Ocular blood flow in preterm neonates

Retinopathy of prematurity (ROP) is a disorder affecting low birthweight, preterm neonates. In the preterm eye, the retina is not fully developed and neovascularization may occur at the margin between the developed vascular retina and undeveloped avascular retina. Without timely treatment by laser or intravitreal anti-vascular endothelial growth factor (VEGF) therapy, this can lead to tractional retinal detachment and blindness. Visualization of the retina in regular examinations by indirect ophthalmoscopy is hence the current standard of care, but the exams are stressful and interpretation of images is subjective. The upregulation of VEGF in ROP would suggest an increase in ocular blood flow. In this report, we evaluate the potential of ultrafast plane-wave Doppler ultrasound (PWU) to detect increased flow velocities in the orbital vessels supplying the eye in a gentle exam with objective findings. We imaged both eyes of 50 low-birthweight preterm neonates using 18 MHz PWU. Flow velocity in the central retinal artery (CRA) and vein (CRV), and the short posterior ciliary arteries were determined and values at each ROP Stage compared. We found significantly increased velocities in the CRA and CRV in Stage 3 ROP eyes, where intervention would be considered. We compared multivariate models for identifying Stage 3 eyes comprised solely of clinical factors, solely of Doppler parameters, and clinical plus Doppler parameters. The respective models provided areas under their respective ROC curves of 0.760, 0.812, and 0.904. PWU Doppler represents a gentle, objective means for identifying neonates at risk for ROP that could complement ophthalmoscopy.

Correlation of Ocular Plane-Wave Doppler With Optical Coherence Tomography Angiography in Preeclampsia

OBJECTIVES

Preeclampsia (PE) is a severe complication of pregnancy characterized by hypertension, proteinuria and compromised fetal blood supply. The eye, like other end organs, is affected by this systemic condition, but unlike in other organs, ocular media transparency allows high-resolution optical visualization of the vascular structure of the retina. Our aim was to assess how ultrasound-determined ocular blood-flow correlates with vascular structure of the retina and choriocapillaris determined by optical coherence tomography angiography (OCTA).

METHODS

Plane-wave ultrasound and OCTA were performed on both eyes of 40 consecutive subjects consisting of normal controls (n = 11), mild PE (n = 5), severe PE (n = 17) and chronic or gestational hypertension (n = 7) within 72 hours following delivery. From ultrasound, we measured pulsatile flow velocity and resistance indices in the central retinal artery (CRA) and vein, the short posterior ciliary arteries (SPCAs) and choroid. From OCTA, we measured vascular density (VD) in the superficial, deep retina and choriocapillaris. We determined differences in Doppler and OCTA parameters among groups and correlations between ultrasound and OCTA.

RESULTS

In severe PE, flow resistance was reduced with respect to controls. Flow velocity and resistance in the and SPCA were moderately correlated with VD in the choriocapillaris and peripapillary retina, but VD in PE did not differ significantly from controls.

CONCLUSIONS

Although OCTA parameters were moderately correlated with Doppler ultrasound, OCTA did not demonstrate significant differences between PE and controls postpartum.

Principles of Ophthalmic Ultrasound

Introduction

Ultrasound imaging of the eye was introduced over 50 years ago. While the physical principles of ultrasound imaging have not changed, technology has undergone tremendous and ongoing development.

Areas covered

The fundamentals of ultrasound physics, biometry (A-scan), structural imaging (B-scan) and blood-flow imaging and measurement (Doppler) will be described. Emphasis will be placed on technological development and potential future advances.

Expert opinion

While A- and B-scan ultrasound of the eye has traditionally been performed with focused single-element transducers, the introduction of annular and linear arrays has enhanced clinical utility. Future advances, especially in multielement arrays, and point-of-care systems promise amazing new capabilities for diagnostic imaging of the eye and orbit.

Ultrasound Assessment of Gaze-induced Posterior Eyewall Deformation in Highly Myopic Eyes

Purpose

To establish a quantitative metric of posterior eyewall deformability in different directions of gaze in highly myopic eyes with and without posterior staphyloma.

Methods

A prospective study was performed on 53 highly myopic patients (106 eyes). Ultrasound scans were acquired in primary, up, downward, nasal, and temporal gazes. A validated intensity-based segmentation algorithm was used to quantify the posterior eyewall geometry on digitalized B-scan images. Posterior eyewall local curvature (K) and distance (L) to the transducer were calculated. The associations between directions of gaze, axial length (AL), and presence of staphyloma with the K and L parameters were assessed.

Results

A total of 53 participants (106 eyes) were studied. Multivariate regression analysis demonstrated that, after accounting for longer AL, and presence of staphyloma, eccentric gaze was often independently associated with various K and L parameters. Specifically, downward gaze was associated with increased posterior eyewall concavity as reflected in the maximum of K (KMax) (β = 0.050, P < 0.001) and absolute value of KMax (β = 0.041, P = 0.011). Both downward gaze and upgaze were independently associated with increase in the derivative of absolute KMax (which is consistent with more apparent, steeper staphyloma ridges), local KMax (which detects KMax at smaller intervals), and Kstd (which represents likelihood of staphyloma presence) and decrease in maximum of L (which represents movement of the staphyloma apex) with all P < 0.05. The β coefficients for downward gaze were consistently greater in magnitude compared with those in upgaze. After accounting for AL and presence of staphyloma, horizontal gazes were independently associated only with decrease in the standard deviation of L (which also represents likelihood of staphyloma presence) and maximum of L.

Conclusions

Downward gaze results in a significant increase in posterior eyewall concavity in highly myopic eyes after accounting for AL and staphyloma presence. In comparison with downward gaze, upgaze resulted in a lower magnitude, but significant changes in staphyloma ridge steepness and the likelihood of staphyloma presence. Horizontal gazes seemed to be associated with less posterior eyewall geometric parameters. Studies are required to further assess the association between downward gaze during near work on posterior eyewall concavity and possible effects on myopia development and progression.

Pulsatility and flow patterns across macro- and microcirculatory arteries of continuous-flow left ventricular assist device patients

BACKGROUND

Reduced arterial pulsatility in continuous-flow left ventricular assist devices (CF-LVAD) patients has been implicated in clinical complications. Consequently, recent improvements in clinical outcomes have been attributed to the "artificial pulse" technology inherent to the HeartMate3 (HM3) LVAD. However, the effect of the "artificial pulse" on arterial flow, transmission of pulsatility into the microcirculation and its association with LVAD pump parameters is not known.

METHODS

The local flow oscillation (pulsatility index, PI) of common carotid arteries (CCAs), middle cerebral arteries (MCAs) and central retinal arteries (CRAs-representing the microcirculation) were quantified by 2D-aligned, angle-corrected Doppler ultrasound in 148 participants: healthy controls, n = 32; heart failure (HF), n = 43; HeartMate II (HMII), n = 32; HM3, n = 41.

RESULTS

In HM3 patients, 2D-Doppler PI in beats with "artificial pulse" and beats with "continuous-flow" was similar to that of HMII patients across the macro- and microcirculation. Additionally, peak systolic velocity did not differ between HM3 and HMII patients. Transmission of PI into the microcirculation was higher in both HM3 (during the beats with "artificial pulse") and in HMII patients compared with HF patients. LVAD pump speed was inversely associated with microvascular PI in HMII and HM3 (HMII, r2 = 0.51, p < 0.0001; HM3 "continuous-flow," r2 = 0.32, p = 0.0009; HM3 "artificial pulse," r2 = 0.23, p = 0.007), while LVAD pump PI was only associated with microcirculatory PI in HMII patients.

CONCLUSIONS

The "artificial pulse" of the HM3 is detectable in the macro- and microcirculation but without creating a significant alteration in PI compared with HMII patients. Increased transmission of pulsatility and the association between pump speed and PI in the microcirculation indicate that the future clinical care of HM3 patients may involve individualized pump settings according to the microcirculatory PI in specific end-organs.

Scleral Thickness in Autosomal Dominant Best Vitelliform Macular Dystrophy

PURPOSE

To investigate the posterior and equatorial scleral thickness in patients with autosomal dominant Best disease, a condition that has chronic subretinal fluid.

METHODS

Retrospective study involving patients with Best disease and age-matched controls. Participants were evaluated with contact B-scan ultrasonography and enhanced depth imaging optical coherence tomography to evaluate scleral thickness in the posterior pole and equator. Univariate analysis and generalized estimating equations were used.

