Commercial Ultrasound System Research Articles

Effects of Acoustic Radiation Force on the Binding Efficiency of BR55, a VEGFR2-Specific Ultrasound Contrast Agent

This work describes an in vivo study analyzing the effect of acoustic radiation force (ARF) on the binding of BR55 VEGFR2-specific contrast-agent microbubbles in a model of prostatic adenocarcinoma in rat. A commercial ultrasound system was modified by implementing high duty-cycle 3.5-MHz center frequency ARF bursts in a scanning configuration. This enabled comparing the effects of ARF on binding in tumor and healthy tissue effectively in the same field of view. Bubble binding was established by measuring late-phase enhancement in amplitude modulation (AM) contrast-specific imaging mode (4 MHz, 150 kPa) 10 min after agent injection when the unbound bubbles were cleared from the circulation. Optimal experimental conditions, such as agent concentration (0.4 × 108–1.6 × 108 bubbles/kg), acoustic pressure amplitude (26–51 kPa) and duty-cycle (20%–95%) of the ARF bursts, were evaluated in their ability to enhance binding in tumor without significantly increasing binding in healthy tissue. Using the optimal conditions (38 kPa peak-negative pressure, 95% duty cycle), ARF-assisted binding of BR55 improved significantly in tumor (by a factor of 7) at a lower agent dose compared with binding without ARF, and it had an insignificant effect on binding in healthy tissue. Thus, the high binding specificity of BR55 microbubbles for targeting VEGFR2 present at sites of active angiogenesis was confirmed by this study. Therefore, it is believed that based on the results obtained in this work, ultrasound molecular imaging using target-specific contrast-agent microbubbles should preferably be performed in combination with ARF.

Optimal sound speed estimation using modified nonlinear anisotropic diffusion to improve spatial resolution in ultrasound imaging

In ultrasound exams of obese patients and the breast, the spatial and contrast resolutions of ultrasound images are severely deteriorated when a constant sound speed corresponding to soft tissue is used in receive dynamic beamformation. This degradation is due to the defocusing of the ultrasound beam because of the disparity in sound speed between soft tissue and fatty layers. To minimize the degradation, this paper proposes a new method of estimating an optimal sound speed that can be used to achieve the best beamforming performance in a region of interest (ROI). The proposed method employs a new focusing quality factor (FQF) as an indicator of how well the focusing is conducted with a given sound speed. The FQF is closely associated with the degree of edge conspicuity, which can be obtained using the proposed modified nonlinear anisotropic diffusion (MNAD) technique. To calculate FQF, ultrasound images are formed with different sound speeds ranging from 1400 to 1600 m/s and, subsequently, the ROI is chosen. In the ROI, the degrees of edge conspicuity (i.e., FQF) are calculated. The sound speed can be considered an optimal one for the ROI if it is used to construct the image that provides the maximum FQF. The performances of the proposed method were evaluated through simulation and in vitro experiments with a tissue-mimicking phantom. The performance was also compared with that of the conventional image-based method employed in a commercial ultrasound imaging system. The experimental results demonstrated that the proposed method is capable of estimating an optimal sound speed with an error of 10 m/s regardless of whether strong targets are included in the ROI or not. On the other hand, the conventional image-based method generated an estimation error of 60 m/s maximally in the case in which there were no strong targets in ROI. This indicates that the proposed method is a useful tool to improve ultrasound image quality for clinical applications, especially for ultrasound exams of obese patients and the breast.

A High-Resolution 3D Ultrasonic System for Rapid Evaluation of the Anterior and Posterior Segment

Traditional ultrasound imaging systems for ophthalmology employ slow, mechanical scanning of a single-element ultrasound transducer. The goal was to demonstrate rapid examination of the anterior and posterior segment with a three-dimensional (3D) commercial ultrasound system incorporating high-resolution linear probe arrays. The 3D images of the porcine eye were generated in approximately 10 seconds by scanning one of two commercial linear arrays (25- and 50-MHz). Healthy enucleated pig eyes were compared with those with induced injury or placement of a foreign material (eg, metal). Rapid, volumetric imaging was also demonstrated in one human eye in vivo. The 50-MHz probe provided exquisite volumetric images of the anterior segment at a depth up to 15 mm and axial resolution of 30 μm. The 25-MHz probe provided a larger field of view (lateral X depth: 20 × 30 mm), sufficient for capturing the entire anterior and posterior segments of the pig eye, at a resolution of 60 μm. A 50-MHz scan through the human eyelid illustrated detailed structures of the Meibomian glands, cilia, cornea, and anterior segment back to the posterior capsule. The 3D system with its high-frequency ultrasound arrays, fast data acquisition, and volume rendering capability shows promise for investigating anterior and posterior structures of the eye.

