Efficient synthetic aperture imaging from a circular aperture with possible application to catheter-based imaging

Ultrasparse Ultrasonic Synthetic Aperture Focus Imaging by Passive Sensing

Ultrasonic synthetic aperture focus techniques (SAFTs) using less than the total number of available array elements to transmit ("sparse" transmissions) have been recently used in both medical imaging and industrial nondestructive testing (NDT) imaging to increase test speed and simplify multiplexer hardware. The challenge of sparse arrays is to obtain a reasonable image quality given the reduced transmitter-receiver combinations available to the beamforming process. This article proposes a "ultrasparse" SAFT method that employs a minimum number of transmitter elements (from one to four elements only) to obtain an entire full-matrix capture (FMC) set of waveforms. Specifically, a "virtual" FMC is obtained from normalized cross-power spectra between each array element pair in an implementation of "passive" ultrasonic sensing. In order to maintain high image quality without sacrificing imaging speed (e.g., applying a minimal initial time delay and keeping a short time recording window), several key steps have to be taken in this "passive" imaging mode, specifically: 1) the use of carefully designed segment-averaged normalized cross-power spectrum (NCPS) for robust passive reconstruction of the ultrasonic impulse response function (IRF) between two receivers; 2) the use of both the causal and acausal portions of the passively reconstructed IRFs; and 3) the compounding of multiple wave modes in the beamforming process. These steps also ensure the elimination of the near-field blind zone hence potentially enabling near-field imaging. The article first reviews the theory of passive IRF reconstruction between two receivers, comparing time-averaged cross correlation versus segment-averaged NCPS, and then demonstrates the application to ultrasparse SAFT FMC imaging of drilled holes in an aluminum block using a linear transducer array where only one to four elements are used in transmission.

Precise Estimation of Intravascular Pressure Gradients

This study presents a method for noninvasive pressure gradient estimation, which allows the detection of small pressure differences with higher precision compared to invasive catheters. It combines a new method for estimating the temporal acceleration of the flowing blood with the Navier-Stokes equation. The acceleration estimation is based on a double cross-correlation approach, which is hypothesized to minimize the influence of noise. Data are acquired using a 256-element, 6.5-MHz GE L3-12-D linear array transducer connected to a Verasonics research scanner. A synthetic aperture (SA) interleaved sequence with 2 ×12 virtual sources evenly distributed over the aperture and permuted in emission order is used in combination with recursive imaging. This enables a temporal resolution between correlation frames equal to the pulse repetition time at a frame rate of half the pulse repetition frequency. The accuracy of the method is evaluated against a computational fluid dynamic simulation. Here, the estimated total pressure difference complies with the CFD reference pressure difference, which yields an R -square of 0.985 and an RMSE of 3.03 Pa. The precision of the method is tested on experimental data, measured on a carotid phantom of the common carotid artery. The volume profile used during measurement was set to mimic flow in the carotid artery with a peak flow rate of 12.9 mL/s. The experimental setup showed that the measured pressure difference changes from -59.4 to 31 Pa throughout a single pulse cycle. This was estimated with a precision of 5.44% (3.22 Pa) across ten pulse cycles. The method was also compared to invasive catheter measurements in a phantom with a 60% cross-sectional area reduction. The ultrasound method detected a maximum pressure difference of 72.3 Pa with a precision of 3.3% (2.22 Pa). The catheters measured a maximum pressure difference of 105 Pa with a precision of 11.2% (11.4 Pa). This was measured over the same constriction and with a peak flow rate of 12.9 mL/s. The double cross-correlation approach revealed no improvement compared to a normal differential operator. The method's strength, thus, lies primarily in the ultrasound sequence, which allows precise and accurate velocity estimations, at which acceleration and pressure differences can be acquired.

Noninvasive estimation of local speed of sound by pulse-echo ultrasound in a rat model of nonalcoholic fatty liver

Objective. Speed of sound has previously been demonstrated to correlate with fat concentration in the liver. However, estimating speed of sound in the liver noninvasively can be biased by the speed of sound of the tissue layers overlying the liver. Here, we demonstrate a noninvasive local speed of sound estimator, which is based on a layered media assumption, that can accurately capture the speed of sound in the liver. We validate the estimator using an obese Zucker rat model of non-alcoholic fatty liver disease and correlate the local speed of sound with liver steatosis.Approach.We estimated the local and global average speed of sound noninvasively in 4 lean Zucker rats fed a normal diet and 16 obese Zucker rats fed a high fat diet for up to 8 weeks. The ground truth speed of sound and fat concentration were measured from the excised liver using established techniques.Main Results. The noninvasive, local speed of sound estimates of the livers were similar in value to their corresponding 'ground truth' measurements, having a slope ± standard error of the regression of 0.82 ± 0.15 (R²= 0.74 andp< 0.001). Measurement of the noninvasive global average speed of sound did not reliably capture the 'ground truth' speed of sound in the liver, having a slope of 0.35 ± 0.07 (R²= 0.74 andp< 0.001). Decreasing local speed of sound was observed with increasing hepatic fat accumulation (approximately -1.7 m s^-1per 1% increase in hepatic fat) and histopathology steatosis grading (approximately -10 to -13 m s^-1per unit increase in steatosis grade). Local speed of sound estimates were highly correlated with steatosis grade, having Pearson and Spearman correlation coefficients both ranging from -0.87 to -0.78. In addition, a lobe-dependent speed of sound in the liver was observed by theex vivomeasurements, with speed of sound differences of up to 25 m s^-1(p< 0.003) observed between lobes in the liver of the same animal.Significance.The findings of this study suggest that local speed of sound estimation has the potential to be used to predict or assist in the measurement of hepatic fat concentration and that the global average speed of sound should be avoided in hepatic fat estimation due to significant bias in the speed of sound estimate.

3D synthetic aperture imaging with a therapeutic spherical random phased array for transcostal applications

Experimental validation of a synthetic aperture imaging technique using a therapeutic random phased array is described, demonstrating the dual nature of imaging and therapy of such an array. The transducer is capable of generating both continuous wave high intensity beams for ablating the tumor and low intensity ultrasound pulses to image the target area. Pulse-echo data is collected from the elements of the phased array to obtain B-mode images of the targets. Since therapeutic arrays are optimized for therapy only with concave apertures having low f-number and large directive elements often coarsely sampled, imaging can not be performed using conventional beamforming. We show that synthetic aperture imaging is capable of processing the acquired RF data to obtain images of the field of interest. Simulations were performed to compare different synthetic aperture imaging techniques to identify the best algorithm in terms of spatial resolution. Experimental validation was performed using a 1 MHz, 256-elements, spherical random phased array with 130 mm radius of curvature. The array was integrated with a research ultrasound scanner via custom connectors to acquire raw RF data for variety of targets. Imaging was implemented using synthetic aperture beamforming to produce images of a rib phantom and ex vivo ribs. The array was shown to resolve spherical targets within ±15 mm of either side of the axis in the focal plane and obtain 3D images of the rib phantom up to ±40 mm of either side of the central axis and at a depth of 3-9 cm from the array surface. The lateral and axial full width half maximum was 1.15 mm and 2.75 mm, respectively. This study was undertaken to emphasize that both therapy and image guidance with a therapeutic random phased array is possible and such a system has the potential to address some major limitations in the existing high intensity focused ultrasound (HIFU) systems. The 3D images obtained with a therapeutic array can be used to identify and locate strong scattering objects aiding to image guidance and treatment planning of the HIFU procedure.

