In vivo ultrasonic measurement of tissue vibration at a stenosis: a case study

A wide range gate data acquisition for diagnosing coronary artery disease

BACKGROUND

The turbulence of blood flow caused by stenosis has an impact on the surrounding coronary artery tissue and creates an audio-frequency vibration to the adjacent myocardial wall. We investigated the diagnostic feasibility of a novel diagnostic method using wide range gate (WRG) ultrasound data acquisition for diagnosing coronary artery disease (CAD). WRG data acquisition detects high-frequency vibrations from coronary artery stenosis, using pulse-wave Doppler ultrasound.

METHODS

We used a Verasonics ultrasound data acquisition system to implement the WRG data acquisition. Investigators performed clinical trials for 80 subjects, with suspected CAD. All enrolled patients participated in WRG data acquisition before coronary angiography (CAG).

RESULTS

As compared with the results of CAG, the sensitivity and specificity of the WRG data analysis were 80% and 84%, respectively. The WRG data analysis showed that the sensitivity and specificity were 81% and 79% in the left anterior descending artery, respectively, 75% and 89% in the left circumflex artery, respectively, and 85% and 82% in the right coronary artery, respectively. In a multivariate analysis, a positive vibrometry result was an independent predictive factor for CAD.

CONCLUSIONS

We proposed a new diagnostic method for detecting CAD using ultrasound. The new data acquisition method showed good potential as an initial diagnostic tool for CAD.

Quest for the Vulnerable Atheroma: Carotid Stenosis and Diametric Strain--A Feasibility Study

The Bernoulli effect may result in eruption of a vulnerable carotid atheroma, causing a stroke. We measured electrocardiography (ECG)-registered QRS intra-stenotic blood velocity and atheroma strain dynamics in carotid artery walls using ultrasonic tissue Doppler methods, providing displacement and time resolutions of 0.1 μm and 3.7 ms. Of 22 arteries, 1 had a peak systolic velocity (PSV) >280 cm/s, 4 had PSVs between 165 and 280 cm/s and 17 had PSVs <165 cm/s. Eight arteries with PSVs <65 cm/s and 4 of 9 with PSVs between 65 and 165 cm/s had normal systolic diametric expansion (0% and 7%) and corresponding systolic wall thinning. The remaining 10 arteries had abnormal systolic strain dynamics, 2 with diametric reduction (>-0.05 mm), 2 with extreme wall expansion (>0.1 mm), 2 with extreme wall thinning (>-0.1 mm) and 4 with combinations. Decreases in systolic diameter and/or extreme systolic arterial wall thickening may indicate imminent atheroma rupture.

Doppler vibrometry: assessment of arterial stenosis by using perivascular tissue vibrations without lumen visualization

PURPOSE

To correlate vibration frequency and duration at Doppler vibrometry with stenosis severity determined at catheter angiography.

MATERIALS AND METHODS

Sixteen patients (eight women) scheduled to undergo abdominal or pelvic angiography were recruited after providing informed consent. An ultrasonography (US) scanner was customized to acquire raw echo data before conventional Doppler processing. Data were acquired from perivascular tissue regions proximal to stenoses, close to the most narrow lumen, and distal to stenoses in the renal, hepatic, common iliac, and superior mesenteric arteries. The data were processed to quantify vibration frequency and duration. The minimum lumen diameter and the pre- and poststenotic lumen diameters were quantified from angiograms. One patient with a hepatic artery stenosis did not yield measurable vibrometry data due to significant bowel gas.

RESULTS

Stenoses (diameter reduction, 43%-91%) were angiographically measured in the six renal arteries, four hepatic arteries, three iliac arteries, and one superior mesenteric artery yielding vibrometry data. Three iliac arteries were normal (<30% diameter reduction at angiography). For these 17 arteries, the vibration frequency was higher with a greater percentage of stenosis [Pearson r = .75; P < .001) and a smaller minimum lumen diameter (r = .72; P < .001). The vibration duration increased with a greater percentage of stenosis (r = .7; P < .001).

