Medical Ultrasound Systems Research Articles

Design and fabrication of a high frequency annular array for medical ultrasound systems

Most high frequency (>15MHz) medical ultrasound systems are based on single element transducers mechanically scanned. These systems can provide images with excellent resolution. However, single element transducers are often limited by the fixed focal point and small depth of field. High frequency medical ultrasound annular arrays consisting of concentric rings of elements are focused electronically. These arrays are desirable to avoid the fixed focal point of the single element transducers and improve the depth of field. This paper demonstrate an over 10MHz annular array transducer with novel piezoelectric single crystal. This transducer exhibits good energy conversion performance with a very low insertion loss at the center frequency. The transducer is promising for intravascular ultrasonic imaging and other applications. This work was supported by the National Natural Science Foundation of China (Grant Nos. 10904093 and 61031003), the Science and Technology Grant Scheme funds from Shenzhen Government (No. 08CXY-23).

DESIGN AND FABRICATION OF A 20 MHz ANNULAR ARRAY FOR MEDICAL ULTRASOUND

Most high frequency (> 15 MHz) medical ultrasound systems are based on single element transducers mechanically scanned. These systems can provide images with excellent resolution. However, single element transducers are often limited by the fixed focal point and small depth of field. Annular arrays consisting of concentric rings of elements are focused electronically. These arrays are desirable to avoid the fixed focal point of the single element transducers and improve the depth of field. This paper reports the design, fabrication, and characterization of a 5-element equal-area annular array transducer. After electrical impedance matching, the average center frequency was 20 MHz and -6 dB bandwidths ranged from 34 to 42%. The ILs for the matched annuli ranged from 6.1 to 26.5 dB.

Design space exploration of adaptive beamforming acceleration for bedside and portable medical ultrasound imaging

The use of adaptive beamforming is a viable solution to provide high-resolution real-time medical ultrasound imaging. However, the increase in image resolution comes at an expense of a significant increase in compute requirement over conventional algorithms. In a bedside diagnosis setting where plug-in power is available, GPUs are promising accelerators to address the processing demand. However, in the case of point-of-care diagnostics where portable ultrasound imaging devices must be used, alternative power-efficient computer systems must be employed, possibly at the expense of lower image resolution in order to maintain real-time performance. This paper presents an initial design space exploration on viable compute architectures that might address the drastically different requirements between bedside and portable medical ultrasound imaging systems using adaptive beamforming. The design and implementation of a GPU accelerator that provides over 45x performance improvement over the equivalent C implementation on a single CPU is presented. Furthermore, and implementation of the beamforming algorithm on a high-performance mobile platform based on an ARM Cortex A8 mobile processor in combination with the built-in NEON accelerator is also presented. The mobile platform delivers over 270x reduction in power consumption when compared to the GPU platform at an expense of much reduced performance. The tradeoffs between power, performance and image quality among the target platforms are studied and future research directions in power-efficient architectures for high-performance medical ultrasound systems are presented.

Development and Application of Piezoelectric Materials for Ultrasound Generation and Detection

The piezoelectric effect and its converse are the primary means used in biomedical ultrasound for converting acoustical energy into electrical energy and vice versa. Piezoelectricity has found many bioengineering applications ranging from ultrasound imaging and therapeutics, to piezoelectric surgery and microelectromechanical systems, and to biomedical implants with associated energy harvesting. Because of its fundamental importance to the proper functioning of most medical ultrasound systems, it is important to gain a general understanding of the effect, the history of its development and from this, an appreciation of its limitations and advantages in the generation and detection of ultrasound. This article describes the historical evolvement associated with its use in relation to most medical ultrasound applications and is intended to serve as an introduction for non-expert readers.

Medical Ultrasound Imaging: To GPU or Not to GPU?

Medical ultrasound imaging stands out from other modalities in providing real-time diagnostic capability at an affordable price while being physically portable. This article explores the suitability of using GPUs as the primary signal and image processors for future medical ultrasound imaging systems. A case study on synthetic aperture (SA) imaging illustrates the promise of using high-performance GPUs in such systems.

Medical ultrasound systems

Medical ultrasound imaging has advanced dramatically since its introduction only a few decades ago. This paper provides a short historical background, and then briefly describes many of the system features and concepts required in a modern commercial ultrasound system. The topics addressed include array beam formation, steering and focusing; array and matrix transducers; echo image formation; tissue harmonic imaging; speckle reduction through frequency and spatial compounding, and image processing; tissue aberration; Doppler flow detection; and system architectures. It then describes some of the more practical aspects of ultrasound system design necessary to be taken into account for today's marketplace. It finally discusses the recent explosion of portable and handheld devices and their potential to expand the clinical footprint of ultrasound into regions of the world where medical care is practically non-existent. Throughout the article reference is made to ways in which ultrasound imaging has benefited from advances in the commercial electronics industry. It is meant to be an overview of the field as an introduction to other more detailed papers in this special issue.

