The Humber catchment and its coastal area: from UK to European perspectives.

The present water quality of the Humber rivers and coastal zone depends on a complex interplay of factors, including physical ones, such as the underlying geology, which influences soil type, climatic ones, such as the rainfall, which influences runoff, socio-economic ones, which influence present-day human activities in the catchment, and the legacy of former activities, such as contaminated sediments from mining. All of these factors affect the fluxes of nutrients and other contaminants to the rivers and coastal zone. The Water Framework Directive (WFD) requires the production of a river basin management plan intended to lead to the achievement of good chemical and ecological status for all water bodies in the catchment over the next two decades. This paper provides an overview of the current environmental and socio-economic state of the Humber catchment and coastal zone, and broadly examines how socio-economic drivers affect the fluxes of nutrients and contaminants to the coastal zone, using the driver-pressure-state-impact-response (DPSIR) approach. This is followed by an overview of future research, describing the use of scenarios to simulate future fluxes and provide a consistent framework to evaluate potential policies to improve water quality in the estuary. The Humber catchment is one of eight case studies within a European research project, EUROCAT (EVK1-CT-2000-00044), which aims to achieve integrated catchment and coastal zone management by analysing the response of the coastal sea to changes in fluxes of nutrients and contaminants from the catchments. For the Humber case study, the research focuses on the fluxes of two nutrient elements, N and P, and four metal contaminants, As, Cu, Pb and Zn. The project requires the integration of scientific and socio-economic approaches, bringing together quantitative environmental data garnered for individual river catchments and coastal zones in previous research programmes, and local and regional socio-economic data, to aid decision-makers in their search for integrated and sustainable coastal zone management strategies.

Microembolic signal description: a reappraisal based on a customized digital postprocessing system.

The high variability in presence and signature of microembolic signals (MES), detected with transcranial Doppler sonography (TCD) in the middle cerebral artery (MCA), cannot be explained with the currently available published data. We applied customized postprocessing on the radiofrequency (RF) signal of a standard TCD system. The spatial resolution was on the order of 2 mm, depending only on the length of the ultrasound (US) burst emitted. The amplitude of clutter-filtered RF signals was color-coded and plotted as a function of time and depth (range 30 mm). Additionally, 128 point fast Fourier transforms (FFTs) (50% temporal overlap) were calculated, visualizing both the background Doppler spectrum and the MES. We evaluated 122 gaseous MES from two patients during cardiac surgery and 52 particulate MES from four patients after carotid endarterectomy. Both MES categories showed comparable properties: they appeared in the RF amplitude plot as rather straight lines of increased intensity, indicating that the velocity remained approximately the same while they passed the US beam. The velocity calculated from the amplitude plot never exceeded that of the background Doppler spectrum. Various "MES patterns" could be identified with respect to the depth range at which the MES were visible. A quarter of the gaseous MES changed their direction at a specific depth, suggesting that the MES entered a branch (e.g., an M2 artery or the anterior cerebral artery). In the FFT analysis, these MES contained both positive and negative frequencies. It is concluded that MES show consistent signature patterns in the amplitude-time plots and that the previously reported variability of MES appearance in conventional Doppler systems is an artefact caused by relatively large signal amplitudes and sample volumes.

Wall shear stress assessment in the common carotid artery of end-stage renal failure patients.

Under physiological circumstances in the common carotid artery (CCA), mean wall shear stress (WSS), defined as mean wall shear rate (WSR) times local whole blood viscosity (WBV), is maintained at approximately 1.5 Pa. In patients with end-stage renal failure (ESRF) whole blood viscosity is low and it is not unlikely that mean WSS is lower in these patients than in control subjects. Moreover, hemodialysis causes an acute increase in blood viscosity with possible effects on WSS. In this study WSS in the CCA was determined with the Shear Rate Estimating System, an apparatus based on ultrasound, in ESRF patients (n = 13) and in presumed healthy age- and sex-matched control subjects (n = 13). Prior to hemodialysis, mean WSS (0.67 +/- 0.23 Pa) was significantly lower (p < 0.05) in patients with ESRF, due to both a lower WBV (2.80 +/- 0.52 mPa.s) and mean WSR (271 +/- 109 s(-1)), than in the control subjects (mean WSS: 1.24 +/- 0.20 Pa; WBV: 3.20 +/- 0.29 mPa.s; WSR: 387 +/- 51 s(-1)). Hemodialysis induced an increase in WBV (up to 3.71 +/- 1.54 mPa.s, p < 0.01), but mean WSS did not change significantly due to a reciprocal decrease in mean wall shear rate. These findings demonstrate that WSS is lower in hemodialysis patients than in control subjects, and that mean WSS is maintained at this low level despite an acute change in blood viscosity.

