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Biomedical subjects

S D Pye

Publications and source records attributed to S D Pye.

At least 19 recordsLinked to original sources
PubMed2006

Nanointerrogation of ultrasonic contrast agent microbubbles using atomic force microscopy.

Predicting the acoustic response of an encapsulated microbubble to ultrasound requires an accurate assessment of the mechanical properties of the microbubble shell. Atomic force microscopy (AFM) provides an unprecedented spatial and force resolution of the order of Angstroms and subnanonewtons, respectively. It is introduced here as a means to interrogate microbubbles manufactured for ultrasonic imaging. The advantage of AFM over scanning electron microscopy (SEM) is that the microbubbles need not be subjected to a low temperature or low-pressure environment. The microbubbles were interrogated in a liquid environment, which could potentially be a simulated physiological environment. AFM was used in tapping mode imaging to reveal topographical detail of biSphere microbubbles. Because microbubbles are large objects compared with the overall size of usual AFM tips, a convolution between the AFM tip and the microbubble was typical of the acquired topographies. However, a part of the top half of the bubble was imaged with nanometer resolution, and roughness measurements are reported. Force-distance curves were captured using contact mode AFM. The range of stiffness or effective spring constant of biSphere was found to be between 1 and 6 N m(-1). In conclusion, the AFM is proposed here for the first time as a tool to image the surface of bubbles at the nanometer range in liquid and to perform reproducible measurements on the mechanical properties of individual microbubbles.

Biomechanical Phenomena↗
PubMed2004

An in vitro study of a microbubble contrast agent using a clinical ultrasound imaging system.

Optimal insonation settings for contrast imaging are yet to be specified, mainly due to the lack of good understanding of the behaviour of the microbubbles. A satisfactory model that explains the behaviour of individual contrast agent scatterers has not yet been reported in the literature. An in vitro system based on a commercial scanner (ATL HDI3000) has been developed to investigate the backscatter of such agents. Suspensions of Definity were introduced in an anechoic tank. The frequency of transmitted ultrasound varied from 1 to 5 MHz, pulse period from 2 to 10 periods and peak negative acoustic pressure from 0.08 to 1.7 MPa. The backscatter at the fundamental and second harmonic frequency windows from the agent was normalized in terms of the corresponding components of backscatter from a blood mimicking fluid suspension. The agent provided a dominant resonance effect at 1.6 MHz transmit frequency. Second harmonic normalized backscatter averaged around 9 dB higher than the fundamental. The normalized fundamental backscatter intensity was linear with peak negative pressure. The second harmonic at resonance peaked at 0.5 MPa suggestive of bubble disruption above such pressure. The system proved capable of illustrating the ultrasonic behaviour of Definity in vitro, and the investigation suggested particular insonation conditions for optimal image enhancement using Definity.

Acoustics↗
PubMed2003

The behaviour of individual contrast agent microbubbles.

In recent years, our knowledge of the behaviour of ultrasonic microbubble contrast agents has improved substantially through in vitro experiments. However, there has been a tendency to use high concentrations of contrast agents in suspension, so that ultrasonic backscatter data are generated by a cloud of microbubbles. Such experiments involve a variety of assumptions with validity that is open to question. In addition, high concentrations of microbubbles cannot be used to understand the behaviour of individual microbubble scatterers. This paper proposes a technique that minimises the number of assumptions that need to be made to interpret in vitro experimental data. The basis of the technique is a dilute suspension of microbubbles that makes single scattering events distinguishable. A commercial scanner was used to collect radio frequency (RF) data from suspensions of two different contrast agents, Quantison and Definity. Backscatter data were collected over a range of acoustic pressures. It was found that Definity provided a constant number of scattering events per unit volume of suspension for almost all applied acoustic pressures. Quantison demonstrated an increasing number of scattering events per unit volume with increasing acoustic pressure. Below 0.6 MPa, Quantison scatterers were not individually detectable and provided levels of backscatter similar to those of a blood-mimicking fluid, which suggests that Quantison microbubbles had almost linear scattering behaviour. At acoustic pressures greater than 0.6 MPa, both agents appeared to provide echoes from free bubbles. The change in the number of scatterers per unit volume with acoustic pressure cannot be demonstrated using high concentrations of contrast agent.

