Continuous assessment of perfusion by tagging including volume and water extraction (CAPTIVE): A steady‐state contrast agent technique for measuring blood flow, relative blood volume fraction, and the water extraction fraction

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Abstract
A new technique, CAPTIVE, that is a synthesis of arterial spin labeling (ASL) blood flow and steady‐state susceptibility con trast relative blood volume imaging is described. Using a single injection of a novel, long half‐life intravascular magnetopharma‐ceutical with a high tissue:blood susceptibility difference (Δ χ) to ΔR1 ratio, changes in tissue transverse relaxivity (ΔR2 or ΔR2*) that arise from changes in blood volume were measured, while preserving the ability to measure blood flow using traditional T1 ‐based ASL techniques. This modification permits the contin uous measurement of both blood flow and blood volume. Also, because the contrast agent can be used to remove the signal from intravascular spins, it is possible to measure the first‐pass water extraction fraction. Contrast‐to‐noise is easily traded off with repetition rate, allowing the use of non‐EPI scanners and more flexible imaging paradigms. The basic theory of these measurements, several experimental scenarios, and validating results are presented. Specifically, the PaCO2‐reactivity of mi‐crovascular and total relative cerebral blood volume (rCBV), ce rebral blood flow (CBF), and the water extraction‐flow product (EF) in rats with the new contrast agent MPEG‐PL‐DyDTPA is measured, and the values are concordant with those of previous literature. As an example of one possible application, continuous flow and volume measurements during transient focal ischemia are presented. It is believed that CAPTIVE imaging will yield a more complete picture of the hemodynamic state of an organ, and has further application for understanding the origins of the BOLD effect.