bluuebunny/crossref_metadata_2025 huggingface dataset:
Scanning Electric Conductivity Gradients with Ultrasonically-Induced Lorentz Force
Authors: Montalibet A. , Jossinet J. , Matias A.
Published: March 2013

The ions in a fluid element oscillating under the effect of a sound wave in the presence of a magnetic field are submitted to Lorentz force. This gives rise to a bulk current density proportional to the medium's electric conductivity. In the present study, the integrality of this interaction current was collected using a pair of plane electrodes located on opposite sides of the sample. A focused transducer produced ultrasound bursts of 10 μs duration, 500 kHz frequency and 1.5 MPa peak pressure. The magnetic field was created by a purpose-built 0.35 T permanent magnet. Wiener inverse filtering was used to retrieve the system response from the recorded waveforms. The final signal was shown to be proportional to the gradient of σ/ρ along ultrasound propagation axis. Electric conductivity, σ, predominantly controls this parameter since mass density, ρ, does not vary in great proportions in biological media. Rectangular blocks of Agar gel and a layered bacon sample were used as models of biological media. The signals obtained in gel blocks had a longitudinal spatial resolution better than 1 mm. The successive layers of the bacon sample were clearly resolved. The advantages of this new modality for tissue characterization include the permeability of body tissue to magnetic field and ultrasound, the harmlessness of the applied fields and the improved spatial resolution in the measurement of a tissue's electric conductivity distribution.
Assessment of Local Pulse Wave Velocity in Arteries Using 2D Distension Waveforms
Authors: Meinders Jan M. , Kornet Lilian , Brands Peter J. , Hoeks Arnold P.G.
Published: March 2013

The reciprocal of the arterial pulse wave velocity contains crucial information about the mechanical characteristics of the arterial wall but is difficult to assess noninvasively in vivo. In this paper, a new method to assess local pulse wave velocity (PWV) is presented. To this end, multiple adjacent distension waveforms are determined simultaneously along a short arterial segment, using a single 2D-vessel wall tracking system with a high frame rate (651 Hz). Each B-mode image consists of 16 echo lines spanning a total width of 15.86 mm. Dedicated software has been developed to extract the end-diastolic diameter from the B-mode image and the distension waveforms from the underlying radiofrequency (rf) information for each echo-line. The PWV is obtained by determining the ratio of the temporal and spatial gradient of adjacent distension velocity waveforms. The proposed method is verified in a phantom and in the common carotid artery (CCA) of humans. Phantom experiments show a high concordance between the PWV obtained from 2D distension velocity waveforms (4.21 ± 0.02 m/s) and the PWV determined using two pressure catheters (4.26 ± 0.02 m/s). Assuming linear spatial gradients, the PWV can also be obtained in vivo for CCA and averages to 5.5 ± 1.5 m/s (intersubject variation, n = 23), which compares well to values found in literature. Furthermore, intrasubject PWV compares well with those calculated using the Bramwell-Hill equation. It can be concluded that the PWV can be obtained from the spatial and temporal gradient if the spatial gradient is linear over the observed length of the artery, i.e. the artery should be homogenous in diameter and distension and the influence of reflections must be small.
Tradeoffs in Elastographic Imaging
Authors: Varghese T. , Ophir J. , Konofagou E. , Kallel F. , Righetti R.
Published: March 2013

This paper presents the tradeoffs in elastographic imaging. Elastography is viewed as a new imaging modality and presented in terms of three fundamental concepts that constitute the basis for the elastographic imaging process. These are the tissue elastic deformation process, the statistical analysis of strain estimation and the image characterization. The first concept involves the use of the contrast transfer efficiency ( CTE) that describes the mapping of a distribution of local tissue elastic moduli into a distribution of local longitudinal tissue strains. The second concept defines the elastographic system and the relationship between ultrasonic and signal processing parameters. This process is described in terms of a stochastic framework (the strain filter) that provides upper and practical performance bounds and their dependence on the various system parameters. Finally, the output image, the elastogram, is characterized by its image parameters, such as signal-to-noise ratio, contrast-to-noise ratio, dynamic range and resolution. Finite-element simulations are used to generate examples of elastograms that are confirmed by the theoretical prediction tools.
Observations of Tissue Response to Acoustic Radiation Force: Opportunities for Imaging
Authors: Nightingale Kathryn , Bentley Rex , Trahey Gregg
Published: March 2013

Acoustic Radiation Force Impulse (ARFI) imaging is a method for characterizing local variations in tissue mechanical properties. In this method, a single ultrasonic transducer array is used to both apply temporally short localized radiation forces within tissue and to track the resulting displacements through time. In an ongoing study of the response of tissue to temporally short radiation force excitation, ARFI datasets have been obtained of ex vivo tissues under various focal configurations. The goal of this paper is to report observations of the response of tissue to radiation force and discuss the implications of these results in the construction of clinical imaging devices.
An Efficient Speckle Tracking Algorithm for Ultrasonic Imaging
Authors: Li Pai-Chi , Lee Wei-Ning
Published: March 2013

