An FPGA-based open platform for ultrasound biomicroscopy

Development of Single-Channel Dual-Element Custom-Made Ultrasound Scanner with Miniature Optical Position Tracker for Freehand Imaging

Handheld ultrasound has great potential in resource-limited areas, and can improve healthcare for rural populations. Single-channel ultrasound has been widely used in many clinical ultrasound applications, and optical tracking is considered accurate and reliable. In this study, we developed a 10 MHz lead magnesium niobate-lead titanate (PMN-PT) dual-element ultrasound transducer combined with a miniature optical position tracker, and then measured the rectus femoris of the thigh, upper arm, and cheek muscles. Compared to single-element transducers, dual-element transducers improve the contrast of near-field signals, effectively reduce noise, and are suitable for measuring curved surfaces. The purpose of position tracking is to calculate the location of the ultrasound transducer during the measurement process. By utilizing positioning information, 2D ultrasound imaging can be achieved while maintaining structural integrity. The dual-element ultrasound scanner presented in this study can enable continuous scanning over a large area without a scanning width limitation. The custom-made dual-element ultrasound scanner has the advantage of being a portable, reliable, and low-cost ultrasound device, and is helpful in popularizing medical care for remote villages.

High-Frequency Ultrasound Imaging With Sub-Nyquist Sampling

Implementation of a high-frequency ultrasound (HFUS) beamformer is computationally challenging because of its high sampling rate. This article introduces an efficient beamformer with sub-Nyquist sampling (or bandpass sampling) that is suitable for HFUS imaging. Our approach used channel radio frequency data sampled at bandpass sampling rate (i.e., 4/ 3f_c ) and postfiltering-based interpolation to reduce the computational complexity. A polyphase structure for interpolation was used to further reduce the computational burden while maintaining an adequate delay resolution ( δ ). The performance of the proposed beamformer (i.e., 4/ 3f_c sampling with sixfold interpolation, δ = 8f_c ) was compared with that of the conventional method (i.e., 4f_c sampling with fourfold interpolation, δ = 16f_c ). Ultrafast coherent compounding imaging was used in simulation, in vitro and in vivo imaging experiments. Axial/lateral resolution and contrast-to-noise ratio (CNR) values were measured for quantitative evaluation. The number of transmit pulse cycles was varied from 1 to 3 using two transducers with different fractional bandwidths (67% and 98%). In the simulation, the proposed and conventional methods showed the similar -6-dB axial beam widths (63.5 and 61.5 μm , respectively) from the two-cycle transmit pulse using the transducer with a bandwidth of 67%. In vitro and in vivo imaging experiments were performed using a Verasonics ultrasound research platform equipped with a high-frequency array transducer (20-46 MHz). The in vitro imaging results using a wire target showed consistent results with the simulation study (i.e., disparity at -6-dB axial resolution). The in vivo feasibility study with a murine mouse model with breast cancer was also performed, and the proposed method yielded a similar image quality compared with the conventional method. From these studies, it was demonstrated that the proposed HFUS beamformer based on the bandpass sampling can substantially reduce the computational complexity while minimizing the loss of spatial resolution for HFUS imaging.

Ultrawide Bandwidth High-Frequency Ultrasonic Transducers With Gradient Acoustic Impedance Matching Layer for Biomedical Imaging

The high-frequency ultrasonic transducers with larger bandwidths yield excellent imaging performance in the biomedical field. However, achieving perfect acoustic impedance matching from the piezo-element to the target medium in the operating frequency spectrum is still a challenge. Conventional matching layers are mostly fabricated by only one or two uniform materials which are limited by their acoustic property. We propose a novel composite matching layer with gradient acoustic impedance based on a 1-3 gradient composite structure and multilevel matching theory. The proposed gradient-composite matching layer applied for ultrasonic transducer provides efficient impedance matching and ultrawide bandwidth which can significantly improve the quality of biomedical imaging. The active aperture size of the matching layer is 5× 5 mm², and the overall thickness for five equivalent layers is 115 [Formula: see text]. The -6-dB bandwidth and the center frequency obtained by the ultrasonic transducer equipped with the 1-3 gradient composite matching layer are 141.7% and 22.3 MHz, respectively. The exceedingly good imaging performance of the fabricated ultrasonic transducer was demonstrated by the tungsten wire phantom and study on the biological tissues of a zebrafish and porcine eyeball. The theoretical and experimental results provide a novel train of thought for improving the quality of biomedical ultrasonic imaging.