In Vivo Evaluation of Plane Wave Imaging for Abdominal Ultrasonography

Although plane wave imaging (PWI) has been extensively employed for ultrafast ultrasound imaging, its potential for sectorial B-mode imaging with a convex array transducer has not yet been widely recognized. Recently, we reported an optimized PWI approach for sector scanning that exploits the dynamic transmit focusing capability. In this paper, we first report the clinical applicability of the optimized PWI for abdominal ultrasonography by in vivo image and video evaluations and compare it with conventional focusing (CF) and diverging wave imaging (DWI), which is another dynamic transmit focusing technique generally used for sectorial imaging. In vivo images and videos of the liver, kidney, and gallbladder were obtained from 30 healthy volunteers using PWI, DWI, and CF. Three radiologists assessed the phantom images, 156 in vivo images, and 66 in vivo videos. PWI showed significantly enhanced (p < 0.05) spatial resolution, contrast, and noise and artifact reduction, and a 4-fold higher acquisition rate compared to CF and provided similar performances compared to DWI. Because the computations required for PWI are considerably lower than that for DWI, PWI may represent a promising technique for sectorial imaging in abdominal ultrasonography that provides better image quality and eliminates the need for focal depth adjustment.

Deconvolution in Intravascular Ultrasound to Improve Lateral Resolution

Intravascular ultrasound (IVUS) is an important diagnostic method for coronary disease. The lateral and axial resolutions of IVUS systems under study are typically ~120 and ~30 µm, respectively. The lateral resolution has a lower quality than the axial one and is restricted by the aperture size of transducers. In addition, this resolution is difficult to further improve physically. However, IVUS is inherently suitable for lateral deconvolution because of its circular imaging area. In this paper, magnitude-based deconvolution was demonstrated to be feasible in IVUS imaging to improve the lateral resolution. The deconvolution process was first simulated to determine the highest feasible resolution. Next, the method was applied to a real system to validate the feasibility. The lateral resolution was improved significantly, that is, 2°-separated targets could be discerned using a transducer with 4.2° -6 dB lateral resolution.

An Optimum Imaging Scheme for IVUS Arrays: Eccentric Cylinder Wave Compounding

With the development of integrated circuit (IC) technologies, complex transmitting and receiving circuits can be integrated into miniature intravascular ultrasound (IVUS) catheters, making it possible to adopt better synthesizing schemes for better imaging. Eccentric cylinder wave compounding should be an optimum synthesizing scheme for the small size cylinder shaped catheter. Eccentric cylinder waves centered at different points are emitted, signals are collected after each emission, and images can be synthesized with easy post processing. Detailed analyses about resolution and grating lobes were made; the optimum eccentric distance was determined. Simulations were done to examine the resolution, signal-to-noise ratio, and resistance to crosstalk and nonuniformity of arrays. Dual apodization and magnitude-based deconvolution were applied to further improve the results.

A Micromachined Pb(Mg1/3Nb2/3)O3-PbTiO3 Single Crystal Composite Circular Array for Intravascular Ultrasound Imaging

This paper describes the design, fabrication, and characterization of a micromachined high-frequency Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) single crystal/epoxy 1-3 composite ultrasound circular array. The 1-3 composites were fabricated by deep reactive ion etching (DRIE) of PMN-PT single crystal. The feature size of single crystal pillars was 18 μm in diameter. The kerf between pillars was less than 4 μm. A 50-element circular array transducer (radially outward) with the pitch of 100 μm was wrapped around a needle resulting in an outer diameter of 1.7 mm. The array test showed that the center frequency reached 39±2 MHz and -6-dB fractional bandwidth was 82±6%. The insertion loss was -41 dB, and crosstalk between adjacent elements was -24 dB. A radial outward imaging testing with phantom wires (D = 50 μm) was conducted. The image was in a dynamic range of 30 dB to show a penetration depth of 6 mm by using the synthetic aperture method. The -6-dB beam width was estimated to be 60 μm in the axial direction at 3.1 mm distance away from the probe. The results suggest that the 40 MHz micromachined 1-3 composite circular array is promising for intravascular ultrasound (IVUS) imaging applications.

An Impulse Radio PWM-Based Wireless Data Acquisition Sensor Interface

A sensor interface circuit based on impulse radio pulse width modulation (IR-PWM) is presented for low power and high throughput wireless data acquisition systems (wDAQ) with extreme size and power constraints. Two triple-slope analog-to-time converters (ATC) convert two analog signals, each up to 5 MHz in bandwidth, into PWM signals, and an impulse radio (IR) transmitted (Tx) with an all-digital power amplifier (PA) combines them while preserving the timing information by transmitting impulses at the PWM rising and falling edges. On the receiver (Rx) side, an RF-LNA followed by an envelope detector recovers the incoming impulses, and a T-flipflop reverts the impulse sequence back to PWM to be digitized by a time-to-digital converter (TDC). Detailed analysis and design guideline on ATC was introduced, and a proof-of-concept prototype was fabricated for a capacitive micromachined ultrasound transducer (CMUT) imaging system in a 0.18-μm HV CMOS process, occupying 0.18 mm² active area and consuming 3.94 mW from a 1.8 V supply. The proposed TDC in this prototype yielded 7-bit resolution, while the entire wDAQ achieved 5.8 effective number of bits (ENOB) at 2 × 10 MS/s.

Efficient Frequency-Domain Synthetic Aperture Focusing Techniques for Imaging With a High-Frequency Single-Element Focused Transducer

Synthetic aperture focusing techniques (SAFT) make the spatial resolution of the conventional ultrasound imaging from a single-element focused transducer more uniform in the lateral direction. In this work, two new frequency-domain (FD-SAFT) algorithms are proposed, which are based on the synthetic aperture radar's wavenumber algorithm, and 2-D matched filtering technique for the image reconstruction. The first algorithm is the FD-SAFT virtual source (FD-SAFT-VS) that treats the focus of a focused transducer as a virtual source having a finite size and the diffraction effect in the far-field is taken into consideration in the image reconstruction. The second algorithm is the FD-SAFT deconvolution (FD-SAFT-DE) that uses the simulated point spread function of the imaging system as a matched filter kernel in the image reconstruction. The performance of the proposed algorithms was studied using a series of simulations and experiments, and it was compared with the conventional B-mode and time-domain SAFT (TD-SAFT) imaging techniques. The image quality was analyzed in terms of spatial resolution, sidelobe level, signal-to-noise ratio (SNR), contrast resolution, contrast-to-speckle ratio, and ex vivo image quality. The results showed that the FD-SAFT-VS had the smallest spatial resolution and FD-SAFT-DE had the second smallest spatial resolution. In addition, FD-SAFT-DE had generally the largest SNR. The computation run time of FD-SAFT-VS and FD-SAFT-DE, depending on the image size, was lower by 4 to 174 times and 4 to 189 times, respectively, compared to the TD-SAFT-virtual point source.

