Interframe clutter filtering for high frequency flow imaging

This study investigates 'interframe' clutter filtering with a high frequency (HF) flow imaging system with the objective of improving the performance of HF microvascular imaging at high frame rates. An interframe filter exploits the correlation of tissue signals on the time scale of the frame rate and is, therefore, insensitive to tissue spectral broadening induced by sweeping a single element transducer over a region of tissue. In vitro experiments were conducted in a tissue-mimicking flow phantom over a range of mean flow velocities (0.5 to 70.0 mm/s). Power Doppler (PD) imaging and color flow (CF) imaging were performed for both slow (0.25 fps) and fast (20 fps) scanning acquisitions. Flow data acquired at 20 fps and interframe filtered had similar velocity and mean Doppler power values as the 0.25 fps single-frame filtered data sets. In vivo validation experiments were conducted using a 500 microm blood vessel in a human finger and detected blood flow of 2 to 3 mm/s. Further in vivo experiments examining experimental murine tumors demonstrated the feasibility of performing HF PD and CF imaging at high frame rates using interframe filtering.     

High-frequency color flow imaging of the microcirculation

The extension of ultrasound (US) color flow imaging (CFI) techniques to high frequencies (> 20 MHz) has the potential to provide valuable noninvasive tools for scientific and clinical investigations of blood flow in the microcirculation. We describe the development of a slow-scan CFI system operating in the 20-100-MHz range that has been optimized to image the microcirculation. The apparatus has incorporated elements of a previously reported pulsed-wave Doppler system and is capable of operating in either CFI or pulsed-wave mode. The performance of the CFI system was evaluated at a center frequency of 50 MHz using two PVDF transducers with -6-dB beam widths of 43 and 60 microm. The -6 dB-axial resolutions were estimated to be 66 and 72 microm, respectively. In vivo validation experiments conducted using the murine ear model demonstrated the detection of flow in vessels down to 15-20 microm in diameter with flow velocities on the order of mm per s. Further experiments examining experimental murine tumors confirmed the successful detection of flow in the tumor microcirculation.     

High-Frequency Ex vivo Ultrasound Imaging of the Auditory System

A 50 MHz array-based imaging system was used to obtain high-resolution images of the ear and auditory system. This previously described custom built imaging system (3, 4 and 2) is capable of 50 μm axial resolution, and lateral resolution varying from 80 μm to 130 μm over a 5.12 mm scan depth. The imaging system is based on a 2 mm diameter, seven-element equal-area annular array, and a digital beamformer that uses high-speed field programmable gate arrays (FPGAs). The images produced by this system have shown far superior depth of field compared with commercially available single-element systems. Ex vivo, three-dimensional (3-D) images were obtained of human cadaveric tissues including the ossicles (stapes, incus, malleus) and the tympanic membrane. In addition, two-dimensional (2-D) images were obtained of an intact cochlea by imaging through the round window membrane. The basilar membrane inside the cochlea could clearly be visualized. These images demonstrate that high-frequency ultrasound imaging of the middle and inner ear can provide valuable diagnostic information using minimally invasive techniques that could potentially be implemented in vivo. (E-mail: J.Brown@dal.ca)     

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.     

Improved objective selection of power Doppler wall-filter cut-off velocity for accurate vascular quantification

The wall-filter selection curve method is proposed to objectively identify a cut-off velocity that minimizes artifacts in power Doppler images. A selection curve, which is constructed by plotting the color pixel density (CPD) as a function of the cut-off velocity, exhibits characteristic intervals hypothesized to include the optimum cut-off velocity. This article presents an improved implementation of the method that automatically detects characteristic intervals in a selection curve and selects an operating point cut-off velocity along a characteristic interval. The method is applied to subregions within the Doppler image to adapt the cut-off velocity to local variations in vascularity. The method's performance is evaluated in 30-MHz power Doppler images of a four-vessel flow phantom. At high (>5 mm/s) flow velocities, qualitative improvements in vessel delineation are achieved and the CPD in the resulting images is accurate to within 3% of the vascular volume fraction of the phantom.     

