When a patient is suspected of having a concussion, or brain injury, doctors often turn to computed tomography, or CT, scans to find out for sure. In a CT scan, a series of X-rays is taken. Each one shows an individual "slice" of the brain, and they can be combined to form a complete three-dimensional picture. Because of the amount of radiation patients are exposed to, however, doctors question the use of the scans, especially for children. Now researchers have proposed an alternative way of diagnosing head injuries: a simple blood test. The test detects proteins that burst out of brain cells into the bloodstream immediately after an injury. It allows doctors to determine whether a patient has suffered head trauma and needs further treatment. What is the main idea of the passage?