Optimization Design of Ultrasonic Transducer With Multimatching Layer

An optimization design strategy is developed for ultrasonic transducer (UT) with multimatching layer to improve its performance. The piezoelectric equivalent circuit model is used to determine the optimization interval of matching layer, and the PiezoCAD software is used to simulate the performance of UT with multimatching layer. The neural network (NN) models are trained by the simulation data to characterize the relationship between the thickness of matching layer and performance of UT. Then, the multiobjective optimality criteria for UT is established based on its performance parameters, including center frequency (CF), -6 dB bandwidth (BW) and pulsewidth (PW). The thickness of matching layer is optimized by particle swarm optimization (PSO) algorithm. According to the designed performance, the optimized copper thickness and parylene thickness are about 17.76 and [Formula: see text], respectively. The simulation results of UT with the optimized multimatching layer well agree with the designed targets. Also, CF, -6 dB BW, and PW of the fabricated UT with the optimized multimatching layer are 5.672 MHz, 50.08%, and [Formula: see text], respectively, which nearly achieve the designed performance. In addition, the performance of UT with the optimized multimatching layer is much better than that of UT without matching layer. Moreover, compared with UT with single or double matching layers determined by the quarter wavelength theory, the UT with the optimized multimatching layer has better comprehensive performance. Finally, the fabricated UT with the optimized multimatching layer is used to measure the thickness of testing block, and the relative errors are all less than 1.0%, which implies that the optimized UT has excellent performance.

Synthetic Aperture Imaging Using High-Frequency Convex Array for Ophthalmic Ultrasound Applications

High-frequency ultrasound (HFUS) imaging has emerged as an essential tool for pre-clinical studies and clinical applications such as ophthalmic and dermatologic imaging. HFUS imaging systems based on array transducers capable of dynamic receive focusing have considerably improved the image quality in terms of spatial resolution and signal-to-noise ratio (SNR) compared to those by the single-element transducer-based one. However, the array system still suffers from low spatial resolution and SNR in out-of-focus regions, resulting in a blurred image and a limited penetration depth. In this paper, we present synthetic aperture imaging with a virtual source (SA-VS) for an ophthalmic application using a high-frequency convex array transducer. The performances of the SA-VS were evaluated with phantom and ex vivo experiments in comparison with the conventional dynamic receive focusing method. Pre-beamformed radio-frequency (RF) data from phantoms and excised bovine eye were acquired using a custom-built 64-channel imaging system. In the phantom experiments, the SA-VS method showed improved lateral resolution (>10%) and sidelobe level (>4.4 dB) compared to those by the conventional method. The SNR was also improved, resulting in an increased penetration depth: 16 mm and 23 mm for the conventional and SA-VS methods, respectively. Ex vivo images with the SA-VS showed improved image quality at the entire depth and visualized structures that were obscured by noise in conventional imaging.

Quantitative characterization of the colorectal cancer in a rabbit model using high-frequency endoscopic ultrasound

PURPOSE

Colonoscopy accompanied with biopsy works as the routine endoscopic strategy for the diagnosis of colorectal cancer (CRC) in clinic; however, the colonoscopy is limited to the tissue surface. During the last decades, enabling technologies are emerging to complement with the colonoscopy for better administration of CRC. The conventional low-frequency (<12 MHz) endoscopic ultrasound (EUS) guided fine-needle aspiration (FNA) has been widely used to assess the lesion penetration. With the high-frequency ultrasound transducer (>20 MHz), EUS allows more precise visualization of the colorectal abnormalities. In order to achieve the accurate detection or in situ characterization of the colorectal lesions, the EUS diagnosis needs more patho-physiological related information in the micro-structural or molecular level. Quantitative ultrasound (QUS) technique, which could extract the micro-structural information from the ultrasound radio-frequency (RF) signal, is promising for the non-invasive tissue characterization. To date, the knowledge of the high-frequency endoscopic QUS for the CRC characterization has not been fully determined.

METHODS

In this work, to our best knowledge, it is the first application of the QUS technique based on a customized high-frequency EUS system (30.5 MHz center frequency) to characterize the colorectal malignancies in a VX2 rabbit CRC model. To eliminate the response from the ultrasound electronic system and transducer, the ultrasound signals from colon tissue were calibrated. And, the resulting quasi-liner ultrasound spectra were fit by the linear regression test. As a result, three spectral parameters, including the slope (k), intercept (I) and Midband Fit (M), were obtained from the best-fit line. The three spectral parameters were compared between the malignant tissue regions and adjacent normal tissue regions of the colon tissue specimen ex vivo. The independent t-test was conducted between the three parameters from the normal and malignant group. The statistical method of Fisher Linear Discriminant (FLD) was used to explore the linear combinations of the three parameters, so as to provide more tissue micro-structural features than the single parameter alone. The three FLD values were derived from three different combinations among k, I and M. The threshold was selected from the statistical analysis to optimize the differentiation criteria between the malignant and the normal tissues. The color-coded images were used to display the local FLD values and combined with the EUS B-mode image.