RESULTS

Of 9 patients with genetically proven Best disease and 23 age-matched controls, there was no significant difference in the age or the gender proportion between groups. Subfoveal choroidal thickness and axial length were not significantly different between groups. Both posterior scleral (OD; 1.38mm vs. 0.89mm, P<.001 and OS; 1.39mm vs. 0.83mm, P<.001) and equatorial scleral (OD; 0.61mm vs. 0.42mm, P=.003, and OS; 0.55mm vs. 0.41mm, P=.017) thicknesses were much greater in cases as compared with controls. Multivariate analysis showed male sex and having Best disease were each significant predictors of posterior scleral thickness and Best disease was the sole significant predictor for equatorial scleral thickness.

CONCLUSION

BEST1 gene may have a developmental role leading to having a thicker sclera, influencing disease manifestation, and contributing to the accumulation of subretinal fluid in Best disease.

Automated Classification and Detection of Staphyloma with Ultrasound Images in Pathologic Myopia Eyes

The aim of this study was to develop an eyewall curvature- and axial length (AxL)-based algorithm to automate detection (clinician-free) of staphyloma ridge and apex locations using ultrasound (US). Forty-six individuals (with emmetropia, high myopia or pathologic myopia) were enrolled in this study (AxL range: 22.3-39.3 mm), yielding 130 images in total. An intensity-based segmentation algorithm automatically tracked the posterior eyewall, calculating the posterior eyewall local curvature (K) and distance (L) to the transducer and the location of the staphyloma apex. By use of the area under the receiver operator characteristic (AUROC) curve to evaluate the diagnostic ability of eight local statistics derived from K, L and AxL, the algorithm successfully quantified non-uniformity of eye shape with an AUROC > 0.70 for most K-based parameters. The performance of binary classification (staphyloma absence vs. presence) was assessed with the best classifier (the combination of AxL, standard deviation of K and standard deviation of L) yielding a diagnostic validation performance of 0.897, which was comparable to the diagnostic performance of junior clinicians. The staphyloma apex was localized with an average error of 1.35 ± 1.34 mm. Combined with the real-time data acquisition capabilities of US, this method can be employed as a screening tool for clinician-free in vivo staphyloma detection.

Ocular Ultrasound: Review of Bioeffects and Safety, Including Fetal and Point of Care Perspective: Review of Bioeffects and Safety, Including Fetal and Point-of-Care Perspective

Ocular ultrasound is an invaluable tool for the evaluation of the eye and orbit. However, the eye and orbit are potentially sensitive to the thermal and mechanical effects of ultrasound. When performing B-mode imaging, dedicated ocular settings should be used. If these settings are not available, limiting the acoustic output to Food and Drug Administration (FDA) recommended maximum levels is strongly advised. Especially important is the acoustic output in spectral (pulsed) and color Doppler modes, which can exceed the FDA's maximum recommended levels for the eye. Adjusting settings to decrease acoustic output and limiting the time of the examination should be done when performing a Doppler examination. The acoustic output of shear wave elastography is significantly higher than FDA guidelines for the eye and should be considered experimental.

High-Frequency Ultrasound Activation of Perfluorocarbon Nanodroplets for Treatment of Glaucoma

Elevated intraocular pressure (IOP) is the most prevalent risk factor for initiation and progression of neurodegeneration in glaucoma. Ocular hypertension results from increased resistance to aqueous fluid outflow caused by reduced porosity and increased stiffness of tissues of the outflow pathway. Acoustic activation and resulting bioeffects of the perfluorocarbon (PFC) nanodroplets (NDs) introduced into the anterior chamber (AC) of the eye could potentially represent a treatment for glaucoma by increasing permeability in the aqueous outflow track. To evaluate the potential of NDs to enter the outflow track, 100-nm diameter perfluoropentane (PFP) NDs with a lipid shell were injected into the AC of ex vivo pig eyes and in vivo rat eyes. The NDs were activated and imaged with 18- and 28-MHz linear arrays to assess their location and diffusion. NDs in the AC could also be visualized using optical coherence tomography (OCT). Because of their higher density with respect to aqueous humor, some NDs settled into the iridocorneal angle where they entered the outflow pathway. After acoustic activation of the NDs at the highest acoustic pressure, small gas bubbles were observed in the AC. After two days, no acoustic activation events were visible in the AC of the rats and their eyes showed no evidence of inflammation.

Ocular Blood Flow in Preterm Neonates: A Preliminary Report

Purpose

Retinopathy of prematurity (ROP) is a vision-threatening complication occurring in pre-term neonates. The standard of care entails regular monitoring by dilated ophthalmoscopy examinations, which entail stress and potential morbidity. In this pilot study, we used plane-wave ultrasound (PWUS) to image, measure, and assess the association of blood-flow velocities in the retrobulbar vessels with ROP stages ranging from stage 0 (immature vessels without ROP) to stage 3.

Methods

Both eyes of 14 preterm neonates at risk for ROP were examined by 18 MHz PWUS. All but two subjects had a follow-up examination. PWUS was acquired for 1.5 seconds at 3000 compound B-scans/sec. Data were postprocessed to form color-flow images and spectrograms depicting flow velocity in the central retinal artery (CRA), central retinal vein (CRV), and the short posterior ciliary arteries (SPCA). Flow parameters derived from spectrograms were compared by ROP stage.

Results

ROP stage was found to correlate with flow velocities. Velocities were significantly elevated with respect to non-ROP eyes in all vessels at stage 3 and in the SPCAs at stage 2.

Conclusions

PWUS measurement of blood flow may provide a quantitative, clinically important, and easily tolerated means for detecting and assessing the risk of ROP in preterm neonates. We speculate that the observed increase in flow velocity results from elevated vascular endothelial growth factor (VEGF) in ROP eyes.

Translational Relevance

PWUS offers a gentle, nonmydriatic method for monitoring neonates at risk for ROP that would complement ophthalmoscopy.

Retrobulbar blood flow in rat eyes during acute elevation of intraocular pressure

Most studies of the effect of acute elevation of intraocular pressure (IOP) on ocular blood-flow have utilized optical coherence tomography (OCT) to characterize retinal and choroidal flow and vascular density. This study investigates the effect of acute IOP elevation on blood flow velocity in the retrobulbar arteries and veins supplying and draining the eye, which, unlike the retinal and choroidal vasculature, are not directly compressed as IOP is increased. By cannulation of the anterior chamber of 20 Sprague-Dawley rats, we increased IOP in 10 mmHg steps from 10 to 60 mmHg and returned to 10 mmHg. After 1 min at each IOP (and 3 min after return to 10 mmHg), we acquired 18 MHz plane-wave ultrasound data at 3000 compound images/sec for 1.5 s. We produced color-flow Doppler images by digital signal processing of the ultrasound data, identified retrobulbar arteries and veins, generated spectrograms depicting flow velocity over the cardiac cycle and characterized changes of vascular density and perfusion in the orbit overall. Systolic, diastolic and mean velocities and resistive and pulsatile indices were determined from arterial spectrograms at each IOP level. Baseline mean arterial and mean venous velocities averaged 30.9 ± 10.8 and 8.5 ± 3.3 mm/s, respectively. Arterial velocity progressively decreased and resistance indices increased at and above an IOP of 30 mmHg. Mean arterial velocity at 60 mmHg dropped by 55% with respect to baseline, while venous velocity decreased by 20%. Arterial and venous velocities and resistance returned to near baseline after IOP was restored to 10 mmHg. Both vascular density and orbital perfusion decreased with IOP, but while perfusion returned to near normal when IOP returned to 10 mmHg, density remained reduced. Our findings are consistent with OCT-based studies showing reduced perfusion of the retina at levels comparable to retrobulbar arterial flow velocity change with increased IOP. The lesser effect on venous flow is possibly attributable to partial collapse of the venous lumen as volumetric venous outflow decreased at high IOP. The continued reduction in orbital vascular density 3 min after restoration of IOP to 10 mmHg might be attributable to persisting narrowing of capillaries, but this needs to be verified in future studies.

Compound Coherent Plane-Wave Ultrasound Imaging of Vascular Malformations of the Orbit

PURPOSE

Prior color-flow Doppler ultrasound studies of the eye have been performed with systems that exceed US Food and Drug Administration permissible ophthalmic ultrasonic energy limits. The authors report a study of orbital vascular malformations using a novel, Food and Drug Administration compliant, ultrafast compound coherent plane-wave ultrasound device to produce power Doppler images.

METHODS

Using a Verasonics Vantage 128 ultrasound engine and a user-developed MATLAB program with a 5-MHz linear-array probe, compound coherent plane-wave ultrasound data were collected on patients with orbital vascular malformations. Real-time color-flow Doppler visualized orbital blood flow. Power Doppler images were produced by post-processing compound coherent plane-wave ultrasound data acquired continuously for 2 seconds.