CONCLUSIONS

Preliminary results indicate that the vibration frequency and duration can be used to quantitatively estimate stenosis severity. Doppler vibrometry is complementary to duplex US and does not require lumen visualization.

Simultaneous imaging of artery-wall strain and blood flow by high frame rate acquisition of RF signals

Mechanical properties of the arterial walls are significantly altered by atherosclerosis, and various studies have been recently conducted to measure the regional elastic properties (radial strain) of the arterial wall. We have developed a phase-sensitive correlation-based method, namely, the phased-tracking method, to measure the regional radial strain. On the other hand, the measurement of blood flow is an important practical routine in the diagnosis of atherosclerosis. It would be useful if the regional strain of the arterial wall as well as blood flow could be assessed simultaneously. Such measurement would require a high frame rate of several kilohertz. In this study, acquisition of ultrasonic RF echoes at a high frame rate (about 3500 Hz) was achieved using parallel beamforming in which plane waves were transmitted only 3 times and receive beamforming created 24 beams for each transmit beam. The accuracy in measurement of the minute radial strain was evaluated by a basic experiment using a cylindrical phantom. The error of the measured strain from the theoretical strain profile and its standard deviation were 4.8% and 9.5%, respectively. Furthermore, the radial strain of a carotid arterial wall and blood flow were simultaneously imaged in vivo.

Direct numerical simulation of transitional flow in a stenosed carotid bifurcation

The blood flow dynamics of a stenosed, subject-specific, carotid bifurcation were numerically simulated using the spectral element method. Pulsatile inlet conditions were based on in vivo color Doppler ultrasound measurements of blood velocity. The results demonstrated the transitional or weakly turbulent state of the blood flow, which featured rapid velocity and pressure fluctuations in the post-stenotic region of the internal carotid artery (ICA) during systole and laminar flow during diastole. High-frequency vortex shedding was greatest downstream of the stenosis during the deceleration phase of systole. Velocity fluctuations had a frequency within the audible range of 100-300Hz. Instantaneous wall shear stress (WSS) within the stenosis was relatively high during systole ( approximately 25-45Pa) compared to that in a healthy carotid. In addition, high spatial gradients of WSS were present due to flow separation on the inner wall. Oscillatory flow reversal and low pressure were observed distal to the stenosis in the ICA. This study predicts the complex flow field, the turbulence levels and the distribution of the biomechanical stresses present in vivo within a stenosed carotid artery.

Contrast-enhanced sonographic characteristics of neovascularization in carotid atherosclerotic plaques

PURPOSE

To evaluate neovascularization within carotid atherosclerotic plaques with contrast-enhanced sonography.

METHODS

We used contrast-enhanced sonography to examine 63 patients with carotid atherosclerotic plaques. The features of neovascularization within the plaques were analyzed and correlated with plaque size and echogenicity.

RESULTS

There were 81 atherosclerotic plaques, 62 of which (43 soft and 19 mixed) enhanced after injection of a contrast agent. The enhancement occurred from the carotid wall to the center of the plaque with a short-line pattern in 36 plaques, whereas 26 plaques enhanced from both the carotid wall and the carotid lumen, with a sparse spot pattern. The arrival time of contrast was shorter (p < 0.001) and time to peak was longer (p < 0.001) in the plaques than in the carotid lumen. Time to peak was shorter, whereas enhanced intensity was greater in soft plaques than in mixed plaques (p < 0.01 and p < 0.05, respectively). Among the 19 unenhanced plaques, 6 were hard, 3 were calcified, 3 were soft, and 7 were mixed. The thickness of the unenhanced plaques was <2.4 mm.

CONCLUSION

Contrast-enhanced sonography allows the noninvasive, dynamic evaluation of neovascularization within carotid plaques, and the presence of neovascularization may correlate with plaque morphology.