High-resolution functional imaging with ultrasound contrast agents based on RF processing in an in vivo kidney experiment.

Knowledge of the relative tissue perfusion distribution is valuable in the diagnosis of numerous diseases. Techniques for the assessment of the relative perfusion distribution, based on ultrasound (US) contrast agents, have several advantages compared to established nuclear techniques. These are, among others, a better spatial and temporal resolution, the lack of exposure of the patient to ionizing radiation and the relatively low cost. In the present study, US radiofrequency (RF) image sequences are acquired, containing the signal intensity changes associated with the transit of a bolus contrast agent through the microvasculature of a dog kidney. The primary objective is to explore the feasibility of calculating functional images with high spatial resolution. The functional images characterize the transit of the contrast agent bolus and represent distributions of peak time, peak value, transit time, peak area, wash-in rate and wash-out decay constant. For the evaluation of the method, dog experiments were performed under optimized conditions where motion artefacts were minimized and an IA injection of the contrast agent Levovist was employed. It was demonstrated that processing of RF signals obtained with a 3.5-MHz echo system can provide functional images with a high spatial resolution of 2 mm in axial resolution, 2 to 5 mm in lateral resolution and a slice thickness of 2 mm. The functional images expose several known aspects of kidney perfusion, like perfusion heterogeneity of the kidney cortex and a different peripheral cortical perfusion compared to the inner cortex. Based on the findings of the present study, and given the results of complimentary studies, it is likely that the functional images reflect the relative perfusion distribution of the kidney.

Experimental investigation of the pulse inversion technique for imaging ultrasound contrast agents.

The application of ultrasound contrast agents aims to detect low velocity blood flow in the microcirculation. To enhance discrimination between tissue and blood containing the contrast agent, harmonic imaging is used. Harmonic imaging requires the application of narrow-band signals and is obscured by high levels of native harmonics generated in an intervening medium. To improve discrimination between contrast agent and native harmonics, a pulse inversion technique has been proposed. Pulse inversion allows wide-band signals, thus preserving the axial resolution. The present study examines the interference of native harmonics and discusses the practical difficulties of wide-band pulse inversion measurements of harmonics by a single transducer. Native harmonics are not eliminated by pulse inversion. Furthermore, only even harmonics remain and are amplified by 6 dB, alleviating the requirement for selective filtering. Finally, it is shown that the contaminating third harmonic contained in the square wave activation signal leaks through in the emitted signal. The spectral location of the contaminating third harmonic is governed by the transducer spectral characteristics while the location of the native and contrast agent second harmonics is not. Thus the contaminating third harmonic and the native and contrast agent second harmonics may overlap and interfere. Optimal discrimination requires a balance between maximal sensitivity for the second harmonic at reception and minimal interference from the contaminating third harmonic.

Angle-independent motion measurement by correlation of ultrasound signals assessed with a single circular-shaped transducer.

In medicine, pulsed ultrasound is a widespread noninvasive technique that measures motion in the direction of the ultrasound beam, i.e., axial motion. The magnitude of the actual motion can be determined only if the angle between the ultrasound beam and the direction of motion (transducer-to-motion angle) is known. For blood flow measurements, current pulsed ultrasound systems assume this angle to be equal to the angle between the ultrasound beam and the longitudinal direction of the vessel, as can be estimated from a two-dimensional brightness-mode (B-mode) image that is obtained prior to the blood flow measurement. For tissue motion measurements, current pulsed ultrasound systems are mostly unable to determine the transducer-to-motion angle. Recently, a model has been derived for the correlation of(analytic) radiofrequency (rf) signals, assessed with a circular-shaped ultrasound transducer along the same line of observation. In the present paper, this model is used to derive estimators, requiring only the calculation of a few correlation coefficients, for the motion components (axial, lateral and actual) and for some of the signal parameters (center frequency, bandwidth and signal-to-noise ratio) of the assessed rf signals. The transducer-to-motion angle can be derived from the estimated motion components. For the evaluation of the estimators, rf signals were acquired with a motion-controlled experimental arrangement. The results of the evaluation study show that the transducer-to-motion angle can be estimated with a mean standard deviation of less than 2 degrees.

