Medical Ultrasound Scanners Research Articles

Performance Evaluation of Displacement Estimators for Real-Time Ultrasonic Strain and Blood Flow Imaging With Improved Spatial Resolution

Ultrasonic strain and blood flow imaging has great potential to improve early tumor detection. Both methods have the same task of estimating echo signal displacements. To be able to resolve tissue stiffness and vascularity of tumors while they are still small, spatial resolution must be improved. Moreover, ultrasonic real-time imaging is vital in clinical practice. In this paper, we evaluate the estimation performances of one novel and several established displacement estimators with real-time capability and compare these with results from cross-correlation-based standard estimators when echo signals that are typically available with a commercial ultrasound scanner set to yield the highest spatial resolution are used. Estimation errors are investigated as a function of sampling rate, signal-to-noise ratio, displacement and strain magnitudes, and frequency-dependent tissue attenuation. Results are from simulations using the FIELD II program to generate ultrasonic echo signals and tissue-mimicking phantom experiments using a Voluson 730 medical ultrasound scanner (GE medical systems Kretztechnik, Austria) that is equipped with a digital research interface that enables the export of beam-formed radio frequency echo data.

Characteristics of the audio sound generated by ultrasound imaging systems

Medical ultrasound scanners use high-energy pulses to probe the human body. The radiation force resulting from the impact of such pulses on an object can vibrate the object, producing a localized high-intensity sound in the audible range. Here, a theoretical model for the audio sound generated by ultrasound scanners is presented. This model describes the temporal and spectral characteristics of the sound. It has been shown that the sound has rich frequency components at the pulse repetition frequency and its harmonics. Experiments have been conducted in a water tank to measure the sound generated by a clinical ultrasound scanner in various operational modes. Results are in general agreement with the theory. It is shown that a typical ultrasound scanner with a typical spatial-peak pulse-average intensity value at 2 MHz may generate a localized sound-pressure level close to 100 dB relative to 20 microPa in the audible (< 20 kHz) range under laboratory conditions. These findings suggest that fetuses may become exposed to a high-intensity audio sound during maternal ultrasound examinations. Therefore, contrary to common beliefs, ultrasound may not be considered a passive tool in fetal imaging.

From bench to bedside: Nonlinear bio-ultrasound

In the early 1960s, David Blackstock developed the weak shock theory which characterized the development and evolution of harmonics in a finite-amplitude propagating plane wave. Later work developed an understanding of beam patterns of the fundamental and higher harmonics from piston radiators. All of these concepts were germane to the field of bio-ultrasound. However, nonlinear effects were largely unexploited in bio-ultrasound until the 1980s, when the role of finite-amplitude mechanisms was recognized as an enhancer of heating and lesion production in high intensity applications. Later, in the 1990s, the role and the advantage of higher harmonics in ultrasonic imaging were developed. In particular, the higher harmonics generated by nonlinear effects, especially in the presence of aberrating tissue, were shown to produce dramatic increases in the resolution and contrast of ultrasound images in cardiology and other applications. Today, every high-end medical ultrasound scanner has the capability of ‘‘harmonic imaging’’ for improved image quality in a number of applications. The physics and technology of this remarkable development will be reviewed, along with the important guiding role of David Blackstock and his collaborators, particularly at UT Austin and at U. Rochester.

Spectral characteristics of the sound generated by ultrasound imaging systems in the human body

Medical ultrasound scanners use high-energy pulses to probe the human body. It can be shown that the radiation force resulting from the impact of such pulses to the object can vibrate the object, producing a localized high-intensity noise in the audible range. When the scanner is used in a maternal examination, and the beam is directed toward the fetal head, this noise can disturb the fetus and cause it to move vigorously [J. Ultrasound Med. 20, 883–889 (2001)]. Here, a theoretical model for the audio noise generated by ultrasound scanners is presented. This model describes the temporal and spectral characteristics of the noise. It has been shown that the noise has rich frequency components at the pulse repetition frequency and its harmonics. Experiments have been conducted in a water tank to measure the noise generated by a clinical ultrasound scanner. Results indicate that the noise pressure level at the focal point of the ultrasound beam on a reflective object can reach up to 120 dB relative to 20 μP. This result is in agreement with the theoretical predictions. The noise was also measured in a human uterus, indicating intensities in the range of 90–100 dB.

Acoustics in medical ultrasound beamformation

Today’s medical ultrasound scanners continue to push the envelope with efforts to improve imaging performance. Increasingly this demands more sophisticated application of acoustics in the design of the beamformers for these scanners. In this paper, limitations of present designs are reviewed by the use of acoustic field simulations. These design limitations include spatial sampling of the transducer aperture and the contributions of two-dimensional apertures and the impact of variations in the speed of sound in the various tissues. The role of beamformation in defining the likely scatterer populations that will be included in any beam will be discussed. The introduction of harmonic imaging into medical ultrasound has changed the landscape significantly especially in fields such as echocardiography. This has generated the need to understand the harmonic generation process of the propagating acoustic wave. Comparisons of the acoustic beams due to the fundamental frequency spectrum and the harmonic spectra are discussed along with means by which the scanners take advantage of them. From this base one can begin to identify areas of further improvement and speculate on their likely impact on the diagnostic process.

