Determination of the velocity of ultrasound in ocular tissues at different temperatures

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.

Influence of Age on Ocular Biomechanical Properties in a Canine Glaucoma Model with ADAMTS10 Mutation

Soft tissue often displays marked age-associated stiffening. This study aims to investigate how age affects scleral biomechanical properties in a canine glaucoma model with ADAMTS10 mutation, whose extracellular matrix is concomitantly influenced by the mutation and an increased mechanical load from an early age. Biomechanical data was acquired from ADAMTS10-mutant dogs (n = 10, 21 to 131 months) and normal dogs (n = 5, 69 to 113 months). Infusion testing was first performed in the whole globes to measure ocular rigidity. After infusion experiments, the corneas were immediately trephined to prepare scleral shells that were mounted on a pressurization chamber to measure strains in the posterior sclera using an inflation testing protocol. Dynamic viscoelastic mechanical testing was then performed on dissected posterior scleral strips and the data were combined with those reported earlier by our group from the same animal model (Palko et al, IOVS 2013). The association between age and scleral biomechanical properties was evaluated using multivariate linear regression. The relationships between scleral properties and the mean and last measured intraocular pressure (IOP) were also evaluated. Our results showed that age was positively associated with complex modulus (p<0.001) and negatively associated with loss tangent (p<0.001) in both the affected and the normal groups, suggesting an increased stiffness and decreased mechanical damping with age. The regression slopes were not different between the groups, although the complex modulus was significantly lower in the affected group (p = 0.041). The posterior circumferential tangential strain was negatively correlated with complex modulus (R = -0.744, p = 0.006) showing consistent mechanical evaluation between the testing methods. Normalized ocular rigidity was negatively correlated with the last IOP in the affected group (p = 0.003). Despite a mutation that affects the extracellular matrix and a chronic IOP elevation in the affected dogs, age-associated scleral stiffening and loss of mechanical damping were still prominent and had a similar rate of change as in the normal dogs.

Objective measurement of accommodative biometric changes using ultrasound biomicroscopy

PURPOSE

To demonstrate that ultrasound biomicroscopy (UBM) can be used for objective quantitative measurements of anterior segment accommodative changes.

SETTING

College of Optometry, University of Houston, Houston, Texas, USA.

DESIGN

Prospective cross-sectional study.

METHODS

Anterior segment biometric changes in response to 0 to 6.0 diopters (D) of accommodative stimuli in 1.0 D steps were measured in eyes of human subjects aged 21 to 36 years. Imaging was performed in the left eye using a 35 MHz UBM (Vumax) and an A-scan ultrasound (A-5500) while the right eye viewed the accommodative stimuli. An automated Matlab image-analysis program was developed to measure the biometry parameters from the UBM images.

RESULTS

The UBM-measured accommodative changes in anterior chamber depth (ACD), lens thickness, anterior lens radius of curvature, posterior lens radius of curvature, and anterior segment length were statistically significantly linearly correlated with accommodative stimulus demands. Standard deviations of the UBM-measured parameters were independent of the accommodative stimulus demands (ACD: 0.0176 mm; lens thickness: 0.0294 mm; anterior lens radius of curvature: 0.3350 mm; posterior lens radius of curvature: 0.1580 mm; and anterior segment length: 0.0340 mm). The mean difference between the A-scan and UBM measurements was -0.070 mm for ACD and 0.166 mm for lens thickness.

CONCLUSIONS

Accommodating phakic eyes imaged using UBM allowed visualization of the accommodative response, and automated image analysis of the UBM images allowed reliable, objective, quantitative measurements of the accommodative intraocular biometric changes.

FINANCIAL DISCLOSURE

Neither author has a financial or proprietary interest in any material or method mentioned.

Evaluation of ultrasound velocity in enucleated equine aqueous humor, lens and vitreous body

BACKGROUND

Sonographic ophthalmic examinations have become increasingly important in veterinary medicine. If the velocity of ultrasound in ocular tissues is known, the A-mode ultrasound method may be used to determine the axial intraocular distances, such as anterior chamber depth, lens thickness, axial length of the vitreous and axial globe length, which are required for intraocular lens (IOL) power calculations. To the authors' knowledge, the velocity of ultrasound in the ocular tissues of the horse was not previously determined. In the present study, 33 lenses, 29 samples of aqueous and 31 of vitreous from 35 healthy equine eyes have been examined. The corresponding ultrasound velocities are reported in dependence of age, temperature, gender and elapsed time after enucleation.

