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Comparison of functional MR and H2 15O positron emission tomography in stimulation of the primary visual cortex
M A Kraut1, S Marenco, B J Soher
1Department of Radiology, Johns Hopkins University, Baltimore, Md 21287-2182, USA.
Purpose:
To locate spoiled gradient-echo functional MR signal changes in relation to brain parenchyma.
Methods:
The region of the primary visual cortex was evaluated using functional MR and H2 15O positron emission tomography in each of six male subjects who were being visually stimulated by means of red light-emitting diode flash goggles.
Results:
The positron emission tomography technique demonstrated substantially greater relative signal change with visual stimulation than did the functional MR technique. Furthermore, the functional MR signal changes were concentrated in loci around the periphery of brain parenchyma exhibiting increased radiotracer activity, as opposed to being collocated.
Conclusions:
Signal changes found using functional MR based on gradient-echo techniques reflect primarily phenomena occurring within small veins and underrepresent activity intrinsic to brain parenchyma, thus introducing potential inaccuracies in locating regions of activated brain tissue. Positron emission tomography, however, directly measures changes in metabolically related activity within the parenchyma.
Insights
Gradient-echo functional MRI (fMRI) signal changes in visual cortex studies may not accurately reflect brain parenchyma activity. Positron emission tomography (PET) offers a more direct measure of parenchymal metabolic changes.
Area of Science:
- Neuroimaging
- Functional Magnetic Resonance Imaging (fMRI)
- Positron Emission Tomography (PET)
Background:
- Functional MRI (fMRI) is a common neuroimaging technique used to detect brain activity.
- Gradient-echo (GE) based fMRI is susceptible to signal alterations that may not directly correlate with neuronal activity.
- Accurate localization of brain activity is crucial for understanding neurological function and disease.
Purpose of the Study:
- To investigate the spatial relationship between gradient-echo functional MRI (fMRI) signal changes and brain parenchyma activity.
- To compare the sensitivity and localization accuracy of fMRI with positron emission tomography (PET) in a visual stimulation task.
Main Methods:
- Six healthy male subjects underwent visual stimulation using red light-emitting diode flash goggles.
- Brain activity was assessed using both functional MRI (fMRI) with gradient-echo sequences and H2 15O positron emission tomography (PET).
- The primary visual cortex region was analyzed for signal changes in both imaging modalities.
Main Results:
- Positron emission tomography (PET) demonstrated significantly greater relative signal changes during visual stimulation compared to functional MRI (fMRI).
- fMRI signal changes were predominantly observed in the periphery of brain parenchyma with increased radiotracer uptake, not collocated with it.
- This suggests fMRI signals in this context are localized around, rather than within, the active brain tissue.
Conclusions:
- Gradient-echo fMRI signal changes primarily reflect activity in small veins, potentially misrepresenting the true location of brain parenchyma activation.
- This spatial discrepancy can lead to inaccuracies in identifying activated brain regions using gradient-echo fMRI.
- Positron emission tomography (PET) directly measures metabolic activity within the brain parenchyma, providing a more accurate localization of functional changes.