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PubMed2002

Clinical evaluation of the dynamic observing tonometer.

PURPOSE: The Dynamic Observing Tonometer (SmartLens, Ophthalmic Development Company AG, Zürich, Switzerland) is a diagnostic contact lens that allows continuous measurement of intraocular pressure, in addition to providing the investigator with a view of the posterior pole and anterior chamber angle. The purpose of this study was to determine the accuracy of this tonometer and the repeatability of the intraocular pressure measurements. PATIENTS AND METHODS: The intraocular pressure was measured by Goldmann applanation tonometry in one randomly chosen eye of 40 subjects (median age 66 years, range 21-77 years). The intraocular pressure, pulse amplitude and 10-second continuous tonometric recordings were then taken using the Dynamic Observing Tonometer and a pneumatonometer. Accuracy was determined by calculating the mean bias and 95% limits of agreement of measurements made with the Dynamic Observing Tonometer against measurements made with the Goldmann and pneumatonometer. Repeatability was evaluated by calculating the differences between pairs of repeated measurements against the mean value and by calculating reliability coefficients. RESULTS: Intraocular pressure measurements made with the Dynamic Observing Tonometer had a mean bias of +2.1 mm Hg (95% limits of agreement: -4.0 to +8.2 mm Hg) compared with Goldmann tonometry. There was a reasonable correlation between Goldmann and Dynamic Observing Tonometer intraocular pressure readings (r = 0.78, P < 0.01). In measuring pulse amplitude, the Dynamic Observing Tonometer was found to have a mean bias of +0.4 mm Hg (95% limits of agreement: -1.6 to +2.3 mm Hg) compared with the pneumatonometer (r = 0.78, P < 0.01). In assessing the repeatability of intraocular pressure measurements, the first Dynamic Observing Tonometer reading was on average 0.4 mm Hg higher than the second (95% limits of agreement: -3.8 to +4.6 mm Hg) with a coefficient of reliability of 0.91. For pulse amplitude readings, the first reading was on average 0.1 mm Hg lower than the second (95% limits of agreement: -1.4 to +1.2 mm Hg) with a coefficient of reliability of 0.90. CONCLUSION: Intraocular pressure measurements taken with the Dynamic Observing Tonometer had a small positive bias compared with Goldmann tonometry. The pulse amplitude values correlated well with those obtained with a pneumatonometer and the repeatability of intraocular pressure measurements was similar to that found in other commercially available tonometers.

Adult↗
PubMed2006

Static versus dynamic loading in the mechanical modulation of vertebral growth.

STUDY DESIGN: Measures of absolute and relative growth modulation were used to determine the effects of static and dynamic asymmetric loading of vertebrae in the rat tail. OBJECTIVES: To quantify the differences between static and dynamic asymmetric loading in vertebral bone growth modulation. SUMMARY OF BACKGROUND DATA: The creation and correction of vertebral wedge deformities have been previously described in a rat-tail model using static loading. The effects of dynamic loading on growth modulation in the spine have not been characterized. METHODS: A total of 36 immature Sprague-Dawley rats were divided among four different groups: static loading (n = 12, 0.0 Hz), dynamic loading (n = 12, 1.0 Hz), sham operated (n = 6), and growth controls (n = 6). An external fixator was placed across the sixth and eighth caudal vertebrae as the unviolated seventh caudal vertebra was evaluated for growth modulation. Static or dynamic asymmetric loads were applied at a loading magnitude of 55% body weight. After 3 weeks of loading, growth modulation was assessed using radiographic measurements of vertebral wedge angles and vertebral body heights. RESULTS: The dynamically loaded rats had a final average wedge deformity of 15.2+/- 6.4 degrees, which was significantly greater than the statically loaded rats whose final deformity averaged 10.3 degrees +/- 3.7 degrees (P < 0.03). The deformity in both groups was statistically greater than the sham-operated (1.1+/- 2.0 degrees) and growth control rats (0.0+/- 1.0 degrees) (P < 0.001). The longitudinal growth was significantly lower on the concavity compared with the convexity in both the dynamically (0.34 +/- 0.23 mm vs. 0.86 +/- 0.23 mm) and statically (0.46 +/- 0.19 mm vs. 0.83 +/- 0.32 mm) loaded rats (P < 0.001). These growth rates were significantly less than the sham operated and growth control rats (P < 0.001). CONCLUSIONS: A variety of fusionless scoliosis implant strategies have been proposed that use both rigid and flexible implants to modulate vertebral bone growth. The results from this study demonstrate that dynamic loading of the vertebrae provides the greatest growth modulation potential.

