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Many of these studies show that the retinal and choroidal vessels respond to various ��stimuli�� differently. For example, in hyperoxia, BOLD increase was larger in the outer choroid layer (12 �� 2%) than the inner retinal layer (7 �� 2%, P http://www.selleckchem.com/products/17-AAG(Geldanamycin).html substantially larger than the outer layer (1.6 �� 1%) [8]. This is likely because hypercapnia has smaller vasodilatory effect on choroidal vessels and/or the oxygen extraction in the choroid is small [24]. These differential layer-specific BOLD responses are consistent with published literature using oxygen electrodes [24]. Mild hypoxia (10% O2) decreases BOLD signal but the magnitude changes are different with ?12 �� 2% in the inner (retinal) layer and ?30 �� 3% in the outer (choroid) layer [11]. Anatomical MRI studies of the retina using contrast agents have been reported including those employing manganese-enhanced [4, 25] and chromium-enhanced [26] MRI. The retinal and choroidal vascular layers http://www.selleck.cn/products/VX-809.html and their respective blood barriers can be visualized by using Gd-DTPA [7, 8] as described above. Blood volume index can be measured by using the blood-pool contrast agent, monocrystalline iron oxide nanoparticles. Blood-volume fMRI responses to hypercapnia and hyperoxia show differential responses between the retinal and choroidal layers [21]. Blood-volume fMRI has been used to evaluate retinal and choroidal responses in rats at graded luminance, frequency, and wavelength of flickering light [22] (Fig. 1c). In the retinal vasculature, increasing luminance increases fMRI signals, red light gives weaker response compared to blue and green, and 10 Hz flicker gives maximal fMRI signals, as expected. The choroidal vascular response is weak and does not depend on flicker parameters, http://www.selleckchem.com/products/lgk-974.html suggesting differential neurovascular coupling between the two vascular layers. Together, these findings offer a mean to probe the unique hemodynamic regulations in the eye. Layer-specific fMRI of light and dark adaptation has been reported using manganese-enhanced [27, 28] and diffusion [29] MRI contrasts. Vascular diameter and vessel intensity changes due to hyperoxia and carbogen inhalation can be detected on angiographic images without contrast agent [30]. 3D angiography without and with contrast agent is capable of visualizing arterial and venous vessels in rats at 42 �� 42 �� 84 ��m [30], providing remarkable details of vessels in the eye and the choriocapillaris.