Whereas fMRI and PET are based on the coupling of neural and vascular (blood flow) activities, electrophysiological methods directly reflect brain cells’ electrical activity. These non-invasive methods include electroencephalography (EEG) and magnetoencephalography (MEG).
EEG measures the electrical activity that is produced by neurons as recorded from electrodes placed along the scalp. MEG maps brain activity by measuring magnetic fields that are generated by neural activity in the brain. Both EEG and MEG provide information about global as well as regional neural activity, but with MEG there is less distortion of the electrical signals. Often one or the other of these electrophysiological methods is combined with fMRI or PET to provide complementary information about normal and disturbed brain function.
EEG is used clinically to measure physiological manifestations of abnormal cortical excitability, primarily in the diagnosis and management of epilepsy and other seizure disorders. It is also used with other many other measures in intensive care to monitor head-injured patients in coma, providing information that helps physicians assess patients’ prognosis. EEG is also used to study sleep disorders. EEG recordings can be conducted while a patient is inside the MR scanner. EEG and fMRI are used together, for instance, to localize where in the brain a seizure starts and where it spreads thereafter.
MEG, by measuring magnetic fields, is used to investigate the basis of sensory processing and motor planning in the brain. MEG is used with MRI in brain tumor patients prior to their surgery to identify the hemisphere controlling language and to precisely locate the areas involved in expressive and receptive language so that surgeons can spare these areas during surgery. Sometimes, patients who will be undergoing this pre-surgical planning will agree to participate during the MEG/MRI procedure in research designed to explore brain processes that may be involved in stuttering, or in memory.
Transcranial magnetic stimulation (TMS) is a non-invasive technique that is used to map cortical functions in the brain, such as identifying motor or speech areas. With TMS, a large electromagnetic coil is placed on the scalp, near the forehead. An electromagnet is then used to create a rapidly changing magnetic field, inducing weak electric currents. It increases plasticity and excitability of neural circuits.
Unlike TMS, repetitive TMS (rTMS) is used as a therapeutic intervention, rather than cortical mapping tool. In stroke patients with motor deficits, rTMS is used to try to restore the balance of excitation between motor cortices in each brain hemisphere. Changes in signals from the motor cortex can be associated with improvements in muscle movements, such as raising a finger. Additionally, as of 2008, rTMS is approved by the Food and Drug Administration for non-invasive treatment of depression. Researchers continue to gain a better understanding of mechanisms of actions, and optimal doses (such as frequency and patterns of delivery). This technique is also being tested experimentally in other neurological conditions such Parkinson’s disease, dystonia and schizophrenia. Like another technique called “deep brain stimulation” (DBS), TMS functions both to provide information on brain functions and to treat some functions.
Deep brain stimulation (DBS) involves implanting electrodes in specific areas in the brain and externally stimulating the electrodes to measure electrical activities of neurons and their electrochemical pathways. DBS is currently approved by the FDA for treatment of intractable Parkinson’s disease and essential tremor. It is also being studied at research centers for treatment of severe intractable depression, obsessive-compulsive disorder, Tourette’s syndrome and other conditions.
In addition, however, in a few highly specialized research centers, neurosurgeons and cognitive scientists are undertaking DBS electrical imaging to begin to explore the neuronal underpinnings of cognition. The studies are undertaken in patients with epilepsy, with their consent, while the patients participate in pre-surgical planning to identify the specific location of the origin of seizures that cannot be controlled with medication, so that essential areas can be spared during surgical treatment for these intractable seizures. DBS electrodes transmit signals from nearby cells when those cells are active in a specific task, such as naming, responding to a happy or sad face, or involved in movement, such as raising a finger.
Laser Doppler Ultrasound is a non-invasive and highly sensitive method for measuring even tiny changes in the rate of blood flow velocity (speed) within arteries throughout the body, including the brain. Its primary use in the brain is for monitoring severely head injured patients, especially those in coma, in intensive care units. It is used in combination with other measures (such as EEG, described above). In fact, it is one of many components of multi-modal monitoring of brain oxygenation and metabolism that help physicians predict a patient’s prognosis and measure patients’ responses to various therapies. It is also used to confirm brain death, the irreversible cessation of all functions of the whole brain.
The technology is based on the principle that sound waves change pitch when combined with motion, in this case movement of red blood cells. The low level laser light is able to penetrate thin areas of the skull, enabling intensive care practitioners to monitor any changes in the ultrasonic signals from patients’ basal cerebral arteries.
Laser Doppler Ultrasound is often used to image the carotid artery to determine whether a major blockage is present that is decreasing blood supply to the brain in patients suspected of having a transient ischemic attack or stroke. In heart disease patients, the technique helps to assess the effects of angiogenesis treatment (promoting the growth of blood vessels in a damaged heart); in cancer patients, the technique measures the effects of anti-angiogenesis treatment (cutting off blood supply to a tumor). Additionally, it is used to follow the migration of injected drug therapies that travel to the abdomen, chest and legs.
Researchers are working to develop ultrasound probes (micro-bubbles that reflect sound from specific molecular targets) to provide molecular information. If this work is successful, ultrasound could become a useful molecular imaging technology.