Effects of transducer, velocity, Doppler angle, and instrument settings on the accuracy of color Doppler ultrasound

The accuracy of a commercial color Doppler ultrasound (US) system was assessed in vitro using a rotating torus phantom. The phantom consisted of a thin rubber tube filled with a blood-mimicking fluid, joined at the ends to form a torus. The torus was mounted on a disk suspended in water, and rotated at constant speeds by a motor. The torus fluid was shown in a previous study to rotate as a solid body, so that the actual fluid velocity was dependent only on the motor speed and sample volume radius. The fluid velocity could, thus, be easily compared to the color Doppler-derived velocity. The effects of instrument settings, velocity and the Doppler angle was assessed in four transducers: a 2.0-MHz phased-array transducer designed for cardiac use, a 4.0-MHz curved-array transducer designed for general thoracic use, and two linear transducers designed for vascular use (one 4.0 MHz and one 6.0 MHz). The color Doppler accuracy was found to be significantly dependent on the transducer used, the pulse-repetition frequency and wall-filter frequency, the actual fluid velocity and the Doppler angle (p < 0.001 by analysis of variance). In particular, the phased array and curved array were observed to be significantly more accurate than the two linear arrays. The torus phantom was found to provide a sensitive measure of color Doppler accuracy. Clinicians need to be aware of these effects when performing color Doppler US exams.     

High-frequency subharmonic pulsed-wave Doppler and color flow imaging of microbubble contrast agents

A recent study has shown the feasibility of subharmonic (SH) flow imaging at a transmit frequency of 20 MHz. This paper builds on these results by examining the performance of SH flow imaging as a function of transmit pressure. Further, we also investigate the feasibility of SH pulsed-wave Doppler (PWD) imaging. In vitro flow experiments were performed with a 1-mm-diameter wall-less vessel cryogel phantom using the ultrasound contrast agent Definity and an imaging frequency of 20 MHz. The phantom results show that there is an identifiable pressure range where accurate flow velocity and power estimates can be made with SH imaging at 10 MHz (SH10), above which velocity estimates are biased by radiation force effects and unstable bubble behavior, and below which velocity and power estimates are degraded by poor SNR. In vivo validation of SH PWD was performed in an arteriole of a rabbit ear, and blood velocity estimates compared well with fundamental (F20) mode PWD. The ability to suppress tissue signals using SH signals may enable the use of higher frame rates and improve sensitivity to microvascular flow or slow velocities near large vessel walls by reducing or eliminating the need for clutter filters.     

A rotating torus phantom for assessing color Doppler accuracy

A rotating torus phantom was designed to assess the accuracy of color Doppler ultrasound. A thin rubber tube was filled with blood analog fluid and joined at the ends to form a torus, then mounted on a disk submerged in water and rotated at constant speeds by a motor. Flow visualization experiments and finite element analyses demonstrated that the fluid accelerates quickly to the speed of the torus and spins as a solid body. The actual fluid velocity was found to be dependent only on the motor speed and location of the sample volume. The phantom was used to assess the accuracy of Doppler-derived velocities during two-dimensional (2-D) color imaging using a commercial ultrasound system. The Doppler-derived velocities averaged 0.81 +/- 0.11 of the imposed velocity, with the variations significantly dependent on velocity, pulse-repetition frequency and wall filter frequency (p < 0.001). The torus phantom was found to have certain advantages over currently available Doppler accuracy phantoms: 1. It has a high maximum velocity; 2. it has low velocity gradients, simplifying the calibration of 2-D color Doppler; and 3. it uses a real moving fluid that gives a realistic backscatter signal.     

An ASIC design for versatile receive front-end electronics of an ultrasonic medical imaging system--16 channel analog inputs and 4 dynamically focused beam outputs

An ultra large-scale ASIC is designed for the receive front-end electronics of an ultrasonic medical imaging system. The chip receives 16 channel analog rf signals and outputs 4 sets of sample-point-wise dynamically focused partial beam data. Four complete beam data sets are obtained in parallel by simply cascading as many chips as needed in an array system. High resolution of the focusing delay is obtained by nonuniformly selecting each channel data from a quadruply-interpolated rf data stream. The proposed ASIC can be applied to most practical array transducers in the frequency range of 2 to 10 MHz. The digital part of the designed ASIC can be implemented on a chip area of 17.9 microm2 with 0.18 mm CMOS technology, leaving sufficient room for 16 ADCs of 8 bits, 50 MHz on the 5.7 mm x 5.7 mm chip with a 208 pin package.     

Synthetic aperture ultrasound imaging with a ring transducer array: preliminary ex vivo results

Purpose

The conventional medical ultrasound imaging has a low lateral spatial resolution, and the image quality depends on the depth of the imaging location. To overcome these problems, this study presents a synthetic aperture (SA) ultrasound imaging method using a ring transducer array.

