Age-related macular degeneration is the leading cause of blindness in the elderly population, with a high demand for early diagnosis since the symptoms are irreversible. Current structural and functional imaging modalities include fundus autofluorescence, optical coherence tomography, and angiography, and are often not sufficient for early stage detection, which is mostly characterized by changes in tissue composition. A technology that enables the in-vivo imaging of the posterior ocular globe is essential for gaining insight into the natural mechanical anatomy of the eye, as well as the changes that take place with ocular diseases. However, in-vivo mechanical imaging of the retina remains a challenge and is currently not available. In this study, we report on the development of acoustic radiation force optical coherence elastography (ARF-OCE) to visualize and quantify the stiffness map of in-vivo retinal tissues based on the Voigt model. We demonstrate the elasticity mapping of an in-vivo rabbit retina, showing the stiffness variations across 5 different layers, ranging from 3 kPa to 16 kPa on the ganglion to the sclera sides. In addition, we introduce a diseased rabbit model based primarily on blue light exposure, and have found a difference in the layered stiffness where inflammation occurred. The results show that the ARF-OCE system has the capability to noninvasively detect tissue abnormalities in-vivo, and represents a significant step toward the development of the ARF-OCE system for clinical use.
Age-related macular degeneration and keratoconus are two ocular diseases occurring in the posterior and anterior eye, respectively. In both conditions, the mechanical elasticity of the respective tissues changes during the early onset of disease. It is necessary to detect these differences and treat the diseases in their early stages to provide proper treatment. Acoustic radiation force optical coherence elastography is a method of elasticity mapping using confocal ultrasound waves for excitation and Doppler optical coherence tomography for detection. We report on an ARF-OCE system that uses modulated compression wave based excitation signals, and detects the spatial and frequency responses of the tissue. First, all components of the system is synchronized and triggered such that the signal is consistent between frames. Next, phantom studies are performed to validate and calibrate the relationship between the resonance frequency and the Young’s modulus. Then the frequency responses of the anterior and posterior eye are detected for porcine and rabbit eyes, and the results correlated to the elasticity. Finally, spatial elastograms are obtained for a porcine retina. Layer segmentation and analysis is performed and correlated to the histology of the retina, where five distinct layers are recognized. The elasticities of the tissue layers will be quantified according to the mean thickness and displacement response for the locations on the retina. This study is a stepping stone to future in-vivo animal studies, where the elastic modulus of the ocular tissue can be quantified and mapped out accordingly.
We present a non-contact “frequency domain photoacoustic microscope” (Fd-PAM) in which photoacoustic signal is generated by an amplitude modulated continuous-wave (CW) laser and detected at the sample surface using two wave mixing (interferometer) in a photorefractive crystal (PRC). The optical detection eliminates the need for a coupling medium, thus making the probe contactless and mitigates loss in signal-to-noise ratio (SNR) resulting from attenuation associated with wave propagation from the sample to the sensor. The single frequency excitation enables the use of extremely narrow band detection techniques like a lock-in amplifier for noise suppression. Our approach also can image multi-layered specimen and directly produce an image that is equivalent to the maximum-intensity projection of the 3D image volume.
Conventional photoacoustic microscopy (PAM) employs light pulses to produce a photoacoustic (PA) effect and detects the resulting acoustic waves using an ultrasound transducer acoustically coupled to the target. The resolution of conventional PAM is limited by the sensitivity and bandwidth of the ultrasound transducer. We investigated a versatile, all-optical PAM (AOPAM) system for characterizing in vivo as well as ex vivo biological specimens. The system employs non-contact interferometric detection of PA signals that overcomes limitations of conventional PAM. A 532-nm pump laser with a pulse duration of 5 ns excites the PA effect in tissue. Resulting acoustic waves produce surface displacements that are sensed using a 532-nm continuous-wave (CW) probe laser in a Michelson interferometer with a 1- GHz bandwidth. The pump and probe beams are coaxially focused using a 50X objective giving a diffraction-limited spot size of 0.48 μm. The phase-encoded probe beam is demodulated using homodyne methods. The detected timedomain signal is time reversed using k-space wave-propagation methods to produce a spatial distribution of PA sources in the target tissue. A minimum surface-displacement sensitivity of 0.19 pm was measured. PA-induced surface displacements are very small; therefore, they impose stringent detection requirements and determine the feasibility of implementing an all-optical PAM in biomedical applications. 3D PA images of ex vivo porcine retina specimens were generated successfully. We believe the AOPAM system potentially is well suited for assessing retinal diseases and other near-surface biomedical applications such as sectionless histology and evaluation of skin burns and pressure or friction ulcers.
