Bias in mean frequency estimation of Doppler signals due to wall clutter filters

Wall clutter filters used in Doppler ultrasound systems give rise to a bias in the mean frequency estimation. This effect has been explored for two second order filters: the initialised infinite impulse response (IIR)-type filter, and the finite impulse response (FIR) echo-canceller filter. These are used in conjunction with two mean frequency estimation algorithms: the FFT-based intensity-weighted-mean algorithm, and the autocorrelation algorithm. The bias is shown to be caused by the finite settling times of the filters. The IIR filter, although having a much better frequency response than the FIR filter, has a greater bias associated with it. A compromise between bias and frequency response is suggested and a simple method of bias correction is described.     

Extracting instantaneous mean frequency information from Doppler signals using the Wigner distribution function

The Wigner distribution function (WDF) does not require the analysed signals to be stationary within the time segment used for analysis, and instantaneous frequency (IF) information can be obtained from it. Because of the influence of the cross-power of the signal components, however, the interpretation of the IF results is physically clear only for monocomponent signals with infinite data lengths. The IF results for multicomponent signals also suffer from spike problems and are quite unstable even when the signal-to-noise ratio is high. It is suggested that a "pseudo-instantaneous mean frequency," which uses the positive part of the WDF to follow the power distribution changes among frequency components, is used as a simple and rapid way to track frequency changes of Doppler signals. Simulation results show that the pseudo-instantaneous mean frequency does not have the same spike problems and gives stable and relatively accurate information about frequency changes when the sampling frequency is properly chosen.     

Transcranial Doppler ultrasound signals associated with prosthetic heart valves: an in vitro study

The application of transcranial Doppler (TCD) ultrasonography to asymptomatic prosthetic heart valve patients can result in detection of localized bursts of high intensity signals, similar to those caused by the passage of emboli. The origin of these signals is not known. In order to investigate this phenomenon in a simplified, more controllable environment, a TCD machine was used to record flow downstream from mechanical prosthetic heart valves in a mock circulatory loop. The model, which uses a saline solution seeded with silk particles (< 15 micrometers) as the circulatory fluid, recreates the principal hydrodynamic characteristics of the left heart and systemic circulation. Reproducibility of the system was established through repeated testing of a Monostrut valve. Three different mechanical valve types, (Monostrut, Medtronic Hall, St. Jude Medical) were tested over a range of simulated cardiac outputs, and the effect of valve size was investigated with four Omniscience tilting disc valves (21, 23, 25 and 29 mm). Average energy of the reflected Doppler signal was used to quantify the amount of high intensity Doppler signal, QTCD. TCD signals recorded in vitro were visually and aurally similar to those found in prosthetic heart valve patients. All valve types generated exponentially more QTCD with increasing simulated cardiac output. Differences amongst valve types were only significant at higher flow outputs, with the Monostrut valve producing the greatest QTCD. Larger valves consistently generated greater QTCD than smaller valves. In conclusion, TCD signals found in prosthetic heart valve patients can be reproduced, at least qualitatively, using a mock circulatory loop which does not incorporate the formed elements of blood.(ABSTRACT TRUNCATED AT 250 WORDS)     

Action of pentamidine-bound nanoparticles against Leishmania on an in vivo model

The efficiency of antileishmanial agents may be enhanced by improving their bioavailability with a colloidal drug carrier. We have investigated the action of free pentamidine, compared with pentamidine bound to polymethacrylate nanoparticles, in a rodent model. BALB/c mice were infected, via the tail vein, with 4 x 10(7) L. major (MON 74) promastigotes. Twelve days after infection, seven groups of mice were treated respectively with methylglucamine antimoniate (Glucantime) 5.56 mg/kg i.p. x 5 d., pentamidine bound nanoparticles (100 microM), unloaded polymethacrylate nanoparticles, unloaded nanoparticles associated with free pentamidine (100 microM) 0.1 ml i.v. x 3 d and free pentamidine isethionate (2.28 mg/kg and 0.17 mg/kg i.v. x 3 d.). Twenty-one days post infection, the mice were sacrificed and the Leishmania load in the liver calculated from the number of amastigotes/500 liver cells and total liver weight in treated and untreated mice. Results demonstrated a 77% amastigote reduction in the group treated with targeted pentamidine relative to the control group. The ratio free pentamidine/bound-pentamidine was approx. 12.     

