AAOS Comprehensive Orthopaedic Review

Section 7 - Spine

Chapter 64. Imaging of the Spine

I. Plain Radiography

A. General information


1. Advantages—Radiographs are the mainstay of orthopaedic imaging. They are easy to obtain, inexpensive, and provide valuable diagnostic information.


2. Disadvantages—A single radiograph provides only a two-dimensional view of the structures being studied. Hence, at least two orthogonal views must be obtained to adequately assess three-dimensional structures.


3. Dense materials absorb more radiation and are brighter on radiographs than less dense structures.


B. Clinical use for spinal conditions


1. Back and neck pain


a. Extremely common reasons for visits to orthopaedic surgeons


b. For patients with nonspecific axial spine pain, a 6-week course of conservative treatment is reasonable before obtaining imaging studies.


c. Warning signs such as increasing pain, constant pain, a history of trauma, constitutional symptoms, a history of tumors, or a history of infection warrant early imaging.


2. Scoliosis



Full-length PA and lateral weight-bearing radiographs are useful for evaluating the patient with scoliosis. To obtain accurate studies, the patient's knees should be locked when obtaining these radiographs.


A 36-inch film, which provides exposure from the occiput to the hips, is needed to adequately evaluate coronal and sagittal alignment.


A plumb line is drawn on the weight-bearing PA and lateral scoliosis radiographs to evaluate malalignment and spinal translation.


Left and right lateral bending studies are used to evaluate structural and nonstructural curves and are critical for preoperative planning.


PA radiographs are preferred to AP views because they subject the sensitive anterior organs, including the breasts, to less radiation.


3. Degenerative spinal conditions


a. Radiographs can provide information about common degenerative conditions such as degenerative disk disease and spondylolisthesis.


b. Lateral flexion and extension views can be used to evaluate for instability in the cervical or lumbar spine.


c. Oblique radiographs of the lumbar spine are useful for evaluating the pars interarticularis and possible pars fractures (spondylolysis). The "Scotty dog" sign will be apparent on an oblique lumbar radiograph in a patient with a pars fracture. The Scotty dog sign is outlined on the radiograph, with the pedicle as the dog's eye, the superior articular facet as the dog's ear, the inferior articular facet as the dog's front leg, and the pars interarticularis as the dog's neck; a lucent line in the position of the dog's collar is a defect in the pars interarticularis (

Figure 1).

II. Computed Tomography

A. General information


1. Axial CT images can be reformatted into sagittal, coronal, and three-dimensional images.


2. Helical (spiral) CT is a recent innovation in CT technology that has dramatically decreased the time needed to obtain a study while also increasing the quality of the studies. Multidetector helical CT provides faster image acquisition by using multiple detectors to obtain data simultaneously as the patient moves through the scanner.


B. Clinical use for spinal conditions


1. CT provides excellent definition of bony architecture for degenerative and traumatic conditions.


2. CT is a cost-effective screening tool following cervical trauma in patients with moderate or high risk of cervical injury.


[Figure 1. Spondylolysis seen on various imaging modalities. A, Oblique radiograph showing lucency (arrow) in the neck of the "Scotty dog" in a patient with L5 spondylolysis. B, Whole body bone scan showing increased unilateral uptake in the region of a left L5 spondylolysis. C, Lumbar single photon emission CT (SPECT) image with increased uptake in the left L5 posterior elements consistent with unilateral spondylolysis.]


Figure 2. Imaging studies of an L3 burst fracture. A, AP radiograph of patient with L3 burst fracture. Note widened interpedicular distance. B, Lateral radiograph showing decreased vertebral body height.C, Axial CT scan showing L3 burst fracture with retropulsed vertebral body and laminar fracture. D, Sagittal CT reconstruction showing loss of vertebral body height and retropulsion of the posterior-superior portion of the vertebral body.]

3. Sagittal and coronal reconstructions should be obtained to evaluate for alignment (Figure 2).


4. Because CT scans are obtained with the patient supine, care must be taken when interpreting for spinal alignment in flexion injuries because the amount of displacement can be underestimated.


5. Degenerative disk and facet disease are well visualized using CT scans.


6. CT myelography involves the injection of radiopaque dye into the thecal sac, allowing visualization of the neural structures and any compressive lesions.


7. CT myelography is especially useful in patients who have previously undergone spinal surgery or in patients who are not candidates for MRI, such as those who have pacemakers.

III. Magnetic Resonance Imaging

A. Biophysics


1. The quality of the initial radio wave and the density and chemical environment of the aligned atoms determine the appearance of the tissue on the MRI scan.


2. The strength of the scanner is usually measured in Tesla (T) units.



Figure 3. Images of an L4-L5 spondylolisthesis demonstrating the benefit of dynamic flexion and extension radiographs. A, Lateral lumbar weight-bearing radiograph in the neutral position showing a minimal L4-L5 spondylolisthesis in a patient with severe low back pain and leg numbness and neurogenic claudication. B, Lateral flexion view showing significantly increased anterior displacement of the L4 vertebral body. C, Sagittal T2-weighted MRI scan showing severe spinal stenosis at L4-L5. Also note that the L4-L5 anterolisthesis appears reduced on this supine MRI scan.]

