Kumar V. Ramnarine

Validation of a New Blood-Mimicking Fluid for Use in Doppler Flow Test Objects

A blood-mimicking fluid (BMF) suitable for use in Doppler flow test objects is described and characterised. The BMF consists of 5 microns diameter nylon scattering particles suspended in a fluid base of water, glycerol, dextran and surfactant. The acoustical properties of various BMF preparations were measured under uniform flow to study the effects of particle size, particle concentration, surfactant concentration, flow rate and stability. The physical properties, (density, viscosity and particle size), and acoustical properties (velocity, backscatter and attenuation) of the BMF are within draft International Electrotechnical Commission requirements.

ASSESSMENT OF THE ACOUSTIC PROPERTIES OF COMMON TISSUE- MIMICKING TEST PHANTOMS

Ultrasound (US) test phantoms incorporating tissue-mimicking materials (TMMs) play an important role in the quality control (QC) and performance testing of US equipment. Three commercially available TMMs (Zerdine from CIRS Inc.; condensed-milk-based gel from Gammex RMI; urethane-rubber-based from ATS Labs) and a noncommercial agar-based TMM, were investigated. Acoustic properties were measured over the frequency range 2.25 to 15 MHz at a range of ambient temperatures (10 to 35 degrees C). The acoustic velocity of the TMMs remained relatively constant with increasing frequency. Only the agar-based TMM had a linear increase of attenuation with frequency, with the other materials exhibiting nonlinear responses to varying degrees (f(1.08) to f(1.83)). The acoustic velocity and attenuation coefficient of all the TMMs varied with temperature, with the urethane-rubber TMM showing the greatest variation of +/- 1.2% for acoustic velocity and +/- 12% for attenuation coefficient. The data obtained in this study highlight the importance of greater knowledge of the acoustic behavior of TMMs to variations in both frequency and temperature, to ensure that accurate and precise measurements are obtained during QC and performance testing.

Construction and geometric stability of physiological flow rate wall-less stenosis phantoms

Wall-less flow phantoms are preferred for ultrasound (US) because tissue-mimicking material (TMM) with good acoustical properties can be made and cast to form anatomical models. The construction and geometrical stability of wall-less TMM flow phantoms is described using a novel method of sealing to prevent leakage of the blood-mimicking fluid (BMF). Wall-less stenosis flow models were constructed using a robust agar-based TMM and sealed using reticulated foam at the inlet and outlet tubes. There was no BMF leakage at the highest flow rate of 2.8 L/min in 0%, 35% and 57% diameter reduction stenoses models. Failure of the 75% stenosis model, due to TMM fracture, occurred at maximum flow rate of 2 L/min (mean velocity 10 m/s within the stenosis). No change of stenosis geometry was measured over 4 days. The construction is simple and effective and extends the possibility for high flow rate studies using robust TMM wall-less phantoms.

Doppler backscatter properties of a blood-mimicking fluid for Doppler performance assessment

The Doppler backscatter properties of a blood-mimickig fluid (BMF) were studied to evaluate its suitability for use in a Doppler flow test object. Measurements were performed using a flow rig with C-flex tubing and BMF flow produced by a roller pump or a gear pump. A SciMed Doppler system was used to measure the backscattered Doppler power with a root-mean-square power meter connected to the audio output. Studies investigated the dependence of the backscattered Doppler power of the BMF with: circulation time; batch and operator preparations; storage; sieve size; flow speed; and pump type. A comparison was made with human red blood cells resuspended in saline. The backscatter properties are stable and within International Electrotechnical Commission requirements. The BMF is suitable for use in a test object for Doppler performance assessment.

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

BackgroundStudies 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.Methods224 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.ResultsAWM 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.ConclusionOur 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.

Embolus Trajectory Through a Physical Replica of the Major Cerebral Arteries

Background and Purpose— The observed distribution of cerebral infarcts varies markedly from expectations based on blood-flow volume or Doppler embolus detection. In this study, we used an in vitro model of the cerebral arteries to test whether embolus microspheres encountering the circle of Willis are carried proportionally to volume flow or express a preferred trajectory related to arterial morphology or embolus size. Methods— Our model consisted of a patient-specific silicone replica of the cerebral macrocirculation featuring physiologically realistic pulsatile flow of a blood-mimicking fluid at approximately 1000 mL/min and an input pressure of approximately 150/70 mm Hg. Particles of 200, 500, and 1000 &mgr;m diameter with equivalent density to thrombus were introduced to the carotid arteries and counted on exiting the model outlets. Results— The middle cerebral arteries (MCAs) of the replica attracted a disproportionate number of emboli compared with the anterior cerebral arteries; 98%±3% of 1000 &mgr;m and 93%±2% of 500 &mgr;m emboli entered the MCA compared with 82%±5% of the flow. The observed distribution of large emboli was consistent with the ratio of MCA:anterior cerebral artery infarcts, approximately 95% of which occur in territories supplied by the MCA. With decreasing embolus size, the distribution of emboli approaches that of the flow (approximately 89% of 200 &mgr;m emboli took the MCA). Conclusions— Embolus trajectory through the cerebral arteries is dependent on embolus size and strongly favors the MCA for large emboli. The 70:30 ratio of MCA:anterior cerebral artery emboli observed by Doppler ultrasound is consistent with the trajectories of small emboli that tend to be asymptomatic.