High-frequency-ultrasound transducers are widely used but are typically based either on planar piezoceramic sections
that are lapped down to smaller thicknesses or on piezopolymers that may be deformed into more complex geometries.
Piezoceramics then require dicing to obtain arrays or can be fractured into spherical geometries to achieve focusing.
Piezopolymers are not as efficient for very small element sizes and are normally available only in discrete thicknesses.
Thick-film (TF) transducers provide a means of overcoming these limits because the piezoelectric film is deposited with
the required thickness, size and geometry, thus avoiding any subsequent machining. Thick-film transducers offer the
potential of a wide range of geometries such as single-elements and annular or linear arrays. Here, a single-element
focused transducer was developed using a piezoceramic composition adapted to high-power operation which is
commonly used at standard MHz frequencies. After fabrication, the transducer was characterized. Using specific
transmit-receive electronics and a water tank adapted to high-frequency devices, the transducer was excited using a short
pulse to evaluate its bandwidth and imaging capabilities. Finally, it was excited by a one-period sine wave using several
power levels to evaluate its capacity to produce high-intensity focused ultrasound at frequencies over 20 MHz.
Convergence of light towards a desired location in optically diffusive and aberrative media is highly relevant to
optical methods of biomedical imaging. In this study, we demonstrated the feasibility of employing photoacoustic
signals originating from an optically absorptive target as feedback for shaping the incident wavefront to increase
optical energy density at the absorptive target. The wavefront of a collimated laser beam was shaped by an array of
two-dimensional MEMS deformable mirrors and then transmitted through optically scattering paraffin. The phase of
light reflected by each mirror was varied (0-2π) iteratively to maximize the amplitude of the photoacoustic signal.
The photoacoustic signal potentially provides a non-invasive and reliable feedback for manipulating spatial phase
distribution of light to achieve focusing in diffusive media and may facilitate optical imaging at greater depths.
Purpose: Ultrasound and optical coherence tomography (OCT) are widely used techniques for diagnostic imaging
of the eye. OCT provides excellent resolution, but limited penetration. Ultrasound provides better penetration, but an
order-of-magnitude poorer resolution than OCT. Photoacoustic imaging is relatively insensitive to scattering, and so
offers a potential means to image deeper than OCT. Furthermore, photoacoustic imaging detects optical absorption,
a parameter that is independent of that detected by conventional ultrasound or OCT. Our aim was to develop a
photoacoustic system suitable for imaging the eye.
Methods: We developed a prototype system utilizing a focused 20 MHz ultrasound probe with a central aperture
through which optics were introduced. The prototype system produced 1-μJ, 5-nsec pulses at 532 or 1064 nm with a
20-μm spot size at a 500 Hz repetition rate. The photoacoustic probe was mounted onto computer-controlled linear
stages and pulse-echo ultrasound and photoacoustic images obtained on ex vivo pig eyes and in vivo mouse eyes.
Results: Lateral resolution was significantly improved by use of a laser spot size much smaller than the acoustic
beamwidth. Photoacoustic signals were obtained primarily from melanin in ex vivo tissues and from melanin and
hemoglobin in vivo. Image fusion allowed superposition of photoacoustic signals upon the anatomic features
detected by conventional ultrasound.
Conclusion: Photoacoustic imaging detects the presence of clinically relevant pigments, such as melanin and oxyand
deoxy-hemoglobin, and, potentially, from other pathologic pigments occurring in disease conditions (tumors,
nevii, macular degeneration). Fine-resolution photoacoustic data provides information not detected in current
ophthalmic imaging modalities.
Purpose: Harmonic imaging has become a well-established technique for ultrasonic imaging at fundamental frequencies of 10 MHz or less. Ophthalmology has benefited from the use of fundamentals of 20 MHz to 50 MHz. Our aim was to explore the ability to generate harmonics for this frequency range, and to generate harmonic images of the eye.
Methods: The presence of harmonics was determined in both water and bovine vitreous propagation media by pulse/echo and hydrophone at a series of increasing excitation pulse intensities and frequencies. Hydrophone measurements were made at the focal point and in the near- and far-fields of 20 MHz and 40 MHz transducers. Harmonic images of the anterior segment of the rabbit eye were obtained by a combination of analog filtering and digital post-processing.
Results: Harmonics were generated nearly identically in both water and vitreous. Hydrophone measurements showed the maximum second harmonic to be -5 dB relative to the 35 MHz fundamental at the focus, while in pulse/echo the maximum harmonic amplitude was -15dB relative to the fundamental. Harmonics were absent in the near-field, but present in the far-field. Harmonic images of the eye showed improved resolution.
