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

Phenotype Characterization of a Mice Genetic Model of Absolute Blindness

Recent technological development requires new approaches to address the problem of blindness. Such approaches need to be able to ensure that no cells with photosensitive capability remain in the retina. The presented model, Opn4^−/− × Pde6b^rd10/rd10 (O×Rd) double mutant murine, is a combination of a mutation in the Pde6b gene (photoreceptor degeneration) together with a deletion of the Opn4 gene (responsible for the expression of melanopsin in the intrinsically photosensitive retinal ganglion cells). This model has been characterized and compared with those of WT mice and murine animal models displaying both mutations separately. A total loss of pupillary reflex was observed. Likewise, behavioral tests demonstrated loss of rejection to illuminated spaces and a complete decrease in visual acuity (optomotor test). Functional recordings showed an absolute disappearance of various wave components of the full-field and pattern electroretinogram (fERG, pERG). Likewise, visual evoked potential (VEP) could not be recorded. Immunohistochemical staining showed marked degeneration of the outer retinal layers and the absence of melanopsin staining. The combination of both mutations has generated an animal model that does not show any photosensitive element in its retina. This model is a potential tool for the study of new ophthalmological approaches such as optosensitive agents.

Morphometric and Microstructural Changes During Murine Retinal Development Characterized Using In Vivo Optical Coherence Tomography

Purpose

The purpose of this study was to develop an in vivo optical coherence tomography (OCT) system capable of imaging the developing mouse retina and its associated morphometric and microstructural changes.

Methods

Thirty-four wild-type mice (129S1/SvlmJ) were anesthetized and imaged between postnatal (P) day 7 and P21. OCT instrumentation was developed to optimize signal intensity and image quality. Semi-automatic segmentation tools were developed to quantify the retinal thickness of the nerve fiber layer (NFL), inner plexiform layer (IPL), inner nuclear layer (INL), and the outer retinal layers (ORL), in addition to the total retina. The retinal maturation was characterized by comparing layer thicknesses between consecutive time points.

Results

From P7 to P10, the IPL increased significantly, consistent with retinal synaptogenesis. From P10 to P12, the IPL and ORL also increased, which is coherent with synaptic connectivity and photoreceptor maturation. In contrast, during these periods, the INL decreased significantly, consistent with cellular densification and selective apoptotic “pruning” of the tissue during nuclear migration. Thereafter from P12 to P21, the INL continued to thin (significantly from P17 to P21) whereas the other layers remained unchanged. No time-dependent changes were observed in the NFL. Overall, changes in the total retina were attributed to those in the IPL, INL, and ORL. Regions of the retina adjacent to the optic nerve head were thinner than distal regions during maturation.

Conclusions

Changes in retinal layer thickness are consistent with retinal developmental mechanisms. Accordingly, this report opens new horizons in using our system in the mouse to characterize longitudinally developmental digressions in models of human diseases.

Ophthalmic Magnetic Resonance Imaging: Where Are We (Heading To)?

Magnetic resonance imaging of the eye and orbit (MReye) is a cross-domain research field, combining (bio)physics, (bio)engineering, physiology, data sciences and ophthalmology. A growing number of reports document technical innovations of MReye and promote their application in preclinical research and clinical science. Realizing the progress and promises, this review outlines current trends in MReye. Examples of MReye strategies and their clinical relevance are demonstrated. Frontier applications in ocular oncology, refractive surgery, ocular muscle disorders and orbital inflammation are presented and their implications for explorations into ophthalmic diseases are provided. Substantial progress in anatomically detailed, high-spatial resolution MReye of the eye, orbit and optic nerve is demonstrated. Recent developments in MReye of ocular tumors are explored, and its value for personalized eye models derived from machine learning in the treatment planning of uveal melanoma and evaluation of retinoblastoma is highlighted. The potential of MReye for monitoring drug distribution and for improving treatment management and the assessment of individual responses is discussed. To open a window into the eye and into (patho)physiological processes that in the past have been largely inaccessible, advances in MReye at ultrahigh magnetic field strengths are discussed. A concluding section ventures a glance beyond the horizon and explores future directions of MReye across multiple scales, including in vivo electrolyte mapping of sodium and other nuclei. This review underscores the need for the (bio)medical imaging and ophthalmic communities to expand efforts to find solutions to the remaining unsolved problems and technical obstacles of MReye, with the objective to transfer methodological advancements driven by MR physics into genuine clinical value.

