Assignment: Brain Imaging Techniques

Assignment: Brain Imaging Techniques

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Assignment: Brain Imaging Techniques

Positron Emission Tomography

Many investigators use positron emission tomography (PET) scans in their studies of brain func- tion because the images produced by PET are much more detailed and, hence, provide more infor- mation than do SPECT scans. In the PET technique, the subject is injected with a radioactive isotope. A number of radioisotopes, including oxygen and carbon, are used in PET scans, but fluo- rine is often the most clinically relevant (Lee et al., 2011). Remember that blood flow in the brain is increased in those regions that are most active. This means that active regions contain the most radioactive isotopes and release the most radiation. A detection device is placed around the sub- ject’s head while the subject is performing a task, such as looking at a word or pointing to a stimu- lus. As the subject participates in the task, certain brain areas become active, and blood flow increases in those areas. Positrons that are emitted from the radioactive isotopes collide with electrons and are annihilated, producing two photons (or gamma rays) that go off in opposite directions and are measured by the detection device. The gamma ray detection device sends the information to a computer, which produces detailed images of the areas of activity in the brain. Studies that use PET technology are producing a wealth of information about how the brain works. More recently, the PET technique has been modified to allow for the study of specific chemicals in the brain (Torigian et al., 2013).

We will examine many more applications of PET imaging in behavioral research in later chapters of this book. The information to be gained from PET studies is important, but unfortunately it is an extremely costly technique. For example, the radioactive isotopes used in PET are very expensive to produce. Very few radioactive isotopes release positrons; most release photons, which makes SPECT a cheaper technique to use. Also, using these isotopes puts the subject and experimenter at considerable health risk, and their use is limited by federal guidelines, which does not make repeated trials on the same subject feasible. PET scans are better at localizing brain functions than are SPECT scans because two photons are produced with each positron emitted, making localiza- tion more precise. However, like SPECT, PET cannot accurately record the time course of many cognitive activities. It takes minutes to make a PET image, and most cognitive functions occur in less than a second.

Functional Magnetic Resonance Imaging

Developed in 1990 by Seiji Ogawa and his colleagues at Bell Laboratories, functional magnetic resonance imaging (fMRI) is a measurement technique that is based on conventional magnetic resonance imaging (MRI) technology (Song, 2012). Whereas MRI is used to produce detailed, static images of the brain, fMRI permits measurement of blood flow through a brain region, which is an indicator of activity in that region. The fMRI technique is designed to detect the differences between oxygenated and deoxygenated blood, based on the fact that hemoglobin carrying oxy- gen has different magnetic properties than deoxygenated hemoglobin. The strange thing about neurons is that they increase their glucose consumption when active, but not their oxygen con- sumption. This means that when blood flow through an active brain region increases, oxygenated hemoglobin builds up in the blood vessels. Functional MRI detects this increase in blood oxygen and thus is able to pinpoint active brain regions.

Photo 1.11 is an fMRI image of the brain of a 32-year-old woman after a massive stroke; the image shows the amount of blood flow received by areas of the brain. As you can see, the images pro- duced by fMRI are as detailed as PET scans, and fMRI has many advantages over PET. For example, the fMRI technique is noninvasive and does not require administration of radioactive chemicals,

Simon Fraser/Science Source

Photo 1.11 This is the brain scan of a 32- year-old woman after she had a stroke. The green and blue areas are receiving normal blood flow, while yellow, red, and black are receiving abnormal blood flow.

wiL81028_01_c01_001-030.indd 20 7/10/13 12:24 PM

CHAPTER 1Section 1.6 Brain Imaging Techniques

which means that subjects can be tested repeatedly without risk of exposing the subjects to radiation. Functional MRI is also a less expensive technique to use than PET. Moreover, fMRI has a time lag of about 1 second (Stehling, Turner, & Mansfield, 1991), which is far superior to that of PET. EEG and MEG are capa- ble of recording brain activity within milliseconds of its occurrence and, hence, provide a more accurate measure of the time course of brain activity than does fMRI. However, fMRI is much better for localizing a specific function in the brain than are EEG or MEG.

One concern when performing fMRI studies is whether or not to compare scanners of different strength levels (Glover et al., 2012). For fMRI experiments, the sub- ject’s entire body must be placed into the scanner, which is shaped like a narrow tube. As a result, some subjects become claustrophobic and uncomfortable during fMRI studies. Any movement by the subject destroys the image being produced, so the subject must lie very still, which increases the subject’s dis- comfort and renders impossible the study of behaviors involving movement of the head, such as speaking. The type of study conducted in the fMRI scanner is also limited by the high magnetic field in the scanner. For example, the instruments used to present stimuli to subjects in PET studies cannot be used in the magnetic environment of the fMRI scanner.