Acoustics↗
PubMed2002

Understanding the limitations of ultrasonic backscatter measurements from microbubble populations.

Despite over ten years of in vitro investigations of ultrasound contrast agents, the level of understanding of their behaviour in ultrasound fields is limited. Several problems associated with these investigations, particular to the nature of contrast agents, are discussed. Using a commercial scanner the RF normalized backscatter of two different contrast agents (Definity and Quantison) was measured at different suspension concentrations and acoustic pressures. Both contrast agents scattered ultrasound nonlinearly and the backscatter showed a dependence on acoustic pressure. In order to assess the average behaviour of the agents across the range of acoustic pressures and microbubble concentrations the experimental data were fitted to a theoretically acceptable model using nonlinear regression analysis. The analysis showed that both the backscatter and the attenuation of the Quantison suspensions displayed a higher order of dependence on acoustic pressure than the Definity suspensions. It was also discovered that Quantison microbubbles did not demonstrate uniform behaviour across the acoustic pressure range. At lower acoustic pressures the behaviour could not follow a model similar to that which predicted the behaviour at higher acoustic pressures, which was mainly due to the fact that free bubbles were released in a fashion dependent on acoustic pressure. The fact that two different populations of scatterers exist in the same suspensions makes the assessment of the behaviour of the particular agent impossible with the high concentrations that are commonly used. Very low concentration suspensions whereby single scattering events can be monitored should be more useful. In conclusion, the approach of using high microbubble concentrations in order to investigate the properties of ultrasonic contrast agents is limited in that the results of such studies cannot be used to understand the behaviour of single microbubbles.

Acoustics↗
PubMed2002

The dependence of ultrasound contrast agents backscatter on acoustic pressure: theory versus experiment.

Experimental investigations have not fully explored the interaction between ultrasound beams and microbubble contrast agents. Moreover theoretical investigations have not solved the problem of the microbubble oscillation. A simple in-vitro system based on a commercial scanner (ATL UM9) was used to insonate (3 MHz transmission) diluted contrast suspensions of Definity and Quantison at different acoustic pressures (0.27-1.52 MPa). The experimental data were referred to a blood mimicking fluid in order to extract an estimate of their scattering cross-section. The results were compared with the solutions of the three main bubble oscillatidn models, Rayleigh-Plesset, Herring and Gilmore. Non-linear solutions of the above models were produced numerically using the Mathematica Package Software. The experiments showed that both agents provided a linear increase in scattering cross-section with increasing acoustic pressure. The thick shelled Quantison provided an increasing number of scatterers with increasing acoustic pressure, which proved that free bubbles leaked out of the shell. At high acoustic pressures both Quantison and Definity scattering cross-sections were almost identical, and were probably that of a free bubble. The Rayleigh-Plesset model provided a scattering cross-section almost independent of acoustic pressure. On the contrary the scattering cross-sections calculated by the Herring and Gilmore models solutions displayed a definite dependence on acoustic pressure of an order higher than one, which is slightly higher than the order of dependence exhibited by the experimental data. However, the increase of the experimentally measured scattering cross-section with acoustic pressure was sharper than the calculated one by the above two models. This is most probably due to the fact that the models simulated damped and not free bubble oscillations. In conclusion the Rayleigh-Plesset model was inadequate in describing the bubble oscillations even at small diagnostic acoustic pressures. The Herring and Gilmore models could simulate the dependence of the scattering cross-section of encapsulated microbubbles on acoustic pressure. However the contribution of free bubble oscillations has still to be modelled.

Journal Article↗
PubMed2002

Guidelines for dosimetry and calibration in ultraviolet radiation therapy: a report of a British Photodermatology Group workshop.

This report examines the dosimetry of ultraviolet (UV) radiation applied to dermatological treatments, and considers the definition of the radiation quantities and their measurement. Guidelines are offered for preferred measurement techniques and standard methods of dosimetry. The recommendations have been graded according to the American Joint Committee on Cancer classification of strength of recommendation and quality of evidence (summarized in Appendix 5).

Humans↗
PubMed2001

PIV applied to eckart streaming produced by a medical ultrasound transducer.