Conclusion: Harmonics can be readily generated at very high frequencies, and at power levels compliant with FDA guidelines for ophthalmology. This technique may yield further improvements to the already impressive resolutions obtainable in this frequency range. Improved imaging of the macular region, in particular, may provide significant improvements in diagnosis of retinal disease.
KEYWORDS: Transducers, Tissues, Spatial frequencies, Spectrum analysis, Ultrasonics, Ultrasonography, Acoustics, Data acquisition, Integration, Medical research
We have conducted a general study that relates calibrated 2-D ultrasonic spectral parameters to the physical properties of sub-resolution tissue scatterers. Our 2-D spectra are computed form digital radio-frequency echo data obtained as the transducer linearly scans along the cross-range (scan direction) with increments smaller than the half beam width. Acquired data are Fourier transformed with respect to range (beam) and cross-range (scan) directions. To quantitatively measure and classify the physical properties of tissues, we have defined two spectral functions and four spectral parameters. The 2-D spectral functions are: radially integrated spectral power (RISP) and angularly integrated spectral power (AISP). The summary parameters are: peak value and 3-dB width of the RISP, slope and intercept of the AISP. These parameter are understood in terms of the beam properties, transducer parameters and the physical properties of the tissue microstructures including size, shape, orientation, concentration and acoustic impedance. Our theoretical model indicates that 1) the 3-dB width of the RISP is predominantly determined by the scatterer size along the beam direction; 2) the slope of the linear fit of the AISP is predominantly determined by the scatterer size along range direction; 3) the concentration and the relative acoustic impedance fluctuation of the scatterers change the overall spectrum magnitude. The predictions of the theoretical model have been verified using beef muscle fibers examined with 40 MHz center frequency.
Objective: To investigate and diagnose testicular pathology in patients with testicular dysfunction using the technique of ultrasound power spectrum analysis. Methods: Testicular ultrasound studies with power spectrum tissue characterization analysis were performed on men with testicular abnormalities as well as normal controls. Semen analysis, biopsy data, microscopic intra-operative findings and data pertaining to testicular function were collected for each surgically evaluated subject. Ultrasound data were analyzed for power spectrum characteristics of microscopic scatterer size and concentration within discrete areas of testicular tissue. Results: Patients with varicoceles and greater than 2x106 sperm/ml on semen analysis had larger average scatterer size (107.7 micrometers ) and lower scatterer concentration (-15.02 dB) than non-obstructed, azoospermic patients with varicoceles (92.4 micrometers and -11.41 dB, respectively). Subjects with obstructed azoospermia had slightly larger average tissue scatterer size (108.1 micrometers ) and lower concentration (-15.73 dB) while normal control data revealed intermediate values of size (102.3 micrometers ) and concentration (-13.1 dB) of scatterers. Spectral data from pure testicular seminoma lesions had the lowest average scatterer size (82.3 micrometers ) with low relative concentration (-14.7 dB). Summary: Ultrasound tissue characterization based on RF spectrum analysis may distinguish different types of testicular pathology including obstructed and non-obstructed azoospermia and tissue changes due to varicocele and tumor.
Purpose: While histopathologic evaluation of ocular melanoma permits assessment of metastatic risk, this is not possible if visual function is to be preserved. In this report, we describe ultrasound methods for non-invasive evaluation of metastatic risk. Methods: Radiofrequency (RF) ultrasound data were acquired prior to enucleation in 117 eyes with untreated malignant melanoma. Extracellular matrix patterns, an indicator of metastatic potential, were identified in histologic sections. We determined calibrated backscatter power spectra, estimates of effective scatterer size and concentration, and the percentage of tumor area comprised of PAS-positive patterns in the anterior, posterior and core regions of the tumors. We compared the spatial correlation of histologic and acoustic properties, performed linear discriminant analysis to define prognostic models, and used receiver operating characteristic (ROC) curves to evaluate models. Results: Statistically significant correlations were found between acoustic parameters and PAS-positive patterns, although their spatial distributions were only weakly related. Stepwise linear discriminant analyses produced models with three to five variables, and ROC areas as high as 0.89. Conclusion: Acoustic spectrum analysis provides information not evident in conventional gray-scale ultrasonograms regarding tissue microstructure. Our results confirm a relationship between spectra and the presence of extracellular matrix patterns associated with metastatic risk.
