Answer: Oxygen

If there ever comes a time in your life where you need a diagnostic image taken of your brain, do take a moment to thank the gods of technology and medical progress that you’re having the procedure done in the 21st century. Why? Because, despite how uncomfortable you might be while getting a CT scan or MRI done today due to the cramped scanning tube, it has nothing on the procedure that preceded it: pneumoencephalography.

Between 1919, when it was introduced by American neurosurgeon Walter E. Dandy, and the late 1970s, when it was displaced by the introduction of CT scanners, pneumoencephalography was the leading brain imaging technique of its day—and one patients universally hated. In order to create better quality X-ray images of the brain, the procedure relied on a lumbar puncture to drain most of the cerebrospinal fluid from around the patient’s brain so that it could be, in turn, replaced with oxygen (or sometimes helium or air). The goal of the procedure was to get X-ray “hindering” fluid out of the way so that the image would come out clearer.

It accomplished that goal, but the procedure itself was agony. The replacement of the cerebrospinal fluid with air typically triggered immediate and intense headaches as well as severe vomiting (both of which often lasted well beyond the duration of the procedure itself). On top of that, the patient would be rotated around to allow the air to displace the remaining cerebrospinal fluid in different areas of the ventricular system and around the brain (like moving the bubble in a carpentry level) for each new image, further contributing to their discomfort and nausea (if not anesthetized).

The introduction of computed tomography scanning and, later, magnetic resonance imaging, lead to the rapid decline of the procedure. Now, outside of very rare uses in research, the procedure is unheard of and we can all, thankfully, get a brain image done without enduring a horror-movie-like encounter with our doctor.