Modern neuroimaging advances in methodology and data modelling allow for brain tissue structure and function to be imaged in increasing detail and specificity. These advances have promise when applied to polycystic ovary syndrome (PCOS), a common multisystem condition associated with psycho-cognitive symptomatology and an increased risk of cerebrovascular disease. This hot-topic review will outline three biological themes that encompass PCOS neurobiology and symptomatology and may benefit from the application of advanced neuroimaging techniques: i) the endocrine system and biorhythms, ii) the cerebrovascular system, and iii) neural structure and function. How modern neuroimaging advances can provide new insights to these areas will be discussed. While knowledge of brain health in PCOS is still an emerging field, advanced neuroimaging offers valuable opportunities to investigate the origins and enduring consequences of PCOS.

Abstract To fully profile how ovarian hormones interact with cerebrovascular function, it is vital to consider, not just resting physiology, but also dynamic aspects of cerebrovasculature that support neural activity. This study uses hypercapnic cerebrovascular reactivity (CVR) and the visually-evoked haemodynamic response function (HRF) to investigate the influence of menstrual-related changes in oestradiol and progesterone on dynamic aspects of the cerebrovascular system. 20 menstruating females (age mean[SD]=23.01[4.01]years) completed a 3T MRI scanning session during the early follicular, late follicular, and mid-luteal phases of their menstrual cycle. Circulating hormones were measured via blood samples. Simultaneous blood oxygen level dependant (BOLD)-CVR and cerebral blood flow (CBF)-CVR data were collected using a pseudocontinuous arterial spin labelling (pCASL) acquisition using a dual-excitation (DEXI) readout during periods of hypercapnia (5% CO 2 ). The HRF was estimated using a whole brain EPI scan during high-contrast radial checkerboard presentation. Both oestradiol and additional progesterone variance were associated with increased CVR (both BOLD-CVR and CBF-CVR; p<0.001) and altered HRF shape (p<0.005). No statistically significant regional effects were found. A secondary experiment investigated the impact of using either canonical or individually mapped HRF in a standard fMRI processing pipeline; namely, population receptive field (pRF) mapping. Results across phases suggest that neither hormone was associated with pRF size when modelled using a canonical HRF (both p>0.05). However, a significant neuroendocrine influence on pRF sizes was discovered when using individually measured HRFs (p<0.001). This study found evidence that dynamic cerebrovascular functions are sensitive to menstrual-related ovarian hormones, which may be a potential mechanism underlying menstrual symptomatology and has implications for fMRI studies that assume intact neurovascular coupling processes in women, regardless of menstrual staging, to make inferences about neural activity.

The ovarian hormones, oestrogen and progesterone, have vaso- and neuroprotective effects, potentially due to cerebrovascular interactions. This study investigates their neuroendocrine influence on cerebral and retinal baseline vascular metrics across a healthy menstrual cycle. Twenty-six menstruating females completed imaging sessions and assessment of circulating hormone levels during their early follicular, late follicular and mid-luteal phase. Perfusion, arterial arrival time (AAT), global oxygen extraction fraction (OEF), cerebrovascular metabolic rate of oxygen (CMRO2), carotid artery radius and carotid pulsatility index (PI) were measured using 3 T MRI. Retinal vessel density and blood flow resistance were assessed with optical coherence tomography angiography (OCT-A). Assessed with linear models, increased oestradiol was related to increased global perfusion (χ2 (1) = 91.623; p = 1.049 × 10-21) and decreased AAT (χ2 (1) = 106.950; p = 4.575 × 10-25). An independent progesterone increase was also associated with increased global perfusion (χ2 (1) = 19.512; p = 9.998 × 10-6) and decreased AAT (χ2 (1) = 40.062; p = 2.46 × 10-10). A relationship was also found between increased oestradiol and decreased retinal blood flow resistance (χ2 (1) = 5.28; p = 0.0215), primarily driven by centrally localised vessels. This study finds that circulating oestrogen increases blood flow in the eye and brain, while progesterone is associated with brain perfusion increases alone. Both are associated with decreased cortical blood arrival speed. These effects suggest a potential pathway for neuroprotective mechanisms.

