Dariush K Nassiri

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.

Online artery diameter measurement in ultrasound images using artificial neural networks.

An automated online technique is described for measurement of artery diameter in flow-mediated dilation (FMD) ultrasound (US) images, using artificial neural networks to identify and track artery walls. This allows FMD results to be calculated without the inherent delay of current retrospective methods. Two networks were trained to identify artery anterior and posterior walls using over 3200 examples from carotid artery images. Both networks correctly classified approximately 97% of the randomly selected test samples. The technique was verified using a physical model with absolute measurement error of -1.16% +/- 1.04% (mean +/- SD) over the diameter range 2 to 8 mm. Advantages of the technique include: online analysis; wall tracking optimisation before the study proper; measurement of diameter changes over the cardiac cycle; low FMD measurement variance; minimal image degradation; and no unwieldy image store. Measurement of artery diameter changes over the cardiac cycle was explored using simulated image sequences generated with a virtual US scanner.

Measurement of thermal and ultrasonic properties of some biological tissues.

The measurement of thermal and ultrasonic properties of biological tissues is essential for the assessment of the temperature rise induced in vivo by diagnostic ultrasound. In this paper, we present measurements of thermal conductivity, thermal diffusivity, speed of sound and ultrasonic attenuation of fresh ex vivo porcine tissue, namely ‘muscle’ (from abdomen and leg), ‘skin with subcutaneous fat’ (from abdomen and leg), ‘abdominal fat’ and ‘bone’. The measurements of the thermal properties of biological tissue samples are based on a transient method. Thermal property measurements show that subcutaneous fat has the lowest thermal conductivity (0.23 W m−1 K−1), while muscle gives the highest values (0.46 W m−1 K−1). Thermal diffusivity of muscle tissue recorded the highest value among the studied tissues (0.16 mm2 s−1) while that of skin with subcutaneous fat gave the lowest value (0.11 mm2 s−1). A scanning acoustic macroscope was used to measure attenuation coefficient and speed of sound for the tissue samples. The results for the speed of sound are broadly similar to those reported in the literature. The power law dependence of the attenuation coefficient of the form η = a f b as a function of frequency was found to be more appropriate than the linear fit in this study.

A rapid and reproducible on line automated technique to determine endothelial function

Objective: To evaluate clinically a new on line, automated technique to measure flow mediated dilatation (FMD) as a marker of endothelial function. Design: Prospective study. Patients: 12 healthy volunteers and 12 patients with significant, angiographically documented coronary artery disease. Interventions: Brachial arteries were imaged using a standard vascular ultrasound system with a 5–12 MHz linear transducer. Arterial diameter was measured on line (in real time) by connecting the ultrasound system to a personal computer equipped with a frame grabber and artery wall detection software (VIA) specially developed by the authors’ group. By using this new technique, FMD was measured following 4.5 minutes of ischaemia of the proximal forearm in all subjects on two separate days. Results: The mean (SD) day to day variability in FMD measurements was 0.90 (0.48)%,which compares very favourably with current methods. The FMD measurement was available within seconds of completing the scan. Conclusions: Personal computer based automated techniques to assess FMD involve image acquisition and recording after which a second (off line) image interpretation session is required. The need for off line analysis makes current methods time consuming and increases the variability of measurement. This on line, automated analysis technique for FMD assessment reduces the variability and greatly increases the speed of measurement. Using this system may mean that fewer patients will be required in clinical trials assessing the effects of interventions on endothelial function. Adopting this method may also facilitate the screening of larger numbers of subjects for endothelial dysfunction.

Hardness and ultrasonic characteristics of the human crystalline lens

Purpose: To evaluate the relationship between hardness of the human crystalline lens and its acoustic characteristics. Setting: St. Georges Hospital Medical School, University of London, London, United Kingdom. Methods: Cataractous lenses from patients who had extracapsular cataract surgery were studied for hardness and ultrasonic characteristics. Lens hardness was assessed with an automated guillotine. Ultrasound velocity and attenuation were measured with a scanning acoustic macroscope using the pulse transmission reflection method. Results: Thirty‐seven lenses from 37 patients (mean age 75.5 years) were evaluated. Lens hardness was associated with ultrasound attenuation (r = 0.65, P < .0001) and attenuation frequency gradient (r = 0.67, P < .0001). The correlation of hardness with mean ultrasound velocity was not significant (r = 0.22, P = .2). Conclusions: The attenuation of ultrasound waves by the human crystalline lens correlated with its hardness. Ultrasonography can be used to evaluate lens hardness.