Measurement of the contrast agent intrinsic and native harmonic response with single transducer pulse waved ultrasound systems.

Ultrasound contrast agents, i.e., small gas filled microbubbles, enhance the echogenicity of blood and have the potential to be used for tissue perfusion assessment. The contrast agents scatter ultrasound in a nonlinear manner and thereby introduce harmonics in the ultrasound signal. This property is exploited in new ultrasound techniques like harmonic imaging, which aims to display only the contrast agent presence. Much attention has already been given to the physical properties of the contrast agent. The present study focuses on practical aspects of the measurement of the intrinsic harmonic response of ultrasound contrast agents with single transducer pulse waved ultrasound systems. Furthermore, the consequences of two other sources of harmonics are discussed. These sources are the nonlinear distortion of ultrasound in a medium generating native harmonics, and the emitted signal itself which might contain contaminating harmonics. It is demonstrated conceptually and by experiments that optimization of the contrast agent harmonic response measured with a single transducer is governed by the transducer spectral sensitivity distribution rather than the resonance properties of the contrast agent. Both native and contaminating harmonics may be of considerable strength and can be misinterpreted as intrinsic harmonics of the contrast agent. Practical difficulties to filter out the harmonic component selectively, without deteriorating the image, may cause misinterpretation of the fundamental as a harmonic.

Effects of a selective A1-adenosine receptor agonist on heart rate and heart rate variability during permanent atrial fibrillation.

BACKGROUND: Mean heart rate and irregularity of the rate, i.e., heart rate variability (HRV), are two aspects of heart rate during atrial fibrillation (AF). An important goal of AF therapy is to control mean heart rate during exercise; the determinants of HRV during AF remain poorly known although its prognostic value has been established. OBJECTIVES: To investigate the effects of a stable, long-acting, selective A1-adenosine receptor agonist, SDZ WAG994, on heart rate during exercise and on HRV. METHODS: In a multicenter, double-blind, randomized, placebo-controlled, parallel group study, patients with permanent AF performed a symptom-limited exercise test and underwent 24-hour ECG monitoring on day 1 during treatment with placebo, and on day 2 during treatment with either placebo or 2 mg SDZWAG994 orally. Changes in mean heart rate during exercise and changes in HRV indices between day 1 and day 2 were compared between the two groups. RESULTS: Thirty-two patients (64 +/- 8 years; 81% male; 25% in NYHA Class II; 38% with no structural heart disease) were included in the study. During active treatments, heart rate remained unchanged at rest and increased significantly during exercise. A significant daytime increase in short-term HRV indices (DpNN50 = 4.5% P = 0.01; DrMSSD = 6% P = 0.03; DSDNN Index = 6% P = 0.02) occurred during active treatment. CONCLUSIONS: Selective A1-adenosine receptor agonism with SDZ/WAG994 limits the increase in mean heart rate during exercise in patients with AF. In addition, this agonist selectively increases short-term HRV indices, suggesting that pNN50, rMSSD, and SDNN reflect vagal influences during AF.

Baseband velocity estimation for second-harmonic signals exploiting the invariance of the Doppler equation.

All Doppler systems, whether conventional Doppler domain or radio frequency (RF) processing is employed, relate the temporal frequency characteristics of the signal at a certain point in depth as function of time to the spatial frequency characteristics of the received signal as function of depth. The mean frequency of the latter may change as a result of depth-dependent attenuation, nonlinear scattering mechanisms, as in harmonic imaging of ultrasound contrast agents, or RF signal demodulation. For all these cases, the relationship between spatial and temporal mean frequency and target velocity is still governed by the familiar Doppler expression if the signal modifications have been properly accounted for. A major drawback of RF signal processing to extract the target velocity is the large number of data points to consider. The computational complexity increases further for harmonic imaging. It is shown conceptually, and demonstrated by signal simulations, that prior to velocity estimation RF demodulation followed by decimation 1) does not affect the Doppler equation, 2) enhances the information content of the samples, 3) reduces the computational load by a factor of four and for harmonic signals by a higher factor, and 4) while demodulation does not have to be actually performed, but can be accounted for by a scaling factor in the cross-correlation function. It is concluded that decimation hardly affects the precision of the velocity estimate if possible frequency aliasing is maintained within bounds, suggesting that the decimation factor is not critical.

Modeling of the correlation of analytic ultrasound radiofrequency signals for angle-independent motion detection.

