Matthew O'Donnell

Internal displacement and strain imaging using ultrasonic speckle tracking

Previous ultrasound speckle tracking methods have been extended, permitting measurement of internal displacement and strain fields over a wide dynamic range of tissue motion. The markedly increased dynamic range of this approach should lead to enhanced contrast resolution in strain and elasticity images. Results of experiments on gelatin-based, tissue equivalent phantoms show the capabilities of the method.<<ETX>>

Thermal strain imaging: a review

Thermal strain imaging (TSI) or temporal strain imaging is an ultrasound application that exploits the temperature dependence of sound speed to create thermal (temporal) strain images. This article provides an overview of the field of TSI for biomedical applications that have appeared in the literature over the past several years. Basic theory in thermal strain is introduced. Two major energy sources appropriate for clinical applications are discussed. Promising biomedical applications are presented throughout the paper, including non-invasive thermometry and tissue characterization. We present some of the limitations and complications of the method. The paper concludes with a discussion of competing technologies.

Arterial Vulnerable Plaque Characterization Using Ultrasound-Induced Thermal Strain Imaging (TSI)

Thermal strain imaging (TSI) is demonstrated in two model systems mimicking two potential clinical applications. First, a custom ultrasound (US) microscope produced high-resolution TSI images of an excised porcine coronary artery. Samples were placed in a temperature-controlled water chamber and scanned transversely and longitudinally. Phase-sensitive, correlation-based speckle tracking was applied to map the spatial distribution of temporal strain across the sample. TSI differentiated fatty tissue from water-based arterial wall and muscle with high contrast and a spatial resolution of 60 mum for a 50-MHz transducer. Both transverse and longitudinal TSI images compared well with B-scans of arterial wall structures, including intima, media, adventitia, and overlying fatty tissue. A second model system was used to test the hypothesis that US can produce the heating pattern required for TSI of internal structures. A 2-D phased array with independent drive electronics was combined with a conventional US scanner (iU22, Philips, Bothell, WA) for these studies. This 513-element array, originally designed for the US therapy, acted as the US heat source. To quantify the temporal strain induced by this system, TSI was performed on a homogeneous rubber phantom. TSI temperature estimates were within 3% error for a 3.2degC temperature rise produced within 2 s using a specially designed beamformer and pulse sequencer. The system was then used to produce TSI scanning of an excised kidney containing an intact piece of fat below the collecting system. These images were validated using an magnetic resonance imaging (MRI) pulse sequence designed for lipid quantification. TSI scans matched well MRI scans and histology both anatomically and quantitatively. Finally, to test the potential of US-induced TSI for a significant clinical problem, images were obtained on an excised canine aorta with fatty tissue inside the lumen. Both longitudinal and transversal TSI agreed well with anatomy. These in vitro results demonstrate the potential of high-resolution US-induced TSI with a small temperature change (<1degC) for plaque characterization.

Identification of vulnerable atherosclerotic plaque using IVUS-based thermal strain imaging

Pathology and autopsy studies have demonstrated that sudden disruption of vulnerable atherosclerotic plaque is responsible for most acute coronary syndromes. These plaques are characterized by a lipid-rich core with abundant inflammatory cells and a thin fibrous cap. Thermal strain imaging (TSI) using intravascular ultrasound (IVUS) has been proposed for high-risk arterial plaque detection, in which image contrast results from the temperature dependence of sound speed. It has the potential to distinguish a lipid-laden lesion from the arterial vascular wall due to its strong contrast between water-bearing and lipid-bearing tissue. Initial simulations indicate plaque identification is possible for a 1/spl deg/C temperature rise. A phantom experiment using an IVUS imaging array further supports the concept, and results agree reasonably well with prediction.

Inducing and Imaging Thermal Strain Using a Single Ultrasound Linear Array

For the first time, the feasibility of inducing and imaging thermal strain using an ultrasound imaging array is demonstrated. A commercial ultrasound scanner was used to heat and image a gelatin phantom with a cylindrical rubber inclusion. The inclusion was successfully characterized as an oil-bearing material using thermal strain imaging.