Ocular Blood Flow Measurements in Diabetic Retinopathy Using 3D Pseudocontinuous Arterial Spin Labeling

BACKGROUND

Distinguishing between the two broad categories of diabetic retinopathy (DR), nonproliferative DR (NPDR) and proliferative DR (PDR), is significant, as the therapeutic strategies for each are completely different.

PURPOSE

To characterize the ocular blood flow (OBF) of DR patients and evaluate the potential utility of OBF values in categorizing DR.

STUDY TYPE

Prospective.

SUBJECTS

A total of 41 DR patients (82 eyes) were recruited in our study. Group 1 comprised 48 eyes with NPDR, and Group 2 comprised 34 eyes with PDR.

FIELD STRENGTH/SEQUENCE

3D pseudocontinuous arterial spin labeling (3D-pcASL) with two postlabeling delays (PLDs) was acquired at 3.0T MR.

ASSESSMENT

OBF values were independently obtained by two doctors from the OBF map.

STATISTICAL TESTS

OBF values and clinical characteristics were compared between the groups using two-sample t-tests and chi-square tests. Receiver operating characteristic (ROC) curves were obtained, and the area under the curve (AUC) was calculated. The consistency of OBF values reported by the two doctors was evaluated using the intraclass correlation coefficient (ICC).

RESULTS

OBF values at PLDs of 1.5 seconds and 2.5 seconds were significantly lower in Group 2 than in Group 1 (P < 0.05 for both PLDs). The OBF values of Group 2 showed a greater increase than those of Group 1 from PLD 1.5 to 2.5 seconds. The AUC of OBF at the 1.5 seconds PLD was 0.90, with a cutoff of 7.73 mL/min/100 g, and the AUC of the OBF at the 2.5 seconds PLD was 0.75, with a cutoff of 8.44 mL/min/100 g. The ICC between the two observers was 0.844 for the OBF at 1.5 seconds PLD and 0.872 for the OBF at 2.5 seconds PLD.

DATA CONCLUSION

PDR can be differentiated from NPDR by the value of OBF as measured by 3D-pcASL.

LEVEL OF EVIDENCE

1 TECHNICAL EFFICACY STAGE: 1.

Multi-parametric MRI of the physiology and optics of the in-vivo mouse lens

The optics of the ocular lens are determined by its geometry (shape and volume) and its inherent gradient of refractive index (water to protein ratio), which are in turn maintained by unique cellular physiology known as the lens internal microcirculation system. Previously, magnetic resonance imaging (MRI) has been used on ex vivo organ cultured bovine lenses to show that pharmacological perturbations to this microcirculation system disrupt ionic and fluid homeostasis and overall lens optics. In this study, we have optimised in vivo MRI protocols for use on wild-type and transgenic mouse models so that the effects of genetically perturbing the lens microcirculation system on lens properties can be studied. In vivo MRI protocols and post-analysis methods for studying the mouse lens were optimised and used to measure the lens geometry, diffusion, T1 and T2, as well as the refractive index (n) calculated from T2, in wild-type mice and the genetically modified Cx50KI46 mouse. In this animal line, gap junctional coupling in the lens is increased by knocking in the gap junction protein Cx46 into the Cx50 locus. Relative to wild-type mice, Cx50KI46 mice showed significantly reduced lens size and radius of curvature, increased T1 and T2 values, and decreased n in the lens nucleus, which was consistent with the developmental and functional changes characterised previously in this lens model. These proof of principle experiments show that in vivo MRI can be applied to transgenic mouse models to gain mechanistic insights into the relationship between lens physiology and optics, and in the future suggest that longitudinal studies can be performed to determine how this relationship is altered by age in mouse models of cataract.

OPTICAL COHERENCE TOMOGRAPHY ANGIOGRAPHY IN PATIENTS WITH RETINITIS PIGMENTOSA

PURPOSE

To evaluate the correlation between the flow density measured by optical coherence tomography angiography and functional parameters in patients with retinitis pigmentosa.

