Lamina-specific anatomic magnetic resonance imaging of the human retina

Imaging the eye as a window to brain health: frontier approaches and future directions

Recent years have seen significant advances in diagnostic testing of central nervous system (CNS) function and disease. However, there remain challenges in developing a comprehensive suite of non- or minimally invasive assays of neural health and disease progression. Due to the direct connection with the CNS, structural changes in the neural retina, retinal vasculature and morphological changes in retinal immune cells can occur in parallel with disease conditions in the brain. The retina can also, uniquely, be assessed directly and non-invasively. For these reasons, the retina may prove to be an important "window" for revealing and understanding brain disease. In this review, we discuss the gross anatomy of the eye, focusing on the sensory and non-sensory cells of the retina, especially microglia, that lend themselves to diagnosing brain disease by imaging the retina. We include a history of ocular imaging to describe the different imaging approaches undertaken in the past and outline current and emerging technologies including retinal autofluorescence imaging, Raman spectroscopy, and artificial intelligence image analysis. These new technologies show promising potential for retinal imaging to be used as a tool for the diagnosis of brain disorders such as Alzheimer's disease and others and the assessment of treatment success.

3T high-resolution magnetic resonance imaging, conventional ultrasonography and ultrasound biomicroscopy of the normal canine eye

Background

Advances in MRI coil technology and increased availability of high-field MRI in veterinary medicine enable the acquisition of images of increasingly high spatial resolution while preserving signal-to-noise ratio.The purpose of the present study was to compare 3T high-resolution magnetic resonance imaging (HR-MRI) with ultrasound (US) and ultrasound biomicroscopy (UBM) in the normal canine eye, to assess its potential to depict normal ocular anatomy.

Results

HR-MRI was compared with US and UBM in 10 eyes from 10 healthy beagle dogs. Ocular structures (cornea, anterior chamber, iridocorneal angle, iris, lens, ciliary body, choroid, vitreous body, posterior wall of the eye, optic nerve and optic nerve sheath, extraocular muscles) were assessed subjectively and central corneal thickness (CCT), anterior chamber depth (ACD), aqueous depth (AQD), anteroposterior, mediolateral and dorsoventral lens diameter (APLD, MLLD, DVLD), anteroposterior diameter of the globe including and excluding the scleroretinal rim (APDSRR, APD), vitreous chamber depth (VCD) and optic nerve sheath diameter (ONSD) were measured in HR-MRI and in US. Optic nerve diameter (OND) was measured in HR-MRI. HR-MRI and UBM appearance of the anterior segment were subjectively compared. Detailed reference high-resolution MRI images of normal eyes of Beagle dogs are provided.

Conclusions

HR-MRI allowed assessment of all structures identified with US and UBM. The MRI examinations were performed under general anesthesia with the addition of a neuromuscular blocking agent, while US and UBM examinations were performed in conscious animals. Visibility of the entire ocular wall, the lens, the structures caudal to the ciliary body and the optic nerve and its sheath was superior with HR-MRI. HR-MRI allowed the distinction of retina, choroid and sclera, and the delineation of structures not previously identified in canine eyes with MRI, including Tenon’s capsule and the sub-Tenon’s space.Plane selection was more accurate with HR-MRI compared to US. In general, the range of measurements was narrower for MRI than for US. CCT, AQD, APLD, MLLD, APD, APDSRR and ONSD differed significantly between HR-MRI and US, respectively (p = 0.005-0.027).Micro-MRI may be useful for the assessment of ocular pathologies in the future.

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.

Motion degradation in optic nerve MRI: A randomized intraindividual comparison study of eye states

PURPOSE

MRI is a powerful tool for optic nerve assessment, but image quality can be degraded by artifacts related to ocular motion. The purpose of this investigation was to evaluate the effect of undergoing MRI with eyes open versus closed on the degree of motion degradation affecting the optic nerves.

METHOD

Patients undergoing 3 Tesla orbital MRI were randomized to undergo the coronal STIR sequence with eyes open and focused on a standardized fixation point, blinking as needed, or with eyes closed. The sequence was then performed again with the other instruction set. Two neuroradiologists rated the intraorbital optic nerves for motion artifact on a 5-point scale (higher numbers reflecting greater motion artifact) in 2 locations of each nerve. Differences were evaluated by the clustered Wilcoxon signed rank test.

