Biomedical subjects

R E Kearney

Publications and source records attributed to R E Kearney.

At least 19 recordsLinked to original sources

PubMed2004

Experimental evaluation of computerised tomography point spread function variability within the field of view: parametric models.

The objective of the paper was to validate non-linear parametric models of computerised tomography point spread function (PSF), to investigate the role of model parameters and to verify the effect of different imaging conditions on estimated parameters. These models were then to be used experimentally to estimate the variation of PSF shape within the field of view of a scanner. Two parametric models of the PSF are presented. The Gaussian model is appropriate when PSF values are positive, and the damped cosine model can account for negative values. These models are non-linear and fully two-dimensional and do not assume radial symmetry. The models were fitted to images of a point source. The models accounted for over 99% of the variance in the PSF signal. Errors in modulation transfer function were limited to 5% when the appropriate model was selected. The difference in the blurring characteristics of three image reconstruction filters was well quantified by shape parameters, and position parameters located the PSF with subpixel accuracy. With a point source located 50mm directly above the centre of the field of view, the PSF was found to be anisotropic.

Humans↗

PubMed2002

Predicting surgical outcome in temporal lobe epilepsy patients using MRI and MRSI.

OBJECTIVE: To develop a classifier that uses MR data to predict surgical outcome in patients with temporal lobe epilepsy (TLE). METHODS: Eighty-one patients with medically refractory TLE who underwent surgical treatment were studied. Patients underwent comprehensive presurgical investigation, including ictal video-EEG recording, 1H MRS imaging, and volumetric MRI. Outcome was measured using Engel's classification system, condensed into two outcome groups. Two approaches were taken. First, outcome was defined as experiencing worthwhile improvement with >90% reduction of seizure frequency (Classes I, II, and III) or not (Class IV). A second approach was to define outcome as experiencing freedom from seizures following surgery (Class I) or not (Classes II, III, and IV). For each approach, a Bayesian classifier was constructed to predict outcome by calculating the probability of a patient's pattern of results from spectroscopic analysis of the temporal lobes and volumetric analysis of the amygdala and hippocampus being associated with the various outcome groups. RESULTS: The worthwhile improvement classifier correctly predicted the surgical outcomes of 60 of 65 (92%) of patients who experienced worthwhile improvement and 10 of 16 (63%) of patients who did not. The seizure-free classifier correctly predicted the surgical outcomes of 39 of 52 (75%) of patients who became seizure free and 21 of 29 (72%) of patients who did not. CONCLUSION: MR features are important markers of surgical outcome in patients with TLE and can provide assistance in identifying surgical candidates.

Adult↗

PubMed2001

Intrinsic and reflex stiffness in normal and spastic, spinal cord injured subjects.

Mechanical changes underlying spastic hypertonia were explored using a parallel cascade system identification technique to evaluate the relative contributions of intrinsic and reflex mechanisms to dynamic ankle stiffness in healthy subjects (controls) and spastic, spinal cord injured (SCI) patients. We examined the modulation of the gain and dynamics of these components with ankle angle for both passive and active conditions. Four main findings emerged. First, intrinsic and reflex stiffness dynamics were qualitatively similar in SCI patients and controls. Intrinsic stiffness dynamics were well modeled by a linear second-order model relating intrinsic torque to joint position, while reflex stiffness dynamics were accurately described by a linear, third-order system relating half-wave rectified velocity to reflex torque. Differences between the two groups were evident in the values of four parameters, the elastic and viscous parameters for intrinsic stiffness and the gain and first-order cut-off frequency for reflex stiffness. Second, reflex stiffness was substantially increased in SCI patients, where it generated as much as 40% of the total torque variance, compared with controls, where reflex contributions never exceeded 7%. Third, differences between SCI patients and controls depended strongly on joint position, becoming larger as the ankle was dorsiflexed. At full plantarflexion, there was no difference between SCI and control subjects; in the mid-range, reflex stiffness was abnormally high in SCI patients; at full dorsiflexion, both reflex and intrinsic stiffness were larger than normal. Fourth, differences between SCI and control subjects were smaller during the active than the passive condition, because intrinsic stiffness increased more in controls than SCI subjects; nevertheless, reflex gain remained abnormally high in SCI patients. These results elucidate the nature and origins of the mechanical abnormalities associated with hypertonia and provide a better understanding of its functional and clinical implications.