RESULTS

Compound coherent plane-wave ultrasound was performed on 3 orbital vascular malformations (1 venolymphatic malformation, 1 infantile hemangioma, and 1 arteriovenous malformation). Compound coherent plane-wave ultrasound produced a high-resolution depiction of orbital blood flow for orbital vascular malformations with high sensitivity to slow flow.

CONCLUSIONS

Analysis of blood flow within orbital lesions informs treatment planning. Compound coherent plane-wave ultrasound is an emerging ultrasound modality that falls within the Food and Drug Administration guidelines for use in the orbit and provides information to characterize orbital vascular malformations.

Plane-Wave Ultrasound Doppler of the Eye in Preeclampsia

Purpose

Pre-eclampsia (PE) is a serious complication of pregnancy characterized by high blood pressure, proteinuria, compromised fetal blood supply, and potential organ damage. The superficial location of the eye makes it an ideal target for characterization hemodynamics. Our aim was to discern the impact of PE on ocular blood flow.

Methods

18 MHz plane-wave ophthalmic ultrasound scanning was performed on subjects with PE (n = 26), chronic or gestational hypertension (n = 8), and normal controls (n = 19) within 72 hours of delivery. Duplicate three-second long scans of the posterior pole including the optic nerve were acquired at 6000 images/sec for evaluation of the central retinal artery and vein and the short posterior ciliary arteries. The choroid was scanned at 1000 images/sec. Doppler analysis provided values of pulsatile flow velocity and resistance indexes.

Results

End diastolic velocity was higher, and pulsatility and resistive indexes were significantly lower in the choroid, central retinal artery and short posterior ciliary arteries in PE than in controls. Blood pressure was elevated in PE with respect to controls and was negatively correlated with resistance.

Conclusions

Although vasoconstriction is considered characteristic of PE, we found reduced resistance in the orbital vessels and choroidal arterioles, implying vasodilation at this level. Future studies incorporating optical coherence tomography angiography for characterization of the retina and choriocapillaris in conjunction with plane-wave ultrasound scanning, particularly in late pregnancy, might address this conundrum.

Translational Relevance

Use of plane-wave ultrasound scanning for evaluation ocular blood flow in women at risk for PE may offer an avenue towards early detection and clinical intervention.

High-Frequency Multipulse, Plane-Wave Acoustic Contrast Imaging

Multipulse (MP) ultrasound contrast agent (UCA) imaging is a method to increase the contrast-to-background (CBR) ratio in regions of blood flow. Plane-wave imaging allows high frame rates, and with high-frequency ultrasound, fine-spatial and temporal resolution. MP and plane-wave imaging have not been applied to high-frequency ultrasound. Here, an 18-MHz linear array was employed to implement the MP methods of pulse inversion (PI) and amplitude modulation (AM) using high-speed, multiangle, compound plane-wave imaging. A flow of the UCA DEFINITY^© at a dilution ratio of 2000:1 circulating through a 2-mm-diameter flow channel in a tissue-mimicking phantom was used to characterize CBR and compared with cases of standard, multiangle compound plane-wave imaging. The relative improvement of PI and AM versus standard plane-wave imaging ranged from 5 to 10 dB. The CBR was observed to be stable over a 60-min time duration for a 2000:1 dilution ratio and a 2000:1 dilution ratio provided an optimal CBR.

Contrast-enhanced plane-wave ultrasound imaging of the rat eye

Preclinical imaging, especially of rodent models, plays a major role in experimental ophthalmology. Our aim was to determine if ultrasound can be used to visualize and measure flow dynamics in the retrobulbar vessels supplying and draining the eye and the potential of contrast microbubbles to provide image and measurement enhancement. To accomplish this, we used a 128-element, 18 MHz linear array ultrasound probe and performed plane-wave imaging of the eyes of Sprague Dawley rats. Compound images were acquired by emitting unfocused wavefronts at multiple angles and combining echo data from all angles to form individual B-scans. Multiple imaging sequences were utilized, compounding up to six angles, with imaging rate of up to 3000 compound B-scans per second and sequence durations from 1.5 to 180 s. Data were acquired before and after intravenous introduction of contrast microbubbles. We found the total power of the Doppler signal in the image plane to increase approximately 20 fold after injection of contrast, followed by an exponential decay to baseline in about 90 s, The best-fit time constant of the decay averaged 41 s. While major vessels and the retinal/choroidal complex were evident pre-contrast, they were dramatically enhanced with contrast present, with details such as choroidal arterioles seen only with contrast. Ocular arteriovenous transit time determined from comparative enhancement curves in arteries and veins was approximately 0.2 s. In conclusion, plane wave ultrasound, especially with enhancement by contrast microbubbles, offers a means for the study of ocular hemodynamics using the rat eye as a model.

Assessment of Vitreous Structure and Visual Function after Neodymium:Yttrium-Aluminum-Garnet Laser Vitreolysis

PURPOSE

Neodymium:yttrium-aluminum-garnet (Nd:YAG) laser treatment is performed on vitreous floaters, but studies of structural and functional effects with objective outcome measures are lacking. This study evaluated Nd:YAG laser effects by comparing participants with vitreous floaters who previously underwent laser treatment with untreated control participants and healthy persons without vitreous floaters using quantitative ultrasonography to evaluate vitreous structure and by measuring visual acuity and contrast sensitivity function to assess vision.

DESIGN

Retrospective, comparative study.

PARTICIPANTS

One eye was enrolled for each of 132 participants: 35 control participants without vitreous floaters, 59 participants with untreated vitreous floaters, and 38 participants with vitreous floaters previously Nd:YAG-treated. Of these, 25 were dissatisfied and sought vitrectomy; 13 were satisfied with observation.

METHODS

The 39-item National Eye Institute Visual Function Questionnaire (NEI-VFQ-39) to assess participant visual well-being, quantitative ultrasonography (QUS) to measure vitreous echodensity, and best-corrected visual acuity (BCVA) and contrast sensitivity function (CSF) to evaluate vision.

MAIN OUTCOME MEASURES

Results of NEI-VFQ-39, QUS, BCVA, and CSF.

RESULTS

Compared with control participants without vitreous floaters, participants with untreated vitreous floaters showed worse NEI-VFQ-39 results, 57% greater vitreous echodensity, and significant (130%) CSF degradation (P < 0.001 for each). Compared with untreated eyes with vitreous floaters, Nd:YAG-treated eyes had 23% less vitreous echodensity (P < 0.001), but no differences in NEI-VFQ-39 (P = 0.51), BCVA (P = 0.42), and CSF (P = 0.17) results. Of 38 participants with vitreous floaters who previously were treated with Nd:YAG, 25 were dissatisfied and seeking vitrectomy, whereas 13 were satisfied with observation. Participants seeking vitrectomy showed 24% greater vitreous echodensity (P = 0.018) and 52% worse CSF (P = 0.006). Multivariate linear regression models confirmed these findings.

CONCLUSIONS

As a group, participants previously treated with Nd:YAG laser for bothersome vitreous floaters showed less dense vitreous, but similar visual function as untreated control participants with vitreous floaters. Because some treated eyes showed less dense vitreous and better visual function than those of untreated control participants, a prospective randomized study of Nd:YAG laser treatment of vitreous is warranted, using uniform laser treatment parameters and objective quantitative outcome measures.

Ultrasound Imaging and Measurement of Choroidal Blood Flow

Purpose

The choroid is a vascular network providing the bulk of the oxygen and nutrient supply to the retina and may play a pivotal role in retinal disease pathogenesis. While optical coherence tomography angiography provides an en face depiction of the choroidal vasculature, it does not reveal flow dynamics. In this report, we describe the use of plane-wave ultrasound to image and characterize choroidal blood flow.

Methods

We scanned both eyes of 12 healthy subjects in a horizontal plane superior to the optic nerve head using an 18-MHz linear array. Plane-wave data were acquired over 10 transmission angles that were coherently compounded to produce 1000 images/sec for 3 seconds. These data were processed to produce a time series of power Doppler images and spectrograms depicting choroidal flow velocity. Analysis of variance was used to characterize peak systolic, and end diastolic velocities and resistive index, and their variability between scans, eyes, and subjects.

Results

Power Doppler images showed distinct arterioles within a more diffuse background. Choroidal flow was moderately pulsatile, with peak systolic velocity averaging approximately 10 mm/sec and resistive index of 0.55. There was no significant difference between left and right eyes, but significant variation among subjects.

Conclusions

Plane-wave ultrasound visualized individual arterioles and allowed measurement of flow over the cardiac cycle. Characterization of choroidal flow dynamics offers a novel means for assessment of the choroid's role in ocular disease.

Translational Relevance

Characterization of choroidal flow dynamics offers a novel means for assessment of the choroid's role in ocular disease.