An efficient speckle tracking algorithm is proposed for motion estimation in ultrasonic imaging. Speckle tracking involves a matching process and a searching process. The matching process of the proposed algorithm is based on a Block Sum Pyramid algorithm that significantly reduces the computational complexity while maintaining the same accuracy as the conventional sum of absolute difference approach. The searching process, on the other hand, is based on a multilevel search strategy rather than the full-search strategy used by most conventional tracking methods. Both simulated speckle images and clinical breast images were used to test the performance of the proposed algorithm. The results show that the computation efficiency is improved by up to a factor of five over the conventional approach. The improved efficiency enables real-time or near-real-time implementation of motion estimation in ultrasonic imaging, which is particularly beneficial in areas such as blood flow estimation, elasticity imaging, speckle image registration, and strain compounding.
Elastographic Imaging Using Staggered Strain Estimates
Authors: Srinivasan S. , Ophir J. , Alam S.K.
Published: March 2013

Conventional techniques in elastography estimate strain as the gradient of the displacement estimates obtained through crosscorrelation of pre- and postcompression rf A-lines. In these techniques, the displacements are estimated over overlapping windows and the strains are estimated as the gradient of the displacement estimates over adjacent windows. The large amount of noise at high window overlaps may result in poor quality elastograms, thus restricting the applicability of conventional strain estimation techniques to low window overlaps, which, in turn, results in a small number of pixels in the image. To overcome this restriction, we propose a multistep strain estimation technique. It computes the first elastogram using nonoverlapped windows. In the next step, the data windows are shifted by a small distance (small fraction of window size) and another elastogram is produced. This is repeated until the cumulative shift equals/exceeds the window size and all the elastograms are staggered to produce the final elastogram.

Simulations and experiments were performed using this technique to demonstrate significant improvement in the elastographic signal-to-noise ratio ( SNRe) and the contrast-to-noise ratio ( CNRe) at high window overlaps over conventional strain estimation techniques, without noticeable loss of spatial resolution. This technique might be suitable for reducing the algorithmic noise in the elastograms at high window overlaps.
Ultrasonic Imaging of Myocardial Strain Using Cardiac Elastography
Authors: Varghese Tomy , Zagzebski J.A. , Rahko P. , Breburda C.S.
Published: March 2013

Clinical assessment of myocardial ischemia based on visually-assessed wall motion scoring from echocardiography is semiquantitative, operator dependent, and heavily weighted by operator experience and expertise. Cardiac motion estimation methods such as tissue Doppler imaging, used to assess myocardial muscle velocity, provides quantitative parameters such as the strain-rate and strain derived from Doppler velocity. However, tissue Doppler imaging does not differentiate between active contraction and simple rotation or translation of the heart wall, nor does it differentiate tethering (passively following) tissue from active contraction. In this paper, we present a strain imaging modality called cardiac elastography that provides two-dimensional strain information. A method for obtaining and displaying both directional and magnitude cardiac elastograms and displaying strain over the entire cross-section of the heart is described. Elastograms from a patient with coronary artery disease are compared with those from a healthy volunteer. Though observational, the differences suggest that cardiac elastography may be a useful tool for assessment of myocardial function. The method is two-dimensional, real time and avoids the disadvantage of observer-dependent judgment of myocardial contraction and relaxation estimated from conventional echocardiography.
Weakly Inhomogeneous Media Tomography
Authors: Ferrière Robert , Mensah Serge , Lefebvre Jean-Pierre
Published: March 2013

Our objective is to develop an ultrasonic scanner for breast imaging. High resolution is obtained by using wide-band spherical waves transmitted and measured in the near field zone (i.e., close to the skin) all around the organ. The tomographic approach that we adopt allows us to use low central frequency waves (3–7 MHz) that are suitable for good penetration while maintaining high resolution and contrast. The procedure is thus suitable for early detection of tumors and increases the chances of total recovery.

The novelty of the present reconstruction procedure is that it associates the signals acquired in transmission to the data measured in reflection over a large aperture. This enables us to correct the phase aberration induced by weak inhomogeneities whose sizes might be several wavelengths. Numerical tests based on Finite Difference Time Domain (FDTD) simulations demonstrate the greater fidelity of the reconstruction.
Real-Time Cylindrical Curvilinear 3-D Ultrasound Imaging
Authors: Pua E.C. , Yen J.T. , Smith S.W.
Published: March 2013