Research interface on a programmable ultrasound scanner

MOTIVATION

Commercial ultrasound machines in the past did not provide the ultrasound researchers access to raw ultrasound data. Lack of this ability has impeded evaluation and clinical testing of novel ultrasound algorithms and applications.

OBJECTIVES

Recently, we developed a flexible ultrasound back-end where all the processing for the conventional ultrasound modes, such as B, M, color flow and spectral Doppler, was performed in software. The back-end has been incorporated into a commercial ultrasound machine, the Hitachi HiVision 5500. The goal of this work is to develop an ultrasound research interface on the back-end for acquiring raw ultrasound data from the machine.

METHODS

The research interface has been designed as a software module on the ultrasound back-end. To increase the amount of raw ultrasound data that can be spooled in the limited memory available on the back-end, we have developed a method that can losslessly compress the ultrasound data in real time.

RESULTS AND DISCUSSION

The raw ultrasound data could be obtained in any conventional ultrasound mode, including duplex and triplex modes. Furthermore, use of the research interface does not decrease the frame rate or otherwise affect the clinical usability of the machine. The lossless compression of the ultrasound data in real time can increase the amount of data spooled by approximately 2.3 times, thus allowing more than 6s of raw ultrasound data to be acquired in all the modes. The interface has been used not only for early testing of new ideas with in vitro data from phantoms, but also for acquiring in vivo data for fine-tuning ultrasound applications and conducting clinical studies. We present several examples of how newer ultrasound applications, such as elastography, vibration imaging and 3D imaging, have benefited from this research interface. Since the research interface is entirely implemented in software, it can be deployed on existing HiVision 5500 ultrasound machines and may be easily upgraded in the future.

CONCLUSIONS

The developed research interface can aid researchers in the rapid testing and clinical evaluation of new ultrasound algorithms and applications. Additionally, we believe that our approach would be applicable to designing research interfaces on other ultrasound machines.

Ultrasonic Doppler Vibrometry: Novel Method for Detection of Left Ventricular Wall Vibrations Caused by Poststenotic Coronary Flow

BACKGROUND

A diastolic coronary flow murmur has been reported for patients with coronary stenoses, yet is rarely appreciated during routine auscultation. We hypothesized that an ultrasonic Doppler method can detect the epicardial vibrations associated with this murmur. Ultrasonic Doppler vibrometry is a pulsed wave echocardiography phase demodulation technique designed for detecting vibrations. We correlated the vibration characteristics measured using vibrometry with the angiographic severity of coronary artery stenosis.

METHODS

In a prospective pilot study, 49 patients were recruited for an ultrasound examination before coronary arteriography. An ultrasound instrument was customized to acquire the raw pulsed wave Doppler echocardiographic data from a range gate placed on the left ventricular myocardium near the path of the epicardial coronary arteries.

RESULTS

Patients with angiographically minor stenosis (tightest stenosis < 50% by quantitative coronary angiography, N = 25) had lower diastolic vibration energy (computed as the median spectral energy of myocardial wall velocity in the 100 approximately 1000-Hz frequency band normalized by a baseline diastolic value) compared with patients with moderate or severe stenosis (any stenosis > 50%, N = 24) (P < .001, area under the receiver operating characteristics curve = 0.84). The vibration energy increased with increasing stenosis severity for less severe narrowing (<70%) but decreased for severe narrowing (>70%) (R(2) = 0.21, P < .0002).

CONCLUSIONS

Preliminary evidence indicates that diastolic left ventricular wall vibrations measured using ultrasonic Doppler vibrometry are related to the severity of coronary artery stenoses. With further refinement and validation, this noninvasive and low-cost method could lead to an early screening and monitoring test for coronary artery stenosis.

