Blood flow and anatomical MRI in a mouse model of retinitis pigmentosa

.. 37 ± 0.5°C via a circulating warm water pad. Respiration rate, heart rate, and oxygen
saturation were continuously monitored (MouseOx, STARR Life Science Corp.,
Oakmont, PA). MRI. MRI studies were performed on a 7 T, 30 …
This study tested the sensitivity of an arterial spin labeling MRI method to image changes in retinal and choroidal blood flow (BF) and anatomical thickness of the retina in the rd10 mouse model of retinitis pigmentosa. High-resolution (42 × 42 μm) MRI was performed on rd10 mice and age-matched controls at 25, 35, and 60 days of age (n = 6 each group) on a 7-T scanner. Anatomical MRI was acquired, and quantitative BF was imaged using arterial spin labeling MRI with a separate cardiac labeling coil. Histology was obtained to confirm thickness changes in the retina. In control mice, the retinal and choroidal vascular layers were quantitatively resolved. In rd10 mice, retinal BF decreased progressively over time, while choroidal BF was unchanged. The rd10 retina became progressively thinner at later time points compared with age-matched controls by anatomical MRI and histology (P < 0.01). BF and anatomical MRI were capable of detecting decreased BF and thickness in the rd10 mouse retina. Because BF is tightly coupled to metabolic function, BF MRI has the potential to noninvasively assess retinal diseases in which metabolism and function are perturbed and to evaluate novel treatments, complementing existing retinal imaging techniques. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc. Retinitis pigmentosa (RP) is a group of inherited retinal diseases which cause retinal degeneration and vision loss, affecting 1.5 million people worldwide (1). It is characterized initially by a progressive loss of photoreceptors, with secondary deterioration of vascular and other cell layers (2). Most RP patients undergo a preliminary loss of peripheral vision and impaired night vision because rods are usually affected first. Secondary degeneration, usually including loss of cones, follows, resulting in the loss of central visual field, color vision, and potentially complete blindness (1). A large number of mutations in various genes which cause RP have been described, including the genes for rhodopsin, mertk, and phosphodiesterase β (2–4). The rd10 mouse is an established animal model of RP (5–7). rd10 mice have a mutation in the Pde6b gene, encoding a subunit of the rod phosphodiesterase (5, 6). Mutations in the gene for the β subunit of the rod phosphodiesterase have been found in human autosomal recessive RP (3). The mutation in the Pde6b gene causes deficient activity of the rod phosphodiesterase, which results in the accumulation of cyclic GMP and death of rod cells (6). Based on histological data, retinal degeneration begins in rd10 mice about postnatal 16 days, and the outer nuclear layer and inner and outer segments completely degenerate by 60 days of age (6, 7). Degeneration of the outer retina begins first with loss of rods, with cone loss and remodeling of the inner retina occurring later (7). Although the genetic aspects and thickness changes of RP are well studied, the lack of noninvasive, depth-resolved imaging techniques has limited the investigation of physiologic changes associated with retinal degeneration in vivo. Clinical examinations of RP include digital fundus photography using the seven stereo fields (8), full-field electroretinography (4), Goldmann visual field with V4e test object, and optical coherence tomography of the macula and optic disc and macular thickness (4). Many potential treatments (9), including vitamin A supplementation, intravitreal administration of growth factors (10), neuroprotective drugs (11), hyperoxia (12), gene therapy (13), and stem cell therapy (14), show potential to slow, halt, or reverse retinal degeneration. Noninvasive imaging technologies that can pinpoint layer-specific cellular and vascular changes may enable longitudinal staging of RP, objective measures of therapeutic interventions, and improved understanding of disease processes in vivo. Vascular changes occur secondary to photoreceptor loss in RP (14–16), with atrophy of the retinal (14, 15) and choroidal (17) vasculature. Histological studies showed leakage in the retinal vessels in Royal College of Surgeons (a model of RP) rats by 2 months of age (18). In a cat model of RP, retinal blood flow (rBF) was compromised while choroidal blood flow (chBF) was not significantly affected (19). In RP patients, diameter, blood velocity, blood flow (BF) in retinal veins, and total rBF were found to be lower (20), subfoveal chBF was lower (17), and chBF was reduced in late, but not early, stages of the disease (21). These findings suggest that there are vascular-specific (retinal and choroidal) changes accompanying retinal degenerations. Improved understanding of the physiological changes accompanying retinal degeneration may enable better understanding of the pathophysiology. BF changes may occur before irreversible degeneration, which could provide a potential objective measure of therapeutic interventions. The goal of this study was to test whether anatomical and BF MRI techniques could detect changes of retinal thickness and possible changes of retinal and choroidal BF at different stages of retinal degeneration in the rd10 mouse model. Anatomical MRI was acquired using a balanced steady state free precession (bSSFP) sequence for fast image acquisition with high signal-to-noise ratio. BF MRI used cardiac spin labeling MRI (22), which is based on the continuous arterial spin labeling technique with a separate cardiac coil to avoid saturation of the imaging signal in the retina due to the small size of mice.