RESULTS AND CONCLUSIONS

The mean Midband Fit (M) and intercept (I) showed significant differences between the malignant and normal tissue regions. The statistical analysis showed that there were significant differences in all the mean FLD values of the spectral parameter combinations (kI, kM and IM) (t test, P < 0.05). And, the combined image result from the B-mode image and color-coded image could visually correlate with the histology result. In conclusion, the high-frequency endoscopic QUS technique was potential to be used as a complementary method to distinguish the colorectal malignancies by leveraging its morphological and micro-structural ultrasound information.

Magnesium Alloy Matching Layer for PMN-PT Single Crystal Transducer Applications

In this paper, we propose using magnesium alloy as the matching layer for the ultrasonic transducer made of 0.68 Pb(Mg_1/3Nb_2/3)O₃-0.32PbTiO₃ (PMN-0.32PT) single crystal. The complete sets of elastic constants of AZ31B, GW83, and ZK60 magnesium alloys have been measured, which is practically a homogeneous material. The AZ31B magnesium alloy has an acoustic impedance of 10.36 MRayl, which is suitable for the development of high-performance ultrasonic transducers. A 3.5-MHz PMN-PT single crystal transducer has been designed and fabricated successfully using AZ31B magnesium alloy as the first quarter-wavelength matching layer. The -6-dB bandwidth and two-way insertion loss at the center frequency of the transducer are about 67% and 11.4 dB, respectively, much superior to the transducer fabricated using 0-3 composite matching layer. The high performance of this transducer indicates that the magnesium alloy is indeed an excellent matching layer material for ultrasonic transducer applications.

PMN-PT Single Crystal Ultrasonic Transducer With Half-Concave Geometric Design for IVUS Imaging

As the key component of intravascular ultrasound (IVUS) imaging systems, traditional commercial side-looking IVUS transducers are flat and unfocused, which limits their lateral resolution. We propose a PMN-PT single crystal IVUS transducer with a half-concave geometry. This unique configuration makes it possible to conduct geometric focusing at a desired depth. To compare performances, the proposed and the traditional flat transducer with similar dimensions were fabricated. We determined that the half-concave transducer has a slightly higher center frequency (35 MHz), significantly broader -6 dB bandwidth (54%) but a higher insertion loss (-22.4 dB) compared to the flat transducer (32 MHz, 28%, and -19.3 dB, respectively). A significant enhancement of the lateral resolution was also confirmed. The experimental results are in agreement with the finite element simulation results. This preliminary investigation suggests that the half-concave geometry design is a promising approach in the development of focused IVUS transducers with broad bandwidth and high lateral resolution.

Development of a Low-Complexity, Cost-Effective Digital Beamformer Architecture for High-Frequency Ultrasound Imaging

This paper presents a low-complexity, cost-effective digital beamformer architecture for a high-frequency ultrasound imaging system. The proposed beamformer uses a lookup table and linear interpolation methods for computing the dynamic receive focusing delays and a postfiltering technique to minimize hardware complexity. In the postfiltering technique, channel radio-frequency data having the same fractional delay (i.e., 16f₀ resolution) are aggregated prior to interpolation. Thus, only four polyphase structure filters are required in the developed beamformer. In addition, a quadrature bandpass filter that generates an analytic signal is utilized as an interpolation filter; this allows decimation during beam formation and a reduction in computational complexity. The proposed method was evaluated through a 20- [Formula: see text] wire phantom experiment, and the -6-dB lateral and axial resolutions obtained therein were measured and compared with those obtained using a conventional method. The same lateral (165 [Formula: see text]) and axial (80 [Formula: see text]) resolutions at a depth of 5.6 mm were obtained using both the methods, and the proposed method could reduce the beamforming points (i.e., computational complexity) by a factor of the decimation factor (≥4). Images from an excised bovine eye were captured; they showed that the proposed beamformer identified fine anatomical structures such as cornea or iris without compromising the spatial resolution and reduced the computational complexity.

Measurement of the Doppler power of flowing blood using ultrasound Doppler devices

Measurement of the Doppler power of signals backscattered from flowing blood (henceforth referred to as the Doppler power of flowing blood) and the echogenicity of flowing blood have been used widely to assess the degree of red blood cell (RBC) aggregation for more than 20 y. Many studies have used Doppler flowmeters based on an analogue circuit design to obtain the Doppler shifts in the signals backscattered from flowing blood; however, some recent studies have mentioned that the analogue Doppler flowmeter exhibits a frequency-response problem whereby the backscattered energy is lost at higher Doppler shift frequencies. Therefore, the measured Doppler power of flowing blood and evaluations of RBC aggregation obtained using an analogue Doppler device may be inaccurate. To overcome this problem, the present study implemented a field-programmable gate array-based digital pulsed-wave Doppler flowmeter to measure the Doppler power of flowing blood, in the aim of providing more accurate assessments of RBC aggregation. A clinical duplex ultrasound imaging system that can acquire pulsed-wave Doppler spectrograms is now available, but its usefulness for estimating the ultrasound scattering properties of blood is still in doubt. Therefore, the echogenicity and Doppler power of flowing blood under the same flow conditions were measured using a laboratory pulser-receiver system and a clinical ultrasound system, respectively, for comparisons. The experiments were carried out using porcine blood under steady laminar flow with both RBC suspensions and whole blood. The experimental results indicated that a clinical ultrasound system used to measure the Doppler spectrograms is not suitable for quantifying Doppler power. However, the Doppler power measured using a digital Doppler flowmeter can reveal the relationship between backscattering signals and the properties of blood cells because the effects of frequency response are eliminated. The measurements of the Doppler power and echogenicity of flowing blood were compared with those obtained in several previous studies.