Observations on seminal vesicle dynamics in an in vivo rat model

PURPOSE: To gain understanding of the seminal vesicle as a muscular organ, seminal vesicle compliance and contractile properties were quantified with an in vivo, microsurgical rat model. MATERIALS AND METHODS: Microsurgical dissection was performed on anesthetized rats to enable simultaneous organ filling and monitoring of intraluminal pressures. The reliability and reproducibility of post-ganglionic hypogastric nerve-induced ipsilateral (4 rats) and bilateral (5 rats) seminal vesicle contractile responses were assessed during repeated nerve stimulation. Seminal vesicle resting compliance was assessed during a constant saline infusion (10 rats). Functional performance curves were obtained at fixed fill-volumes by measuring organ contraction after nerve stimulation (4 rats). RESULTS: A reproducible seminal vesicle contractile response was obtained with a nerve stimulation interval > 15 minutes. Bilateral seminal vesicle responses were observed with unilateral nerve stimulation. The resting organ compliance curve with saline filling exhibited a characteristic, triphasic response. Functional performance studies revealed that contractile performance improves as the fill-volume increases until the distensibility limit of the organ is reached. CONCLUSIONS: A reliable, in vivo, rat model of seminal vesicle organ compliance and contractility is described. The seminal vesicle is a highly contractile, compliant smooth muscular organ with dynamic properties analogous to that of the urinary bladder. This experimental system may allow for the investigation of pharmacologic and other physiological influences on in vivo organ activity.     

Doppler quantification of echo-contrast injections in vivo

It is difficult to quantify myocardial perfusion using contrast echocardiography because the echogenicity of injected contrast is unknown. We propose that a measurement of Doppler amplitude from blood in a systemic artery during the passage of contrast could define the needed input function. Time-amplitude curves from pulsed Doppler cuffs on coronary and carotid arteries of 7 dogs were analyzed during aortic root and left atrial injections of Albunex. We found in individual animals that the areas under the Doppler time-amplitude curves were correlated to the amount of Albunex injected (R = 0.87-0.99), inversely correlated to cardiac output (R = 0.83), and uncorrelated to coronary flow (R = 0.18). Due to better mixing, the coronary and carotid response areas correlated better for left atrial injections (R = 0.96) than for aortic root injections (R = 0.56). We conclude that Doppler amplitude detection can be used to quantify the passage of echo-contrast agents, that the measurements comply with indicator-dilution principles, and that systemic measurements in the carotid artery could be used to predict the coronary input function for injection sites with good systemic mixing.     

Doppler microembolic signals in children with prosthetic cardiac valves

BACKGROUND AND PURPOSE: The aim of this study was the evaluation of the prevalence and counts of Doppler microembolic signals (MES) in children with prosthetic cardiac valves and their comparison to those obtained in corresponding adult patients. PATIENTS AND METHODS: Nine children and 43 adults with ATS valves implanted in the aortic position were monitored over both middle cerebral arteries with transcranial Doppler ultrasound. MES were identified on-line according to standard criteria. Heart rate and rhythm, valve type, size and duration, patients' height, International Normalized Ratio, and prevalence of neurological complications were obtained from all study participants. RESULTS: MES prevalence and counts were significantly higher in children compared with adult patients (100% versus 25.5% and 58 [18.5 to 115.5] versus 55 [2 to 10.5], median, 95% CI, respectively). No corresponding differences in valve duration of valve implant were evident, but children has heart rates and were significantly smaller compared with adults. A positive correlation between patients' size, heart rate, and MES counts was noted. CONCLUSIONS: MES counts in children with mechanical prosthetic valves are significantly higher compared with those in corresponding adults. We hypothesize that this is due to (1) the shorter distance between aortic valve and middle cerebral artery, since cavitation bubbles have a short life span and are bound to dissolve with time, and (2) the faster heart rate in children, resulting in a higher number of valve closures per minute.     

