CHAPTER SYNOPSIS:
The evaluation of the spine should always begin with a comprehensive patient history and physical examination, followed by the acquisition of basic and advanced imaging studies. Multiple imaging modalities are available for the evaluation of the spine: conventional radiographs, computed tomography, nuclear scintigraphy, magnetic resonance imaging, functional and metabolic imaging, and interventional radiology techniques. This chapter outlines a frequently used algorithm for selecting the study type, the sequence in which to obtain the studies, and the basic techniques of interpreting imaging studies.
IMPORTANT POINTS:
- •
Imaging studies are invaluable to the orthopaedic surgeon when diagnosing and planning treatment for spine pathology.
- •
The spine surgeon should know the indications and contraindications of each type of advanced imaging technique to select appropriately the most useful test(s) for each patient.
- •
The radiologist and spine surgeon should work as a team when reviewing advanced imaging of the spine to obtain the most accurate interpretation of the study findings, and to correlate those findings with the patient’s history and physical examination so that the most likely diagnoses are identified.
CLINICAL PEARLS:
- •
Conventional radiographs should be used for the evaluation of coronal (scoliosis) and sagittal (spondylolisthesis, kyphosis) plane deformities, for the evaluation of osseous destruction secondary to tumor or infection, for the localization of levels before and during spine surgery, and for the evaluation of instrumentation and fusion after surgery.
- •
Computed tomography should be used for the detailed evaluation of osseous abnormalities or destruction in patients with trauma, tumor, or infection; with reconstructive techniques to obtain a three-dimensional view of the spine; or in conjunction with myelography to evaluate for stenosis.
- •
Nuclear scintigraphy should be used for the evaluation of metabolically active disease, such as primary or metastatic tumor or infection.
- •
Magnetic resonance imaging is the preferred and most commonly used advanced imaging modality for the spine. It allows for excellent evaluation of neural elements, degree of stenosis, and degenerative disc disease, and is often critical for the evaluation of spinal tumors and infectious processes.
CLINICAL PITFALLS:
- •
The selection of an inappropriate imaging modality or the failure to correlate imaging with clinical findings can result in inaccurate conclusions, and lead to misdiagnosis and potential failure of the treatment plan.
- •
Inadequate skill or experience in the evaluation of imaging studies can result in an incorrect diagnosis.
- •
Lack of consultation with radiology colleagues or failure to review radiology imaging reports can result in missed vital information.
VIDEO AVAILABLE:
N/A
Together with a comprehensive history and physical examination, imaging is essential for the evaluation of the patient who has suspected or known spine pathology. It facilitates preoperative assessment of a patient, generation of a differential diagnosis, selection of the most likely diagnosis, postoperative evaluation of a patient, and monitoring of disease progression. Experience may help the clinician decide which imaging modality is best for a patient, evaluate imaging studies effectively, determine the most likely diagnosis, and use the imaging studies in conjunction with clinical findings to guide surgical decision making. However, we suggest that a team approach in conjunction with a radiologist will help the surgeon systematically evaluate each imaging study, correlate the findings with the clinical information more accurately, reduce the incidence of unnecessary imaging, and increase the likelihood of making the correct and clinically relevant diagnosis and treatment decision. The purpose of this chapter is to review the imaging modalities currently used for evaluation of the spine in terms of evolving technique, image quality, speed, cost, availability, and safety, with a specific focus on the evaluation of degenerative diseases of the spine.
IMAGING MODALITIES
Conventional Radiography
Conventional radiography involves the use of ionizing radiation and a radiation detector. Until recently, the detector was primarily a film cassette, but digital image acquisition has replaced the cassette method at many institutions. With digital acquisition, an imaging plate and image processor are used to convert radiation energy to light energy and eventually to a digital image. Advantages of digital radiography include improved image quality and speed, availability of postimaging processing and optimization, and ease of image storage and retrieval.
Conventional radiography (cassette or digital) is a widely available and cost-effective modality for the initial evaluation of the spine; it is valuable for trauma assessment, determination of coronal and sagittal deformity, identification of spondylosis and spondylolisthesis (and their progression), and detection of osteolytic and osteoblastic lesions suggestive of malignancy. Initial evaluation often begins with anteroposterior (AP) and lateral views of the area of interest ( Fig. 2-1 A, B ). The need for additional studies, such as oblique, flexion, or extension views, is determined by the clinical situation. Radiographs can be used to determine the level of abnormality before and during surgery, and to provide a rapid evaluation of instrumentation placement and deformity correction during and after surgery.
Although conventional radiography provides a relatively effective assessment of the osseous structures and their alignment, it is limited in its ability to visualize soft tissues, the spinal cord, the occipitocervical and cervicothoracic junctions, and bone marrow involvement. Furthermore, reductions in bone mass are evident on radiographs only after a 30% to 50% decrease in bone mineral density.
Computed Tomography
Like conventional radiography, computed tomography (CT) uses ionizing radiation. However, unlike radiography, this modality acquires images in the axial plane, produces cross-sectional images, and allows for sagittal and coronal three-dimensional reconstructions via postimage acquisition processing. Multidetector-row CT can acquire all of the necessary data for a chest and abdomen study during a single breath hold, or 15 seconds.
