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PubMedDate not supplied

A robust time-varying identification algorithm using basis functions.

We extend a recently developed time invariant (TIV) model order search criterion named the optimal parameter search algorithm (OPS) for identification of time varying (TV) autoregressive (AR) and autoregressive moving average (ARMA) models. Using the TV algorithm is facilitated by the fact that expanding each TV coefficient onto a finite set of basis sequences permits TV parameters to become TIV. Taking advantage of this TIV feature of expansion parameters exploits the features of the OPS, which has been shown to provide accurate model order selection as well as extraction of only the significant model terms. Another advantage of the new algorithm is its ability to discriminate insignificant basis sequences thereby reducing the number of expansion parameters to be estimated. Due to these features, the resulting algorithm can accurately estimate TV AR or ARMA models and determine their orders. Indeed, comparison via computer simulations of AR models between the proposed method and one of the well-known iterative methods, recursive least squares, shows the greater capability of the new method to track TV parameters. Furthermore, application of the new method to experimentally obtained renal blood flow signals shows that the new method provides higher-resolution time-varying spectral capability than does the short-time Fourier transform (STFT), concomitant with fewer spurious frequency peaks than obtained with the STFT spectrogram.

Algorithms↗

PubMed1991

Passive material properties of intact ventricular myocardium determined from a cylindrical model.

The equatorial region of the canine left ventricle was modeled as a thick-walled cylinder consisting of an incompressible hyperelastic material with homogeneous exponential properties. The anisotropic properties of the passive myocardium were assumed to be locally transversely isotropic with respect to a fiber axis whose orientation varied linearly across the wall. Simultaneous inflation, extension, and torsion were applied to the cylinder to produce epicardial strains that were measured previously in the potassium-arrested dog heart. Residual stress in the unloaded state was included by considering the stress-free configuration to be a warped cylindrical arc. In the special case of isotropic material properties, torsion and residual stress both significantly reduced the high circumferential stress peaks predicted at the endocardium by previous models. However, a resultant axial force and moment were necessary to cause the observed epicardial deformations. Therefore, the anisotropic material parameters were found that minimized these resultants and allowed the prescribed displacements to occur subject to the known ventricular pressure loads. The global minimum solution of this parameter optimization problem indicated that the stiffness of passive myocardium (defined for a 20 percent equibiaxial extension) would be 2.4 to 6.6 times greater in the fiber direction than in the transverse plane for a broad range of assumed fiber angle distributions and residual stresses. This agrees with the results of biaxial tissue testing. The predicted transmural distributions of fiber stress were relatively flat with slight peaks in the subepicardium, and the fiber strain profiles agreed closely with experimentally observed sarcomere length distributions. The results indicate that torsion, residual stress and material anisotropy associated with the fiber architecture all can act to reduce endocardial stress gradients in the passive left ventricle.

Animals↗

PubMedDate not supplied

On the physics of laser-induced selective photothermolysis of hair follicles: Influence of wavelength, pulse duration, and epidermal cooling.

The physical basis for optimization of wavelength, pulse duration, and cooling for laser-induced selective photothermolysis of hair follicles in human skin is discussed. The results indicate that the most important optimization parameter is the cooling efficiency of the technique utilized for epidermal protection. The optical penetration is approximately the same for lasers at 694, 755, and 800 nm. The penetration of radiation from Nd:yttrium-aluminum-garnet lasers at 1064 nm is, however, somewhat larger. Photothermal damage to the follicle is shown to be almost independent of laser pulse duration up to 100 ms. The results reveal that epidermal cooling by a 30-80-ms-long cryogen spurt immediately before laser exposure is the only efficient technique for laser pulse durations less than 10 ms. For longer pulse durations in the 30-100 ms range, protection can be done efficiently by skin cooling during laser exposure. For laser pulses of 100 ms, an extended precooling period, e.g., by bringing a cold object into good thermal contact with the skin for about 1 s, can be of value. Thermal quenching of laser induced epidermal temperature rise after pulsed exposure can most efficiently be done with a 20 ms cryogen spurt applied immediately after irradiation.