Conventional pulsed ultrasound systems are able to assess motion of scatterers in the direction of the ultrasound beam, i.e., axial motion, by determining the lag at which the maximum correlation occurs between consecutively-received radiofrequency (rf) signals. The accuracy, resolution, and processing time of this technique is improved by making use of a model for the correlation of rf signals. All previously-described correlation models only include axial motion, but it is common knowledge that lateral motion, i.e., motion in the plane perpendicular to the beam axis, reduces the correlation of rf signals in time. In the present paper, a model for the correlation of analytic rf signals in depth and time is derived and verified. It also includes, aside of some signal and transducer parameters, both axial and lateral motion. The influence of lateral motion on the correlation of (analytic) rf signals is strongly related to local phase and amplitude characteristics of the ultrasound beam. It is shown how the correlation model, making use of an ultrasound transducer with a circular beam shape, can be applied to estimate, independent of angle, the magnitude of the actual motion. Furthermore, it is shown that the model can be applied to estimate the local signal-to-noise ratio and rf bandwidth.

A noninvasive method to estimate pulse wave velocity in arteries locally by means of ultrasound.

Noninvasive evaluation of vessel wall properties in humans is hampered by the absence of methods to assess directly local distensibility, compliance, and Young's modulus. Contemporary ultrasound methods are capable of assessing end-diastolic artery diameter, the local change in artery diameter as a function of time, and local wall thickness. However, to assess vessel wall properties of the carotid artery, for example, the pulse pressure in the brachial artery still must be used as a substitute for local pulse pressure. The assessment of local pulse wave velocity as described in the present article provides a direct estimate of local vessel wall properties (distensibility, compliance, and Young's modulus) and, in combination with the relative change in artery cross-sectional area, an estimate of the local pulse pressure. The local pulse wave velocity is obtained by processing radio frequency ultrasound signals acquired simultaneously along two M-lines spaced at a known distance along the artery. A full derivation and mathematical description of the method to assess local pulse wave velocity, using the temporal and longitudinal gradients of the change in diameter, are presented. A performance evaluation of the method was carried out by means of experiments in an elastic tube under pulsatile pressure conditions. It is concluded that, in a phantom set-up, the assessed local pulse wave velocity provides reliable estimates for local distensibility.

Experimental verification of the correlation behavior of analytic ultrasound radiofrequency signals received from moving structures.

Conventional pulsed ultrasound systems are only able to detect motion along the ultrasound beam (i.e., axial motion). If the angle between the actual motion direction and the ultrasound beam is known, then the magnitude of the actual motion can be derived. This technique can be applied for laminar blood-flow measurements in straight vessels, but for tissue motion it is inadequate because the local tissue motion direction is unknown and may be position-dependent. Assessment of both the axial motion and the lateral motion (i.e., in the direction perpendicular to the ultrasound beam) makes angle-independent assessment of the magnitude of the actual motion feasible. Information about the axial and lateral motion is available in a set of radiofrequency (RF) signals obtained along the same line of observation (M-mode). The experiments described in the present paper show that axial and lateral motion can be estimated from the shape of the envelope of the 2-D (spatial and temporal) correlation function of analytic M-mode RF signals. Furthermore, it is demonstrated that the shape is also affected by the Band width of the received RF signals, signal-to-noise ratio, and local amplitude and phase characteristics of the ultrasound beam.

A single bit RF domain complex cross-correlation velocity estimator for color flow mapping.

This paper evaluates the performance of a one bit mean frequency estimator to estimate blood flow velocity for ultrasound color flow mapping. This one bit mean frequency estimator, referred to as BC3 estimator, is derived from the recently introduced complex cross-correlation model (C3M) employing the full dynamic data range. The C3M velocity estimator is not suitable for application in color flow mapping because of its high hardware complexity and associated computational load. The BC3 estimator estimates the mean blood flow velocity using only two complex cross-correlation coefficients. For this purpose the latter are computed by means of a complex one bit cross-correlation operation. Each sample of the RF signals is converted into an one bit representation based on the sign of the real and imaginary part of the RF samples. A full derivation and mathematical description of the BC3 estimator is presented. In addition, a thorough performance evaluation of the BC3 estimator in comparison with the full dynamic range C3M velocity estimator is carried out by means of signal simulations to document the effect of signal to noise ratio, sample frequency and bandwidth. For the specific simulation conditions considered the standard deviation of both estimators (C3M and BC3) is comparable. The bias of the BC3 estimator appears to be a function of velocity, while the full dynamic range C3M velocity estimator exhibits no bias. The simulation results are confirmed by evaluation of data from an in vivo measurement. Taking into account the low hardware complexity and computational load in combination with the achieved precision, it may be concluded that the BC3 estimator is well suited for implementation in color flow mapping.