Particle image velocimetry (PIV) is applied for the first time to study Eckart streaming induced by a medical ultrasonic transducer operating at a frequency of 3.3 MHz and effective acoustic intensities of 0.25 and 3 Wcm(-2). A temporal series of velocities in a two-dimensional plane were recorded resulting in an experimental set comprising over half a million velocity data points. These enabled average and fluctuating properties to be determined and clearly indicated the quasi-steady nature of the flow. The average large scale velocity fluctuations along the axis caused by this quasi-steady property were calculated to be 2 and 20 m ms(-1) at effective intensities of 0.25 and 3 Wcm(-2) respectively corresponding to approximately 25% of the peak flow velocity in both cases. Furthermore averaged shear rates were calculated with peak values of 1 and 8 s(-1) for the low and high intensities respectively. The present investigation indicates the usefulness of PIV for such studies and serves as a prelude to investigations of streaming in biological type fluids.

Rheology↗
PubMed2001

Three-dimensional forward-viewing intravascular ultrasound imaging of human arteries in vitro.

The aim of this work was to investigate the suitability of a novel forward-viewing intravascular ultrasound (IVUS) technique for three-dimensional imaging of severely stenosed or totally occluded vessels, where the conventional side-viewing IVUS systems are of limited use. A stiff 3.8 mm diameter forward-viewing catheter was manufactured to scan a 72 degrees sector ahead of its tip. Conical volume data were acquired by rotating the catheter over 180 degrees by means of a motorised mechanical system. Operating at 30 MHz, the catheter was integrated with an IVUS scanner and a radiofrequency data acquisition system. Postmortem carotid and femoral arteries were scanned in vitro. Correlation of the reconstructed images with histology demonstrated the ability of this forward-viewing IVUS system to visualise healthy lumens, bifurcations, thickened atherosclerotic walls and, most importantly, severe and complete vessel occlusions. A rotating-sector forward-viewing IVUS system is suitable for anatomical assessment of severely diseased vessels in three dimensions.

Arterial Occlusive Diseases↗
PubMed2001

Contrast agent stability: a continuous B-mode imaging approach.

The stability of contrast agents in suspensions with various dissolved gas levels has not been reported in the literature. An in vitro investigation has been carried out that studied the combined effect of varying the acoustic pressure along with degassing the suspension environment. In this study, the contrast agents were introduced into suspensions with different oxygen concentration levels, and their relative performance was assessed in terms of decay rate of their backscatter echoes. The partial pressures of oxygen in those solutions ranged between 1.5 and 26 kPa. Two IV and one arterial contrast agents were used: Definity, Quantison, and Myomap. It was found that Quantison and Myomap released free bubbles at high acoustic pressure that also dissolved faster in degassed suspensions. The backscatter decay for Definity did not depend on the air content of the suspensions. The destruction of bubbles was dependent on acoustic pressure. Different backscatter performance was observed by different populations of bubbles of the last two agents. The physical quantity of "overall backscatter" (OB) was defined as the integral of the decay rate over time of the backscatter of the contrast suspensions, and improved significantly the understanding of the behaviour of the agents. A quantitative analysis of the backscatter properties of contrast agents using a continuous imaging approach was difficult to achieve. This is due to the fact that the backscatter in the field of view is representative of a bubble population affected by the ultrasound (US) field, but this bubble population is not representative of the contrast suspension in the whole tank. Single frame insonation is suggested to avoid the effects of decay due to the ultrasonic field, and to measure a tank-representative backscatter. The definition of OB was useful, however, in understanding the behaviour of the agents.

Acoustics↗
PubMed2001

An in vitro system for the study of ultrasound contrast agents using a commercial imaging system.

An in vitro system for the investigation of the behaviour of contrast microbubbles in an ultrasound field, that provides a full diagnostic range of settings, is yet to be presented in the literature. The evaluation of a good compromise of such a system is presented in this paper. It is based on (a) an HD13000 ATL scanner (Bothell, WA, USA) externally controlled by a PC and (b) on the use of well-defined reference materials. The suspensions of the reference ultrasonic scattering material are placed in an anechoic tank. The pulse length ranges from 2 to 10 cycles, the acoustic pressure from 0.08 to 1.8 MPa, the transmit frequency from 1 to 4.3 MHz, and the receive frequency from 1 to 8 MHz. The collection of 256 samples of RF data, at an offset distance from the transducer face, was performed at 20 MHz digitization rate, which corresponds to approximately 1 cm depth in water. Two particle suspensions are also presented for use as reference scatterers for contrast studies: (a) a suspension of Orgasol (ELF Atochem, Paris, France) particles (approximately 5 microm mean diameter) and (b) a suspension of Eccosphere (New Metals & Chemicals Ltd, Essex, UK) particles (approximately 50 microm mean diameter). A preliminary experiment with the contrast agent Definity (DuPont Pharmaceutical Co, Waltham, MA) showed that the above two materials are suitable for use as a reference for contrast backscatter.

