The effect of resonance on transient microbubble acoustic response: Experimental observations and numerical simulations.

A large number of acoustic signals from single lipid-shelled Definity® (Lantheus Medical Imaging, N. Billerica, MA) microbubbles have been measured using a calibrated microacoustic system, and a unique transient characteristic of resonance has been identified in the onset of scatter. Comparison of the numerically obtained response of microbubbles with acoustic measurements provides good agreement for a soft shell that is characterized by small area dilatation modulus and strain softening behavior, and identifies time to maximum radial excursion and scatter as a robust marker of resonance during transient response. As the sound amplitude increases a two-population pattern emerges in the time delay vs the fundamental acoustic scatter plots, consisting of an initial part pertaining to microbubbles with less than resonant rest radii, which corresponds to the weaker second harmonic resonance, and the dominant resonant envelope pertaining to microbubbles with resonant and greater than resonant rest radii, which corresponds to the primary and subharmonic resonances. Consequently, a wider resonant spectrum is observed. It is a result of the strain softening nature of soft lipid shells, based on which the microbubble sizes corresponding to the above resonances decrease as the sound amplitude increases. This bares an impact on the selection of an optimal microbubble size pertaining to subharmonic imaging.

Putting corannulene in its place. Reactivity studies comparing corannulene with other aromatic hydrocarbons.

A series of aromatic hydrocarbons were investigated so as to compare the reactivity of corannulene with planar aromatic hydrocarbons. Corannulene was found to be more reactive than benzene, naphthalene and triphenylene to Friedel-Crafts acylation whilst electrophilic aromatic bromination was also used to confirm that triphenylene was less reactive than corannulene and that pyrene, perylene and acenaphthene were more so. The stabilisation of a neighbouring carbocation by the various aromatic systems was investigated through consideration of the rates of methanolysis of a series of benzylic alcohols. The reactivity series was found to parallel that observed for the electrophilic aromatic substitutions and both series are supported by computational studies. As such, a reactivity scale was devised that showed that corannulene was less reactive than would be expected for an aromatic planar species of similar pi electron count.

Phase measurements on terahertz waves.

The change in phase of the free space terahertz (THz) electric field as a sample of material introduced into the THz beampath of a CW THz system is measured and used to calculate the index of refraction of materials at 250 GHz.

A prospective gating method to acquire a diverse set of free-breathing CT images for model-based 4DCT.

Breathing motion modeling requires observation of tissues at sufficiently distinct respiratory states for proper 4D characterization. This work proposes a method to improve sampling of the breathing cycle with limited imaging dose. We designed and tested a prospective free-breathing acquisition protocol with a simulation using datasets from five patients imaged with a model-based 4DCT technique. Each dataset contained 25 free-breathing fast helical CT scans with simultaneous breathing surrogate measurements. Tissue displacements were measured using deformable image registration. A correspondence model related tissue displacement to the surrogate. Model residual was computed by comparing predicted displacements to image registration results. To determine a stopping criteria for the prospective protocol, i.e. when the breathing cycle had been sufficiently sampled, subsets of N scans where 5 ⩽ N ⩽ 9 were used to fit reduced models for each patient. A previously published metric was employed to describe the phase coverage, or 'spread', of the respiratory trajectories of each subset. Minimum phase coverage necessary to achieve mean model residual within 0.5 mm of the full 25-scan model was determined and used as the stopping criteria. Using the patient breathing traces, a prospective acquisition protocol was simulated. In all patients, phase coverage greater than the threshold necessary for model accuracy within 0.5 mm of the 25 scan model was achieved in six or fewer scans. The prospectively selected respiratory trajectories ranked in the (97.5 ± 4.2)th percentile among subsets of the originally sampled scans on average. Simulation results suggest that the proposed prospective method provides an effective means to sample the breathing cycle with limited free-breathing scans. One application of the method is to reduce the imaging dose of a previously published model-based 4DCT protocol to 25% of its original value while achieving mean model residual within 0.5 mm.

Is there an ideal set of prospective scan acquisition phases for fast-helical based 4D-CT?

The article aims to determine if a prospective acquisition algorithm can be used to find the ideal set of free-breathing phases for fast-helical model-based 4D-CT. A retrospective five-patient dataset that consisted of 25 repeated free breathing CT scans per patient was used. The sum of the square root amplitude difference between all the breathing phases was defined as an objective function to determine the optimality of sets of breathing phases. The objective function was intended to determine if a specific set of breathing phases would yield a motion model that could accurately predict the motion in all 25 CT scans. Voxel specific motion models were calculated using all combinations of N scans from 25 breathing trajectories, (3 ⩽ N ⩽ 25), and the minimum number of scans required to absolutely characterize the motion model was analyzed. This analysis suggests that the number of scans could potentially be reduced to as few as five scans. When the objective function was large, the resulting motion model provided an excellent approximation to the motion model created using all 25 scans.

