The brain is the most complex organ in the human body, both in terms of its structure and its chemical composition.
The brain is largely made up of two cell types: neurons and glial cells. Neurons are the basic working unit of the brain and nervous system. They are highly specialized for the function of conducting messages. Glial cells support the various processes involved in moving messages between neurons.
The constant exchange of messages between neurons is called neurotransmission. Some neurons send messages in the form of a chemical signal (called a neurotransmitter) and others send an electrical signal. A neural circuit is formed by many neurons working together and multiple circuits form regions of the brain.
Neurotransmission involves these basic steps:
When this process does not work properly, mental illness can occur. Many brain disorders, such as Parkinson’s disease and depression, are linked to a disruption in brain circuitry. For example, depression is associated with either an excess or a shortage of neurotransmitters, such as serotonin, noradrenaline and glutamate.
When electrical signals in the areas of the brain that control movement are abnormal, tremors may occur. Surges in electrical signals inside the brain may cause recurring seizures, or epilepsy.
Brain cells deprived of oxygenated blood during a stroke die, causing permanent damage. In Alzheimer’s disease and other types of dementia, large numbers of neurons stop functioning, lose connections with other neurons and die.
Both malignant and benign tumors also affect the brain, destroying brain cells and causing pressure on other parts of the brain and increased pressure within the skull.
Researchers are using molecular imaging to gain a better understanding of the brain and to develop treatments for:
Molecular imaging technologies play an important role in neuroimaging because they provide a ‘window’ into the living brain. Where CT and conventional MR imaging offer structural information on the brain, technologies such as PET and SPECT allow scientists to visualize brain function and to measure its chemical processes. Abnormal brain function is often detected by molecular imaging before the structural changes resulting from brain cell death can be seen on CT or MRI.
By measuring blood flow and cellular activity such as metabolism, PET and SPECT scanning are useful for: detecting the early onset of neurological disorders pinpointing areas of the brain causing epileptic seizures assisting in the diagnosis of stroke and tumors.
For example, brain cells affected by dementia are less active and have lower glucose metabolic rates than normal cells. Areas of decreased glucose metabolism may also indicate the area giving rise to seizures. Decreased oxygen use and blood flow may indicate a stroke; abnormal patterns of glucose metabolism and an accumulation of amino acid may signal the presence of a brain tumor.
Molecular imaging has become part of standard care for brain cancer, assisting physicians in staging, determining the extent of the disease, and in detecting recurrences.
One of the most active uses of neuroimaging is in the study of brain circuitry, the interconnected brain cells or neurons that constantly exchange messages in a process called neurotransmission. Many mental illnesses and other brain disorders are linked to a disruption in brain circuitry. Understanding how neurotransmitters, synapses and circuits are altered in people with brain disorders is critical to developing new treatments and prevention strategies.
Another active area of research is the use of molecular imaging for Alzheimer’s disease (AD). New radiotracers are being developed that offer the potential to detect the AD in its earliest stages
Imaging the build-up of beta-amyloid plaques in the living brain was a major breakthrough in dementia research. Researchers found that PET scanning with a radiotracer called carbon-11-labeled Pittsburgh Compound B (C-11 PIB) was able to image the accumulation of beta-amyloid plaque, which is associated with Alzheimer’s disease. Studies with PET and C-11 PIB have showed that adults with AD take up more PIB in the brain than do cognitively healthy older people.
While PIB continues to be used in research institutions across the country, other similar PET radiotracers are under development. These radiotracers have a longer half-life than PIB, which will facilitate their use in more locations across the country.
Early detection of AD through molecular imaging techniques will assist the development and evaluation of medications to slow the progression of the disease and give patients access to therapies that are more effective in the beginning stages of the disease.
While molecular imaging technologies such as beta-amyloid imaging with PET are currently only used as research tools, they may soon help physicians to:
A major research initiative called the National Institute on Aging’s Alzheimer’s Disease Neuroimaging Initiative (ADNI), followed hundreds of cognitively healthy individuals and others with MCI and early AD over at least five years. Participants underwent annual MRI and PET scans to allow researchers to assess changes in both the normal aging brain and in individuals with MCI and AD. By correlating images and other test results from the study’s participants, such as cognitive evaluations and fluid and urine samples, researchers hope to identify valuable biomarkers of the disease process. Data from the ADNI is now available to researchers worldwide.