RESULTS

The velocity of ultrasound at 36°C in equine aqueous, lens and vitreous are 1529 ±10 m/s, 1654± 29 m/s and 1527 ±16 m/s respectively, and the corresponding conversion factors are 0.998± 0.007, 1.008 ±0.018 and 0.997 ±0.010. A linear increase of the speed of ultrasound with increasing temperature has been determined for aqueous and vitreous. No temperature dependence was found for the speed of ultrasound in the lens. The ultrasound velocity did not significantly differ (95%) on the basis of gender, age or time after enucleation during the first 72 hours after death.

CONCLUSIONS

Compared to human eyes, the ultrasound velocity in equine lental tissue deviates by one percent. Therefore, axial length measurements obtained with ultrasound velocities for the human eye must be corrected using conversion factors. For the aqueous and vitreous, deviations are below one percent and can be neglected in clinical settings.

Correlation between biomechanical responses of posterior sclera and IOP elevations during micro intraocular volume change

PURPOSE

This study tested the hypothesis that intraocular pressure (IOP) elevations, induced by controlled increase of intraocular volume, are correlated with the biomechanical responses of the posterior sclera.

METHODS

Porcine globes were tested within 48 hours postmortem. The first group of globes (n = 11) was infused with 15 μL of phosphate-buffered saline at three different rates to investigate rate-dependent IOP elevations. The second group (n = 16) was first infused at the fast rate and then underwent inflation tests to investigate the relationship between IOP elevations (ΔIOP) and scleral strains. The strains in the superotemporal region of the posterior sclera were measured by ultrasound speckle tracking. Linear regression was used to examine the association between ΔIOP due to micro-volumetric infusion and the scleral strains at a specific inflation pressure.

RESULTS

The average ΔIOP was 14.9 ± 4.3 mm Hg for the infusion of 15 μL in 1 second. The ΔIOP was greater for the faster infusion rates but highly correlated across different rates (P < 0.001). A significant negative association was found between the ΔIOP and the tangential strains in both the circumferential (R(2) = 0.54, P = 0.003) and meridian (R(2) = 0.53, P = 0.002) directions in the posterior sclera.

CONCLUSIONS

This study showed a substantial increase in IOP, with a large intersubject variance during micro-volumetric change. A stiffer response of the sclera was associated with larger IOP spikes, providing experimental evidence linking corneoscleral biomechanics to IOP fluctuation. In vivo measurement of corneoscleral biomechanics may help better predict the dynamic profile of IOP.

Variance of speed of sound and correlation with acoustic impedance in canine corneas

The clinical standard for measuring corneal thickness is ultrasound pachymetry that assumes a constant speed of sound. The purpose of this study was to examine the variance of speed of sound and its relationship with acoustic impedance in healthy eyes of canines with a large age span. Corneal speed of sound and acoustic impedance were measured in 34 canine eyes at room temperature (21 ± 1°C). The mean speed of sound was 1577 ± 10 m/s ranging from 1553 to 1594 m/s. There was a strong correlation between speed of sound and acoustic impedance (R = 0.84, p < 0.001). Corneal speed of sound had a small variance in healthy canines over 1-year-old, but was significantly lower in younger canines suggesting an age effect. The strong correlation between corneal speed of sound and acoustic impedance may offer a potential means to noninvasively detect abnormal speed of sound for more accurate corneal thickness estimation.

