MRI-based parameter inference for cerebral perfusion modelling in health and ischaemic stroke

Cerebral perfusion modelling is a promising tool to predict the impact of acute ischaemic stroke treatments on the spatial distribution of cerebral blood flow (CBF) in the human brain. To estimate treatment efficacy based on CBF, perfusion simulations need to become suitable for group-level investigations and thus account for physiological variability between individuals. However, computational perfusion modelling to date has been restricted to a few patient-specific cases. This study set out to establish automated parameter inference for perfusion modelling based on neuroimaging data and thus enable CBF simulations of groups. Magnetic resonance imaging (MRI) data from 75 healthy senior adults were utilised. Brain geometries were computed from healthy reference subjects' T1-weighted MRI. Haemodynamic model parameters were determined from spatial CBF maps measured by arterial spin labelling (ASL) perfusion MRI. Thereafter, perfusion simulations were conducted in 75 healthy cases followed by 150 acute ischaemic stroke cases representing an occlusion and CBF cessation in the left and right middle cerebral arteries. The anatomical fitness of the brain geometries was evaluated by comparing the simulated grey (GM) and white matter (WM) volumes to measurements in healthy reference subjects. Strong positive correlations were found in both tissue types (GM: Pearson's r 0.74, P<0.001; WM: Pearson's r 0.84, P<0.001). Haemodynamic parameter tuning was verified by comparing the total volumetric blood flow rate to the brain in healthy reference subjects and simulations (Pearson's r 0.89, P<0.001). In acute ischaemic stroke cases, the simulated infarct volume using a perfusion-based estimate was 197±25 ml. Computational predictions were in agreement with anatomical and haemodynamic values from the literature concerning T1-weighted, T2-weighted, and phase-contrast MRI measurements in healthy scenarios and acute ischaemic stroke cases. The acute stroke simulations did not capture small infarcts (left tail of the distribution), which could be explained by neglected compensatory mechanisms, e.g. collaterals. The proposed parameter inference method provides a foundation for group-level CBF simulations and for in silico clinical stroke trials which could assist in medical device and drug development.

On the Sensitivity Analysis of Porous Finite Element Models for Cerebral Perfusion Estimation

Computational physiological models are promising tools to enhance the design of clinical trials and to assist in decision making. Organ-scale haemodynamic models are gaining popularity to evaluate perfusion in a virtual environment both in healthy and diseased patients. Recently, the principles of verification, validation, and uncertainty quantification of such physiological models have been laid down to ensure safe applications of engineering software in the medical device industry. The present study sets out to establish guidelines for the usage of a three-dimensional steady state porous cerebral perfusion model of the human brain following principles detailed in the verification and validation (V&V 40) standard of the American Society of Mechanical Engineers. The model relies on the finite element method and has been developed specifically to estimate how brain perfusion is altered in ischaemic stroke patients before, during, and after treatments. Simulations are compared with exact analytical solutions and a thorough sensitivity analysis is presented covering every numerical and physiological model parameter. The results suggest that such porous models can approximate blood pressure and perfusion distributions reliably even on a coarse grid with first order elements. On the other hand, higher order elements are essential to mitigate errors in volumetric blood flow rate estimation through cortical surface regions. Matching the volumetric flow rate corresponding to major cerebral arteries is identified as a validation milestone. It is found that inlet velocity boundary conditions are hard to obtain and that constant pressure inlet boundary conditions are feasible alternatives. A one-dimensional model is presented which can serve as a computationally inexpensive replacement of the three-dimensional brain model to ease parameter optimisation, sensitivity analyses and uncertainty quantification. The findings of the present study can be generalised to organ-scale porous perfusion models. The results increase the applicability of computational tools regarding treatment development for stroke and other cerebrovascular conditions.

Simulation-based optimisation to quantify heterogeneity of specific ventilation and perfusion in the lung by the Inspired Sinewave Test