Properties of ion channels formed by Staphylococcus aureus delta-toxin.

The delta-toxin of Staphylococcus aureus has been investigated in terms of its potential to form ion channels in planar lipid bilayers formed at the tip of patch electrodes. Channel formation has been shown to occur for delta-toxin concentrations in the range 0.1 to 2.0 microM. In 0.5 M KCl, two major classes of channels were seen--'small' with conductances of 70-100 pS, and 'large' with a conductance of approx. 450 pS. Current-voltage relationships for lipid bilayers containing several delta-toxin channels revealed both voltage-dependent and independent components to channel gating. Reversal potential measurements showed the channels to be cation selective. In the presence of 3.0 M KCl, the channel gating kinetics were complex, with multiple open and closed states. The results are interpreted in terms of a model for the channel consisting of a hexameric cluster of alpha-helical delta-toxin molecules.

Crystallization of the delta toxin of Staphylococcus aureus.

Delta toxin is a small cytolytic polypeptide produced and secreted by the organism Staphylococcus aureus and belongs to a family of surface-active toxins that exhibit pronounced effects on a wide variety of cellular membranes. Although this class of proteins has been much studied by a wide variety of physical techniques, no consensus has been reached on their mode of action. Therefore, in order to investigate their role in causing membrane damage, a structural analysis of the delta toxin has been initiated. Crystals of this protein have been grown by dialysis against mixtures of 2-methylpentan-2,4-diol and water. These crystals are relatively insensitive to radiation damage and diffract to high resolution. The results of this study should provide a valuable insight into the cytolytic properties of these molecules.

Crystallization of an NADP+-dependent malic enzyme from rat liver.

Crystals of a tetrameric NADP+-dependent malic enzyme from rat liver have been grown in the presence of NADP+ using the hanging-drop method of vapour diffusion with ammonium sulphate as the precipitant. Measurement of the crystal density and calculation of the values of Vm for different numbers of polypeptide chains in the unit cell indicate that the asymmetric unit of the crystal contains a complete tetramer, allowing the application of non-crystallographic symmetry to the determination of the molecular structure of this enzyme. This structure would provide only the second example for an enzyme involved in oxidative decarboxylation, the other being 6-phosphogluconate dehydrogenase. In addition, then, to providing an insight into the structure-function relationship in malic enzyme, the successful structure determination would permit valuable comparisons to be made between these two and other enzymes with this catalytic activity.

Chronic aldosterone therapy and the control of transepithelial transport of ions and water by the colon and coprodeum of the domestic fowl (Gallus domesticus) in vivo.

White Leghorn laying hens were maintained on commercial poultry food (medium-Na+ diet) and fresh water. Birds maintained on a high-Na+ diet received, in addition, 10 ml 9% (w/v) NaCl/kg by stomach load for 2 days before the perfusion experiments. Some birds on each dietary Na+ level also received aldosterone injections (60 micrograms/kg per day, i.m.) for 2 days before the perfusion experiments. The lumen of the coprodeum and colon of anaesthetized birds was perfused with solutions resembling ureteral urine, with systematically varied Na+, NH4+, Cl- and osmotic concentrations. Aldosterone enhanced net Na+ absorption (JNa) and associated net Cl- absorption and K+ secretion, and induced (in birds on medium- but not on high-Na+ diets) the appearance of a saturable JNa component dependent on the luminal concentration of Na+. Aldosterone enhanced net absorption of NH4+ and decreased the transmural potential difference in birds on a high-Na+ diet only; water and phosphate fluxes were not affected. Disparities between aldosterone- and Na+-depletion-induced effects suggested that one or more factors (in addition to aldosterone) are involved in the normal mediation of the responses of the lower intestine of the domestic fowl to varied Na+ intake.

Acute effects of aldosterone on water and electrolyte transport in the colon and coprodeum of the domestic fowl (Gallus domesticus) in vivo.

White Leghorn laying hens were maintained on commercial poultry food (medium-Na+ diet) and fresh water. High-Na+ diet birds received, in addition, 10 ml 9% (w/v) NaCl/kg by stomach load for 2 days before perfusion experiments. The lumen of the coprodeum and colon of anaesthetized birds was perfused with a hyperosmotic solution resembling ureteral urine. Transmural solute and water fluxes and potential difference (p.d.) values were measured for 2.5 h before and for 8 h after i.v. injection of aldosterone (120 microgram/kg). After administration of aldosterone, the lag, increasing and stabilized plateau phases were identified for Na+, K+ and Cl- fluxes (which together formed an electroneutral ion-exchange system); plateau flux values were significantly greater than preinjection values and were comparable to values for birds maintained on low-Na+ diets in parallel experiments. Ammonium, phosphate and water fluxes were unresponsive to aldosterone and p.d. values showed a transient increase in medium-Na+ diet birds only. In parallel experiments on birds on low sodium diets the ammonium flux and p.d. increased but the osmotic flow and phosphate transfer did not respond. Therefore acute injection of aldosterone reproduced some but not all of the responses to dietary Na+ restriction in fowls.