Cold Temperature↗

PubMed2004

Scanning-slit photon counting x-ray imaging system using a microchannel plate detector.

An experimental prototype of a novel photon counting x-ray imaging system was evaluated. This system is based on an "edge-on" microchannel plate (MCP) detector and utilizes scanning slit imaging configuration. The detector is capable of photon counting, direct conversion, high spatial resolution, controllable physical charge amplification, quantum limited and scatter free operation. The detector provides a 60 mm wide field of view (FOV) and its count rate is 200 kHz for the entire FOV. The count rate of the current system is limited by the position encoding electronics, which has a single input for all events from the entire detector, and incorporates a single channel ADC with 1 micros conversion time. It is shown that the count rate can potentially be improved to clinically acceptable levels using multichannel application specific integrated circuit (ASIC) electronics and multi-slit image acquisition geometry. For a typical acquisition time used in this study, the image noise was measured to be less than the typically acceptable noise level for medical x-ray imaging. It is anticipated that the noise level will be also low after the implementation of the ASIC electronics. The quantum efficiency of the detector was measured to be 40%-56% for an energy range of 50-90 kVp for MCPs used in this study and can be improved to > 80% using MCPs with the optimized parameters. Images of resolution and anthropomorphic phantoms were acquired at an x-ray tube voltage of 50 kVp. The value of contrast transfer function for the detector was measured to be 0.5 at a spatial frequency of 5 lp/mm. The intrinsic spatial resolution of the system is 28 microm FWHM and was limited by the accuracy of the time-to-digital conversion of the position encoding electronics. Given the advantages of the edge-on MCP detector such as direct conversion and physical charge amplification, it can potentially be applied to mammography and chest radiography.

Equipment Design↗

PubMed2004

Modifications to the IMFAST leaf sequencing optimization algorithm.

The optimizing leaf sequencer IMFAST minimizes intensity modulated treatment times. However, algorithm modifications can yield improved results. Currently, during segment extraction, the largest extract for a given number of levels is chosen. The modification chooses an extract that yields the fewest segments and levels when the rod-pushing algorithm is applied to the difference between the original map and the extract. Also, successive optimization parameter values are now allowed to increase. These modifications reduced the number of segments and the relative fluence from the original algorithm by an average of 7%-11% and 8%-17%, respectively, depending on whether interdigitation and/or tongue-and-groove constraints were considered. The tests were done on two clinical head and neck intensity modulated radiation therapy cases. Compared to the sweeping window algorithm, a reduction of 35%-55% of the number of segments is possible with a change in the relative fluence of -9% - 16%, depending on the constraints. Compared to other previously published algorithms that deal with the constraints tested here, the modified IMFAST algorithm provides the greatest reduction in the number of segments with the minimum increase in the relative fluence.

Algorithms↗

PubMedDate not supplied

Semiempirical model for generating tungsten target x-ray spectra.

A semiempirical model for generating tungsten target x-ray spectra is presented. This model extends earlier work in two significant areas. First, both bremsstrahlung and characteristic x-ray production are assumed to occur at varying depths within the target. Second, optimal parameters for the model were determined from experimental spectra utilizing nonlinear least-squares techniques. As a result, good agreement is obtained between calculated and measured x-ray tube spectra and output for different target angles and a wide range of x-ray tube potentials. Such is not the case with previously published models.

Mathematics↗

PubMed1995

A modeling approach for quantifying tumor hypoxia with [F-18]fluoromisonidazole PET time-activity data.

[F-18]fluoromisonidazole (FMISO), a positron-emitting nitroimidazole, binds preferentially to hypoxic cells. It has been used to image hypoxia in human tumors with positron emission tomography (PET). In order to quantify tumor oxygenation status from these PET data, a kinetic model of FMISO cellular bioreduction has been developed to relate cellular oxygen concentration to the cellular FMISO reaction rate constant, kappa A. Also, a compartmental model of FMISO transport and metabolism has been developed to compute the volume average kappa A in tissue regions from [F-18]FMISO PET time-activity data. This compartmental model was characterized using Monte Carlo simulations and [F-18]FMISO PET time-activity data. The model performed well in Monte Carlo simulations; performance was enhanced by fixing three of the seven model parameters at physiologically reasonable values. The four parameters optimized were blood flow rate, kappa A for two partial volume/spillover correction factors. The model was able to accurately determine kappa A for a variety of computer-generated time-activity curv including those for hypothetical heterogeneous tissue regions and poorly perfused tissue regions. The model was also able to fit [H-3]FMISO time-activity data from 36B-10 rat tumors as well as [F-18]FMISO PET time-activity data from a human patient with a base of the tongue squamous cell carcinoma. The kappa A values in muscles ROIs were comparable to those in well-oxygenated cell monolayers while kappa A values in tumor ROIs were greater, suggesting the presence of hypoxic cells in the tumor.

