Biomedical subjects

Kathryn R Nightingale

Publications and source records attributed to Kathryn R Nightingale.

14 recordsLinked to original sources

PubMed2006

Dynamic mechanical response of elastic spherical inclusions to impulsive acoustic radiation force excitation.

Acoustic radiation force impulse imaging has been used clinically to study the dynamic response of lesions relative to their background material to focused, impulsive acoustic radiation force excitations through the generation of dynamic displacement field images. Dynamic displacement data are typically displayed as a set of parametric images, including displacement immediately after excitation, maximum displacement, time to peak displacement, and recovery time from peak displacement. To date, however, no definitive trends have been established between these parametric images and the tissues' mechanical properties. This work demonstrates that displacement magnitude, time to peak displacement, and recovery time are all inversely related to the Young's modulus in homogeneous elastic media. Experimentally, pulse repetition frequency during displacement tracking limits stiffness resolution using the time to peak displacement parameter. The excitation pulse duration also impacts the time to peak parameter, with longer pulses reducing the inertial effects present during impulsive excitations. Material density affects tissue dynamics, but is not expected to play a significant role in biological tissues. The presence of an elastic spherical inclusion in the imaged medium significantly alters the tissue dynamics in response to impulsive, focused acoustic radiation force excitations. Times to peak displacement for excitations within and outside an elastic inclusion are still indicative of local material stiffness; however, recovery times are altered due to the reflection and transmission of shear waves at the inclusion boundaries. These shear wave interactions cause stiffer inclusions to appear to be displaced longer than the more compliant background material. The magnitude of shear waves reflected at elastic lesion boundaries is dependent on the stiffness contrast between the inclusion and the background material, and the stiffness and size of the inclusion dictate when shear wave reflections within the lesion will interfere with one another. Jitter and bias associated with the ultrasonic displacement tracking also impact the estimation of a tissue's dynamic response to acoustic radiation force excitation.

Acoustics↗

PubMed2006

Ultrasonic tracking of acoustic radiation force-induced displacements in homogeneous media.

The use of ultrasonic methods to track the tissue deformation generated by acoustic radiation force is subject to jitter and displacement underestimation errors, with displacement underestimation being primarily caused by lateral and elevation shearing within the point spread function (PSF) of the ultrasonic beam. Models have been developed using finite element methods and Field II, a linear acoustic field simulation package, to study the impact of focal configuration, tracking frequency, and material properties on the accuracy of ultrasonically tracking the tissue deformation generated by acoustic radiation force excitations. These models demonstrate that lateral and elevation shearing underneath the PSF of the tracking beam leads to displacement underestimation in the focal zone. Displacement underestimation can be reduced by using tracking beams that are narrower than the spatial extent of the displacement fields. Displacement underestimation and jitter decrease with time after excitation as shear wave propagation away from the region of excitation reduces shearing in the lateral and elevation dimensions. The use of higher tracking frequencies in broadband transducers, along with 2D focusing in the elevation dimension, will reduce jitter and improve displacement tracking accuracy. Relative displacement underestimation remains constant as a function of applied force, whereas jitter increases with applied force. Underdeveloped speckle (SNR < 1.91) leads to greater levels of jitter and peak displacement underestimation. Axial shearing is minimal over the tracking kernel lengths used in acoustic radiation force impulse imaging and thus does not impact displacement tracking.

Acoustics↗

PubMed2006

Characterizing acoustic attenuation of homogeneous media using focused impulsive acoustic radiation force.

A new method to characterize a material's attenuation using acoustic radiation force is proposed. Comparison of displacement magnitudes generated in a homogeneous material by acoustic radiation force excitations can be used to estimate the material's attenuation when the excitations are applied over a range of focal depths while maintaining a constant lateral focal configuration. Acoustic attenuations are related to the inverse of the excitation focal depth that yields the greatest focal zone displacement for this protocol. Experimental studies in calibrated tissue-mimicking phantoms are presented to demonstrate the feasibility of this method. Attenuations ranging from 0.3-1.5 dB/cm/MHz were characterized over excitation focal depths ranging from 5-30 mm, with an accuracy of 0.1 +/- 0.15 dB/cm/MHz. As currently implemented, this method is limited to characterizing materials that have homogeneous material properties and acoustic attenuations. This method for characterizing acoustic attenuation can be performed using conventional diagnostic scanners without any additional hardware and could also be performed concurrently with acoustic radiation force-based imaging modalities to generate images of mechanical properties and attenuation that are spatially co-registered with B-mode images.