Contrast Media↗
PubMed2000

Quantification of microbubble destruction of three fluorocarbon-filled ultrasonic contrast agents.

The assessment of myocardial blood velocity using ultrasonic contrast agents is based on the premise that the vast majority of contrast microbubbles within a myocardial region can be destroyed by an acoustic pulse of sufficient magnitude. Determination of the period of time after destruction that a region of myocardium needs to reperfuse may be used to assess myocardial blood velocity. In this study, we investigated the acoustic pressure sensitivity of three solutions of intravenous fluorocarbon-filled contrast agents and the magnitude of acoustic pulse required to destroy the contrast agent microbubbles. A novel tissue-mimicking phantom was designed and manufactured to investigate the relationships between mean integrated backscatter, incident acoustic pressure and number of frames of insonation for three fluorocarbon-filled contrast agents (Definity(R), Optison(R), and Sonazoid(R), formerly NC100100). Using a routine clinical ultrasound (US) scanner (Acuson XP-10), modified to allow access to the unprocessed US data, the contrast agents were scanned at the four acoustic output powers. All three agents initially demonstrated a linear relationship between mean integrated backscatter and number of frames of insonation. For all three agents, mean integrated backscatter decreased more rapidly at higher acoustic pressures, suggesting a more rapid destruction of the microbubbles. In spite of the fact that there was no movement of microbubbles into or out of the beam, only the results from Definity(R) suggested that a complete destruction of the contrast agent microbubbles had occurred within the total duration of insonation in this study.

Albumins↗
PubMed2000

Scanning techniques for three-dimensional forward-viewing intravascular ultrasound imaging.

Intravascular ultrasound (US) imaging is a useful tool for assessing arterial disease and aiding treatment procedures. Forward-viewing intravascular US imaging could be of particular use in severely stenosed or totally occluded arteries, where the current side-viewing intravascular US systems are limited by their inability to access the site of interest. In this study, five 3-D forward-viewing intravascular scanning patterns were investigated. The work was carried out using scaled-up vessel phantoms constructed from tissue-mimicking material and a PC-controlled scanning and acquisition system. The scanning patterns were examined and evaluated with regard to the image quality of dense and sparse data sets, the accuracy of quantitative measurements of lumen dimensions and the potential for clinical use. The relative merits and drawbacks of the different patterns are discussed and a preferred scanning pattern is recommended.

Arterial Occlusive Diseases↗
PubMed2000

Evaluation of an experimental system for the in vitro assessment of ultrasonic contrast agents.

The ultrasonic properties of microbubble contrast agents need to be fully understood if reproducible images and quantitative results are to be produced. Additional aspects of the physical and chemical environment into which the contrast agents are introduced also need to be taken into account, and their effect on contrast agent performance evaluated. A setup that provides an accurate and reproducible data-acquisition system is presented and evaluated in this paper. The linear range of this system is assessed, as well as its accuracy and precision. A new approach to the investigation of contrast agents, based on normalised backscatter, is discussed. Also, a common technique of degassing, widely used in other areas, is described and evaluated to determine its appropriateness to contrast agent studies.

Contrast Media↗
PubMed2000

A study of the directional response of ultraviolet radiometers: I. Practical evaluation and implications for ultraviolet measurement standards.

The directional responses of a range of ultraviolet radiometers commonly used for irradiance measurements of UVB and UVC have been studied. Radiometers with 24 diffuser/filter combinations were assessed using a deuterium source, and three different diffuser/filter designs were assessed using a monochromatic source. The directional responses of the radiometers have been calculated and expressed in terms of figures of merit similar to those described for (photopic) illuminance meters in BS 667 and CIE 69. Those radiometers that performed best for the measurement of both small and extended sources of UVB and UVC had raised PTFE diffusers. We conclude that UV radiometers with a directional response error f2 < 10% are readily available commercially, and that it would be appropriate for future ultraviolet standards to set an upper limit of 5% on f2. This would ensure that the overall uncertainty in irradiance measurements of extended ultraviolet sources is not dominated by the error in the directional response of the radiometer.