The degree of specific ventilatory heterogeneity (spatial unevenness of ventilation) of the lung is a useful marker of early structural lung changes which has the potential to detect early-onset disease. The Inspired Sinewave Test (IST) is an established noninvasive 'gas-distribution' type of respiratory test capable of measuring the cardiopulmonary parameters. We developed a simulation-based optimisation for the IST, with a simulation of a realistic heterogeneous lung, namely a lognormal distribution of spatial ventilation and perfusion. We tested this method in datasets from 13 anaesthetised pigs (pre and post-lung injury) and 104 human subjects (32 healthy and 72 COPD subjects). The 72 COPD subjects were classified into four COPD phenotypes based on 'GOLD' classification. This method allowed IST to identify and quantify heterogeneity of both ventilation and perfusion, permitting diagnostic distinction between health and disease states. In healthy volunteers, we show a linear relationship between the ventilatory heterogeneity versus age ([Formula: see text]). In a mechanically ventilated pig, IST ventilatory heterogeneity in noninjured and injured lungs was significantly different (p < 0.0001). Additionally, measured indices could accurately identify patients with COPD (area under the receiver operating characteristic curve is 0.76, p < 0.0001). The IST also could distinguish different phenotypes of COPD with 73% agreement with spirometry.

A porous circulation model of the human brain for in silico clinical trials in ischaemic stroke

The advancement of ischaemic stroke treatment relies on resource-intensive experiments and clinical trials. In order to improve ischaemic stroke treatments, such as thrombolysis and thrombectomy, we target the development of computational tools for in silico trials which can partially replace these animal and human experiments with fast simulations. This study proposes a model that will serve as part of a predictive unit within an in silico clinical trial estimating patient outcome as a function of treatment. In particular, the present work aims at the development and evaluation of an organ-scale microcirculation model of the human brain for perfusion prediction. The model relies on a three-compartment porous continuum approach. Firstly, a fast and robust method is established to compute the anisotropic permeability tensors representing arterioles and venules. Secondly, vessel encoded arterial spin labelling magnetic resonance imaging and clustering are employed to create an anatomically accurate mapping between the microcirculation and large arteries by identifying superficial perfusion territories. Thirdly, the parameter space of the problem is reduced by analysing the governing equations and experimental data. Fourthly, a parameter optimization is conducted. Finally, simulations are performed with the tuned model to obtain perfusion maps corresponding to an open and an occluded (ischaemic stroke) scenario. The perfusion map in the occluded vessel scenario shows promising qualitative agreement with computed tomography images of a patient with ischaemic stroke caused by large vessel occlusion. The results highlight that in the case of vessel occlusion (i) identifying perfusion territories is essential to capture the location and extent of underperfused regions and (ii) anisotropic permeability tensors are required to give quantitatively realistic estimation of perfusion change. In the future, the model will be thoroughly validated against experiments.

Identifying the myogenic and metabolic components of cerebral autoregulation

Cerebral autoregulation is the term used to describe a number of mechanisms that act together to maintain a near constant cerebral blood flow in response to changes in arterial blood pressure. These mechanisms are complex and known to be affected in a range of cerebrovascular diseases. However, it can be difficult to assign an alteration in cerebral autoregulation to one of the underlying physiological mechanisms without the use of a complex mathematical model. In this paper, we thus set out a new approach that enables these mechanisms to be related to the autoregulation behaviour and hence inferred from experimental measurements. We show that the arteriolar response is a function of just three parameters, which we term the elastic, the myogenic and the metabolic sensitivity coefficients, and that the full vascular response is dependent upon only seven parameters. The ratio of the strengths of the myogenic and the metabolic responses is found to be in the range 2.5 to 5 over a wide range of pressure, indicating that the balance between the two appears to lie within this range. We validate the model with existing experimental data both at the level of an individual vessel and across the whole vasculature, and show that the results are consistent with findings from the literature. We then conduct a sensitivity analysis of the model to demonstrate which parameters are most important in determining the strength of static autoregulation, showing that autoregulation strength is predominantly set by the arteriolar sensitivity coefficients. This new approach could be used in future studies to help to interpret the components of the autoregulation response and how they are affected under different conditions, providing a greater insight into the fundamental processes that govern autoregulation.

A model for generating synthetic arterial blood pressure

A new model capable of simulating many important aspects of human arterial blood pressure (ABP) is proposed. Both data-driven approach and physiological principles have been applied to describe the time series of diastolic, systolic, dicrotic notch and dicrotic peak pressure points. Major static and dynamic features of the model can be prescribed by the user, including heart rate, mean systolic and diastolic pressure, and the corresponding physiological control quantities, such as baroreflex sensitivity coefficient and Windkessel time constant. A realistic ABP generator can be used to compile a virtual database of signals reflecting individuals with different clinical conditions and signals containing common artefacts. The ABP model permits to create a platform to assess a wide range of biomedical signal processing approaches and be used in conjunction with, e.g. Kalman filters to improve the quality of ABP signals.