Acoustics↗

PubMed2006

Streaming detection for evaluation of indeterminate sonographic breast masses: a pilot study.

OBJECTIVE: Streaming detection is a novel sonography technique that uses ultrasonic energy to induce movement in cyst fluid that is detected on Doppler sonography. This pilot study evaluates the utility of streaming detection for differentiating cysts from solid masses in breast lesions that are indeterminate on sonography. SUBJECTS AND METHODS: Thirty-nine lesions-11 simple cysts and seven solid masses (control group) and 21 masses with indeterminate findings for the diagnosis of a cyst versus a solid lesion (study group)-in 34 patients were evaluated using streaming detection. All lesions underwent cyst aspiration or biopsy (n = 35) or were diagnosed simple cysts (n = 4) on sonography. Lesion size and depth were recorded. Streaming detection software was placed on conventional sonography units. Acoustic pulses were focused on the lesion, and if fluid movement was generated, it was seen on the spectral Doppler display as velocity away from the transducer. Lesions were then aspirated or underwent biopsy, and the viscosity of the aspirated fluid was recorded. The sensitivity and specificity of the technique and the effect of cyst size, cyst depth, and fluid viscosity in diagnosing fluid-filled cysts were assessed. RESULTS: Overall, 31 cysts and eight solid masses (seven benign, one carcinoma) were diagnosed in the study and control groups. Aspiration of indeterminate lesions resulted in 20 cysts and one solid mass. Lesions ranged in size from 4 to 47 mm and in depth from 4 to 29 mm. In the control group, streaming detection correctly showed nine of the 11 simple cysts (sensitivity, 82%; positive predictive value, 100%), and acoustic streaming was absent in all seven solid masses (specificity, 100%; negative predictive value, 78%). Of the indeterminate lesions, streaming detection allowed correct identification of 10 of 20 cysts (sensitivity, 50%; positive predictive value, 100%). Acoustic streaming was not detected in the one solid study group lesion. Neither cyst size or depth nor fluid viscosity had a significant effect on the ability to detect fluid. CONCLUSION: The streaming detection technique improved differentiation of cysts from solid masses in indeterminate lesions and has potential for reducing the number of recommended cyst aspirations for the diagnosis of indeterminate breast masses.

Adult↗

PubMed2005

Acoustic radiation force impulse imaging of the abdomen: demonstration of feasibility and utility.

The feasibility of utilizing acoustic radiation force impulse (ARFI) imaging to assess the mechanical properties of abdominal tissues was investigated. The thermal safety of the technique was also evaluated through the use of finite element method models. ARFI imaging was shown to be capable of imaging abdominal tissues at clinically realistic depths. Correspondence between anatomical structures in B-mode and ARFI images was observed. ARFI images showed similar tumor contrast when compared with B-mode images of ex vivo abdominal cancers. Finite element method models and in vitro measurements confirmed the thermal safety of ARFI imaging at depth. ARFI imaging is inexpensive, safe and convenient and is a promising modality for use in abdominal imaging.

Abdomen↗

PubMed2005

Acoustic radiation force impulse imaging of myocardial radiofrequency ablation: initial in vivo results.

Acoustic radiation force impulse (ARFI) imaging techniques were used to monitor radiofrequency (RF) ablation of ovine cardiac tissue in vivo. Additionally, ARFI M-mode imaging methods were used to interrogate both healthy and ablated regions of myocardial tissue. Although induced cardiac lesions were not visualized well in conventional B-mode images, ARFI images of ablation procedures allowed determination of lesion location, shape, and relative size through time. The ARFI M-mode images were capable of distinguishing differences in behavior through the cardiac cycle between healthy and damaged tissue regions. As conventional sonography is often used to guide ablation catheters, ARFI imaging, which requires no additional equipment, may be a convenient modality for monitoring lesion formation in vivo.

Animals↗

PubMed2005

A finite-element method model of soft tissue response to impulsive acoustic radiation force.