Enhancement of Doppler signals in aortic and mitral valve diseases

Ultrasound contrast media increase backscatter from blood, thus improving the signal-to-noise ratio. Potential clinical applications of intravenous ultrasound contrast are reviewed. Contrast enhancement of continuous wave Doppler is indicated when the native recordings are noisy and no complete envelope of the Doppler spectrum is obtained. In aortic stenosis several investigations showed good agreement between the gradient calculated from Doppler measurements and the results of cardiac catheterization. In mitral insufficiency maximum area of the regurgitant jet is a widely used parameter for estimation of the severity of the regurgitation. However, assessment of the maximum jet area may not be possible because of poor acoustic windows. Contrast enhancement provides complete display of the regurgitant jet in most of the patients. The diagnostic confidence of the Doppler investigation is further improved by the recording of the pulmonary venous flow, which can be recorded in most of the patients following contrast injection. Therefore contrast enhanced transthoracic Doppler is an alternative to transesophageal Doppler investigation in patients with poor transthoracic windows.     

Doppler velocimetry of growth-restricted fetuses in an ovine model of placental insufficiency

Crush syndrome is the severe systemic manifestation of prolonged muscle compression and compartment syndrome. Careful patient assessment, early diagnosis, and aggressive treatment are vital to prevent multiorgan failure and death. Medical management of systemic complications, along with operative procedures of fasciotomy and debridement, are indicated with accompanying compartment syndrome. Debridement of necrotic and nonviable tissue is necessary; significant risks of infection and hemorrhage remain until the wounds can be subsequently closed or covered with skin graft. Crush syndrome and muscle necrosis in a closed injury without compartment syndrome may be followed clinically until healing or demarcation of a gangrenous part occurs, providing the patient's general medical condition, including renal function, can be maintained. Fasciotomy and hyperbaric oxygen will not reverse necrosis of muscle in the absence of compartment syndrome and therefore do not affect outcome of the extremity. Overall, prognosis is improved by early diagnosis and treatment, but outcome of the crushed extremity is poor and Volkmann's contracture often results.     

Autocorrelation of integrated power Doppler signals and its application

The integrated power Doppler signal arising from blood flow is a random process. In this article, a general approach to model this random process is studied. Both the theoretical and experimental results show that the temporal autocorrelation function of the integrated power Doppler signals is directly related to properties of the insonified medium, such as the scattering strengths and velocities of all moving scatterers, and as well as the properties of the Doppler imaging system, such as the point spread function (psf) of the power Doppler images. Some initial experiments are performed to test the proposed model. Its potential application for flow measurement, such as perfusion evaluation, is also discussed.     

Monitoring of radio frequency tissue ablation in an interventional magnetic resonance environment. Preliminary ex vivo and in vivo results

