Positron Emission Tomography (PET) is a method of medical imaging that works by detecting gamma rays produced by the interaction of electrons with positrons after a tracer radionuclide is injected into the body. The tracer radionucleotide used is dependent on the part of the body that is being investigated, in some cases it is modified glucose, in others, it is a receptor ligand. Common uses of PET scans are cardiac studies that measure the oxygenation of the tissues of the heart and the function of the heart muscle, to detect skeletal trauma and bone cancers, kidney imaging to study the renal function and morphology, the study of the lungs to assess suspected pulmonary emboli, and to diagnose and stage various tumors. PET is usually performed with fluorine fluorodeoxyglyucose (FDG) as the tracer to perform functional tests that provide valuable information regarding tumor staging and treatment monitoring. Modern scanners combine PET scans with computed tomography X-rays to produce vivid three-dimensional views of the patient’s pathology.
PET scans are differentiated from other imaging techniques insofar as that they show the functions of specific organs or regions of anatomy. X-Rays and MRIs are very good at showing the structures and the blood flow of the systems that are being investigated, but they do not give any indication of its function. By showing the metabolism of a specific region or how a ligand is taken up by a target tissue, we are able to establish a functional analysis of the region in question.
In recent times, optogenetic techniques have been developed that allow for very specific study of neuronal circuits. The technique employs light activated proteins that allows for control of an individual neuron and allows scientists to determine the exact pathways of activation in targeted brain regions. A modified virus delivers the light sensitive protein to specific areas of the brain and then using a fiber optic cable driven through a cannula, brain activity can be monitored.
Traditionally, in order to assess functional activity an fMRI image was employed, however, this required that the animal being examined be sedated and thereby reducing the neuronal activity. In a method combining optogenetics and PET FDG scanning, Thanos, et al., were able to assess in vivo metabolism in the nucleus accumbens, which is a key part of the reward center in the brain and is involved in drug addiction, in rats. Thanos, et al., used an embedded cation channel that was activated when exposed to blue light at a specific wavelength. Upon activation of the channel, they were able to observe increased metabolism in the regions associated with the nucleus accumbens where they implanted the cation channel. Regions with observed activity included the basal ganglia, and the limbic system. Interestingly, they also showed a decrease of activity in the posterior cingulate gyrus, anterior cingulate gyrus and the secondary motor cortex. These deactivated regions correspond to regions that are deactivated when engaging in a task. This indicates that activation of the nucleus accumbens inhibits the “default mode network.”
In another remarkable study by Marzluff, et al (2012), the neuronal circuitry underlying the crow’s perception of human faces was imaged. The crow has an ability to distiniguish human faces. The exact mechanism of the facial recognition was not known but it was believed that the circuits were similar to human brains. The experiment involved crows captured while wearing a mask, and then while the crows were in captivity, people in another mask cared for them and fed them. After a few weeks the crows were given FDG either by a person wearing a capturing mask, a person wearing a caring mask or in an empty room. The birds were then anesthetized and PET images were made of their brains. The pattern of FDG uptake showed activation of the tectofugal visual pathway and other forebrain areas and a response from the thalamus and the entopallium.
There are many advantages to human use of PET scans with very few drawbacks. Among the advantages are the beautiful functional images produced. The images provide a detailed view of the functions of the system being examined. The procedure is relatively fast, taking about 1 hour for uptake of the tracer material and another hour for the scan to actually be performed. PET is able to distinguish between malignant cancers and benign tumors and when used for neuro-diagnostics is the only method that can determine diseases such as Alzheimer which otherwise is only diagnosable at autopsy. Finally, it is the preferred method of determining cancer remission and recidivism. The disadvantages are relatively few, but include expense as each scan costs between 800 and 4000 dollars which is only reimbursed by insurance in the case of certain malignant cancers. Furthermore, the radiation dose is about double that of a normal x-ray, but this is still a relatively small dose of radiation. A final drawback to PET scans is the relatively high number of false-positives it gives due to various tissues taking up the tracer dye.
As seen from the two experiments described above, using DFG and PET scanning can reveal a lot of functional imagery of the brains of animals. We are now able to visualize the pathways involved in learning and reward behavior which enhances our understanding of the brain and the mind. By creating maps of these pathways, it is hoped that one day we will be able to fix any derangements of these pathways should they appear.
References:
Brookhaven National Laboratory. "Lights, chemistry, action: New method for mapping
brain activity." ScienceDaily, 10 Apr. 2013. Web. 23 Apr. 2013.
Marzluff, John M., and Robert Miyaoka, Satoshi Minoshima, Donald J. Cross. “Brain
imaging reveals neuronal circuitry underlying the crow’s perception of human faces.” PNAS 109.39 (2012): 15912-15917. Online. www.pnas.org/cgi/doi/10.1073/pnas.1206109109
“Positron Emission Tomography.” Wikipedia: The Free Encyclopedia. Wikimedia
Foundation, Inc. 20 April 2013. Web. 22 April 2013.
Thanos, Panayotis K., and, Lisa Robison, Eric J. Nestler, Ronald Kim, Michael
Michaelides, Mary-Kay Lobo, Nora D. Volkow. “Mapping Brain Metabolic Connectivity in Awake Rats with PET and Optogenetic Stimulation.” The Journal of Neuroscience 33.15 (2013): 6343-6349. Online. DOI:10.1523/JNEUROSCI.4997-12.2013