RESULTS

Seventy-seven orbits were included. Interrater reliability was high (weighted kappa = 0.78). The anterior intraorbital optic nerves were rated with less motion artifact when eyes were open and focused during acquisition than when closed (p = 0.006), but this was not the case for the posterior intraorbital optic nerve (p = 0.69). For example, at the anterior intraorbital optic nerve, motion artifact of mean grade better than 2 was seen in 60% of eyes-open vs. 32% of eyes-closed acquisitions, while mean grade 4 or worse was seen in 4% of eyes-open vs. 12% of eyes-closed acquisitions.

CONCLUSION

Undergoing orbital MRI with eyes open and focused rather than closed reduces motion artifact at the anterior intraorbital segment of the optic nerve.

Comparison of choroidal thickness in systemic hypertensive subjects with healthy individuals by spectral domain optical coherence tomography

Purpose:

The purpose of the study was to compare the choroidal thickness in normal population and hypertensive patients and to assess the possible effect of hypertension on choroidal thickness using Spectral Domain Optical Coherence Tomography (SD-OCT).

Methods:

This was a comparative cross-sectional study. A total of 68 eyes of 34 individuals in the age group of 40–60 years were included in both the hypertensive group and control group. Individuals with refractive error beyond ± 3 D and posterior segment pathology were excluded. The choroidal thickness was measured at the sub-foveal region, 500 μm nasal and 500 μm temporal to the fovea on SD-OCT with enhanced depth imaging (EDI) mode. Systolic blood pressure (SBP), Diastolic blood pressure (DBP), and Mean arterial pressure (MAP) were recorded in all individuals. Duration of hypertension was also noted in hypertensive individuals.

Results:

The choroidal thickness at all locations was significantly lower in the hypertensive group (subfoveal, nasal, temporal and mean choroidal thickness 253.24 ± 63.96 μm, 249.35 ± 63.57 μm, 250.01 ± 63.37 μm, 250.87 ± 63.38 μm, respectively) as compared to the control group (subfoveal, nasal, temporal and mean choroidal thickness 301.25 ± 55.79 μm, 298.97 ± 57.07 μm, 299.49 ± 55.06 μm, 299.90 ± 55.50 μm, respectively). The choroidal thickness in the hypertensive group also had a significant negative correlation with the SBP (Spearman correlation coefficient, rho = –0.35, P = 0.003) and the duration of hypertension (rho = -0.25, P = 0.037).

Conclusion:

The study demonstrated decreased choroidal thickness in systemic hypertensive subjects as compared to age-matched healthy individuals. The choroidal thickness in hypertensive subjects also had a significant but weak negative correlation with SBP and duration of hypertension.

3-Dimensional magnetic resonance imaging of the freely moving human eye

Eye motion is a major confound for magnetic resonance imaging (MRI) in neuroscience or ophthalmology. Currently, solutions toward eye stabilisation include participants fixating or administration of paralytics/anaesthetics. We developed a novel MRI protocol for acquiring 3-dimensional images while the eye freely moves. Eye motion serves as the basis for image reconstruction, rather than an impediment. We fully reconstruct videos of the moving eye and head. We quantitatively validate data quality with millimetre resolution in two ways for individual participants. First, eye position based on reconstructed images correlated with simultaneous eye-tracking. Second, the reconstructed images preserve anatomical properties; the eye's axial length measured from MRI images matched that obtained with ocular biometry. The technique operates on a standard clinical setup, without necessitating specialized hardware, facilitating wide deployment. In clinical practice, we anticipate that this may help reduce burdens on both patients and infrastructure, by integrating multiple varieties of assessments into a single comprehensive session. More generally, our protocol is a harbinger for removing the necessity of fixation, thereby opening new opportunities for ethologically-valid, naturalistic paradigms, the inclusion of populations typically unable to stably fixate, and increased translational research such as in awake animals whose eye movements constitute an accessible behavioural readout.