The impact of sound speed errors on medical ultrasound imaging.

The results of a quantitative study of the impact of sound speed errors on the spatial resolution and amplitude sensitivity of a commercial medical ultrasound scanner are presented in the context of their clinical significance. The beamforming parameters of the scanner were manipulated to produce sound speed errors ranging over +/-8% while imaging a wire target and an attenuating, speckle-generating phantom. For the wire target, these errors produced increases in lateral beam width of up to 320% and reductions in peak echo amplitude of up to 10.5 dB. In the speckle-generating phantom, these errors produced increases in speckle intensity correlation cell area of up to 92% and reductions in mean speckle brightness of up to 5.6 dB. These results are applied in statistical analyses of two detection tasks of clinical relevance. The first is of low contrast lesion detectability, predicting the changes in the correct decision probability as a function of lesion size, contrast, and sound speed error. The second is of point target detectability, predicting the changes in the correct decision probability as function of point target reflectivity and sound speed error. Representative results of these analyses are presented and their implications for clinical imaging are discussed. In general, sound speed errors have a more significant impact on point target detectability over lesion detectability by these analyses, producing up to a 22% reduction in correct decisions for a typical error.

Flow field visualization of sediment-laden flow using ultrasonic imaging

This investigation concerns the application of particle image velocimetry (PIV) to ultrasonic images of sediment-laden flow in order to obtain accurate, quantitative, two dimensional vector maps of the flow field within the sediment suspension. Experiments were carried out using a medical ultrasound scanner to obtain ultrasonic images in a kaolin/salt water suspension of mean concentration 16 g/l and to compare results obtained with the ultrasound scanner in clear water with those from established measurement techniques. Measurements of mud were obtained, and the comparison with established methods indicated that the application of PIV to the analysis of ultrasound images yields velocity information accurate to within ±15%. This measurement technique will be of great value in furthering the understanding of the behavior of sediment-laden flows.

Numerical simulation of the acoustic field of a phased-array medical ultrasound scanner

In order to evaluate the possible side effects of diagnostic medical ultrasound, and in particular the effects of nonlinear propagation of sound, it is necessary to characterize the scanners used. To this end, the acoustic field emitted by a phased-array Hitachi EUB-26 ultrasound scanner operating in a strongly focused mode was measured a small distance from the scanner head, and the velocity field at the surface of the head deduced. A finite difference model was then used to propagate the pressure field beyond the focus, taking into account nonlinear effects, attenuation and diffraction, and the result compared with observations in water, giving reasonable agreement out to the scanplane focus. The extrapolated field was then examined in detail, revealing those regions in which most power is lost to the medium, some of which are off-axis, and so might not be detected by normal calibration procedures.

Multi-dimensional velocity estimation with ultrasound using spatial quadrature

Conventional ultrasonic motion or flow velocity estimation is limited to the axial velocity component. A novel technique, called "spatial quadrature," for the simultaneous estimation of axial and lateral velocity components is introduced. This technique utilizes even and odd spatial weightings of the point spread function that have a quandrature relationship. These weightings produce modulation in the received echo that can be decoded to quantify lateral motion. This estimator can be implemented for two-dimensional velocity estimation with a conventional transducer array and medical ultrasound scanner, preferably one with parallel receive processing. The technique also can be extended to three-dimensional velocity estimation given an appropriate 2-D transducer and scanner architecture. Discussed are space and frequency domain descriptions of this estimator in the context of a brief overview of 2-D flow estimation methods, including analyses characterizing the performance envelope of this estimator. In addition, the application of this technique in CW and PW regimes, and with both radio frequency and baseband quadrature (I- and-Q) data, are discussed. Phantom studies demonstrate spatial quadrature in 2-D using a clinical scanner.

Comparison of selected ultrasound performance tests with varying overall receiver gain and dynamic range, using conventional and magnified field of view.

Most ultrasound (US) scanner vendors currently offer a feature that allows a region of the ultrasound image to be magnified or zoomed. Although the methods of magnification vary among vendors, the ability to "zoom in" on a selected portion of the image has gained clinical acceptance. However, using this feature introduces additional steps in the quality assurance (QA) measurement procedures. No studies exist that demonstrate the advantage of a magnified field of view (FOV) over the conventional FOV for QA purposes. The purpose of this study was to investigate the influence of a magnified versus nonmagnified FOV on various common QA performance tests as a function of the overall receiver gain and dynamic range. Additionally, since the performance tests are subject to variations caused by scanner settings, sets of QC tests were recorded using several different scanner settings to investigate any change in the sensitivity of the QC measurements with respect to the magnified and nonmagnified fields of view. The lateral and axial resolution, slice thickness, and caliper accuracy (vertical and horizontal) as a function of varying overall receiver gain and dynamic range, were obtained using conventional (no zoom) as well as a magnified (zoom) field of view (FOV). Each measurement was performed three times by a single observer using a 4 MHz "vector" format transducer on a single diagnostic medical ultrasound scanner. The results show no statistical significance in the variability of most recorded masurements when using the conventional versus the magnified FOV. However, in some cases, such as lateral resolution, the average value measured using the magnified FOV was typically 0.5 mm lower than when using a conventional FOV.