Mathematical model of the effect of ischemia-reperfusion on brain capillary collapse and tissue swelling

Restoration of an adequate cerebral blood supply after an ischemic attack is a primary clinical goal. However, the blood-brain barrier may break down after a prolonged ischemia causing the fluid in the blood plasma to filtrate and accumulate into the cerebral tissue interstitial space. Accumulation of this filtration fluid causes the cerebral tissue to swell, a condition known as vasogenic oedema. Tissue swelling causes the cerebral microvessels to be compressed, which may further obstruct the blood flow into the tissue, thus leading to the no-reflow phenomenon or a secondary ischemic stroke. The actual mechanism of this however is still not fully understood. A new model is developed here to study the effect of reperfusion on the formation of vasogenic oedema and cerebral microvessel collapse. The formation of vasogenic oedema is modelled using the capillary filtration equation while vessel collapse is modelled using the tube law of microvessel. Tissue swelling is quantified in terms of displacement, which is modelled using poroelastic theory. The results show that there is an increase in tissue displacement and interstitial pressure after reperfusion. In addition, the results also show that vessel collapse can occur at high value of reperfusion pressure, low blood osmotic pressure, high cerebral capillary permeability and low cerebral capillary stiffness. This model provides insight on the formation of ischemia-reperfusion injury by tissue swelling and vessel collapse.

Comparing different analysis methods for quantifying the MRI amide proton transfer (APT) effect in hyperacute stroke patients

Amide proton transfer (APT) imaging is a pH mapping method based on the chemical exchange saturation transfer phenomenon that has potential for penumbra identification following stroke. The majority of the literature thus far has focused on generating pH-weighted contrast using magnetization transfer ratio asymmetry analysis instead of quantitative pH mapping. In this study, the widely used asymmetry analysis and a model-based analysis were both assessed on APT data collected from healthy subjects (n = 2) and hyperacute stroke patients (n = 6, median imaging time after onset = 2 hours 59 minutes). It was found that the model-based approach was able to quantify the APT effect with the lowest variation in grey and white matter (≤ 13.8 %) and the smallest average contrast between these two tissue types (3.48 %) in the healthy volunteers. The model-based approach also performed quantitatively better than the other measures in the hyperacute stroke patient APT data, where the quantified APT effect in the infarct core was consistently lower than in the contralateral normal appearing tissue for all the patients recruited, with the group average of the quantified APT effect being 1.5 ± 0.3 % (infarct core) and 1.9 ± 0.4 % (contralateral). Based on the fitted parameters from the model-based analysis and a previously published pH and amide proton exchange rate relationship, quantitative pH maps for hyperacute stroke patients were generated, for the first time, using APT imaging.

Genetic testing in inherited polyposis syndromes - how and why?

There have been recent advances in genetic testing enabling accurate diagnosis of polyposis syndromes by identifying causative gene mutations, which is essential in the management of individuals with polyposis syndrome and predictive genetic testing of their extended families. There are some similarities in clinical presentation of various polyposis syndromes, which may pose a challenge to diagnosis. In this review, we discuss the clinical presentation of the main polyposis syndromes and the process of genetic testing, including the latest advancement and future of genetic testing. We aim to reiterate the importance of genetic testing in the management of polyposis syndromes, potential pitfalls associated with genetic testing and recommendations for healthcare professionals involved with the care of polyposis patients.

Removal of acidic pharmaceuticals within a nitrifying recirculating biofilter

The fate of pharmaceutically active compounds (PhACs) in wastewater treatment systems is an area of increasing concern. Little research has been done to understand this issue in rural or decentralized communities. The objective of this research was to examine the ability of a bench scale nitrifying recirculating biofilter (RBF) to remove four acidic PhACs: gemfibrozil, naproxen, ibuprofen and diclofenac from secondary treated municipal wastewater at concentrations of 20 and 200μg/L. The average removals in this study were between 92 and 99% for ibuprofen, 89 and 99% for naproxen, 62 and 92% for gemfibrozil and 40 and 76% for diclofenac, which is consistent with literature. Ibuprofen and naproxen were largely removed through biological transformation; whereas gemfibrozil and diclofenac showed more variable removal, likely due to both biological transformation and sorption processes. PhAC removal in the RBFs was repeatable between trials, robust and responsive to system upsets, and the presence of PhACs as a single compound versus mixtures had no impact on PhAC removal efficiency. In summary, this study indicates that RBFs as a nitrifying stage of a multi-stage filtration process could be a viable technology for removal of some acidic pharmaceuticals in small onsite wastewater treatment facilities.

