from L1 to L5. The articular facets at L5/S1 are often in the coronal plane. In cases where a count is still uncertain but necessary, a single cervicothoracic sagittal scout can be obtained.³⁰

Figure 4.05 • MR imaging of the lumbar spine showing T1- and T2-weighted, mid-sagittal slices, demonstrating the osseous and soft tissue anatomy visible. The key to the identification of structures is given in Box 4.01.

Figure 4.06 • T1-weighted parasagittal MR imaging of the lumbar spine at the level of the pedicles. The key to the identification of structures is given in Box 4.01.

Coronal images are not routinely used in evaluation of the lumbar spine except as ‘scouts’ and in scoliotic patients. Severe curvatures can prevent full visualization of all intraspinal structures in all three planes; the scout can allow data to be acquired in an oblique plane, making interpretation simpler (Figure 4.07).

Figure 4.07 • MR imaging of the lumbar spine in a patient with a severe scoliosis. By taking a coronal plan scan (A), it was possible to modify the T1-weighted sagittal images to compensate for the curve, avoid volume averaging and to prevent the area of anatomical and clinical interest falling outside the plane of imaging (B and insert). The key to the identification of structures is given in Box 4.01.

Contrast media in lumbar spine imaging are routinely used in three situations, details of which are given below.

Following disc operations, gadolinium is used to differentiate between postsurgical scar tissue and recurrent herniation. Scar tissue is vascularized and will enhance with contrast injection, whilst disc material is avascular and will generally not enhance.³⁶

When enhancement occurs in a disc fragment, it will typically be present around the periphery (rim enhancement) and corresponds to diffusion of the contrast medium from the adjacent tissues. The rate of diffusion into a fragment is much slower, allowing for distinction between the scar and fragment. Because of this, it is necessary to perform T1-weighted axial sequence imaging immediately after contrast injection.³⁰

In the case of intraspinal tumours, contrast is utilized for the early detection, characterization, localization and extent evaluation of the lesion ^10,³⁰; gadolinium is the modality of choice when investigating primary tumours of the central nervous system.³⁷^–³⁹ When basic MR imaging of an infection is ambiguous, contrast will be used to demonstrate the extent and activity of the lesion and to inform treatment options.⁴⁰