OBJECTIVE:Polycystic ovary syndrome (PCOS) is associated with an increased risk of cerebrovascular disease, but the effects on cerebrovascular function are unknown. In this pilot study, we sought to compare cerebrovascular perfusion, pulsatility, reactivity and metabolism between women with PCOS and healthy volunteers using MRI, and investigated the influence of testosterone and insulin resistance on these parameters. DESIGN:Case-control pilot study. PATIENTS:Fifteen patients with PCOS (age: 32.0 ± 7.4 years; body mass index [BMI]: 31.8 ± 5.7 kg/m2) and 12 healthy controls (HC) (age: 30.7 ± 6.4 years; BMI: 30.2 ± 5.8 kg/m2). MEASUREMENTS:We used 3T magnetic resonance imaging (MRI) to assess several aspects of cerebrovascular function: (1) perfusion (cerebral blood flow [CBF] and arterial arrival time [AAT]; PCOS N = 15; HC N = 12), (2) pulsatility index (PCOS N = 15; HC N = 12), (3) breath-hold induced cerebrovascular reactivity (CVR; PCOS N = 15; HC N = 10), and (4) global oxygen metabolism (oxygen extraction fraction [OEF] and cerebral metabolic rate of oxygen [CMRO2]; PCOS N = 9; HC N = 8). Linear regression models investigated the contribution of PCOS status, serum testosterone and Homeostatic Model Assessment for Insulin Resistance (HOMA2-IR). Regional analysis underwent false discovery rate (FDR) correction for multiple comparisons. RESULTS:Overall baseline CBF was not statistically different in PCOS patients compared to controls after adjustment for other variables (χ2(1) = 3.29; p = 0.07) but did show evidence for regional reduction with PCOS status in the transverse temporal gyrus (χ2(84) = 110.31; p = 0.03; -29.05 mL/100 g/min ± 7.26 [standard error; SE]). Similarly, while PCOS status was not associated with overall CVR (χ2(1) = 0.78; p = 0.38), there was evidence of a regional interaction (χ2(84) = 154.25; p < 0.001) in the parahippocampus (1.37% signal change ± 0.27 [SE]) and pericalcarine cortex (1.04% signal change ± 0.26 [SE]). Neither testosterone nor HOMA2-IR was associated with any outcome measure. CONCLUSIONS:We observed regional reduction in cerebral blood flow and regional increase in cerebrovascular reactivity in women with PCOS compared to healthy controls. However, due to the limited statistical power and unclear menstrual timing in this pilot study, these results require further replication.

OBJECTIVE:Despite an increased cerebrovascular disease risk, the impact of Polycystic Ovary Syndrome (PCOS) on cerebrovascular haemodynamics and function is unknown. This study characterised cerebrovascular haemodynamics and function in women with PCOS versus healthy controls. DESIGN:Case-control study. PATIENTS:Fifteen women with PCOS (age: 31 ± 6 years; body mass index (BMI): 31.8 ± 5.7 kg/m2) and 16 controls (age: 30 ± 7 years; BMI: 29.9 ± 5.5 kg/m2). MEASUREMENTS:Resting global cerebral blood (CBF) was assessed by 3T MRI. Middle- and posterior cerebral artery blood velocities (MCAv, PCAv) were measured by Doppler ultrasound and pulsatility index (MCAPI, PCAPI) calculated. Neurovascular coupling (NVC), internal carotid artery cerebrovascular reactivity (CVRCO2) and dynamic cerebral autoregulation (dCA) directional sensitivity were assessed using a visual stimulus, 6% fixed-inspired CO2 and repeated squat-stand manoeuvres, respectively. RESULTS:Resting CBF (PCOS: 57.2 ± 7.5 ml/100 g/min; controls: 61.6 ± 11.6 ml/100 g/min, p = 0.25) and MCAv, PCAv, MCAPI and PCAPI (all p > 0.05) were similar between groups. NVC (14 ± 4.9% vs. 13 ± 3.4%, p = 0.45), CVRCO2 (5.1 ± 1.9% vs. 6.5 ± 2.9%, p = 0.20) and dCA directional sensitivity were similar between groups. However, women with PCOS had elevated relative PCAPI during NVC (PCOS: 12.0 ± 5.6% vs. controls: 7.0 ± 3.8%, p = 0.04), and impaired vasodilation of the internal carotid artery during CVRCO2 (PCOS: -0.10 ± 0.22 mm vs. controls: 0.18 ± 0.24 mm, p < 0.01). CONCLUSIONS:Cerebrovascular function is largely preserved in women with PCOS, although elevated arterial pulsatility and impaired vasodilatory response to carbon dioxide may indicate early endothelial dysfunction in the cerebral vasculature. Larger studies are needed to confirm this in view of our limited study power.

