Structural Neuroimaging Techniques

17.2.1 Autopsy

The most straightforward imaging technique, used for thousands of years, is the autopsy, derived from the Greek word autopsia, meaning "the act of seeing with one's own eyes." It is the ultimate in invasiveness and is nonrepeatable, but it is the only imaging technique in which one looks directly at the tissue of interest. The main disadvantage of the technique from a research perspective is that the participant must be dead, which precludes inferences about functional mechanisms.

X-rays were discovered by Wilhelm C. Roentgen in 1895 and became the first relatively noninvasive neuroimaging technique. Unlike an autopsy, x-ray scans were repeatable and did no obvious damage to the tissue being imaged. However, researchers later determined that x-rays produce both acute local tissue damage and chronic genetic damage that increases the risk of cancer. These side effects restrict the use of x-ray exams to circumstances in which there is a strong clinical justification and argue against repeating exams unnecessarily. X-rays have a spatial resolution of about 3 mm. They are transmitted through soft tissue relatively unimpeded, although dense tissue like bone will to some degree both absorb and scatter them. The major limitation of x-rays as an imaging modality is that they act similarly to a light source transmitted through the entire thickness of the head. Consequently, subtle changes in tissue density may be masked by the shadow of intervening bone, so it is necessary to view a region of interest from multiple angles.

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