Several groups are studying acoustic radiation force and its ability to image the mechanical properties of tissue. Acoustic radiation force impulse (ARFI) imaging is one modality using standard diagnostic ultrasound scanners to generate localized, impulsive, acoustic radiation forces in tissue. The dynamic response of tissue is measured via conventional ultrasonic speckle-tracking methods and provides information about the mechanical properties of tissue. A finite-element method (FEM) model has been developed that simulates the dynamic response of tissues, with and without spherical inclusions, to an impulsive acoustic radiation force excitation from a linear array transducer. These FEM models were validated with calibrated phantoms. Shear wave speed, and therefore elasticity, dictates tissue relaxation following ARFI excitation, but Poisson's ratio and density do not significantly alter tissue relaxation rates. Increased acoustic attenuation in tissue increases the relative amount of tissue displacement in the near field compared with the focal depth, but relaxation rates are not altered. Applications of this model include improving image quality, and distilling material and structural information from tissue's dynamic response to ARFI excitation. Future work on these models includes incorporation of viscous material properties and modeling the ultrasonic tracking of displaced scatterers.

Acoustics↗

PubMed2005

Acoustic radiation force impulse (ARFI) imaging of the gastrointestinal tract.

The evaluation of lesions in the gastrointestinal (GI) tract using ultrasound can suffer from poor contrast between healthy and diseased tissue. Acoustic Radiation Force Impulse (ARFI) imaging provides information about the mechanical properties of tissue using brief, high-intensity, focused ultrasound to generate radiation force and ultrasonic correlation-based methods to track the resulting tissue displacement. Using conventional linear arrays, ARFI imaging has shown improved contrast over B-mode images when applied to solid masses in the breast and liver. The purpose of this work is to (1) investigate the potential for ARFI imaging to provide improvements over conventional B-mode imaging of GI lesions and (2) demonstrate that ARFI imaging can be performed with an endocavity probe. ARFI images of an adenocarcinoma of the gastroesophageal (GE) junction, status-post chemotherapy and radiation treatment, demonstrate better contrast between healthy and fibrotic/malignant tissue than standard B-mode images. ARFI images of healthy gastric, esophageal, and colonic tissue specimens differentiate normal anatomic tissue layers (i.e., mucosal, muscularis and adventitial layers), as confirmed by histologic evaluation. ARFI imaging of ex vivo colon and small bowel tumors portray interesting contrast and structure that are not as well defined in B-mode images. An endocavity probe created ARFI images to a depth of over 2 cm in tissue-mimicking phantoms, with maximum displacements of 4 microm. These findings support the clinical feasibility of endocavity ARFI imaging to guide diagnosis and staging of disease processes in the GI tract.

Adenocarcinoma↗

PubMed2004

Acoustic radiation force impulse imaging of thermally- and chemically-induced lesions in soft tissues: preliminary ex vivo results.

The ability of acoustic radiation force impulse (ARFI) imaging to visualize thermally- and chemically-induced lesions in soft tissues was investigated. Lesions were induced in freshly excised bovine liver samples. Chemical lesions were induced via the injection of formaldehyde and thermal lesions were created using a radiofrequency (RF) ablation system. Although conventional sonography was unable to visualize induced lesions, ARFI imaging was capable of monitoring lesion size and boundaries. Agreement was observed between lesion size in ARFI images and in results from pathology. The fact that ARFI imaging requires no additional equipment aside from that needed for conventional ultrasonic imaging makes it a promising modality for monitoring lesion development in situations where sonography is already involved as a guiding mechanism, such as in procedures requiring precise catheter placement.

Animals↗

PubMed2004

Acoustic radiation force impulse imaging of the mechanical properties of arteries: in vivo and ex vivo results.

We present results of a pilot study of ex vivo and in vivo acoustic radiation force impulse (ARFI) imaging demonstrating measurements of the mechanical properties of the carotid and popliteal arteries. The results were obtained on a modified commercial scanner, providing coregistered B-mode and color Doppler images. 2D and 1D through time images are formed from the measurements of tissues' response to very brief and localized applications of radiation force. The images show good correlation with B-mode and, in ex vivo studies, pathology-based characterizations of vessel geometry and plaque stiffness. In vivo measurements of arterial response during both systole and diastole are presented. We address implementation issues and discuss potential applications of this new vascular imaging method.

Biomechanical Phenomena↗

PubMed2004

Experimental studies of the thermal effects associated with radiation force imaging of soft tissue.