Feasibility of low-frequency ultrasound imaging using pulse compressed parametric ultrasound

When using high-frequency (HF) ultrasound in sonography, attenuation due to the viscosity of the medium limits the available imaging depth, and strong reflection and scattering from hard tissue, such as bone, render biological diagnosis very difficult. In order to resolve these problems, the feasibility of low-frequency (LF) parametric ultrasound imaging with high directivity was explored in the present study. A pulse compression technique was applied to chirp-modulated parametric ultrasound waves in the frequency band of 100-500 kHz generated from modulated primary ultrasound waves with a center frequency of 2.8 MHz in order to improve the signal-to-noise ratio (SNR). Low-frequency ultrasound images of brass rods obtained using pulse compressed parametric ultrasound exhibit accurate target distances, a 3-mm range resolution, which agrees well with the theoretical value, and an 8-dB improvement in SNR. Parametric ultrasound imaging with pulse compression makes easy to separate overlapping targets in comparison with HF ultrasound imaging, and indicates the image with brightness independent of distance in comparison with directly radiated LF ultrasound imaging. These results reveal that pulse compressed LF parametric ultrasound is not only a useful method for improving the SNR and providing accurate distance measurements, but also enables imaging of overlapping targets.

Intravascular Ultrasound Imaging With Virtual Source Synthetic Aperture Focusing and Coherence Factor Weighting

Intravascular ultrasound (IVUS) has been frequently used for coronary artery imaging clinically. More importantly, IVUS is the fundamental image modality for most advanced multimodality intravascular imaging techniques, since it provides a more comprehensive picture of vessel anatomy on which other imaging data can be superimposed. However, image quality in the deeper region is poor because of the downgraded lateral resolution and contrast-to-noise ratio (CNR). In this paper, we report on the application of an ultrasound beamforming method that combines virtual source synthetic aperture (VSSA) focusing and coherence factor weighting (CFW) to improve the IVUS image quality. The natural focal point of conventional IVUS transducer was treated as a virtual source that emits spherical waves within a certain region. Mono-static synthetic aperture focusing was conducted to achieve higher resolution. Coherence factor was calculated using delayed RF signals and applied to the synthesized beam to increase the CNR and focusing quality. The proposed method was tested through simulations in Field II and imaging experiments in both linear and rotational scans. The lateral resolution for linear scan mode is improved from 165-524 to 126-143 μm ; resolution for rotational scan mode improves by up to 42%. CNR improvement by up to 1.5 was observed on the anechoic cysts of different sizes and at different locations. Herein, it is demonstrated that the beamforming method, which combines VSSA and CFW, can significantly improve the IVUS image quality. This approach can be readily integrated into the current IVUS imaging system for enhanced clinical diagnosis.

Synthetic aperture focusing technique for photoacoustic endoscopy

Photoacoustic endoscopy (PAE) is a promising tool for the detection of atherosclerotic plaque. In this work, we propose a novel design of a side-viewing PAE probe based on a synthetic aperture focusing technique (SAFT) to enable high transverse resolution over large depth of focus (DOF) along the radial direction. A point-like ultrasonic detector is used to ensure a wide detection angle and thus a large synthetic aperture for SAFT. We first perform numerical simulation to optimize the PAE probe design, which involves the placement of the point-like detector and the diameter of a reflection rod mirror. Then, experiments are conducted based on the optimized probe design. High transverse resolution of 115-190 µm over large DOF of 3.5 mm along the radial direction is experimentally obtained. The SAFT-based PAE holds promise for endoscopic imaging with a high transverse resolution for both the surface and deep regions of tissue.

Accurate 3D reconstruction of bony surfaces using ultrasonic synthetic aperture techniques for robotic knee arthroplasty

Robotically guided knee arthroplasty systems generally require an individualized, preoperative 3D model of the knee joint. This is typically measured using Computed Tomography (CT) which provides the required accuracy for preoperative surgical intervention planning. Ultrasound imaging presents an attractive alternative to CT, allowing for reductions in cost and the elimination of doses of ionizing radiation, whilst maintaining the accuracy of the 3D model reconstruction of the joint. Traditional phased array ultrasound imaging methods, however, are susceptible to poor resolution and signal to noise ratios (SNR). Alleviating these weaknesses by offering superior focusing power, synthetic aperture methods have been investigated extensively within ultrasonic non-destructive testing. Despite this, they have yet to be fully exploited in medical imaging. In this paper, the ability of a robotic deployed ultrasound imaging system based on synthetic aperture methods to accurately reconstruct bony surfaces is investigated. Employing the Total Focussing Method (TFM) and the Synthetic Aperture Focussing Technique (SAFT), two samples were imaged which were representative of the bones of the knee joint: a human-shaped, composite distal femur and a bovine distal femur. Data were captured using a 5MHz, 128 element 1D phased array, which was manipulated around the samples using a robotic positioning system. Three dimensional surface reconstructions were then produced and compared with reference models measured using a precision laser scanner. Mean errors of 0.82mm and 0.88mm were obtained for the composite and bovine samples, respectively, thus demonstrating the feasibility of the approach to deliver the sub-millimetre accuracy required for the application.

Towards a Reduced-Wire Interface for CMUT-Based Intravascular Ultrasound Imaging Systems

Having intravascular ultrasound (IVUS) imaging capability on guide wires used in cardiovascular interventions may eliminate the need for separate IVUS catheters and expand the use of IVUS in a larger portion of the vasculature. High frequency capacitive micro machined ultrasonic transducer (CMUT) arrays should be integrated with interface electronics and placed on the guide wire for this purpose. Besides small size, this system-on-a-chip (SoC) front-end should connect to the back-end imaging system with a minimum number of wires to preserve the critical mechanical properties of the guide wire. We present a 40 MHz CMUT array interface SoC, which will eventually use only two wires for power delivery and transmits image data using a combination of analog-to-time conversion (ATC) and an impulse radio ultra-wideband (IR-UWB) wireless link. The proof-of-concept prototype ASIC consumes only 52.8 mW and occupies 4.07 [Formula: see text] in a 0.35- [Formula: see text] standard CMOS process. A rectifier and regulator power the rest of the SoC at 3.3 V from a 10 MHz power carrier that is supplied through a 2.4 m micro-coax cable with an overall efficiency of 49.1%. Echo signals from an 8-element CMUT array are amplified by a transimpedance amplifier (TIA) array and down-converted to baseband by quadrature sampling using a 40 MHz clock, derived from the power carrier. The ATC generates pulse-width-modulated (PWM) samples at 2 × 10 MS/s with 6 bit resolution, while the entire system achieved 5.1 ENOB. Preliminary images from the prototype system are presented, and alternative data transmission and possible future directions towards practical implementation are discussed.

