Pneumoencephalography

Pneumoencephalography
Medical diagnostics
Pneumoencephalography
ICD-9-CM	87.01
MeSH	D011011
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Pneumoencephalography (sometimes abbreviated PEG; also referred to as an "air study") was a common medical procedure in which most of the cerebrospinal fluid (CSF) was drained from around the brain by means of a lumbar puncture and replaced with air, oxygen, or helium to allow the structure of the brain to show up more clearly on an X-ray image. It was derived from ventriculography, an earlier and more primitive method where the air is injected through holes drilled in the skull.

The procedure was introduced in 1919 by the American neurosurgeon Walter Dandy and was performed extensively until the late 1970s, when it was replaced by more-sophisticated and less-invasive modern neuroimaging techniques.

Though pneumoencephalography was the single most important way of localizing brain lesions of its time, it was, nevertheless, extremely painful and generally not well tolerated by conscious patients. Pneumoencephalography was associated with a wide range of side-effects, including headaches and severe vomiting, often lasting well past the procedure. During the study, the patient's entire body would be rotated into different positions in order to allow air to displace the CSF in different areas of the ventricular system and around the brain. This further added to the patient's already heightened level of discomfort (if not anesthetized). Video of the procedure is documented in a BBC documentary of an early EMI CT installation. [1]. A related procedure is pneumomyelography, where gas is used similarly to investigate the spinal canal.

Pneumoencephalography makes use of plain X-ray images. Unfortunately, these are very poor at resolving soft tissues, such as the brain. Moreover, all the structures captured in the image are superimposed on top of each other, which makes it difficult to pick out individual items of interest (unlike modern scanners which are able to produce fine virtual slices of the body, including of soft tissues). Therefore, pneumoencephalography did not usually image abnormalities directly, rather their secondary effects. The overall structure of the brain contains crevices and cavities which are filled by the CSF. Unfortunately, both the brain and the CSF produce similar signals on an X-ray image. However, draining the CSF allows for greater contrast between the brain matter and the (now drained) crevices in and around it, which then show up as dark shadows on the X-ray image. The aim of pneumoencephalography is to outline these shadow-forming air-filled structures so that their shape and anatomical location can be examined. Following the procedure, an experienced radiologist reviews the X-ray films to see if the shape or location of these structures have been distorted or shifted by the presence of certain kinds of lesions. Unfortunately, this also means that in order to show up on the images, lesions have to either be located right on the edge of the structures or if located elsewhere in the brain, be large enough to push on surrounding healthy tissues to an extent necessary to cause a distortion in the shape of the more distant air-filled cavities (and hence distal tumors detected this way tended to be fairly large).