Velocity-selective arterial spin labelling bolus duration measurements: Implications for consensus recommendations.

Area of Science:

• Neuroimaging
• Physiology
• Medical Physics

Background:

• Velocity-selective arterial spin labelling (VSASL) is valuable for studying brain perfusion due to its insensitivity to prolonged arterial transit times.
• This technique is advantageous in conditions like aging, vascular pathologies, cancer, and cerebrovascular reactivity (CVR) measurements where arterial transit times vary.
• Accurate cerebral blood flow (CBF) calculation using VSASL relies on the assumption that the vascular crushing module defines the bolus duration before the labeled blood's trailing edge arrives.

Purpose of the Study:

• To measure the duration of the labeled blood bolus in healthy adult human brains at rest and during hypercapnia.
• To investigate the impact of altered CBF during CO2 breathing on bolus duration and CVR measurements.
• To evaluate the implications of observed bolus duration changes for current VSASL imaging protocols and recommendations.

Main Methods:

• VSASL MRI was employed to measure grey matter bolus duration in healthy adults.
• Measurements were performed both at rest and during 5% inspired CO2 breathing to induce hypercapnia and elevate CBF.
• Bolus duration was analyzed across different velocity cutoffs and spatial regions (frontal, temporal, occipital lobes).

Main Results:

• At rest, grey matter bolus duration was approximately 2.05–2.22 seconds, consistent with previous findings.
• During hypercapnia, a significant decrease in bolus duration was observed, with a matched reduction in CVR above a labelling delay of 1.2 seconds.
• Hypercapnic bolus duration was spatially heterogeneous, being shorter in the frontal and temporal lobes compared to the occipital lobe.

Conclusions:

• The assumption of a stable bolus duration in VSASL is challenged by physiological changes like hypercapnia.
• Current consensus recommendations for VSASL imaging timing (e.g., 1.4 seconds post-label) may lead to CBF underestimation, particularly during CVR measurements or in conditions with rapid blood flow.
• Optimizing imaging timing based on measured bolus duration is crucial for accurate perfusion quantification in VSASL studies.