Methods

An experimental ring transducer array imaging system was constructed. The array was composed of 2048 transducer elements, and had a diameter of 200 mm and an inter-element pitch of 0.325 mm. The imaging object was placed in the center of the ring transducer array, which was immersed in water. SA ultrasound imaging was then employed to scan the object and reconstruct the reflection image.

Results

Both wire phantom and ex vivo experiments were conducted. The proposed method was found to be capable of producing isotropic high-resolution images of the wire phantom. In addition, preliminary ex vivo experiments using porcine organs demonstrated the ability of the method to reconstruct high-quality images without any depth dependence.

Conclusion

The proposed ring transducer array and SA ultrasound imaging method were shown to be capable of producing isotropic high-resolution images whose quality was independent of depth.

     

A high-frequency continuous-wave Doppler ultrasound system for the detection of blood flow in the microcirculation

Basic ultrasound physics and several clinical and experimental observations suggest that high-frequency Doppler ultrasound (HFD) operating in the frequency range 20–100 MHz holds the promise of detecting blood flow in the microcirculation. This article describes a directional, continuous-wave (CW), 1- to 200-MHz Doppler ultrasound system. The system electronics have a dynamic range of 100 dB, a noise floor of 10 nV and a directional isolation of 50 dB. The development of a 40-MHz Doppler transducer composed of two, 81-μm-thick, lithium niobate crystals that have been air-backed and transmission-line tuned for maximum sensitivity is described. This device is used to test the CW Doppler system using string and capillary phantoms and in vivo tissue. We show that HFD can detect and measure velocities on the order of the blood velocities found in the capillaries (1 mm/s) and arterioles (5 mm/s) with suitable velocity (50–500 μm/s) and temporal (20–250 ms) resolutions. In vivo measurements demonstrate that HFD is sensitive to the detection of blood flow in small vessels.     

Apodized adaptive beamformer

Purpose

A number of studies aimed at improvement of ultrasound image quality, such as spatial resolution and contrast, have been conducted. Apodization is known as an important factor that determines image quality. However, in the case of amplitude and phase estimation (APES) beamforming, a kind of adaptive beamformer that has been employed in medical ultrasound recently, only rectangular apodization has been used in the previous studies. In this study, apodization was employed in adaptive beamforming, and its effects on image quality were examined in phantom experiments.

Methods

We recently proposed a modified APES beamformer that reduces the computational complexity significantly using sub-aperture beamforming. In this study, the total receiving aperture was divided into four sub-apertures, and the APES beamforming was applied to the output from the four sub-apertures. Before the delay-and-sum (DAS) beamforming in each sub-aperture, echoes received by individual transducer elements were apodized with rectangular, Gaussian, and two Hanning functions, where the apodization with two Hanning functions realized lateral modulation of the ultrasonic field. The lateral spatial resolution was evaluated by the full width at half maximum of an echo from a string phantom, and the image contrast was evaluated using a cyst phantom.

Results

The modified APES beamformer realized a significantly better spatial resolution of 0.38 mm than that of the conventional delay-and-sum beamformer (0.67 mm), even with rectangular apodization. Using Gaussian apodization, the spatial resolution was further improved to 0.34 mm, and contrast was also improved from 4.3 to 5.1 dB. Furthermore, an image obtained by the modified APES beamformer with apodization consisting of two Hanning functions was better “tagged” as compared with the conventional DAS beamformer with the same apodization.

Conclusion

Apodization was shown to be effective in adaptive beamforming, and an image obtained by the adaptive beamformer with lateral modulation seemed to have potential for improvement of the accuracy in measurement of tissue lateral motion.

     

Real-time rectilinear volumetric imaging using a periodic array

Current real-time phased array volumetric scanners use a 2-D array to scan a pyramidal volume comprised of many sector scans stacked in the elevation direction. This scan format is primarily useful for cardiac imaging to avoid interference from the ribs. However, a real-time rectilinear volumetric scan with a wider field-of-view (FOV) close to the transducer could prove more useful for abdominal, breast or vascular imaging. In our previous work, a 94 x 94 Mills cross array operating at 5 MHz was fabricated, and the first real-time rectilinear volumetric images were made using a 2-D array and the Duke real-time 3-D scanner. The FOV for the Mills cross was 30 x 8 x 60 mm. Despite reasonable success with the Mills cross array, the array had limitations of poor off-axis sensitivity and a smaller FOV in one direction. To overcome these limitations, a new rectilinear array containing over 65,500 elements was developed with a periodic geometry to increase the FOV to 30 mm x 30 mm x 60 mm and improve the off-axis sensitivity. Images of tissue-mimicking phantoms and the carotid artery in vivo were obtained. In addition, spectral and color flow Doppler results from a pulsatile flow phantom were obtained.     