Animals↗

PubMed2004

Stereotactic IMRT for prostate cancer: dosimetric impact of multileaf collimator leaf width in the treatment of prostate cancer with IMRT.

The focus of this work is the dosimetric impact of multileaf collimator (MLC) leaf width on the treatment of prostate cancer with intensity-modulated radiation therapy (IMRT). Ten patients with prostate cancer were planned for IMRT delivery using two different MLC leaf widths--4mm and 10mm--representing the Radionics micro-multileaf collimator (mMLC) and Siemens MLC, respectively. Treatment planning was performed on the XKnifeRT2 treatment-planning system (Radionics, Burlington, MA). All beams and optimization parameters were identical for the mMLC and MLC plans. All the plans were normalized to ensure that 95% of the planning target volume (PTV) received 100% of the prescribed dose. The differences in dose distribution between the two different plans were assessed by dose-volume histogram (DVH) analysis of the target and critical organs. We specifically compared the volume of rectum receiving 40 Gy (V40), 50 Gy (V50), 60 Gy (V60), the dose received by 17% and 35% of rectum (D17 and D35), and the maximum dose to 1 cm3 of the rectum for a prescription dose of 74 Gy. For the urinary bladder, the dose received by 25% of bladder (D25), V40, and the maximum dose to 1 cm3 of the organ were recorded. For PTV we compared the maximum dose to the "hottest" 1 cm3 (Dmax1 cm3) and the dose to 99% of the PTV (D99). The dose inhomogeneity in the target, defined as the ratio of the difference in Dmax1 cm3 and D99 to the prescribed dose, was also compared between the two plans. In all cases studied, significant reductions in the volume of rectum receiving doses less than 65 Gy were seen using the mMLC. The average decrease in the volume of the rectum receiving 40 Gy, 50 Gy, and 60 Gy using the mMLC plans was 40.2%, 33.4%, and 17.7%, respectively, with p < 0.0001 for V40 and V50 and p < 0.012 for V60. The mean dose reductions for D17 and D35 for the rectum using the mMLC were 20.4% (p < 0.0001) and 18.3% (p < 0.0002), respectively. There were consistent reductions in all dose indices studied for the bladder. The target dose inhomogeneity was improved in the mMLC plans by an average of 29%. In the high-dose range, there was no significant difference in the dose deposited in the "hottest" 1 cm3 of the rectum between the two plans for all cases (p > 0.78). In conclusion, the use of the mMLC for IMRT of the prostate resulted in significant improvement in the DVH parameters of the prostate and critical organs, which may improve the therapeutic ratio.

Equipment Design↗

PubMed2001

On the annoyance caused by impulse sounds produced by small, medium-large, and large firearms.