Shear Wave Elastography Assessment of Carotid Plaque Stiffness: In Vitro Reproducibility Study

This study assessed inter- and intra-observer reproducibility of shear wave elastography (SWE) measurements in vessel phantoms simulating soft and hard carotid plaque under steady and pulsatile flow conditions. Supersonic SWE was used to acquire cine-loop data and quantify Youngs modulus in cryogel vessel phantoms. Data were acquired by two observers, each performing three repeat measurements. Mean Youngs modulus was quantified within 2-mm regions of interest averaged across five frames and, depending on vessel model and observer, ranged from 28 to 240 kPa. The mean inter-frame coefficient of variation (CV) was 0.13 (range: 0.07-0.18) for observer 1 and 0.14 (range: 0.12-0.16) for observer 2, with mean intra-class correlation coefficients (ICCs) of 0.84 and 0.83, respectively. The mean inter-operator CV was 0.13 (range: 0.08-0.20), with a mean ICC of 0.76 (range: 0.69-0.82). Our findings indicate that SWE can quantify Youngs modulus of carotid plaque phantoms with good reproducibility, even in the presence of pulsatile flow.

Improved characterisation of focal liver tumours: dynamic power Doppler imaging using NC100100 echo-enhancer

OBJECTIVE To assess the vascularisation of focal hepatic tumours using NC100100, enhanced power Doppler imaging. METHODS Twenty-two patients with focal liver tumours (12 metastases and ten hemangiomas) were studied. Using standardised settings, power Doppler imaging with ATL HDI3000 was performed before and after intravenous administration of NC100100 contrast agent. The video-recorded examinations were digitised for off-line analysis on a personal computer. Regions of interest were defined over the entire tumour and a neighbouring area of the normal liver parenchyma. The temporal changes of the mean power Doppler signal intensity (PDSI) was quantified to provide contrast agent wash-in (PDSI-time) curves for the initial 40 s. RESULTS Liver metastases were characterised by a rapid increase in PDSI, while the PDSI-time curves within hemangiomas were flat. The PDSI within the tumour increased significantly in ten subjects with liver metastases and only one subject with hemangioma. An enhanced rim around hemangiomas was seen in four subjects. There was no clear relationship between the contrast agent dose and the peak PDSI within metastases. CONCLUSIONS Power Doppler imaging with NC100100 contrast agent enhances tumour visualisation and may aid differential diagnosis of focal liver lesions.

Enhanced Detection of Thromboemboli With the Use of Targeted Microbubbles

Background and Purpose— Targeted ultrasound contrast agents have recently been developed to adhere selectively to specific pathogenic materials such as plaque or thrombus. Administration of such microbubbles has potential to aid transcranial Doppler ultrasound (TCD) detection of emboli and to act as markers for distinguishing one embolic material from another. The purpose of this study was to investigate whether TCD detection of circulating thrombus emboli would be enhanced by the addition of targeted microbubbles. Methods— Binding of microbubbles to the surface of the thrombus was confirmed by scanning electron microscopy. Targeted and control bubbles were then introduced to thrombus and tissue-mimicking material circulated under pulsatile-flow conditions in an in vitro flow rig. Embolic signal intensities before and after introduction of the bubbles were measured by TCD. Results— Targeted microbubbles enhanced TCD signal intensities from thrombus emboli by up to 13 dB. The bubbles were capable of binding to moving thrombus when injected into the flow circuit in low concentrations (≈36 bubbles per 100 mL) and were retained on the thrombus under pulsatile-flow conditions. Signal intensities from similarly sized pieces of tissue-mimicking material were not enhanced by injection of targeted bubbles. Conclusions— Injection of appropriately targeted microbubbles significantly enhances TCD detection of circulating thrombus emboli in vitro.

Contrast-Enhanced Doppler Perfusion Index Clinical and Experimental Evaluation

Objective. To assess the potential of the power Doppler signal intensity rate of enhancement due to contrast agent wash‐in for assessment of hepatic hemodynamics. Methods. With the use of standardized settings, power Doppler sonography was performed before and after administration of a contrast agent. Video‐recorded examinations were digitized for offline analysis on a personal computer. The temporal changes of the power Doppler signal intensity were quantified to provide contrast agent wash‐in curves. The contrast‐enhanced Doppler perfusion index was defined by the ratio of the wash‐in gradient of the hepatic artery and portal vein as contrast‐enhanced Doppler perfusion index = hepatic artery gradient/(hepatic artery gradient + portal vein gradient). The contrast‐enhanced Doppler perfusion index was evaluated at 4 contrast agent doses in each of 14 patients with liver metastases and 3 patients with hemangiomas. An in vitro flow model was used to determine the relationships between the power Doppler rate of enhancement and flow in vessels of 4, 8, and 12 mm in diameter. Results. In vivo, there was a significantly higher (P < .0001) mean contrast enhanced Doppler perfusion index in patients with liver metastases (mean, 0.59; 95% confidence interval, 0.54–0.63), compared with patients with hemangiomas (mean, 0.33; 95% confidence interval, 0.24–0.41). The corresponding coefficients of variations were 25% for patients with liver metastases and 31% for patients with hemangiomas. In vitro, the power Doppler rate of enhancement was proportional to flow speed and independent of vessel diameter. Conclusions. Measurement of the contrast‐enhanced Doppler perfusion index may have potential in assessment of hepatic hemodynamics and focal liver disease.