Purpose: The cornea acts as the window of the eye's optical system, and its chief refractive component. The lack of intervening tissues makes the cornea accessible to very high frequency ultrasonic study. In this report, we detail use of radiofrequency (RF) signal processing methods to characterize corneal pathology and to enhance biometric precision. Methods: Using a 50 MHz PVDF transducer, we scanned the cornea using an arc motion so as to maintain normality and consistent range. RF data were acquired at a sample rate of 500 MHz. Deconvolution against a glass-plate echo allowed biometric enhancement (by effectively broadening the bandwidth) and measurement of tissue backscatter spectra. Results: Calibrated spectrum analysis was used to quantitatively measure backscatter in corneal scars and other pathologies. Signal processing allowed us to attain reproducibility for repeated measurements of the corneal epithelium (approximately 50 microns thick) to 1 micron. By combining measurements from a series of planes, maps of the thickness of the individual layers comprising the cornea were produced. Conclusion: The layers of the cornea have different optical refractive indices, and thus their thicknesses directly affect visual acuity. The scattering of light by a corneal scar is caused by inhomogeneities or irregularities that may result in acoustic backscatter as well. The ability of ultrasound to quantify backscatter and corneal layer thickness provides a new avenue for diagnosis of corneal disease and refractive abnormalities.
The first high frequency ultrasound system able to image blood flow in the microcirculation in real-time has been developed. 2D color flow frames are rapidly acquired using a recently reported method to achieve frame rates approaching 10 fps. A new flow phantom was constructed in order to tune the wall filter order, cutoff and attenuation for a 25 MHz, f/2 transducer. RF data were acquired in both M-mode and swept-mode, and processed in order to tune the wall filter. These filters were then used in making controlled measurements of flow velocity and volume flow rate for a typical PRF of 500 Hz (1 mm/sec scan speed). Over th einput of mean axial velocities ranging from 0.3 to 3.0 mm/sec (0.88 to 8.8 mm/sec angle corrected), the measured mean and maximum flow velocities were linear, with slight over-estimation of mean velocities due to the wall filter cutoff. Without correction for finite beam size, the volume flow rates were over-estimated by a factor of 2. The color flow settings were then applied to image microcirculatory flow within the nail bed of a human finger, where they were tested and optimized for a variety of vessel sizes and flow velocities.
Two- and three-dimensional depictions of ultrasound echo signal data have potential for helping to detect and diagnose disease and to plan and monitor therapy. The utilization of very-high-frequency ultrasound and spectrum analysis of radio- frequency echo signals extends the capabilities of ultrasonic imaging for these purposes. Images generated using these techniques can present tissue architecture with exquisite resolution and can provide information on underlying properties of scatterers in the tissue. Changes in properties over time can be used to monitor disease progression or response to therapy. Relating tissue echo-signal parameters obtained from unknown tissue to database values of known tissue types can provide means of characterizing tissue for the purposes of detection or diagnosis and treatment planning. These potential applications are illustrated using examples from plaque, ophthalmic, skin, and prostate studies.
Very high frequency (50 MHz) ultrasound provides spatial resolution on the order of 30 microns axially by 60 microns laterally. Our aim was to reconstruct the three-dimensional anatomy of the eye in the full detail permitted by this fine- scale transducer resolution. We scanned the eyes of human subjects and anesthetized rabbits in a sequence of parallel planes 50 microns apart. Within each scan plane, vectors were also spaced 50 microns apart. Radio-frequency data were digitized at a rate of 250 MHz or higher. A series of spectrum analysis and segmentation algorithms was applied to data acquired in each plane; the outputs of these procedures were used to produce color-coded 3-D representations of the sclera, iris and ciliary processes to enhance 3-D volume rendered presentation. We visualized the radial pattern of individual ciliary processes in humans and rabbits and the geodetic web of supporting connections between the ciliary processes and iris that exist only in the rabbit. By acquiring data such that adjacent vectors and planes are separated by less than the transducer's lateral resolution, we were able to visualize structures, such as the ciliary web, that had not been seen before in-vivo. Our techniques offer the possibility of high- precision imaging and measurement of anterior segment structures. This would be relevant in monitoring of glaucoma, tumors, foreign bodies and other clinical conditions.
This report describes ultrasonic transducers that are designed to expedite thermal-necrosis treatment of tumors, particularly in the eye. The spherical-cap transducers employ pairs of parallel strip-electrodes to generate focal-zone beam patterns that are narrow in one direction and exhibit a number ofprominent lobes over a larger width in the orthogonal direction. Diffraction analysis and thermal modelling are employed to derive information for designing such asymmetric beams and producing continuous, asymmetric thermal lesions within tumors. Compared with typical ellipsoidal lesions, the "chicklet" shaped lesions produced by these beams permit a larger tissue volume to be treated as the transducer is scanned across a tumor. This fact can reduce the number ofscans required to treat entire tumors and may significantly reduce overall clinical treatment times. Keywords: ultrasonic therapy, ultrasonic transducers, thermal modelling, thermal necrosis, high-intensity focused ultrasound
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