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Brain Incorporation of ¹¹C-Arachidonic Acid, Blood Volume, and Blood Flow in Healthy Aging: A Study With Partial-Volume Correction

¹ Brain Physiology and Metabolism Section, National Institute on Aging, National Institutes of Health, Bethesda, Maryland
² Postgraduate Specialty School in Nuclear Medicine, University of Pisa Medical School, Pisa, Italy
³ Human Motor Control Section, National Institute on Neurological Diseases and Stroke, National Institutes of Health, Bethesda, Maryland
⁴ Clinical Epilepsy Section, National Institute on Neurological Diseases and Stroke, National Institutes of Health, Bethesda, Maryland
⁵ PET Department, Clinical Center, National Institutes of Health, Bethesda, Maryland

PET with ¹¹C-arachidonic acid (AA) can be used to quantify neural signaling related to phospholipase A₂ (PLA₂). Animal studies suggest reduction in the activity of this signaling system with age. The aim of this study was to evaluate the effect of healthy aging on brain incorporation of ¹¹C-AA, before and after partial-volume correction (PVC). Methods: Absolute measurements of cerebral blood flow (CBF) were obtained in 8 young and 7 old healthy subjects (mean age ± SD, 27 ± 5 y and 65 ± 9 y) with bolus injection of ¹⁵O-water. About 15 min later, dynamic 60-min 3-dimensional scans were acquired after the injection of ¹¹C-AA. Radioactivity frames of ¹¹C-AA were corrected for head motion and registered to magnetic resonance (MR) images. A 3-segment (3S) and a 2-segment (2S) PVC was applied pixel-by-pixel to the activity frames. For the 3S method, the white matter value was estimated using a new automatic method by extrapolating the activity values of pixels with white matter membership > 0.99. Parametric images of the brain incorporation rate of ¹¹C-AA (K*) and cerebral blood volume (V_b), as well as CBF, were generated and regional gray matter values were obtained. Results: Among cortical areas, there were no significant differences (uncorrected P < 0.05) in K* or V_b absolute values between young and old subjects before or after PVC. A significant reduction of CBF was detected in the frontal cortex of the elderly group. After normalization to the global gray average, K*, V_b, and CBF values revealed significant reductions in the frontal lobe of old subjects; none of these differences were significant after PVC. Conclusion: These results confirm previous PET findings that brain function at rest is minimally affected by healthy aging. Proper PVC methodology is of critical importance in accurate quantitative assessment of PET physiologic measures.

Key Words: arachidonic acid • blood volume • aging • partial-volume correction • PET

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