A laboratory study was designed in which the annoyance was investigated for 14 different impulse sound types produced by various firearms ranging in caliber from 7.62 to 155 mm. Sixteen subjects rated the annoyance for the simulated conditions of (1) being outdoors, and (2) being indoors with the windows closed. In the latter case, a representative outdoor-to-indoor reduction in sound level was applied. It was anticipated that the presumed additional annoyance caused by the "heaviness" of the impulse sounds might be predicted from the difference between the C-weighted sound exposure level (CSEL; LCE) and the A-weighted sound exposure level (ASEL; LAE). In the outdoor rating conditions, the annoyance was almost entirely determined by ASEL. The explained variance, r2, in the mean ratings by ASEL was 0.95. In the indoor rating conditions, however, the explained variance in the annoyance ratings by (outdoor) ASEL was significantly increased from r2 = 0.87 to r2= 0.97 by adding the product (LCE-LAE)(LAE-alpha) as a second variable. In combination with a 12-dB adjustment for small firearms, the present results showed that for the entire set of impulse sounds rated indoors with windows closed, the rating sound level, Lr, is given by Lr=LAE +12dB+beta(LCE-LAE)(LAE-alpha), with alpha=45dB and beta=0.015dB(-1). For the outdoor rating condition, the optimal parameter values were equal to alpha=57 dB and, again, beta=0.015 dB(-1). In validation studies, in which the effects of the present rating procedure will be compared to field data, it has to be determined to what extent the constants alpha and beta have to be adjusted.

Adolescent↗

PubMed2001

DPOAE suppression tuning: cochlear immaturity in premature neonates or auditory aging in normal-hearing adults?

Previous work has shown that distortion product otoacoustic emission (DPOAE) suppression tuning curves (STCs) recorded from premature neonates are narrower than adult STCs at both low and high frequencies. This has been interpreted to indicate an immaturity in cochlear function prior to term birth. However, an alternative explanation for this finding is that adult DPOAE STCs are broadened and reflect cochlear hair cell loss in normal-hearing adults due to aging, and natural exposure to noise and ototoxins. This alternative hypothesis can be tested by studying suppression tuning in normal-hearing school-aged children. If normal-hearing children, who have not aged significantly or been exposed to noise/ototoxins, have DPOAE suppression tuning similar to adults, the auditory aging hypothesis can be ruled out. However, if children have tuning similar to premature neonates and dissimilar from adults, it implicates aging or other factors intrinsic to the adult cochlea. DPOAE STCs were recorded at 1500, 3000, and 6000 Hz using optimal parameters in normal-hearing children and adults. DPOAE STCs collected previously from premature neonates were used for age comparisons. In general, results indicate that tuning curves from children are comparable to adult STCs and significantly different from neonatal STCS at 1500 and 6000 Hz. Only the growth of suppression was not adultlike in children and only at 6000 Hz. These findings do not strongly support the auditory aging hypothesis as a primary explanation for previously observed neonatal-adult differences in DPOAE suppression tuning. It suggests that these age differences are most likely due to immaturities in the neonatal cochlea. However, nonadultlike suppression growth observed in children at 6000 Hz warrants further attention and may be indicative of subtle alternations in the adult cochlea at high frequencies.

Age Factors↗

PubMed2002

Experimental studies of a drumlike silencer.

The theoretical finding of the broadband performance of a reactive silencer is validated experimentally. The silencer consists of two highly stretched membranes lining part of the duct and backed by two long and shallow cavities. The test rig was built with a small square duct of 5 cm in dimension, and each cavity is 5 cm deep and 25 cm long. Two types of metal foils, stainless steel and copper, were used, and the lowest membrane-to-air mass ratio was 1.3. A transmission loss in excess of 10 dB was achieved over more than one octave band. For one configuration close to the optimal parameters, the predicted ratio of the frequency band limits is 2.47, while the experiment gave 2.35. Three spectral peaks were found in the stopband, as predicted, but the peaks were broader than prediction, indicating the presence of significant sound energy dissipation mechanisms. Comparison with theoretical simulation shows that the cavity damping dominates over membrane friction. Tests using heavier membranes and membrane with different levels of tension also agree with predictions. Issues of practical implementation of the concept as a flow-through silencer are also addressed.

Journal Article↗

PubMed1992

An objective quality assessment method for bit-reduction coding of wideband speech.

This paper proposes a new objective quality assessment method for bit-reduction coding of wideband speech taking into account the masking effect of quantizing noise. First, this paper analyzes the reliability and sensitivity of the speech quality assessment method, based on a paired-comparison test with a modulated noise reference signal, for the bit-reduction coding of high-quality wideband speech. Then, the perception of quantizing noise is studied using speech with noise synthesized similar to the quantizing noise. The detection of quantizing noise is found to be influenced by masking by the source signal. This leads to a new method of objectively estimating the quality of coding speech by multiple regression analysis. The factors for the estimation are segmental signal-to-noise ratio, spectrum envelope distance between source signal and quantizing noise, and the similarity of the noise power envelope to the source signal in the time domain. This estimation method is applied to the parameter optimization of wideband coding systems.

