10 Fundamentals
Clinical Aspects
Computed tomography (CT) is widely used in the diagnosis of spinal disorders, providing in-formation of extremely high quality for a variety of indications. At the same time, CT is a prime example of the problems involved in examining a predominantly longitudinal organ system with a modality that is designed for imaging on axial planes. Unlike magnetic resonance imaging (MRI), CT cannot furnish a primary survey of the vertebral column with a few sagittal scans, followed by the selective evaluation of sites with apparent abnormalities. This limits the value of spinal CT in several respects:
• As a screening examination
• For imaging pathology with significant longitudinal extent
• For finding lesions located at an unknown level
Because of these limitations, the clinical findings and any additional imaging studies have a crucial role in focusing the investigation on a specific region. If there is no clinical information from the referring physician and such information cannot be easily obtained, the radiologist should try to narrow the examination based on the history, diagnostic findings, and previous imaging studies if the patient’s condition permits and the situation is not an emergency.
CT should not be used to investigate nonspecific symptoms such as “unexplained back pain” unless all other diagnostic options have been exhausted.
Frequent Indications for Spinal Computed Tomography
Disk Herniation
CT is most commonly used in the spine to evaluate for an intervertebral disk protrusion or herniation encroaching on nerve roots, cauda equina fibers, or the spinal cord. This indication is much more common in the lumbar spine than in the cervical or thoracic areas.
Clinical Manifestations
A basic distinction is drawn between the following clinical presentations:
• Low back pain or neck pain with a non-segmental or radicular pattern of pain projection and no neurologic deficits.
• Lumboischialgia or cervicobrachialgia, which can usually be attributed to nerve root compression during the physical examination.
Nerve root compression may be caused by a central herniation of the disk above the nerve root or by a lateral, intraforaminal disk herniation at the same level. This pattern defines a limited region for CT evaluation.
In patients with symptoms of L1–3 lumbar root compression and normal CT findings at the adjacent levels, the compression may be caused in rare cases by a lateral disk herniation at a more caudal level. The herniated disk impinges on the descending, paravertebral portion of the root after it has exited the neuroforamen. In these cases, the radiologist may have to extend the examination to the lower adjacent levels after consulting with the referring physician.
Examination Technique and Sources of Error
In patients examined for a herniated disk, the standard CT technique for visualizing the intervertebral disks is to acquire single slices directed parallel to the disk spaces. This requires readjusting the gantry angle for each level imaged. Because most units have a maximum gantry angle of 20–25°, the lowest level often cannot be imaged precisely parallel to the disk space. This should be taken into account when evaluating L5–S1 herniations that extend beyond the disk space. It is one reason why spiral (helical) CT scanning without gantry angulation is often recommended in patients undergoing evaluation for a herniated disk.
The need for multiple gantry angulations creates another potential source of misinterpretation: adjusting the gantry angle separately for each level causes the slices on the posterior side to overlap. In some circumstances, this may cause a herniated disk to be imaged twice, as the apparently contiguous craniocaudal images portray two ruptured disks separated by redundant, normal-appearing spinal segments. In reality, the scans merely cut the same herniated disk twice at different angles.
To evaluate the imaged structures and detect or exclude a herniated disk, it is also help ful to keep in mind the causal mechanism of beam-hardening artifacts (see p. 288).
Differential Diagnosis
When examining a patient with low back pain, it is important to evaluate all imaging findings critically and consider the full range of diagnostic possibilities. These include:
• A narrow spinal canal
• Bony lesions of the sacrum
• Abnormalities of bone density and trabecular structure, especially in patients with osteoporosis
• A defect in the vertebral arch (spondylolysis)
• Other potential causes of nerve root compression
• Retroperitoneal lymph nodes
• Aortic aneurysm
Fractures and Other Trauma
Another frequent indication for CT examination of the spine is a search for traumatic changes. At some trauma centers, CT is used as a primary screening study in injured patients. Spiral CT is used to evaluate the head, chest, and abdomen, depending on the pattern of the injury.
Next, the regions that have the highest statistical involvement for a particular clinical presentation or trauma mechanism are selectively reexamined.
The lesions of primary interest in spinal trauma are vertebral body fractures. These injuries are difficult to detect in whole-body examinations, however, and they cannot be reliably excluded unless selective thin CT slices are acquired.
Clinical signs such as pain and neurologic deficits, as well as secondary fracture signs such as paravertebral or retroperitoneal hematomas in spiral CT scans of the chest or abdomen, always warrant an additional bone-window examination of the adjacent vertebral bodies using thin CT slices.
Two-dimensional reformatted images are often helpful as well. Sagittal reformatting is particularly useful for mapping fracture patterns in the sagittal plane.
Whenever possible, the radiologist should correlate the CT images with conventional radiographs.
The primary goals in the CT diagnosis of intraspinal masses are to define the precise level of the mass and establish its identity. Intravenous contrast administration is therefore usually recommended. A dynamic contrast study can help in making a differential-diagnostic classification.
The most important criterion in the differential diagnosis of an intraspinal mass, however, is its location. Tumor masses are classified as follows:
• Extradural
• Intradural extramedullary
• Intramedullary
The most common entities within these groups are listed in Table 10.1.
| Location | Entity | |
|---|---|---|
| Extradural (55 %) | • Metastases (e.g., from breast, prostatic or bronchial carcinoma, gastrointestinal tract tumors, melanoma) | |
• Osteogenic tumors (e.g., chordoma, osteoma, bone cyst) | ||
• Tumors that invade the spinal canal secondarily from a paravertebral location (e.g., retroperitoneal paravertebral lymphoma, pleural mesothelioma, neuroblastoma) | ||
• Rare location of predominantly intradural masses (e.g., meningioma) | ||
| Intradural extra medullary (40 %) Intramedullary (5 %) | Meningioma, schwannoma, epidermoid Astrocytoma, ependymoma, angioblastoma |
Evaluation and Interpretation of Findings
Particularly in patients undergoing examination for degenerative disk disease, it is important to evaluate and interpret the morphologic imaging findings correctly in relation to the patient’s clinical symptoms, since degenerative changes can be found in many asymptomatic patients. The detection of a disk protrusion in itself does not mean that the protrusion is causing the symptoms. Familiarity with the clinical symptoms is thus important both as a plausibility check and as a way of judging the significance of the findings.
At a time when cost efficiency greatly influences the choice of diagnostic procedures, the CT findings should not only help furnish a diagnosis, but should also influence patient management. Like other imaging modalities, CT should be part of the diagnostic algorithm and help the referring physician decide which surgical or nonsurgical options are appropriate for treating a particular set of clinical symptoms.
Since CT was introduced clinically in the early 1970s, rapid progress in hardware and software development and advances in computer technology have made the method an important tool in the basic work-up of spinal disorders. Despite the advent of MRI, the role of CT has become even more firmly established.
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