Determination of inorganic cations and ammonium in environmental waters by ion chromatography with a high-capacity cation-exchange column.

While alkali and alkaline earth cations are commonly determined by using spectrometric techniques such as atomic absorption spectrometry or inductively coupled plasma, ammonium cation in the same sample must be measured separately by a wet chemical technique such as colorimetry, titrimetry, or ammonia-selective electrode. In a single 25-min run ion chromatography can determine all of the important inorganic cations including lithium, sodium, ammonium, potassium, magnesium and calcium. In this paper, we describe the use of ion chromatography with a new high-capacity cation-exchange column (the IonPac CS16), an electrolytically-generated methanesulfonic acid eluent and suppressed conductivity detection to determine dissolved alkali and alkaline earth cations and ammonium in drinking water wastewater and aqueous soil extracts. The IonPac CS16 is a high-capacity cation-exchange column that incorporates recent advances in polymer chemistry to enable trace-level determinations of cations even in high-ionic-strength matrices. We discuss the linear range, method detection limits, and analyte recoveries obtained with this column, and evaluate the effect of potential interferences on method performance during the analysis of typical environmental samples.

Laser-induced dispersed fluorescence detection of polycyclic aromatic compounds in soil extracts separated by capillary electrochromatography.

Polycyclic aromatic hydrocarbons (PAHs) and nitrogen containing aromatic compounds (NCACs) are characterized in soil extracts and laboratory standards by capillary electrochromatography (CEC) with laser-induced dispersed fluorescence (LIDF) detection using a liquid-nitrogen cooled charge-coupled device detector. The LIDF detection technique provides information on compound identity and, when coupled with the high separation efficiencies of the CEC technique, proves useful in the analysis of complex mixtures. Differences in fluorescence spectra also provide a means of identifying co-eluting compounds by using deconvolution algorithms. Detection limits range from 0.5 to 96x10(-10) M for selected PAHs and 0.9-3.7x10(-10) M for selected NCACs. Soil extracts are also injected onto the CEC column to evaluate chromatographic method performance with respect to complex samples and the ability to withstand exposure to environmental samples.

New block-grafted anion exchanger for environmental water analysis by ion chromatography.

The IonPac AS14A is a recently developed stationary phase that was produced using a new block-grafting technique, which enables the preparation of high-water-content anion exchangers with excellent peak shape and good chromatographic efficiency. The performance of this column for the analysis of inorganic anions was compared to that obtained using an IonPac AS4A column, which is specified in US Environmental Protection Agency Method 300.0, in addition to another commonly used alternative: the AS14 column. The AS14A column is available in two different formats: 250x4 mm I.D. (7.0 microm diameter particle) and 150x3 mm I.D. (5.5 microm diameter particle). The IonPac AS14A (in 4 mm I.D. format) was found to provide similar performance to the AS14 column with increased peak efficiency and better pH stability and is a suitable alternative for the analysis of anions in moderate- to high-ionic-strength samples. The IonPac AS14A (in 3 mm I.D. format) provides comparable run times to the AS4A column with better overall peak selectivity and improved fluoride resolution, hence this column would be a suitable column to substitute in place of either the AS4A or AS14 columns for the analysis of inorganic anions in low- to moderate-ionic-strength environmental waters. The AS14A column used with an Atlas electrolytic suppressor provides equivalent method detection limits to those obtained when using a micromembrane suppressor but with the operational convenience of a self-regenerating suppressor.

Determination of hexavalent chromium at the level of the California Public Health Goal by ion chromatography.

Chromium is a primary drinking water contaminant in the USA with hexavalent chromium, Cr(VI), being the most toxic form of the metal. As a required step in developing a revised state drinking water standard for chromium, the California Department of Health Services recently issued a new Public Health Goal (PHG) of 2.5 microg/l for total chromium and 0.2 microg/l for Cr(VI). Hexavalent chromium can be determined (as chromate) by ion chromatography, as described in US Evironmental Protection Agency Method 218.6; however, the method as originally published does not allow sufficient sensitivity for analysis at the California PHG level of 0.2 microg/l. Modification of the conditions described in Method 218.6, including the use of a lower eluent flow-rate, larger reaction coil, and larger injection volume, significantly increases the method sensitivity. The modified method, which uses IonPac NG1 and AS7 guard and analytical columns, an eluent of 250 mM ammonium sulfate-100 mM ammonium hydroxide operated at 1.0 ml/min, a 1000 microl injection volume, and postcolumn reaction with 2 mM diphenylcarbazide-10% methanol-0.5 M sulfuric acid (using a 750 microl reaction coil) followed by UV-Vis detection at 530 nm, permits a method detection limit for chromate of 0.02 microg/l. This results in a quantitation limit of 0.06 microg/l, which is more than sufficient for analysis at the California PHG level. Calibration is linear over the range of 0.1-10 microg/l and quantitative recoveries (>80%) are obtained for chromate spiked at 0.2 microg/l in drinking water. The modified method provides acceptable performance, in terms of chromate peak shape and recovery, in the presence of up to 1000 mg/l chloride or 2000 mg/l sulfate.