RATIONALE AND OBJECTIVES: The authors evaluate the feasibility of monitoring radio frequency (RF) ablation in an interventional, open-configuration, 0.5-tesla magnetic resonance (MR) environment. METHODS: Ex vivo and in vivo RF coagulation necrosis were induced in porcine paraspinal muscle tissue using a 300 kHz monopolar RF generator applying 5 to 20 W over 3 to 9 minutes. Images were acquired simultaneous to RF application, after RF application, and in an intermittent mode (60 seconds of RF followed by 15 seconds of MR imaging). Temperature changes were monitored based on amplitude (ex vivo) and phase alterations (in vivo) of a T1-weighted graded refocused echo (GRE) sequence enabling an update every 2.5 seconds. A standardized color-coded subtraction technique enhanced signal changes. Additionally, T2- and T1-weighted spin echo (SE) images were acquired with and without intravenous contrast. Macroscopic coagulation size was compared with lesion size seen on MR images. RESULTS: Although lesion diameters were related directly to applied RF power, the application mode had no significant impact on coagulation size (P > 0.05). As could be expected, MR imaging during RF ablation resulted in major image distortion. Radio frequency effects were seen on images acquired in the continuous and intermittent modes. Coagulation size seen on GRE images correlated well with macroscopy both ex vivo (r = 0.89) and in vivo (r = 0.92). Poorer correlation was found with postinterventional SE sequences (r = 0.78-0.84). CONCLUSIONS: Magnetic resonance monitoring of RF effects is feasible both ex vivo as well as in vivo using temperature-sensitive sequences in an open-configuration MR environment.     

Influence of muscle activity on the forces in the femur: an in vivo study

Experiments were performed on two patients with custom-made instrumented massive proximal femoral prostheses implanted after tumour resection. In vivo axial forces transmitted along the prostheses were telemetered during level walking, single- and double-leg stance, and isometric exercises of the hip muscles. These activities varied the lever arms available to the external loads: minimum for double-leg stance and maximum for hip isometric exercises. Kinematic, force plate, EMG and telemetered force data were recorded simultaneously. The force magnification ratio (FMR; the ratio of the telemetered axial force to the external force) was calculated. The FMRs ranged from 1.3 (during double-leg stance) to 29.8 (during abductors test), indicating that a major part of the axial force in the long bones is a response to muscle activity, the strength of which depends on the lever arms available to the external loads. From these results, it was shown that the bulk of the bending moment along limbs is transmitted by a combination of tensile forces in muscles and compressive forces in bones, so moments transmitted by the bones are smaller than the limb moments. It was concluded that appropriate simulation of muscle forces is important in experimental or theoretical studies of load transmission along bones.     

Detection of asymptomatic cerebral embolic signals with Doppler ultrasound

Can asymptomatic cerebral emboli be detected? With transcranial Doppler ultrasonography of the middle cerebral artery, short-duration high-intensity asymptomatic embolic signals were detected in 6 of 25 patients with carotid stenosis and in 9 of 24 with prosthetic cardiac valves, but not in 20 normal controls. In carotid stenosis the signals were usually unilateral and ipsilateral to the stenosis. Embolic signals were significantly more common in patients with mechanical valves than with pig xenografts (8/13 vs 1/11). With mechanical valves embolic signals were usually bilateral. Detection of asymptomatic emboli may allow identification of and preventive treatment in at-risk patients.     

Influence of mating frequency on sow reproductive performance

Gilts and sows were bred one, two, or three times during a single estrous period in a commercial herd for evaluating the effect of mating frequency on reproductive performance. Estrus detection started at approximately 0630 daily by applying back pressure to females with the presence of a mature boar. Natural mating was used. Gilts detected in estrus were mated in the morning of d 1 (AM), the morning of d 1 and 2 (AM-AM), and the morning and afternoon of d 1 and morning of d 2 (AM-PM-AM) for mating frequencies 1, 2, and 3, respectively. Sows were bred in the AM, AM-AM, AM-PM-AM (1), and morning of d 1 and morning and afternoon of d 2 [AM-AM-PM (2)] for mating frequency 1, 2, 3 (1), and 3 (2), respectively. Breeding events in the morning and afternoon started at approximately 0730 and 1530. Females were randomly assigned to a mating frequency. Boars were randomly assigned to each breeding event. In total, 256 gilts and 766 sows were involved in the study. Gilts with a single mating (76.5%, P = .06) and triple matings (80.4%, P < .05) had higher farrowing rates than those with double matings (65.3%). No differences (P > .1) in the farrowing rates of sows were observed between mating frequencies 1, 2, 3 (1), and 3 (2). Double-mated gilts had more (P < .03) total born (9.7 vs 8.6) and pigs born alive (9.3 vs 8.2) than did single-mated gilts. There were no differences (P > .3) in total born and pigs born alive in sows between mating frequencies. We concluded that triple-mating gilts and sows did not improve farrowing rate and litter size compared with single and double matings. There were no differences in farrowing rate and litter size between double- and single-mated sows. Gilts with double matings had a larger litter size than those with a single mating.     