METHODS

Twenty eyes of 20 patients with retinitis pigmentosa and 21 eyes of 21 healthy subjects were prospectively included in this study. Optical coherence tomography angiography was performed using RTVue XR Avanti with AngioVue (Optovue Inc). The macula was imaged with a 6 × 6-mm scan, whereas for the optic nerve head a 4.5 × 4.5-mm scan was taken. Visual acuity, visual field parameters (mean deviation and visual field index), full-field electroretinography, and multifocal electroretinography were tested for correlation with flow density data.

RESULTS

The flow density (whole en face) in the superficial/deep retinal OCT angiograms and in the optical coherence tomography angiography of the optic nerve head was significantly lower in the retinitis pigmentosa group when compared with the control group (P < 0.001). The flow density in the superficial retinal OCT angiogram (fovea) correlated significantly with the visual acuity (rSpearman = -0.77, P < 0.001) and the visual field parameters (visual field index: rSpearman = 0.56, P = 0.01; mean deviation: rSpearman = 0.54, P = 0.01).

CONCLUSION

Patients with retinitis pigmentosa show a decreased macular and optic nerve head perfusion compared with healthy subjects. The flow density measured using optical coherence tomography angiography correlated with subjective and objective functional parameters.

Effect of Choroidal Vessel Density on the Ellipsoid Zone and Visual Function in Retinitis Pigmentosa Using Optical Coherence Tomography Angiography

Purpose

We evaluate the effect of choroidal vessel density on the residual length of the ellipsoid zone (EZ) and visual function in patients with retinitis pigmentosa (RP) using optical coherence tomography angiography (OCTA).

Methods

Fifty-three patients with RP (n = 101 eyes) and 53 normal participants (n = 76 eyes) were enrolled in this study. Patients with RP were assigned to three groups according to their best-corrected visual acuity (BCVA). All patients underwent ophthalmologic examinations, including BCVA, fundus examination performed with a slit-lamp using an indirect 90 diopter (D) lens, OCTA, full-field electroretinogram (ERG), and visual field. The choroidal vessel density in the choriocapillaris-Sattler's layer (DC-S), Haller's layer (DH), horizontal length of the ellipsoid (HEL), and vertical length of the ellipsoid (VEL) were assessed using OCTA and Adobe Photoshop CS3 extended software.

Results

A significantly increasing impairment of choroidal vessel density (DC-S and DH) was characterized in the RP groups compared to those of the controls (P < 0.05 for all). The magnitude of the reduction in the DC-S and DH was much easier to identify for more severely impaired BCVA in the RP groups (P < 0.05 for all). The DC-S had the strongest correlation with the HEL, VEL, BCVA, visual field, and b-wave amplitude (r = 0.735, r = 0.753, r = -0.843, r = 0.579, and r = 0.671, respectively).

Conclusions

Using noninvasive OCTA, choroidal microcirculation, especially in the small/middle choroidal vessel layers, was a prominent factor affecting the EZ, visual acuity, visual field, and recordable ERG b-wave amplitude of patients with RP. This may provide new insights into the progress mechanism and treatment of RP.

Projection-Resolved Optical Coherence Tomographic Angiography of Retinal Plexuses in Retinitis Pigmentosa

PURPOSE

To use projection-resolved optical coherence tomographic angiography (PR-OCTA) to characterize the microvascular changes in 3 distinct retinal plexuses in retinitis pigmentosa (RP) patients.

DESIGN

Prospective cross-sectional study.

METHODS

A commercial 70-kHz spectral-domain optical coherence tomography (OCT) system was used to acquire 6-mm macular scans from RP patients and age-matched healthy participants at a tertiary academic center. Blood flow was detected using a commercial version of split-spectrum amplitude-decorrelation angiography (SSADA) algorithm. The PR-OCTA algorithm was used to suppress projection artifacts and resolve microvasculature in 3 plexuses around the macula. Vessel density was calculated from en face OCTA of the parafoveal and perifoveal regions in each of the 3 plexuses, as well as the all-plexus inner retinal slab. Inner and outer retinal thicknesses were measured form structural OCT scans. Generalized estimating equations and Spearman's rank correlation statistical methods were used.