Animals↗
PubMedDate not supplied

Localization of prostate cancer using 3T MRI: comparison of T2-weighted and dynamic contrast-enhanced imaging.

OBJECTIVE: To compare dynamic contrast-enhanced imaging and T2-weighted imaging using a 3T MR unit for the localization of prostate cancer. METHODS: Twenty consecutive patients with biopsy-proven prostate cancer underwent both T2-weighted imaging and dynamic contrast-enhanced imaging. At T2-weighted imaging and dynamic contrast-enhanced imaging, the presence or absence of prostate cancer confined within the prostate without extracapsular or adjacent organ invasion was evaluated in the peripheral zones of base, mid-gland, and apex on each side. Final decisions on prostate cancer localization were made by consensus between two radiologists. Degrees of depiction of tumor borders were graded as poor, fair, or excellent. RESULTS: Prostate cancer was pathologically detected in 64 (53%) of 120 peripheral zone areas. The sensitivity, specificity, and accuracy for prostate cancer detection were 55%, 88% and 70% for T2-weighted imaging and 73%, 77%, and 75% for dynamic contrast-enhanced imaging, respectively. Three cancer areas were detected only by T2-weighted imaging, 15 only by dynamic contrast-enhanced imaging, and 34 by both T2-weighted imaging and dynamic contrast-enhanced imaging. A fair or excellent degree at depicting tumor border was achieved in 67% by T2-weighted imaging and in 90% by dynamic contrast-enhanced imaging (P<0.05). CONCLUSIONS: Dynamic contrast-enhanced imaging at 3T MRI is superior to T2-weighted imaging for the detection and depiction of prostate cancer and thus is likely to be more useful for preoperative staging.

Aged↗
PubMed2003

Critical dynamics of the Gaussian model with multispin transitions.

In this paper, we present a multispin transition mechanism, which is an extension of the Glauber one, to investigate critical dynamics. By exactly solving the master equation, the influence of the multispin transition mechanism on the dynamic critical behavior is studied for the Gaussian model with nearest-neighbor interactions on d-dimensional lattices (d=1, 2, and 3). The time evolution of magnetization is exactly calculated, and the exact results of relaxation time and dynamic critical exponent are obtained. Our models are divided into two kinds: one is the spin-cluster transition and the other is the arbitrary multispin transition. It is found that there are different relaxation times, but the same dynamical critical exponent for different kinds of multispin transitions. The results show that the dynamical critical exponents are independent of spatial dimensions and configurations of transitional spins, and that the dynamical critical exponent is the same as that of the Glauber dynamics, and thus give a strong support to the simple single-spin-transition dynamics. Finally, we give a brief discussion on the results.

Journal Article↗
PubMed2003

Dynamic equivalence between soft- and hard-core Brownian fluids.

In this work, we demonstrate the dynamic equivalence between the members of the family of Brownian fluids whose particles interact through strongly repulsive radially symmetric soft-core potentials. We specifically consider pair potentials proportional to inverse powers of (r/sigma). This equivalence is the dynamic extension of the static equivalence between all these pair potentials and the hard-sphere fluid, assumed in the treatment of soft-core reference potentials in the classical (Weeks-Chandler-Andersen or Barker-Henderson) perturbation theories of simple liquids. In contrast with the strict hard-sphere Brownian system, in the case of soft-sphere potentials the conventional Brownian dynamics algorithm is indeed well defined. We find that, except for small values of nu, and/or very short times, the dynamic properties of all these systems collapse into a single universal curve, upon a well-defined rescaling of the time and distance variables. This family of systems includes the hard-sphere limit. This observation permits a conceptually simple, new, and accurate Brownian dynamics algorithm to simulate the dynamic properties of the hard-sphere model dispersion without hydrodynamic interactions. Such an algorithm consists of the straightforward rescaling of the Brownian-dynamics simulated properties of any of the dynamically equivalent soft-sphere systems.