Estimation of in situ ultrasound exposure during obstetric examinations

Layered tissue models are developed to estimate in situ ultrasound intensity during common obstetric examinations by incorporation of measured overlying tissue thicknesses with data on the attenuation properties of tissues. Results are compared with attenuation models recommended by the United States Food and Drug Administration (FDA) and National Council on Radiation Protection and Measurements (NCRP). For abdominal and transvaginal scanning in the first trimester, fixed attenuation models based on attenuation values of 1.2 dB/MHz and 0.6 dB/MHz, respectively, are representative of worst-case exposure conditions. For second and third trimesters, a fixed attenuation value of 0.8 dB/MHz is representative of worst-case exposure conditions. A fixed attenuation value of 1.0 dB/MHz is suggested for common Doppler examinations. The study suggests that the FDA-derating factor of 0.3 dB/cm.MHz may not give a conservative estimate of in situ intensity for certain obstetric examinations.

Microemboli detection using ultrasound backscatter

Microemboli detection and characterisation have recently received great attention due to its clinical importance in the management of cerebrovascular disease. The new method presented in this paper is directly based on the idea that the ultrasound (US) backscattered signal from flowing blood is chaotic (El-Brawany and Nassiri 2002). The detection technique involves building a nonlinear model of the deterministic characteristics of the chaotic backscatter signal from blood, and the use of this model to look at the prediction error as a primary decision-making criterion for the microemboli detector. A complementary feature to the prediction error, namely, the degree of coherence between the US excitation pulse and the prediction error signal is used to enhance the detection process. The detector has been built using a feed-forward neural network with error back-propagation. The detection technique is tested successfully using a vascular flow phantom with solid spheres and bubbles of known sizes introduced in the flow circuit to mimic solid and gaseous emboli. Receiver operating characteristic (ROC) curve is used to assess the performance of the detection process. The total classification rate ranges from 88% to 96%.

The relationship between nuclear colour and opalescence on the LOCSIII scale and physical characteristics of cataract nuclei.

Purpose To evaluate the compression characteristics of the human lens nucleocortex in relation to its LOCSIII clinical grading.Methods Sixteen subjects undergoing planned extracapsular cataract surgery had pre-operative slit-lamp examination and assessment of cataract LOCSIII grade followed by postoperative in vitro evaluation of the nucleus with measurement of ‘linear compressibility’ by a purpose-designed caliper incorporating a strain gauge, enabling the derivation of a graph of nuclear compression (D (mm) against applied force (F (N)).Results Nuclear colour correlates with the force required to compress a lens to 75% of its original depth (F75) (R = 0.625, P = 0.017). Nuclear opalescence correlates with the force required to compress a lens to 75% of its original depth (R = 0.651, P = 0.012) and inversely with linear compressibility (ΔD/ΔF, the slope of the graph of nuclear compression against applied force) (R = −0.610, P = 0.014). F75 is a direct and linear compressibility is an inverse related parameter of lens nucleus ‘hardness’.Conclusion A new instrument is described which allows measurement of ‘hardness’-related compression characteristics of the human cataract in vitro. There is a relationship between the LOCSIII clinical classification of nuclear cataracts and mechanical compression characteristics of the cataractous lens. LOCSIII classification may aid the preoperative planning of an appropriate surgical approach to an individual cataract.

New approach for modelling ultrasound blood backscatter signal

The ultrasound (US) scattered signal from blood has been treated as a random signal by many investigators. However, the degree of randomness of a medium is a relative term that can change considerably with the resolution of the sensor. In this study, the backscattered signal from blood has been looked at as a chaotic signal. By this treatment, according to Takens theorem, a single variable (e.g., amplitude of the blood-backscattered signal) can be used to reconstruct the nonlinear dynamics of the blood-scattered signal. Multilayer perceptron neural network architecture, with error back-propagation, has been formulated and used as a basis for building and testing the chaotic model of the backscattered signal. This chaotic model is used successfully as a short-term predictor of the backscattered signal from blood-mimicking fluid (BMF) flowing in a vascular flow phantom under pulsatile flow. This modelling approach can be useful, for example, in detecting blood-borne emboli.

The Automated Assessment of Ultrasound Scanner Lateral and Slice Thickness Resolution: Use of the Step Response

The imaging performance assessment of ultrasound scanners based on traditional phantoms is limited by repeatability, subjectivity and systematic errors giving low confidence in results. A new approach to the automated measurement of scanner resolution is described. The method utilises a step change in backscatter to derive resolution from the imaging system line spread function and has been used to calculate resolution in two dimensions as a continuous function of depth. Resolution data was used to calculate resolution integrals for both lateral and slice thickness independently. For resolution integral repeatability, analysis of variance showed no significant difference between operators (p=0.05) with intra and inter-operator repeatability (+/-1 standard deviation) of 1.5% and 1.5% for lateral resolution, respectively, and 2.6% and 3.3% for slice thickness, respectively. Low contrast penetration was also calculated automatically and the worst case operator repeatability was 1.3%. The acoustic properties of the phantom were validated. The possibility of extending the technique to axial resolution is discussed.