Tuesday, August 25, 2009

One technique of imaging neuroanatomy is computerized tomography (CT) scan (or computerized axial tomography (CAT)) scan. It employs a narrow beam (an axis) of X rays that is aimed through the patient’s head and directed a detector on the opposite side. The beam moves in a slow circular arc and the detector moves along with it. Since difference brain tissues vary in density, they block the X rays to different degrees. A computer eventually constructs a composite picture, based on X ray views from different angles.

The most widely used neuroimaging technique is magnetic resonance imaging (MRI). MRI scans are safer because they use no X rays, relying instead upon a physical principle called nuclear magnetic resonance. This method exploits the fact that nuclei at centre of atoms have different resonant frequencies. If you perturb them, they sing like tuning forks as they bounce back to the normal. Because different parts of the brain have different chemical structures, each part sings differently. An MRI scan disturbs these atoms, by passing on a very high frequency alternating magnetic field through the brain, by means of electromagnets surrounding the patient’s head. As the magnetic field fluctuates, the chorus of atomic voices is detected by magnetic sensors within the scanner and the computer then assembles the data to produce a magnificently detained picture of the brain. The Nobel Prize in Medicine for 2003 went to two researchers for developing MRI. Names aside, as again another prominent contributor to the MRI research was sidelined.

CT and MRI scans typically allow anatomical depictions: they can scan brain structures but not whether these structures are active or are participating in a behavior. To reveal function, the experimenter uses several techniques to measure brain physiology. The earliest developed was electroencephalogram (EEG) that detects tiny electrical currents generated by neurons on the surface of the brain and only requires placing tiny electrodes on the top and sides of the head. The EEG’s reactions to a repeated stimulus can also be averaged by yielding event-related potentials (also called evoked potentials). EEGs find their greatest use in diagnosing epilepsy and sleep disorders; event-related potentials are useful in detecting intactness of our sensory and neuromuscular pathways.

For a time, it seems that investigators could view anatomy in depth (with CT or MRI) but they could only do so on the brain surface. The breakthrough that allowed in-depth 3D localization of brain function was positron emission tomography (PET). In a PET scan, the participant is injected with a safe dose of radioactive sugar that resembles glucose (the only metabolic fuel the brain can use) but which emits subatomic particles called positrons. Brain cells that are particularly active at any time will take up more of this substance and thus give off more positrons, which are then detected and from which a computer image is assembled (much like CT or MRI scans).

But since the radioactive element must be injected, a given PET scan session cannot last too long, lest the patient receive too much radiation. Moreover, the radioactive material takes time to be absorbed by active brain cells and to wash out later. This means that the PET scan images change too slowly, to capture fact-changing brain events. Both these problems are addressed in functional MRI. In an fMRI examination, you will perform a particular task during the imaging process, causing increased metabolic activity in the area of the brain responsible for the task. This activity, which includes expanding blood vessels, chemical changes and the delivery of extra oxygen, can then be recorded on MRI images. Blood-oxygen-level dependent (BOLD) activity, as it is called, accompanies increased neural activity.

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