Phase-rectified signal averaging for intrapartum electronic fetal heart rate monitoring is related to acidaemia at birth

OBJECTIVE

Recent studies suggest that phase-rectified signal averaging (PRSA), measured in antepartum fetal heart rate (FHR) traces, may sensitively indicate fetal status; however, its value has not been assessed during labour. We determined whether PRSA relates to acidaemia in labour, and compare its performance to short-term variation (STV), a related computerised FHR feature.

DESIGN

Historical cohort.

SETTING

Large UK teaching hospital.

POPULATION

All 7568 Oxford deliveries that met the study criteria from April 1993 to February 2008.

METHODS

We analysed the last 30 minutes of the FHR and associated outcomes of infants. We used computerised analysis to calculate PRSA decelerative capacity (DC(PRSA)), and its ability to predict umbilical arterial blood pH ≤ 7.05 using receiver operator characteristic (ROC) curves and event rate estimates (EveREst). We compared DC(PRSA) with STV calculated on the same traces.

MAIN OUTCOME MEASURE

Umbilical arterial blood pH ≤ 7.05.

RESULTS

We found that PRSA could be measured in all cases. DC(PRSA) predicted acidaemia significantly better than STV: the area under the ROC curve was 0.665 (95% CI 0.632-0.699) for DC(PRSA), and 0.606 (0.573-0.639) for STV (P = 0.007). EveREst plots showed that in the worst fifth centile of cases, the incidence of low pH was 17.75% for DC(PRSA) but 11.00% for STV (P < 0.001). DC(PRSA) was not highly correlated with STV.

CONCLUSIONS

DC(PRSA) of the FHR can be measured in labour, and appears to predict acidaemia more accurately than STV. Further prospective evaluation is warranted to assess whether this could be clinically useful. The weak correlation between DC(PRSA) and STV suggests that they could be combined in multivariate FHR analyses.

Optimal sampling schedule for chemical exchange saturation transfer

The sampling schedule for chemical exchange saturation transfer imaging is normally uniformly distributed across the saturation frequency offsets. When this kind of evenly distributed sampling schedule is used to quantify the chemical exchange saturation transfer effect using model-based analysis, some of the collected data are minimally informative to the parameters of interest. For example, changes in labile proton exchange rate and concentration mainly affect the magnetization near the resonance frequency of the labile pool. In this study, an optimal sampling schedule was designed for a more accurate quantification of amine proton exchange rate and concentration, and water center frequency shift based on an algorithm previously applied to magnetization transfer and arterial spin labeling. The resulting optimal sampling schedule samples repeatedly around the resonance frequency of the amine pool and also near to the water resonance to maximize the information present within the data for quantitative model-based analysis. Simulation and experimental results on tissue-like phantoms showed that greater accuracy and precision (>30% and >46%, respectively, for some cases) were achieved in the parameters of interest when using optimal sampling schedule compared with evenly distributed sampling schedule. Hence, the proposed optimal sampling schedule could replace evenly distributed sampling schedule in chemical exchange saturation transfer imaging to improve the quantification of the chemical exchange saturation transfer effect and parameter estimation.

Evaluating the use of a continuous approximation for model-based quantification of pulsed chemical exchange saturation transfer (CEST)

Many potential clinical applications of chemical exchange saturation transfer (CEST) have been studied in recent years. However, due to various limitations such as specific absorption rate guidelines and scanner hardware constraints, most of the proposed applications have yet to be translated into routine diagnostic tools. Currently, pulsed CEST which uses multiple short pulses to perform the saturation is the only viable irradiation scheme for clinical translation. However, performing quantitative model-based analysis on pulsed CEST is time consuming because it is necessary to account for the time dependent amplitude of the saturation pulses. As a result, pulsed CEST is generally treated as continuous CEST by finding its equivalent average field or power. Nevertheless, theoretical analysis and simulations reveal that the resulting magnetization is different when the different irradiation schemes are applied. In this study, the quantification of important model parameters such as the amine proton exchange rate from a pulsed CEST experiment using quantitative model-based analyses were examined. Two model-based approaches were considered - discretized and continuous approximation to the time dependent RF irradiation pulses. The results showed that the discretized method was able to fit the experimental data substantially better than its continuous counterpart, but the smaller fitted error of the former did not translate to significantly better fit for the important model parameters. For quantification of the endogenous CEST effect, such as in amide proton transfer imaging, a model-based approach using the average power equivalent saturation can thus be used in place of the discretized approximation.