Journal Article↗
PubMed2004

Nonequilibrium critical dynamics of the relaxational models C and D.

We investigate the critical dynamics of the n-component relaxational models C and D, which incorporate the coupling of a nonconserved and conserved order parameter S, respectively, to the conserved energy density rho, under nonequilibrium conditions by means of the dynamical renormalization group. Detailed balance violations can be implemented isotropically by allowing for different effective temperatures for the heat baths coupling to the slow modes. In the case of model D with conserved order parameter, the energy density fluctuations can be integrated out, leaving no trace of the nonequilibrium perturbations in the asymptotic regime. For model C with scalar order parameter, in equilibrium governed by strong dynamic scaling (z(S)=z(rho)), we find no genuine nonequilibrium fixed point either. The nonequilibrium critical dynamics of model C with n=1 thus follows the behavior of other systems with nonconserved order parameter wherein detailed balance becomes effectively restored at the phase transition. For n> or =4, the energy density generally decouples from the order parameter. However, for n=2 and n=3, in the weak dynamic scaling regime (z(S)< or =z(rho)) entire lines of genuine nonequilibrium model C fixed points emerge to one-loop order, which are characterized by continuously varying static and dynamic critical exponents. Similarly, the nonequilibrium model C with spatially anisotropic noise and n<4 allows for continuously varying exponents, yet with strong dynamic scaling. Subjecting model D to anisotropic nonequilibrium perturbations leads to genuinely different critical behavior with softening only in subsectors of momentum space and correspondingly anisotropic scaling exponents. Similar to the two-temperature model B (randomly driven diffusive systems) the effective theory at criticality can be cast into an equilibrium model D dynamics, albeit incorporating long-range interactions of the uniaxial dipolar or ferroelastic type.

Journal Article↗
PubMed2004

Broad relaxation spectrum and the field theory of glassy dynamics for pinned elastic systems.

We study thermally activated, low-temperature equilibrium dynamics of elastic systems pinned by disorder using one loop functional renormalization group (FRG). Through a series of increasingly complete approximations, we investigate how the field theory reveals the glassy nature of the dynamics, in particular divergent barriers and barrier distributions controling the spectrum of relaxation times. First, we naively assume a single relaxation time tau(k) for each wave vector k, leading to analytical expressions for equilibrium dynamical response and correlations. These exhibit two distinct scaling regimes (scaling variables T k(theta) ln t and t/ tau(k), respectively, with T the temperature, theta the energy fluctuation exponent, and tau(k) approximately e(c k(-theta) /T) ) and are easily extended to quasiequilibrium and aging regimes. A careful study of the dynamical operators encoding for fluctuations of the relaxation times shows that this first approach is unsatisfactory. A second stage of approximation including these fluctuations, based on a truncation of the dynamical effective action to a random friction model, yields a size (L) dependent log-normal distribution of relaxation times (effective barriers centered around Ltheta and of fluctuations approximately L(theta/2) ) and some procedure to estimate dynamical scaling functions. Finally, we study the full structure of the running dynamical effective action within the field theory. We find that relaxation time distributions are nontrivial (broad but not log normal) and encoded in a closed hierarchy of FRG equations divided into levels p=0,1, em leader, corresponding to vertices proportional to the pth power of frequency omega(p). We show how each level p can be solved independently of higher ones, the lowest one (p=0) comprising the statics. A thermal boundary layer ansatz (TBLA) appears as a consistent solution. It extends the one discovered in the statics which was shown to embody droplet thermal fluctuations. Although perturbative control remains a challenge, the structure of the dynamical TBLA which encodes barrier distributions opens the way for deeper understanding of the field theory approach to glasses.

Journal Article↗
PubMedDate not supplied

Dynamic map labeling.