Non-invasive estimation of hyperthermia temperatures with ultrasound

Ultrasound is an attractive modality for temperature monitoring because it is non-ionizing, convenient, inexpensive and has relatively simple signal processing requirements. This modality may be useful for temperature estimation if a temperature-dependent ultrasonic parameter can be identified, measured and calibrated. The most prominent methods for using ultrasound as a non-invasive thermometer exploit either (1) echo shifts due to changes in tissue thermal expansion and speed of sound (SOS), (2) variation in the attenuation coefficient or (3) change in backscattered energy from tissue inhomogeneities. The use of echo shifts has received the most attention in the last decade. By tracking scattering volumes and measuring the time shift of received echoes, investigators have been able to predict the temperature from a region of interest both theoretically and experimentally in phantoms, in isolated tissue regions in vitro and preliminary in vivo studies. A limitation of this method for general temperature monitoring is that prior knowledge of both SOS and thermal-expansion coefficients is necessary. Acoustic attenuation is dependent on temperature, but with significant changes occurring only at temperatures above 50 degrees C, which may lead to its use in thermal ablation therapies. Minimal change in attenuation, however, below this temperature range reduces its attractiveness for use in clinical hyperthermia. Models and measurements of the change in backscattered energy suggest that, over the clinical hyperthermia temperature range, changes in backscattered energy are dependent on the properties of individual scatterers or scattering regions. Calibration of the backscattered energy from different tissue regions is an important goal of this approach. All methods must be able to cope with motion of the image features on which temperature estimates are based. A crucial step in identifying a viable ultrasonic approach to temperature estimation is its performance during in vivo tests.

Use of a bovine eye lens for observation of HIFU-induced lesions in real-time

Study of coagulative lesion formation by high intensity focused ultrasound (HIFU) in tissue usually requires performing a sequence of experiments under different exposure conditions followed by tissue sectioning. This paper, inspired by the pioneering work of Frederic L. Lizzi, reports on the use of the bovine eye lens as a laboratory model to observe visually the development of HIFU-induced lesions. The first part of this work describes the measurement of the lens shape, density, sound speed and attenuation. The measured values were within the range of previously published values. In the second part, HIFU-induced lesion development was observed in real-time and compared with good agreement with theoretical simulation. Theoretical modeling included acoustic propagation, absorptive heating and thermal dose, as well as the experimentally measured lens characteristics. Thus, the transparent eye lens can be used as a laboratory phantom to facilitate the understanding of HIFU treatment in other tissues.

ULTRASONOGRAPHIC EVALUATION OF PRE- AND POSTNATAL DEVELOPMENT OF THE EYES IN BEAGLES

Transabdominal ultrasonography was performed to visualize the development of intraocular structures of both eyes of four different fetuses in each of two pregnant beagles. Postnatally the development of both eyes of 11 pups was visualized with ultrasonography. Furthermore, biometric measurements of the length of the eye, the depth of cornea, the anterior chamber, the lens (anteroposterior depth and equatorial diameter), and the vitreous body of the fetuses and pups were obtained when these structures could be identified with ultrasonography. In all fetuses the eyes with lens, vitreous body, hyaloid artery, and scleroretinal rim could be clearly identified from day 37 of pregnancy. Postnatally also the cornea, anterior chamber, iris, ciliary body, and optic disc were visible. Biometric measurements revealed, both pre- and postnatally, a continuous growth of the depth of the eye, anterior chamber, lens (anteroposterior depth and equatorial diameter), and vitreous body.

Noninvasive temperature estimation based on the energy of backscattered ultrasound

Hyperthermia has been used as a cancer treatment in which tumors are elevated to cytotoxic temperatures to aid in their control. A noninvasive method for volumetrically determining temperature distribution during treatment would greatly enhance the ability to uniformly heat tumors at therapeutic levels. Ultrasound is an attractive modality for this purpose. We investigated changes in backscattered energy (CBE) from pulsed ultrasound with temperature. Our predicted changes in backscattered energy were matched by in vitro measurements in samples of bovine liver, turkey breast, and pork rib muscle. We studied CBE in tissue regions with multiple scatterers, of isolated individual scatterers, and in collections of individual scatterers. The latter appears to have the most potential. We measured the CBE with a focused circular transducer with a center frequency of 7.5 MHz. The standard deviation of the CBE of 75-125 scattering regions from 0.3 to 0.5 cm3 volumes increased nearly monotonically from 37 degrees C to 50 degrees C in each tissue type. Although the slopes were different, the curve for each type of tissue was well matched by a second-degree polynomial, with a correlation coefficient of 0.99 in each case. Thus the use of the CBE of ultrasound for temperature estimation may have clinical promise with a convenient, low cost modality. Because our approach exploits the inhomogeneities present in tissue, we believe that if it is successful in vitro, it holds promise for in vivo application.

Ultrasound biomicroscopy examination of posterior chamber phakic intraocular lens position

OBJECTIVE

To better elucidate the in vivo position of the Collamer posterior chamber phakic intraocular lens (PCPIOL) and its relationship to the iris and the crystalline lens and to analyze possible variations over time.