完全3D打印技术制作MRI质量控制体模

目的 设计并完全通过3D打印技术制造测量精度高、制作速度快的MRI质量控制（以下简称质控）体模。方法 设计符合MRI质量控制标准的体模整体和测试组件。采用设计制图软件Pro/Engineer,通过工程制图,设计出MRI质控体模的各模块和外壳的3D模型;导出STL格式文件,并采用3D光敏成型打印机（LITE600HD）整体打印并组装。采用1.5T MR成像系统对体模进行测试。结果 星形测试模块测得相位编码方向极限分辨率为6.55 LP/cm,频率编码方向为4.19 LP/cm。几何畸变测试模块测量图像畸变率为9.94%。层厚测试模块实际计算层厚为5.63~6.51 mm,偏差为+1.51 mm,图像纵横比为0.984。结论 通过3D打印制造MRI质量控制体模测试精度高,成本低,加工速度快,能够满足MRI日常质控和定制化体模的需要。     

In vivo cardiac imaging of adult zebrafish using high frequency ultrasound (45-75 MHz)

The zebrafish has emerged as an excellent genetic model organism for studies of cardiovascular development. Optical transparency and external development during embryogenesis allow for visual analysis in the early development. However, to understand the cardiovascular structures and functions beyond the early stage requires a high-resolution, real-time, noninvasive imaging alternative due to the opacity of adult zebrafish. In this research, we report the development of a high frequency ultrasonic system for adult zebrafish cardiac imaging, capable of 75 MHz B-mode imaging at a spatial resolution of 25 microm and 45 MHz pulsed-wave Doppler measurement. The system allows for real-time delineation of detailed cardiac structures, estimation of cardiac dimensions, as well as image-guided Doppler blood flow measurements. In vivo imaging studies showed the identification of the atrium, ventricle, bulbus arteriosus, atrioventricular valve and bulboventricular valve in real-time images, with cardiac measurement at various stages. Doppler waveforms acquired at the ventricle and the bulbus arteriosus demonstrated the utility of this system to study the zebrafish cardiovascular hemodynamics. This high frequency ultrasonic system offers a multitude of opportunities for cardiovascular researchers. In addition, the detection of E-flow and A-flow during the ventricular filling and the appearance of diastolic flow reversal at bulbus arteriosus suggested the functional similarity of zebrafish heart to that of higher vertebrates.     

多普勒组织显像评估严重心力衰竭而QRS间期正常患者的左室收缩不同步

目的:应用多普勒组织显像(DTI)评估严重心衰而QRS间期正常的患者左室收缩不同步的发生率。方法:应用DTI分析20例QRS间期正常的严重心衰患者和20例正常人的左室侧壁、后间隔、前壁、下壁、前间隔及后壁的基底段和中段共12个节段的脉冲组织多普勒频谱,测量每例的QRS波起始至各个左室壁节段多普勒频谱收缩峰起始的时间(以Ts表示)。计算每例最大Ts与最小Ts的差值(Ts-MD)、Ts的标准差(Ts-SD)和Ts的变异系数(Ts-CV),用以评估两组对象左室收缩不同步的差异。结果:HF组的Ts-MD、Ts-SD和Ts-CV均较正常对照组明显增大(P<0.001),表明QRS间期正常的心衰患者存在左室壁收缩不同步。以TS-MD53.08 ms、Ts-SD18.08 ms和Ts-CV0.91(正常值单侧95%可信区间,x 1.65s)作为切断值,HF组左室收缩不同步的发生率分别为55.0%(11/20)、55.0%(11/20)和55.0%(11/20),显著高于对照组(P均<0.001);且QRS间期正常的心衰患者最大收缩延迟部位亦不完全相同。结论:DTI显示左室收缩不同步在严重心衰而QRS间期正常的患者中常见。这个结果支持心衰患者即使QRS间期正常也可能从CRT中受益。     