In patients who are obese or exhibit signs of pulmonary disease, standard transthoracic scanning may yield poor quality cardiac images. For these conditions, two-dimensional transesophageal echocardiography (TEE) is established as an essential diagnostic tool. Current techniques in transesophageal scanning, though, are limited by incomplete visualization of cardiac structures in close proximity to the transducer. Thus, we propose a 2D curvilinear array for 3D transesophageal echocardiography in order to widen the field of view and increase visualization close to the transducer face. In this project, a 440 channel 5 MHz two-dimensional array with a 12.6 mm aperture diameter on a flexible interconnect circuit has been molded to a 4 mm radius of curvature. A 75% element yield was achieved during fabrication and an average −6dB bandwidth of 30% was observed in pulse-echo tests. Using this transducer in conjunction with modifications to the beam former delay software and scan converter display software of the our 3D scanner, we obtained cylindrical real-time curvilinear volumetric scans of tissue phantoms, including a field of view of greater than 120° in the curved, azimuth direction and 65° phased array sector scans in the elevation direction. These images were achieved using a stepped subaperture across the cylindrical curvilinear direction of the transducer face and phased array sector scanning in the noncurved plane. In addition, real-time volume rendered images of a tissue mimicking phantom with holes ranging from 1 cm to less than 4 mm have been obtained. 3D color flow Doppler results have also been acquired. This configuration can theoretically achieve volumes displaying 180° by 120.° The transducer is also capable of obtaining images through a curvilinear stepped subaperture in azimuth in conjunction with a rectilinear stepped subaperture in elevation, further increasing the field of view close to the transducer face. Future work includes development of an array for adapting these modifications to a 6 mm diameter en do scope probe.
Blood Flow Estimation Error with Intravascular Ultrasound Due to In-Plane Component of Flow
Authors: de Ana F.J. , O'Donnell M.
Published: March 2013

Previously, we presented a real-time method to measure blood flow perpendicular to the image plane of an intravascular ultrasound (IVUS) imaging system using a slow-time FIR (finite impulse response) filter bank. Any in-plane flow introduces error in the flow measurement using the filter bank algorithm. Simulations show that for a flow angle of ±10° and velocities between 200 mm/s and 300 mm/s, the energy within the lowest frequency band filter is 6.92 to 7.80 times higher than for perpendicular flow in the worst case. We present a variation of the FIR filter bank algorithm, applying filter coefficients in a tilted fashion to slow-time signals (i.e., combining slow-time and fast-time). An appropriate tilt, which depends on the flow angle and velocity, corrects for the increased energy under the frequency bands. In this case, the energy under the lowest frequency band filter for an angle of ±10° and velocities ranging from 200 mm/s to 300 mm/s is 2.09 to 2.94 times higher than for perpendicular flow, yielding greater than a factor of three improvement in the worst case over the original slow-time method. Moreover, the average energy over the vessel determined with the appropriate tilt is within 2–3% of the true value.
Adaptive Scan Sequence for Real-Time Ultrasound 3-D Imaging
Authors: Hashimoto Hiroshi , Takeuchi Yasuhito
Published: March 2013

An adaptive scan sequence is proposed for real-time ultrasound three-dimensional (3-D) imaging. Generally, the refresh rate of 3-D imaging is limited by the view window, resolution and sound speed in the human body. If the object is transformed or moved during the data set acquisition, the 3-D image will be distorted. To realize a high refresh rate for 3-D imaging, a reduction of the number of transmissions is required. Using a small number of transmissions results in a small view window or low resolution. In this paper, we propose a method to adaptively modify the spatial and temporal beam density according to the local motion of the object. We present 3-D images of a phantom with high frame rate, high resolution and wide view-angle window, significantly reducing the size of the full real scan raw data set.
Detection of Lesions Using Differential Rates of Speckle Decorrelation
Authors: Ha Jun S. , Hossack John A.
Published: March 2013

Breast lesions that may be cancerous generally possess a higher elastic modulus than the surrounding tissue. Thus, they are sensed to move differently from the surrounding tissue during finger-based palpation. Since the palpation is subjective in nature, a method using ultrasound for detection of the differential motion associated with hard lesions is proposed. It is intended that regions of detected ‘lump’ be highlighted on top of a conventional B-mode image so that the user can positively associate palpated lumps with specific regions on the anatomical B-mode image. We detect the differential motion associated with a lesion by detecting a local abnormality in a map of image-to-image correlations as the transducer is swept in an elevational direction. Experiments were performed with three different tissue mimicking phantoms. The results indicate that the location of the mobile lesion can be successfully estimated whereas immobile lesions are not very detectable. This approach is simple and requires only a short period of time for each scan. It has practically no theoretical incremental cost.
Linear Arrays for Vibro-Acoustography: A Numerical Simulation Study
Authors: Silva Glauber T. , Greenleaf James F. , Fatemi Mostafa
Published: March 2013

In this work, we study linear array beamforming for vibro-acoustography systems. The goal of this research is to assess the feasibility of vibro-acoustography imaging systems based on linear arrays for clinical applications. The vibro-acoustography transducer is designed with two arrays driven by two continuous-wave or tone-burst signals at slightly different frequencies. The system beamforming is modeled in a homogeneous and lossless fluid. In this model, the point-spread function (psf) of the system is obtained as the product of the spatial impulse function of each linear array. Three linear array configurations are analyzed for vibro-acoustography systems. Simulations considering 64 elements in each array are performed. Aspects related to clinical applications of vibro-acoustography, such as system spatial resolution, sidelobes, grating lobes and image frame-rate are discussed. It is concluded that linear array transducers can produce acceptable spatial resolution for clinical applications.
Elastographic Imaging of Thermal Lesions in Liver In-Vivo Using Diaphragmatic Stimuli
Authors: Varghese Tomy , Shi Hairong
Published: March 2013