DESIGN

Prospective observational case series.

PARTICIPANTS

Eighteen eyes of nine patients were included.

INTERVENTION

A Staar Collamer implantable PCPIOL was implanted for the correction of high myopia.

MAIN OUTCOME MEASURES

The eyes were studied with a 50-MHz ultrasound biomicroscopy UBM 840. The exact PCPIOL position and the distances between it and the crystalline lens were measured at 3, 6, and 12 months after surgery.

RESULTS

There were no intraoperative complications. In 13 eyes (72.22%), contact between the PCPIOL and the crystalline lens was found at some time during follow-up. In 3 eyes (16.6%), central contact could be demonstrated. We also observed that the contact zone and its extension can vary over time. In 2 eyes, rotation of the lens was observed.

CONCLUSIONS

We found contact between the PCPIOL and the crystalline lens in a high percentage of cases. There was also mobility of the lens in the posterior chamber, especially in the anteroposterior plane, and, as a consequence, both the contact zone and its extension would vary over time.

Hardness and ultrasonic characteristics of the human crystalline lens

PURPOSE

To evaluate the relationship between hardness of the human crystalline lens and its acoustic characteristics.

SETTING

St. George's Hospital Medical School, University of London, London, United Kingdom.

METHODS

Cataractous lenses from patients who had extracapsular cataract surgery were studied for hardness and ultrasonic characteristics. Lens hardness was assessed with an automated guillotine. Ultrasound velocity and attenuation were measured with a scanning acoustic macroscope using the pulse transmission reflection method.

RESULTS

Thirty-seven lenses from 37 patients (mean age 75.5 years) were evaluated. Lens hardness was associated with ultrasound attenuation (r = 0.65, P <.0001) and attenuation frequency gradient (r = 0.67, P <.0001). The correlation of hardness with mean ultrasound velocity was not significant (r = 0.22, P =.2).

CONCLUSIONS

The attenuation of ultrasound waves by the human crystalline lens correlated with its hardness. Ultrasonography can be used to evaluate lens hardness.

Ultrasound velocity in the aqueous and vitreous humours of the one-humped camel (Camelus Dromedarius)

BACKGROUND: There are no previous reports on the velocity of ultrasound through the aqueous and vitreous humours of the one-humped camel. This information is required for determination of the depth of the aqueous and vitreous chambers and therefore the axial length. This knowledge together with other, yet to be determined, ocular dimensions will ultimately be used in drawing up a schematic eye for the camel. This will provide a tool for studying the visual capability of this animal. METHOD: A-mode ultrasonography was used to determine the velocity of sound through the aqueous and vitreous fluids of the one-humped camel. The fluids were obtained from freshly enucleated eyes. RESULTS: The average velocity of ultrasound through 20 aqueous and 22 vitreous humour samples kept at 20 degrees Celsius was 1,499 +/- 23 m/s and 1,497 +/- 24 m/s, respectively. CONCLUSION: We recommend that at 20 degrees Celsius a common value of 1,498 m/s should be adopted as the velocity of ultrasound through the aqueous and vitreous humours of the one-humped camel. This value is slower than in humans but similar to that of the cow and pig.

Theoretical estimation of the temperature dependence of backscattered ultrasonic power for noninvasive thermometry

The backscattered signal received from an insonified volume of tissue depends on tissue properties, such as attenuation, velocity, density, and backscatter coefficient and on the characteristics of the transducer at the insonified volume. Analysis of scattering in response to a burst of insonification showed that the temperature dependence of backscattered power was dominated by the effect of temperature on the backscatter coefficient. The temperature dependence of attenuation had a small effect on backscattered power. Backscattered power was independent of effects of temperature on velocity. These results were seen in the analysis of two types of inhomogeneity: 1) an aqueous scatterer in a water-based medium and 2) a lipid-based scatterer in the same water-based medium. The temperature dependence of the backscatter coefficient was inferred assuming that the backscatter coefficient was proportional to the scattering cross-section of a small scatterer. Backscattered power increased nearly logarithmically with temperature over the range from 37 degrees to 50 degrees C. Our model predicted a change of 5 dB for the lipid scatterer and a change of up to 3 db for the aqueous-based scatterer over that temperature range. For situations in which temperature dependence of the backscattered power can be calibrated, it may be possible to use the backscattered power level to track temperature distributions in tissue.