Motion artifact reduction for IVUS-based thermal strain imaging

Thermal strain imaging (TSI) using intravascular ultrasound (IVUS) has the potential to identify lipid pools within rupture-prone arterial plaques and serve as a valuable supplement to current IVUS systems in diagnosing acute coronary syndromes. The major challenge for in vivo application of TSI will be cardiac motion, including bulk motion and tissue deformation. Simulations based on an artery model, including a lipid-filled plaque, demonstrate that effective bulk motion compensation can be achieved within a certain motion range using spatial interpolation. We also propose a practical imaging scheme to minimize mechanical strains caused by tissue deformation based on a linear least squares fitting strategy. This scheme was tested on clinical data by artificially superimposing thermal displacements corresponding to different temperature rises. Results suggest a 1-2/spl deg/C temperature rise is required to detect lipids in an atherosclerotic plaque in vivo.

The feasibility of using thermal strain imaging to regulate energy delivery during intracardiac radio-frequency ablation

A method is introduced to monitor cardiac ablative therapy by examining slope changes in the thermal strain curve caused by speed of sound variations with temperature. The sound speed of water-bearing tissue such as cardiac muscle increases with temperature. However, at temperatures above about 50°C, there is no further increase in the sound speed and the temperature coefficient may become slightly negative. For ablation therapy, an irreversible injury to tissue and a complete heart block occurs in the range of 48 to 50°C for a short period in accordance with the well-known Arrhenius equation. Using these two properties, we propose a potential tool to detect the moment when tissue damage occurs by using the reduced slope in the thermal strain curve as a function of heating time. We have illustrated the feasibility of this method initially using porcine myocardium in vitro. The method was further demonstrated in vivo, using a specially equipped ablation tip and an 11-MHz microlinear intracardiac echocardiography (ICE) array mounted on the tip of a catheter. The thermal strain curves showed a plateau, strongly suggesting that the temperature reached at least 50°C.

Ultrasonic thermal imaging of microwave absorption

We propose a new ultrasonic imaging method for tissue differentiation based on thermal and dielectric properties of tissue. Electromagnetic radiation, when applied to tissue over a short period, causes a small temperature rise depending on tissue dielectric constant. Ultrasound speckle displacements related to this temperature change can be determined from local changes in the sound speed using speckle tracking. A more detailed analysis revealed that microwave absorption contrast and speed contrast (specifically, temperature derivative of the sound speed) are the primary contributing factors to final image contrast. In this paper, we present the theoretical background and initial experimental evaluation of this imaging modality, which suggests that it may supplement existing intravascular ultrasound (IVUS) imaging systems to identify vulnerable arterial plaque due to the strong contrast between water-bearing and lipid-bearing tissue.

Thermoelastic generation of continuous Lamb waves for microfluidic devices

We use the thermoelastic effect as non-contact method to generate Lamb waves for applications in microfluidics. Our system uses a regeneratively amplified ultrafast Ti:Sapphire laser producing 150 fs pulses at a 250 kHz repetition rate. These pulses are focused onto a commercially available 2 /spl mu/m thick nitrocellulose membrane coated with gold. Strongly chirped Lamb waves are generated using a cylindrical lens to produce a single line focus. We measure extremely slow phase velocities ranging from 80 to 150 m/s within a frequency range from 250 kHz to 2 MHz. Adjusting the spacing of a multiple line source produces CW Lamb waves at frequencies of 250 kHz, 500 kHz, and 750 kHz. These results show that the thermoelastic effect is a promising non-contact method to generate CW Lamb waves for microfluidic devices.

Monitoring radiofrequency catheter ablation using thermal strain imaging

A method to monitor ablative therapy by examining slope changes in the thermal strain curve caused by speed of sound with temperature is introduced. The variation of sound speed with temperature rise for most soft tissue follows a similar pattern to that of water. Unlike most liquids, the sound speed of tissue increases with temperature. However, at temperatures above about 50 °C, there is no further increase in the sound speed and the temperature coefficient may become slightly negative. For ablation therapy, an irreversible injury to tissue and a complete heart block occurs in the range of 48–50 °C for a short period in accordance with the well known Arrhenius equation. Using these two properties, we propose a potential tool to detect the moment when tissue damage occurs using the reduced slope in the thermal strain curve as a function of heating time. Using a prototype intracardiac echocardiography (ICE) array for imaging and a catheter for RF ablation, we were able to observe an obvious slope change in the thermal strain curve in an excised tissue sample. The method was further tested in-vivo, using a specially equipped ablation tip and an 11 MHz microlinear (ML) ICE array mounted on the tip of a catheter. As with in-vitro experiments, the thermal strain curve showed a plateau and a change in the sign of the slope.