The ovarian hormones, oestrogen and progesterone, have vaso- and neuroprotective effects, likely due to interactions with the cerebrovascular system. This study investigates their neuroendocrine influence on a range of cerebral and retinal vascular functions across a healthy menstrual cycle. Twenty-six healthy, menstruating females completed imaging sessions and assessment of circulating hormone levels during their early follicular, late follicular, and mid-luteal phase (1-4, 10-12 and 20-22 days after menses onset). Cerebral blood flow (CBF), arterial arrival time (AAT), global oxygen extraction fraction (OEF), cerebrovascular metabolic rate of oxygen (CMRO2), carotid artery radius and carotid pulsatility index (PI) were measured using 3T MRI. Retinal vessel density and blood flow resistance were assessed with optical coherence tomography angiography (OCT-A). Assessed with linear models, increased oestradiol was related to increased global CBF (Chi2(1)=35.05; p=3.2x10-9) and increased AAT (Chi2(1)=5.87; p=0.015). Increased progesterone was associated with increased global CBF (Chi2(1)=13.00; p=0.0003). In the retina, a relationship was found between oestradiol and decreased retinal blood flow resistance (Chi2(1)=5.28; p=0.0215), which was primarily driven by centrally localised vessels. This study finds that circulating oestrogen increases blood flow in the eye and brain, while progesterone significantly impacts the brain alone. These effects suggest a potential pathway for neuroprotective mechanisms. ### Competing Interest Statement The authors have declared no competing interest.

Gender-affirming hormone replacement therapy (gaHRT) is an important step for many in the gender diverse community, associated with increased quality-of-life and lower self-reported scores of depression and anxiety. However, considering the interactions that the involved sex hormones have on vasculature (with oestrogen and testosterone demonstrating vasodilatory and vasoconstricting properties, respectively), it is important for transgender healthcare research to examine how the manipulation of these hormones interact with cerebrovascular structure and functioning. There is a stark lack of research in this area. This mini-review outlines the research suggesting a vascular impact of these sex hormones using evidence from a range of cohorts (e.g., menopause, polycystic ovary syndrome) and discusses the work that has been done into cerebrovascular changes following gaHRT. Finally, recommendations for future research into cerebrovascular health in transgender cohorts following gaHRT are outlined.

New FindingsWhat is the central question of this study?Gonadal hormones modulate cerebrovascular function while insulin-like growth factor 1 (IGF-1) facilitates exercise-mediated cerebral angiogenesis; puberty is a critical period of neurodevelopment alongside elevated gonadal hormone and IGF-1 activity: but whether exercise training across puberty enhances cerebrovascular function is unkown.What is the main finding and its importance?Cerebral blood flow is elevated in endurance trained adolescent males when compared to untrained counterparts. However, cerebrovascular reactivity to hypercapnia is faster in trained vs. untrained children, but not adolescents. Exercise-induced improvements in cerebrovascular function are attainable as early as the first decade of life.AbstractGlobal cerebral blood flow (gCBF) and cerebrovascular reactivity to hypercapnia (CVRCO2${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$) are modulated by gonadal hormone activity, while insulin-like growth factor 1 facilitates exercise-mediated cerebral angiogenesis in adults. Whether critical periods of heightened hormonal and neural development during puberty represent an opportunity to further enhance gCBF and CVRCO2${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ is currently unknown. Therefore, we used duplex ultrasound to assess gCBF and CVRCO2${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ in n = 128 adolescents characterised as endurance-exercise trained (males: n = 30, females: n = 36) or untrained (males: n = 29, females: n = 33). Participants were further categorised as pre- (males: n = 35, females: n = 33) or post- (males: n = 24, females: n = 36) peak height velocity (PHV) to determine pubertal or 'maturity' status. Three-factor ANOVA was used to identify main and interaction effects of maturity status, biological sex and training status on gCBF and CVRCO2${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$. Data are reported as group means (SD). Pre-PHV youth demonstrated elevated gCBF and slower CVRCO2${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ mean response times than post-PHV counterparts (both: P & LE; 0.001). gCBF was only elevated in post-PHV trained males when compared to untrained counterparts (634 (43) vs. 578 (46) ml min-1; P = 0.007). However, CVRCO2${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ mean response time was faster in pre- (72 (20) vs. 95 (29) s; P & LE; 0.001), but not post-PHV (P = 0.721) trained youth when compared to untrained counterparts. Cardiorespiratory fitness was associated with gCBF in post-PHV youth (r2 = 0.19; P & LE; 0.001) and CVRCO2${\mathrm{CV}}{{\mathrm{R}}_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ mean response time in pre-PHV youth (r2 = 0.13; P = 0.014). Higher cardiorespiratory fitness during adolescence can elevate gCBF while exercise training during childhood primes the development of cerebrovascular function, highlighting the importance of exercise training during the early stages of life in shaping the cerebrovascular phenotype.

A wealth of research has investigated the aging brain using blood oxygenation level dependent functional MRI [Blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI)]. However, many studies do not consider the aging of the cerebrovascular system, which can influence the BOLD signal independently from neural activity, limiting what can be inferred when comparing age groups. Here, we discuss the ways in which the aging neurovascular system can impact BOLD fMRI, the consequences for age-group comparisons and possible strategies for mitigation. While BOLD fMRI is a valuable tool in this context, this review highlights the importance of consideration of vascular confounds.