Kyle W. Hollman

Coherence factor of speckle from a multi-row probe

The coherence factor provides a quantitative measure of image quality. It is defined as the ratio of the coherent sum across array elements to the incoherent sum and measures the distribution of ultrasonic energy between the main beam and side lobes of a radiation pattern. Values range from 0 to 1. For low values most of the energy is outside of the main beam, and for high values it is in the main beam. The authors have applied the van Cittert-Zernike theorem to determine analytic solutions of the coherence factor for single and multi-row arrays. The solution depends only on the number of rows and columns in a transducer array. With a multi-row probe, the authors imaged a uniform tissue-mimicking phantom and saved coherent signals. Images of the phantom were produced based on coherent and incoherent summations of array elements. They then combined the two images to produce a coherence factor image. Within the focal region, average coherence was 0.50 for the phantom which compares favorably to a value of 0.53 from the analytic solution. Next, phase distortions of pi/2 and pi radians were electronically introduced at specific elements, and the phantom was imaged again. Phase distortion greatly effects energy distribution for coherent summations but has a minimal effect on incoherent summations. An introduced distortion of pi/2 decreased the average coherence factor to 0.33. A distortion of pi further decreased it to 0.11. Results of human studies showed decreased average coherence factors compared to undistorted phantom images. These results suggest that the coherence factor provides a quantitative measure of beam quality for in vivo imaging.

Strain imaging of corneal tissue with an ultrasound elasticity microscope

Purpose. We hypothesize that high-resolution elasticity measurements can guide corrective refractive surgery of the cornea. Elasticity measurements would improve surgical outcomes by adding biomechanical information not used in existing clinical nomograms. As an initial investigation, we determined the usefulness and evaluated the ability of our ultrasound elasticity microscope by measuring strain ex vivo in an intact porcine eye globe. Methods. Strain was predicted with a finite element model guided by direct mechanical measurements of corneal elasticity. Next, a porcine cornea was deformed with a slitted plate while being imaged with ultrasound. For high spatial resolution, the ultrasound elasticity microscope uses a 50 MHz transducer with a 1.4 f/number. It produces high-quality conventional ultrasonic B-scans over large thicknesses by confocal processing. Strain was calculated from tracking speckle in these images after deformation. This technique is compatible with in vivo measurements. Results. Compressional and expansional deformations were the same order of magnitude from −3.5% to as great as +3.5%. Strain imaging indicated the stroma expanded into the slit of the deformation plate while Bowmans layer compressed. This bipolar variation within a specimen is unusual. Within the stroma, a variation of strain with depth was measured suggesting a distribution of elasticity. Results compared favorably with the finite element model. Conclusion. An ultrasound elasticity microscope can produce high-resolution strain images throughout the corneal depth. Various layers with different elastic properties appeared as different strains in the images.

Bubble-based acoustic radiation force elasticity imaging

Acoustic radiation force is applied to bubbles generated by laser-induced optical breakdown (LIOB) to study viscoelastic properties of the surrounding medium. In this investigation, femtosecond laser pulses are focused in the volume of gelatin phantoms of different concentrations to form bubbles. A two-element confocal ultrasonic transducer generates acoustic radiation force on individual bubbles while monitoring their displacement within a viscoelastic medium. Tone burst pushes of varying duration have been applied by the outer element at 1.5 MHz. The inner element receives pulse-echo recordings at 7.44 MHz before, during, and after the excitation bursts, arid cross-correlation processing is performed offline to monitor bubble position. Maximum bubble displacements are inversely related to the Youngs moduli for different gel phantoms, with a maximum bubble displacement of over 200 /spl mu/m in a gel phantom with a Youngs modulus of 1.7 kPa. Bubble displacements scale with the applied acoustic radiation force and displacements can be normalized to correct for differences in bubble size. Exponential time constants for bubble displacement curves are independent of bubble radius and follow a decreasing trend with the Youngs modulus of the surrounding medium. These results demonstrate the potential for bubble-based acoustic radiation force methods to measure tissue viscoelastic properties.

