|
Functional brain-imaging technology includes the following five techniques: functional magnetic resonance imaging (fMRI), positron emission tomography (PET), single photon emission computed tomography (SPECT), magnetoencephalography (MEG), magnetic resonance spectroscopy (MRS), and topographic electroencephalography (TEEG).
These imaging techniques measure the following:
- Glucose metabolism during PET scans, conducted with a form of glucose detectable by the PET scanner
- Regional cerebral blood flow (rCBF) during PET scans, conducted with a detectable form of water or during SPECT with various
tracers
- The blood-oxygen-level-dependent (BOLD) signal that relates both regional cerebral blood flow and regional cerebral blood volume during fMRI scans. (See sidebar on page 7 for more information about the risks of MRI for cochlear implant users.)
When designing studies using these techniques, clinicians and researchers consider the spatial and temporal resolutions associated with the images. Spatial resolutions may be as small as 1–2 mm when using fMRI scans or as great as 8–12 mm when using PET scans conducted with labeled water.
Temporal resolution refers to the length of time a stimulus presentation is needed to sample cortical responses to a task. Image acquisition may be very quick as in the case of fMRI (less than 100 milliseconds), or long—30 minutes—for PET scans of glucose metabolism.
The test environment also is an important factor. PET and fMRI scans typically are run in scanners, and fMRI scans produce very loud noises and require heavy acoustic shielding. SPECT scans, on the other hand, may be run in a quiet, acoustically controlled environment.
For studies with individuals who use cochlear implants, fMRI is not an option. Therefore, the spatial and temporal resolution of SPECT or PET most influence study-design issues. SPECT provides the greatest control of the study environment but limits investigators to only one study (experimental condition) per day. PET scans with labeled water provide the greatest experimental flexibility in that six or more study conditions (cognitive conditions) can be examined in one session in the scanner. The temporal resolution is similar for both techniques, about 60 sec for rCBF with PET, and about 30 sec for rCBF with SPECT. Also, the spatial resolution is similar, though generally a little better for SPECT—about 6–8 mm for SPECT and 8–12 mm for PET.
Functional brain-imaging studies require a contrast between cortical function measured under at least two test conditions. The conditions may be simple—eyes opened or eyes closed—or may be more complex manipulations that tease apart the subtleties of auditory perceptions. For example, many of our studies require a contrast between watching and listening to a recording of a person reading a story versus only watching a person reading a story. The listening portion of the test may be varied to present signals to two ears, one ear, or to present manipulated (filtered, etc.) sounds in some fashion.
The colored blobs typically observed in functional brain images may represent simple mathematical operations (e.g., subtracting images acquired in two conditions to obtain a difference score) or more sophisticated statistical constructs regulating the power or reliability of the observations by setting the probability levels and size of the effects.
PET and SPECT studies also involve the injection of a radioisotope that is taken up by the brain fairly rapidly, such as SPECT scans integrating activities over 20 seconds in length, or more slowly, such as PET scans with labeled water integrating activities over two minutes. The physical half-life of the radioisotopes also varies from fairly long (>6 hours) or relatively short (<1 minute). Thus, the temporal and spatial resolution of neuroimaging studies is based on the careful control of the stimulus to be studied, how an individual responds to the stimulus, the statistical manipulation of the data, and the test environment.
|