Aris Dermitzakis

The Acoustic Scatter from Single biSphere Microbubbles

Single microbubble acoustic acquisitions provide information on the behaviour of microbubble populations by enabling the generation of large amounts of data. Acoustic signals from single polylactide-shelled and albumin coated biSphere™ microbubbles have been acquired. The responses observed from a range of incident frequencies and acoustic pressures varied in duration. Partial echoes shorter than the incident pulse duration have been observed for low frequency pulses of sufficient amplitude, suggesting release of gas from bubbles. The results presented suggest that the mechanism of scatter from hard shelled agents may be shell disruption and gas release, or partly from gas leaking from defected shell sites, which has previously not been observed optically. These results can provide the basis for improved imaging through optimization of incident pulse parameters, with potential benefits to both diagnostic and therapeutic techniques.

Modeling of small carbon fiber-reinforced polymers for X-ray imaging simulation

A methodology for generation of realistic three-dimensional software models of carbon fiber-reinforced polymer (CFRP) structures, dedicated for use in simulation studies of advanced X-ray imaging techniques for non-destructive testing (NDT), has been developed, implemented, and evaluated. Two CFRP models are presented in this paper, one built as a set of stacked layers that contain continuous carbon bundles and a second as a braided textile from woven carbon bundles. The following CFRP defects were modeled: porosity, missing carbon bundles, and non-carbon inclusions. X-ray projection images were generated using an in-house developed X-ray imaging simulator. The obtained preliminary visual and quantitative validation results showed an overall good correlation of characteristics between synthetic and experimental data radiographs and justify the use of this model for research in CFRP X-ray imaging. The application of the CFRP model is demonstrated in a feasibility study that aims to computationally evaluate the appropriateness of two advanced X-ray imaging techniques: cone-beam CT (CBCT) and tomosynthesis (limited arc tomography), as inspection techniques for NDT of CFRP parts. The simulation showed that in all cases the CBCT approach outperformed both conventional radiography and tomosynthesis in terms of defect characterization and visualization.

Survival of single microbubbles insonated in solution and in narrow tubes

To optimise the use of contrast agents clinically, it is necessary to have a thorough understanding of their behaviour. We have assessed the acoustic response of biSphere¿ microbubbles in both free and 200 ¿m rigid tube environments. An experimental system utilising a Philips research scanner Sonos 5500, previously described, has been used for the assessment of the acoustic response of microbubble contrast agent in solution. More recently a version of this system which enabled the investigation of the acoustic response of single microbubbles in tubes from 200¿m to 12¿m diameter has been TM designed. BiSphere microbubbles were insonated with 7 identical consecutive pulses, at 1ms intervals, using a 6-cycle, 1.6MHz ultrasound pulse for peak negative acoustic pressures ranging 160-1000 kPa. The tubes used for the data described below were 200¿m cellulose tubes with a gravity feed inducing the flow. TM For biSphere , data for 800 single bubbles in tubes and 800 free single bubbles has been analysed by decomposing the calibrated backscattered RF signal into fundamental and harmonic components. The fundamental RMS backscatter from free microbubbles was generally higher than from tubed microbubbles. For rigid shelled biSphere microbubbles, more destruction was observed in tubed microbubbles than in free. The presence of the tube is not thought to have a significant effect on the incident acoustic field. The behaviour of free and microbubbles in 200¿m tubes is not identical and this should be addressed theoretically. Future work will collect data from a range of tube diameters to determine microbubble behaviour at the range of vessel diameters encountered in vivo.

New Line Contrast Figure of Merit for image quality assessment

A novel Figure of Merit (FoM) is presented which is based on analysis of data extracted from line profiles within the image. This FoM is referred as Line Contrast (LC). In order to evaluate its performance, the proposed FoM along with Contrast to Noise Ratio (CNR), are applied in a simulated phase contrast image and the resulting figures are compared. Phase contrast image is created using the XRAYImagingSimulator an in-house developed software platform for x-ray imaging simulation. A semi-cylindrical phantom especially designed for phase contrast applications that contains materials mimicing the refractive properties (indexes) of breast tissues was used. Images were produced at 20 keV with source to isocenter distance (SID) 23m and object to detector distance (ODD) 400mm and pixel size of 5μm. Results from the application of both FoMs shows that LC comes into full accordance with visual assessment and that it overpass the traditionally used CNR, especially in the ability of detecting the edge enhancement due to phase contrast, of the different structures within the image. The edge detection features make it optimal for phase contrast investigation where the effect on the edge enhancement is of outmost importance.

Super-Resolved Ultrasound Echo Spectra With Simultaneous Localization Using Parametric Statistical Estimation

Ultrasound contrast imaging (UCI) aims to detect flow changes in the vascular bed that can help differentiate normal from diseased tissues thus providing an early screening tool for diagnosis or treatment monitoring. Ultrasound contrast agents (UCAs), used in UCI, are microbubbles that scatter ultrasound non-linearly. To date the signal processing research has successfully subtracted signals from the linear response of tissue (linear signals), but, in general, has not provided a sensitive detection that is specific to the UCA signal. This paper develops a method for the temporal and spectral estimation of linear and non-linear ultrasound echo signals. This technique is based on non-parametric methods for coarse estimation, followed by a parametric method within a Bayesian framework for estimation refinement. The results show that the pulse location can be estimated to within ±3 sample points accuracy for signals consisting of ≈ 80 sample points depending on the signal type, while the frequency content can be estimated to within 0.050 MHz deviations for frequencies in the 1 to 4 MHz range. This parametric spectral estimation achieved a 5-fold improvement in the frequency resolution compared with Fourier-based methods, and revealed previously unresolved frequency information that led to over 80% correct signal classification for linear and non-linear echo signals.