Ultrasonic interrogation of tissue vibrations in arterial and organ injuries: preliminary in vivo results

Soft tissues surrounding vascular injuries are known to vibrate at audible and palpable frequencies, producing bruits and thrills. We report the results of a feasibility study where Doppler ultrasound (US) was used to quantitatively estimate the tissue vibrations after induced trauma in an animal model. A software-programmable US system was used to acquire quadrature-demodulated ensembles of received US echoes bypassing clutter filtering and other conventional Doppler processing stages. The waveforms of tissue velocity surrounding the injury site were then estimated from the clutter data using autocorrelation and analyzed to determine vibration characteristics. Six New Zealand white rabbits and two juvenile pigs were used for the study. The femoral artery of the anesthetized animal was punctured with an 18-gauge needle to model a peripheral arterial trauma, and the liver was surgically exposed and incised to model organ trauma. Two types of oscillatory tissue motion were observed: "vibrations" with high frequency (>50 Hz) and low peak-peak amplitude (<1 microm) and "flutter" with low frequency (<50 Hz) and high peak-peak amplitude (>1 microm). Active bleeding in femoral artery punctures produced tissue vibrations at the frequency of 323 +/- 214 Hz (mean +/- standard deviation, pooled for both rabbits and pigs) and the amplitude of 0.24 +/- 0.15 microm. Active bleeding in liver incisions produced vibrations at the frequency of 120 +/- 47 Hz and the amplitude of 0.33 +/- 0.25 microm. Flutter was observed in punctured arteries at the frequency of 28 +/- 13 Hz the amplitude of 2.92 +/- 1.75 microm, and in incised livers at the frequency of 26 +/- 6 Hz and the amplitude of 1.53 +/- 0.76 microm. In a punctured artery, the vibration frequency and phase of tissue surrounding the artery were highly correlated between neighboring locations in tissue (correlation coefficient = 0.98), and with the flow oscillations in the lumen (correlation coefficient = 0.96). This preliminary study indicates that tissue vibrations could provide additional physiologic information for detecting, localizing and monitoring internal bleeding using US.

Acoustic radiation from a fluid-filled, subsurface vascular tube with internal turbulent flow due to a constriction

The vibration of a thin-walled cylindrical, compliant viscoelastic tube with internal turbulent flow due to an axisymmetric constriction is studied theoretically and experimentally. Vibration of the tube is considered with internal fluid coupling only, and with coupling to internal-flowing fluid and external stagnant fluid or external tissue-like viscoelastic material. The theoretical analysis includes the adaptation of a model for turbulence in the internal fluid and its vibratory excitation of and interaction with the tube wall and surrounding viscoelastic medium. Analytical predictions are compared with experimental measurements conducted on a flow model system using laser Doppler vibrometry to measure tube vibration and the vibration of the surrounding viscoelastic medium. Fluid pressure within the tube was measured with miniature hydrophones. Discrepancies between theory and experiment, as well as the coupled nature of the fluid-structure interaction, are described. This study is relevant to and may lead to further insight into the patency and mechanisms of vascular failure, as well as diagnostic techniques utilizing noninvasive acoustic measurements.

Ultrasonic vibration dectection with wavelets: preliminary results

Several arterial disorders are known to cause systolic audio vibrations in tissue: they include stenoses, vasospasm, aneurysms, bleeds and arteriovenous fistulas. High-amplitude vibrations can be discovered with conventional Doppler ultrasound (US) instruments; however, differentiating brief, low-amplitude vibrations from other nonstationary echo sources is difficult. Further, characterizing the frequency and amplitude of vibrations is not feasible with conventional Doppler US. The automated detection and estimation of both the frequency and amplitude of vibrations with durations less than 100 ms and amplitudes of a micrometer or less have remained a signal-processing challenge. These vibrations may be associated with both nonstationary colored noise and strong low-frequency clutter. The normalized continuous Morlet wavelet power-spectrum analysis of quadrature Doppler echoes, followed by a binary hypothesis test for noise, results in simulated detection rates above 99.9%, with 0.1% false alarms for signal-on signal-to-noise ratios (SNRs) as low as one. Two clinical examples are included.

A Minor Experimental Stenosis in Porcine Descending Thoracic Aorta Affects the Spectral Content of Pressure Pulse Wave

OBJECTIVE

To carry out research into the possible changes of the spectral content of pressure pulse wave after the creation of an experimentally induced stenosis in the pig descending thoracic aorta.