Deuterium↗
PubMed2000

A study of the directional response of ultraviolet radiometers: II. Implications for ultraviolet phototherapy derived from computer simulations.

A theoretical model has been used to simulate irradiances for ultraviolet (UV) phototherapy cabinets and other sources. The accuracy of the simulation results has been checked by comparison with experimental measurements. The simulations have been used to study the influence of different factors on UV phototherapy exposure and to develop recommendations for the operation and calibration of phototherapy cabinets. Many radiometers used in the evaluation of skin doses have input optics with directional responses that are not proportional to the cosine of the angle of incidence for the UV radiation. Data on radiometer directional responses have been incorporated into the simulations, which show that the poor directional responses for some radiometers currently in use will give errors of 20-50% in the assessment of irradiance. The influence of lamp source geometries employed for radiometer calibration has been investigated. UV phototherapy dosimetry commonly uses a spectroradiometer and a radiometer in the transfer of irradiance calibrations from a small standard UV lamp to a large-area source with a different UV spectrum. Recommendations are given on the range of acceptability for radiometer directional responses and a method is described for determining whether these are fulfilled. Recommendations are made on the techniques that should be used for calibration.

Calibration↗
PubMed2000

Do phototherapy hoods really protect the neonate?

UNLABELLED: The objective of the current study was to evaluate the protection given to the eyes of neonates by an Amber 300 phototherapy hood during blue-light phototherapy from Drager Phototherapie 800 units, and to make recommendations for clinical practice. Hazard-weighted blue-light radiance of phototherapy lamps was measured inside neonatal incubators, with and without the use of a protective phototherapy hood. The study was carried out in a tertiary referral neonatal unit. No patients were involved. A mannequin was used as model of a jaundiced neonate being treated with blue-light phototherapy. The study shows that hazard-weighted blue-light radiance levels detectable from within the space enclosed by the hood may be several times greater than accepted industrial threshold limits for adults. CONCLUSIONS: Nursing and medical staff must ensure meticulous care in the positioning of infants, so that the caudal edge of the shadow cast by the hood is always at least 30 mm inferior to the infants' lower eyelids. The vulnerability of the neonatal retina and the relatively high levels of blue-light radiance visible from within the shadow of the hood may make it advisable to use more effective eye protection. Triple phototherapy using lamps at the foot-end of the incubator is clearly hazardous and should not be carried out unless the infant's eyes are protected by eye-patches.

Equipment Safety↗
PubMed1999

Characterization of cavitational activity in lithotripsy fields using a robust electromagnetic probe.

A robust electromagnetic probe has been used to investigate cavitational activity in vitro in the fields of two extracorporeal lithotripters and one intracorporeal lithotripter. Some aspects of the electromagnetic probe design and characteristics are described. A series of experiments have been carried out with results indicating that the probe head moves in response to the pressure gradient generated by radial motion of cavitation bubbles. Empirical expressions have been derived for the cavitational force acting on the probe head, and for the low-frequency sawtooth pressure wave generated by inertial cavitation. This is the first time that the existence of a low-frequency sawtooth wave produced by inertial cavitation has been described. A linear relationship exists between the negative pressure amplitude of the sawtooth wave and the lifetime of the bubbles. Close to the cavitation site, substantial negative pressure is maintained throughout bubble expansion. This can easily exceed the tensile strength of urinary calculi, and may be considered to be an important mechanism for disintegrating these relatively weak structures. A pilot study has also been carried out involving three patients treated by extracorporeal lithotripsy. Signals similar to those recorded during the in vitro cavitation experiments were detected. We conclude that the electromagnetic probe is capable of detecting and quantifying aspects of cavitational activity in vitro, and potentially also in vivo. The observation that the probe responds directly to the motion of cavitation bubbles, coupled with its ability to detect cavitation at a distance, give it the potential for use in a range of medical and industrial applications.

Electromagnetic Phenomena↗