In Vivo Elasticity Mapping of Posterior Ocular Layers Using Acoustic Radiation Force Optical Coherence Elastography

Purpose

We used acoustic radiation force optical coherence elastography (ARF-OCE) to map out the elasticity of retinal layers in healthy and diseased in vivo rabbit models for the first time.

Methods

A healthy rabbit eye was proptosed and imaged using ARF-OCE, by measuring the tissue deformation after an acoustic force is applied. A diseased retinal inflammation model was used to observe the contrast before and after disease formation. Retinal histologic analysis was performed to identify layers of the retina corresponding with the optical images.

Results

The general trend of the retinal layer elasticity is increasing stiffness from the ganglion side to the photoreceptor side, with the stiffest layer being the sclera. In a healthy rabbit model, the mechanical properties varied from 3 to 16 kPa for the five layers that were identified via optical imaging and histology (3.09 ± 0.46, 3.82 ± 0.88, 4.53 ± 0.74, 6.59 ± 2.27, 16.11 ± 5.13 kPa). In the diseased model, we have induced optical damage in a live rabbit and observed a change in the stiffness trend in its retina.

Conclusions

High sensitivity elasticity maps can be obtained using the ARF-OCE system to differentiate different retinal layers. Subtle changes in the mechanical properties during the onset of diseases, such as retinal degeneration, can be measured and aid in early clinical diagnosis. This study validates our imaging system for the characterization of retinal elasticity for the detection of retinal diseases in vivo.

A frequency-domain non-contact photoacoustic microscope based on an adaptive interferometer

A frequency-domain, non-contact approach to photoacoustic microscopy (PAM) that employs amplitude-modulated (0.1-1 MHz) laser for excitation (638-nm pump) in conjunction with a 2-wave mixing interferometer (532-nm probe) for non-contact detection of photoacoustic waves at the specimen surface is presented. A lock-in amplifier is employed to detect the photoacoustic signal. Illustrative images of tissue-mimicking phantoms, red-blood cells and retinal vasculature are presented. Single-frequency modulation of the pump beam directly provides an image that is equivalent to the 2-dimensional projection of the image volume. Targets located superficially produce phase modulations in the surface-reflected probe beam due to surface vibrations as well as direct intensity modulation in the backscattered probe light due to local changes in pressure and/or temperature. In comparison, the observed modulations in the probe beam due to targets located deeper in the specimen, for example, beyond the ballistic photon regime, predominantly consist of phase modulation.

Treatment of Macular Degeneration with Sildenafil: Results of a Two-Year Trial

OBJECTIVE

To evaluate PDE5/6 inhibition with sildenafil to reduce choroidal ischemia and treat age-related macular degeneration.

METHODS

Sildenafil was prescribed to treat participants with macular degenerations or macular dystrophies measured by spectral-domain optical coherence tomography, color fundus photography, enhanced depth imaging, and best-corrected visual acuity.

RESULTS

No change in calcified drusen was noted. Vitelliform-type soft drusen were not substantially changed. A participant with Best vitelliform macular dystrophy had a significant improvement in vision as well as in photoreceptor and ellipsoid layers.

CONCLUSIONS

Our research supports sildenafil as a safe treatment for age-related and vitelliform macular degenerations. Thickened Bruch's membrane reduces the beneficial effect of perfusion increase, but all eyes appear to benefit from PDE6. Notably, maintenance or improvement in the photoreceptor layer may be the most significant result of sildenafil and is consistent with PDE6 inhibition. Thus, sil-denafil treatment of macular degeneration offers significant potential for vision retention and recovery.

Correlation of Microcirculation Architecture with Ultrasound Backscatter Parameters of Uveal Melanoma

Ultrasound tissue characterization parameters of size and acoustic concentration can be used to sub-classify uveal melanoma and estimate the patient's risk of death. Histologically determined microcirculation patterns have also been found to sub-classify uveal melanoma and predict patient's risk of death. This study examines the spatial relationship between tumor features detected by the two techniques. Three dimensional ultrasound images that depict the size and relative concentration of scattering elements in uveal melanoma within a range of approximately 50-120 microns were compared with PAS (without hematoxylin) stained histological sections graded and localized according to tumor microvascular patterns. Both ultrasound parameter imaging and histopathology were accomplished by workers masked to the other procedure. In three of five patients vascular networks were identified histopathologically. The predominant ultrasound feature seen in the regions of the histologically identified networks were clusters of scatterers in the range of approximately 50-80 microns. In the two cases without a network vascular pattern, lower range scatterers dominated the tumor volume. All the cases could be distinguished by the size and distribution of contiguous spatial clusters of acoustic scatterers. Scatterer size features detected in three dimensional ultrasound parameter images can identify tumor regions containing a specific microvascular pattern. Ultrasound tissue characterization features used to sub-classify uveal melanoma may have a biophysical basis related to patterns of tumor microcirculation.

Improved High-Frequency Ultrasound Corneal Biometric Accuracy by Micrometer-Resolution Acoustic-Property Maps of the Cornea

Purpose

Mapping of epithelial thickness (ET) is useful for detection of keratoconus, a disease characterized by corneal thinning and bulging in which epithelial thinning occurs over the apex. In prior clinical studies, optical coherence tomography (OCT) measurements of ET were systematically thinner than those obtained by 40-MHz high-frequency ultrasound (HFU) where a constant speed of sound (c) of 1636 m/s was used for all corneal layers. The purpose of this work was to study the acoustic properties, that is, c, acoustic impedance (Z), and attenuation (α) of the corneal epithelium and stroma independently using a scanning acoustic microscope (SAM) to investigate the discrepancy between OCT and HFU estimates of ET.

Methods

Twelve unfixed pig corneas were snap-frozen and 6-μm sections were scanned using a custom-built SAM with an F-1.08, 500-MHz transducer and a 264-MHz bandwidth. Two-dimensional maps of c, Z, and α with a spatial resolution of 4 μm were derived.

Results

SAM showed that the value of c in the epithelium (i.e., 1548 ± 18 m/s) is substantially lower than the value of c in the stroma (i.e., 1686 ± 33 m/s).

Conclusion

SAM results demonstrated that the assumption of a constant value of c for all corneal layers is incorrect and explains the prior discrepancy between OCT and HFU ET determinations.

Translational Relevance

The findings of this study have important implications for HFU-based ET measurements and will improve future keratoconus diagnosis by providing more-accurate ET estimates.

Gene Therapy Restores Mfrp and Corrects Axial Eye Length

Hyperopia (farsightedness) is a common and significant cause of visual impairment, and extreme hyperopia (nanophthalmos) is a consequence of loss-of-function MFRP mutations. MFRP deficiency causes abnormal eye growth along the visual axis and significant visual comorbidities, such as angle closure glaucoma, cystic macular edema, and exudative retinal detachment. The Mfrp rd6 /Mfrp rd6 mouse is used as a pre-clinical animal model of retinal degeneration, and we found it was also hyperopic. To test the effect of restoring Mfrp expression, we delivered a wild-type Mfrp to the retinal pigmented epithelium (RPE) of Mfrp rd6 /Mfrp rd6 mice via adeno-associated viral (AAV) gene therapy. Phenotypic rescue was evaluated using non-invasive, human clinical testing, including fundus auto-fluorescence, optical coherence tomography, electroretinography, and ultrasound. These analyses showed gene therapy restored retinal function and normalized axial length. Proteomic analysis of RPE tissue revealed rescue of specific proteins associated with eye growth and normal retinal and RPE function. The favorable response to gene therapy in Mfrp rd6 /Mfrp rd6 mice suggests hyperopia and associated refractive errors may be amenable to AAV gene therapy.

Material Properties of Human Ocular Tissue at 7-µm Resolution

Quantitative assessment of the material properties of ocular tissues can provide valuable information for investigating several ophthalmic diseases. Quantitative acoustic microscopy (QAM) offers a means of obtaining such information, but few QAM investigations have been conducted on human ocular tissue. We imaged the optic nerve (ON) and iridocorneal angle in 12-µm deparaffinized sections of the human eye using a custom-built acoustic microscope with a 250-MHz transducer (7-µm lateral resolution). The two-dimensional QAM maps of ultrasound attenuation (α), speed of sound ( c), acoustic impedance ( Z), bulk modulus ( K), and mass density (ρ) were generated. Scanned samples were then stained and imaged by light microscopy for comparison with QAM maps. The spatial resolution and contrast of scanning acoustic microscopy (SAM) maps were sufficient to resolve anatomic layers of the retina (Re); anatomic features in SAM maps corresponded to those seen by light microscopy. Significant variations of the acoustic parameters were found. For example, the sclera was 220 MPa stiffer than Re, choroid, and ON tissue. To the authors' knowledge, this is the first systematic study to assess c, Z, K, ρ, and α of human ocular tissue at the high ultrasound frequencies used in this study.