High-contrast resolution and multiplanar reconstruction, the most important advantages of CT, permit an excellent evaluation of the spine for accurate characterization of the osseous details of a lesion, the degree of bony destruction, and spinal alignment ( Fig. 2-2 A, B ). Three-dimensional reconstructions can assist in careful fracture evaluation, preoperative planning, and examination of complex deformity. Furthermore, CT plays an important role in myelography and biopsy of various lesions. However, despite such advantages, CT has several drawbacks: poor soft-tissue contrast, the use of ionizing radiation, and sensitivity to motion and metal artifact.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) generates multiplanar images that have excellent anatomic and spatial resolution, but unlike conventional radiography and CT, it does not involve ionizing radiation. MRI uses a generated magnetic field, the application of radiofrequency pulses and the absorption of that energy by atomic nuclei of various tissues, the detection of that energy released by the same nuclei, and the translation of the released energy to create discrete regions of varying signal intensities or brightness on the images. The intensity of the signal depends on the number of protons within different tissues and, therefore, the water content of those tissues. Thus, tissues such as joint fluid or cerebrospinal fluid appear bright on T2-weighted images, whereas air and cortical bone appear dark.
The fundamental advantage of MRI is its ability to provide a high-resolution depiction of osseous and soft-tissue structures. With respect to the spine, MRI provides excellent visualization of the vertebral body, intervertebral discs, spinal canal, posterior elements, ligaments, paraspinal muscles, nerve roots, and the spinal cord. With multiplanar imaging and the use of various pulse sequences, MRI facilitates abnormality characterization and has been shown to have high sensitivity and specificity for the detection of various disease processes (e.g., 93% and 94%, respectively, for vertebral osteomyelitis ). MRI has also proved invaluable in the assessment of neoplasms of the spine, with a greater accuracy than CT ( Fig. 2-3 A, B ). For the degenerative spine, MRI allows for excellent evaluation of the degree of central and foraminal stenosis, as well as the degree of other degenerative changes such as facet arthropathy and degenerative disc disease.
The disadvantages of using MRI include the inability to scan patients with cardiac pacemakers or other embedded ferromagnetic material ; problems in obtaining a scan in patients with claustrophobia; limited image quality for patients with instrumentation (although that is not a contraindication) because of metal artifact, even with less ferromagnetic metals (such as titanium) that produce less artifact ; and compared with CT, an inferior ability to assess detail of osseous or calcified structures. MRI is contraindicated in patients with pacemakers or other implantable devices. Relative contraindications to MRI include claustrophobia and the first trimester of pregnancy. Instrumentation is not a contraindication to MRI, but it can cause excessive scatter, which can substantially reduce the quality of the images.
Myelography
Myelography is defined as radiography of the spine after injection of a nonionic contrast material into the subarachnoid space, via a lumbar or cervical puncture. The major indications for its use include imaging of a patient for whom MRI is contraindicated (because of claustrophobia, presence of pacemaker, among other factors) and imaging in the presence of spinal instrumentation. After the contrast is injected, conventional radiographs and, in most cases, CT images are obtained ( Fig. 2-4 ). In the degenerative lumbar and cervical spine, it is important to trace the pathway of each nerve root sleeve carefully to evaluate for foraminal stenosis, and to examine the axial CT images carefully to evaluate for central stenosis. Compared with the increasing use of MRI, the use of myelography is not as common. For this reason, less experienced clinicians may not be familiar with the evaluation of these imaging studies and may consult with a radiologist experienced in this technique as necessary.
Disadvantages of myelography include the potential for allergic reaction, use of ionizing radiation, a lower seizure threshold, bleeding, risk for infection, headache, nausea, the invasiveness of and pain associated with the examination, the time and expertise needed to perform the study, the risk for neural damage, and the inability to determine areas of compression below blocks in contrast flow. The primary contraindication to myelography is allergy to the nonionic contrast agent. Relative contraindications include seizure disorder, bleeding disorders, concurrent use of anticoagulants, infection or skin disease over the site of injection, and pregnancy.
Except for those situations in which metal artifact is a factor, MRI is superior to myelography for the evaluation of spinal abnormality and has been shown to have a substantially greater accuracy in the detection of herniated discs and a lower false-positive rate. Some clinicians think that CT myelography allows for better evaluation of foraminal stenosis than MRI; however, with improvements in MRI techniques, both imaging modalities are effective in this regard.
Nuclear Scintigraphy
Nuclear scintigraphy (also termed a radionuclide bone scan ) provides anatomic and physiologic information via the administration of a radiopharmaceutical compound into a patient’s venous system. The radioisotope is preferentially deposited in regions of increased bone remodeling or activity, which allows areas of increased or decreased bone turnover to be differentiated by a scintillation camera that detects and localizes the gamma radiation emitted by the injected agent. This modality is particularly useful for the evaluation of metabolic bone disorders, stress fractures, primary and metastatic neoplasms, infections, and degenerative disorders. Nuclear scintigraphy can also be used in lieu of MRI to help determine the age of vertebral compression fractures ( Fig. 2-5 A, B ).