Ultrasound Vector Flow Imaging-Part II: Parallel Systems

This paper gives a review of the current state-of-the-art in ultrasound parallel acquisition systems for flow imaging using spherical and plane waves emissions. The imaging methods are explained along with the advantages of using these very fast and sensitive velocity estimators. These experimental systems are capable of acquiring thousands of images per second for fast moving flow as well as yielding the estimates of low velocity flow. These emerging techniques allow the vector flow systems to assess highly complex flow with transitory vortices and moving tissue, and they can also be used in functional ultrasound imaging for studying brain function in animals. This paper explains the underlying acquisition and estimation methods for fast 2-D and 3-D velocity imaging and gives a number of examples. Future challenges and the potentials of parallel acquisition systems for flow imaging are also discussed.

Optimized Plane Wave Imaging for Fast and High-Quality Ultrasound Imaging

This paper presents a method for optimizing parameters affecting the image quality in plane wave imaging. More specifically, the number of emissions and steering angles is optimized to attain the best images with the highest frame rate possible. The method is applied to a specific problem, where image quality for a λ -pitch transducer is compared with a λ /2-pitch transducer. Grating lobe artifacts for λ -pitch transducers degrade the contrast in plane wave images, and the impact on frame rate is studied. Field II simulations of plane wave images are made for all combinations of the parameters, and the optimal setup is selected based on Pareto optimality. The optimal setup for a simulated 4.1-MHz λ -pitch transducer uses 61 emissions and a maximum steering angle of 20° for depths from 0 to 60 mm. The achieved lateral full-width at half-maximum (FWHM) is 1.5λ and the contrast is -29 dB for a scatterer at 9 mm ( 24λ ). Using a λ /2-pitch transducer and only 21 emissions within the same angle range, the image quality is improved in terms of contrast, which is -37 dB. For imaging in regions deeper than 25 mm ( 66λ ), only 21 emissions are optimal for both the transducers, resulting in a -36 dB contrast at 34 mm ( 90λ ). Measurements are performed using the experimental SARUS scanner connected to a λ -pitch and λ /2-pitch transducer. A wire phantom and a tissue mimicking phantom containing anechoic cysts are scanned and show the performance using the optimized sequences for the transducers. FWHM is 1.6λ and contrast is -25 dB for a wire at 9 mm using the λ -pitch transducer. For the λ /2-pitch transducer, contrast is -29 dB. In vivo scans of the carotid artery of a healthy volunteer show improved contrast and present fewer artifacts, when using the λ /2-pitch transducer compared with the λ -pitch. It is demonstrated with a frame rate, which is three times higher for the λ /2-pitch transducer.

Impact of element pitch on synthetic aperture ultrasound imaging

PURPOSE

Synthetic aperture imaging was introduced in medical ultrasound to obtain high-quality images. In synthetic aperture ultrasound imaging, spherical transmit waves illuminate a target region from different positions, resulting in low-resolution images for each transmission. By coherent compounding of the resulting low-resolution images, a high-resolution image is obtained. Multiple steered receiving beams need to be created to obtain each low-resolution image and, thus, grating lobes should influence the image quality. In the present study, an array ultrasonic probe with a small element pitch was introduced to reduce the influences of grating lobes, and the effect of element pitch on image quality was examined in detail.

METHOD

A linear array ultrasonic probe at a nominal center frequency of 7.5 MHz with an element pitch of 0.1 mm has been introduced. This probe does not produce grating lobes within the imaging region in theory because the element pitch of this probe is half of the ultrasonic wavelength. The contrast of an ultrasonic image was evaluated using a cyst phantom.

RESULTS

The contrasts obtained by synthetic aperture imaging with element pitches of 0.1 and 0.2 mm were 4.88 and 4.69 dB, respectively, which were similar to the 4.67 dB obtained by conventional beamforming with focused transmit beams, when the number of transmissions was 121. The contrast obtained with an element pitch of 0.1 mm was similar (4.34 dB) even when the number of transmissions was decreased to 61. However, the contrast obtained with an element pitch of 0.2 mm showed a larger degradation (3.77 dB) at 31 transmissions.

DISCUSSION AND CONCLUSION

Even with larger element pitches, good image contrast could be obtained when the number of transmissions was large. This is because echoes from grating lobes are incoherent among transmissions, and they are suppressed by compounding low-resolution images obtained by individual transmissions. On the other hand, an array probe with smaller element pitches achieves good image contrast even with a smaller number of transmissions and, thus, it would be preferable to realize a higher frame rate.

Microbubble-mediated intravascular ultrasound imaging and drug delivery

Intravascular ultrasound (IVUS) provides radiation-free, real-time imaging and assessment of atherosclerotic disease in terms of anatomical, functional, and molecular composition. The primary clinical applications of IVUS imaging include assessment of luminal plaque volume and real-time image guidance for stent placement. When paired with microbubble contrast agents, IVUS technology may be extended to provide nonlinear imaging, molecular imaging, and therapeutic delivery modes. In this review, we discuss the development of emerging imaging and therapeutic applications that are enabled by the combination of IVUS imaging technology and microbubble contrast agents.

Synthetic aperture imaging for multilayer cylindrical object using an exterior rotating transducer

The synthetic aperture focusing technique (SAFT) with significant improvements in lateral resolution has been adapted for ultrasound imaging of multilayer objects. To apply SAFT to imaging of cylindrical objects such as solid axles or pipes with small diameter, exterior cylindrical scan is much preferred. In this paper, a frequency-domain algorithm is proposed for such cylindrical scan performed with an exterior rotating transducer. The algorithm is derived from Fourier-domain solutions to the waveequation in cylindrical coordinates, and then extended to the multilayer case. A simulation model for multilayer structure is established, and the algorithm is demonstrated for both simulated and experimental data. Compared with the raw images, the reconstructed images with proposed algorithm attain better lateral resolution for multilayer objects. It is shown that the attainable angular resolution for each layer is approximately consistent with that achieved in the single-layer case, as long as the transmission factors are approximately uniform within the divergence angle of the transducer. The performance of proposed algorithm is verified with experimental C-scan image and demonstrates that it is capable of improving the lateral resolution in both scanning directions.