Table 1.2: Comparison of brain recording and brain imaging techniques

Technique Benefits Drawbacks

EEG Noninvasive; relatively low cost of equipment; accurately records brain activity within milliseconds

Difficult to localize exact source of electrical activity; some distortion as electrical currents pass through skull

MEG Noninvasive, no distortion as magnetic fields pass through bone; accurately records brain activity within milliseconds

Expensive equipment; does not allow precise localization of brain activity; cannot pick up deep signals in the brain

SPECT Better than EEG or MEG in localizing brain activity; cheaper than PET imaging

Requires administration of a radio- isotope; time lag > 20 seconds; can- not be used in studies of cognition

PET Better localization of brain activity than SPECT; can be used to localize specific neurotransmitter receptors in the brain

Assignment: Brain Imaging Techniques

Extremely expensive radioisotopes required; some health risk associ- ated with radioisotopes; time lag > 1 min

fMRI Noninvasive; precise localization of brain activity; less expensive than PET; time lag < 1 second, better than PET

Time lag does not permit study of cognitive processes, subject must remain very still during imaging

Positron Emission Tomography

Many investigators use positron emission tomography (PET) scans in their studies of brain func- tion because the images produced by PET are much more detailed and, hence, provide more infor- mation than do SPECT scans. In the PET technique, the subject is injected with a radioactive isotope. A number of radioisotopes, including oxygen and carbon, are used in PET scans, but fluo- rine is often the most clinically relevant (Lee et al., 2011). Remember that blood flow in the brain is increased in those regions that are most active. This means that active regions contain the most radioactive isotopes and release the most radiation. A detection device is placed around the sub- ject’s head while the subject is performing a task, such as looking at a word or pointing to a stimu- lus. As the subject participates in the task, certain brain areas become active, and blood flow increases in those areas. Positrons that are emitted from the radioactive isotopes collide with electrons and are annihilated, producing two photons (or gamma rays) that go off in opposite directions and are measured by the detection device. The gamma ray detection device sends the information to a computer, which produces detailed images of the areas of activity in the brain. Studies that use PET technology are producing a wealth of information about how the brain works. More recently, the PET technique has been modified to allow for the study of specific chemicals in the brain (Torigian et al., 2013).

Assignment: Brain Imaging Techniques

We will examine many more applications of PET imaging in behavioral research in later chapters of this book. The information to be gained from PET studies is important, but unfortunately it is an extremely costly technique. For example, the radioactive isotopes used in PET are very expensive to produce. Very few radioactive isotopes release positrons; most release photons, which makes SPECT a cheaper technique to use. Also, using these isotopes puts the subject and experimenter at considerable health risk, and their use is limited by federal guidelines, which does not make repeated trials on the same subject feasible. PET scans are better at localizing brain functions than are SPECT scans because two photons are produced with each positron emitted, making localiza- tion more precise. However, like SPECT, PET cannot accurately record the time course of many cognitive activities. It takes minutes to make a PET image, and most cognitive functions occur in less than a second.

Functional Magnetic Resonance Imaging

Developed in 1990 by Seiji Ogawa and his colleagues at Bell Laboratories, functional magnetic resonance imaging (fMRI) is a measurement technique that is based on conventional magnetic resonance imaging (MRI) technology (Song, 2012). Whereas MRI is used to produce detailed, static images of the brain, fMRI permits measurement of blood flow through a brain region, which is an indicator of activity in that region. The fMRI technique is designed to detect the differences between oxygenated and deoxygenated blood, based on the fact that hemoglobin carrying oxy- gen has different magnetic properties than deoxygenated hemoglobin. The strange thing about neurons is that they increase their glucose consumption when active, but not their oxygen con- sumption. This means that when blood flow through an active brain region increases, oxygenated hemoglobin builds up in the blood vessels. Functional MRI detects this increase in blood oxygen and thus is able to pinpoint active brain regions.

Photo 1.11 is an fMRI image of the brain of a 32-year-old woman after a massive stroke; the image shows the amount of blood flow received by areas of the brain. As you can see, the images pro- duced by fMRI are as detailed as PET scans, and fMRI has many advantages over PET. For example, the fMRI technique is noninvasive and does not require administration of radioactive chemicals,

Simon Fraser/Science Source

Photo 1.11 This is the brain scan of a 32- year-old woman after she had a stroke. The green and blue areas are receiving normal blood flow, while yellow, red, and black are receiving abnormal blood flow.

wiL81028_01_c01_001-030.indd 21 7/10/13 12:24 PM