Multiband diffusion-weighted MRI of the eye and orbit free of geometric distortions using a RARE-EPI hybrid

Diffusion-weighted imaging (DWI) provides information on tissue microstructure. Single-shot echo planar imaging (EPI) is the most common technique for DWI applications in the brain, but is prone to geometric distortions and signal voids. Rapid acquisition with relaxation enhancement [RARE, also known as fast spin echo (FSE)] imaging presents a valuable alternative to DWI with high anatomical accuracy. This work proposes a multi-shot diffusion-weighted RARE-EPI hybrid pulse sequence, combining the anatomical integrity of RARE with the imaging speed and radiofrequency (RF) power deposition advantage of EPI. The anatomical integrity of RARE-EPI was demonstrated and quantified by center of gravity analysis for both morphological images and diffusion-weighted acquisitions in phantom and in vivo experiments at 3.0 T and 7.0 T. The results indicate that half of the RARE echoes in the echo train can be replaced by EPI echoes whilst maintaining anatomical accuracy. The reduced RF power deposition of RARE-EPI enabled multiband RF pulses facilitating simultaneous multi-slice imaging. This study shows that diffusion-weighted RARE-EPI has the capability to acquire high fidelity, distortion-free images of the eye and the orbit. It is shown that RARE-EPI maintains the immunity to B₀ inhomogeneities reported for RARE imaging. This benefit can be exploited for the assessment of ocular masses and pathological changes of the eye and the orbit.

Track-weighted imaging for neuroretina: Evaluations in healthy volunteers and ischemic optic neuropathy

BACKGROUND

The use of MRI-tractography to explore the human neuroretina is yet to be reported. Track-weighted imaging (TWI) was recently introduced as a qualitative tractography-based method with high anatomical contrast.

PURPOSE

To explore the human retina in healthy volunteers and patients with anterior ischemic optic neuropathy (AION) using TWI reconstructions.

STUDY TYPE

Prospective.

POPULATION

Twenty AION patients compared with 20 healthy volunteers.

FIELD STRENGTH/SEQUENCE

3.0T MRI diffusion-weighted imaging (DWI) with b-value of 1000 s/mm2 and 60 diffusion-weighting noncollinear directions.

ASSESSMENT

We performed constrained spherical deconvolution from the diffusion-weighted signal and volumetric tractography method, whereby 10 million streamlines are initiated from seed points randomly distributed throughout the orbital area. We then reconstructed TWI maps with isotropic voxel size of 300 μm.

STATISTICAL TESTS

We tested the effect of the number of diffusion-weighting directions, ocular laterality, and ocular dominance on healthy retinal fascicles distribution. We then performed factorial analysis of variance to test the effects of the presence/absence of the fascicles on the visual field defect in patients.

RESULTS

In healthy volunteers, we found more temporal fascicle in right eyes (P = 0.001), more superior fascicles in dominant eyes (P = 0.014), and fewer fascicles with tractography maps based on 30 directions than those based on 45 directions (P = 9 × 10-8 ) and 60 directions (P = 6 × 10-7 ). Eight out of 20 AION patients presented with complete absence of neuroretinal fascicle, side of the disease, which was correlated with visual field mean deviation at the 6-month visit [F(1,17) = 6.97, P = 0.016]. Seven patients presented with a temporal fascicle in the injured eye; this fascicle presence was linked to visual field mean deviation at the 6-month visit [F(1,17) = 8.43, P = 0.009].

DATA CONCLUSION

In AION patients, the presence of the temporal neuroretinal fascicle in the affected eye provides an objective outcome radiological sign correlated with visual performance.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018.

Past, Present, and Future Concepts of the Choroidal Scleral Interface Morphology on Optical Coherence Tomography

The choroid is the most vascular tissue in the eye and it has been implicated in the pathophysiology of a variety of ocular diseases. A new era of research in the choroid began with the improved ability to visualize this layer and its inner and outer boundaries using spectral domain optical coherence tomography (OCT) with enhanced depth imaging and swept source OCT. The accuracy and precision of qualitative and quan-titative assessments of the choroidal layer support the potential use of OCT-derived choroidal parameters for diagnosis, monitoring of disease progression, planning surgical access, and evaluating treatment response. Although there is increasing interest in measuring choroidal thickness, there is currently no consensus nomenclature to classify choroidal layers and boundaries. Furthermore, the definition and description of the choroidal scleral interface is inconsistent in the literature, contributing to interstudy variation in choroidal thickness measurements. The purpose of this review is to provide an overview of the literature on the definition of choroidal layers and choroidal scleral boundary, review the discrepan-cies, and harmonize the terminology so that a consensus nomenclature can be proposed.