Auditory Perception↗

PubMed1994

Acoustics of the tenor high voice.

The spectra of six tenors were analyzed at high pitches, F4 to B4. Because of the wide separation between harmonics, formant frequencies could not be extracted in the traditional way. Rather, an analysis-by-synthesis technique was used to match the spectra of a model to the measured spectra, using parameter optimization. Results suggest that tenors maintain their first formant frequencies well above the fundamental for all vowels except [u]. The purpose of this seems to be to distribute the acoustic energy between harmonics 2, 3, and 4 rather than to boost the fundamental. Tuning the first formant to the fundamental is a technique used effectively by sopranos but seems to be deliberately avoided by tenors in order to preserve a male quality.

Humans↗

PubMed1976

Effect of antibiotics on L-form induction of Neisseria meningitidis.

Conditions have been developed that permit highly efficient conversion in vitro of all serogroups of Neisseria meningitidis to the L-form. During the course of the studies, optimal parameters for cultivation and propogation were determined and compared with those found in studies done by previous workers on meningococci. The plasma expander polyvinylpyrrolidone (PVP) (molecular weight, 40,000) has been incorporated into the medium as an osmotic stabilizer. A method is outlined for removing toxicity (due to impurities in PVP) reported in some earlier studies done in this laboratory. Light and scanning electron microscopy of the L-form inductants and of a stable L-form have shown characteristics distinctive among the strains observed. Finally, the effects of both cell wall and other antibiotics were studied. It was shown that cell wall antibiotics other than the commonly used potassium penicillin G could induce N. meningitidis to the L-form, using the PVP-stabilized medium base. Furthermore, the antibiotics vancomycin, colistin, nystatin, and trimethoprim, used at the concentrations found in commercially prepared Transgrow formulations, did not adversely affect meningococcal L-form induction. The system described herein will hopefully serve as an effective tool, specifically in the study of "aseptic" meningitis and possible meningococcal L-form etiology and, more generally, in evaluating the ability of bacterial L-forms to cause disease.

Anti-Bacterial Agents↗

PubMed1971

Rapid methods for determining decarboxylase activity: ornithine and lysine decarboxylases.

A rapid biochemical method for the determination of ornithine and lysine decarboxylase (EC 4.1.1.18) activity has been developed for use in the routine clinical laboratory. It is based on the detection of the amine end product produced in response to the single key amino acid added to a synthetic medium. A modified ninhydrin reagent is used to detect the amine after a chloroform extraction. This procedure can be used with a 1- to 4-hr incubation period (utilizing an initial concentrated inoculum) or with an overnight culture. Thus, measurements based on the alkalinization of the medium after a lengthy incubation period are avoided. Optimal parameters for enzyme activity are discussed.

Amines↗

PubMed1987

Transformation of vegetative cells of Bacillus thuringiensis by plasmid DNA.

Plasmid DNA-mediated transformation of vegetative cells of Bacillus thuringiensis was studied with the following two plasmids: pBC16 coding for tetracycline resistance and pC194 expressing chloramphenicol resistance. A key step was the induction of competence by treatment of the bacteria with 50 mM Tris hydrochloride buffer (pH 8.9) containing 30% sucrose. Transformation frequency was strongly influenced by culture density during the uptake of DNA and required the presence of polyethylene glycol. Growth in a minimal medium supplemented with Casamino Acids gave 35 times more transformants than growth in a rich medium. The highest frequencies were obtained with covalently closed circular DNA. With all parameters optimized, the frequency was 10(-3) transformants per viable cell or 10(4) transformants per microgram of DNA. Cells previously frozen were also used as recipients in transformation experiments; such cells gave frequencies similar to those obtained with freshly grown cells. The procedure was optimized for B. thuringiensis subsp. gelechiae, but B. thuringiensis subsp. kurstaki, B. thuringiensis subsp. galleriae, B. thuringiensis subsp. thuringiensis, and B. thuringiensis subsp. israelensis were also transformed. Compared with protoplast transformation, our method is much faster and 3 orders of magnitude more efficient per microgram of added DNA.