Radiofrequency or microwave ablations are interstitial focal ablative therapies that can be used in a percutaneous fashion for treating tumors in the liver, kidney, and prostate. These modalities provide in situ destruction of tumors. We present a method for in-vivo elastographic visualization of the ablated regions in the liver during and after thermal therapy. In-vivo elastographic imaging uses compressions of the liver due to movement of the diaphragm during the respiratory cycle. Elastography of the liver and other abdominal organs has not been attempted previously due to the difficulty in providing controlled compressions. Gating of the data acquisition to the respiratory waveform would provide access to data where the compression increments are similar in both magnitude and direction, thereby enabling reproducible imaging of the thermal lesion or tumor. Comparison of elastograms with gross-pathology of ablated tissue illustrates the correspondence between elastographic image features and pathology. Ultrasound is routinely used to guide the rf ablation procedure, so the same imaging system could be used for elastographic imaging. Since the technique utilizes physiological motion of the diaphragm due to respiration, it may also be employed in the visualization of cancerous tumors in the liver.
A Digital Ultrasonic System for Small Animal Imaging
Authors: Li Meng-Lin , Chen Yen-Fu , Guan Wei-Jung , Li Pai-Chi
Published: March 2013

In this paper, a 50 MHz digital ultrasonic imaging system designed for small animals and its applications to mouse embryos and tumors are presented. Utilizing the programmability of a fully digital system, several advanced imaging techniques are developed and implemented. A synthetic aperture focusing technique is implemented to increase the penetration and extend the depth of focus of a mechanical scan system using a high frequency single crystal transducer. An adaptive weighting technique is introduced to further reduce the sidelobes and enhance image contrast. Experimental results demonstrate efficacy of the proposed two-step adaptive focusing technique. For flow imaging, several flow estimation algorithms are evaluated. Simulations and flow phantom experiments show that the butterfly search technique offers superior flow imaging performance particularly when the SNR is low and the pulse bandwidth is high. The system is tested by imaging mouse embryos and microcirculation of a mouse tumor in vivo. It is demonstrated that the system is capable of simultaneously acquiring B-mode, color Doppler mode and power Doppler mode small animal images. The system set-up, adaptive focusing technique, flow estimation algorithms and imaging results are shown.
Adaptive Spectral Strain Estimators for Elastography
Authors: Alam S. Kaisar , Lizzi Frederic L. , Varghese Tomy , Feleppa Ernest J. , Ramachandran Sarayu
Published: March 2013

In conventional elastography, internal tissue deformations, induced by external compression applied to the tissue surface, are estimated by cross-correlation analysis of echo signals obtained before and after compression. Conventionally, strains are estimated by computing the gradient of estimated displacement. However, gradient-based algorithms are highly susceptible to noise and decorrelation, which could limit their utility. We previously developed strain estimators based on a frequency-domain (spectral) formulation that were shown to be more robust but less precise compared to conventional strain estimators. In this paper, we introduce a novel spectral strain estimator that estimates local strain by maximizing the correlation between the spectra of pre- and postcompression echo signals using iterative frequency-scaling of the latter; we also discuss a variation of this algorithm that may be computationally more efficient but less precise. The adaptive spectral strain estimator combines the advantages of time- and frequency-domain methods and has outperformed conventional estimators in experiments and 2-D finite-element simulations.
Spectral Normalization for Ultrasonic Contrast Microbubble Detection
Authors: Chen Shigao , McMahon Eileen M. , Fatemi Mostafa , Greenleaf James F. , Belohlavek Marek
Published: March 2013

Ultrasonic contrast agents consisting of microbubbles are used to assess tissue perfusion. The microbubbles are highly reflective and nonlinear and thus produce harmonics that are stronger than those from tissues. However, the magnitude of harmonic signals resulting from a region with microbubbles also depends on the acoustic pressure of incident ultrasound and the attenuation of intervening tissues in the ultrasound path. Therefore, the harmonic magnitude, as used in traditional harmonic imaging, may not be a reliable indicator of the presence or absence of microbubbles, and hence, tissue perfusion. To compensate for these effects, we present two parameters defined as the ratio of the harmonic to the fundamental component ( HFR) and the ratio of the harmonic to squared fundamental ( HSFR). A simplified model is used to illustrate the usefulness of these two parameters. Experiments show that both parameters improve detection of microbubbles and that HSFR performs better than HFR.
Real-Time 3D Color Flow Doppler for Guidance of Vibrating Interventional Devices
Authors: Fronheiser Matthew P. , Wolf Patrick D. , Idriss Salim F. , Nelson Rendon C. , Lee Warren , Smith Stephen W.
Published: March 2013