Acoustic cavitation and bioeffects in the vascular system.

An in vivo bolus generation system has provided information concerning acoustic cavitation in blood and associated bioeffects. Microbubble boluses produced in the canine abdominal aorta by tone bursts of 1.8 MHz focused ultrasound were detected in distal arteries using medical ultrasound scanners and a resonant bubble detector. At the maximum acoustic intensity, 19,000 W/cm2(ISPTA), 125–250 ms bursts generated contrast enhancement persisting several cardiac cycles. Intensities down to 3,700 W/ cm2 yielded progressively weaker boluses while shorter and weaker boluses were produced with 64–0.5 ms bursts at the maximum acoustic intensity. There was a 50% occurrence of vascular wall effects (by histology) but no immediate mechanical failure. Oxygen ventilation has been shown to destabilize IV injected contrast agents and similarly, cavitation thresholds were elevated in the vena cava. Resonant bubble dectection indicated the presence of 7 μm diameter bubbles but the bubble size range is unknown. These results provide insight into bubble production by acoustic cavitation but any bioeffects must be examined in the context of arterial bolus production where the risk of significant damage may be less than that of alternative procedures. [Work supported in part by USPHS 5 RO1 DK42290.] a)Current address: Acuson, Inc., Sunnyvale, CA.

Optoelectronic transmitters for medical ultrasound transducers

Optoelectronics and fiber optics can be used to miniaturize and improve the flexibility of the transducer cable and transducer handle of medical diagnostic ultrasound scanners. The reduction in size has gained importance as 2-D array transducers with up to 1000 independent channels become accepted to improve diagnostic ultrasound images. The authors describe the analysis, design, fabrication and testing of a prototype silicon photoconductive semiconductor switch (PCSS). The monolithic silicon PCSS was used in combination with an infrared semiconductor diode laser with a fiber optic "pigtail" to shock excite and burst excite a 2-D array transducer element resonant at 2.5 MHz. Optically controlled voltage, current, and ultrasound pulses are compared to those from conventional electronic shock excitation and narrow band Doppler pulses. The optically triggered ultrasound pulse for single shock excitation produced 30 V spikes at the 2-D array element with a fall time of 200 nsec and a rise time of 2 /spl mu/sec with a peak current through the transducer element of 34 mA. An optically produced burst of eight pulses at a frequency of 2.5 MHz produced 11 V spikes at the transducer with a fall time under 100 nsec and a rise time of approximately 300 nsec. The peak current per pulse was 25 mA through the transducer element. These results show the feasibility of applying optoelectronic technology to replace conventional electronic transmitter technology.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Use of a medical ultrasonic scanner for the inspection of high voltage insulation

It is shown that standard medical ultrasound scanners can provide rapid inspection of insulating materials, detecting possible areas of field-induced degradation. The position and the extent of degradation within the material can be given. This work was performed on practical insulating structures and on pieces fabricated in the laboratory. The samples contain defects that simulate conditions that can occur in production processes and during the in-service life of insulation. The results are affected by fillers and by the structure of the material. Excellent results were obtained from cast polyurethane and epoxy resins containing the usual particular fillers in the usual proportions. >

Methods for Estimation of Statistical Properties of Envelopes of Ultrasonic Echoes from Myocardium

Several investigators have characterized various forms of heart disease from the statistical properties of envelopes of ultrasonic echos from myocardium. In particular, the mean-to-standard deviation ratio (MSR), skewness, and kurtosis of the envelope probability density function have been used for the detection of myocardial ischemia, infarction, reperfusion, and hypertrophy. In this paper, the effects of phenomena other than tissue acoustic properties upon estimates of statistical parameters are investigated. These include system characteristics (center frequency, bandwidth, beam width, etc.), sample volume dimensions, and tissue velocity. In myocardium, relatively small amounts of tissue are available for interrogation. It is shown that, under these limited data acquisition conditions, substantial systematic biases in the estimates of statistical parameters may occur. Analytic forms for errors in the envelope variance estimate are derived. Estimation of the envelope mean, variance, MSR, skewness, and kurtosis is investigated experimentally, using a commercial medical ultrasound scanner and a tissue-mimicking phantom.

High resolution multiline ultrasonic beamformer

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