In vitro validation of volumetric blood flow measurement using Doppler flow wire

Determination of any volumetric blood flow requires assessment of mean blood flow velocity and vessel cross-sectional area. For evaluation of coronary blood flow and flow reserve, however, assessment of average peak velocity alone is widely used, but changes in velocity profile and vessel area are not taken into account. We studied the feasibility of a new method for calculation of volumetric blood flow by Doppler power using a Doppler flow wire. An in vitro model with serially connected silicone tubes of known lumen diameters (1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 mm) and pulsatile blood flow ranging from 10 to 200 mL/min was used. A Doppler flow wire was connected to a commercially available Doppler system (FloMap(R), Cardiometrics) for online calculation of the zeroth (M(0)) and the first (M(1)) Doppler moment, as well as mean flow velocity (V(m)). Two different groups of sample volumes (at different gate depths) were used: 1. two proximal sample volumes lying completely within the vessel were required to evaluate the effect of scattering and attenuation on Doppler power, and 2. distal sample volumes intersecting completely the vessel lumen to assess the vessel cross-sectional area. Area (using M(0)) and V(m) (using M(1)/M(0)) obtained from the distal gates were corrected for scattering and attenuation by the data obtained from the proximal gates, allowing calculation of absolute volumetric flow. These results were compared to the respective time collected flow. Correlation between time collected and Doppler-derived flow measurements was 0.98 (p < 0.0001), with a regression line close to the line of equality indicating an excellent agreement of the two measurements in each individual tube. The mean paired flow difference between the two techniques was 1.5 +/- 9.0 mL/min (ns). Direct volumetric blood flow measurement from received Doppler power using a Doppler flow wire system is feasible. This technique may potentially be of great clinical value because it allows an accurate assessment of coronary flow and flow reserve with a commercially available flow wire system.     

High-Frequency Chirp Ultrasound Imaging with an Annular Array for Ophthalmologic and Small-Animal Imaging

High-frequency ultrasound (HFU, >20 MHz) is an attractive means of obtaining fine-resolution images of biological tissues for ophthalmologic, dermatological and small-animal imaging applications. Even with current improvements in circuit designs and high-frequency equipment, HFU has two inherent limitations. First, HFU images have a limited depth-of-field (DOF) because of the short wavelength and the low fixed F-number of conventional HFU transducers. Second, HFU is usually limited to shallow imaging because of the significant attenuation in most tissues. In a previous study, a five-element annular array with a 17-MHz center frequency was excited using chirp-coded signals, and a synthetic-focusing algorithm was used to extend the DOF and increase penetration depth. In the present study, a similar approach with two different five-element annular arrays operating near a center frequency of 35 MHz is implemented and validated. Following validation studies, the chirp-imaging methods were applied to imaging vitreous-hemorrhage-mimicking phantoms and mouse embryos. Images of the vitreous phantom showed increased sensitivity using the chirp method compared with a standard monocycle imaging method, and blood droplets could be visualized 4 mm deeper into the phantom. Three-dimensional datasets of 12.5-day-old mouse embryo heads were acquired in utero using chirp and conventional excitations. Images were formed and brain ventricles were segmented and reconstructed in three dimensions. The brain ventricle volumes for the monocycle excitation exhibited artifacts that were not apparent on the chirp-based dataset reconstruction. (E-mail: mamou@rrinyc.org)     

Performance tests of Doppler ultrasound equipment with a tissue and blood-mimicking phantom

A tissue- and blood-mimicking phantom was assembled for assessing the performance of ultrasound Doppler equipment. The phantom is in the shape of a rectangular parallelepiped with a 10 x 20 cm scanning window and a depth of 16 cm. Components of the phantom include a tissue-mimicking material, 7.9 mm diameter simulated vessels, a fluid with similar back-scatter as whole blood, and a peristaltic pumping system producing peak scatterer velocities greater than 1 m/sec. Performance tests done with the phantom are outlined. These include assessments of the maximum depth of penetration and of the directional discrimination capabilities of the instrument, determinations of the accuracy of displayed flow velocities, and accuracy assessments of the displayed position of the Doppler sample volume.     

A novel in vivo procedure for volumetric flow measurements

We report on a novel procedure for invasive volumetric blood flow measurements using a commercially available Doppler flow wire system, which could, until now, only measure flow velocity. We here describe a method applicable in vivo to generate both velocity and cross-sectional area information from the same pulsed-wave Doppler signal for volumetric flow assessment. We demonstrate its feasibility and validation in vivo in pig coronary arteries. Our Doppler-derived volumetric flow measurements were compared with the respective transit-time flow and showed an excellent correlation (r = 0.969; p < 0.0001). Agreement between transit-time and Doppler-derived flow measurements could be observed for flow conditions ranging from 30 to 180 mL/min. The mean values for the two methods were 71.4 +/- 43.7 mL/min and 71.3 +/- 42.2 mL/min, respectively. We conclude that this technique might possibly be introduced into future clinical practice as an invasive procedure of choice for the assessment of volumetric blood flow.