Non-invasive MRI Assessments of Tissue Microstructures and Macromolecules in the Eye upon Biomechanical or Biochemical Modulation

The microstructural organization and composition of the corneoscleral shell (CSS) determine the biomechanical behavior of the eye, and are important in diseases such as glaucoma and myopia. However, limited techniques can assess these properties globally, non-invasively and quantitatively. In this study, we hypothesized that multi-modal magnetic resonance imaging (MRI) can reveal the effects of biomechanical or biochemical modulation on CSS. Upon intraocular pressure (IOP) elevation, CSS appeared hyperintense in both freshly prepared ovine eyes and living rat eyes using T2-weighted MRI. Quantitatively, transverse relaxation time (T2) of CSS increased non-linearly with IOP at 0-40 mmHg and remained longer than unloaded tissues after being unpressurized. IOP loading also increased fractional anisotropy of CSS in diffusion tensor MRI without apparent change in magnetization transfer MRI, suggestive of straightening of microstructural fibers without modification of macromolecular contents. Lastly, treatments with increasing glyceraldehyde (mimicking crosslinking conditions) and chondroitinase-ABC concentrations (mimicking glycosaminoglycan depletion) decreased diffusivities and increased magnetization transfer in cornea, whereas glyceraldehyde also increased magnetization transfer in sclera. In summary, we demonstrated the changing profiles of MRI contrast mechanisms resulting from biomechanical or biochemical modulation of the eye non-invasively. Multi-modal MRI may help evaluate the pathophysiological mechanisms in CSS and the efficacy of corneoscleral treatments.

Risk factors for poor outcomes in patients with open-globe injuries

Purpose

The aim of this study was to identify the risk factors that are predictive of poor outcomes in penetrating globe trauma.

Patients and methods

This retrospective case series evaluated 103 eyes that had been surgically treated for an open-globe injury from 2007 to 2010 at the eye clinic of the University of Virginia. A total of 64 eyes with complete medical records and at least 6 months of follow-up were included in the study. Four risk factors (preoperative best-corrected visual acuity [pre-op BCVA], ocular trauma score [OTS], zone of injury [ZOI], and time lapse [TL] between injury and primary repair) and three outcomes (final BCVA, monthly rate of additional surgeries [MRAS], and enucleation) were identified for analysis.

Results

Pre-op BCVA was positively associated with MRAS, final BCVA, and enucleation. Calculated OTS was negatively associated with the outcome variables. No association was found between TL and ZOI with the outcome variables. Further, age and predictor variable-adjusted analyses showed pre-op BCVA to be independently positively associated with MRAS (P=0.008) and with final BCVA (P<0.001), while the calculated OTS was independently negatively associated with final BCVA (P<0.001), but not uniquely associated with MRAS (P=0.530).

Conclusion

Pre-op BCVA and OTS are best correlated with prognosis in open-globe injuries. However, no conventional features reliably predict the outcome of traumatized eyes.

Incorporating dixon multi-echo fat water separation for novel quantitative magnetization transfer of the human optic nerve in vivo

PURPOSE

The optic nerve (ON) represents the sole pathway between the eyes and brain; consequently, diseases of the ON can have dramatic effects on vision. However, quantitative magnetization transfer (qMT) applications in the ON have been limited to ex vivo studies, in part because of the fatty connective tissue that surrounds the ON, confounding the magnetization transfer (MT) experiment. Therefore, the aim of this study was to implement a multi-echo Dixon fat-water separation approach to remove the fat component from MT images.

METHODS

MT measurements were taken in a single slice of the ON and frontal lobe using a three-echo Dixon readout, and the water and out-of-phase images were applied to a two-pool model in ON tissue and brain white matter to evaluate the effectiveness of using Dixon fat-water separation to remove fatty tissue from MT images.

RESULTS

White matter data showed no significant differences between image types; however, there was a significant increase (p < 0.05) in variation in the out-of-phase images in the ON relative to the water images.

CONCLUSIONS

The results of this study demonstrate that Dixon fat-water separation can be robustly used for accurate MT quantification of anatomies susceptible to partial volume effects resulting from fat. Magn Reson Med 77:707-716, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

MRI of rod cell compartment-specific function in disease and treatment in vivo

Rod cell oxidative stress is a major pathogenic factor in retinal disease, such as diabetic retinopathy (DR) and retinitis pigmentosa (RP). Personalized, non-destructive, and targeted treatment for these diseases remains elusive since current imaging methods cannot analytically measure treatment efficacy against rod cell compartment-specific oxidative stress in vivo. Over the last decade, novel MRI-based approaches that address this technology gap have been developed. This review summarizes progress in the development of MRI since 2006 that enables earlier evaluation of the impact of disease on rod cell compartment-specific function and the efficacy of anti-oxidant treatment than is currently possible with other methods. Most of the new assays of rod cell compartment-specific function are based on endogenous contrast mechanisms, and this is expected to facilitate their translation into patients with DR and RP, and other oxidative stress-based retinal diseases.

