Functional Imaging of Craving : Part 2

by National Institute of Health

(Page 2 of 6)

Single Photon Emission Computed Tomography

SPECT is the oldest functional imaging technique and generally provides images with the lowest resolution, although newer SPECT equipment provides images with significantly greater detail. With this approach, the subject is injected with a weakly radioactive tracer - usually 99mtechnetium-hexamethylpropylene- amineoxide - to measure blood flow. This technique allows the investigator to visualize the blood flow distribution within the brain during the first 1 or 2 minutes after 99mTc-HMPAO injection. The major advantage of this approach is that it is more widely available and much less expensive than other imaging techniques, such as PET.

SPECT also has substantial disadvantages, however. The biggest disadvantage is that because 99mTc-HMPAO decays slowly (has a long half-life), the two images needed to produce a difference image must be recorded several days apart, leading to two practical problems. First, each time a subject is placed in the machine that measures the radioactivity and generates the image, his or her positioning will be slightly different. This inconsistency complicates the generation of an accurate difference image, because the two original images must be aligned so that their size, shape, and orientation match perfectly. Second, mental states and the resulting brain states are likely to vary more when they are recorded a few days apart than when they are recorded a few minutes or hours apart. As a result of the long separation between scans required by SPECT, difference images can reflect changes in brain states merely associated with the passage of time rather than specifically with the mental state under investigation.

Positron Emission Tomography

For PET, subjects are injected with molecules that naturally occur in the tissues, such as water (H2O) and glucose, and which have been tagged with a radioactive variant (isotope) of one of their atoms. These radioactive variants are taken up into the tissues just like the normal molecules, and their distribution can be measured using a scanner. The most commonly employed PET technique measures blood flow using radioactively labeled H2O. Each PET image represents the blood flow during the first 1 or 2 minutes following 15O administration. This approach offers several advantages over SPECT. For example, because 15O has a half-life of only a few minutes, six to eight images can be produced during one imaging session. This feature reduces imaging problems that result from the subject's movement, incorrect alignment of the images, and day-to-day variations in mental state.

Another tracer used in PET imaging is radioactively labeled glucose. This compound, which is taken up by cells that are in the process of generating energy from glucose breakdown (metabolically active cells), is then trapped within the cells so that its location can be imaged. In contrast to 15O PET's short measurement period, FDG PET measures neuronal glucose use during a period of 20 to 30 minutes. This extended measurement allows the generation of more accurate images compared with 15O PET. A disadvantage of the prolonged measurement period needed to generate one image, however, is that FDG PET allows the generation of only one pair of images, and thus of one difference image, per day.

A major advantage of PET over SPECT is that it generates images with a greater resolution: The decay of each radioactive isotope used in PET eventually generates two photons that can be detected by a special camera. Conversely, the decay of each tracer molecule used in SPECT generates only one photon, thereby allowing less precise localization of the tracer molecules.

PET also is associated with two major disadvantages, however. First, because the 15O and 18F tracers have such short half-lives, they must be generated immediately before use - a labor-intensive and expensive process. Thus, a machine to generate these tracers (a cyclotron) must be located on-site at every PET facility. Second, because researchers can produce only a few images per day for each subject, statistical analyses combining data from several subjects are necessary to generate reliable results. Images produced from a group of subjects, however, have considerably lower resolution than images produced from a single subject, because human brain anatomy varies slightly from person to person.

Functional Magnetic Resonance Imaging

fMRI is the newest functional imaging technique. Like normal MRI, this approach does not use radioactive tracers, but exposes the subject to radio waves in the presence of a strong magnetic field. This treatment causes the nuclei of certain atoms in the brain to emit signals that can be detected by a scanner. The most commonly used form of fMRI analyzes the distribution of hemoglobin, the molecule in red blood cells that helps transport oxygen to the tissues. Specifically, this approach determines the ratio of hemoglobin that is still carrying oxygen (oxygenated hemoglobin) to hemoglobin that has already delivered its oxygen to the tissues (deoxygenated hemoglobin).

This fMRI approach is based on the fact that oxygenated and deoxygenated hemoglobin differ slightly in their magnetic properties, resulting in the emission of a signal that can be detected by special scanners. These scanners are so sensitive that they can detect signals from a volume (voxel) of brain tissue as small as a few millimeters on each side. The signal, which is proportional to the ratio of the concentration of oxygenated to deoxygenated hemoglobin inside the voxel, is called the blood oxygen level-dependent (BOLD) signal.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.