Ultrasonic spectroscopy of the porcine eye lens

The purpose of the work is to measure and study the acoustic characteristics of the porcine eye lens and find correlations with chemical and optical parameters, obtained from literature. Ultrasonic spectroscopy was performed by using a scanning acoustic macroscope (frequency 20 MHz, resolution 150 microns). The transducer performed a two-dimensional scan over a central slice (1 mm thickness) of porcine lens (number of lenses = 10). A double transmission pulse-echo method was used to acquire the ultrasonic data from the lens. Two-dimensional images were reconstructed of the local ultrasound velocity and the frequency-dependent ultrasound attenuation. Axial and equatorial profiles of these parameters were calculated from the images. The acoustic parameters are not constant, but show a systematic dependence on the location within the lens. The profiles of the acoustic parameters are similar in shape to profiles of the protein and water contents of eye lens and to the profiles of the optical refractive index. A thorough quantitative correlation study is indicated, which should be based on detailed protein content data in porcine lenses.

Acoustic velocity and attenuation of eye tissues at 20 MHz

The ultrasound velocity and frequency-dependent attenuation of human and porcine eye tissues (cornea, lens, retina, choroid, sclera, vitreous body) were measured in the frequency range from 17 to 23 MHz. The results for the ultrasound velocity were compared to values taken from the literature and appeared to be in the same range. A comparison made between the acoustic parameters of human and porcine eyes showed that the porcine eye can serve as an animal model for the human eye. A mathematical operation is proposed to extrapolate the attenuation to the lower frequencies that are commonly used in clinical equipment. Finally, a first attempt was made to investigate the age dependence of the acoustic parameters of human tissues: some tissues showed a significant age effect.

The history of ultrasound techniques in ophthalmology

The history of 50 years of applications of ultrasound in ophthalmology is described. This period started in 1938 with a study of the possible effects of high intensity ultrasound on eyes. The measurement of biological effects for the assessment of potential hazards characterizes the first decades. More recently, the therapeutic use of ultrasound by hyperthermia has gained much interest in ophthalmology. Further topics are: the measurement of acoustic characteristics of ocular tissues; the biometry of the eye ball, the results of which are used to calculate the optical power of artificial lens implants; the development of diagnostic instruments in various parts of the world; and ultrasonic tissue characterization for the differential diagnosis of pathology. The final topic is concerned with some recent developments which present a look into the future.

Repeatability, reproducibility and intersession variability of the Allergan Humphrey ultrasonic biometer

We evaluated repeatability and reproducibility of the Allergan Humphrey model 820 ultrasonic biometer. Sixty eyes (30 subjects) free from ophthalmological abnormality were examined on two separate occasions by two experimenters, one experienced and one inexperienced. The 95% confidence limits for repeatability lie between +0.12/-0.13 mm and reproducibility between +0.15/-0.14 mm for AC depth, between +0.11/-0.13 mm and +0.12/-0.16 mm for lens thickness, between +0.17/-0.19 mm and +0.19/-0.18 mm for right axial length and between +0.26/-0.40 mm and +0.22/-0.25 mm for left axial length. The 40% gain used to measure left axial lengths produced poor retinal echospikes, which may account for reduced accuracy compared to the right eyes using 60% gain. Ten subjects (20 eyes) returned for 3 further visits and the variability over 5 sessions was examined using a two-factor ANOVA with repeated measures on one factor. There were no significant experimenter or session effects and no interaction effect. This biometer gives clinically repeatable and reproducible results and is convenient to use.

Investigation of ultrasound axially traversing the human eye

A ray tracing model for ultrasonic propagation through the human eye, including the lens, has been developed on the assumptions of lossless media and non-reflecting interfaces. Measurement of the distribution of an ultrasonic beam before and after traversing specimens of human eyes in vitro, and of the velocity of ultrasound in the various dissected media, has permitted some comparison of the predictions of the model with experiment. The agreement is good although there are significant limitations involved and these are discussed. For imaging systems the effect of the eye arises largely from the lens which acts as a defocussing lens of focal length approx. 13.5 cm. Although the experiments were performed at approx. 4 MHz, the validity of the ray tracing model is largely frequency independent and will be appropriate at the higher frequencies commonly used in ophthalmology.