Magic angle-enhanced MRI of fibrous microstructures in sclera and cornea with and without intraocular pressure loading

PURPOSE

The structure and biomechanics of the sclera and cornea are central to several eye diseases such as glaucoma and myopia. However, their roles remain unclear, partly because of limited noninvasive techniques to assess their fibrous microstructures globally, longitudinally, and quantitatively. We hypothesized that magic angle-enhanced magnetic resonance imaging (MRI) can reveal the structural details of the corneoscleral shell and their changes upon intraocular pressure (IOP) elevation.

METHODS

Seven ovine eyes were extracted and fixed at IOP = 50 mm Hg to mimic ocular hypertension, and another 11 eyes were unpressurized. The sclera and cornea were scanned at different angular orientations relative to the main magnetic field inside a 9.4-Tesla MRI scanner. Relative MRI signal intensities and intrinsic transverse relaxation times (T2 and T2*) were determined to quantify the magic angle effect on the corneoscleral shells. Three loaded and eight unloaded tendon samples were scanned as controls.

RESULTS

At magic angle, high-resolution MRI revealed distinct scleral and corneal lamellar fibers, and light/dark bands indicative of collagen fiber crimps in the sclera and tendon. Magic angle enhancement effect was the strongest in tendon and the least strong in cornea. Loaded sclera, cornea, and tendon possessed significantly higher T2 and T2* than unloaded tissues at magic angle.

CONCLUSIONS

Magic angle-enhanced MRI can detect ocular fibrous microstructures without contrast agents or coatings and can reveal their MR tissue property changes with IOP loading. This technique may open up new avenues for assessment of the biomechanical and biochemical properties of ocular tissues in aging and in diseases involving the corneoscleral shell.

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.

High-resolution MRI of uveal melanoma using a microcoil phased array at 7 T

High-field MRI is a promising technique for the characterisation of ocular tumours, both in vivo and after enucleation. For in vivo imaging at 7 T, a dedicated three-element microcoil array was constructed as a high-sensitivity receive-only device. Using a dedicated blink/fixation protocol, high-resolution in vivo images could be acquired within 3 min in volunteers and patients with no requirement for post-acquisition image registration. Quantitative measures of axial length, aqueous depth and lens thickness in a healthy volunteer were found to agree well with standard ocular biometric techniques. In a patient with uveal melanoma, in vivo MRI gave excellent tumour/aqueous body contrast. Ex vivo imaging of the enucleated eye showed significant heterogeneity within the tumour.

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.

Human vitreous: MR imaging of oxygen partial pressure

PURPOSE

To develop a magnetic resonance (MR) imaging approach to noninvasively image quantitative Po(2) in the human vitreous.

MATERIALS AND METHODS

Human studies were approved by the institutional review board with informed consent obtained from all subjects and were HIPAA compliant. Animal studies were performed with animal care committee approval. An MR imaging method to measure the longitudinal relaxation rate, or R1, of water was implemented with a 3.0-T MR imager. R1 was calibrated in water phantoms at multiple Po(2) and temperature conditions (n = 10) and in ex vivo animal vitreous (n = 2). Vitreous Po(2) was imaged in three human volunteers (age range, 26-28 years) in multiple sessions on separate days to evaluate reproducibility. The effects of temperature and ambient air were evaluated by acquiring data with the eye open and closed. Statistical analysis consisted of t tests, with P less than .05 indicating significant difference.

RESULTS

Calibrations of phantoms and ex vivo vitreous yielded an R1 association with oxygen of 0.209 sec(-1) + Po(2) ⋅ 2.07 × 10(-4) sec(-1)/mm Hg at 37°C, and an association with temperature (Δ[1/R1]/ΔTemperature) of 0.106 sec/°C ± 0.009 (standard deviation). A difference in R1 was found between the phantoms and vitreous. If uncorrected, vitreal Po(2) would be significantly overestimated (P < .001). In vivo human vitreous Po(2) maps were spatially heterogeneous, with a whole vitreous Po(2) of 16.7 mm Hg ± 6.5 (eye closed). Measurements between open and closed eyes showed spatially dependent R1 differences, which translated to temperature differences of 0.34°-0.83°C across the eye.

