Brain, PET scan

These four PET scans show how blood flow to different parts of the brain is affected by various activities. In this case, an oxygen isotope that is taken up by the hemoglobin in blood is used as a source of positrons. 

This patient is about to undergo a PET scan of brain function. 

A PET scan requires a substance called a tracer. A suitable tracer must accumulate in the target organ, and it must be modified to contain unstable radioactive atoms that emit positrons. Glucose is used for brain imaging, because the brain processes glucose as the fuel for mental and neural activities. A common tracer for PET brain scans is glucose modified to contain radioactive fluorine atoms. Our molecular inset shows a simplified model of this modified glucose molecule. 

In preparation for a PET scan, a patient is injected with a dose of the tracer, which quickly accumulates in the brain. The patient is placed inside the PET scanner, and the instrument detects the gamma rays emitted by the tracer. The result is an image showing the distribution of glucose in the brain, which indicates where brain activity is greatest. 

In the first of these studies, the researchers identified the areas of the brain that are associated with normal sadness. They asked volunteer subjects to think about some very sad personal experiences - and about some emotionally neutral experiences - while their brains were being imaged in a PET scanner. When thinking about the sad experiences, the volunteers reported feeling intense sadness, and many of them became tearful. The PET scans showed the changes in brain activity that accompanied these sad feelings. They demonstrated increased blood flow in the limbic system - a part of the brain that is involved in the control of emotion - and decreased blood flow in parts of the brain that are involved in the control of attention. 

Fig,1, An [ F]FDG-PET scan depicting glucose metabolism in a monozygotic twin pairand in a control subject. Note the clear reduction in [ F]FDG uptake especially in temporoparietal areas (thick arrows) in the cotwin having AD (on the left), and similar, but less pronounced reduction in same brain areas (thin arrows) in the cognitively healthy cotwin (in the middle) as compared with the uptake in a control subject (on the right), (See Colour Plate Section at the end of this book,)... 

How, we wonder, would a PET scan of Aldous Huxley s brain in deep reflection compare with the images collected in outwardly attentive waking, deep sleep, and that most easily obtained altered state of consciousness, REM sleep dreaming My guess is that it would look more like REM than deep sleep or waking. 

Long before PET scan studies revealed the dramatic shift in cortico-limbic balance (in favor of the limbic system in REM), affective neuroscientists like Richard Davidson had proposed just such shifts to account for data describing differences in the experience and expression of positive and negative affects. Some people are optimistic and outgoing, whereas others are pessimistic and introverted. Davidson s work has begun to explain these differences in terms of differential brain activation. 

There has been considerable controversy over the dangerousness of ecstasy. Some researchers, based on animal studies or positron emission tomography (PET) scans of users brains, believe the drug causes a destruction of the brain cells responsible for producing serotonin, an important neurotransmitter. However, it is unclear how lasting or dangerous these effects are in humans. Ecstasy can also cause severe dehydration and unpredictable cardiovascular effects. 

A 1998 study using data from positron emission tomography or PET scans on humans also demonstrated that ketamine stimulates the release of dopamine, the brain s pleasure chemical. Most drugs of abuse spur the forced release of dopamine, reinforcing pleasurable associations in the user. 

In 1993, Zametkin and colleagues published a follow-up study in which they performed PET scanning to measure glucose metabolism in the brains of ADHD and non-ADHD adolescents. This time, they found no difference between the brains of the ADHD adolescents and the non-ADHD adolescents. 

Even Zametkin has admitted that PET scanning cannot tell whether a person is ADHD. One commonly asked question of our brain imaging studies, he has stated, is whether PET scanning can be used to diagnose ADHD. Unfortunately, this is not currently possible because there is considerable overlap in our study between normal and ADHD brain metabolism. 13... 

In summary, no reliable scientific evidence for the biological cause of ADHD has been found to date. Zametkin, the researcher behind the PET scan studies on the brain, agrees ... 

On a physiological level, Ritalin causes a 23 to 30% decrease of the overall blood flow in all areas of the human brain, as measured by a PET scan. This reduction is believed to be caused by a constriction of the blood vessels, probably... 

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