Reproduction of antiviral effect in an in vivo model of human cytomegalovirus retinal infection

Fibroblast growth factors (FGFs) play important roles in a variety of developmental processes in mammals. The dependence of their activity on heparin binding has been a puzzle that, in recent years, has been the subject of active investigation. Recent structural analyses on complexes of FGFs with heparin fragments or heparin analogs have unveiled the extreme complexity of these systems.     

The chick embryo chorioallantoic membrane as an in vivo wound healing model

The chick embryo chorioallantoic membrane (CAM) was used as an in vivo wound healing model. A full excision of a 1 mm2 CAM area was filled by a granulation tissue after 96-120 h, which eventually formed a scar in 75% of the cases. In the remaining 25%, a solution of continuity was left which, however, was smaller in size than the one observed immediately after the excision. Under the microscope, the CAM area involved in the repair process showed: i. hyperplasia of the chorionic epithelium; ii. about three times as many microvessels and fibroblasts in the mesenchyme as in the normal adjacent control regions; iii. an inflammatory infiltrate mostly consisting of macrophages; and iv. a strong positivity for fibronectin in the extracellular matrix. The validity of this experimental model appears to be confirmed by the fact that we were able to reproduce all the critical events controlling the wound healing process, such as re-epithelization, angiogenesis, formation of an inflammatory infiltrate and deposition of one of the main constituents of the extracellular matrix, such as fibronectin.     

Acute hyperglycaemia causes severe disturbances of mesenteric microcirculation in an in vivo rat model

BACKGROUND: Chronic hyperglycaemia in diabetes is suggested to be a major cause of diabetic angiopathy. Up until now, the effects of acutely induced hyperglycaemia in non-diabetic subjects as well as hyperglycaemic effects in early diabetes, on mesenteric microcirculation and leucocyte endothelial cell interaction have not been investigated extensively. METHODS: The aim of this study was to examine rat mesenteric microcirculatory parameters such as leucocyte adhesion, leucocyte emigration, venular shear rate and leucocyte rolling velocity using a new rat model both with continuous venous glucose infusion and with continuous arterial measurement of blood glucose concentration while observing mesenteric microcirculation with in vivo capillary microscopy in the non-diabetic and diabetic state. RESULTS: In normal non-diabetic rats, acute elevation of glucose concentration resulted in a highly significant, rapid and step-by- step enhancement of adhesion and emigration in a dose dependent manner. Leucocyte rolling velocity was reduced with rising glucose levels. Venular shear rate showed a similar reduction at all hyperglycaemic levels. In streptozotocin-induced diabetes, adhesion and emigration were significantly enhanced while shear rate and leucocyte rolling velocity were severely reduced, resembling the effects of glucose infusion experiments. Longer duration of diabetes resulted in a further enhancement of leucocyte adhesion and reduction of leucocyte rolling velocity while emigration and shear rate were not influenced by a longer period of diabetes manifestation. CONCLUSION: Experiments using different mannitol concentrations revealed that most of the observed glucose effects can be mimicked by mannitol and are therefore - at least in part - due to changes in osmolarity by yet unknown mechanisms.     

