Alcoholism and the Brain : Brain Damage, Part 2

by National Institute of Health

(Page 4 of 5)

Imaging of Brain Structure. With neuroimaging techniques such as computerized tomography and magnetic resonance imaging (MRI), which allow brain structures to be viewed inside the skull, researchers can study brain anatomy in living patients. CT scans rely on x-ray beams passing through different types of tissue in the body at different angles. Pictures of the "inner structure" of the brain are based on computerized reconstruction of the paths and relative strength of the x-ray beams. CT scans of alcoholics have revealed diffuse atrophy of brain tissue, with the frontal lobes showing the earliest and most extensive shrinkage.

MRI techniques have greatly influenced the field of brain imaging because they allow noninvasive measurement of both the anatomy (using structural MRI) and the functioning (using functional magnetic resonance imaging [fMRI], described below) of the brain with great precision. Structural MRI scans are based on the observation that the protons derived from hydrogen atoms, which are richly represented in the body because of its high water content, can be aligned by a magnetic field like small compass needles. When pulses are emitted at a particular frequency, the protons briefly switch their alignment and "relax" back into their original state at slightly different times in different types of tissue. The signals they emit are detected by the scanner and converted into highly precise images of the tissue. MRI methods have confirmed and extended findings from post mortem and CT scan studies - namely, that chronic use of alcohol results in brain shrinkage. This shrinkage is most marked in the frontal regions and especially in older alcoholics. Other brain regions, including portions of the limbic system and the cerebellum, also are vulnerable to shrinkage.

Imaging of Brain Function: Hemodynamic Methods. Hemodynamic methods create images by tracking changes in blood flow, blood volume, blood oxygenation, and energy metabolism that occur in the brain in response to neural activity. PET and SPECT are used to map increased energy consumption by the specific brain regions that are engaged as a patient performs a task. One example of this mapping involves glucose, the main energy source for the brain. When a dose of a radioactively labeled glucose (a form of glucose that is absorbed normally but cannot be fully metabolized, thus remaining "trapped" in a cell) is injected into the bloodstream of a patient performing a memory task, those brain areas that accumulate more glucose will be implicated in memory functions. Indeed, PET and SPECT studies have confirmed and extended earlier findings that the prefrontal regions are particularly susceptible to decreased metabolism in alcoholic patients. It is important to keep in mind, however, that frontal brain systems are connected to other regions of the brain, and frontal abnormalities may therefore reflect pathology elsewhere.

Even though using low doses of radioactive substances that decay quickly minimizes the risks of radiation exposure, newer and safer methods have emerged, such as MRI methods. MRI is noninvasive, involves no radioactive risks, and provides both anatomical and functional information with high precision. The fMRI method is sensitive to metabolic changes in the parts of the brain that are activated during a particular task. A local increase in metabolic rate results in an increased delivery of blood and increased oxygenation of the region participating in a task. The blood oxygenation level-dependent (BOLD) effect is the basis of the fMRI signal. Like PET and SPECT, fMRI permits observing the brain "in action," as a person performs cognitive tasks or experiences emotions.

In addition to obtaining structural and functional information about the brain, MRI methodology has been used for other specialized investigations of the effects of alcohol on the brain. For example, structural MRI can clearly delineate gray matter from white matter but cannot detect damage to individual nerve fibers forming the white matter. By tracking the diffusion of water molecules along neuronal fibers, an MRI technique known as diffusion tensor imaging (DTI) can provide information about orientations and integrity of nerve pathways, confirming earlier findings from post mortem studies which suggested that heavy drinking disrupts the microstructure of nerve fibers. Moreover, the findings correlate with behavioral tests of attention and memory. These nerve pathways are critically important because thoughts and goal-oriented behavior depend on the concerted activity of many brain areas.

Another type of MRI application, magnetic resonance spectroscopy imaging (MRSI), provides information about the neurochemistry of the living brain. MRSI can evaluate neuronal health and degeneration and can detect the presence and distribution of alcohol, certain metabolites, and neurotransmitters.

Imaging of Brain Function: Electromagnetic Methods. In spite of their excellent spatial resolution - that is, the ability to show precisely where the activation changes are occurring in the brain - hemodynamic methods such as PET, SPECT, and fMRI have limitations in showing the time sequence of these changes. Activation maps can reveal brain areas involved in a particular task, but they cannot show exactly when these areas made their respective contributions. This is because they measure hemodynamic changes (blood flow and oxygenation), indicating the neuronal activation only indirectly and with a lag of more than a second. Yet, it is important to understand the order and timing of thoughts, feelings, and behaviors, as well as the contributions of different brain areas.

The only methods capable of online detection of the electrical currents in neuronal activity are electromagnetic methods such electroencephalography (EEG), event-related brain potentials (ERP), and magnetoencephalography (MEG). (The ERP method is considered derived from electroencephalography.) EEG reflects electrical activity measured by small electrodes attached to the scalp. Event-related potentials are obtained by averaging EEG voltage changes that are time-locked to the presentation of a stimulus such as a tone, image, or word. MEG uses sensors in a machine that resembles a large hair dryer to measure magnetic fields generated by brain electrical activity. These techniques are harmless and give us insight into the dynamic moment-to-moment changes in electrical activity of the brain. They show when the critical changes are occurring, but their spatial resolution is ambiguous and limited.

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