Adult↗

PubMed2001

Identification of physiological systems: estimation of linear time-varying dynamics with non-white inputs and noisy outputs.

A new technique to identify linear time-varying systems from ensembles of input-output realisations is presented. First, a correlation-based least-squares method is derived. This method consists of solving, for each sampling time, a matrix equation involving estimates of the input autocorrelation and input-output cross-correlation functions computed from data across the ensemble. Then, the matrix inverse needed to solve this matrix equation is replaced with a pseudo-inverse. The model is thus constrained to describe only those components of the dynamics that can be reliably identified. Ignoring 'unidentifiable' components has virtually no adverse effect on the predicted outputs. Simulation results demonstrate that the pseudoinverse technique yields more reliable estimates of the dynamics than a previously proposed least-squares technique when the inputs are coloured and the output signal-to-noise ratio (SNR) is low. With the input spectrum flat up to approximately 10% of the sampling rate and an output SNR of 5dB, the mean variance accounted for (VAF) between the true instantaneous impulse response functions (IRFs) and the instantaneous IRFs estimated with the least-squares technique was 0.2%. In contrast, the mean VAF between the true instantaneous IRFs and the instantaneous IRFs estimated with the pseudoinverse technique was 89.0%.

Ankle Joint↗

PubMed2001

Identification of time-varying Hammerstein systems from ensemble data.

In this paper, we describe a new technique to identify rapidly time-varying Hammerstein systems from ensembles of input-output realizations. The technique involves two steps. A correlation approach is first used to obtain initial estimates of the linear subsystem parameters for every sampling time. An iterative optimization algorithm is then employed to produce final estimates of the system parameters. The input does not need to be white. The technique was tested on simulated data and was found to produce excellent results under realistic conditions.

Algorithms↗

PubMed2001

Separable least squares identification of nonlinear Hammerstein models: application to stretch reflex dynamics.

The Hammerstein cascade, consisting of a zero-memory nonlinearity followed by a linear filter, is often used to model nonlinear biological systems. This structure can represent some high-order nonlinear systems accurately with relatively few parameters. However, it is not possible, in general, to estimate the parameters of a Hammerstein cascade in closed form. The most effective method available to date uses an iterative approach, which alternates between estimating the linear element from a crosscorrelation, and then fitting a polynomial to the nonlinearity via linear regression. This paper proposes the use of separable least squares optimization methods to estimate the linear and nonlinear elements simultaneously in a least squares framework. A separable least squares algorithm for the identification of Hammerstein cascades is developed and used to analyze stretch reflex electromyogram data from two experimental subjects. The results show that in each case the proposed algorithm produced a better model, in that it predicted the system's response to novel inputs more accurately, than did models estimated using the traditional iterative algorithm. Monte-Carlo simulations demonstrated that when the input is a non-Gaussian, nonwhite signal, as is often the case experimentally, the traditional iterative identification approach produces biased models, whereas the separable least squares approach proposed in this paper does not.

Algorithms↗

PubMed2000

Intrinsic and reflex contributions to human ankle stiffness: variation with activation level and position.

A parallel-cascade system identification method was used to identify intrinsic and reflex contributions to dynamic ankle stiffness over a wide range of tonic voluntary contraction levels and ankle positions in healthy human subjects. Intrinsic stiffness dynamics were described well by a linear pathway having elastic, viscous, and inertial properties. A velocity-sensitive pathway comprising a delay, a static non-linearity, resembling a half-wave rectifier, followed by a low-pass filter, described reflex stiffness dynamics. The absolute magnitude of intrinsic and reflex stiffness parameters varied from subject to subject but the relative changes with contraction level and position were consistent. Intrinsic stiffness increased monotonically with contraction level while reflex stiffness was maximal at low contraction levels and then decreased. Intrinsic and reflex stiffness both increased as the ankle was dorsiflexed. As a result, reflex mechanics made their largest relative contributions near the neutral position at low levels of activity. The size of the maximum reflex contribution varied widely among subjects, in some it was so small (ca 1%) that it would be unlikely to have any functional importance; however, in other subjects, reflex contributions were large enough (as high as 55% in one case) to play a significant role in the control of posture and movement. This variability may have arisen because stretch reflexes were not useful for the torque-matching task in these experiments. It will be of interest to examine other tasks where stretch reflexes would have a direct impact on performance.