RESULTS

Forty-four eyes from 26 RP patients and 34 eyes from 26 healthy subjects were included. Significant reduction in vessel density was detected in the perifovea but not the parafovea of inner retinal slab of RP patients (P = .001 and P = .56, respectively) compared to controls. We also found deeper retinal plexuses (intermediate and deep capillary plexuses, ICP and DCP) were primarily damaged by RP, compared to superficial vascular complex (SVC). Significant thickening of the inner retina and thinning of the outer retina were also observed. Strong correlation was found between the vessel density in the perifoveal ICP and DCP and outer retinal thickness in RP patients with no history of cystoid macular edema.

CONCLUSIONS

PR-OCTA enables the detection of microvascular changes in the perifoveal regions of the ICP and DCP in RP, with relative sparing of the SVC. OCT and OCTA parameters might be able to provide better understanding of the pathophysiology of the disease, as well as monitoring disease progression and the response to experimental treatments.

Characterisation of microvascular abnormalities using OCT angiography in patients with biallelic variants in USH2A and MYO7A

AIMS

Using optical coherence tomography angiography (OCTA) to characterise microvascular changes in the retinal plexuses and choriocapillaris (CC) of patients with MYO7A and USH2A mutations and correlate with genotype, retinal structure and function.

METHODS

Twenty-seven patients with molecularly confirmed USH2A (n=21) and MYO7A (n=6) mutations underwent macular 6×6 mm OCTA using the AngioVue. Heidelberg spectral-domain OCT scans and MAIA microperimetry were also performed, the preserved ellipsoid zone (EZ) band width and mean macular sensitivity (MS) were recorded. OCTA of the inner retina, superficial capillary plexus (SCP), deep capillary plexus (DCP) and CC were analysed. Vessel density (VD) was calculated from the en face OCT angiograms of retinal circulation.

RESULTS

Forty-eight eyes with either USH2A (n=37, mean age: 34.4±12.2 years) or MYO7A (n=11, mean age: 37.1±12.4 years), and 35 eyes from 18 age-matched healthy participants were included. VD was significantly decreased in the retinal circulation of patients with USH2A and MYO7A mutations compared with controls (p<0.001). Changes were observed in both the SCP and DCP, but no differences in retinal perfusion were detected between USH2A and MYO7A groups. No vascular defects were detected in CC of the USH2A group, but peripheral defects were detected in older MYO7A patients from the fourth decade of life. VD in the DCP showed strong association with MS and EZ width (Spearman's rho =0.64 and 0.59, respectively, p<0.001).

CONCLUSION

OCTA was able to detect similar retinal microvascular changes in patients with USH2A and MYO7A mutations. The CC was generally affected in MYO7A mutations. OCT angiography may further enhance our understanding of inherited eye diseases and their phenotype-genotype associations.

Quantitative progression of retinitis pigmentosa by optical coherence tomography angiography

Optical coherence tomography angiography (OCT-A) is a non-invasive alternative to fluorescein angiography that allows for the study of the retinal and choroidal vasculatures. In this retrospective cohort study of 28 patients with retinitis pigmentosa (RP), we used OCT-A to quantify changes in perfusion density, foveal avascular zone (FAZ) area, and choriocapillaris blood flow over time and correlated these variables with ellipsoid zone (EZ) line width and best-corrected visual acuity (BCVA). Perfusion density decreased by 2.42 ± 0.62% per year at the superior capillary plexus (SCP) (P = 0.001) and 2.41 ± 0.76% per year at the deep capillary plexus (DCP) (P = 0.004). FAZ area increased by 0.078 ± 0.021 mm² per year (P = 0.001) at the SCP and 0.152 ± 0.039 mm² per year (P = 0.001) at the DCP. No changes were observed in the choriocapillaris blood flow. EZ line width had the strongest correlation to perfusion density at the SCP (r = 0.660 and 0.635, first and second visit, respectively, P = 0.001), while BCVA most strongly correlated with FAZ area at the SCP (r = 0.679 and 0.548, P = 0.001 and 0.003). Our results suggest that OCT-A is a useful tool for monitoring RP disease progression and may be used to measure retinal vascular parameters as outcomes in clinical trials.

