Computed Tomography

Since its introduction in the 1970s, CT has enjoyed wide application within all the radiologic subspecialties. In fact, CT has effectively replaced conventional tomography and many other radiologic procedures (e.g., lymphangiography and pneumoencephalography). CT has undergone major changes in the past few years, with incremental improvements in hardware and software technologies, including refinement of spiral CT systems, overcoming limitations. In a typical modern spiral or "helical" CT scan, instead of obtaining data using sequential single exposures by moving the gantry, the patient is moved through a rotating, continuous fan-beam exposure, and a block of data in the form of a corkscrew or helix is obtained.1 Improvements, such as the introduction of higher heat capacity X-ray tubes, subsecond X-ray tube rotation times, detector technologies, and realtime image reconstruction computer hardware and software, transformed CT scan into a very fast, large-volume, multi-beam acquisition technology.

CT, however, currently plays a limited role in brain tumor imaging. Its availability in the emergency department makes it the first-line technique for evaluating patients presenting with signs and symptoms of increased intracranial pressure, seizures, and other neurologic symptoms that could be caused intracranial neoplasms. Inevitably, if a tumor is discovered on

Magnetic resonance imaging has become the mainstay of diagnosis in the evaluation of primary and metastatic brain tumors. Unfortunately, MRI is very sensitive but not very specific. Although it provides excellent anatomic detail, MRI remains incapable of accurately grading tumors. Extension of T2 signal abnormalities, involvement of the corpus callosum, enhancement pattern, cortical involvement, intra- versus extraaxial localization, mass effect, and the age of the patient are some of the factors that allow the radiologist to narrow the differential diagnosis of a brain lesion.

New techniques are constantly being developed to increase the specificity of MRI. Among the most promising of those new techniques are fast fluid-attenuated inversion recovery (FLAIR) imaging, diffusion-weighted imaging (DWI), perfusion imaging, and magnetic resonance spectroscopy (MRS). Functional MRI (fMRI), diffusion tensor imaging, magnetization transfer (MT) imaging, and perfusion imaging with arterial spin labeling are applied clinically only infrequently.

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