Adult↗

PubMed1999

Fitting cytosolic ADP recovery after exercise with a step response function.

Phosphorus magnetic resonance (MR) spectroscopy was used to measure the recovery kinetics of calculated cytoplasmic metabolically active adenosine diphosphate (ADP) after exercise in normal subjects and patients with mitochondrial myopathies. These kinetics have previously been fitted with a single exponential function, despite a complex time-dependent undershoot in many subjects. By considering the transition from ischemic-exercise to perfused-recovery as a step function input, a second-order linear system was developed yielding a step response function to fit the ADP recovery. Using this method, an average improvement in fit of 23% resulted in a significant improvement in the characterization of ADP recovery for all normal subjects with substantial undershoot. The patient group had a comparable improvement in fit of 11%. Fitting the ADP recovery with a second-order step response function can provide significantly better characterization of muscle oxidative metabolism in vivo.

Adenosine Diphosphate↗

PubMed1999

Modulation of stretch reflexes during imposed walking movements of the human ankle.

Our overall objectives were to examine the role of peripheral afferents from the ankle in modulating stretch reflexes during imposed walking movements and to assess the mechanical consequences of this reflex activity. Specifically we sought to define the changes in the electromyographic (EMG) and mechanical responses to a stretch as a function of the phase of the step cycle. We recorded the ankle position of a normal subject walking on a treadmill at 3 km/h and used a hydraulic actuator to impose the same movements on supine subjects generating a constant level of ankle torque. Small pulse displacements, superimposed on the simulated walking movement, evoked stretch reflexes at different phases of the cycle. Three major findings resulted: 1) soleus reflex EMG responses were influenced strongly by imposed walking movements. The response amplitude was substantially smaller than that observed during steady-state conditions and was modulated throughout the step cycle. This modulation was qualitatively similar to that observed during active walking. Because central factors were held constant during the imposed walking experiments, we conclude that peripheral mechanisms were capable of both reducing the amplitude of the reflex EMG and producing its modulation throughout the movement. 2) Pulse disturbances applied from early to midstance of the imposed walking cycle generated large reflex torques, suggesting that the stretch reflex could help to resist unexpected perturbations during this phase of walking. In contrast, pulses applied during late stance and swing phase generated little reflex torque. 3) Reflex EMG and reflex torque were modulated differently throughout the imposed walking cycle. In fact, at the time when the reflex EMG response was largest, the corresponding reflex torque was negligible. Thus movement not only changes the reflex EMG but greatly modifies the mechanical output that results.

Adult↗

PubMed1998

Benign symmetric lipomatosis (Madelung's disease).

Benign symmetric lipomatosis, also known as Madelung's disease, is a rare condition characterized by massive fatty deposits arranged symmetrically around the neck, shoulders, and arms. These patients might present for liposuction and body contouring. Although infrequently encountered in the average plastic surgeon's practice, this condition should be considered when evaluating candidates for these procedures. The deformity is associated with chronic alcohol use and also with malignant tumors of the upper airways. The deformity is prone to recurrence and its surgical treatment often results in less than optimal aesthetic outcomes. Despite this fact, surgical removal via either direct excision or suction-assisted lipectomy provides the only real hope of palliation. This report describes a patient with this deformity and a review of the literature.

Female↗

PubMed1997

Experimental correlation-based identification of X-ray CT point spread function. Part 1: Method and experimental results.

Knowledge of the point spread function (PSF) of an imaging system is important when studying the characteristics of the blur present in the images. Published experimental PSF identification techniques adapt classical one-dimensional linear system identification strategies using impulse, step function or periodic input signals. This study proposes and successfully applies a correlation method based on the Wiener-Hopf equation to identify the PSF of a CT scanner. The input consists of a series of pseudorandomly located holes. Results are found to be statistically equivalent to those obtained with the impulse method at a 90% two-sided confidence interval. Like the impulse method, it readily yields two-dimensional estimate, but the larger input circumvents the major objection to the use of a wire input. Furthermore, it is relatively immune to output noise, offering an advantage over edge methods. This resistance to noise may prove helpful for nuclear medicine imaging techniques, for which the signal-to-noise ratio is much lower than that X-ray for CT.