Acoustic characterization of microbubble dynamics in laser-induced optical breakdown

A real-time acoustic technique to characterize microbubbles produced by laser-induced optical breakdown (LIOB) in water was developed. Femtosecond laser pulses are focused just inside the surface of a small liquid tank. A tightly focused, high frequency, single-element ultrasonic transducer is positioned so its focus coincides axially and laterally with this laser focus. When optical breakdown occurs, a bubble forms and a pressure wave is emitted (i.e., acoustic emission). In addition to this acoustic signal, the microbubble is actively probed with pulse-echo measurements from the same transducer. After the bubble forms, received pulse-echo signals have an extra pulse, describing the bubble location and providing a measure of axial bubble size. Wavefield plots of successive recordings illustrate the generation, growth, and collapse of cavitation bubbles due to optical breakdown. These same plots also can be used to quantify LIOB thresholds.

Acoustic detection of microbubble formation in enhanced optical breakdown of silver/dendrimer nanocomposites

We utilize a real-time acoustic technique, based on pulse-echo measurements to detect formation of microbubbles in an aqueous solution of a silver/dendrimer nanocomposite ~DNC!. Wave-field plots of successive recordings illustrate the generation and behavior of bubbles created by the optical breakdown process. A significant threshold reduction is achieved with DNC particles compared to its host dendrimer, enabling a diverse field of low-threshold breakdown applications.

Bubble-based acoustic radiation force for monitoring intraocular lens elasticity

We test the hypothesis that local viscoelastic properties of the intraocular lens can be measured by applying acoustic radiation force to laser-generated bubbles. Presbyopia is an age-related condition resulting from increased stiffness of the lens, reducing its ability to accommodate. A technique to measure local lens viscoelastic properties is needed to better understand the onset of presbyopia and guide potential correction procedures. Laser-induced optical breakdown (LIOB) is used to create bubbles within porcine intraocular lenses. Optical breakdown occurs when sufficiently high threshold fluence is attained at the focus of femtosecond pulsed lasers, inducing plasma formation and bubble generation. The small transient gas bubbles can be used as targets for acoustic radiation force measurements prior to their ultimate collapse. While ultrasonic speckle is extremely limited within the lens, LIOB bubbles provide strong ultrasonic backscatter to measure lens viscoelastic properties. In this investigation, explanted porcine lenses are embedded within a gelatin phantom (5 w/w%) prior to laser treatment. An integrated optical-acoustical system has been constructed enabling simultaneous bubble creation and radiation force experiments. A two-element confocal ultrasonic transducer generates acoustic radiation force with the 1.5 MHz outer element while monitoring the bubble displacement within the lens using the 7.44 MHz inner element. Preliminary experiments have demonstrated the ability to create LIOB bubbles within explanted porcine lenses with lifetimes on the order of a few minutes and at any depth within the lens. Acoustic radiation force experiments with LIOB bubbles in porcine lenses exhibit exponential responses with time constants near 2 ms and maximum displacements on the order of 100 /spl mu/m. These results advance the development of an in vivo technique to measure local lens viscoelastic properties.

Measurement of corneal elasticity with an acoustic radiation force elasticity microscope.

To investigate the role of collagen structure in corneal biomechanics, measurement of localized corneal elasticity with minimal destruction to the tissue is necessary. We adopted the recently developed acoustic radiation force elastic microscopy (ARFEM) technique to measure localize biomechanical properties of the human cornea. In ARFEM, a low-frequency, high-intensity acoustic force is used to displace a femtosecond laser-generated microbubble, while high-frequency, low-intensity ultrasound is used to monitor the position of the microbubble within the cornea. Two ex vivo human corneas from a single donor were dehydrated to physiologic thickness, embedded in gelatin and then evaluated using the ARFEM technique. In the direction perpendicular to the corneal surface, ARFEM measurements provided elasticity values of E = 1.39 ± 0.28 kPa for the central anterior cornea and E = 0.71 ± 0.21 kPa for the central posterior cornea in pilot studies. The increased value of corneal elasticity in the anterior cornea correlates with the higher density of interweaving lamellae in this region.