Influence of upper airway pressure changes on respiratory frequency

Influences of pressure changes within the upper airway on respiratory frequency were studied in anesthetized rabbits. These reflex effects were investigated in two ways: (1) by applying sustained negative or positive pressures to isolated upper airway in vagally intact animals during tracheostomy breathing, and (2) by briefly occluding the nasal airways of vagotomized animals at end expiration. Negative pressure applied to isolated upper airway decreased the respiratory frequency (P less than 0.01). Decrease in respiratory frequency correlated with magnitude of pressure change. In contrast, positive pressures produced an increase in respiratory frequency (P less than 0.05). But, the increase in frequency correlated poorly with magnitude of positive pressure change. A post-stimulus effect lasting several breaths was seen following the release of both negative and positive pressures (P less than 0.01). Nasal occlusion in vagotomized animals was associated with an increase in Ti and Ttot of the first nasally occluded breath (P less than 0.05). Increase in Ti was not associated with an increase in peak diaphragmatic activity. This would result in decreased inspiratory load on the upper airway patency maintaining muscles by reducing the mean inspiratory pressure. Hence, these reflex responses to airway pressure could play a functional role in the maintenance of upper airway patency.     

Endopeptidase inhibitors decrease myocardial ischemia/reperfusion injury in an in vivo rabbit model

Periods of ischemia followed by reperfusion of the ischemic tissue are associated with myocardial damage and ventricular arrhythmia. Angiotensin converting enzyme inhibitors limit the occurrence of these arrhythmias. The protective effects of angiotensin converting enzyme inhibitors may be due to inhibition of bradykinin (BK) degradation, rather than inhibition of angiotensin II formation. Other enzymes which catabolize BK include the endopeptidases EP24.11 and EP24.15. The purpose of this study was to determine if inhibitors of EP24.11 and EP24.15 decrease ischemia/reperfusion injury and if this protection is mediated by BK receptors. Rabbits were anesthetized and prepared for recording of cardiovascular parameters. The chest was opened and a left ventricular artery occluded for 30 min, followed by a 2-hr reperfusion period. Infarct size was determined using triphenyl tetrazolium chloride staining immediately after reperfusion. The enzyme inhibitors, ramiprilat, N-[1-(R,S)-carboxy-3-phenylpropyl]-Phe-pAB, and N[1-(R,S)-carboxy-3-phenylpropyl]-Ala-Ala-Phe-pAb, singly and in combinations were administered 3 min before reperfusion. Compared to saline (32.1 +/- 2.1), ramiprilat (18.3 +/- 2.8) and the EP inhibitors (14.4 +/- 1.4 for the combination) significantly decreased infarct size, with the greatest decrease occurring when all three inhibitors were combined (10.6 +/- 1.5). The protective effect of the EP inhibitors was blocked by the BK2 receptor antagonist, HOE 140 (30.1 +/- 2.6). Enzyme assays demonstrated EP24.11 and EP24.15 in the rabbit heart. We conclude that the EP inhibitors decreased ischemia/reperfusion injury by protecting BK from metabolism and that a combination of inhibitors provides superior protection to that given by a single agent.     

Mathematical description of photobleaching in vivo describing the influence of tissue optics on measured fluorescence signals

The observed decrease in the fluorescence signal during photodynamic therapy (PDT) may contain dosimetric information as this photobleaching provides direct information on the photodynamic processes occurring in the tissue. A correct interpretation of the photobleaching signal, however, is crucial for its use in dosimetry. In this study the influence of scattering and absorption phenomena in tissue on the emitted fluorescence signal are described mathematically. Analytical solutions of the resulting expression show a difference from the single-decaying-exponential function generally used for describing photobleaching signals. The solutions are a function of the fluence rate at the inner side of tissue boundary psi(0*), the photobleaching dose constant beta, the incident irradiation power I0 and time. The accuracy of the results was investigated by comparison of the analytic solutions with numerical calculations using fluence rate profiles and escape functions obtained by Monte Carlo (MC) simulations. Good resemblance is observed when the value for psi(0*) calculated by the MC simulations is used in the analytical solutions. Experimental results in this study indicate the photobleaching dose constant of ALA-induced PpIX to be 33 +/- 3 J cm-2. Determination of beta for different types of photosensitizer and the development of an accurate method to determine psi(0*) can make monitoring of photobleaching during PDT valuable for dosimetry.