Ultrafast Ultrasound Imaging of Ocular Anatomy and Blood Flow

Purpose

Ophthalmic ultrasound imaging is currently performed with mechanically scanned single-element probes. These probes have limited capabilities overall and lack the ability to image blood flow. Linear-array systems are able to detect blood flow, but these systems exceed ophthalmic acoustic intensity safety guidelines. Our aim was to implement and evaluate a new linear-array–based technology, compound coherent plane-wave ultrasound, which offers ultrafast imaging and depiction of blood flow at safe acoustic intensity levels.

Methods

We compared acoustic intensity generated by a 128-element, 18-MHz linear array operated in conventionally focused and plane-wave modes and characterized signal-to-noise ratio (SNR) and lateral resolution. We developed plane-wave B-mode, real-time color-flow, and high-resolution depiction of slow flow in postprocessed data collected continuously at a rate of 20,000 frames/s. We acquired in vivo images of the posterior pole of the eye by compounding plane-wave images acquired over ±10° and produced images depicting orbital and choroidal blood flow.

Results

With the array operated conventionally, Doppler modes exceeded Food and Drug Administration safety guidelines, but plane-wave modalities were well within guidelines. Plane-wave data allowed generation of high-quality compound B-mode images, with SNR increasing with the number of compounded frames. Real-time color-flow Doppler readily visualized orbital blood flow. Postprocessing of continuously acquired data blocks of 1.6-second duration allowed high-resolution depiction of orbital and choroidal flow over the cardiac cycle.

Conclusions

Newly developed high-frequency linear arrays in combination with plane-wave techniques present opportunities for the evaluation of ocular anatomy and blood flow, as well as visualization and analysis of other transient phenomena such as vessel wall motion over the cardiac cycle and saccade-induced vitreous motion.

In vivo imaging of ocular blood flow using high-speed ultrasound

Clinical ophthalmic ultrasound is currently performed with mechanically scanned, single-element probes, but these are unable to provide useful information about blood flow with Doppler techniques. Linear arrays are well-suited for the detection of blood flow, but commercial systems generally exceed FDA ophthalmic safety limits. A high-speed plane-wave ultrasound approach with an 18-MHz linear array was utilized to characterize blood flow in the orbit and choroid. Acoustic intensity was measured and the plane-wave mode was within FDA limits. Data were acquired for up to 2 sec and up to 20,000 frames/s with sets of steered plane-wave transmissions that spanned 2*θ degrees where 0 degrees was normal to the array. Lateral resolution was characterized using compounding from 1 to 50 transmissions and -6-dB lateral beamwidths ranged from 320 to 180 μm, respectively. Compounded high-frame-rate data were post-processed using a singular value decomposition spatiotemporal filter and then flow was estimated at each pixel using standard Doppler processing methods. A 1-cm diameter rotating scattering phantom and a 2-mm diameter tube with a flow of blood-mimicking fluid were utilized to validate the flow-estimation algorithms. In vivo data were obtained from the posterior pole of the human eye which revealed regions of flow in the choroid and major orbital vessels supplying the eye.

Focused ultrasound in ophthalmology

The use of focused ultrasound to obtain diagnostically significant information about the eye goes back to the 1950s. This review describes the historical and technological development of ophthalmic ultrasound and its clinical application and impact. Ultrasound, like light, can be focused, which is crucial for formation of high-resolution, diagnostically useful images. Focused, single-element, mechanically scanned transducers are most common in ophthalmology. Specially designed transducers have been used to generate focused, high-intensity ultrasound that through thermal effects has been used to treat glaucoma (via ciliodestruction), tumors, and other pathologies. Linear and annular transducer arrays offer synthetic focusing in which precise timing of the excitation of independently addressable array elements allows formation of a converging wavefront to create a focus at one or more programmable depths. Most recently, linear array-based plane-wave ultrasound, in which the array emits an unfocused wavefront and focusing is performed solely on received data, has been demonstrated for imaging ocular anatomy and blood flow. While the history of ophthalmic ultrasound extends back over half-a-century, new and powerful technologic advances continue to be made, offering the prospect of novel diagnostic capabilities.

Detection of Keratoconus in Clinically and Algorithmically Topographically Normal Fellow Eyes Using Epithelial Thickness Analysis

PURPOSE

To assess the effectiveness of a keratoconus-detection algorithm derived from Artemis very high-frequency (VHF) digital ultrasound (ArcScan Inc., Morrison, CO) epithelial thickness maps in the fellow eye from a series of patients with unilateral keratoconus.

METHODS

The study included 10 patients with moderate to advanced keratoconus in one eye but a clinically and algorithmically topographically normal fellow eye. VHF digital ultrasound epithelial thickness data were acquired and a previously developed classification model was applied for identification of keratoconus to the clinically normal fellow eyes. Pentacam (Oculus Optikgeräte, Wetzlar, Germany) Belin-Ambrósio Enhanced Ectasia Display "D" score (BAD-D) data (5 of 10 eyes), and Orbscan (Bausch & Lomb, Rochester, NY) SCORE data (9 of 10 eyes) were also evaluated.

RESULTS

Five of the 10 fellow eyes were classified as keratoconic by the VHF digital ultrasound epithelium model. Five of 9 fellow eyes were classified as keratoconic by the SCORE model. For the 5 fellow eyes with Pentacam and VHF digital ultrasound data, one was classified as keratoconic by the VHF digital ultrasound model, one (different) eye by a combined VHF digital ultrasound and Pentacam model, and none by BAD-D alone.

CONCLUSIONS

Under the assumption that keratoconus is a bilateral but asymmetric disease, half of the 'normal' fellow eyes could be found to have keratoconus using epithelial thickness maps. The Orbscan SCORE or the combination of topographic BAD-D criteria with epithelial maps did not perform better.

A Novel Quantitative 500-MHz Acoustic Microscopy System for Ophthalmologic Tissues

OBJECTIVE

This paper describes development of a novel 500-MHz scanning acoustic microscope (SAM) for assessing the mechanical properties of ocular tissues at fine resolution. The mechanical properties of some ocular tissues, such as lamina cribrosa (LC) in the optic nerve head, are believed to play a pivotal role in eye pathogenesis.

METHODS

A novel etching technology was used to fabricate silicon-based lens for a 500-MHz transducer. The transducer was tested in a custom-designed scanning system on human eyes. Two-dimensional (2-D) maps of bulk modulus (K) and mass density (ρ) were derived using improved versions of current state-of-the-art signal processing approaches.

RESULTS

The transducer employed a lens radius of 125 μm and had a center frequency of 479 MHz with a -6-dB bandwidth of 264 MHz and a lateral resolution of 4 μm. The LC, Bruch's membrane (BM) at the interface of the retina and choroid, and Bowman's layer (BL) at the interface of the corneal epithelium and stroma, were successfully imaged and resolved. Analysis of the 2-D parameter maps revealed average values of LC, BM, and BL with KLC = 2.81 ±0.17; GPa, KBM = 2.89 ±0.18; GPa, KBL = 2.6 ±0.09 ; GPa, ρ LC = 0.96 ±0.03 g/cm³; ρ BM = 0.97 ±0.04 g/cm³; ρ BL = 0.98 ±0.04 g/cm³.

SIGNIFICANCE

This novel SAM was shown to be capable of measuring mechanical properties of soft biological tissues at microscopic resolution; it is currently the only system that allows simultaneous measurement of K, ρ, and attenuation in large lateral scales (field area >9 mm²) and at fine resolutions.

No Evidence for Association of SCO2 Heterozygosity with High-Grade Myopia or Other Diseases with Possible Mitochondrial Dysfunction

SCO2 mutations cause recessively inherited cytochrome c oxidase deficiency. Recently Tran-Viet et al. proposed that heterozygosity for pathogenic SCO2 variants, including the common E140K variant, causes high-grade myopia. To investigate the association of SCO2 mutations with myopia, ophthalmic examinations were performed on 35 E140K carriers, one homozygous infant, and on a mouse model of Sco2 deficiency. Additionally, a screen for other putative effects of SCO2 heterozygosity was carried out by comparing the prevalence of the common E140K variant in a population of patients with undiagnosed diseases compatible with SCO2-related pathogenesis to that in a general population sample. High-grade myopia was not identified in any of the studied individuals. Of the carriers, 17 were emmetropic, and 18 possessed refractive errors. Additionally, no significant axial elongation indicative of high-grade myopia was found in mice carrying E129K (corresponding to E140K in humans) knock-in mutations. The prevalence of E140K carriers in the symptomatic cohort was evaluated as 1:103 (CI: 0.44-2.09) and did not differ significantly from the population prevalence (1:147, CI: 0.45-1.04).Our study demonstrates that heterozygosity for pathogenic SCO2 variants is not associated with high-grade myopia in either human patients or in mice.