Acoustic detection of controlled laser-induced microbubble creation in gelatin

A high-frequency (85 MHz) acoustic technique is used to identify system parameters for controlled laser-induced microbubble creation inside tissue-mimicking, gelatin phantoms. Microbubbles are generated at the focus of an ultrafast 793-nm laser source and simultaneously monitored through ultrasonic pulse-echo recordings. Displayed in wavefield form, these recordings illustrate microbubble creation, and integrated backscatter plots provide specifics about microbubble characteristics arid dissolution behavior. By varying laser parameters, including pulse fluence (or pulse energy flux, J/cm/sup 2/), total number of pulses delivered, and the period between pulses, the size, lifetime, and dissolution dynamics of laser-induced microbubbles may be independently controlled. Pulse fluence is the main size-controlling parameter, whereas both increases in pulse fluence and pulse number can lengthen microbubble lifetime from tens to hundreds of milliseconds. In short, a microbubble of particular lifetime does not necessarily have to be of a particular size. Microbubble behavior, furthermore, is independent of pulse periods below a fluence-dependent threshold value, but it exhibits stochastic behavior if pulse repetition is too slow. These results demonstrate that laser pulse fluence, number, and period may be varied to deposit energy in a specific temporal manner, creating arid stabilizing microbubbles with particular characteristics and, therefore, potential uses in sensitive acoustic detection and manipulation schemes.

Using an Ultrasound Elasticity Microscope to Map Three-Dimensional Strain in a Porcine Cornea

An ultrasound elasticity microscope was used to map 3-D strain volume in an ex vivo porcine cornea to illustrate its ability to measure the mechanical properties of this tissue. Mechanical properties of the cornea play an important role in its function and, therefore, also in ophthalmic diseases such as kerataconus and corneal ectasia. The ultrasound elasticity microscope combines a tightly focused high-frequency transducer with confocal scanning to produce high-quality speckle over the entire volume of tissue. This system and the analysis were able to generate volume maps of compressional strain in all three directions for porcine corneal tissue, more information than any previous study has reported. Strain volume maps indicated features of the cornea and mechanical behavior as expected. These results constitute a step toward better understanding of corneal mechanics and better treatment of corneal diseases.

COMPARISON OF SCANNING ACOUSTIC MICROSCOPY AND HISTOLOGY IMAGES IN CHARACTERIZING SURFACE IRREGULARITIES AMONG ENGINEERED HUMAN ORAL MUCOSAL TISSUES

Acoustic microscopy was used to monitor an ex vivo produced oral mucosal equivalent (EVPOME) developed on acellular cadaveric dermis (AlloDerm®). As seeded cells adhered and grew, they filled in and smoothed out the surface irregularities, followed by the production of a keratinized protective outermost layer. If noninvasive in vitro ultrasonic monitoring of these cellular changes could be developed, then tissue cultivation could be adjusted in-process to account for biologic variations in the development of these stratified cell layers. Cultured keratinocytes (from freshly obtained oral mucosa) were harvested and seeded onto AlloDerm® coated with human type IV collagen and cultured 11 days. EVPOMEs were imaged on the 11th day post-seeding using a scanning acoustic microscope (SAM) that consists of a single-element transducer: 61 MHz center frequency, 32 MHz bandwidth, 1.52 f-number. The specimen surface was determined by thresholding the magnitude of the signal at the first axial incidence of a value safely above noise: 20-40 dB above the signal for the water and 2-dimensional (2-D) ultrasonic images were created using confocal image reconstruction. A known area from each micrograph was divided into 12-40 even segments and examined for surface irregularities. These irregularities were quantified and one-way analysis of variance (ANOVA) and linear regression analysis were performed to correlate the surface profiles for both the AlloDerm® and EVPOME specimens imaged by SAM. Histology micrographs of the AlloDerm® and EVPOME specimens were also prepared and examined for surface irregularities. Unseeded AlloDerm® averaged seven to nine surface changes per 400 μm. The number of changes in surface irregularities decreased to two to three per 400 μm on the mature EVPOMEs. The numbers of surface irregularities between the unseeded AlloDerm® vs. developing EVPOME are similar for both histology and SAM 2-D B-scan images. For the EVPOME 2-D B-scan micrographs produced by SAM, the decrease in surface irregularities is indicative of the stratified epithelium formed by seeded oral keratinocytes; verified in the histology images between the AlloDerm® and EVPOME. A near 1:1 linear correlation shows the similarities between the two imaging modalities. SAM demonstrates its ability to discern the cell development and differentiation occurring on the EVPOME devices. Unlike histology, SAM measurements are noninvasive and can be used to monitor tissue graft development without damaging any cells/tissues.