Lu Hanzhang, Golay Xavier, Pekar James J, Van Zijl Peter C M
Department of Radiology, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA.
Magn Reson Med. 2003 Aug;50(2):263-74. doi: 10.1002/mrm.10519.
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.
在大脑激活过程中，通过微血管的扩张和收缩促进了局部氧输送的控制。本文报道了一种对这些微血管调节敏感的新型功能磁共振成像（fMRI）方法。这种对比是通过以一种独立于血液氧合和血流的方式消除血液信号来实现的。因此，无需外源性对比剂或任何其他侵入性程序，就可以通过剩余血管外水信号（即实质组织的信号）的变化来评估脑血容量（CBV）的变化。这种基于血管空间占据（VASO）的功能磁共振成像（fMRI）方法在视觉刺激、屏气（高碳酸血症）和过度通气（低碳酸血症）方面的可行性得到了验证。在视觉刺激和屏气过程中，VASO信号与刺激范式呈负相关，这与局部血管扩张一致。在过度通气过程中，这种效应会逆转。对基于VASO-fMRI、基于脑血流量（CBF）的fMRI和基于血氧水平依赖（BOLD）的fMRI的血流动力学反应进行比较，结果表明VASO中存在小动脉和小静脉的时间特征。计算得出水交换率变化和脑脊液部分容积污染的影响可以忽略不计。在常用的3.75×3.75×5 mm³的fMRI分辨率下，VASO-fMRI的对比噪声比（CNR）与基于CBF的fMRI相当，但比BOLD-fMRI低3倍。文中提供了支持VASO-fMRI方法比BOLD能更好地定位灰质的论据。
