Myelography

Myelography is an invasive procedure that has been largely abandoned for CT and MRI in most cases and is reserved at pres ent for occasions when CT and MRI are unavailable. It is performed by injecting water-soluble contrast material containing iodine into the subarachnoid space by a spinal tap. The contrast enhances the cerebrospinal fluid (CSF) and delineates the dural sac and the nerve root sleeves emanating into the intervertebral foraminae (Figures 2-9A and 2-9B). Myelography, especially combined with CT in the so-called computerized assisted myelography (CAM), or myelo-CT, can be helpful for diagnosis of disc herniation, osteophytic impingement on cervical and lumbar roots, and cord compression. Some radiologists and surgeons insist on obtaining myelo-CT prior to surgery because it depicts finer details of the patho-anatomy. It is also helpful in detecting subarachnoid tumoral spread, drop metastasis, and arachnoiditis, as well as dilated vessels indicating vascular malformation when MRI is not available or is contraindicated (Figures 2-9C-F). Finally, CAM is sometime necessary to ascertain diagnoses such as prolapse or hernia-tion of the spinal cord.

COMPUTED TOMOGRAPHY (CT)

Tomography was introduced to radiology during the 1930s. Whereas conventional radiological techniques produce summed images of an object, tomographic scanners rotate to divide the object and organize it into spatially consecutive, parallel image sections. The process, which was originally totally mechanical, has been improved upon by new technology. In computed tomography (CT), a computer stores a large amount of data—attenuation values—from a selected region of the body, making it possible to determine the spatial relationship of the radiation-absorbing structures within it. The computed tomogram consists of a matrix of attenuation values depicted in various shades of gray, thereby creating a spatial image of the scanned object. Attenuation is measured by detectors aligned behind the patient, opposite the X-ray source. The attenuation values measured by CT are reproducible. Important diagnostic information about tissues in the scanned regions of interest is obtained.

For technical realization of computed tomograms, the number and quality of data from individual picture elements, or the degree of spatial resolution, increases in proportion with the number of attenuation measurements taken from different angles.

There are basically four different types of CT scanning systems:

1. Single-detector rotate-translate systems (first generation). A fine X-ray beam scans the body through 180 steps of 1°. The intensity of the beam is measured by individual contralateral detector elements. After each angular increment, a (linear) translation is made as the beam traverses the body. A minimum of several minutes is required for scanning.

2. Multiple-detector rotate-translate systems (second generation). A detector array with 5 to 50 elements is located contralateral to the X-ray source. A pencil X-ray beam or a fan beam reduces the number of angular increments required

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