Sequential beamforming for synthetic aperture imaging

Synthetic aperture sequential beamforming (SASB) is a novel technique which allows to implement synthetic aperture beamforming on a system with a restricted complexity, and without storing RF-data. The objective is to improve lateral resolution and obtain a more depth independent resolution compared to conventional ultrasound imaging. SASB is a two-stage procedure using two separate beamformers. The initial step is to construct and store a set of B-mode image lines using a single focal point in both transmit and receive. The focal points are considered virtual sources and virtual receivers making up a virtual array. The second stage applies the focused image lines from the first stage as input data, and take advantage of the virtual array in the delay and sum beamforming. The size of the virtual array is dynamically expanded and the image is dynamically focused in both transmit and receive and a range independent lateral resolution is obtained. The SASB method has been investigated using simulations in Field II and by off-line processing of data acquired with a commercial scanner. The lateral resolution increases with a decreasing F#. Grating lobes appear if F#≤2 for a linear array with λ-pitch. The performance of SASB with the virtual source at 20mm and F#=1.5 is compared with conventional dynamic receive focusing (DRF). The axial resolution is the same for the two methods. For the lateral resolution there is improvement in FWHM of at least a factor of 2 and the improvement at -40dB is at least a factor of 3. With SASB the resolution is almost constant throughout the range. For DRF the FWHM increases almost linearly with range and the resolution at -40dB is fluctuating with range. The theoretical potential improvement in SNR of SASB over DRF has been estimated. An improvement is attained at the entire range, and at a depth of 80mm the improvement is 8dB.

A method for direct localized sound speed estimates using registered virtual detectors

Accurate sound speed estimates are desirable in a number of fields. In an effort to increase the spatial resolution of sound speed estimates, a new method is proposed for direct measurement of sound speed between arbitrary spatial locations. The method uses the sound speed estimator developed by Anderson and Trahey. Their least squares fit of the received waveform's curvature provides an estimate of the wave's point of origin. The point of origin and the delay profile calculated from the fit are used to arrive at a spatially registered virtual detector. Between a pair of registered virtual detectors, a spherical wave is propagated. By beamforming the data, the time-of-flight between the two virtual sources can be calculated. From this information, the local sound speed can be estimated. Validation of the estimator is made using phantom and simulation data. The set of test phantoms consisted of both homogeneous and inhomogeneous media. Several different inhomogeneous phantom configurations were used for the physical validation. The simulation validation focused on the limits of target depth and signal-to-noise ratio on virtual detector registration. The simulations also compare the impact of two- and three-layer inhomogeneous media. The phantom results varied based on signal-to-noise ratio and geometry. The results for all cases were generally less than 1% mean error and standard deviation. The simulation results varied somewhat with depth and F/#, but primarily, they varied with signal-to-noise ratio and geometry. With two-layer geometries, the algorithm has a worst-case spatial registration bias of 0.02%. With three-layer geometries, the axial registration error gets worse with a bias magnitude up to 2.1% but is otherwise relatively stable over depth. The stability over depth of the bias in a given medium still allows for accurate sound speed estimates with a mean relative error less than 0.2%.

Optimizing circular ring arrays for forward-looking IVUS imaging

Forward-looking (FL) catheter-based imaging systems are highly desirable for guiding interventions in intra- vascular ultrasound (IVUS) applications. One of the main challenges of array-based FL-IVUS systems is the large channel count, which results in increased system complexity. Synthetic phased-array processing with a reduced firing count simplifies the front-end and, hence, can enable 3-D real-time imaging. Recently, we have investigated dual-ring arrays suitable for IVUS imaging, in which the two concentric circular arrays are used separately as transmit (Tx) and receive (Rx) arrays. In this study, we present different optimized array designs based on dual and single circular rings which are suitable for synthetic phased-array processing with a reduced number of firings. To obtain an optimal firing set that produces low side lobes in the wideband response, we use a simulated annealing algorithm. In the simulations, we use 1.2-mm-diameter array configurations with 64 Tx and 58 Rx elements, a center frequency of 20 MHz and fractional bandwidths of 50% and 80%. The results show that optimized dual-ring arrays provide 8 dB improvements in peak near side-lobe level with no widening in the main lobe width when compared with full and other sparse co-arrays.

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.

High-frame-rate echocardiography using diverging transmit beams and parallel receive beamforming

PURPOSE

Echocardiography is a widely used modality for diagnosis of the heart. It enables observation of the shape of the heart and estimation of global heart function based on B-mode and M-mode imaging. Subsequently, methods for estimating myocardial strain and strain rate have been developed to evaluate regional heart function. Furthermore, it has recently been shown that measurements of transmural transition of myocardial contraction/relaxation and propagation of vibration caused by closure of a heart valve would be useful for evaluation of myocardial function and viscoelasticity. However, such measurements require a frame rate much higher than that achieved by conventional ultrasonic diagnostic equipment. In the present study, a method based on parallel receive beamforming was developed to achieve high-frame-rate (over 300 Hz) echocardiography.

METHODS

To increase the frame rate, the number of transmits was reduced to 15 with angular intervals of 6°, and 16 receiving beams were created for each transmission to obtain the same number and density of scan lines as realized by conventional sector scanning. In addition, several transmits were compounded to obtain each scan line to reduce the differences in transmit-receive sensitivities among scan lines. The number of transmits for compounding was determined by considering the width of the transmit beam. For transmission, plane waves and diverging waves were investigated. Diverging waves showed better performance than plane waves because the widths of plane waves did not increase with the range distance from the ultrasonic probe, whereas lateral intervals of scan lines increased with range distance.

RESULTS

The spatial resolution of the proposed method was validated using fine nylon wires. Although the widths at half-maxima of the point spread functions obtained by diverging waves were slightly larger than those obtained by conventional beamforming and parallel beamforming with plane waves, point spread functions very similar to those obtained by conventional beamforming could be realized by parallel beamforming with diverging beams and compounding. However, there was an increase in the lateral sidelobe level in the case of parallel beamforming with plane and diverging waves. Furthermore, the heart of a 23-year-old healthy male was measured.

CONCLUSION

Although the contrast of the B-mode image obtained by the proposed method was degraded due to the increased sidelobe level, a frame rate of 316 Hz, much higher than that realized by conventional sector scanning of several tens of Hertz, was realized with a full lateral field of view of 90°.

Ultrasonic microscanning

A detailed review is given of the application of high-frequency ultrasound (HFUS) at frequencies of 20 MHz and above for high-resolution, cross-sectional imaging of biological soft tissue. The state of the art of HFUS imaging systems is discussed with respect to the underlying engineering concepts, system designs, and available transducer technology. Furthermore, the dependency of the spatial resolution on the system's parameters is analysed. Skin imaging, eye imaging, small animal imaging for preclinical research, and intravascular ultrasound in coronary arteries for arteriosclerotic disease diagnostics are presented as examples for the application of HFUS imaging in medical diagnostics. It is shown that, in the frame of the indicated applications, ultrasound in the frequency range 20-100MHz gives a good compromise between the contrary demands for a good spatial resolution and a sufficiently large penetration depth of ultrasound waves into the tissue. Scanning schemes for the imaging of tissue morphology are considered, including spatial compounding as a multidirectional imaging technique.

Display pixel-based synthetic aperture focusing method for intravascular ultrasound imaging

An intravascular ultrasound image reconstruction technique that combines synthetic aperture focusing and display pixel-based focusing methods is presented. Although the synthetic aperture focusing method can improve intravascular ultrasound image quality, the final displayed images are usually blurry in the angular direction due to the limitations of the digital scan converter. The display pixel-based focusing method can eliminate blurring effects caused by the digital scan converter. Therefore, the image quality can be further improved by applying the display pixel-based focusing method to the synthetic aperture focusing method, especially for intravascular ultrasound images. The experimental studies were performed to evaluate display pixel-based synthetic aperture focusing method. The computational complexity of the display pixel-based synthetic aperture focusing method was discussed in comparison with that of the synthetic aperture focusing method.