Algorithms and Results of Eye Tissues Differentiation Based on RF Ultrasound

Algorithms and software were developed for analysis of B-scan ultrasonic signals acquired from commercial diagnostic ultrasound system. The algorithms process raw ultrasonic signals in backscattered spectrum domain, which is obtained using two time-frequency methods: short-time Fourier and Hilbert-Huang transformations. The signals from selected regions of eye tissues are characterized by parameters: B-scan envelope amplitude, approximated spectral slope, approximated spectral intercept, mean instantaneous frequency, mean instantaneous bandwidth, and parameters of Nakagami distribution characterizing Hilbert-Huang transformation output. The backscattered ultrasound signal parameters characterizing intraocular and orbit tissues were processed by decision tree data mining algorithm. The pilot trial proved that applied methods are able to correctly classify signals from corpus vitreum blood, extraocular muscle, and orbit tissues. In 26 cases of ocular tissues classification, one error occurred, when tissues were classified into classes of corpus vitreum blood, extraocular muscle, and orbit tissue. In this pilot classification parameters of spectral intercept and Nakagami parameter for instantaneous frequencies distribution of the 1st intrinsic mode function were found specific for corpus vitreum blood, orbit and extraocular muscle tissues. We conclude that ultrasound data should be further collected in clinical database to establish background for decision support system for ocular tissue noninvasive differentiation.

In vivo three-dimensional photoacoustic imaging based on a clinical matrix array ultrasound probe

We present an integrated photoacoustic and ultrasonic three-dimensional (3-D) volumetric imaging system based on a two-dimensional (2-D) matrix array ultrasound probe. A wavelength-tunable dye laser pumped by a Q-switched Nd:YAG laser serves as the light source and a modified commercial ultrasound imaging system (iU22, Philips Healthcare) with a 2-D array transducer (X7-2, Philips Healthcare) detects both the pulse-echo ultrasound and photoacoustic signals. A multichannel data acquisition system acquires the RF channel data. The imaging system enables rendering of co-registered 3-D ultrasound and photoacoustic images without mechanical scanning. The resolution along the azimuth, elevation, and axial direction are measured to be 0.69, 0.90 and 0.84 mm for photoacoustic imaging. In vivo 3-D photoacoustic mapping of the sentinel lymph node was demonstrated in a rat model using methylene blue dye. These results highlight the clinical potential of 3-D PA imaging for identification of sentinel lymph nodes for cancer staging in humans.

P2-10-02: Assessment of Tumor Response to Neoadjuvant Chemotherapy Using Ultrasound-Guided Near Infrared Light.