Bacillus thuringiensis↗

PubMed2003

Choroidal vascular remodelling in central serous chorioretinopathy after indocyanine green guided photodynamic therapy with verteporfin: a novel treatment at the primary disease level.

AIMS: To evaluate the changes in the choroidal vasculature in central serous chorioretinopathy (CSC) after photodynamic therapy (PDT) with verteporfin and to assess its potential role as a treatment option. METHODS: A prospective, non-comparative, interventional study was performed in eyes with persistent CSC or chronic CSC that had fluorescein leakage at the fovea. All eyes received one single session of PDT with verteporfin (6 mg/m2 body surface area) followed by application of 50 J/cm2 laser at 689 nm. The laser spot size was guided by findings in ICG-A. RESULTS: Six eyes from six patients with a mean follow up of 12.7 months were analysed. Narrowing of the original dilated choroidal vessels and decrease in extravascular leakage could be demonstrated in all (100%) PDT treated eyes. 3 months after PDT, the mean diameter of the dilated choroidal vessel reduced from 546 microm to 371 microm (p=0.028). Five (83%) patients had improvement in visual symptoms and best corrected visual acuity. Fluorescence leakage stopped at the 1 month follow up in five eyes (83%) and at 3 months in all six eyes (100%). One eye developed choroidal neovascularisation at 3 month follow up. There was no other serious ocular or systemic complication. CONCLUSIONS: PDT is successful in stopping the fluorescein leakage in all six patients without recurrence of CSC. The ICG-A findings of choroidal vascular remodelling and decreased choroidal permeability after PDT are encouraging. As the sample size is small and the mean follow up period is short, further trials of PDT with verteporfin for CSC are required to address the optimal parameters in ensuring longer term safety and efficacy outcome.

Adult↗

PubMed2006

Microwave ablation: results with a 2.45-GHz applicator in ex vivo bovine and in vivo porcine liver.

PURPOSE: To characterize the relationship between applied power and treatment duration in their effect on extent of coagulation produced with a 2.45-GHz microwave applicator in both an ex vivo and a perfused in vivo liver model. MATERIALS AND METHODS: All experimentation was approved by the Institute of Animal Care and Use Committee. Multiple tissue ablations were performed in ex vivo bovine liver (120 ablations) and in vivo porcine liver (45 ablations) with a 5.7-mm-diameter 2.45-GHz microwave applicator. The applied power was varied from 50 to 150 W (in 25-W increments ex vivo and 50-W increments in vivo), while treatment duration varied from 2 to 20 minutes (in eight time increments for ex vivo and five for in vivo liver). Three-dimensional contour maps of the resultant short- and long-axis coagulation diameters were constructed to identify the optimal parameters to achieve maximum coagulation in both ex vivo and in vivo models. Multivariate analysis was performed to characterize the relationship between applied power and treatment duration. RESULTS: Power and treatment duration were both associated with coagulation diameter in a sigmoidal fashion (ex vivo, R(2) = 0.78; in vivo, R(2) = 0.74). For ex vivo liver, the maximum short-axis coagulation diameter (7.6 cm +/- 0.2 [standard deviation] by 12.3 cm +/- 0.8) was achieved at greatest power (150 W) and duration (20 minutes). In vivo studies revealed a sigmoidal relationship between duration and coagulation size, with a relative plateau in coagulation size achieved within 8 minutes duration at all power levels. After 8 minutes of treatment at 150 W, the mean short-axis coagulation diameter for in vivo liver was 5.7 cm +/- 0.2 by 6.5 cm +/- 1.7, which was significantly larger than the corresponding result for ex vivo liver (P < .05). CONCLUSION: Large zones of ablation can be achieved with the 2.45-GHz microwave applicator used by the authors. For higher-power ablations, larger zones of coagulation were achieved for in vivo liver than for ex vivo liver with short energy applications, a finding previously not seen with other ablation devices, to the authors' knowledge.

Animals↗