Electricity↗

PubMed1997

Experimental correlation-based identification of X-ray CT point spread function. Part 2: Simulation and design of input signal.

The preferred signals for non-parametric correlation-based point spread function identification are white noise or pseudo-random binary sequences (PRBSs). Given the difficulty of building a phantom based on either of these signals, a new input is devised that corresponds to pseudo-randomly located holes. The positions of the holes correspond to zeros in a 2-D PRBS. To optimise the design of the phantom and to ensure proper imaging procedure, a number of simulations are conducted. The effects of the following parameters on identification quality are investigated: the size of the holes and their minimum separation, the period of the PRBS, input-output translational and rotational mis-registration, pixel size and the presence of cupping. The factors affecting identification quality the most are rotational alignment, hole size and separation, as well as sequence length. During simulations, a point spread function offering characteristics similar to the Philips Tomoscan CX is identified. Optimal results are obtained when the signal consists of 0.6 mm holes, separated by 0.9 mm, whose position is based on a 32 x 32 PRBS generated with a ten-stage shift-register. When adequate rotational alignment is provided, it is shown that the pseudo-randomly located holes signal is a good substitute for a purely white signal when identifying the PSF of a CT scanner.

Humans↗

PubMed1997

Identification of physiological systems: a robust method for non-parametric impulse response estimation.

The identification of non-parametric impulse response functions (IRFs) from noisy finite-length data records is analysed using the techniques of matrix perturbation theory. Based on these findings, a method for IRF estimation is developed that is more robust than existing techniques, particularly when the input is non-white. Furthermore, methods are developed for computing confidence bounds on the resulting IRF estimates. Monte Carlo simulations are used to assess the capabilities of this new method and to demonstrate its superiority over classical techniques. An application to the identification of dynamic ankle stiffness in humans is presented.

Ankle Joint↗

PubMed1997

Identification of time-varying stiffness dynamics of the human ankle joint during an imposed movement.

The time-varying stiffness dynamics of the human ankle joint were identified during a large stretch imposed upon the active triceps surae muscles. Small stochastic position perturbations were superimposed upon many repetitions of the larger movement and an ensemble time-varying identification technique was then used to characterize the relationship between the small perturbation and the torque it evoked at each sample in time throughout the movement. This technique was found to provide an excellent description of the ankle stiffness dynamics throughout this movement, with the identified stiffness impulse response functions accounting for more than 80% of the torque variance at all times. The average low-frequency stiffness values (K(low)) derived from the stiffness impulse responses at each sample in time are believed to reflect primarily the instantaneous elastic properties of active crossbridges. These properties, which reflect the contractile state of the muscles more directly than force or torque measurements, have not been obtained previously from an intact muscle-joint system. We found that stiffness actually increased during the later portion of the large imposed stretch, indicating the triceps surae muscles did not yield significantly, and that the post-stretch steady-state stiffness level was approximately 60% higher than prior to the stretch. Reflex activity evoked by the large stretch did not produce a detectable change in K(low), even though this activity did produce a clear twitch-like response in joint torque beginning approximately 60 ms following stretch onset. A second-order mechanical model was found to provide an adequate characterization of stiffness dynamics for steady-state periods before and well after the imposed movement, but it could not adequately describe the observed changes in stiffness dynamics during the movement itself. However, the variation of model parameters indicated that the torque evoked by the stochastic displacement was predominantly elastic in nature. The stiffness behavior during stretch observed here for the intact human ankle joint is largely consistent with previous studies performed in isolated muscle preparations.

Adult↗

PubMed1997

Identification of intrinsic and reflex contributions to human ankle stiffness dynamics.

We have examined dynamic stiffness at the human ankle using position perturbations which were designed to provide a wide-bandwidth input with low average velocity. A parallel-cascade, nonlinear system identification technique was used to separate overall stiffness into intrinsic and reflex components. Intrinsic stiffness was described by a linear, second-order system similar to that demonstrated previously. Reflex stiffness dynamics were more complex, comprising a delay, a unidirectional rate-sensitive element and then lowpass dynamics. Reflex mechanisms were found to be most important at frequencies of 5-10 Hz. The gain and dynamics of reflex stiffness varied strongly with the parameters of the perturbation, the gain decreasing as the mean velocity of the perturbation increased. Under some conditions, torques generated by reflex mechanisms were of the same magnitude as those from intrinsic mechanisms. It is concluded that reflex stiffness can be large enough to be important functionally, but that its effects will depend strongly upon the particular conditions.