Comparison of Corneal Epithelial Thickness Measurement Between Fourier-Domain OCT and Very High-Frequency Digital Ultrasound

PURPOSE

To compare measurements of corneal epithelial thickness using optical coherence tomography (OCT) and very high-frequency digital ultrasound (VHFDU).

METHODS

Retrospective analysis of 189 virgin corneas and 175 post-laser refractive surgery (LRS) corneas that had corneal epithelial thickness measurement with RTVue Fourier-domain OCT (Optovue, Inc., Fremont, CA) (tear film included) and Artemis VHFDU (ArcScan Inc., Morrison, CO) (tear film excluded). Averages were calculated for the central 2-mm diameter zone and for two further concentric annuli of 1.5- and 0.5-mm width, each divided into eight sectors. Agreement was analyzed by mean difference (OCT - VHFDU), 95% limits of agreement (LoA) (1.96 standard deviation of the difference), and Bland-Altman analysis.

RESULTS

In virgin epithelium, mean central thickness was 53.4 ± 3.20 µm (range: 46 to 62 µm) with OCT and 54.1 ± 2.96 µm (range: 48 to 61 µm) with VHFDU; OCT measured thinnest in 70% with a mean difference of -0.71 µm (95% LoA of ± 3.94 µm, P < .001). In post-LRS epithelium, mean central thickness was 57.9 ± 6.08 µm (range: 42 to 77 µm) with OCT and 60.5 ± 6.47 µm (range: 42 to 79 µm) with VHFDU; OCT measured thinnest in 88%, with a mean difference of -2.48 µm (95% LoA of ± 5.33 µm, P < .001). A larger difference between methods was more common with thicker epithelium.

CONCLUSIONS

Corneal epithelial thickness measurements using OCT were found to be slightly thinner than for VHFDU. In contrast to VHFDU, OCT measurement includes the tear film, so the true difference is probably approximately 4 µm more than reported. The difference was greatest inferiorly and higher for post-LRS eyes and in thicker epithelium.

Fine-resolution maps of acoustic properties at 250 MHz of unstained fixed murine retinal layers

Ex vivo assessment of microscale tissue biomechanical properties of the mammalian retina could offer insights into diseases such as keratoconus, and macular degeneration. A 250-MHz scanning acoustic microscope (7-μm resolution) has been constructed to derive two-dimensional quantitative maps of attenuation (α), speed of sound (c), acoustic impedance (Z), bulk modulus (B), and mass density ( ρ). The two-dimensional maps were compared to coregistered hematoxylin-and-eosin stained sections. This study is the first to quantitatively assess  α, c, Z, B, and ρ of individual retinal layers of mammalian animals at high ultrasound frequencies. Significant differences in these parameters between the layers were demonstrated.

Ultrasound-based quantification of vitreous floaters correlates with contrast sensitivity and quality of life

PURPOSE

Clinical evaluation of floaters lacks quantitative assessment of vitreous structure. This study used quantitative ultrasound (QUS) to measure vitreous opacities. Since floaters reduce contrast sensitivity (CS) and quality of life (Visual Function Questionnaire [VFQ]), it is hypothesized that QUS will correlate with CS and VFQ in patients with floaters.

METHODS

Twenty-two eyes (22 subjects; age = 57 ± 19 years) with floaters were evaluated with Freiburg acuity contrast testing (FrACT; %Weber) and VFQ. Ultrasonography used a customized probe (15-MHz center frequency, 20-mm focal length, 7-mm aperture) with longitudinal and transverse scans taken in primary gaze and a horizontal longitudinal scan through premacular vitreous in temporal gaze. Each scan set had 100 frames of log-compressed envelope data. Within each frame, two regions of interest (ROIs) were analyzed (whole-central and posterior vitreous) to yield three parameters (energy, E; mean amplitude, M; and percentage of vitreous filled by echodensities, P50) averaged over the entire 100-frame dataset. Statistical analyses evaluated E, M, and P50 correlations with CS and VFQ.

RESULTS

Contrast sensitivity ranged from 1.19%W (normal) to 5.59%W. All QUS parameters in two scan positions within the whole-central ROI correlated with CS (R > 0.67, P < 0.001). P50 in the nasal longitudinal position had R = 0.867 (P < 0.001). Correlations with VFQ ranged from R = 0.52 (P < 0.013) to R = 0.65 (P < 0.001).

CONCLUSIONS

Quantitative ultrasound provides quantitative measures of vitreous echodensity that correlate with CS and VFQ, providing objective assessment of vitreous structure underlying the functional disturbances induced by floaters, useful to quantify vitreous disease severity and the response to therapy.

Acoustic radiation force for noninvasive evaluation of corneal biomechanical changes induced by cross-linking therapy

OBJECTIVES

To noninvasively measure changes in corneal biomechanical properties induced by ultraviolet-activated riboflavin cross-linking therapy using acoustic radiation force (ARF).

METHODS

Cross-linking was performed on the right eyes of 6 rabbits, with the left eyes serving as controls. Acoustic radiation force was used to assess corneal stiffness before treatment and weekly for 4 weeks after treatment. Acoustic power levels were within US Food and Drug Administration guidelines for ophthalmic safety. Strain, determined from ARF-induced displacement of the front and back surfaces of the cornea, was fit to the Kelvin-Voigt model to determine the elastic modulus (E) and coefficient of viscosity (η). The stiffness factor, the ratio of E after treatment to E before treatment, was calculated for treated and control eyes. At the end of 4 weeks, ex vivo thermal shrinkage temperature analysis was performed for comparison with in vivo stiffness measurements. One-way analysis of variance and Student t tests were performed to test for differences in E, η, the stiffness factor, and corneal thickness.

RESULTS

Biomechanical stiffening was immediately evident in cross-linking-treated corneas. At 4 weeks after treatment, treated corneas were 1.3 times stiffer and showed significant changes in E (P= .006) and η (P= .007), with no significant effect in controls. Corneal thickness increased immediately after treatment but did not differ significantly from the pretreatment value at 4 weeks.

CONCLUSIONS

Our findings demonstrate a statistically significant increase in stiffness in cross-linking-treated rabbit corneas based on in vivo axial stress/strain measurements obtained using ARF. The capacity to noninvasively monitor corneal stiffness offers the potential for clinical monitoring of cross-linking therapy.

Epithelial remodeling as basis for machine-based identification of keratoconus

PURPOSE

To develop and evaluate automated computerized algorithms for differentiation of normal and keratoconus corneas based solely on epithelial and stromal thickness data.

METHODS

Maps of the corneal epithelial and stromal thickness were generated from Artemis-1 very high-frequency ultrasound arc-scans of 130 normal and 74 keratoconic subjects diagnosed by combined topography and tomography examination. Keratoconus severity was graded based on anterior curvature, minimum corneal thickness, and refractive error. Computer analysis of maps produced 161 features for one randomly selected eye per subject. Stepwise linear discriminant analysis (LDA) and neural network (NN) analysis were then performed to develop multivariate models based on combinations of selected features to correctly classify cases. The sensitivity, specificity, and area under the receiver operating characteristic curve (AUC) were determined for each classifier.

RESULTS

Stepwise LDA resulted in a six-variable model that provided an AUC of 100%, indicative of complete separation of keratoconic from normal corneas. Leave-one-out analysis resulted in 99.2% specificity and 94.6% sensitivity. Neural network analysis using the same six variables resulted in an AUC of 100% for the training set. Test set performance averaged over 10 trials gave a specificity of 99.5 ± 1.5% and sensitivity of 98.9 ± 1.9%. The LDA function values correlated with keratoconus severity grade.

CONCLUSIONS

The results demonstrate that epithelial remodeling in keratoconus represents an independent means for differentiation of normal from advanced keratoconus corneas.

Anterior-segment imaging for assessment of glaucoma

This article summarizes the physics, technology and clinical application of ultrasound biomicroscopy (UBM) and optical coherence tomography (OCT) for assessment of the anterior segment in glaucoma. UBM systems use frequencies ranging from approximately 35 to 80 MHz, as compared with typical 10-MHz systems used for general-purpose ophthalmic imaging. OCT systems use low-coherence, near-infrared light to provide detailed images of anterior segment structures at resolutions exceeding that of UBM. Both technologies allow visualization of the iridocorneal angle and, thus, can contribute to the diagnosis and management of glaucoma. OCT systems are advantageous, being noncontact proceedures and providing finer resolution than UBM, but UBM systems are superior for the visualization of retroiridal structures, including the ciliary body, posterior chamber and zonules, which can provide crucial diagnostic information for the assessment of glaucoma.

