The confound of hemodynamic response function variability in human resting-state functional MRI studies

Cerebrovascular reactivity increases across development in multiple networks as revealed by a breath-holding task: A longitudinal fMRI study

Functional magnetic resonance imaging (fMRI) has been widely used to understand the neurodevelopmental changes that occur in cognition and behavior across childhood. The blood-oxygen-level-dependent (BOLD) signal obtained from fMRI is understood to be comprised of both neuronal and vascular information. However, it is unclear whether the vascular response is altered across age in studies investigating development in children. Since the breath-hold (BH) task is commonly used to understand cerebrovascular reactivity (CVR) in fMRI studies, it can be used to account for developmental differences in vascular response. This study examines how the cerebrovascular response changes over age in a longitudinal children's BH data set from the Nathan Kline Institute (NKI) Rockland Sample (aged 6-18 years old at enrollment). A general linear model approach was applied to derive CVR from BH data. To model both the longitudinal and cross-sectional effects of age on BH response, we used mixed-effects modeling with the following terms: linear, quadratic, logarithmic, and quadratic-logarithmic, to find the best-fitting model. We observed increased BH BOLD signals in multiple networks across age, in which linear and logarithmic mixed-effects models provided the best fit with the lowest Akaike information criterion scores. This shows that the cerebrovascular response increases across development in a brain network-specific manner. Therefore, fMRI studies investigating the developmental period should account for cerebrovascular changes that occur with age.

Ophthalmic artery reversal predicts contralateral body weakness symptoms better than carotid Doppler velocity-preliminary results

Background

The measurement of blood velocity in the carotid artery has been the most popular noninvasive method of identifying and classifying carotid stenosis for half a century. Carotid stenosis is an indicator of elevated risk of stroke; anatomic revascularization reduces the chance of stroke by more than half. Controversy persists on how patients with severe carotid stenosis should be selected for anatomic revascularization. Patients with a connected circle of Willis (coW) might not benefit from anatomic revascularization; patients with two segments missing in the coW are most likely to benefit from revascularization.

Methods

Based on this analysis of data from carotid duplex examinations and transcranial Doppler examinations including ophthalmic artery (OA) direction in 28 patients, a refined carotid examination protocol is proposed. This refinement includes Doppler measurement of OA flow direction and documentation of internal carotid artery (ICA) bruit in addition to the adoption of an ICA peak systolic velocity (PSV) criterion exceeding 350 cm/s for identification of the patient most likely to benefit from carotid stenosis treatment.

Results

Sensitivity and specificity of OA direction or carotid bruit are 84.6%±5.4%, 71.4%±2.1% and for PSV >350 cm/s are 84.6%±5.4%, 59.5%±2.3% for predicting contralateral body weakness.

Conclusions

The proposed examination can be performed with the same duplex scanner and scan head currently used for carotid examinations with little additional time.