Harriet O. Lloyd

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.

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.

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.

Acoustic-property maps of the cornea for improved high-frequency ultrasound corneal biometric accuracy

Early diagnosis of keratoconus, a progressive disease characterized by corneal thinning and bulging, is important for avoiding corneal refractive surgery and for treatment planning. Current gold-standard procedures rely on surface topography and corneal-thickness measurements using high-frequency ultrasound (HFU) or optical coherence tomography (OCT). In a previous study of more than 200 normal corneas, OCT measurements of epithelial thickness were systematically thinner than those obtained from 40-MHz HFU measurements. In the present study, acoustic impedance (Z), attenuation (a) and speed of sound (c) of the corneal epithelium and stroma were independently measured using a scanning acoustic microscope (SAM) to investigate the discrepancy in thickness estimates. Corneas of two pigs were snap-frozen and 12-μm thick sections were scanned using a custom-built SAM with an F-1.16, 250-MHz transducer with a 160-MHz bandwidth. 2D maps of c, Z and a, with a spatial resolution of 7 μm were derived. These maps were used to model HFU propagation in silico and to provide more-accurate estimates of corneal and epithelial thicknesses. HFU significantly overestimated epithelial thickness by 1.2 to 2.2 μm because a single value of c (1636 m/s) was used to estimate thickness for all corneal layers. SAM showed that the value of c in the epithelium is substantially lower (i.e., 1539 ± 18 m/s) than the value of c in the stroma (i.e., 1591 ± 28 m/s). After using the SAM-based values in the simulations, no significant difference between HFU and OCT thickness determinations occurred, which showed that the assumption of a constant value of c for all corneal layers is incorrect. SAM permitted obtaining reliable thickness measurements because it provides accurate acoustic-property estimates at fine resolution.

IV.D. Physiology of Accommodation and Role of the Vitreous Body

The eye is a three-chambered system, consisting of the anterior chamber, posterior chamber, and vitreous compartment. The anterior and posterior chambers are separated by the iris and filled with aqueous fluid produced by the ciliary epithelium and flowing from the posterior to the anterior chamber through the pupil. Fluid is constantly replenished and “turned over” in both the aqueous and the vitreous. The fluid “relief valve” is primarily movement of the aqueous from the eye via the trabecular meshwork and Schlemm’s canal, but also is transported from the eye via a secondary uveoscleral outflow route [1–3] involving the ciliary body, choroid, sclera, and episcleral tissues. The anterior and posterior chambers are normally in communication through the pupil and hence in pressure equilibrium. The vitreous is normally also in pressure equilibrium with the aqueous. Remarkably, this pressure equilibrium is maintained during growth and development when there are significant changes in anatomy.

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 choroids role in ocular disease. Translational Relevance Characterization of choroidal flow dynamics offers a novel means for assessment of the choroids role in ocular disease.

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.

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.

An adaptive interferometric sensor for all-optical photoacoustic microscopy

Conventional photoacoustic microscopy (PAM) involves detection of optically induced thermo-elastic waves using ultrasound transducers. This approach requires acoustic coupling and the spatial resolution is limited by the focusing properties of the transducer. We present an all-optical PAM approach that involved detection of the photoacoustically induced surface displacements using an adaptive, two-wave mixing interferometer. The interferometer consisted of a 532-nm, CW laser and a Bismuth Silicon Oxide photorefractive crystal (PRC) that was 5×5×5 mm3. The laser beam was expanded to 3 mm and split into two paths, a reference beam that passed directly through the PRC and a signal beam that was focused at the surface through a 100-X, infinity-corrected objective and returned to the PRC. The PRC matched the wave front of the reference beam to that of the signal beam for optimal interference. The interference of the two beams produced optical-intensity modulations that were correlated with surface displacements. A GHz-bandwidth photoreceiver, a low-noise 20-dB amplifier, and a 12-bit digitizer were employed for time-resolved detection of the surface-displacement signals. In combination with a 5-ns, 532-nm pump laser, the interferometric probe was employed for imaging ink patterns, such as a fingerprint, on a glass slide. The signal beam was focused at a reflective cover slip that was separated from the fingerprint by 5 mm of acoustic-coupling gel. A 3×5 mm2 area of the coverslip was raster scanned with 100-μm steps and surface-displacement signals at each location were averaged 20 times. Image reconstruction based on time reversal of the PA-induced displacement signals produced the photoacoustic image of the ink patterns. The reconstructed image of the fingerprint was consistent with its photograph, which demonstrated the ability of our system to resolve micron-scaled features at a depth of 5 mm.