F.L. Lizzi

System of therapeutic ultrasound and real-time ultrasonic scanning

A system is described for obtaining in real-time cross-sectional and 3-dimensional images of a body under study using ultrasonic energy. A piezoelectric transducer is positioned to emit ultrasonic energy and receive echo pulses. The transducer is electronically swept or physically rotated to produce a series of sectored scan planes which are separated by a known angular distance. The echo pulses are processed to produce an ultrasonic image in pseudo 3-dimensional display. By using data from one scan plane, processed as a B-scan image, cross-sectional data can be obtained. Such is combined in a display with an overlay to visually portray the object and positioning information or comparative data. The system is combined with a computer for data analysis and a therapeutic transducer for treatment.

Ultrasonic Hyperthermia for Ophthalmic Therapy

Absfrucct-A theoretical model was developed to compute the spatiotemporal features of temperature rises induced in ocular tissue during exposure to high-intensity focused ultrasound. The model incorporates damage integral evaluation to predict the Occurrence and dimensions of thermally induced lesions. Experimental data confirmed the accuracy of computed lesion dimensions for scleral and chorioretinal lesions. Computed results also are in agreement with measured threshold exposure levels needed to produce minimal chorioretinal lesions. The model is being improved to account for blood-flow cooling effects during longterm hyperthermia. Data were collected using this long-term mode on Greene’s melanoma implanted in the rabbit choroid, and these data indicate that tumor regression can be achieved at levels that are consistent with damage integral predictions.

Very-High Frequency Ultrasonic Imaging and Spectral Assays of the Eye

During the past several years, focused ultrasonic transducers operating at center frequencies near 50 MHz have become available for pulse-echo imaging.1 The large bandwidths (e.g., 30 MHz) and narrow beamwidths (e.g., 75 urn) afforded by these transducers have greatly increased the resolution attainable for examining superficial segments of the body.

High-frequency harmonic imaging of the eye

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.

Harmonic Band Spectrum Analysis of Backscattered Ultrasound from Lesioned and Normal Tissue

HIFU dose curves (lesion size vs. exposure parameters) exhibit scatter because of local variations in the acoustic properties of tissue. Therefore, clinical applications of HIFU, such as cardiac and cancer ablation, will benefit from the ability to distinguish treated from normal tissue, which can provide the surgeon with lesion monitoring. However, HIFU lesions, especially protein‐denaturing lesions (PDLs), may be difficult to visualize with conventional B‐mode ultrasound. In this study, spectrum analysis of backscattered radiofrequency (RF) ultrasound was successful in imaging lesions. HIFU lesions were formed at 5 MHz for various intensities and durations in model tissues including degassed chicken breast in vitro, fresh rabbit liver ex vivo, and canine cardiac left ventricle in vivo. The tissues were scanned pre‐ and post‐exposure using confocal array and single‐element diagnostic probes incorporated into the HIFU transducer assembly. The diagnostic probes were excited with a monocycle pulse under con...

Feasibility of In‐Vivo Cardiac HIFU Ablation

The potential for cardiac applications of HIFU remains largely unexplored. In order to create reproducible lesions in a beating heart, it is necessary to maintain focusing at a certain position within moving myocardial tissue. One technique is to use multiple short HIFU exposures (0.2 s) and to synchronize them with an EKG signal and respiration. In order to investigate the interaction of HIFU exposures and cardiac tissues, a series of in‐vitro experiments was conducted. The left ventricular free wall (LVFW) of calf hearts were cut into 4‐cm cubes, degassed in phosphate buffer saline (PBS), and heated to 37C. Several transducers were employed. Most experiments used a 33‐mm diameter spherical‐cap transducer with focal length of 35 mm, operated at a frequency of 5.075 MHz and a focused intensity of 13 kW/cm2 (in‐situ spatial average over the half‐power points of the focused beam). The transducer was coupled to the LVFW using degassed PBS. First, the effects of pericardial fat, focal depth, and temperature o...

Computer simulations of ultrasonic heating for ocular therapy

Abstract Computer simulations are being performed to model the temperature patterns produced during ultrasonically induced hyperthermia of ocular tumours. The software package for these simulations incorporates operator interaction and uses tissue geometry obtained from B‐mode data. Previous studies used geometric approximations for the incident beams used for hyperthermia. In the current study, these beams were computed using diffraction analysis to obtain more realistic simulations of clinical exposures.

A System Integrating HIFU Exposure Capabilities with Multiple Modes of Synchronous Ultrasonic Monitoring

A versatile biomedical ultrasound system has been developed and tested. The system controls and monitors high‐intensity focused ultrasound (HIFU) exposures designed to produce therapeutic tissue lesions primarily by thermal phenomena. The system is used with custom HIFU transducer arrays that contain central diagnostic transducer arrays. The diagnostic and visualization functions are performed using a subsystem that provides full digital control over high‐resolution diagnostic ultrasound arrays. Custom software controls all aspects of the imaging (e.g., electronic focusing and frame rates) with scripts and a graphical user interface. The HIFU transducer is excited using a 16‐channel power amplifier controlled by a digital subsystem and waveform synthesizers. Software operator control is also provided for desired HIFU exposure parameters (apodization, frequency, time duration, focal length, intensity) and a variety of synchronous excitation modes for the HIFU and diagnostic arrays.

Relationship of 2D ultrasonic spectral parameters to the physical properties of soft tissue scatterers

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.

Ultrasonic Theraphy and Imaging in Ophthalmology

The term “medical ultrasound”usually brings to mind the diagnostic ultrasonography systems that have become an internationally accepted modality for examining virtually all organs of the body. However, some of the earliest research with medical ultrasound was directed at studying how intense ultrasound could be used to modify tissue structures for possible therapy. For example, early work, reviewed by Kremkau (1), investigated how ultrasound might be used to treat cancer, and pioneering efforts in the laboratories of Fry, Dunn, and Lele (2,3) showed how focused ultrasound could be used to produce focal lesions for treating brain tumors and other disorders. Recently, there has been a renewed interest in therapeutic ultrasound, especially as a modality for inducing hyperthermia for cancer therapy (4).