In vitro estimation of mean sound speed based on minimum average phase variance in medical ultrasound imaging

Effective receive beamforming in medical ultrasound imaging is important for enhancing spatial and contrast resolution. In current ultrasound receive beamforming, a constant sound speed (e.g., 1540 m/s) is assumed. However, the variations of sound speed in soft tissues could introduce phase distortions, leading to degradation in spatial and contrast resolution. This degradation becomes even more severe in imaging fatty tissues (e.g., breast) and with obese patients. In this paper, a mean sound speed estimation method where phase variance of radio-frequency channel data in the region of interest is evaluated is presented for improving spatial and contrast resolution. The proposed estimation method was validated by the Field II simulation and the tissue mimicking phantom experiments. In the simulation, the sound speed of the medium was set to 1450 m/s and the proposed method was capable of capturing this value correctly. From the phantom experiments, the −18-dB lateral resolution of the point target at 50 mm obtained with the estimated mean sound speed was improved by a factor of 1.3, i.e., from 3.9 mm to 2.9 mm. The proposed estimation method also provides an improvement of 0.4 in the contrast-to-noise ratio, i.e., from 2.4 to 2.8. These results indicate that the proposed mean sound speed estimation method could enhance the spatial and contrast resolution in the medical ultrasound imaging systems.

Development of advanced medical ultrasound systems

Development of advanced medical ultrasound systems

Inverter-based 1 V analog front-end amplifiers in 90 nm CMOS for medical ultrasound imaging

In this paper, we present the design and experimental evaluation of 1 V analog front-end amplifiers designed in 90 nm CMOS technology for capacitive micro-machined ultrasound transducers (CMUTs) for medical ultrasound imaging systems. We propose two front-end amplifier topologies based on an inverter-based cascode amplifier; the first is a continuous time amplifier and the second is a charge sampling amplifier (CSA). The proposed front-end amplifiers are designed to amplify the signals from CMUTs in the frequency bandwidth from 15 to 45 MHz with a centre frequency of 30 MHz. From the measurements, the continuous time single-ended transimpedance amplifier achieves a voltage gain of 19 dB, an output noise power spectral density of 0.042 (μV)/SQRT(Hz) at a centre-frequency of 30 MHz, and a total harmonic distortion of −23 dB at 450 mV p–p output voltage at 30 MHz input signal frequency. It draws only 598 μA per amplifier from a 1 V power supply. Its area measured only about 32 μm × 32 μm per amplifier. On the other hand, a sampling based front-end amplifier [CSA] achieves a transfer gain of 17.4 dB at an input signal frequency of 30 MHz and an upper 3 dB cut-off frequency of 46 MHz at a sampling clock frequency of 100 MHz. It consumes 586 μA per amplifier from a 1 V power supply and achieves a signal-to-noise (SNR) ratio of 45.7 dB with a peak-to-peak output signal amplitude of 500 mV at a sampling frequency of 100 MHz. It occupies an area of 1470.2 μm2 (which is equivalent to 38 μm × 38 μm), which also includes the area of the switches for the CSA that will be used for the single CMUT element.

Parametric imaging for characterizing focal liver lesions in contrast-enhanced ultrasound

The differentiation between benign and malignant focal liver lesions plays an important role in diagnosis of liver disease and therapeutic planning of local or general disease. This differentiation, based on characterization, relies on the observation of the dynamic vascular patterns (DVP) of lesions with respect to adjacent parenchyma, and may be assessed during contrast-enhanced ultrasound imaging after a bolus injection. For instance, hemangiomas (i.e., benign lesions) exhibit hyper-enhanced signatures over time, whereas metastases (i.e., malignant lesions) frequently present hyperenhanced foci during the arterial phase and always become hypo-enhanced afterwards. The objective of this work was to develop a new parametric imaging technique, aimed at mapping the DVP signatures into a single image called a DVP parametric image, conceived as a diagnostic aid tool for characterizing lesion types. The methodology consisted in processing a time sequence of images (DICOM video data) using four consecutive steps: (1) pre-processing combining image motion correction and linearization to derive an echo-power signal, in each pixel, proportional to local contrast agent concentration over time; (2) signal modeling, by means of a curve-fitting optimization, to compute a difference signal in each pixel, as the subtraction of adjacent parenchyma kinetic from the echopower signal; (3) classification of difference signals; and (4) parametric image rendering to represent classified pixels as a support for diagnosis. DVP parametric imaging was the object of a clinical assessment on a total of 146 lesions, imaged using different medical ultrasound systems. The resulting sensitivity and specificity were 97% and 91%, respectively, which compare favorably with scores of 81 to 95% and 80 to 95% reported in medical literature for sensitivity and specificity, respectively.