The goal of this investigation was to examine the feasibility of guiding interventional devices using piezoelectric buzzers to create velocity sources, which were imaged and tracked with real-time 3D color flow Doppler. The interventional devices examined in this study included a pacemaker lead, Brockenbrough needle for cardiac septal puncture, cardiac guidewire and radiofrequency ablation needles for cancer therapy. Each was mechanically coupled to a piezoelectric buzzer and was imaged using a commercial real-time 3D ultrasound system with either a 2.5 MHz matrix array transducer or a 5 MHz, 22 F catheter transducer equipped with a tool port. In vitro images acquired in tissue phantoms, excised liver with a ‘tumor’ target and an excised sheep heart show strong vibration signals in 3D color flow Doppler, enabling real-time tracking and guidance of all the devices in three dimensions. In a sheep model, in vivo tracking of the pacing lead was performed in the superior vena cava as well as the right atrium using RT3D color flow Doppler images. The vibrating rf ablation needles were guided through the liver toward “tumor” targets in vivo with real-time 3D color flow Doppler images.
Real-Time Calibration of Temperature Estimates during Radiofrequency Ablation
Authors: Varghese T. , Daniels M.J.
Published: March 2013

Radiofrequency ablation is an interstitial focal ablative therapy that can be used in a percutaneous fashion and permits in situ destruction of hepatic tumors. Recurrence rates after rf therapy are as high as 34–55%, due to difficulties in accurately identifying the zone of necrosis (thermal lesion) because of the low intrinsic acoustic contrast between normal and ablated liver tissue. Our goal is to provide real-time ultrasonic tracking of temperature changes over the large range of temperatures traditionally used (40–100°C) in rf ablation procedures using an external ultrasound transducer. Temperature estimates are obtained using a cross-correlation algorithm applied to rf ultrasound echo signal data acquired at discrete intervals during heating. Apparent tissue displacement estimates obtained at these discrete time-intervals are accumulated to obtain a cumulative displacement map, whose gradient provides after appropriate scaling provides a temperature map at the specified elapsed ablation duration. Temperature maps are used to display the initial temperature rise and to continuously update a thermal map of the treated region. In this paper, we develop calibration curves that relate the echo shift due to the change in the speed of sound and thermal expansion to the corresponding temperature increase on in-vitro tissue specimens. These calibration curves can then be utilized for the real time calibration and analysis of temperature estimates obtained from the rf echo signals during ablation. Temperature maps obtained using the calibration curve compare favorably to temperature estimates observed using the invasive thermosensor readings on the ablation electrode and previous results that utilized a linear calibration factor.
Clinical Evaluation of Combined Spatial Compounding and Adaptive Imaging in Breast Tissue
Authors: Dahl Jeremy J. , Soo Mary S. , Trahey Gregg E.
Published: March 2013

When spatial compounding is applied to targets with significant acoustic velocity inhomogeneities, the correlation between speckle patterns of the images to be averaged decreases, thereby increasing the speckle reduction nominally obtained. Phase correction applied to these targets improves the coherence of the wavefield and restores image spatial frequencies. Combining these two modes can be used to effectively increase the contrast-to-noise ratio (CNR) of imaging targets and improve the general image quality of these targets over spatial compounding alone. This paper presents a clinical evaluation of combined spatial compounding and adaptive imaging in breast tissue and compares this combined technique to conventional imaging and to adaptive imaging and spatial compounding operating independently. Experiments were performed on a 1.75-D, 8 × 96 array attached to a commercially-available scanner. Cysts, microcalcifications and other breast structures were targeted in order to assess the impact of the combined mode on CNR, target width, target brightness and target peak-to-background ratio (PBR). In general, phase correction improved cyst CNR by 7.7%, decreased target width by 18.7%, increased target brightness by 30.1% and increased PBR by 17.9%. Compounding alone, using three overlapping 9.71 mm subapertures, increased cyst CNR by 24.6%, but increased target width by 25.4% and decreased PBR by 13.2%. Combining both modes, however, increased cyst CNR by 32.6%, inappreciably increased target width by 1.1% and marginally decreased PBR by 2.8%. The increase in target brightness with this combined mode was 20.0%
An Autoregressive Model-Based Method for Contrast Agent Detection in Ultrasound Radiofrequency Images
Authors: Dydenko Igor , Durning Bruno , Jamal Fadi , Cachard Christian , Friboulet Denis
Published: March 2013

This paper presents a spectral autoregressive method dedicated to the detection of ultrasound contrast agents (USCA) from radiofrequency (rf) data. The method is based on second-order autoregressive (AR) modeling of the rf signal. Contrast agents induce a second harmonic, which may be efficiently detected through the AR spectrum using the magnitude of the second AR spectral peak (SM2). In contrast to multipulse methods that process two or more rf frames, our method processes a single rf frame. The method is tested by numerical simulation and on in vitro data for contrast agent concentrations ranging from 103 to 50 × 103 bubbles/ml (2 × 10-6 to 10-4 volumic concentration) and mechanical index (MI) ranging from 0.1 to 0.36. The results show that the proposed parameter SM2 enables one to detect correctly the contrast agent, in particular at low concentration and MI (the minimum difference in SM2 between tissue and USCA is 10 dB). Furthermore, the in-vitro data demonstrates that an adapted smoothing technique reduces the variability of SM2 and provides accurate and stable segmentation of the contrast agent perfusion region.
Ultrasonic Noninvasive Temperature Estimation Using Echoshift Gradient Maps: Simulation Results
Authors: Techavipoo Udomchai , Chen Quan , Varghese Tomy
Published: March 2013