Many groups are studying acoustic radiation force-based imaging modalities to determine the mechanical properties of tissue. Acoustic Radiation Force Impulse (ARFI) imaging is one of these modalities that uses standard diagnostic ultrasound scanners to generate localized, impulsive, acoustic radiation force in tissue. This radiation force generates tissue displacements that are tracked using conventional correlation-based ultrasound methods. The dynamic response of tissue to this impulsive radiation force provides information about the mechanical properties of the tissue. The generation of micron-scale displacements using acoustic radiation force in tissue requires the use of high-intensity acoustic beams, and the soft tissue heating associated with these high-intensity beams must be evaluated to ensure safety when performing ARFI imaging in vivo. Experimental studies using thermocouples have validated Finite Element Method (FEM) models that simulate the heating of soft tissue during ARFI imaging. Spatial maps of heating measured with the thermocouples are in good agreement with FEM model predictions, with cooling time constants measured and modeled to be on the order of several seconds. Transducer heating during ARFI imaging has been measured to be less than 1 degrees C for current clinical implementations. These validated FEM models can now be used to simulate soft tissue heating associated with different transducers, beam spacing, focal configurations and thermal material properties. These experiments confirm that ARFI imaging of soft tissue is safe, although thermal response must be monitored when developing ARFI beam sequences for specific tissue types and organsystems.

Acoustics↗

PubMed2004

On the thermal effects associated with radiation force imaging of soft tissue.

Several laboratories are investigating the use of acoustic radiation force to image the mechanical properties of tissue. Acoustic Radiation Force Impulse (ARFI) imaging is one approach that uses brief, high-intensity, focused ultrasound pulses to generate radiation force in tissue. This radiation force generates tissue displacements that are tracked using conventional correlation-based ultrasound methods. The tissue response provides a mechanism to discern mechanical properties of the tissue. The acoustic energy that is absorbed by tissue generates radiation force and tissue heating. A finite element methods model of acoustic heating has been developed that models the thermal response of different tissues during short duration radiation force application. The beam sequences and focal configurations used during ARFI imaging are modeled herein; the results of these thermal models can be extended to the heating due to absorption associated with other radiation force-based imaging modalities. ARFI-induced thermal diffusivity patterns are functions of the transducer f-number, the tissue absorption, and the temporal and spatial spacing of adjacent ARFI interrogations. Cooling time constants are on the order of several seconds. Tissue displacement due to thermal expansion is negligible for ARFI imaging. Changes in sound speed due to temperature changes can be appreciable. These thermal models demonstrate that ARFI imaging of soft tissue is safe, although thermal response must be monitored when ARFI beam sequences are being developed.

Animals↗

PubMed2003

BSS-based filtering of physiological and ARFI-induced tissue and blood motion.

Blind source separation (BSS) for adaptive filtering is presented in application to imaging both physiological and acoustic radiation force impulse (ARFI)-induced tissue and blood motion in the common carotid artery. The collected raw radiofrequency (RF) data includes vessel wall motion, blood flow and ARFI-induced motion. In the context of these complex motion patterns, the same BSS adaptive filtering method was employed for three diverse applications: 1. clutter filtering ensembles prior to blood velocity estimation, 2. extracting small axial velocity components from noisy velocity measurements given large flow angles and 3. reducing noise in measured ARFI-induced tissue displacement profiles to enhance differentiation of local tissue structures. The filter separated physiological vessel wall motion from axial blood flow and ARFI-induced motion; successful filter performance is demonstrated in velocity estimates, color flow images and ARFI displacement profiles. The results demonstrate the breadth of applications for BSS adaptive filtering in the clinical imaging environment.

Blood Flow Velocity↗

PubMed2003

Estimates of echo correlation and measurement bias in acoustic radiation force impulse imaging.

Acoustic radiation force impulse (ARFI) imaging is a novel imaging modality in which pulses from a diagnostic ultrasound scanner are used to displace tissue and track its motion. The region displaced has lateral and elevational dimensions of similar scale to the ultrasound beams used to track the motion. Therefore, there is a range of tissue displacements present within the tracking beam, leading to decorrelation of the echo signal. Expressions are derived for the expected value of the displacement estimate and the cross-correlation at the expected displacement. Numerical simulations confirm the analytical model.

Computer Simulation↗