Line Contrast Figure of Merit for Dual Energy X-ray Image Quality Assessment: Initial Results

Various Figures of Merits (FoMs) are used in order to evaluate the quality of x-ray images. A new Figure of Merit (FoM) is presented, which is based on analysis of data extracted from line profiles within the image. This FoM is referred as Line Contrast (LC). In order to evaluate its performance, the proposed FoM along with Contrast to Noise Ratio (CNR) and Contrast Michelson are applied in simulated Brain CT images and the resulting FoM values are compared. Brain CT images are created using the XRAYImagingSimulator, an in-house developed software platform for x-ray imaging simulation. A 3D brain phantom especially modified for x-ray imaging that contains volumes of gray, white, skull and cerebrospinal fluid was used. Presented Dual Energy CT images were produced with the low/high energy combination of 100/110 keV monochromatic beams. All images were evaluated using CNR, Contrast Michelson and LC FoM, both with and without the simulated presence of noise. Initial results from the application of FoMs show that LC comes into full accordance with visual assessment and that it overpasses the traditionally used CNR and Contrast Michelson. Initial results are presented here, while further studies on the evaluation of the proposed FoM are in progress.

Biomedical Engineering Education: Need for Harmonisation

Medical technology has radically reshaped the way healthcare is delivered today and continues to improve it in an accelerated pace. Biomedical Engineering (BME) is a multidisciplinary field lying in the cross-section of medical/biological sciences and engineering. Healthcare today is technology-driven and delivered by teams rather than individuals. Biomedical Engineers (BMEs) as professionals are playing a vital role in these developments, being behind the recent advances and involved during the whole life cycle of Medical Devices (MDs), from the innovative idea to their final use. More than five hundred thousand MDs models are available in the world market today. As a result, the BME profession is expected to be the first in demand amongst all other engineering subspecialties in the years to come. However, this rapid evolution creates a constant pressure for new knowledge and skills for the BMEs and therefore for continuous curriculum updates in order to meet R&D and market demands, but also for harmonisation of studies worldwide that will facilitate staff and students’ mobility and collaboration.

The polydisperse acoustic signature of rigid microbubbles

Microbubbles are used in medical ultrasound imaging as contrast agents to image the vascular bed under the mode of Ultrasound Contrast Imaging (UCI). The microbubble shell determines the acoustic response and hence the signal that is utilized to form the images in UCI. Single microbubble signals from BiSphere™ (POINT Biomedical, San Carlos, CA, USA) microbubbles were captured using a clinical ultrasound system. Three main typical responses of microbubbles were identified, a) full duration echo, b) echo with duration shorter than the incident pulse and c) echo that in part resembles that in (b) and in addition prior to that another short duration initial lower amplitude signal. These data corroborate that the shell structural and nanomechanical property provide the different responses at different microbubble sizes. These different signals present an opportunity for tracking the movement of well differentiated single microbubbles particularly with novel super-resolution imaging methods that require sparse microbubble populations.

A setup for the assessment of the effect of tubular confinement on the acoustic response of microbubbles.

Ultrasound contrast agents are gas filled microbubbles which produced enhanced echoes in ultrasound imaging thus allowing the acquisition of detailed information on the path of blood. It is theoretically known that the size of a vessel affects the behavior of a microbubble, which could potentially be used to discriminate different sized vessels. This information would be useful in the monitoring of neovascularization in tumor growth or treatment. However, currently it is not possible to identify the vessel diameter by any means of signal processing of microbubble echoes. In order to assess microbubble behavior when confined in tubes we compared the acoustic backscatter from biSphere™ microbubbles both free in water and flowing in 200 μm diameter tubes that are similar in size to arterioles. Experimental systems that allow the interrogation of individual microbubbles were designed and modified to allow investigation of both free microbubbles and those in tubes. Unprocessed single microbubble RF data were collected, allowing the calculation of both the fundamental and second harmonic components of the backscattered signal. Microbubbles confined in tubes had lower amplitude response compared to unconfined microbubbles. On consecutive insonations of the same microbubble, free microbubbles produced echoes above noise more often than confined microbubbles. This setup may be used to investigate microbubble behavior in a range of smaller tubes with diameters similar to capillaries thus enabling signal processing design for vessel differentiation.

Modelling of small CFRP aerostructure parts for X-ray imaging simulation

Purpose – The purpose of this paper is to develop a realistic computational model of carbon fibre reinforced polymer (CFRP) structures dedicated for in-silico investigations of the use of X-ray-based imaging techniques as non-destructive testing (NDT) of CFRP parts. Design/methodology/approach – CFRPs contain layers of carbon-fibres bundles within resin. Bundles’ orientation in the different layers is arranged with respect to each other at a well-defined primary direction. In the model, the bundle was simulated as a circular cylinder. The resulted model is a stack of layers of unidirectional bundles having orientation of 0°/90°/45°/−45°. Two CFRP structures were modelled: a flat CFRP part and a real shaped CFRP clip. A porous layer and non-carbon fibres were inserted within each model, respectively. X-ray projection images were generated with a dedicated simulation programme. Three setups were investigated: radiography, tomosynthesis and cone-beam CT (CBCT). Findings – Results showed that porosity and non-carbon fibres were visible with all X-ray-based techniques. Tomosynthesis and CBCT, however, provide higher quality image of defects. Practical implications – The CFRP computational model is a valuable tool in design, testing and optimization phase of X-ray-based imaging techniques for use in NDT of composite materials. Simulated images are generated within a short time; thus results from virtual optimization and testing are obtained very fast and at low cost. Originality/value – An innovative computational model of CFRP structures, dedicated for X-ray imaging simulations, has been developed. The model is characterized by simplicity in its creation and realistic visual appearance of the produced X-ray images.