Photoacoustic tomography: a potential new tool for prostate cancer

The feasibility of photoacoustic tomography (PAT) for noninvasive imaging of prostate cancer was explored through the study on a canine model in vivo. Imaging of blood-rich lesions mimicking prostate tumors was achieved using a commercial medical ultrasound (US) system without affecting its original imaging functions. Based on the optical contrast between hemoglobin and other tissues, PAT has demonstrated good sensitivity and high contrast-to-noise ratio in visualizing deep lesions; while US has presented the morphological features including the boundary and the urethral of the prostate. PAT of prostate cancer may facilitate improved tumor localization, staging of disease, and detection of recurrences.

Principle and performance of the transverse oscillation vector velocity technique in medical ultrasound.

Medical ultrasound systems measure the blood velocity by tracking the blood cells motion along the ultrasound field. This is done by pulsing in the same direction a number of times and then finding e.g., the shift in phase between consecutive pulses. Properly normalized, this is directly proportional to the axial blood velocity. A major drawback is that only the axial velocity component is found. Often the lateral component is most important as blood vessels run parallel to the skin surface. The talk presents the transverse oscillation approach, which also can find the lateral velocity component by using a double oscillating field. A special estimator is then used for finding both the axial and lateral velocity components, so that both magnitude and phase can be calculated. The method for generating double oscillating ultrasound fields and the special estimator are described, and its performance revealed for a flow rig setup. Several examples from the clinical use of the approach are shown. From these it is seen that both velocity magnitude and angle varies temporally and spatially across the cardiac cycle, and it is, thus, important to estimate both continuously over the image region and time.

Estimation of speed of sound in dual-layered media using medical ultrasound image deconvolution

The speed of sound in soft tissues is usually assumed to be 1540 m/s in medical pulse-echo ultrasound imaging systems. When the true speed is different, the mismatch can lead to distortions in the acquired images, and so reduce their clinical value. Previously we reported a new method of sound-speed estimation in the context of image deconvolution. Unlike most other sound-speed estimation methods, this enables the use of unmodified ultrasound machines and a normal scanning pattern. Our approach was validated for largely homogeneous media with single sound speeds. In this article, we demonstrate that sound speeds of dual-layered media can also be estimated through image deconvolution. An ultrasound simulator has been developed for layered media assuming that, for moderate speed differences, the reflection at the interface may be neglected. We have applied our dual-layer algorithm to simulations and in vitro phantoms. The speed of the top layer is estimated by our aforesaid method for homogeneous media. Then, when the layer boundary position is known, a series of deconvolutions are carried out with dual-layered point-spread functions having different lower-layer speeds. The best restoration is selected using a correlation metric. The error level (e.g., a mean error of −9 m/s with a standard deviation of 16 m/s) for in vitro phantoms is found to be not as good as that of our single-speed algorithm, but is comparable to other local speed estimation methods where the data acquisition may not be as simple as in our proposed method.

Stereoscopic three or four dimensional ultrasound imaging

Medical diagnostic ultrasound stereo imaging is provided. A medical diagnostic ultrasound system operable to scan a body with ultrasound is also operable to generate a three dimensional stereoscopic view of the body. The video processing unit and/or display device create the stereoscopic display.

Polyimide-Based Conformal Ultrasound Transducer Array for Needle Guidance

A conformal ultrasound imaging system has been developed that may potentially decrease the user variability associated with operation of rigid transducer-based medical ultrasound systems. Conformal transducer arrays were built and characterized, featuring bulk piezoelectric elements mounted on an array of silicon islands, connected with flexible polyimide joints. The joints successfully supported electrodes while bending, and were sufficiently soft to enable both conformation to body surfaces and needle insertion for ultrasound guidance procedures. A signal-to-noise ratio (SNR) of 39 dB was achieved from a needle target in a soft tissue phantom. A space-time backward propagation image reconstruction algorithm was simulated to demonstrate needle detection using a conformal transducer with an arbitrary array architecture.