We address the problem of filtering, selecting and placing labels on a dynamic map, which is characterized by continuous zooming and panning capabilities. This consists of two interrelated issues. The first is to avoid label popping and other artifacts that cause confusion and interrupt navigation, and the second is to label at interactive speed. In most formulations the static map labeling problem is NP-hard, and a fast approximation might have O(nlogn) complexity. Even this is too slow during interaction, when the number of labels shown can be several orders of magnitude less than the number in the map. In this paper we introduce a set of desiderata for "consistent" dynamic map labeling, which has qualities desirable for navigation. We develop a new framework for dynamic labeling that achieves the desiderata and allows for fast interactive display by moving all of the selection and placement decisions into the preprocessing phase. This framework is general enough to accommodate a variety of selection and placement algorithms. It does not appear possible to achieve our desiderata using previous frameworks. Prior to this paper, there were no formal models of dynamic maps or of dynamic labels; our paper introduces both. We formulate a general optimization problem for dynamic map labeling and give a solution to a simple version of the problem. The simple version is based on label priorities and a versatile and intuitive class of dynamic label placements we call "invariant point placements". Despite these restrictions, our approach gives a useful and practical solution. Our implementation is incorporated into the G-Vis system which is a full-detail dynamic map of the continental USA. This demo is available through any browser.

Journal Article↗
PubMed2005

Dynamics of hope in adults living with HIV/AIDS: a substantive theory.

AIM: The aim of this paper was to discuss a substantive theory on the dynamics of hope in adults living with the human immunodeficiency virus/acquired immune deficiency syndrome. BACKGROUND: Existing literature describes how the dynamics of hope have profound connections with human essence and life, wellbeing and health, suffering and illness, and nursing care, in general as well as in the context of human immunodeficiency virus/acquired immune deficiency syndrome. METHOD: The data consisted of five original articles describing the dynamics of hope in people living with human immunodeficiency virus/acquired immune deficiency syndrome and their significant others in Finland. Meta-synthesis was adapted to synthesize the original studies into a substantive theory. FINDINGS: The core category that emerged was the dynamically alternating balance between interconnected hope, despair and hopelessness based on folding (closing down of) and unfolding (opening up of) possibilities with regard to the dynamics of hope in dealing with the changing self and life with human immunodeficiency virus/acquired immune deficiency syndrome. The dynamics of hope include the subprocesses of hope, despair and hopelessness. CONCLUSION: The dynamics of hope in adults living with human immunodeficiency virus/acquired immune deficiency syndrome is an important phenomenon in nursing practice and nursing science. In taking care of people living with acquired immune deficiency syndrome, factors contributing to folding possibilities should be minimized, if possible, and factors contributing to unfolding possibilities should be nourished. The present study adds to the knowledge-base of nursing science about the dynamics of hope in people living with acquired immune deficiency syndrome. Future research should focus on testing this theory in people living with acquired immune deficiency syndrome.

Adaptation, Psychological↗
PubMed2007

Effects of bronchodilators on dynamic hyperinflation following hyperventilation in patients with COPD.

BACKGROUND AND OBJECTIVE: The present study was performed to examine the occurrence of dynamic hyperinflation following hyperventilation in COPD patients and former smokers without COPD, and the efficacy of short-acting anticholinergic agents (SAAC) and beta2-agonists (SABA) for lung hyperinflation following metronome-paced hyperventilation in COPD. METHODS: Fifty-nine patients with COPD, 20 ex-smokers without COPD and 20 healthy subjects who had never smoked were examined for dynamic hyperinflation by metronome-paced hyperventilation with respiratory rate increasing from 20 to 30 and 40 tidal breaths/min. Dynamic hyperinflation was evaluated as the decrease in inspiratory capacity (IC) following hyperventilation, and the effects of SAAC and SABA on dynamic hyperinflation were assessed. RESULTS: COPD patients showed a significant increase in end-expiratory lung volume and a decrease in IC following hyperventilation, and ex-smokers without COPD also showed mild but significant dynamic hyperinflation. Multiple stepwise linear regression analysis revealed that the carbon monoxide transfer coefficient (DLco/VA) and RV/TLC were significant and independent determinants of dynamic hyperinflation in COPD. Treatment with SAAC and SABA significantly increased IC at each respiratory rate, independently of the increases in FEV1. Furthermore, SABA significantly inhibited the decrease in IC due to hyperventilation. CONCLUSIONS: These findings suggest that lung hyperinflation following hyperventilation may be a useful method for detecting dynamic hyperinflation observed not only in patients with COPD but also in ex-smokers without COPD, and both SAAC and SABA are effective in reducing dynamic hyperinflation in COPD.