Fast parametric beamformer for synthetic aperture imaging

This paper describes the design and implementation of a real-time delay-and-sum synthetic aperture beamformer. The beamforming delays and apodization coefficients are described parametrically. The image is viewed as a set of independent lines that are defined in 3D by their origin, direction, and inter-sample distance. The delay calculation is recursive and inspired by the coordinate rotation digital computer (CORDIC) algorithm. Only 3 parameters per channel and line are needed for their generation. The calculation of apodization coefficients is based on a piece- wise linear approximation. The implementation of the beamformer is optimized with respect to the architecture of a novel synthetic aperture real-time ultrasound scanner (SARUS), in which 4 channels are processed by the same set of field-programmable gate arrays (FPGA). In synthetic transmit aperture imaging, low-resolution images are formed after every emission. Summing all low-resolution images produces a perfectly focused high-resolution image. The design of the beamformer is modular, and a single beamformation unit can produce 4600 low-resolution images per second, each consisting of 32 lines and 1024 complex samples per line. In its present incarnation, 3 such modules fit in a single device. The summation of low-resolution images is performed internally in the FPGA to reduce the required bandwidth. The delays are calculated with a precision of 1/16th of a sample, and the apodization coefficients with 7-bit precision. The accumulation of low-resolution images is performed with 24-bit precision. The level of the side- and grating lobes, introduced by the use of integer numbers in the calculations and truncation of intermediate results, is below -86 dB from the peak.

Spatial encoding using a code division technique for fast ultrasound imaging

This paper describes a method for spatial encoding in synthetic transmit aperture ultrasound imaging. This allows several ultrasonic sources to be active simultaneously. The method is based on transmitting pseudorandom sequences to spatially encode the transmitters. The data can be decoded after only one transmission using the knowledge of the transmitted code sequences as opposed to other spatial encoding techniques, such as Hadamard or Golay encoding. This makes the method less sensitive to motion, and data can be acquired using fewer transmissions. The aim of this paper is to analyze the underlying theory and to test the feasibility in a physical system. The method has been evaluated in simulations using Field II in which the point-spread functions were simulated for different depths for a 7 MHz linear array transducer. A signal-to-noise ratio (SNR) simulation also was included in the study in which an improvement in SNR of approximately 1.5 dB was attained compared to the standard synthetic transmit aperture (STA) firing scheme. Considering the amount of energy transmitted, this value is low. A plausible explanation is given that is verified in simulation. The method also was tested in an experimental ultrasound scanner and compared to a synthetic transmit aperture ultrasound imaging scheme using a sinusoidal excitation. The performance of the proposed method was comparable to the reference with respect to axial and lateral resolution, but it displayed poorer contrast with sidelobe levels at approximately - 40 dB compared to the mainlobe.

Designing waveforms for temporal encoding using a frequency sampling method

In this paper a method for designing waveforms for temporal encoding in medical ultrasound imaging is described. The method is based on least squares optimization and is used to design nonlinear frequency modulated signals for synthetic transmit aperture imaging. By using the proposed design method, the amplitude spectrum of the transmitted waveform can be optimized, such that most of the energy is transmitted where the transducer has large amplification. To test the design method, a waveform was designed for a BK8804 linear array transducer. The resulting nonlinear frequency modulated waveform was compared to a linear frequency modulated signal with amplitude tapering, previously used in clinical studies for synthetic transmit aperture imaging. The latter had a relatively flat spectrum which implied that the waveform tried to excite all frequencies including ones with low amplification. The proposed waveform, on the other hand, was designed so that only frequencies where the transducer had a large amplification were excited. Hereby, unnecessary heating of the transducer could be avoided and the signal-to-noise ratio could be increased. The experimental ultrasound scanner RASMUS was used to evaluate the method experimentally. Due to the careful waveform design optimized for the transducer at hand, a theoretic gain in signal-to-noise ratio of 4.9 dB compared to the reference excitation was found, even though the energy of the nonlinear frequency modulated signal was 71% of the energy of the reference signal. This was supported by a signal-to-noise ratio measurement and comparison in penetration depth, where an increase of 1 cm was found in favor for the proposed waveform. Axial and lateral resolutions at full-width half-maximum were compared in a water phantom at depths of 42, 62, 82, and 102 mm. The axial resolutions of the nonlinear frequency modulated signal were 0.62, 0.69, 0.60, and 0.60 mm, respectively. The corresponding axial resolutions for the reference waveform were 0.58, 0.65, 0.62, and 0.60 mm, respectively. The compression properties of the matched filter (mismatched filter for the linear frequency modulated signal) were tested for both waveforms in simulation with respect to the Doppler frequency shift occurring when probing moving objects. It was concluded that the Doppler effect of moving targets does not significantly degrade the filtered output. Finally, in vivo measurements are shown for both methods, wherein the common carotid artery on a 27-year-old healthy male was scanned.

In-vivo evaluation of convex array synthetic aperture imaging

This paper presents an in-vivo study of synthetic transmit aperture (STA) imaging in comparison with conventional imaging, evaluating whether STA imaging is feasible in-vivo and whether the image quality obtained is comparable with traditional scanned imaging in terms of penetration depth, spatial resolution, contrast resolution and artifacts. Acquisition was performed using our research scanner RASMUS and a 5.5 MHz convex array transducer. STA imaging was acquired using circular wave emulation by 33-element subapertures and a 20 micros linear FM signal as excitation pulse. For conventional imaging, a 64 element aperture was used in transmit and receive with a 1.5 cycle sinusoid excitation pulse. Conventional and STA images were acquired interleaved, ensuring that the exact same anatomical location was scanned. Image sequences were recorded in real time and processed off-line. Seven male volunteers were scanned abdominally and the resulting images were compared by three medical doctors using randomized blinded presentation. Penetration and image quality were scored and evaluated statistically. Results showed slightly but significantly (0.48 cm, p = 0.008) increased penetration using STA. Image quality was also highly significantly (p < 0.001) increased. Results show that in-vivo ultrasound imaging using STA is feasible for abdominal imaging without severe motion artifacts.