The response of hepatocyte cell volume to hyperthermia and its role in oedema

A novel mathematical model for hepatocytes and surrounding volume is presented here; in addition to tracking ion transport and diffusion the new model allows for changing cell volume. Using temporally and spatially varying temperature as an input, this paper shows how differences between diffusion coefficients directly influence increases in cell volume. The multiscale nature of the model presents a possible link from established cellular equations to the observed clinical result of oedema present in thermal treatments of cancer.

Effects of arterial blood gas levels on cerebral blood flow and oxygen transport

Near Infra-Red Spectroscopy (NIRS) is a non-invasive technique which can be used to investigate cerebral haemodynamics and oxygenation with high temporal resolution. When combined with measures of Cerebral Blood Flow (CBF), it has the potential to provide information about oxygen delivery, utilization and metabolism. However, the interpretation of experimental results is complex. Measured NIRS signals reflect both scalp and cerebral haemodynamics and are influenced by many factors. The relationship between Arterial Blood Pressure (ABP) and CBF has been widely investigated and it central to cerebral autoregulation. Changes in arterial blood gas levels have a significant effect on ABP and CBF and these relationships have been quantified previously. The relationship between ABP and NIRS signals, however, has not been fully characterized. In this paper, we thus investigate the influence of changes in arterial blood gas levels both experimentally and theoretically, using an extended mathematical model of cerebral blood flow and metabolism, in terms of the phase angle at 0.1 Hz. The autoregulation response is found to be strongly dependent upon the carbon dioxide (CO2) partial pressure but much less so upon changes in arterial oxygen saturation (SaO2). The results for phase angle sensitivity to CO2 show good agreement between experimental and theory, but a poorer agreement is found for the sensitivity to SaO2.

Effects of arterial blood gas levels on cerebral blood flow and oxygen transport

Near Infra-Red Spectroscopy (NIRS) is a non-invasive technique which can be used to investigate cerebral haemodynamics and oxygenation with high temporal resolution. When combined with measures of Cerebral Blood Flow (CBF), it has the potential to provide information about oxygen delivery, utilization and metabolism. However, the interpretation of experimental results is complex. Measured NIRS signals reflect both scalp and cerebral haemodynamics and are influenced by many factors. The relationship between Arterial Blood Pressure (ABP) and CBF has been widely investigated and it central to cerebral autoregulation. Changes in arterial blood gas levels have a significant effect on ABP and CBF and these relationships have been quantified previously. The relationship between ABP and NIRS signals, however, has not been fully characterized. In this paper, we thus investigate the influence of changes in arterial blood gas levels both experimentally and theoretically, using an extended mathematical model of cerebral blood flow and metabolism, in terms of the phase angle at 0.1 Hz. The autoregulation response is found to be strongly dependent upon the carbon dioxide (CO2) partial pressure but much less so upon changes in arterial oxygen saturation (SaO2). The results for phase angle sensitivity to CO2 show good agreement between experimental and theory, but a poorer agreement is found for the sensitivity to SaO2.

Hereditary mixed polyposis syndrome due to a BMPR1A mutation

The conditions Juvenile Polyposis Syndrome (JPS) and Hereditary Mixed Polyposis Syndrome (HMPS) are associated with an increased risk of colorectal carcinoma. The genetic mechanisms which explain these conditions have until recently been poorly understood. Recent interest has focused on the transforming growth factor (TGF)-beta signalling pathway and, in particular, on mutations in the SMAD4 gene. However, not all cases of JPS and HMPS have mutations in SMAD4 and focus has now shifted to other components of the TGF-beta pathway to clarify the genetic mechanisms involved in these conditions. In this report, we describe the significance of a bone morphogenetic protein receptor type 1A gene mutation in an Irish family.

