Technical NoteFunctional MR angiography with 7.0 T: Is direct observation of arterial response during neural activity possible?
Introduction
As it is known, a blood oxygen level dependent (BOLD) signal in functional magnetic resonance imaging (fMRI), a well-known technique for the study of brain functions, depends on the level of deoxyhemoglobin, which can be affected by blood flow, blood volume, as well as oxygen extraction (Ogawa et al., 1990). That is, the vascular system supports the increased neural activity with a blood flow as well as an increased blood volume, which increases the essential metabolic substrates, such as oxygen and glucose (Sandman et al., 1984). Many physiological studies have been conducted to understand the mechanisms of the vascular response to neural activity, and they arrived at the conclusion that arteriole or small arteries, among others, may regulate and control the flow of blood of the capillary bed during a neural activity (Heuttel et al., 2004), but their behavior in the human brain in-vivo has not been explored in detail due to the difficulty of direct observations (Iadecola et al., 1997, Ngai et al., 1995). There have been previous studies using functional brain imaging techniques for revealing vascular responses to somatosensory stimuli (Belle et al., 1995). Their resolution was not high enough, so that they were unable to observe the functional changes in the target vessels with their MRA images. A more recent study indicated that there was indeed an arterial flow that affects the hemodynamic or BOLD response during brain activation (Vazquez et al., 2006). These previous studies, however, relied on indirect measurements of the vascular change during stimuli, such as the perfusion imaging technique called arterial spin labeling, which provides the indirect measurement of the gross cerebral blood flow (CBF) or volume (CBV), but not the change of individual micro-vessels that are involved in the local neural activity (Calamante et al., 1999, Sardashti et al., 1990, Williams et al., 1992).
The arterial responses during neural activities have been studied in animals using optical imaging spectroscopy, laser Doppler flowmetry as well as fMRI, but little is known about the human brain in-vivo and its direct visualization of vascular signal changes during stimuli (Devor et al., 2003, Kim et al., 2007, Moon et al., 2007, Vanzetta et al., 2005, Zheng et al., 2005). It is probably due to the technique, which is unable to visualize the dynamical changes of blood vessels in the human brain in-vivo during the process of the neural activities, i.e., dynamic angiography, micro- or small vessels imaging. Current MRI technology using conventional MRI (≤ 3.0 T) simply could not support such a high resolution functional angiography of the sub-millimeter order. In this study, for the first time, using 7.0 T MRI, we introduce a technique to examine the brain function in-vivo by measuring directly the changes in the individual blood vessel by external stimulation, that is, functional MR angiography similar to that of fMRI.
Section snippets
Subjects and MR system
Fourteen young healthy volunteers (mean age: 23.14 ± 5.17, 8 females and 6 males) from the local universities participated in the study. All subjects participated in the study after signing the form of informed consent. The experiments were approved by the institutional review board (IRB). MRA imaging was performed using a 7.0 T MR Scanner (Magnetom, Siemens AG, Berlin, Germany) consisting of a 90 cm bore superconducting magnet (Magnex Magnet Technology, Oxford, UK) equipped with Siemens Syngo
Results and discussion
Encouraged by the previous results of a micro-vascular study of the lenticulostriate artery (Cho et al., 2008), we have attempted an externally stimulated functional angiography of near micro-vascular structures or small vessels, that is, fMRA. In this fMRA study, for the first time, using an ultra-high field 7.0 T MR system, we have observed and were able to directly measure the changes in the human cerebral vessels, which are supplying blood to the area of central sulcus during hand movements
Acknowledgments
This work is supported by the Gil Foundation, Incheon, Korea and the SRC/ERC program of MOST/KOSEF (R11-2005-014), a grant (M103KV010026-07K2201-02610) from the Brain Research Center of the 21st Century Frontier Research Program, and Nano-Bio Technology program (M10530010001-06N3001-00110) funded by the Ministry of Science and Technology, Republic of Korea.
Author contributions
Z.H.C., C.K.K., J.Y.H., S.H.K. and C.A.P. contributed to the MR imaging. K.N.K. and S.M.H. made contributions to the
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