Explaining The Current Role Of High Frequency Ultrasound In Ophthalmic Diagnosis (Ophthalmic Ultrasound)

Ultrasound has become as indispensable as indirect ophthalmoscopy or slit lamp in evaluation of the eye. It is an important adjuvant for the clinical assessment of a variety of ocular and orbital diseases. Advances in instrumentation, higher frequencies and more sensitivity and resolution have resulted in continuous improvement in image quality.Very high frequency ultrasound uses frequencies in the range of 35 to 100 MHz to show greater detail of the anterior segment. Penetration is limited for these higher frequencies to only a few millimeters and thus only the anterior vitreous behind the ciliary body and lens can be imaged. High frequency ultrasound in the range of 20 to 30 MHz has a penetration of about 10 mm and can be used for posterior pole evaluation of the retina and choroid.

Age-related macular degeneration: choroidal ischaemia?

Aim

Our aim is to use ultrasound to non-invasively detect differences in choroidal microarchitecture possibly related to ischaemia among normal eyes and those with wet and dry age-related macular degeneration (AMD).

Design

Prospective case series of subjects with dry AMD, wet AMD and age-matched controls.

Methods

Digitised 20 MHz B-scan radiofrequency ultrasound data of the region of the macula were segmented to extract the signal from the retina and choroid. This signal was processed by a wavelet transform, and statistical modelling was applied to the wavelet coefficients to examine differences among dry, wet and non-AMD eyes. Receiver operating characteristic (ROC) analysis was used to evaluate a multivariate classifier.

Results

In the 69 eyes of 52 patients, 18 did not have AMD, 23 had dry AMD and 28 had wet AMD. Multivariate models showed statistically significant differences between groups. Multiclass ROC analysis of the best model showed an excellent volume-under-curve of 0.892±0.17. The classifier is consistent with ischaemia in dry AMD.

Conclusions

Wavelet augmented ultrasound is sensitive to the organisational elements of choroidal microarchitecture relating to scatter and fluid tissue boundaries such as seen in ischaemia and inflammation, allowing statistically significant differentiation of dry, wet and non-AMD eyes. This study further supports the association of ischaemia with dry AMD and provides a rationale for treating dry AMD with pharmacological agents to increase choroidal perfusion.

ClinicalTrials.gov registration

NCT00277784.

Measurement of choroidal perfusion and thickness following systemic sildenafil (Viagra(®) )

PURPOSE

To demonstrate anatomic and physiologic changes in the human choroid following systemic sildenafil citrate (Viagra®) using enhanced depth imaging spectral domain-optical coherence tomography (EDI-OCT) and swept-scan high-frequency digital ultrasound.

METHODS

Seven healthy male subjects (mean age 32.7 years) were evaluated at baseline and 2 hr after ingesting 50 mg of sildenafil. Swept-scan high-frequency digital ultrasound and EDI-OCT were utilized to measure choroidal perfusion and thickness, respectively. Results were read by masked observers. The Wilcoxon signed-rank test and t-test were used to analyse differences in choroidal flow and thickness at baseline and 2 hr after ingestion of sildenafil.

RESULTS

Two hours following sildenafil, increased choroidal perfusion was observed in 11 of 12 eyes measured by swept-scan high-frequency digital ultrasound. The mean increase was 3.46 (±2.00) times baseline with a range of 0.47-7.80 times baseline (p = 0.004). Increased choroidal thickness was observed in 12 of 12 eyes measured with EDI-OCT. The average choroidal thickness increased by 11.6% temporal to the fovea, 9.3% nasal to the fovea and 10.7% underneath the fovea (p < 0.001 for all values).

CONCLUSIONS

Choroidal perfusion and thickness both increase in response to systemic sildenafil. These changes could secondarily affect retinal function, explain previously reported clinical symptoms and potentially be a useful adjunct for treatment of ocular diseases that would benefit from increased choroidal blood flow.

Effect of ultrasound radiation force on the choroid

PURPOSE

While visualization of the retina and choroid has made great progress, functional imaging techniques have been lacking. Our aim was to utilize acoustic radiation force impulse (ARFI) response to probe functional properties of these tissues.

METHODS

A single element 18-MHz ultrasound transducer was focused upon the retina of the rabbit eye. The procedure was performed with the eye proptosed and with the eye seated normally in the orbit. The transducer was excited to emit ARFI over a 10-ms period with a 25% duty cycle. Phase resolved pulse/echo data were acquired before, during, and following ARFI.

RESULTS

In the proptosed eye, ARFI exposure produced tissue displacements ranging from 0 to 10 μm, and an immediate increase in choroidal echo amplitude to over 6 dB, decaying to baseline after about 1 second. In the normally seated eye, ultrasound phase shifts consistent with flow were observed in the choroid, but enhanced backscatter following ARFI rarely occurred. ARFI-induced displacements of about 10 μm were observed at the choroidal margins. Larger displacements occurred within the choroid and in orbital tissues.

CONCLUSIONS

We hypothesize that elevated intraocular pressure occurring during proptosis induced choroidal ischemia and that acoustic radiation force produced a transient local decompression and reperfusion. With the eye normally seated, choroidal flow was observed and little alteration in backscatter resulted from exposure. Clinical application of this technique may provide new insights into diseases characterized by altered choroidal hemodynamics, including maculopathies, diabetic retinopathy, and glaucoma.

Comparison of artemis 2 ultrasound and Visante optical coherence tomography corneal thickness profiles

PURPOSE

To compare corneal thickness profiles of cross-sections of cornea determined by arc-scanned immersion ultrasound and optical coherence tomography (OCT).

METHODS

Corneas of 28 eyes from 14 participants were scanned in triplicate using the Artemis 2 high-frequency arc-scanned ultrasound system (ArcScan Inc) and the Visante OCT system (Carl Zeiss Meditec). Corneal thickness and reproducibility were compared within 3.5 mm of central cornea in the horizontal plane.

RESULTS

Although highly correlated, Visante central and peripheral corneal thickness values were systematically thinner than Artemis 2 values. Within the central 0.5 mm, the difference was approximately 8 μm, but the difference increased with distance from the center. Reproducibility for each instrument was comparable, measuring <4 μm centrally and increasing peripherally.

CONCLUSIONS

Visante OCT measurements of corneal thickness are thinner than Artemis 2 ultrasound values centrally with an increasing difference with peripheral position. Measurement reproducibility was comparable for the two techniques.

Non-planar pad-printed thick-film focused high-frequency ultrasonic transducers for imaging and therapeutic applications

Pad-printed thick-film transducers have been shown to be an interesting alternative to lapped bulk piezoceramics, because the film is deposited with the required thickness, size, and geometry, thus avoiding any subsequent machining to achieve geometrical focusing. Their electromechanical properties are close to those of bulk ceramics with similar composition despite having a higher porosity. In this paper, padprinted high-frequency transducers based on a low-loss piezoceramic composition are designed and fabricated. High-porosity ceramic cylinders with a spherical top surface are used as the backing substrate. The transducers are characterized in view of imaging applications and their imaging capabilities are evaluated with phantoms containing spherical inclusions and in different biological tissues. In addition, the transducers are evaluated for their capability to produce high-acoustic intensities at frequencies around 20 MHz. High-intensity measurements, obtained with a calibrated hydrophone, show that transducer performance is promising for applications that would require the same device to be used for imaging and for therapy. Nevertheless, the transducer design can be improved, and simulation studies are performed to find a better compromise between low-power and high-power performance. The size, geometry, and constitutive materials of optimized configurations are proposed and their feasibility is discussed.

Pulse-encoded ultrasound imaging of the vitreous with an annular array

The vitreous body is nearly transparent both optically and ultrasonically. Conventional 10- to 12-MHz diagnostic ultrasound can detect vitreous inhomogeneities at high gain settings, but has limited resolution and sensitivity, especially outside the fixed focal zone near the retina. To improve visualization of faint intravitreal fluid/gel interfaces, the authors fabricated a spherically curved 20-MHz five-element annular array ultrasound transducer, implemented a synthetic-focusing algorithm to extend the depth-of-field, and used a pulse-encoding strategy to increase sensitivity. The authors evaluated a human subject with a recent posterior vitreous detachment and compared the annular array with conventional 10-MHz ultrasound and spectral-domain optical coherence tomography. With synthetic focusing and chirp pulse-encoding, the array allowed visualization of the formed and fluid components of the vitreous with improved sensitivity and resolution compared with the conventional B-scan. Although optical coherence tomography allowed assessment of the posterior vitreoretinal interface, the ultrasound array allowed evaluation of the entire vitreous body.

