CT Scan

CT scanning utilizes the information from multiple radiographs rotated around a patient at a given "slice" to create three-dimensional "slices" through a patient. By processing the data from the multiple radiographs, every point within that slice may be "triangulated" and reconstructed to form a CT scan "allowing one to see the shape and position of the egg yolk without breaking the egg." What is demonstrated is only the "location, shape and density" of the yolk and not its internal architecture (Bogduk 2003). This slicing is repeated at various levels along a patient to form several such images, much like cutting ham slices. Because CT scan fundamentally uses radiographs, the same basic four gray shades of density are used to define the contrast that is obtained (bone, fat, soft tissue, and air); however, because multiple images are used in reconstruction, averaging of shades permits subtleties in shading with the potential for visualizing small details (Fig. 6.2).

CT scans, like ham, come in a variety of styles including helical (spiral) and single slice. Helical CT uses data from multiple slices by traveling in a spiral fashion around the patient. As a result it provides better resolution of high-contrast structures such as bone and air and thus is useful in identifying bony fractures (as in facial fractures) and pulmonary emboli. Single slice CT scanning is advantageous over helical CT in providing detail.

There are two fundamental uses for CT scans in pain management: identification of intracavitary tumors (such as within the thorax, abdomen, and pelvis) and imaging of the spine (Table 6.2). With respect to tumors, CT scan can provide data on progression and recurrence of primary and secondary tumors pointing to potential causes of pain. Back pain without neurologic findings is initially managed conservatively without initial imaging. MRI of the such as computer tomography (CT) more dimensional view, radiographs 6.1 lists conditions for which plain

Figure 6.2 CT scan fundamentally uses radiographs, the same basic four gray shades of density are used to define the contrast that is obtained (bone, fat, soft tissue, and air). However, because multiple images are used in reconstruction, averaging of shades permits subtleties in shading with the potential for visualizing small details.

Figure 6.2 CT scan fundamentally uses radiographs, the same basic four gray shades of density are used to define the contrast that is obtained (bone, fat, soft tissue, and air). However, because multiple images are used in reconstruction, averaging of shades permits subtleties in shading with the potential for visualizing small details.

Table 6.2 Conditions for which computed tomography (CT) scan may be used as initial or follow-up test.

CT scan as primary modality

□ Suspected pulmonary embolism

Low back pain with radiculopathy [if magnetic resonance imaging (MRI) not possible] CT scan after initial plain radiograph

□ Facial fracture

Adapted from Mettler (2005).

spine is indicated when back pain occurs with a neurologic finding, such as radiculopathy. When MRI is contraindicated and spinal imaging is needed, CT myelography remains an option and results maybe equivalent in detecting lesions (Modic 1986). CT may also be more useful when spinal hardware is present because of MRI artifacts.

Much as in plain radiography, contrast agents may be used to enhance detail and such is the principle behind CT myelography. A needle is inserted into the spinal fluid and contrast agent given. A plain radiograph or CT scan of the spine is performed and the outline of the contrast observed. Any extradural indentations of the thecal sac (such as from herni-ated disks, bone spurs, spinal stenosis, or tumors) suggest that a lesion is present, although the detail of that lesion may not be discernible (Kleefield 2004). Lesions that are present but do not impinge upon the thecal sac may be missed. Minor risks of this procedure include headache from the spinal injection. One of the more dreaded complications includes arachnoiditis, in which the nerves of the cauda equina become matted down from scarring and inflammation of the arachnoid as a reaction to the contrast agent (Eldevik 1978). The symptoms of this complication may be far worse than the condition for which this study was ordered.

While the typical CT scan provides images along an axial plane, it is possible through reconstruction of the computerized data to provide coronal and sagittal views as well. CT scans, however, are inferior to MRI in providing detail of soft tissues and thus fail to provide information about intrinsic problems within the spinal cord itself.

In MRI, a strong external magnetic field is applied to tissues to align the atoms in those tissues with the magnetic field. The field is then released and the atoms lose their alignment. Different tissues lose their alignment at different rates and this loss results in the production of radio waves. Because each type of tissue produces a different frequency of radio waves, different tissues can be distinguished by the different frequencies they produce. Two fundamental types of decay, T1 and T2, produce distinct frequencies when the magnetic field within tissues relaxes. Also, consequently, each of these produces different contrasts within tissue which is reconstructed to define the boundaries and innards of these tissues (Table 6.3).

Table 6.3 Relative weighting under T1 and T2 imaging of magnetic resonance imaging (MRI).

Tissue

T1 Image

T2 Image

Fat

White

Black

Water (or CSF)

Black

White

Brain and muscle

Gray

Gray

CSF = cerebrospinal fluid.

CSF = cerebrospinal fluid.