Preclinical Arterial Spin Labeling Measurement of Cerebral Blood Flow

Magnetic resonance imaging has been utilized as a quantitative and noninvasive method to image blood flow. Arterial spin labeling (ASL) is an MRI technique that images blood flow using arterial blood water as an endogenous tracer. Herein we describe the use of ASL to measure cerebral blood flow completely noninvasively in rodents, including methods, analysis, and important considerations when utilizing this technique.

Application of Arterial Spin Labelling in the Assessment of Ocular Tissues

Arterial spin labelling (ASL) is a noninvasive magnetic resonance imaging (MRI) modality, capable of measuring blood perfusion without the use of a contrast agent. While ASL implementation for imaging the brain and monitoring cerebral blood flow has been reviewed in depth, the technique is yet to be widely used for ocular tissue imaging. The human retina is a very thin but highly stratified structure and it is also situated close to the surface of the body which is not ideal for MR imaging. Hence, the application of MR imaging and ASL in particular has been very challenging for ocular tissues and retina. That is despite the fact that almost all of retinal pathologies are accompanied by blood perfusion irregularities. In this review article, we have focused on the technical aspects of the ASL and their implications for its optimum adaptation for retinal blood perfusion monitoring. Retinal blood perfusion has been assessed through qualitative or invasive quantitative methods but the prospect of imaging flow using ASL would increase monitoring and assessment of retinal pathologies. The review provides details of ASL application in human ocular blood flow assessment.

Magnetic Resonance Imaging of Cerebral Blood Flow in Animal Stroke Models

Perfusion could provide useful information on the metabolic status and functional status of tissues and organs. This review summarizes the most commonly used perfusion measurement methods: Dynamic susceptibility contrast (DSC) and arterial spin labeling (ASL) and their applications in experimental stroke. Some new developments of cerebral blood flow (CBF) techniques in animal models are also discussed.

Steady pulsed imaging and labeling scheme for noninvasive perfusion imaging

PURPOSE

A steady pulsed imaging and labeling (SPIL) scheme is proposed to obtain high-resolution multislice perfusion images of mice brain using standard preclinical MRI equipment.

THEORY AND METHODS

The SPIL scheme repeats a pulsed arterial spin labeling (PASL) module together with a short mixing time to extend the temporal duration of the generated PASL bolus to the total experimental time. Multislice image acquisition takes place during the mixing times. The mixing time is also used for magnetization recovery following image acquisition. The new scheme is able to yield multislice perfusion images rapidly. The perfusion kinetic curve can be measured by a multipulsed imaging and labeling (MPIL) scheme, i.e., acquiring single-slice ASL signals before reaching steady-state in the SPIL sequence.

RESULTS

When applying the SPIL method to normal mice, and to mice with unilateral ischemia, high-resolution multislice (five slices) CBF images could be obtained in 8 min. Perfusion data from ischemic mice showed clear CBF reductions in ischemic regions. The SPIL method was also applied to postmortem mice, showing that the method is free from magnetization transfer confounds.

CONCLUSION

The new SPIL scheme provides for robust measurement of CBF with multislice imaging capability in small animals.

Association of retinal vessel attenuation with visual function in eyes with retinitis pigmentosa

Purpose

To investigate the association between visual changes and retinal vessel attenuation in patients with retinitis pigmentosa (RP).

Design

A retrospective, longitudinal, observational cohort study.

Methods

We analyzed 45 eyes from 45 subjects who were followed-up for ≥3 years at our clinic. Using the computer-based Interactive Vessel Analysis program, central retinal artery equivalent (CRAE) and central retinal vein equivalent (CRVE) were determined. Age- and sex-matched controls from normal subjects were selected from our archived fundus photograph library. Visual acuity, visual field area (Goldmann perimetry, V4e white test light), mean deviation (Humphrey perimetry, central 10-2 program), and central macular thickness (optical coherence tomography) were analyzed for correlations with CRAE and CRVE.