Abstract Background Neoadjuvant chemotherapy has been increasingly used in treating breast cancers. Because breast cancer is a heterogeneous disease, it is important to effectively monitor the tumor response to assist in tailoring treatments to response. In our early study, we have introduced a novel ultrasound (US)-guided diffused light imaging in the near infrared (NIR) spectrum to monitor tumor vascular changes which correlated to tumor response. The objectives of this study are: (a) to validate the initial findings with a larger patient pool, and (b) to assess vascular changes at every treatment cycle and to correlate early vascular changes with the tumor pathological response. Methods: 33 patients who underwent neoadjuvant treatment were recruited from Dec. 2007 to May 2011 and their tumor vascular content was assessed with a combined imager consisting of a commercial US system coupled to a NIR imager. Patients were imaged before their treatment, at the end of each treatment cycle and before their surgery. The co-registered US was used to localize the tumor and the NIR imager was used to map the tumor vascular distribution which was assessed based on a percentage total hemoglobin (%tHb) concentration normalized to the pre-treatment level. 18 patients were treated with AC followed by Taxol. This group of patients was monitored every two weeks at the end of each treatment. The remaining patients were on 3-week cycles of chemotherapy and monitored every 3 weeks. 6 patients were treated with TC without Adriamycin (TC), or with Adriamycin (TAC), 6 HER2 positive patients were treated with TC and Herceptin (TCH); and 3 patients were treated with AC/Bevacizumab. Pathologic response was graded based on Miller and Payne system as grade 1: non-responders (A); grades 2 and 3: partial responders (B); 4: near-complete and 5: complete responders (C). Results: In the AC/Taxol group (n=18), there were 5 responders (C), 9 partial (B) and 4 non-responders (A). The statistical significance based on %tHb between groups A and C was achieved at the end of cycle 5 and the rest of the treatment cycles (p<0.05), however, the statistical significance between A and B was only obtained at end of cycle 5 (p<0.05) and not maintained for cycles 6–8. The statistical significance between B and C was only achieved at the end the treatment (p<0.05). For the TC,TAC and TCH group (n=12), there were 6 responders(C) and 6 partial responders (B). The statistical significance between these two groups was achieved at the end of cycle 3 and the rest of the treatment cycles (p<0.05). For the 3 patients who were treated with AC/Bevacizumab, 2 patients achieved complete response and one partial with grade 2. The complete responders had more than 50% reduction in %tHb at the end of cycles 3–4; while the partial one showed only 10–15% reduction during the entire treatment course. Discussion: Our findings indicate that tumor vascular changes assessed by %tHb can be used to predict the tumor pathological response. This is a powerful tool to help predict responsiveness to therapy. Interestingly, dramatic and early responses were noted in the patients who received the biologic agents (bevacizumab and herceptin) and this may be very valuable in following responses using these agents. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P2-10-02.

Acoustic Radiation Force Impulse (ARFI) Imaging: a Review.

Acoustic radiation force based elasticity imaging methods are under investigation by many groups. These methods differ from traditional ultrasonic elasticity imaging methods in that they do not require compression of the transducer, and are thus expected to be less operator dependent. Methods have been developed that utilize impulsive (i.e. < 1 ms), harmonic (pulsed), and steady state radiation force excitations. The work discussed herein utilizes impulsive methods, for which two imaging approaches have been pursued: 1) monitoring the tissue response within the radiation force region of excitation (ROE) and generating images of relative differences in tissue stiffness (Acoustic Radiation Force Impulse (ARFI) imaging); and 2) monitoring the speed of shear wave propagation away from the ROE to quantify tissue stiffness (Shear Wave Elasticity Imaging (SWEI)). For these methods, a single ultrasound transducer on a commercial ultrasound system can be used to both generate acoustic radiation force in tissue, and to monitor the tissue displacement response. The response of tissue to this transient excitation is complicated and depends upon tissue geometry, radiation force field geometry, and tissue mechanical and acoustic properties. Higher shear wave speeds and smaller displacements are associated with stiffer tissues, and slower shear wave speeds and larger displacements occur with more compliant tissues. ARFI images have spatial resolution comparable to that of B-mode, often with greater contrast, providing matched, adjunctive information. SWEI images provide quantitative information about the tissue stiffness, typically with lower spatial resolution. A review these methods and examples of clinical applications are presented herein.

Dual-pulse frequency compounded superharmonic imaging

Tissue second-harmonic imaging is currently the default mode in commercial diagnostic ultrasound systems. A new modality, superharmonic imaging (SHI), combines the third through fifth harmonics originating from nonlinear wave propagation through tissue. SHI could further improve the resolution and quality of echographic images. The superharmonics have gaps between the harmonics because the transducer has a limited bandwidth of about 70% to 80%. This causes ghost reflection artifacts in the superharmonic echo image. In this work, a new dual-pulse frequency compounding (DPFC) method to eliminate these artifacts is introduced. In the DPFC SHI method, each trace is constructed by summing two firings with slightly different center frequencies. The feasibility of the method was established using a single-element transducer. Its acoustic field was modeled in KZK simulations and compared with the corresponding measurements obtained with a hydrophone apparatus. Subsequently, the method was implemented on and optimized for a setup consisting of an interleaved phased-array transducer (44 elements at 1 MHz and 44 elements at 3.7 MHz, optimized for echocardiography) and a programmable ultrasound system. DPFC SHI effectively suppresses the ghost reflection artifacts associated with imaging using multiple harmonics. Moreover, compared with the single-pulse third harmonic, DPFC SHI improved the axial resolution by 3.1 and 1.6 times at the -6-dB and -20-dB levels, respectively. Hence, DPFC offers the possibility of generating harmonic images of a higher quality at a cost of a moderate frame rate reduction.