Administration, Inhalation↗
PubMed2006

Hierarchical Bayesian methods for estimation of parameters in a longitudinal HIV dynamic system.

HIV dynamics studies have significantly contributed to the understanding of HIV infection and antiviral treatment strategies. But most studies are limited to short-term viral dynamics due to the difficulty of establishing a relationship of antiviral response with multiple treatment factors such as drug exposure and drug susceptibility during long-term treatment. In this article, a mechanism-based dynamic model is proposed for characterizing long-term viral dynamics with antiretroviral therapy, described by a set of nonlinear differential equations without closed-form solutions. In this model we directly incorporate drug concentration, adherence, and drug susceptibility into a function of treatment efficacy, defined as an inhibition rate of virus replication. We investigate a Bayesian approach under the framework of hierarchical Bayesian (mixed-effects) models for estimating unknown dynamic parameters. In particular, interest focuses on estimating individual dynamic parameters. The proposed methods not only help to alleviate the difficulty in parameter identifiability, but also flexibly deal with sparse and unbalanced longitudinal data from individual subjects. For illustration purposes, we present one simulation example to implement the proposed approach and apply the methodology to a data set from an AIDS clinical trial. The basic concept of the longitudinal HIV dynamic systems and the proposed methodologies are generally applicable to any other biomedical dynamic systems.

Antiretroviral Therapy, Highly Active↗
PubMed1977

Static and dynamic fusimotor action on the response of Ia fibres to low frequency sinusoidal stretching of widely ranging amplitude.

1. Single fusimotor fibres were stimulated repetitively to test their action on the responsiveness of muscle spindle primary endings in the cat soleus to sinusoidal stretching of both large and small amplitude. Frequencies of 0.06-4 Hz were used at amplitudes from 10 mum to 3 mm.2. The response was assessed by fitting a sinusoid to the cycle histogram of the afferent firing throughout the course of the cycle; this linear approximation measures the fundamental of the response and ignores any harmonics. The sine was allowed to project to negative values and any empty bins in the histogram were ignored when fitting.3. With small amplitudes of stretching the histograms were reasonably sinusoidal, but with large amplitudes they showed appreciable distortion of the wave form for the passive ending and during dynamic fusimotor stimulation. Non-linearity of response manifested itself also, with increasing amplitude of stretching, by an increase in the phase advance of the response, by increasing r.m.s. deviation of the histogram points from the fitted sine and (for dynamic stimulation) by an increase in the mean value of the fitted sine.4. With increasing amplitude the response modulation ceased to increase proportionately with the stimulus, so that the sensitivity of the ending to a large stretch (defined as afferent modulation/stretch amplitude) was appreciably less than for a small stretch. This effect was most pronounced for the passive ending.5. Whatever the amplitude of movement the modulation during static stimulation was less than that for the passive or during dynamic stimulation. For small amplitudes the response during dynamic stimulation was less than that of the passive, but for large amplitudes the response during dynamic stimulation was always the greater. At some intermediate cross-over amplitude the two responses were the same size, though still differing slightly in other respects. The value of the cross-over amplitude was usually about 200 mum at 1 Hz, and increased on lowering the frequency. Thus dynamic fusimotor action does not uniformly produce either an increase or a decrease in the sensitivity of the ending in relation to the passive.6. Bode plots, for each amplitude, of sensitivity and phase against frequency suggested that(a) under all conditions the ending is relatively insensitive to frequency in the range studied, for the slope of the log-log sensitivity lines was only 0.15-0.2 (3.5-6 db/decade);(b) the mechanism which makes for non-linearity is not particularly frequency sensitive;(c) static fusimotor stimulation does not change the frequency sensitivity of the ending;(d) dynamic fusimotor stimulation very slightly increases the frequency sensitivity of the ending for large amplitudes.In reaching these conclusions more attention was paid to the slope of the sensitivity lines than to the values of phase.7. It appears that the major effect of fusimotor action, whether static or dynamic, is to regulate the sensitivity of the primary ending to stretching for all amplitudes of movement (i.e. gain) rather than to control the relative values of its sensitivity to length and to velocity (i.e. crudely, the damping in a feed-back loop).