Results

Both CRAE and CRVE were significantly decreased in RP eyes (94.9±13.5 μm and 155.6±20.0 μm, respectively) compared with control eyes (138.1±14.7 μm and 215.0±20.4 μm, respectively, both P<0.001). After 3 years of follow-up, visual field area was associated with both CRAE (r=0.584, P<0.01) and CRVE (r=0.500, P=0.008). A significant association was also observed between mean deviation and CRAE (r=0.298, P=0.047). In eyes with RP, a narrower vessel caliber at baseline was associated with a larger decline in visual acuity over the 3-year follow-up interval (CRAE: r=−0.344, P=0.021; CRVE: r=−0.314, P=0.035).

Conclusion

Retinal vessel caliber is associated with some visual functions in patients with RP.

Magnetic resonance imaging of the retina: from mice to men

This mini-review provides an overview of magnetic resonance imaging (MRI) applications to study rodent, cat, non-human primate, and human retinas. These techniques include T(1) - and T(2) -weighted anatomical, diffusion, blood flow, blood volume, blood-oxygenation level dependent, manganese-enhanced, physiological, and functional MRI. Applications to study the retinas in diabetic retinopathy, glaucoma, and retinal degeneration are also reviewed. MRI offers some unique advantages compared with existing imaging techniques and has the potential to further our understanding of physiology and function in healthy and diseased retinas.

Choroidal blood flow decreases with age: an MRI study

PURPOSE

To verify that a visual fixation protocol with cued eye blinks achieves sufficient stability for magnetic resonance imaging (MRI) blood-flow measurements and to determine if choroidal blood flow (ChBF) changes with age in humans.

METHODS

The visual fixation stability achievable during an MRI scan was measured in five normal subjects using an eye-tracking camera outside the MRI scanner. Subjects were instructed to blink immediately after recorded MRI sound cues but to otherwise maintain stable visual fixation on a small target. Using this fixation protocol, ChBF was measured with MRI using a 3 Tesla clinical scanner in 17 normal subjects (24-68 years old). Arterial and intraocular pressures (IOP) were measured to calculate perfusion pressure in the same subjects.

RESULTS

The mean temporal fluctuations (standard deviation) of the horizontal and vertical displacements were 29 ± 9 μm and 38 ± 11 μm within individual fixation periods, and 50 ± 34 μm and 48 ± 19 μm across different fixation periods. The absolute displacements were 67 ± 31 μm and 81 ± 26 μm. ChBF was negatively correlated with age (R = -0.7, p = 0.003), declining 2.7 ml/100 ml/min per year. There were no significant correlations between ChBF versus perfusion pressure, arterial pressure, or IOP. There were also no significant correlations between age versus perfusion pressure, arterial pressure, or IOP. Multiple regression analysis indicated that age was the only measured independent variable that was significantly correlated with ChBF (p = 0.03).

CONCLUSIONS

The visual fixation protocol with cued eye blinks was effective in achieving sufficient stability for MRI measurements. ChBF had a significant negative correlation with age.

Comparison of retinal and cerebral blood flow between continuous arterial spin labeling MRI and fluorescent microsphere techniques

PURPOSE

To compare basal retinal and cerebral blood flow (BF) values using continuous arterial spin labeling (CASL) MRI and fluorescent microspheres.

MATERIALS AND METHODS

A total of 41 animals were used. BF was measured using an established microsphere technique (a mixture of 2.5 million 8 μm green and 0.5 million 10 μm blue fluorescent microspheres) and CASL MRI blood flow measurement in the rat retina and brain at 7 Tesla (T) and 11.7T, respectively.

RESULTS

Retinal BF by MRI was 1.18 ± 0.57 mL/g/min and choroidal BF was 8.14 ± 1.8 mL/g/min (n = 6). Microsphere retinal BF was 9.12 ± 2.8 μL/min per tissue and choroidal BF was 73.38 ± 44 μL/min per tissue (n = 18), corresponding to a retinal BF value of 1.22 ± 0.36 mL/g/min by means of a wet weight conversion. The wet-weight of the choroid could not be determined. To corroborate our findings, cerebral BF (CBF) by MRI was also analyzed. In the cerebral cortices, CBF was 0.91 ± 0.29 mL/g/min (n = 14) by CASL MRI and 1.09 ± 0.37 mL/g/min (n = 6) by microspheres. There were no significant differences found between MRI and microsphere blood flow in the retina and brain.