Three-dimensional structure of lactoferrin in various functional states.

The three-dimensional structures of various forms of lactoferrin, determined by high resolution crystallographic studies, have been compared in order to determine the relationship between structure and biological function. These comparisons include human apo and diferric lactoferrins, metal and anion substituted lactoferrins, the N-terminal half molecule of human lactoferrin, and bovine diferric lactoferrin. The structures themselves define the nature and location of the iron binding sites and allow anti-bacterial and putative receptor-binding regions to be mapped on to the molecular surface. The structural comparisons show that small internal adjustments can allow the accommodation of different metals and anions without altering the overall molecular structure, whereas large-scale conformational changes are associated with metal binding and release, and smaller, but significant, movements accompany species variations. The results also focus on differences in flexibility between the two lobes, and on the importance of interactions in the inter-lobe region in modulating iron release from the N-lobe and in possibly enabling binding at one site to be signalled to the other.

The interface between self medication and the NHS.

Cost and convenience seem to be major factors in determining whether, given the choice, patients purchase a medicine over the counter or obtain it on prescription. With current arrangements, exemption from prescription charges provides an incentive to continue to obtain products on NHS prescription even when they are available over the counter. There is therefore no simple relation between the availability of over the counter medicines and the level of prescribing of deregulated products. The appropriate use of over the counter medicines--particularly those that have only recently been deregulated--places a burden of care on community pharmacists and calls for closer working relationships with general practitioners. In particular, systems for referral and for recording details of both prescribed and over the counter medicines need to be developed, and a direct route needs to be established for community pharmacists to report adverse drug reactions to over the counter products.

The acoustic signature of decaying resonant phospholipid microbubbles.

Sub-capillary sized microbubbles offer improved techniques for diagnosis and therapy of vascular related disease using ultrasound. Their physical interaction with ultrasound remains an active research field that aims to optimize techniques. The aim of this study is to investigate whether controlled microbubble disruption upon exposure to consecutive ultrasound exposures can be achieved. Single lipid-shelled microbubble scattered echoes have been measured in response to two consecutive imaging pulses, using a calibrated micro-acoustic system. The nonlinear evolution of microbubble echoes provides an exact signature above and below primary and secondary resonance, which has been identified using theoretical results based on the Mooney-Rivlin strain softening shell model. Decaying microbubbles follow an irreversible trajectory through the resonance peak, causing the evolution of specific microbubble spectral signatures. The characteristics of the microbubble motion causes varying amounts of shell material to be lost during microbubble decay. Incident ultrasound field parameters can thus accurately manipulate the regulated shedding of shell material, which has applications for both imaging applications and localized drug delivery strategies.

Imaging of the ovine corpus luteum microcirculation with contrast ultrasound.

Ultrasound contrast agents have been the subject of microvascular imaging research. The sheep corpus luteum (CL) is a microvascular tissue that provides a natural angiogenic and antiangiogenic process, which changes during the luteal phase of the estrous cycle of the ewe. It can also be controlled and monitored endocrinologically, providing a very attractive in vivo model for the study and development of microvascular measurement. The perfusion of the fully developed CL between days 8 and 12 of the estrous cycle was studied in six ewes. A Philips iU22 ultrasound scanner (Bothell, WA, USA) with the linear array probe L9-3 was used to capture contrast-enhanced images after an intravenous bolus injection of 2.4 mL SonoVue (Bracco S.P.A., Milan, Italy). Time-intensity curves of a region of interest inside the CL were formed from linearized image data. A lagged-normal model to simulate the compartmental kinetics of the microvascular flow was used to fit the data, and the wash-in time was measured. Good contrast enhancement was observed in the CLs of all animals and the wash-in time averaged at 5.5 s with 9% uncertainty. The regression coefficient was highly significant for all fits. These data correlated with stained endothelial area in the histology performed postmortem. Two ewes were injected with prostaglandin F2alpha to induce CL regression, which resulted in an increase of wash-in time after a few hours. The CL of the ewe is thus proposed as an ideal model for the study and development of microvascular measurements using contrast ultrasound. Our initial results demonstrate a highly reproducible model for the study of the microvascular hemodynamics in a range of tissues and organs.

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.