Epithelial thickness after hyperopic LASIK: three-dimensional display with Artemis very high-frequency digital ultrasound

PURPOSE

To characterize the epithelial thickness profile in a population of eyes after LASIK for hyperopia or hyperopic astigmatism.

METHODS

The epithelial thickness profile was measured in vivo by Artemis very high-frequency (VHF) digital ultrasound scanning (ArcScan Inc) across the central 10-mm diameter of the cornea on 65 eyes at least 3 months after hyperopic LASIK using a 7-mm ablation zone with the MEL 80 excimer laser (Carl Zeiss Meditec). Maps of the average, standard deviation, minimum, maximum, and range of epithelial thickness were plotted. The cross-sectional hemi-meridional epithelial thickness profile was calculated using annular averaging. Linear regression analysis was performed to evaluate correlations between epithelial thickness, spherical equivalent refraction treated, and maximum simulated keratometry.

RESULTS

The mean thinnest epithelial thickness was 39.7 +/- 5.6 microm and the mean thickest epithelial thickness was 89.3 +/- 14.6 microm. The average epithelial thickness profile showed an epithelial doughnut pattern characterized by localized central thinning within the 4-mm diameter zone surrounded by an annulus of thick epithelium, with the thickest epithelium at the 3.4-mm radius. The epithelium was on average 10-microm thicker temporally than nasally at the 3.4-mm radius. Central epithelium was thinner and paracentral epithelium was thicker for higher hyperopic corrections and steeper maximum simulated keratometry.

CONCLUSIONS

Three-dimensional high-resolution ultrasound mapping of epithelial thickness profile after LASIK for hyperopia demonstrated thinner epithelium centrally and thicker epithelium paracentrally. Presumably, the paracentral epithelial thickening compensated in part for the stromal tissue removed by the hyperopic ablation, whereas the central epithelial thinning compensated for the localized increase in corneal curvature.

Photoacoustic-guided convergence of light through optically diffusive media

We demonstrate that laser beams can be converged toward a light-absorbing target through optically diffusive media by using photoacoustic-guided interferometric focusing. The convergence of light is achieved by shaping the wavefront of the incident light with a deformable mirror to maximize the photoacoustic signal, which is proportional to the scattered light intensity at the light absorber.

The Flvr-encoded murine oligoadenylate synthetase 1b (Oas1b) suppresses 2-5A synthesis in intact cells

Resistance to flavivirus-induced disease in mice is conferred by the autosomal gene Flv, identified as 2'-5' oligoadenylate synthetase 1b (Oas1b). Resistant mice express a full-length Oas1b protein while susceptible mice express the truncated Oas1btr. In this study, Oas1b was shown to be an inactive synthetase. Although the Oas/RNase L pathway was previously shown to have an antiviral role during flavivirus infections, Oas1b protein inhibited Oas1a in vitro synthetase activity in a dose-dependent manner and reduced 2-5A production in vivo in response to poly(I:C). These findings suggest that negative regulation of 2-5A by inactive Oas1 proteins may fine tune the RNase L response that if not tightly controlled could cause significant damage in cells. The results also indicate that flavivirus resistance conferred by Oas1b is not mediated by 2-5A. Instead, Oas1b inhibits flavivirus replication by an alternative mechanism that overrides the proviral effect of reducing 2-5A accumulation and RNase L activation.

Repeatability of layered corneal pachymetry with the artemis very high-frequency digital ultrasound arc-scanner

PURPOSE

To assess the three-dimensional repeatability of thickness measurements for epithelium, stroma, cornea, flap, and residual stromal bed using the Artemis very high-frequency (VHF) digital ultrasound arc-scanner (ArcScan Inc).

METHODS

Five consecutive measurements were obtained for 10 eyes of 10 patients 1 year after LASIK using the Artemis VHF digital ultrasound arc-scanner across the central 10-mm diameter of the cornea. Repeatability analysis was performed for thickness measurements for each corneal layer-epithelium, stroma, cornea, flap, and residual stromal bed. The standard deviation of repeated measurements (point-repeatability) was calculated for each measurement location in 0.1-mm steps for the 10×10-mm matrix. The pooled standard deviation of the point-repeatability for each measurement location within the central 1-, 2-, and 3-mm radius was calculated (region-repeatability). The corneal thickness of the baseline scan set was compared to that of subsequent scan sets within the same session and plotted over time to assess any possible hydration effects of the immersion technique.

RESULTS

The repeatability at the corneal vertex was 0.58 μm for epithelium, 1.78 μm for stroma, 1.68 μm for cornea, 1.68 μm for flap, and 2.27 μm for residual stromal bed. The region-repeatability within the central 1-mm radius was 1.01 μm for epithelium, 3.44 μm for stroma, 3.35 μm for cornea, 2.81 μm for flap, and 3.97 μm for residual stromal bed. The mean difference in corneal thickness from the baseline value was within 1.25 μm for each of the subsequent four scan sets over a 5-minute immersion period.

CONCLUSIONS

Layered pachymetry of the epithelium, stroma, cornea, flap, and residual stromal bed showed high repeatability with the Artemis VHF digital ultrasound arc-scanner. The high repeatability validates the use of the Artemis for in vivo layered pachymetry.

High-resolution photoacoustic imaging of ocular tissues

Optical coherence tomography (OCT) and ultrasound (US) are methods widely used for diagnostic imaging of the eye. These techniques detect discontinuities in optical refractive index and acoustic impedance, respectively. Because these both relate to variations in tissue density or composition, OCT and US images share a qualitatively similar appearance. In photoacoustic imaging (PAI), short light pulses are directed at tissues, pressure is generated due to a rapid energy deposition in the tissue volume and thermoelastic expansion results in generation of broadband US. PAI thus depicts optical absorption, which is independent of the tissue characteristics imaged by OCT or US. Our aim was to demonstrate the application of PAI in ocular tissues and to do so with lateral resolution comparable to OCT. We developed two PAI assemblies, both of which used single-element US transducers and lasers sharing a common focus. The first assembly had optical and 35-MHz US axes offset by a 30 degrees angle. The second assembly consisted of a 20-MHz ring transducer with a coaxial optics. The laser emitted 5-ns pulses at either 532 nm or 1064 nm, with spot sizes at the focus of 35 microm for the angled probe and 20 microm for the coaxial probe. We compared lateral resolution by scanning 12.5 microm diameter wire targets with pulse/echo US and PAI at each wavelength. We then imaged the anterior segment in whole ex vivo pig eyes and the choroid and ciliary body region in sectioned eyes. PAI data obtained at 1064 nm in the near infrared had higher penetration but reduced signal amplitude compared to that obtained using the 532 nm green wavelength. Images were obtained of the iris, choroid and ciliary processes. The zonules and anterior cornea and lens surfaces were seen at 532 nm. Because the laser spot size was significantly smaller than the US beamwidth at the focus, PAI images had superior resolution than those obtained using conventional US.

Epithelial, stromal, and total corneal thickness in keratoconus: three-dimensional display with artemis very-high frequency digital ultrasound

PURPOSE

To characterize the epithelial, stromal, and total corneal thickness profile in a population of eyes with keratoconus.

METHODS

Epithelial, stromal, and total corneal thickness profiles were measured in vivo by Artemis very high-frequency (VHF) digital ultrasound scanning (ArcScan) across the central 6- to 10-mm diameter of the cornea on 54 keratoconic eyes. Maps of the average, standard deviation, minimum, maximum, and range of epithelial, stromal, and total corneal thickness were plotted. The average location of the thinnest epithelium, stroma, and total cornea were found. The cross-sectional semi-meridional stromal and total corneal thickness profiles were calculated using annular averaging. The absolute stromal and total corneal thickness progressions relative to the thinnest point were calculated using annular averaging as well as for 8 semi-meridians individually.

RESULTS

The mean corneal vertex epithelial, stromal, and total corneal thicknesses were 45.7+/-5.9 microm, 426.4+/-38.5 microm, and 472.2+/-41.4 microm, respectively. The average epithelial thickness profile showed an epithelial doughnut pattern characterized by localized central thinning surrounded by an annulus of thick epithelium. The thinnest epithelium, stroma, and total cornea were displaced on average by 0.48+/-0.66 mm temporally and 0.32+/-0.67 mm inferiorly, 0.31+/-0.45 mm temporally and 0.54+/-0.37 mm inferiorly, and 0.31+/-0.43 mm temporally and 0.50+/-0.35 mm inferiorly, respectively, with reference to the corneal vertex. The increase in semi-meridional absolute stromal and total corneal thickness progressions was greatest inferiorly and lowest temporally.

CONCLUSIONS

Three-dimensional thickness mapping of the epithelial, stromal, and total corneal thickness profiles characterized thickness changes associated with keratoconus and may help in early diagnosis of keratoconus.