The applicability of ultrasound dynamic receive beamformers to photoacoustic imaging

Ultrasound array transducers offer several advantages over mechanically-scanned transducers for photoacoustic imaging, including high imaging frame rates and dynamic focusing. Development of a photoacoustic array system can be accelerated by adapting existing commercial ultrasound systems and harnessing their performance-enhancing aspects such as parallel beamforming. One challenge faced when adapting commercial ultrasound systems for photoacoustic imaging is that the dynamic delay sequences required for focusing must account for one-way rather than two-way ultrasound wave propagation. Modifying the hardware may be difficult for developers and impossible for users, but some ultrasound systems provide a parameter, c: the speed of sound used to calculate these delays. A linear-array based ultrasound platform with parallel channel acquisition is used to compare experimental point-spread functions produced using an ultrasound beamformer with a scaled value of c to those produced by a photoacoustic beamformer. Scaling c by a factor of √2 provides the best image quality compared with adjustments by 1 and 2, but requires image rescaling, which can be done post-acquisition or by modification of the rendering software. Although optimal focusing is achieved for linear scanning, this is not the case for sector scanning, which requires angular and depth rescaling.

Ultrasound Functional Imaging Using Harmonic to Fundamental Ratio (HFR) - Simulation study

Nonlinearity of ultrasound relies upon the characteristics of a medium such as a B/A-ratio (BAR) and sound velocity, causing generating harmonic components of transmitted ultrasound. Also, it is known that a certain disease leads to the change in a BAR of a medium. There have been several attempts of nonlinearity parameter B/A imaging, that is based on tomography and thus may be limited in practical application. As an alternative, this paper proposes a new nonlinearity parameter imaging method that can be easily implemented in a commercial ultrasound imaging system.

Estimation of blood velocity, volumetric flow and wall shear rate using Doppler ultrasound

Commercial ultrasound systems can make a number of measurements related to haemodynamics which are relevant to clinical practice and to clinical research. These include maximum velocity, volumetric flow and wall shear rate. Using appropriate protocols, measurements can be made averaged over the cardiac cycle, or as a function of time through the cardiac cycle. Maximum velocity underpins most of these measurements. Maximum velocity is overestimated as a result of geometric spectral broadening, by typically up to 30%, but by much larger amounts as the angle approaches 90°. Though not used in clinical practice, a simple correction technique using a string phantom can substantially reduce these errors. For volumetric flow and wall shear rate, methods such as specialist multi-gate ultrasound systems, magnetic resonance imaging and image guided modelling are available. Before resorting to these more complex methods users might consider that, with care and attention to procedure, high quality information may be obtained using commercial ultrasound systems. Manufacturers could make more use of the colour flow image for quantification of velocity, and adopt vector Doppler techniques.

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.

Monitoring Radio-Frequency Thermal Ablation with Ultrasound by Low Frequency Acoustic Emissions – In Vitro and In Vivo Study