Percutaneous ultrasound-image-guided radiofrequency (rf) ablation is an effective treatment for patients with hepatic malignancies that are excluded from surgical resection due to other complications. However, ablated regions are not clearly differentiated from normal untreated regions using conventional ultrasound imaging due to similar echogenic tissue properties. In this paper, we investigate the statistics that govern the relationship between temperature elevation and the corresponding temperature map obtained from the gradient of the echoshifts obtained using consecutive ultrasound radiofrequency signals. A relationship derived using experimental data on the sound speed and tissue expansion variations measured on canine liver tissue samples at different elevated temperatures is utilized to generate ultrasound radiofrequency simulated data. The simulated data set is then utilized to statistically estimate the accuracy and precision of the temperature distributions obtained. The results show that temperature increases between 37 and 67°C can be estimated with standard deviations of ± 3 °C. Our results also indicate that the correlation coefficient between consecutive radiofrequency signals should be greater than 0.85 to obtain accurate temperature estimates.
Soft tissue Absorption Tomography with Correction for Scattering Aberrations
Authors: Franceschini Emilie , Pauzin Marie-Christine , Mensah Serge , Groby Jean-Philippe
Published: March 2013

Among the many factors involved in ultrasound attenuation phenomena, scattering effects play a major role, even in the unexpected case of soft tissues. It is proposed in this study to quantitatively evaluate the scattering affecting the measurements before reconstructing the absorption parameter alone.

The reconstruction procedure involves three steps: (1) Estimating the sound speed map using a transmission tomography algorithm. This estimation procedure provides a numerical phantom of the organ probed, cleared of all dissipative components. This absorption free phantom mimics the (viscoacoustic) tissues imaged except for the density and absorption characteristics: the density a priori equals 1000 kg/m3 and the absorption is not taken into account. The impedance fluctuations in the object are therefore approximated on the basis of the sound speed contrast. (2) Synthesing the field scattered by the absorption free phantom; the attenuation observed here results solely from the scattering phenomenon. The synthesis is carried out using a finite-element time domain code simulating the ultrasonic propagation through the phantom. It provides the scattering distortion reference introduced into the log spectral absorption estimator. (3) Reducing the scattering distortions affecting the integrated absorption measured along the ray paths using a log spectral procedure.

The corrected integrated absorption is then processed using a tomographic reconstruction procedure that provides an estimate of the absorption distribution. Simple numerical simulations show the improvement obtained in the absorption estimates with this approach.
Investigation of Optimal Method for Inducing Harmonic Motion in Tissue Using a Linear Ultrasound Phased Array — A Simulation Study
Authors: Heikkilä Janne , Hynynen Kullervo
Published: March 2013

Many noninvasive ultrasound techniques have been developed to explore mechanical properties of soft tissues. One of these methods, Localized Harmonic Motion Imaging (LHMI), has been proposed to be used for ultrasound surgery monitoring. In LHMI, dynamic ultrasound radiation-force stimulation induces displacements in a target that can be measured using pulse-echo imaging and used to estimate the elastic properties of the target. In this initial, simulation study, the use of a one-dimensional phased array is explored for the induction of the tissue motion. The study compares three different dual-frequency and amplitude-modulated single-frequency methods for the inducing tissue motion. Simulations were computed in a homogeneous soft-tissue volume. The Rayleigh integral was used in the simulations of the ultrasound fields and the tissue displacements were computed using a finite-element method (FEM). The simulations showed that amplitude-modulated sonication using a single frequency produced the largest vibration amplitude of the target tissue. These simulations demonstrate that the properties of the tissue motion are highly dependent on the sonication method and that it is important to consider the full three-dimensional distribution of the ultrasound field for controlling the induction of tissue motion.
Autocorrelation-Based Technique for Automated Segmentation of Endocardial Border in 3D Echocardiograms
Authors: Morda Louis S. , Lim Wei Keat , Konofagou Elisa E.
Published: March 2013

Left-ventricular (LV) segmentation is essential in the early detection of heart disease, where left-ventricular wall motion is being tracked in order to detect ischemia. In this paper, a new method for automated segmentation of the left-ventricular chamber is described. An autocorrelation-based technique isolates the LV cavity from the myocardial wall on 2-D slices of 3D short-axis echocardiograms. A morphological closing function and median filtering are used to generate a uniform border. The proposed segmentation technique is designed to be used in identifying the endocardial border and estimating the motion of the endocardial wall over a cardiac cycle. To this purpose, the proposed technique is particularly successful in border delineation by tracing around structures like papillary muscles and the mitral valve, which constitute the typical obstacle in LV segmentation techniques. The results using this new technique are compared to the manual detection results in short-axis views obtained at the papillary muscle level from 3D datasets in human and canine experiments in vivo. Qualitatively, the automatically-detected borders are highly comparable to the manually-detected borders enclosing regions in the left-ventricular cavity with a relative error within the range of 4.2%-6%. Thus, the new technique constitutes a robust segmentation method for automated segmentation of endocardial borders and is suitable for wall motion tracking for automated detection of ischemia in vivo.
75 MHz Ultrasound Biomicroscopy of Anterior Segment of Eye
Authors: Silverman Ronald H. , Cannata Jonathan , Shung K. Kirk , Gal Omer , Patel Monica , Lloyd Harriet O. , Feleppa Ernest J. , Coleman D. Jackson
Published: March 2013