Directional velocity estimation using a spatio-temporal encoding technique based on frequency division for synthetic transmit aperture ultrasound

This paper investigates the possibility of flow estimation using spatio-temporal encoding of the transmissions in synthetic transmit aperture imaging (STA). The spatial encoding is based on a frequency division approach. In STA, a major disadvantage is that only a single transmitter (denoting single transducer element or a virtual source) is used in every transmission. The transmitted acoustic energy will be low compared to a conventional focused transmission in which a large part of the aperture is used. By using several transmitters simultaneously, the total transmitted energy can be increased. However, to focus the data properly, the signals originating from the different transmitters must be separated. To do so, the pass band of the transducer is divided into a number of subbands with disjoint spectral support. At every transmission, each transmitter is assigned one of the subbands. In receive, the signals are separated using a simple filtering operation. To attain high axial resolution, broadband spectra must be synthesized for each of the transmitters. By multiplexing the different waveforms on different transmitters over a number of transmissions, this can be accomplished. To further increase the transmitted energy, the waveforms are designed as linear frequency modulated signals. Therefore, the full excitation amplitude can be used during most of the transmission. The method has been evaluated for blood velocity estimation for several different velocities and incident angles. The program Field II was used. A 128-element transducer with a center frequency of 7 MHz was simulated. The 64 transmitting elements were used as the transmitting aperture and 128 elements were used as the receiving aperture. Four virtual sources were created in every transmission. By beamforming lines in the flow direction, directional data were extracted and correlated. Hereby, the velocity of the blood was estimated. The pulse repetition frequency was 16 kHz. Three different setups were investigated with flow angles of 45, 60, and 75 degrees with respect to the acoustic axis. Four different velocities were simulated for each angle at 0.10, 0.25, 0.50, and 1.00 m/s. The mean relative bias with respect to the peak flow for the three angles was less than 2%, 2%, and 4%, respectively.

Frequency division transmission imaging and synthetic aperture reconstruction

In synthetic transmit aperture imaging only a few transducer elements are used in every transmission, which limits the signal-to-noise ratio (SNR). The penetration depth can be increased by using all transmitters in every transmission. In this paper, a method for exciting all transmitters in every transmission and separating them at the receiver is proposed. The coding is done by designing narrow-band linearly frequency modulated signals, which are approximately disjointed in the frequency domain and assigning one waveform to each transmitter. By designing a filterbank consisting of the matched filters corresponding to the excitation waveforms, the different transmitters can be decoded at the receiver. The matched filter of a specific waveform will allow information only from this waveform to pass through, thereby separating it from the other waveforms. This means that all transmitters can be used in every transmission, and the information from the different transmitters can be separated instantaneously. Compared to traditional synthetic transmit aperture (STA) imaging, in which the different transmitters are excited sequentially, more energy is transmitted in every transmission, and a better signal-to-noise-ratio is attained. The method has been tested in simulation, in which the resolution and contrast was compared to a standard synthetic transmit aperture system with a single sinusoid excitation. The resolution and contrast was comparable for the two systems. The method also has been tested using the experimental ultrasound scanner RASMUS. The resolution was evaluated using a string phantom. The method was compared to a conventional STA using both sinusoidal excitation and linear frequency modulated (FM) signals as excitation. The system using the FM signals and the frequency division approach yielded the same performance concerning both axial (of approximately equal to 3 wavelengths) and lateral resolution (of approximately equal to 4.5 wavelengths). A SNR measurement showed an increase in SNR of 6.5 dB compared to the system using the conventional STA method and FM signal excitation.

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 degrees C temperature rise is required to detect lipids in an atherosclerotic plaque in vivo.

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 degrees C temperature rise. A phantom experiment using an IVUS imaging array further supports the concept, and results agree reasonably well with prediction.

Coherent-array imaging using phased subarrays. Part I: basic principles

The front-end hardware complexity of a coherent array imaging system scales with the number of active array elements that are simultaneously used for transmission or reception of signals. Different imaging methods use different numbers of active channels and data collection strategies. Conventional full phased array (FPA) imaging produces the best image quality using all elements for both transmission and reception, and it has high front-end hardware complexity. In contrast, classical synthetic aperture (CSA) imaging only transmits on and receives from a single element at a time, minimizing the hardware complexity but achieving poor image quality. We propose a new coherent array imaging method--phased subarray (PSA) imaging--that performs partial transmit and receive beam-forming using a subset of adjacent elements at each firing step. This method reduces the number of active channels to the number of subarray elements; these channels are multiplexed across the full array and a reduced number of beams are acquired from each subarray. The low-resolution subarray images are laterally upsampled, interpolated, weighted, and coherently summed to form the final high-resolution PSA image. The PSA imaging reduces the complexity of the front-end hardware while achieving image quality approaching that of FPA imaging.

In-vivo synthetic aperture flow imaging in medical ultrasound

A new method for acquiring flow images using synthetic aperture techniques in medical ultrasound is presented. The new approach makes it possible to have a continuous acquisition of flow data throughout the whole image simultaneously, and this can significantly improve blood velocity estimation. Any type of filter can be used for discrimination between tissue and blood flow without initialization, and the number of lines used for velocity estimation is limited only by the nonstationarity of the flow. The new approach is investigated through both simulations and measurements. A flow rig is used for generating a parabolic laminar flow, and a research scanner is used for acquiring RF data from individual transducer elements. A reference profile is calculated from a mass flow meter. The parabolic velocity profile is estimated using the new approach with a relative standard deviation of 2.2% and a mean relative bias of 3.4% using 24 pulse emissions at a flow angle of 45 degrees. The 24 emissions can be used for making a full-color flow map image. An in-vivo image of flow in the carotid artery for a 29-year-old male also is presented. The full image is acquired using 24 emissions.

Dynamic-transmit focusing using time-dependent focal zone and center frequency

A method for achieving dynamic-transmit focus is presented. Within an initial high bandwidth pulse, successively higher frequencies are focused to successively closer focal zones along a transmitted beam line. During the receive operation, a time varying bandpass filter initially passes the higher frequencies that are focused closer in and successively passes lower center frequencies as time evolves and the focal zone moves out. The dynamically focused receive pulses are digitally sampled and processed by a matched digital filter to minimize phase anomalies. In this way, an improved resolution image is obtained with no loss of frame rate. The method is evaluated using comprehensive simulations that account for realistic levels of phase aberration and tissue attenuation. The method is relatively robust with respect to these perturbations. When the appropriate conditions apply, the new method can achieve an improvement in mean lateral resolution similar to that found in a multiple transmit zone implementation but without the frame rate penalty. A discussion of implementation considerations and limitations is presented.

An integrated compliant balloon ultrasound catheter for intravascular strain imaging

An integrated compliant balloon ultrasound catheter was developed to allow greater deformations in strain imaging with intravascular ultrasound. A 64-element circumferential array was placed inside a compliant silicone balloon catheter to capture real-time, phase-sensitive radio frequency (RF) data during deformation experiments. Strains over 40% could be applied to normal arterial wall tissue with intracatheter pressures as low as 200 kPa (2 atm). Strain images of a hard-soft rubber phantom, thrombus, and fibrotic plaque were produced using the integrated balloon ultrasound catheter. Results show that this catheter can apply large deformations at low pressures and image various vascular pathologies ex vivo. Potentially, it can serve as a multifunctional, intravascular therapeutic device to guide angioplasty and stent deployment.