The object of this study was to evaluate the monitoring of thermal ablation therapy by measuring the nonlinear response to ultrasound insonation at the region being treated. Previous reports have shown that during tissue heating, microbubbles are formed. Under the application of ultrasound, these microbubbles may be driven into nonlinear motion that produces acoustic emissions at sub-harmonic frequencies and a general increase of emissions at low frequencies. These low frequency emissions may be used to monitor ablation surgery. In this study, a modified commercial ultrasound system was used for transmitting ultrasound pulses and for recording raw RF-lines from a scan plane in porcine ( in vitro) and rabbit ( in vivo) livers during radio-frequency ablation (RFA). The transmission pulse was 15 cycles in length at 4 MHz ( in vitro) and 3.6 MHz ( in vivo). Thermocouples were used for monitoring temperatures during the RFA treatment.In the in vitro experiments, recorded RF signals (A-lines) were segmented, and the total energy was measured at two different frequency bands: at a low frequency band (LFB) of 1–2.5 MHz and at the transmission frequency band (TFB) of 3.5–4.5 MHz. The mean energy at the LFB and at the TFB increased substantially in areas adjacent to the RF needle. These energies also changed abruptly at higher temperatures, thus, producing great variance in the received energy. Mean energies in areas distant from RF needle showed little change and variation during treatment. It was also shown that a 3 dB increase of energy at the low frequency band was typically obtained in regions in which temperature was above 53.3 ± 5°C. Thus, this may help in evaluating regions undergoing hyperthermia. In the in vivo experiments, an imaging algorithm based on measuring the LFB energy was used. The algorithm performs a moving average of the LFB energies measured at segments within the scan plane.Results show that a colored region is formed on the image and that it is similar in size to a measurement of the lesion from gross pathology, with a correlation coefficient of 0.743. (E-mail: Itai.Winkler@RKDiagnostics.co.il)

Clinical assessment of shockwave lithotripsy accuracy.

Kidney stone movement primarily due to patient respiration compromises shock wave lithotripsy (SWL) targeting and efficacy. The objective of this study is to describe the use of B‐mode ultrasound to evaluate the accuracy of targeting during SWL. Patients undergoing electrohydraulic SWL were enrolled into this institutionally approved research study. A commercial diagnostic ultrasound imaging system, either Philips HDI 5000 or iU‐22, was used to intermittently visualize and detect any shockwave‐induced motion of the stone during 1–3 min periods. Four patients (mean age 52.7) underwent treatment of seven renal stones with mean individual stone size of 10.41 ± 4.5 mm. A mean of 2937 shocks (range 2750–3000) were delivered at a rate of 1–2 Hz and charging voltage of 14–26 kV. Stone oscillation or jumping at the exact time of individual shock delivery was visualized with ultrasound: no stones completely failed to move. Accurate alignment, as interpreted by positive stone motion, occurred in a mean of 50 ± 20.4% of shockwaves. Ultrasound imaging represents a method of real‐time assessment of accuracy in SWL and may provide the basis for devices to control targeting so that shockwaves are only delivered when the stone is in focus. [Work supported by NIH DK43881 and DK086371.]

The Verasonics ultrasound system as a pedagogic tool in teaching wave propagation, scattering, beamforming, and signal processing concepts in physics and engineering.

The Verasonics ultrasound system is a highly programmable data acquisition and processing platform designed to facilitate development of new medical ultrasound imaging methods. In contrast to conventional commercial ultrasound systems, individual element digitized rf data are available to the developer. All beamforming and postprocessing are done in software, and both the hardware data acquisition sequence and the host computer processing flow are programmable by the user using a MATLAB interface. Because the system is designed to be highly flexible, it can also be useful as a practical tool in teaching acoustic wave physics, transducer and array design, and data processing concepts, using benchtop scale homemade acoustic and elastic media, including flow models. For script evaluation and testing, the Verasonics system includes a hardware simulator that uses a simple point scatterer numerical model to compute rf backscatter data. rf data can also be recorded during a hardware acquisition, and then reprocessed using different user-developed algorithms for comparative study. Because the system is easy to learn, many fundamental concepts can be explored in a laboratory setting, using scattering media or custom transducers fabricated as part of the student experimental plan. The system enables sophisticated hands-on experience with acoustics beyond the numerical world.

The Acute Effects of Smoking on the Cyclic Variations in Blood Echogenicity of Carotid Artery

The objective of this research is to study the cyclic variations in echogenicity (CVE) as an acute response to smoking. CVEs, caused by the aggregation of red blood cells (RBC) were measured from the cross-sectional images of the common carotid artery using coded harmonic imaging of a commercial ultrasound system. The amplitude of the CVE (A cve) was analyzed among 28 smokers before and after smoking. A cve was increased in 22 smokers and decreased in six smokers after 1–2 cigarettes were smoked. Heart rate (HR) was also estimated from the ultrasonic images before and after smoking. The smokers were optimally divided into two clusters with respect to the change in A cve and the intrinsic characteristics of smokers ( i.e., daily consumed cigarettes and smoking years) through a two-step cluster analysis (TSCA). The increase in A cve after smoking was significantly higher in the heavy smoker cluster compared with the light smoker cluster. The results suggest that the acute changes in A cve in response to smoking are different between heavy smokers and light smokers. This preliminary study demonstrates the potential application of coded harmonic ultrasound imaging to detect or characterize RBC aggregation. In addition, the results may be useful for understanding the acute physiologic changes caused by smoking. (E-mail: paeng@jejunu.ac.kr)

