Arterial cerebral blood volume (aCBV) weighted inflow vascular space occupancy (iVASO) provides complementary hemodynamic information to dynamic susceptibility contrast in patients with stenotic artery disease
Introduction. Active and passive mechanisms for controlling cerebral blood volume (CBV) at the arterial, capillary and venous levels are incompletely understood, yet central to cerebral blood flow (CBF) regulation. Furthermore, knowledge of CBV changes in different vascular compartments would be important for investigations of fMRI contrast and in clinical scenarios where flow volume coupling may be impaired. Recently, inflow vascular space occupancy (iVASO) was proposed as a non invasive approach for isolating arterial CBV (aCBV) reactivity [1] and absolute aCBV [2,3] Specifically, by acquiring an image with and without inflowing blood water signal, a difference image (DM/M0) can be obtained which is proportional to aCBV. This iVASO with dynamic subtraction (iVASO DS) principle has been proposed [2,3] however it has not been validated against MRstandard dynamic susceptibility contrast (DSC) measurements, nor has it been compared against other hemodynamic measures such as CBF, total CBV and mean transit time (MTT=CBV/CBF) Here, we compare iVASO DS contrast with DSCmeasured CBF, CBV, and MTT in patients with varying degrees of steno occlusive disease of the internal carotid artery (ICA) Methods. All patients (n=17; age=72.6+ 7.9 yrs) provided informed, written consent and were scanned at 3.0T (Siemens) using a protocol consisting of DWI, T1weighted (T1w) DSC, and iVASO DS. Patient preparation. Stenosis degree in left and right ICAs were measured by duplex ultrasound; patients were grouped as mild stenosis (stenosis<60% (n=8) and moderate to severe stenosis (stenosis>=60% (n=9) Experiment. Scan parameters were optimized for each modality: DWI: TR/TE=4400/93, 1.6x1.6x3.0 mm, b value=1000 s/mm. 3D T1w MPRAGE: TR/TI/TE=1800/900/4.4 ms, 1.7x1.7x2.0 mm. DSC with intravenous Gd DTPA injection [4] TR/TE=1481/30 ms, 1.7x1.7x5.0 mm. iVASO DS [2] TR/TE/TI=1778/18/989 ms, 2.4x2.4x5.0 mm. Processing. All data were corrected for motion and baseline drift [5] spatial smoothing (FWHM=5 mm) was applied to DSC and iVASO DS images, and gray matter (GM) masks were generated. GM quantification was performed separately in affected (aff) and unaffected (unaff) brain hemispheres, defined as contralateral or ipsilateral to maximum ICA stenosis burden, respectively. For DSC, PErfusioN Graphical User Interface (Penguin) software with a singular value decomposition was applied to calculate CBF, CBV and MTT [6] Quantification is sensitive to arterial input function (AIF) choice; we used automated AIF detection available in the software and conservatively characterize all maps in arbitrary units (a.u. iVASO DM/M0=(control null)/M0 was calculated, with white matter signal in the absence of preparation pulses being used for M0 calibration. DM/M0 scales linearly with aCBV when TI= capillary arrival time (CAT; time for nulled blood to reach capillaries) [2,3] which at TI=989 ms has been assumed to be approximately correct. Additionally, we attempt to account for CAT asymmetries using the DSC measured MTT, which in the absence of collateral flow would be expected to be proportional to flow velocity and hence to CAT. This yields a new, corrected DM/M0 ratio: [1]