A Fully Programmable Computing Architecture for Medical Ultrasound Machines

Application-specific ICs have been traditionally used to support the high computational and data rate requirements in medical ultrasound systems, particularly in receive beamforming. Utilizing the previously developed efficient front-end algorithms, in this paper, we present a simple programmable computing architecture, consisting of a field-programmable gate array (FPGA) and a digital signal processor (DSP), to support core ultrasound signal processing. It was found that 97.3% and 51.8% of the FPGA and DSP resources are, respectively, needed to support all the front-end and back-end processing for B-mode imaging with 64 channels and 120 scanlines per frame at 30 frames/s. These results indicate that this programmable architecture can meet the requirements of low- and medium-level ultrasound machines while providing a flexible platform for supporting the development and deployment of new algorithms and emerging clinical applications.

Feedback Biasing in Nanoscale CMOS Technologies

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This work rediscovers the attractiveness of feedback biasing when applied to circuits designed in nanoscale CMOS technologies. It is shown that very compact amplifiers can be obtained by utilizing a type of biasing that imposes minimal area overhead. We discuss how the undesired features of the nanoscale CMOS technologies actually help in the revival of this simple biasing method in newer technology generations. The measurement results of prototyped common-source (CS) amplifiers utilizing feedback biasing for application in medical ultrasound imaging systems are presented in this brief. The proposed feedback biasing is also suitable for amplifying signals from high-impedance sources that pose challenges on maintaining high input impedance for the voltage amplifiers while maintaining a very low input capacitance value. Measurements show that the proposed amplifier achieves a voltage gain of 28 dB, an output noise power spectral density of 0.11 <formula formulatype="inline"><tex Notation="TeX">$(\mu\hbox{V})^{2}/ \hbox{Hz}$</tex></formula> at center-frequency, and a total harmonic distortion of <formula formulatype="inline"> <tex Notation="TeX">$-$</tex></formula>30 dB, with the full-scale output at 30 MHz, while drawing 120 <formula formulatype="inline"> <tex Notation="TeX">$\mu\hbox{A}$</tex></formula> from a 1-V power supply. The amplifiers were fabricated in 90-nm CMOS technology and measured to be just <formula formulatype="inline"><tex Notation="TeX">$20\ \mu\hbox{m} \times 10\ \mu\hbox{m}$</tex></formula>. </para>

Micro flexible ultrasound imaging array systems.

Medical ultrasonography has continued to be an important biomedical imaging modality for its various advantages. Conventional medical ultrasound systems typically utilize a rigid transceiver phase array, with a relatively small transceiver contact area. A micro conformal system increases observation aperture by using a large, flexible transducer array that conforms to curved body surfaces, providing increased angular coverage and 3‐D imagery. In addition, the frequency‐modulated continuous wave (FMCW) signaling format requires lower peak power than pulse‐echo systems and exhibit more favorable receiver noise characteristics. This paper provides an overview of the hardware development of the micro flexible ultrasound array system. The image formation procedure is first presented in the form of pulse‐echo imaging modality. Subsequently, image reconstruction algorithms are formulated for the FMCW imaging format. In addition, the equivalence between the pulse‐echo and FMCW imaging modes is developed. This enables us to conduct image reconstruction process in both space as well as spatial‐frequency domain. The presentation includes theoretical analysis, image formation algorithms, and laboratory experiments. [This research project was supported by the Telemedicine and Advanced Technology Research Center (TATRC), Department of Defense under award numbers W81XWH‐07‐1‐0672 and W81XWH‐07‐1‐0668.]

Diagnostic medical ultrasound systems and transducers utilizing micro-mechanical components

Diagnostic medical ultrasound systems and transducers utilizing micro-mechanical components

Timing-error-difference calibration using reciprocal signals

Timing errors in transmission and reception electronic channels of medical ultrasound imaging systems are generally smaller than one-tenth of a wavelength and do not influence the focusing quality of the system. However, these errors influence the performance of the near-field-signal-redundancy algorithm for correcting phase-aberrations generated by speed heterogeneity in the medium due to its high sensitivity to errors. The effect of timing errors is to make the transmission and reception phase-aberration profiles different. When the difference is much smaller than the period of the signal, an algorithm has been proposed in a previous work to measure the average of the transmission and reception phase-aberration profiles, and it can be used as an approximation to correct phase-aberrations on both transmission and reception. However, when the difference is large, the transmission and reception phase-aberration profiles need to be measured separately. In this paper, several algorithms that use reciprocal signals are proposed to measure the difference profile of the transmission and reception phase-aberration profiles. Their performances are theoretically analyzed, simulated, and experimentally tested. From the measured average and difference profiles, the transmission and reception phase-aberration profiles can be derived separately and used to correct phase-aberrations on transmission and reception, respectively.