METHODS

Eight healthy, normotensive Landrace pigs were subjected to thoracotomy under sterile conditions. At the upper segment of the descending thoracic aorta, a 5-mm-long circumferential symmetric constriction was imposed and stabilized; hence, a 15-20 mm Hg pressure gradient was established. Pressure tip catheters were used in order to monitor the pressure gradient. Blood flow disturbances were recorded through a bidirectional Doppler flow meter at pre- and poststenotic areas (A and B, respectively). Measurements were carried out before, 10 min after, and 90 days after the creation of the stenosis. The recorded waveforms were analyzed mathematically by using Fourier Transform, in order to determine their spectral component. Eight sham-operated pigs were used as controls.

RESULTS

Fourier Transform analysis showed a significant increase (P < 0.05) of spectral content in A and B areas. Also, the "relative" harmonic amplitudes in nonstenotic subjects were higher than in stenotic animals (P < 0.05).

CONCLUSIONS

A minor experimental constriction located at the level of the descending thoracic aorta increased the spectral content of the pressure pulse wave, indicating that spectral analysis may detect slight flow disturbances before developing remarkable signs of an impaired circulation system.

Tissue Doppler imaging of carotid plaque wall motion: a pilot study

Background

Studies suggest the physical and mechanical properties of vessel walls and plaque may be of clinical value in the diagnosis and treatment of cardiovascular atherosclerotic disease. The purpose of this pilot study was to investigate the potential clinical application of ultrasound Tissue Doppler Imaging (TDI) of Arterial Wall Motion (AWM) and to quantify simple wall motion indices in normal and diseased carotid arteries.

Methods

224 normal and diseased carotid arteries (0–100% stenoses) were imaged in 126 patients (age 25–88 years, mean 68 ± 11). Longitudinal sections of the carotid bifurcation were imaged using a Philips HDI5000 scanner and L12-5 probe under optimized TDI settings. Temporal and spatial AWMs were analyzed to evaluate the vessel wall displacements and spatial gradients at peak systole averaged over 5 cardiac cycles.

Results

AWM data were successfully extracted in 91% of cases. Within the carotid bifurcation/plaque region, the maximum wall dilation at peak systole ranged from -100 to 750 microns, mean 335 ± 138 microns. Maximum wall dilation spatial gradients ranged 0–0.49, mean 0.14 ± 0.08. The AWM parameters showed a wide variation and had poor correlation with stenoses severity. Case studies illustrated a variety of pertinent qualitative and quantitative wall motion features related to the biophysics of arterial disease.

Conclusion

Our clinical experience, using a challenging but realistic imaging protocol, suggests the use of simple quantitative AWM measures may have limitations due to high variability. Despite this, pertinent features of AWM in normal and diseased arteries demonstrate the potential clinical benefit of the biomechanical information provided by TDI.

Angle-independent estimation of maximum velocity through stenoses using vector Doppler ultrasound

Categorisation for arterial stenoses treatment is determined primarily by the degree of occlusion, which is often estimated ultrasonically from blood velocity measurements. In current single-beam ultrasound (US) systems, this estimate can suffer from gross errors due to angle-dependence. The purpose of this study was to find out if an experimental dual-beam US system could reduce the angle-dependence of the velocity estimates. We compared four dual-beam velocity estimation algorithms on both a string phantom and straight tube wall-less flow phantoms incorporating symmetrical and asymmetrical stenoses from 0% to 91% by area. The estimated maximum velocity varied, on average, by 7.6% for beam-vessel angles from 40 degrees to 80 degrees. The fluctuation in the magnitude estimate was reduced by a factor of 2.6 using a hybrid single-dual-beam algorithm. We conclude that, when the true velocity lies in the scan plane, the dual-beam system reduces the angle-dependence and, thus, has the potential to improve categorisation of patients with arterial stenoses.