Efficient three-dimensional imaging from a small cylindrical aperture

Small-diameter cylindrical imaging platforms, such as those being considered in the development of in vivo ultrasonic microprobes, pose unique image formation challenges. The curved apertures they provide are incompatible with many of the commonly used frequency-domain synthetic aperture imaging algorithms. At the same time, their frequently small diameters place limits on the available aperture and the angular resolution that may be achieved. We obtain a three-dimensional, frequency-domain imaging algorithm for this geometry by making suitable approximations to the point spread function for wave propagation in cylindrical coordinates and obtaining its Fourier transform by analogy with the equivalent problem in Cartesian coordinates. For the most effective use of aperture, we propose using a focused transducer to place a virtual source a short distance from the probe. The focus is treated as a diverging source by the imaging algorithm, which then forms images on deeper cylindrical shells. This approach retains the simplicity and potential angular resolution of a single element, yet permits full use of the available probe aperture and a higher energy output. Computer simulations and experimental results using wire targets show that this imaging technique attains the resolution limit dictated by the operating wavelength and the transducer characteristics.

Synthetic aperture imaging with arrays of arbitrary shape--part II: The annular array

It has been previously shown that full aperture resolution can be achieved with an annular transducer array by transmitting and receiving between all pairs of elements around the array circumference and applying an appropriate weighting function. If there are N elements in the array, this requires N transmissions. This paper shows that full aperture resolution can be obtained with a much smaller number of transmissions (two to four) by using a certain aperture phase weighting on transmit and receive. Thus, full aperture, real-time imaging from an annular array should be feasible.

Filter-based synthetic transmit and receive focusing

Most diagnostic ultrasonic imaging systems perform fixed focusing on transmit and dynamic focusing on receive. Such systems suffer from image quality degradation at depths away from the transmit focal zone. Several dynamic transmit focusing techniques have been previously investigated. Among them, a filter-based, retrospective focusing technique was proposed to increase the length of the transmit focal zone. In this paper, the filter-based technique is extended from dynamic receive focusing to fixed receive focusing. It is shown that the filtering technique with fixed receive focusing can achieve an image quality similar to that of dynamic receive focusing with filtering. The performance of the proposed approach is verified using real ultrasound data. It is shown that the proposed approach with fixed receive focusing requires a longer filter than that with dynamic receive focusing. Nonetheless, system complexity is greatly reduced with synthetic transmit and receive focusing because the dynamic receive focusing circuit is no longer needed.

Synthetic aperture-based beam compression for intravascular ultrasound imaging

In this paper, intravascular ultrasound (IVUS) images acquired with a 64-element array transducer using a multistatic acquisition scheme are presented. The images are reconstructed from a collection of pulse-echo measurements using a synthetic aperture array imaging technique. The main limitations of IVUS imaging are a poor lateral resolution and elevated grating lobes caused by the imaging geometry. We propose a Synthetic Aperture Focusing Technique (SAFT), which uses a limited number of A-scan signals. The focusing process, which is performed in the Fourier domain, requires far less computation time than conventional delay-and-sum methods. Two different reconstruction kernel functions have been derived and are compared for the processing of experimental data.

Simulation of circular array ultrasound transducers for intravascular applications

The beam shape of a circular array transducer that is commonly used in intravascular ultrasound catheters was investigated in linear mode of operation. For this use, a simulation program which can simulate the radio frequency (rf)-response of a number of scatterers has been developed. The program is based on the impulse response method, which is implemented in the frequency domain. Due to the unusual geometry of the transducer, the far field gets peculiarly shaped for large apertures. Instead of having a far field with its maximum intensity in a single lobe on the acoustical axis, the far field splits up into a dual-lobe far field with maximum intensity in two lobes off the acoustical axis. A formula is derived that predicts the occurrence of these beam shapes.

Rotary encoding for intravascular ultrasonic imaging systems

Images produced with an intravascular ultrasound system (IVUS) can be distorted because of uncertainty in the instantaneous angular position of a rotating ultrasonic transducer. A rotary encoder placed in proximity to the transducer is required to detect the problem; however, size constraints make a conventional electromechanical or optomechanical encoder difficult to implement. Measurements that test the feasibility of a software-derived encoder, based of the rate of decorrelation of ultrasonic RF lines with angle, are reported. Provided that the instantaneous angular velocity of the transducer can be measured, adjustments can be made to the pulse rate of the transducer, which would eliminate the image distortion.

Correlation processing for correction of phase distortions in subaperture imaging

Ultrasonic subaperture imaging combines synthetic aperture and phased array approaches and permits low-cost systems with improved image quality. In subaperture processing, a large array is synthesized using echo signals collected from a number of receive subapertures by multiple firings of a phased transmit subaperture. Tissue inhomogeneities and displacements in subaperture imaging may cause significant phase distortions on received echo signals. Correlation processing on reference echo signals can be used for correction of the phase distortions, for which the accuracy and robustness are critically limited by the signal correlation. In this study, we explore correlation processing techniques for adaptive subaperture imaging with phase correction for motion and tissue inhomogeneities. The proposed techniques use new subaperture data acquisition schemes to produce reference signal sets with improved signal correlation. The experimental test results were obtained using raw radio frequency (RF) data acquired from two different phantoms with 3.5 MHz, 128-element transducer array. The results show that phase distortions can effectively be compensated by the proposed techniques in real-time adaptive subaperture imaging.

Subaperture processing for ultrasonic imaging

Ultrasonic subaperture processing using aperture synthesis and beam space interpolation is presented. The number of beam lines scanning the image plane for a given transmit-receive subaperture combination is chosen according to the spatial sampling criteria for that combination. On each beam line, echo signals over the entire array are collected through electronic multiplexing of array channels, where the transmit subaperture at the transducer center is fired K successive times, with K equal to the number of nonoverlapping receive subapertures. For every receive subaperture, the number of beam lines is increased through digital interpolation using a linear filter with spatial frequency band associated with the subaperture. Interpolated beam lines from all receive subapertures are then added to obtain a high resolution sector image. The efficiency of subaperture processing for different system configurations is tested on experimental rf data acquired from two different phantoms using a 3.5 MHz, 128-element transducer array. The proposed subaperture processing reduces the number of firings for data acquisition, and thus allows real-time imaging with very low susceptibility to motion artifacts.

Adaptive multi-element synthetic aperture imaging with motion and phase aberration correction

Multi-element synthetic aperture techniques employing subaperture processing over successive firing steps can produce good image quality with simple front-end hardware but are susceptible to motion and phase aberration artifacts. We explore correlation processing using fully common spatial frequencies of overlapping subapertures to adapt beamforming for motion and phase aberrations. Signals derived from the subset of elements representing common spatial frequencies exhibit significantly higher correlation coefficients than those from signals computed using the entire subaperture. In addition, the correlation coefficient decreases linearly with subaperture separation for complete subaperture signals, but remains nearly constant with subaperture separation if only common spatial frequencies are used. Adaptive multi-element synthetic aperture imaging with correlation processing using fully common spatial frequencies is tested on experimental RF data acquired from a diffuse scattering phantom using a 3.5 MHz, 128-element transducer array. The results indicate that common spatial frequencies can be used efficiently for correlation processing to correct motion and phase aberration for adaptive multi-element synthetic aperture imaging.