CONCLUSION

BF values in the rat retina and cerebral cortex by MRI are in agreement with those obtained by the microsphere technique.

Balanced steady state free precession for arterial spin labeling MRI: Initial experience for blood flow mapping in human brain, retina, and kidney

We implemented pseudo-continuous ASL (pCASL) with 2D and 3D balanced steady state free precession (bSSFP) readout for mapping blood flow in the human brain, retina, and kidney, free of distortion and signal dropout, which are typically observed in the most commonly used echo-planar imaging acquisition. High resolution functional brain imaging in the human visual cortex was feasible with 3D bSSFP pCASL. Blood flow of the human retina could be imaged with pCASL and bSSFP in conjunction with a phase cycling approach to suppress the banding artifacts associated with bSSFP. Furthermore, bSSFP based pCASL enabled us to map renal blood flow within a single breath hold. Control and test-retest experiments suggested that the measured blood flow values in retina and kidney were reliable. Because there is no specific imaging tool for mapping human retina blood flow and the standard contrast agent technique for mapping renal blood flow can cause problems for patients with kidney dysfunction, bSSFP based pCASL may provide a useful tool for the diagnosis of retinal and renal diseases and can complement existing imaging techniques.

Decreased retinal-choroidal blood flow in retinitis pigmentosa as measured by MRI

PURPOSE

To evaluate retinal and choroidal blood flow (BF) using high-resolution magnetic resonance imaging (MRI) as well as visual function measured by the electroretinogram (ERG) in patients with retinitis pigmentosa (RP).

METHODS

MRI studies were performed in 6 RP patients (29-67 years) and 5 healthy volunteers (29-64 years) on a 3-Tesla scanner with a custom-made surface coil. Quantitative BF was measured using the pseudo-continuous arterial spin-labeling technique at 0.5 × 0.8 × 6.0 mm. Full-field ERGs of all patients were recorded. Amplitudes and implicit times of standard ERGs were analyzed.

RESULTS

Basal BF in the posterior retinal-choroid was 142 ± 16 ml/100ml/min (or 1.14 ± 0.13 μl/mm(2)/min) in the control group and was 70 ±19 ml/100ml/min (or 0.56 ± 0.15 μl/mm(2)/min) in the RP group. Retinal-choroidal BF was significantly reduced by 52 ± 8 % in RP patients compared to controls (P<0.05). ERG a- and b-wave amplitudes of RP patients were reduced, and b-wave implicit times were delayed. There were statistically significant correlations between a-wave amplitude and BF value (r=0.9, P<0.05) but not between b-wave amplitude and BF value (r =0.7, P=0.2).

CONCLUSIONS

This study demonstrates a novel non-invasive MRI approach to measure quantitative retinal and choroidal BF in RP patients. We found that retinal-choroidal BF was markedly reduced and significantly correlated with reduced amplitudes of the a-wave of the standard combined ERG.

Layer-specific blood-flow MRI of retinitis pigmentosa in RCS rats

The Royal College of Surgeons (RCS) rat is an established animal model of retinitis pigmentosa, a family of inherited retinal diseases which starts with loss of peripheral vision and progresses to eventual blindness. Blood flow (BF), an important physiological parameter, is intricately coupled to metabolic function under normal physiological conditions and is perturbed in many neurological and retinal diseases. This study reports non-invasive high-resolution MRI (44 × 44 × 600 μm) to image quantitative retinal and choroidal BF and layer-specific retinal thicknesses in RCS rat retinas at different stages of retinal degeneration compared with age-matched controls. The unique ability to separate retinal and choroidal BF was made possible by the depth-resolved MRI technique. RBF decreased with progressive retinal degeneration, but ChBF did not change in RCS rats up to post-natal day 90. We concluded that choroidal and retinal circulations have different susceptibility to progressive retinal degeneration in RCS rats. Layer-specific retinal thickness became progressively thinner and was corroborated by histological analysis in the same animals. MRI can detect progressive anatomical and BF changes during retinal degeneration with laminar resolution.