What improvement made brain surgery more viable during WW1?

The advancement that significantly improved the viability of brain surgery during WW1 was the introduction of saline for shock and blood transfusions. This development was critical because many soldiers experienced severe blood loss and shock due to injuries sustained on the battlefield. The ability to manage shock through saline solutions and transfusions helped stabilize patients, allowing surgeons to carry out more complex surgical procedures, including those on the brain. Prior to this advancement, the significant risk associated with blood loss made brain surgery exceedingly dangerous.

The other options, while pertinent to the broader context of medical advancements, did not have the same immediate impact on brain surgery during that period. Advanced imaging techniques were not widely available at the time, and non-invasive techniques were still in their infancy. Additionally, while anesthetics were crucial for pain management, the specific context of head injuries required addressing blood loss and shock to improve surgical outcomes. Thus, the introduction of saline for shock and blood transfusions stands out as a key improvement for brain surgery during WW1.