Biomedical subjects

Hanzhang Lu

Publications and source records attributed to Hanzhang Lu.

21 records · Page 2Linked to original sources

PubMed2003

Functional magnetic resonance imaging based on changes in vascular space occupancy.

During brain activation, local control of oxygen delivery is facilitated through microvascular dilatation and constriction. A new functional MRI (fMRI) methodology is reported that is sensitive to these microvascular adjustments. This contrast is accomplished by eliminating the blood signal in a manner that is independent of blood oxygenation and flow. As a consequence, changes in cerebral blood volume (CBV) can be assessed through changes in the remaining extravascular water signal (i.e., that of parenchymal tissue) without need for exogenous contrast agents or any other invasive procedures. The feasibility of this vascular space occupancy (VASO)-dependent functional MRI (fMRI) approach is demonstrated for visual stimulation, breath-hold (hypercapnia), and hyperventilation (hypocapnia). During visual stimulation and breath-hold, the VASO signal shows an inverse correlation with the stimulus paradigm, consistent with local vasodilatation. This effect is reversed during hyperventilation. Comparison of the hemodynamic responses of VASO-fMRI, cerebral blood flow (CBF)-based fMRI, and blood oxygenation level-dependent (BOLD) fMRI indicates both arteriolar and venular temporal characteristics in VASO. The effect of changes in water exchange rate and partial volume contamination with CSF were calculated to be negligible. At the commonly-used fMRI resolution of 3.75 x 3.75 x 5 mm(3), the contrast-to-noise-ratio (CNR) of VASO-fMRI was comparable to that of CBF-based fMRI, but a factor of 3 lower than for BOLD-fMRI. Arguments supporting a better gray matter localization for the VASO-fMRI approach compared to BOLD are provided.

Adult↗

PubMed2003

Measurements of cerebral perfusion and arterial hemodynamics during visual stimulation using TURBO-TILT.

Estimation of cerebral blood flow (CBF) in functional perfusion imaging could benefit from a method capable of separating effects of arterial arrival time and trailing edge. To accomplish this, the transfer insensitive labeling technique (TILT) was combined with a train of 13 consecutive acquisitions, called TURBO-TILT. Visual activation maps obtained at 13 postlabeling delay times (TI) showed a spatial shift from regions surrounding the arterial vasculature at short TI to brain parenchyma at longer delay times. High baseline CBF and short arrival times were found for the voxels with maximum activation at short TI (<1200 ms), while CBF values (43 ml / 100 g tissue/min) and its increase upon activation (55%) at longer TI were in agreement with literature data on regional cerebral perfusion.

Adult↗

PubMed2002

Intervoxel heterogeneity of event-related functional magnetic resonance imaging responses as a function of T(1) weighting.

Inflow effects on activation-related BOLD signal changes in event-related fMRI experiments were assessed by varying the repetition time (TR) and flip angle (FA) values for gradient-echo echo-planar imaging (GE-EPI). Surprisingly, both increases and decreases were detected in these signal changes with increased T(1) weighting (reduced TR, increased FA). The well-known "positive" effect is attributed to inflow of fresh spins in the slice, leading to an apparent reduction in T(1). The "negative" effect is attributed to voxels containing pure parenchyma, where large-vessel inflow effects are very small and the BOLD effect is dominated by microvascular blood volume and oxygenation changes. Because blood T(1) is greater than tissue T(1) at 1.5 T, the fractional BOLD effect decreases with increased T(1) weighting. To aid in the interpretation of these experimental results, numerical simulations were performed based on a physiological multicompartment model, including pure tissue, large vessels (arteries, veins), microvessels (arterioles, capillaries, venules), and cerebrospinal fluid.

Adult↗