Studies in humans and several other nonrodent species show that retinal and optic nerve BF increases during retinal visual stimulation.
21,22,57 Typically the optic nerve response is greater than the retinal response, though both exhibit roughly exponential responses to luminance and bell-shaped responses to frequency and wavelength using laser Doppler flowmetry, oxygen tension measurements, and electroretinogarphy.
10,58 –60 In keeping with the literature,
Figure 5 shows that the retinal MION Δ
R 2*response was greatest in the optic disc region, and
Figure 6 shows robust retinal MION Δ
R 2*responses to luminance, frequency, and wavelength. Consistent with the literature, our results show the choroid was largely unresponsive to flicker stimulation and the modulation of the flicker parameters.
57
In conclusion, the use of a blood-pool contrast agent and a high-field-strength scanner provides sufficient resolution and sensitivity to resolve the rat retinal and choroidal circulations and detect visually evoked retinal and choroidal responses. Retinal vessels respond to changing luminance, frequency, and wavelength, whereas choroidal vessels do not. The key disadvantage of BV fMRI is poorer spatiotemporal resolution and signal-to-noise ratio compared to optically based imaging techniques. Other MRI techniques have been used to image anatomy,
26,61 relative blood oxygenation,
26 BF,
28 and functional parameters
52,53 in the retina. Because MRI can give simultaneous, global information about the ocular circulations without depth limitation and can be done repeatedly, this imaging approach could have applications in early detection and longitudinal monitoring of retinal diseases, such as retinal ischemia, glaucoma, diabetic retinopathy, and retinitis pigmentosa, where retinal and choroidal hemodynamics and neurovascular coupling may be perturbed differently in different diseases and at various disease stages.
11 This approach may have clinical applications because similar iron oxide contrast agents are approved for clinical use. Anatomic, BF, and functional MRI studies of the human retina have recently been reported, albeit not yet with laminar resolution.
62 –64 In addition, the retinal anatomy with multiple well-defined cellular and vascular layers serves an excellent model to advance MRI technologies to push the boundary of MRI and fMRI spatial resolution.
Supported in part by the NIH/NEI (R01 EY014211, EY018855, and R01EY09702), MERIT Award from the Department of Veterans Affairs, and a grant from the San Antonio Life Science Institute to TQD. Y.-Y.I.S. is supported by American Heart Association (10POST4290091) and San Antonio Area Foundation.