Action Potentials↗
PubMed1977

Effects of combining static and dynamic fusimotor stimulation on the response of the muscle spindle primary ending to sinusoidal stretching.

1. A pair of fusimotor fibres, one static and the other dynamic, were stimulated simultaneously to test their combined action on the response of muscle spindle primary endings in the cat soleus to sinusoidal stretching. A frequency of 1 Hz was chiefly used, with a wide range of amplitudes (10 micronm-2 mm). The response of the ending was assessed from the parameters of the sine fitted to its firing averaged throughout the course of the cycle; this was felt useful even though the responses to the larger stretches showed certain non-linear features. 2. With small stretches (up to about 50 micronm amplitude) static action dominated, and the modulation of firing during conbined stimulation was little or no larger than that found during the static stimulation on its own, and much smaller than that found during the static stimulation on its own, and much smaller than that found during the dynamic stimulation. The phase of the response was, however, much the same for all three conditions. 3. With larger stretches the modulation with combined stimulation was intermediate between the values found on stimulating either fusimotor fibre on its own; the dynamic contribution increased progressively with the amplitude of stretching. 4. With larger stretches the phase of the response during combined stimulation was appreciably closer to that for static action than to that for dynamic action. But the differences between the various conditions were small (below 20 degrees) and seem attributable to various distortions of the response wave from away from a true sinusoid, rather than betokening a difference in the ratio of velocity to length sensitivity under the various conditions. This view was supported by the effects on phase of grading the rate of stimulation of one fusimotor fibre while holding that of the other constant. 5. Detailed comparison of the cycle histograms obtained under different conditions showed an interestingly asymmetrical pattern of summation and occlusion of the effects of the two kinds of fusimotor fibre. At the peak of the response to a large stretch static action summed with dynamic action, which was here the stronger, so that at this phase of the cycle the firing was greater with the combined stimulation than with either fibre on its own. But, in the trough of the response to the same stretch static action occluded any dynamic action, which was now the weaker, so that at this phase of the cycle the firing with combined stimulation was virtually the same as that with static stimulation on its own. With a small stretch, static action normally occluded dynamic action throughout the cycle; this is in line with the firing during static action now usually being greater than that during dynamic action for all phases of the cycle.

Action Potentials↗
PubMed1991

Responses of cat muscle spindles which lack a dynamic fusimotor supply.

1. The experiments reported here support the view that some spindles in the peroneus tertius muscle of the anaesthetized cat lack a nuclear bag1 intrafusal fibre. 2. The bag1 fibre is characterized by the fact that it is innervated exclusively by dynamic fusimotor axons. A method was devised to test each spindle in peroneus tertius for a dynamic fusimotor innervation. The ventral roots containing the muscle's motor supply were subdivided into five portions, approximately equal in terms of the tension they generated, and each piece was stimulated in turn, repetitively, at fusimotor strength, during ramp stretch of the muscle, to look for a large increase in dynamic response. 3. The method allowed confirmation that the majority of spindles in peroneus tertius had a dynamic fusimotor innervation. However, where the dynamic effect was weak and accompanied by a strong static fusimotor action and extrafusal unloading, it risked being overlooked. 4. The confirmatory test for the presence of a bag1 fibre was whether or not the spindle showed a large increase in dynamic response in the presence of the drug succinyl choline (SCh) injected arterially close to the muscle in which the spindle is located. SCh is known to induce a contracture in the bag1 fibre and therefore mimics tonic dynamic fusimotor stimulation. 5. In five experiments, of a total of forty-two spindles with afferents conducting within the group I range, five examples were encountered where there was no increase in dynamic response, either with ventral root stimulation or perfusion with SCh. It was concluded that these were spindles which lacked a bag1 fibre. 6. Passive stretch of such spindles revealed no feature in the response which allowed them to be distinguished from spindles in which the bag1 fibre was present. This conclusion posed the question, what contribution, if any, does the bag1 fibre make to the stretch response? 7. It was possible to show that under some conditions the bag1 fibre did contribute to the response to stretch. If the spindle was conditioned by repetitive nerve stimulation, at fusimotor strength, at a length longer than that at which a test stretch was applied, the response to the stretch was delayed, and it began at a lower rate than after conditioning stimulation at the test length. The delayed response was attributed to the presence of slack in intrafusal fibres.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2005

Dynamic bilateral contrast-enhanced MR imaging of the breast: trade-off between spatial and temporal resolution.

