Background <p>Ascorbic acid, a crucial antioxidant, is transported from blood to cerebrospinal fluid (CSF) via the choroid plexus (ChP). A noninvasive tool to visualize and quantify ascorbic acid transport process in the human brain is currently lacking. We aimed to determine whether 6-deoxy-6-[¹⁸F]fluoro-L-ascorbic acid ([¹⁸F]FAA) PET can visualize and quantify ascorbic acid transport from blood to CSF via the ChP in humans.</p> Methods <p>This prospective secondary analysis evaluated participants between May 2024 and September 2024. Eight participants underwent a 45-minute dynamic brain [¹⁸F]FAA PET/CT scan; 153 participants underwent a static [¹⁸F]FAA PET/CT scan. Tracer accumulation in ChP and CSF were measured in the lateral and fourth ventricles. Univariable and multivariable linear regression models were used to investigate the associated factors of [¹⁸F]FAA PET metrics.</p> Results <p>One hundred sixty-one participants (mean age, 48.0 years ± 15.7 [SD]; 90 women) were evaluated. Dynamic [¹⁸F]FAA PET showed rapid ChP tracer accumulation in the lateral and fourth ventricles, peaking at about 5&#xa0;min post-injection followed by declining, while CSF [¹⁸F]FAA standardized uptake value progressively increased. The area under the curve of the time-activity curves from 0 to 40&#xa0;min was significantly higher in the young group (&lt;60 years, <i>n</i> = 4) than in the elderly group (≥ 60 years, <i>n</i> = 4) in the ChP. Multiple linear regression analyses showed age was independently and negatively associated with [¹⁸F]FAA accumulation in the ChP and CSF.</p> Conclusions <p>[¹⁸F]FAA PET is a non-invasive tool for visualizing and assessing choroid plexus ascorbic acid transport in humans.</p> Clinical trial number <p>(ChiCTR2300076126: <a href="http://www.chictr.org.cn/">http://www.chictr.org.cn/</a>) with the Clinical Trial Registry (2023-09-25).</p>