Piezoelectric single crystals for ultrasonic transducers in biomedical applications

Piezoelectric single crystals, which have excellent piezoelectric properties, have extensively been employed for various sensors and actuators applications. In this paper, the state-of-art in piezoelectric single crystals for ultrasonic transducer applications is reviewed. Firstly, the basic principles and design considerations of piezoelectric ultrasonic transducers will be addressed. Then, the popular piezoelectric single crystals used for ultrasonic transducer applications, including LiNbO₃ (LN), PMN-PT and PIN-PMN-PT, will be introduced. After describing the preparation and performance of the single crystals, the recent development of both the single-element and array transducers fabricated using the single crystals will be presented. Finally, various biomedical applications including eye imaging, intravascular imaging, blood flow measurement, photoacoustic imaging, and microbeam applications of the single crystal transducers will be discussed.

A digital multigate Doppler method for high frequency ultrasound

Noninvasive visualization of blood flow with high frequency Doppler ultrasound has been extensively used to assess the morphology and hemodynamics of the microcirculation. A completely digital implementation of multigate pulsed-wave (PW) Doppler method was proposed in this paper for high frequency ultrasound applications. Analog mixer was eliminated by a digital demodulator and the same data acquisition path was shared with traditional B-mode imaging which made the design compact and flexible. Hilbert transform based quadrature demodulation scheme was employed to achieve the multigate Doppler acquisition. A programmable high frequency ultrasound platform was also proposed to facilitate the multigate flow visualization. Experimental results showed good performance of the proposed method. Parabolic velocity gradient inside the vessel and velocity profile with different time slots were acquired to demonstrate the functionality of the multigate Doppler. Slow wall motion was also recorded by the proposed method.

Multiple-site hemodynamic analysis of Doppler ultrasound with an adaptive color relation classifier for arteriovenous access occlusion evaluation

This study proposes multiple-site hemodynamic analysis of Doppler ultrasound with an adaptive color relation classifier for arteriovenous access occlusion evaluation in routine examinations. The hemodynamic analysis is used to express the properties of blood flow through a vital access or a tube, using dimensionless numbers. An acoustic measurement is carried out to detect the peak-systolic and peak-diastolic velocities of blood flow from the arterial anastomosis sites (A) to the venous anastomosis sites (V). The ratio of the supracritical Reynolds (Re(supra)) number and the resistive (Res) index quantitates the degrees of stenosis (DOS) at multiple measurement sites. Then, an adaptive color relation classifier is designed as a nonlinear estimate model to survey the occlusion level in monthly examinations. For 30 long-term follow-up patients, the experimental results show the proposed screening model efficiently evaluates access occlusion.

A flexible annular-array imaging platform for micro-ultrasound

Micro-ultrasound is an invaluable imaging tool for many clinical and preclinical applications requiring high resolution (approximately several tens of micrometers). Imaging systems for micro-ultrasound, including single-element imaging systems and linear-array imaging systems, have been developed extensively in recent years. Single-element systems are cheaper, but linear-array systems give much better image quality at a higher expense. Annular-array-based systems provide a third alternative, striking a balance between image quality and expense. This paper presents the development of a novel programmable and real-time annular-array imaging platform for micro-ultrasound. It supports multi-channel dynamic beamforming techniques for large-depth-of-field imaging. The major image processing algorithms were achieved by a novel field-programmable gate array technology for high speed and flexibility. Real-time imaging was achieved by fast processing algorithms and high-speed data transfer interface. The platform utilizes a printed circuit board scheme incorporating state-of-the-art electronics for compactness and cost effectiveness. Extensive tests including hardware, algorithms, wire phantom, and tissue mimicking phantom measurements were conducted to demonstrate good performance of the platform. The calculated contrast-to-noise ratio (CNR) of the tissue phantom measurements were higher than 1.2 in the range of 3.8 to 8.7 mm imaging depth. The platform supported more than 25 images per second for real-time image acquisition. The depth-of-field had about 2.5-fold improvement compared to single-element transducer imaging.