To diagnose MS, or to tell if a person has it, a doctor will determine what kind of symptoms are present and how often they occur. The most common guidelines used for this are called the McDonald criteria, which define the symptoms of MS and how often they must occur in order to make a diagnosis. A doctor can also order tests to be done by a laboratory, which can determine how active the immune system is in the patient. A special machine called an MRI can photograph the inside of the central nervous system to show if the person has lesions from damaged myelin. Certain types of neurons can be checked to see how responsive they are. Neurons with damaged myelin around their axons will respond more slowly than normal neurons.