Very high frequency ultrasound (35–50 MHz) has had a significant impact upon clinical imaging of the anterior segment of the eye, offering an axial resolution as small as 30 μm. Higher frequencies, while potentially offering even finer resolution, are more affected by absorption in ocular tissues and even in the fluid coupling medium. Our aim was to develop and apply improved transducer technology utilizing frequencies beyond those routinely used for ultrasound biomicroscopy of the eye.

A 75-MHz lithium niobate transducer with 2 mm aperture and 6 mm focal length was fabricated. We scanned the ciliary body and cornea of a human eye six years post-LASIK. Spectral parameter images were produced from the midband fit to local calibrated power spectra. Images were compared with those produced using a 35 MHz lithium niobate transducer of similar fractional bandwidth and focal ratio. The 75-MHz transducer was found to have a fractional bandwidth (-6 dB) of 61%. Images of the post-LASIK cornea showed higher stromal backscatter at 75 MHz than at 35 MHz. The improved lateral resolution resulted in better visualization of discontinuities in Bowman's layer, indicative of microfolds or breaks occurring at the time of surgery. The LASIK surface was evident as a discontinuity in stromal backscatter between the stromal component of the flap and the residual stroma. The iris and ciliary body were visualized despite attenuation by the overlying sclera.

Very high frequency ultrasound imaging of the anterior segment of the eye has been restricted to the 35–50 MHz band for over a decade. We showed that higher frequencies can be used in vivo to image the cornea and anterior segment. This improvement in resolution and high sensitivity to backscatter from the corneal stroma will provide benefits in clinical diagnostic imaging of the anterior segment.
Shear-Modulus Estimation by Application of Spatially-Modulated Impulsive Acoustic Radiation Force
Authors: McAleavey Stephen A. , Menon Manoj , Orszulak Jarrod
Published: March 2013

We present a method for determining the shear modulus of an elastic material wherein a spatially-modulated acoustic radiation force is used to generate a disturbance of known spatial frequency (wavelength). The propagation of this initial displacement as a shear wave is measured using ultrasound tracking methods and the temporal frequency of the shear wave is estimated. Given the known wavelength and material density and the measured estimate of temporal frequency, the shear modulus at the point of excitation may be calculated easily. Using this method, the shear moduli of two gelatin phantoms was estimated to be 1.4 and 5.8 kPa, in good agreement with 1.5 and 5.6 kPa values determined though quasistatic material testing.
Effects of Phase Aberration and Noise on Extended High Frame Rate Imaging
Authors: Wang Jing , Lu Jian-Yu
Published: March 2013

Based on the high frame rate (HFR) imaging theories, an extended HFR imaging method has been developed recently in our lab where multiple limited-diffraction array beams or steered plane waves are used in transmissions to reconstruct a high quality image of an equivalent dynamic focusing in both transmissions and receptions. The method has the potential to simplify imaging systems because the fast Fourier transform and square-wave aperture weightings can be used. The method is also flexible in using different numbers of transmissions for a continuous trade-off between image quality and frame rate. In this paper, we study the effects of phase aberration and noise on the extended HFR imaging method with in vitro experiments and compare the results with those obtained with a conventional delay-and-sum (D&S) method of a fixed-transmission focus and a dynamically-focused reception. In the experiments, an ATS539 tissue-mimicking phantom and an Acuson V2 phase array transducer (128 elements, 2.5 MHz, and 0.15-mm pitch) were used. The transducer was driven by a homemade general-purpose HFR imaging system that was capable of producing both the limited-diffraction array beams and steered plane waves and echo data were acquired with the same system and then transferred to a personal computer via a universal serial bus (USB) 2.0 link for image reconstructions. The phase aberration was introduced by adding random phase shifts to both transmission and reception beams. The random noise was added to the received radiofrequency echo data. Results show that the phase aberration and noise degrade both the extended HFR and the conventional delay-and-sum (D&S) imaging method. However, images reconstructed with the extended HFR imaging method have an overall higher quality than those with the D&S method given the phase aberration and noise models studied.
Ultrasound Probe with Integrated ECG Lead: Feasibility Study
Authors: Ramireddy Archana , Light Edward D. , Smith Stephen W.
Published: March 2013

We integrated electrocardiogram (ECG) leads onto the face of a cardiac ultrasound transducer and the exterior chassis of a simulated small, portable scanner to minimize the number of devices needed to collect cardiac information. Since the ECG leads were not placed on their standard locations, a precordial ECG was recorded.
Direction of Arrival Filters for Improved Aberration Estimation
Authors: Dahl Jeremy J. , Feehan Thomas J.
Published: March 2013