3. Magnetic coils placed over the body part being examined can improve the images obtained by the MRI scanner.


4. The time it takes for the hydrogen atoms to return to their initial equilibrium is called the relaxation time. The two relaxation times studied are the longitudinal (T1) and transverse (T2), and these are related to the physical characteristics of the studied tissue.


a. T1-weighted images—Fat, proteinaceous fluids, and subacute hematomas are bright on these images.


b. T2-weighted images—Structures with increased water content such as cysts, inflamed tissue, cerebrospinal fluid, acute fractures, and tumors are bright and well evaluated on these images.


B. Clinical use for spinal conditions


1. MRI provides excellent visualization of soft tissues, including intervertebral disks, ligaments, facet joints, nerves, and the spinal cord.


2. MRI is useful for trauma evaluations, including ligamentous disruption and spinal cord signal changes.


a. Changes in the signal seen in the spinal cord posttraumatically can signify hemorrhage or edema.


b. Hemorrhage carries a significantly worse prognosis than edema.


c. Acute and subacute compression fractures have increased signal on T2-weighted images and short T1 inversion recovery (STIR) images.


3. Intravenous gadolinium contrast


a. Useful in tumors, infections, and in the previously operated spine


b. Structures with increased vascularity appear hyperintense on postcontrast T1-weighted images.


c. In the postoperative spine, recurrent disk herniation is avascular and therefore appears hypointense, whereas scar tissue is vascular and appears hyperintense.


4. MR angiography


a. MR angiography creates a vascular map in multiple planes and is useful to evaluate vascularity in and around the spine.


b. MR angiography should be considered when planning complex spinal surgery in patients with abnormal anatomy (eg, revision surgery, trauma patients, and patients with tumors or other destructive lesions).


5. Standing and dynamic MRI scanners


a. A large number of spinal disorders are related to axial loading or spinal motion.


b. Specially designed scanners allow MRIs to be obtained with the patient in the standing position, in flexion, and in extension (Figure 3).


6. Clinical significance


a. Care must be taken when interpreting MRI results in asymptomatic patients. Multiple studies have shown rates of positive MRI findings in asymptomatic patients as high as 80%.


b. MRI findings must be correlated with the patient's symptoms when using them to define treatment protocols because multiple studies have reported positive MRI scans in asymptomatic patients.

IV. Nuclear Medicine

A. Biophysics


1. Nuclear medicine studies involve the intravenous injection of a radioactive marker called a radionuclide.


2. Radionuclides are bound to different carrier complexes that interact characteristically within the body.


3. Areas where the radionuclides collect show up as bright, or "hot," whereas lack of deposition is dark, or "cold."


B. Clinical use for spinal conditions


1. Nuclear medicine studies provide images of the entire body and hence are useful screening tests.


2. They are used for metastatic tumors, fractures, and spondylolysis.


3. Nuclear medicine studies can be combined with CT or MRI to provide a three-dimensional view of areas with radionuclide uptake with increased sensitivity.


C. Types of nuclear medicine studies


1. Bone scan


a. Technetium Tc 99m methylene diphosphonate is injected and localizes to areas of high bone turnover. The body is then scanned with a gamma camera, which is sensitive to the emitted radiation.


b. Tumors, fractures, and infections show up as bright, or "hot."


2. Gallium scan


a. Gallium-67 is injected and localizes to areas of infection and rapid cell division such as tumors.


b. Scans are obtained 24, 48, and occasionally 72 hours postinjection.


c. A gallium scan can be combined with CT to provide cross-sectional imaging.


3. Indium 111-labeled leukocyte scan


a. Blood is drawn from the patient, and the white blood cells (WBCs) are labeled with indium 111.


b. The labeled WBCs are injected back into the patient, where they localize to areas of infection.


c. The scan is usually obtained within 24 hours following reinjection.


d. This study is frequently performed in conjunction with technetium bone scans.

Top Testing Facts

1. Plain radiographs create a two-dimensional view, and two orthogonal images are required to adequately visualize the examined structures.


2. PA radiographs are preferred to AP views when evaluating patients with scoliosis because they minimize the amount of radiation absorbed by vital organ systems.


3. Dense materials absorb more radiation and are brighter on radiographs than less dense structures.


4. Helical CT provides faster acquisition of images than previous technology.


5. CT myelography is useful for evaluating neural structures in patients with contraindications to MRI, such as those with pacemakers.


6. MRI is useful for evaluating soft tissues, including disks, nerves, and the spinal cord, and for diagnosing disk herniations, spinal stenosis, and acute or subacute fractures.


7. MRI findings must be correlated with patient symptoms because multiple studies have reported positive MRI scans in asymptomatic patients.


8. Intravenous gadolinium contrast is useful in MRI of patients with tumors, infections, and previous surgery.


9. Nuclear medicine studies have different imaging characteristics, depending on the type of radionuclide used.


10. Nuclear medicine studies can be performed in conjunction with cross-sectional imaging such as CT or MRI to provide more detailed studies.


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