Ankle Joint↗

PubMed1995

Effect of maintained stretch on the range of motion of the human ankle joint.

The effectiveness of maintained stretch in expanding the range of motion of the human ankle joint was assessed in a population of normal adults. Controlled movements were imposed upon the ankle, and triceps surae and tibialis anterior electromyograms were monitored to ensure that only passive joint properties generated ankle torque. We found that a majority of subjects (7 of 12) showed evidence of muscle activity sufficient to distort a subjective assessment of changes in range of motion. For the remaining five subjects, a 60-s maintained stretch produced a small decrease in the torque subsequently generated by an imposed dorsiflexing movement, but this effect was transient and largely disappeared following 300 s of rest at a neutral position. This short-term effect is consistent with the viscoelastic properties of collagenous material stretched during such treatment and is unlikely to lead to long-term increases in range of motion. RELEVANCE: The results of these experiments indicate that subjective assessments of changes in joint range of motion may be distorted by voluntary and reflexive muscle activation. Moreover the presumed increases in range of motion produced by maintained stretch in our normal subjects were small and transient. These results suggest that future assessments of the long-term efficacy of this treatment must monitor muscle activity and take into account known viscoelastic properties of collagenous materials.

Journal Article↗

PubMed1995

Psychological and rheumatic symptoms of women requesting silicone breast implant removal.

In this case series, we describe the psychological and rheumatic symptoms of 52 women requesting breast implant removal. Main outcome measures include psychological evaluation with the Brief Symptom Inventory and the Beck Depression Inventory. Clinical evaluation was performed using a rheumatic symptom survey. Results noted elevation of all Brief Symptom Inventory subscales (highest in Somatization and Obsessive-Compulsive), and the Beck Depression Inventory showed mild depression. Rheumatic survey noted a mean of 12 of 23 symptoms. In conclusion, psychological distress and mild depression are noted in this group, who also may experience rheumatic symptoms of connective tissue disease. There was a moderate positive correlation between rheumatic symptoms and psychological distress.

Breast Implants↗

PubMed1995

Bioavailability of fluconazole in surgical intensive care unit patients: a study comparing routes of administration.

OBJECTIVE: Anticandidal therapy is commonly used in the surgical intensive care unit (SICU). Unfortunately, it is expensive because it is generally given intravenously, as acute trauma and abdominal surgery are often accompanied by impaired gastrointestinal function. We compared the systemic availability of fluconazole given enterally or intravenously in trauma and surgery SICU patients to determine the reliability of enteral administration. METHODS: Nine adult trauma (Injury Severity Score (> or = 18) and nine adult abdominal surgery SICU patients were randomized to receive fluconazole 100 mg via the intravenous (IV) or enteral route. Patients with a bilirubin > 4.0 mg/dL or creatinine clearance < 60 mL/min were excluded. Enteral fluconazole was crushed, dissolved, and flushed through a nasogastric or feeding tube. Eleven serial blood samples were drawn over 72 hours. Area under the curve (AUC), elimination rate constant (Ke), and terminal half-life (T1/2) were determined and compared (t test). Relative bioavailability was estimated (AUC(enteral)/AUCIV). RESULTS: Peak concentrations occurred within 2 hours after enteral dosing and 15 minutes after IV dosing. The relative bioavailability was 77%. Weight, AUC, Ke, and T1/2 did not differ between enteral and IV dosing. CONCLUSIONS: Fluconazole is significantly absorbed when crushed, dissolved, and given via a nasogastric or feeding tube in SICU patients. Nonsignificant trends toward lower systemic availability with enteral administration can be overcome with slightly higher doses. Since enteral administration of fluconazole costs 10% of fluconazole given intravenously, more liberal use of enteral administration offers tremendous savings. Such savings moderate the cost concerns of antifungal therapy.

Abdomen↗