Successful adaptive imaging requires accurate measurements of the aberration profile across the array surface. Two-dimensional spatial filters are used to obtain more accurate estimates of aberrating layers by suppressing wavefronts emanating from off-axis scatterers. Application of these filters to the rf signals of the individual elements rejects wavefronts arriving from angles other than the look direction of the array and results in an increase in element-to-element correlation. Spatial filtering reduced the amount of error in the measured aberration profiles and adaptive spatial filtering further improved the estimates. The improvements in aberration estimation obtained with these methods are verified using simulations and experiments in tissue-mimicking phantoms. The technique is applied to signals obtained from in vivo human thyroid.
A Low-Cost B-Mode USB Ultrasound Probe
Authors: Richard William D. , Zar David M. , Solek Roman
Published: March 2013

The Universal Serial Bus (USB) is now the ubiquitous interface bus of choice for connecting peripherals to personal computers and laptops. USB 2.0 is a half-duplex bus running at 480 Mb/s and each peripheral can draw as much as 500 mA of current at a nominal 5 V from the USB connector. We have developed a family of USB-based, B-mode probes that connect directly to a personal computer or laptop and that draw as little as 250 mA (1.25 W) when forming ten 5 MHz images/second. The pulser/receiver, high voltage supply, analog-to-digital converter, servo and USB interface are implemented on a small circuit board inside the probe body. After raw data are transferred to the computer, gain compensation, interpolation, filtering and other data processing are performed by the host processor. This gives flexibility to developers and allows enhancements to the system to be incorporated via software updates. In addition, the raw data are available for storage and later postprocessing. There are several advantages to this architectural approach to B-mode imaging, including low cost, portability and optimal signal-to-noise performance. This paper describes the advantages of the architecture of the probe family, discusses the hardware/software division of the required processing steps and presents example images from a 12.5 MHz ophthalmic probe.
Performance Evaluation of Ultrasonic Nakagami Image in Tissue Characterization
Authors: Tsui Po-Hsiang , Yeh Chih-Kuang , Chang Chien-Cheng , Chen Wen-Shiang
Published: March 2013

Conventional ultrasonic B-mode images qualitatively describe tissue structures but are unsuitable for quantitative analyses of scatterer properties. We have recently developed an ultrasonic parametric imaging technique based on the Nakagami statistical distribution that is able to quantify scatterer concentrations. The aim of the present study is to further explore both the behavior of a Nakagami image in characterizing different scatterer structures at different signal-to-noise ratios (SNRs) and the feasibility of Nakagami imaging using a general commercial ultrasound scanner for tissue examinations. Simulations, experiments on a tissue-mimicking phantom and in vitro measurements on a muscle tissue before and after microwave treatment were carried out. The SNR and contrast-to-noise ratio (CNR) were estimated to quantify image performance. The results demonstrate that a Nakagami image can differentiate different scatterer concentrations for single, hypoechoic and hyperechoic targets. Also, a Nakagami image, when combined with an ultrasound scanner, can complement the B-scan to characterize tissue and to identify the region of interest with a larger CNR. However, the noise effect can degrade the performance of a Nakagami image. When the signal SNR decreased to 15 dB in simulations and to 8 dB in experiments, the CNR of the hyperechoic Nakagami image decreased by 4% and 27%, respectively, and that of the hypoechoic one decreased by 42% and 80%, respectively. These results indicate that a Nakagami image behaves well in identifying regions with high scatterer concentrations but does not perform well when both the scatterer concentration and SNR are low.
An Integrated Indenter-ARFI Imaging System for Tissue Stiffness Quantification
Authors: Zhai Liang , Palmeri Mark L. , Bouchard Richard R. , Nightingale Roger W. , Nightingale Kathryn R.
Published: March 2013

The goal of this work is to develop and characterize an integrated indenter-ARFI (acoustic radiation force impulse) imaging system. This system is capable of acquiring matched datasets of ARFI images and stiffness profiles from ex vivo tissue samples, which will facilitate correlation of ARFI images of tissue samples with independently-characterized material properties. For large and homogeneous samples, the indenter can be used to measure the Young's moduli by using Boussinesq's solution for a load on the surface of a semi-infinite isotropic elastic medium. Experiments and finite element method (FEM) models were designed to determine the maximum indentation depth and minimum sample size for accurate modulus reconstruction using this solution. Applying these findings, indentation measurements were performed on three calibrated commercial tissue-mimicking phantoms and the results were in good agreement with the calibrated stiffness. For heterogeneous tissue samples, indentation can be used independently to characterize relative stiffness variation across the sample surface, which can then be used to validate the stiffness variation in registered ARFI images. Tests were performed on heterogeneous phantoms and freshly-excised colon cancer specimens to detect the relative stiffness and lesion sizes using the combined system. Normalized displacement curves across the lesion surface were calculated and compared. Good agreement of the lesion profiles was observed between indentation and ARFI imaging.