PURPOSE: To investigate prospectively the trade-off between temporal and spatial resolution in dynamic contrast material-enhanced bilateral magnetic resonance (MR) imaging of the breast. MATERIALS AND METHODS: Informed consent and institutional review board approval were obtained. An intraindividual comparative study was performed in 30 patients (mean age, 53 years; age range, 27-70 years) with a total of 54 enhancing lesions (28 benign and 26 malignant) who underwent dynamic MR imaging of the breast twice, once with a standard dynamic protocol (256 x 256 matrix, 69 seconds per acquisition) and once on a separate day with a modified dynamic protocol (400 x 512 matrix, 116 seconds per acquisition). Systematic qualitative analysis of morphologic features and region-of-interest-based analysis of enhancement kinetics were performed. RESULTS: A statistically significant difference (generalized linear modeling) in enhancement rates of benign versus malignant lesions was lost when moving from the standard to the modified dynamic protocol. Kinetic information on signal intensity time course patterns was preserved. Delineation of lesion margins and internal architecture was clearly superior with the modified dynamic protocol, which allowed identification of lesion features associated with high positive predictive value or high negative predictive value for breast cancer. Ten benign lesions classified as Breast Imaging Reporting and Data System (BI-RADS) category 3 with the standard protocol were correctly downgraded to BI-RADS category 2 with the modified protocol owing to visualization of internal septations. Thirteen malignant lesions categorized as BI-RADS category 3 or 4 with the standard protocol were correctly upgraded to BI-RADS category 4 or 5 with the modified protocol owing to visualization of spicules or rim enhancement. Receiver operating characteristic analysis revealed a significantly larger area under the curve for results obtained with the modified dynamic protocol. CONCLUSION: Increased spatial resolution significantly improves diagnostic confidence and accuracy at dynamic MR imaging, even if this improvement occurs at the expense of temporal resolution. Loss of kinetic information regarding enhancement rates proved to be not diagnostically relevant because enhancement rates showed broad overlap between benign and malignant lesions and were therefore of only limited diagnostic use in the individual patient. Kinetic information regarding time course pattern was preserved and confirmed as having high specificity and high positive predictive value.

Adult↗
PubMed1998

Evaluation of chemotherapy in advanced urinary bladder cancer with fast dynamic contrast-enhanced MR imaging.

PURPOSE: To evaluate if the failure of chemotherapy in patients with advanced urinary bladder cancer can be predicted early in the course of chemotherapy with fast dynamic contrast material-enhanced magnetic resonance (MR) imaging. MATERIALS AND METHODS: In this prospective study, 22 consecutive patients with histologically proved advanced urinary bladder cancer underwent MR imaging before and after two, four, and six cycles of chemotherapy with methotrexate, vinblastine, adriamycin, and cisplatin (MVAC). The response after two chemotherapy cycles was evaluated by using conventional tumor size parameters at unenhanced MR imaging and with changes in the time to the start of tumor or lymph node enhancement at fast dynamic contrast-enhanced MR imaging. The results obtained with these techniques were compared with the findings at histopathology in cystectomy (n = 9) or multiple transurethral resection (n = 13) specimens obtained after completion of chemotherapy. RESULTS: After two MVAC cycles, the accuracy, sensitivity, and specificity in distinguishing responders from nonresponders with conventional MR imaging were 73%, 79%, and 63%, respectively. With the dynamic technique, these were 95%, 93%, and 100%, respectively. Although the differences between these values are not significant (P = .48 for sensitivity, .25 for specificity, and .07 for accuracy), the data indicate that dynamic enhanced MR imaging performed better than unenhanced MR imaging. Dynamic imaging yielded correct results after two MVAC cycles in 21 cases, and in all cases after four cycles. After four MVAC cycles, the accuracy of dynamic MR imaging was significantly better (P < .05). Persisting early enhancement after four MVAC cycles correctly corresponded with lack of response in all nine cases, and after two cycles in eight of these cases. The unenhanced MR technique showed initial tumor size reduction in three of these cases. CONCLUSION: Conventional and dynamic enhanced MR imaging were used to evaluate chemotherapy after two, four, and six cycles of MVAC in 22 patients with bladder cancer. After two cycles, dynamic MR imaging helped detect 13 of 14 responders and eight of eight nonresponders. It helped detect five of seven lymph node responders and two of two nonresponders. Thus, it may be possible to predict after two MVAC cycles whether a patient will respond to chemotherapy.