CONCLUSION

This study established an MR imaging protocol to image quantitative vitreous Po(2) noninvasively and evaluated effects from vitreal macromolecules, temperature gradients, and ambient air on vitreal Po(2) values. Measurement of vitreous Po(2) with MR imaging has the potential to be used to study eye diseases noninvasively.

Review: magnetic resonance imaging techniques in ophthalmology

Imaging the eye with magnetic resonance imaging (MRI) has proved difficult due to the eye's propensity to move involuntarily over typical imaging timescales, obscuring the fine structure in the eye due to the resulting motion artifacts. However, advances in MRI technology help to mitigate such drawbacks, enabling the acquisition of high spatiotemporal resolution images with a variety of contrast mechanisms. This review aims to classify the MRI techniques used to date in clinical and preclinical ophthalmologic studies, describing the qualitative and quantitative information that may be extracted and how this may inform on ocular pathophysiology.

Layer-specific manganese-enhanced MRI of the retina in light and dark adaptation

PURPOSE

To employ functional manganese-enhanced MRI (MEMRI) to image layer-specific changes in calcium-dependent activities in the rat retina during light versus dark adaptation.

METHODS

Functional MEMRI at 20 × 20 × 700 μm was used to study light and dark adaptation in the same animals (N = 10) in which one eye was covered and the fellow eye was not. The activity encoding of the light and dark adaptation was achieved in awake conditions and imaged under anesthesia. T(1)-weighted MRI at 11.7 tesla (T) was performed using two identical radiofrequency transceiver coils to allow interleaved MRI acquisitions of the two eyes. An intravascular contrast agent was also used to verify layer assignments.

RESULTS

MEMRI detected contrasts among the inner retina, outer retina, and choroid. Independent confirmation of the vascular layers and boundaries between layers was documented with an intravascular contrast agent. The retinal layer thicknesses agreed with published data. The outer retina had lower MEMRI activity in light compared with dark adaption (P < 0.001), consistent with the increased metabolic demand associated with the "dark current." The inner retina had higher MEMRI activity in light compared with dark adaption (P < 0.05). The choroid MEMRI activity was not statistically different between light and dark adaptation (P > 0.05).

CONCLUSIONS

This study demonstrated a high-resolution MEMRI protocol to image functional activities among different layers of the retinas in awake animals during light and dark adaptation. This approach could have potential applications in animal models of retinal dysfunction.

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

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.

Manganese-enhanced MRI reveals multiple cellular and vascular layers in normal and degenerated retinas

PURPOSE

To use manganese-enhanced magnetic resonance imaging (MEMRI) at 25 × 25 × 800 μm(3) to image different retinal and vascular layers in the rat retinas.

MATERIALS AND METHODS

Manganese-chloride was injected intraocularly in normal (n = 5) and Royal College of Surgeons (RCS, an model of photoreceptor degeneration) (n = 5) rats at postnatal day 90. MEMRI at 4.7 T was performed 24 hours later. MRI was repeated following intravenous Gd-DTPA in the same animals to highlight the vasculatures. Layer assignment and thickness were compared to histology.

RESULTS

MEMRI 24 hours after intravitreal manganese-chloride injection revealed seven bands of alternating hyper- and hypointensities, corresponding histologically to the ganglion cell layer, inner plexiform layer, inner nuclear layer, outer plexiform layer, outer nuclear layer, photoreceptor-segment layer, and choroidal vascular layer. Intravenous Gd-DTPA-which does not cross the blood-retinal barrier and the retinal pigment epithelium-further enhanced the two layers bounding the retina, corresponding to the retinal and choroidal vascular layers, but not the avascular outer nuclear layer and the photoreceptor-segment layer. MEMRI of the RCS retinas revealed the loss of the outer plexiform layer, outer nuclear layer, and photoreceptor-segment layer. Histological analysis corroborated the MRI laminar assignments and thicknesses.

CONCLUSION

Lamina-specific retinal structures neurodegenerative changes to structure in retinal diseases can be detected using MEMRI.