Photoacoustic Imaging With a Commercial Ultrasound System and a Custom Probe

Building photoacoustic imaging (PAI) systems by using stand-alone ultrasound (US) units makes it convenient to take advantage of the state-of-the-art ultrasonic technologies. However, the sometimes limited receiving sensitivity and the comparatively narrow bandwidth of commercial US probes may not be sufficient to acquire high quality photoacoustic images. In this work, a high-speed PAI system has been developed using a commercial US unit and a custom built 128-element piezoelectric-polymer array (PPA) probe using a P(VDF-TrFE) film and flexible circuit to define the elements. Since the US unit supports simultaneous signal acquisition from 64 parallel receive channels, PAI data for synthetic image formation from a 64- or 128-element array aperture can be acquired after a single or dual laser firing, respectively. Therefore, two-dimensional (2-D) B-scan imaging can be achieved with a maximum frame rate up to 10 Hz, limited only by the laser repetition rate. The uniquely properties of P(VDF-TrFE) facilitated a wide –6 dB receiving bandwidth of over 120% for the array. A specially designed 128-channel preamplifier board made the connection between the array and the system cable, which not only enabled element electrical impedance matching but also further elevated the signal-to-noise ratio (SNR) to further enhance the detection of weak photoacoustic signals. Through the experiments on phantoms and rabbit ears, the good performance of this PAI system was demonstrated. (E-mail: xdwang@umich.edu)

Quantificação semiautomática da velocidade do sangue em imagens de ultrassonografia Doppler

Blood velocity vs. time graphic images acquired by ultrasound Doppler technique can be used to reveal relationships between data within those graphics and the presence of cardiovascular diseases, among other applications. To facilitate this kind of study in clinical protocols involving hundreds of patients, a computational tool was developed to extract quantitative data from the graphics. After calibration and selection of the region of interest defined by the user, the algorithm automatically detects the baseline and the curve envelope to calculate the peak velocities and the velocity-time integral (VTI). Image processing techniques, such as Connected Components, Gaussian and Median filters were applied to improve the robustness of the envelope detection. A comparative analysis between measurements obtained with commercial ultrasound systems operated by specialists (named as 'A') and by the present methodology (named as 'B') included systolic peak velocities and VTIs of common carotid and brachial arteries under basal condition, brachial arteries in the reactive hyperemic response and echocardiographic exams. The results showed a small mean difference, calculated as d = A - B, in the Bland-Altman analysis and a high correlation for both: peak velocity (d = 0.02 m/s; r > 0.99; p 0.99; p < 0.01; 75 samples). These findings indicate the reliability of this methodology which, in addition to the user friendly graphical interface of this new tool, can be easily used by the clinicians for their large-scale studies based on Doppler ultrasound images.

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.

Validation of Endoscopic Ultrasound Measured Flow Rate in the Azygos Vein Using a Flow Phantom

Increase in flow rate within the azygos vein may be used as an indicator of the degree of liver cirrhosis. The aim of this study was to evaluate the error in measurement of flow rate using a commercial endoscopic ultrasound system, using a flow phantom that mimicked azygos vein depth, diameter and flow rate. Diameter was underestimated in all cases, with an average underestimation of 0.09 cm. Maximum velocity was overestimated, by 4 ± 4% at 50°, 11 ± 3% at 60° and 23 ± 7% at 70°. The increase in error with beam-vessel angle is consistent with the error as arising from geometric spectral broadening. Flow was underestimated by amounts up to 33%, and it is noted that the overestimation caused by geometric spectral broadening is in part compensated by underestimation of diameter. It was concluded that measurement of flow rate using a commercially available endoscopic ultrasound system is dependent on the beam-vessel angle, with errors up to 33% for typical vessel depths, diameter and beam-vessel angle. (E-mail: P.Hoskins@ed.ac.uk)