Adult↗
PubMed1998

Dynamic restitution of action potential duration during electrical alternans and ventricular fibrillation.

The restitution kinetics of action potential duration (APD) were investigated in paced canine Purkinje fibers (P; n = 9) and endocardial muscle (M; n = 9), in isolated, perfused canine left ventricles during ventricular fibrillation (VF; n = 4), and in endocardial muscle paced at VF cycle lengths (simulated VF; n = 4). Restitution was assessed with the use of two protocols: delivery of a single extrastimulus after a train of stimuli at cycle length = 300 ms (standard protocol), and fixed pacing at short cycle lengths (100-300 ms) that induced APD alternans (dynamic protocol). The dynamic protocol yielded a monotone increasing restitution function with a maximal slope of 1.13 +/- 0.13 in M and 1.14 +/- 0.17 in P. Iteration of this function reproduced the APD dynamics found experimentally, including persistent APD alternans. In contrast, the standard protocol yielded a restitution relation with a maximal slope of 0.57 +/- 0.18 in M and 0.84 +/- 0.20 in P, and iteration of this function did not reproduce the APD dynamics. During VF, the restitution kinetics at short diastolic interval were similar to those determined with the dynamic protocol (maximal slope: 1.72 +/- 0.47 in VF and 1.44 +/- 0.49 in simulated VF). Thus APD dynamics at short coupling intervals during fixed pacing and during VF were accounted for by the dynamic, but not the standard, restitution relation. These results provide further evidence for a strong relationship among the kinetics of electrical restitution, the occurrence of APD alternans, and complex APD dynamics during VF.

Action Potentials↗
PubMed2002

Cardiovascular responses to static and dynamic contraction during comparable workloads in humans.

Previous studies suggest that the blood pressure response to static contraction is greater than that caused by dynamic exercise. In anesthetized cats, however, pressor responses to electrically induced static and dynamic contraction of the same muscle group are similar during equivalent workloads and peak tension development [i.e., similar tension-time index (TTI)]. To determine if the same relationship exists in humans, where contraction is voluntary and central command is present, dynamic (180 s; 1/s) and static (90 s) contractions at 30% of maximal voluntary contraction (MVC) were performed. Dynamic contraction also was repeated at the same TTI for 90 s at 60% MVC. Mean arterial pressure (MAP), heart rate (HR), cardiac output (CO), MAP during postexercise arterial occlusion (an index of the metaboreceptor-induced activation of the exercise pressor reflex), and relative perceived exertion (RPE) (an index of central command) were assessed. No differences in these variables were found between static and dynamic contraction at a tension of 30% MVC. During dynamic contraction at 60% MVC, changes in MAP (16 +/- 3 vs. 19 +/- 4 mmHg) and absolute HR (92 +/- 6 vs. 69 +/- 5 beats/min), CO (7.9 +/- 0.4 vs. 6.3 +/- 0.3 l/min), RPE (16 +/- 1 vs. 13 +/- 1), and MAP during postexercise arterial occlusion (115 +/- 3 vs. 100 +/- 4 mmHg) were greater than during static contraction (P < 0.05). Thus increases in MAP and HR, activation of central command, and muscle metabolite-induced stimulation of the exercise pressor reflex during static and dynamic contraction in humans seem to be similar when peak tension and TTI are equal. Augmented responses to dynamic contraction at 60% MVC are likely related to greater activation of these two mechanisms.

Adult↗