Compared to both plain radiography and CT, MRI is most useful for depicting the structure of soft tissues (Fig. 6.3). It is not as useful for depicting bone; any visualization of bone (i.e., white on T1-weighted images) really represents the marrow fat within bone and not the calcium itself (Mettler 2005). For bone structures, plain radiographs and CT scan remain the preferred modality. Furthermore, because MRI does not use ionizing radiation, it is potentially safer in some individuals (e.g., pregnant women beyond the first trimester (Wilkinson and Paley 2008)). Unfortunately, any ferromagnetic materials within or on a person may be pulled violently into the field and this remains a major contraindication to the use of MRI. In very high fields, people have reported seeing spherical visual hallucinations and have even had vertigo (Wilkinson and Paley 2008).

Specifically, the following remain major contraindications to the use ofMRI:

• Cardiac pacemaker;

• Implanted cardiac defibrillator;

Figure 6.3 Compared to both plain radiography and CT, MRI is most useful for depicting the structure of soft tissues.

• Carotid artery vascular clamp;

• Neurostimulator;

Insulin or infusion pump;

• Implanted drug infusion device;

• Bone growth/fusion stimulator;

• Cochlear, otologic, or ear implant (DCMR-Contraindications to MRI 2009).

Interestingly, presence of an implanted intrathecal pumps used for pain, such as a Medtronic Synchromed® pump (Medtronic Corporation, Minneapolis, MN, USA), do not comprise an absolute contraindication to MRI. The pump itself will stop functioning during the MRI exposure but will resume thereafter. It is recommended that the pump programming be checked immediately after the MRI to verify that it has not been altered (Important Safety Information for Drug Delivery Systems 2009).

Because of its superiority in demonstrating the intrinsic detail of soft tissue and because of its sensitivity to movement, MRI is most useful in imaging static soft tissue structures such as the spine, brain, and joints (Table 6.4). As such, it is the preferred means of evaluating back pain with radiculopathy. Furthermore, it is useful for detecting abnormalities within single joints and osteomyelitis as it provides soft tissue contrast in relation to the surrounding bone.

Table 6.4 Conditions for which magnetic resonance imaging (MRI) may be used as initial or follow-up test.

MRI as primary modality

□ Low back pain with radiculopathy

□ Occult hip fracture

□ Occult knee fracture MRI following plain radiograph

□ Joint pain, monoarticular

□ Osteomyelitis

Adapted from Mettler (2005).

When plain radiographs fail to demonstrate a hip or knee fracture, MRI is also useful for delineating an occult fracture.

Many patients complain of claustrophobia with MRI. To help alleviate this problem, open MRI systems have been developed. The strength of an MRI magnet is measured in Tesla (T) (Wilkinson and Paley 2008); most MRI scanners are in the range of 0.2-3.0 T, with the majority being 1.5 T. To generate a field above 1 T, a superconducting magnet must be cooled with liquid helium. Such a system requires a closed MRI system, i.e., a non-open MRI system (Wilkinson and Paley 2008). The open systems use weaker magnets and consequently provide lower resolution and less contrast, both of which affect diagnostic ability.

Much as in CT scanning, contrast agents enhance images. Gadolinium is the most commonly used contrast agent in MRI scanning. It is a toxic metal that is rendered safe by combining it with a chelating agent (Wilkinson and Paley 2008). This contrast agent accumulates where the blood-brain barrier has been compromised as in the case oftumors, abscesses, and demyelination (Wilkinson and Paley 2008). Their greatest value is in the distinguishing of tumor from edema.

Although the complications associated with gadolinium are far less than those with the contrast agents used in CT scanning, some still do exist (Prince et al. 1996). The most common reactions are simple allergic-like reactions that include pruritus, rash, hives, and facial swelling. Patients with pre-existing renal insufficiency or failure are at risk for developing the fibrosing condition known as nephrogenic systemic fibrosis caused by deposition of gadolinium in the tissues (Information for Healthcare Professionals Gadolinium-Based Contrast Agents for Magnetic Resonance Imaging 2009). It is characterized by a scleroderma-like syndrome involving the following organ symptoms: "burning or itching, reddened or darkened patches, and/or skin swelling, hardening and/or tightening [of the skin], yellow raised spots on the whites of the eyes, joint stiffness, limited range of motion in the arms, hands, legs, or feet; pain deep in the hip bone or ribs; and/or muscle weakness (Information for Healthcare Professionals Gadolinium-Based Contrast Agents for Magnetic Resonance Imaging 2009)." No known treatment exists.

Dealing With Back Pain

Dealing With Back Pain

Deal With Your Pain, Lead A Wonderful Life An Live Like A 'Normal' Person. Before I really start telling you anything about me or finding out anything about you, I want you to know that I sympathize with you. Not only is it one of the most painful experiences to have backpain. Not only is it the number one excuse for employees not coming into work. But perhaps just as significantly, it is something that I suffered from for years.

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