Reduction of the clutter component in Doppler ultrasound signals based on singular value decomposition: a simulation study.

In pulsed Doppler ultrasound systems, the ultrasound radiofrequency (RF) signals received can be employed to estimate noninvasively the time-dependent blood velocity distribution within and artery. The RF signals are composed of signals originating from clutter (e.g., vessel walls) and scatterers (e.g., red blood cells). The clutter, which is induced by stationary or slowly-moving structure interfaces, must be suppressed to get reliable estimates of the mean blood flow velocities. In conventional pulsed Doppler systems, this is achieved with a static temporal high-pass filter. The static cut-off frequency and the roll-off of these filters cause the culture not always to be optimally suppressed. This paper introduces a clutter removal filter that is based on Singular Value Decomposition (SVD). Unlike conventional high-pass filters, which take into account only the information of the temporal direction, the SVD filter makes use of the information of the temporal and spatial directions. The advantage of this approach is that it does not matter where the clutter is located in the RF signal. The performance of the SVD filter is examined with computer-generated Doppler RF signals. The results are compared with those of standard linear regression (SLR) filter. The performance of the SVD filter is good, especially if a large temporal window (i.e., approximately 100 RF signals) is applied, which improves the performance for low blood flow velocities, A major disadvantage of the SVD filter is its computational complexity, which increases considerably for larger temporal windows.

A radio frequency domain complex cross-correlation model to estimate blood flow velocity and tissue motion by means of ultrasound.

This article introduces a mean frequency estimator based on a radio frequency (RF) domain complex cross-correlation model (C3M). The C3M estimator differs from the real cross-correlation model (CCM) estimator in two respects; it is an unbiased estimator of blood flow velocity and/or tissue motion independent of the bandwidth of the RF ultrasound signals, and it provides an estimate of the spatial bandwidth of the RF-signal. The estimators derived from the complex cross-correlation model (mean spatial frequency, mean temporal frequency, spatial bandwidth and signal-to-noise ratio) are based on three complex cross-correlation coefficients. A full derivation and mathematical description of both estimators (C3M and CCM), starting from a Gaussian model of the complex power spectral density distribution of sampled RF signals, are presented. In addition, a thorough performance evaluation of the C3M estimator in comparison with the CCM estimator is carried out by means of simulations to document the effect of signal-to-noise ratio, bandwidth and sample frequency. In the context of the specific simulation conditions considered, the quality of the C3M estimator is shown to offer the best performance (no bias, low standard deviation of the estimate). Taking into account the computational load and the robustness of the C3M estimator, it may be concluded that the C3M estimator combines high quality and modest complexity.

Alterations in c-fos expression after different experimental procedures of sleep deprivation in the cat.

In the present study, we sought to examine the expression of the c-fos proto-oncogene in the cat brain after two different procedures of 24 h sleep deprivation. A first group of cats was gently sleep-deprived; they were awoken by a gentle touch of the hand (n = 5). A second group was sleep-deprived by the water tank technique which is a stressful deprivation (n = 4). A third group was placed 2 h on the water tank and was therefore stressed but not sleep-deprived (n = 2). A fourth group (control group of basal and unspecific Fos expression) was not sleep-deprived (n = 5). These four groups allowed us to separate Fos expression due to stress from Fos expression due to sleep deprivation. On the one hand, compared with controls cats, an important increase in Fos expression, detected by immunohistochemistry, was observed in the preoptic area of sleep-deprived cats by both gentle and stressful methods. On the other hand, there was a significant increase in Fos expression in the lateral hypothalamus of gently deprived cats as compared with control cats. These data indicate that c-fos expression can be employed as a marker of some putative homeostatic mechanism regulating sleep. The only sites in which there was a significant increased number of c-fos expressing neurons were located in the preoptic area which is known to be involved in sleep and in the lateral hypothalamic area.

A physical study of vibrating membranes in developing a middle ear prosthesis.

We present a prosthesis meant to replace the tympano-ossicular set as well as the measuring apparatus for developing and characterizing this prosthesis. Such biocompatible materials as silicone elastomer and Teflon are used. Effects due to aging and sterilization have been negligible. Comparisons with anatomic specimens have yielded good results. Furthermore, the first clinical trials with this prosthesis have been promising.