Mathematical modeling of thermal ablation

Ablation techniques have become a widespread choice for the treatment of cancerous tumors for which surgical resection techniques have a poor prognosis. The minimally invasive nature and high success rate when performed by experienced clinicians mean that ablation is likely to remain a core technique. However, the success rate can drop off dramatically when less-experienced operators are involved, and it is particularly difficult to kill all of the tumor and only the tumor, given the dynamic nature of the processes that lead to cell death. Mathematical modeling of the response to ablation treatment has a long history. Since the seminal paper of Pennes in 1948, there have been numerous attempts to propose models that are both physiologically accurate and computationally inexpensive. All of these models are based on different principles and assumptions, which may make them suitable only for particular applications. This makes choosing a model very difficult because of the lack of understanding about what the limitations of different assumptions are likely to be and how this influences the necessary computational resources. Here we review the models available in the literature, illustrating how the different assumptions impact upon both their accuracy and computational expense.The primary intentions are to provide a critical scientific review and a practical guide for researchers wishing to use such models in clinical applications.

A two-equation coupled system model for determination of liver tissue temperature during radio frequency ablation

A model is presented that is an alternative approach to the bio-heat equation for use in radio frequency heating of the liver. The model comprises both a tissue subvolume and a blood subvolume. Separate bio-heat equations are determined for each subvolume, but with an additional term exchanging heat between them, thus creating a coupled system. The derivation for the two coupled differential equations is outlined and sample simulations are presented to demonstrate the importance of considering the two subvolumes separately, even when the blood subvolume is a small fraction of the tissue subvolume.

Sequential UV- and chlorine-based disinfection to mitigate Escherichia coli in drinking water biofilms

This study was designed to examine the potential downstream benefits of sequential disinfection to control the persistence of Escherichia coli under conditions relevant to drinking water distribution systems. Eight annular reactors (four polycarbonate and four cast iron) were setup in parallel to address various factors that could influence biofilm growth in distribution systems. Eight reactors were treated with chlorine, chlorine dioxide and monochloramine alone or in combination with UV to examine the effects on Escherichia coli growth and persistence in both the effluent and biofilm. In general, UV-treated systems in combination with chlorine or chlorine dioxide and monochloramine achieved greater log reductions in both effluent and biofilm than systems treated with chlorine-based disinfectants alone. However, during UV-low chlorine disinfection, E. coli was found to persist at low levels, suggesting that the UV treatment had instigated an adaptive mutation. During UV-chlorine-dioxide treatment, the E. coli that was initially below the detection limit reappeared during a low level of disinfection (0.2 mg/L) in the cast iron systems. Chloramine was shown to be effective in disinfecting suspended E. coli in the effluent but was unable to reduce biofilm counts to below the detection limit. Issues such as repair mechanism of E. coli and nitrification could help explain some of these aberrations. Improved understanding of the ability of chlorine-based disinfectant in combination with UV to provide sufficient disinfection will ultimately effect in improved management and safety of drinking water.

Multiple mitochondrial DNA deletions in monozygotic twins with OPMD

BACKGROUND

Oculopharyngeal muscular dystrophy (OPMD) is caused by expansions of the poly (A) binding protein 2 (PABP2) gene. Previous histological analyses have revealed mitochondrial abnormalities in the muscles of OPMD patients but their significance remains uncertain.

OBJECTIVE

We had the rare opportunity to study monozygotic twins with identical expansions of the PABP2 gene but with markedly different severities of OPMD. Both had histological features of mitochondrial myopathy. We determined whether mitochondrial DNA abnormalities underlay these changes.

METHODS

Clinical information was obtained by history and examination. Muscle biopsies were obtained from each subject and genetic analysis was performed using long-range PCR and Southern blotting.

RESULTS

We demonstrate, for the first time, the presence of mitochondrial DNA (mtDNA) deletions by Southern blotting in individuals with OPMD. This correlates with the presence of mitochondrial myopathy in both twins. Moreover, both twins had different mtDNA deletions, which might explain their phenotypic differences.

CONCLUSION

We hypothesise that mitochondrial dysfunction may occur as a consequence of PABP2 gene mutations, and that this dysfunction may affect the phenotypic manifestations of OPMD.

Biallelic mutation of MSH2 in primary human cells is associated with sensitivity to irradiation and altered RAD51 foci kinetics

BACKGROUND

Reports of differential mutagen sensitivity conferred by a defect in the mismatch repair (MMR) pathway are inconsistent in their conclusions. Previous studies have investigated cells established from immortalised human colorectal tumour lines or cells from animal models.

METHODS

We examined primary human MSH2-deficient neonatal cells, bearing a biallelic truncating mutation in MSH2, for viability and chromosomal damage after exposure to DNA-damaging agents.

RESULTS

MSH2-deficient cells exhibit no response to interstrand DNA cross-linking agents but do show reduced viability in response to irradiation. They also show increased chromosome damage and exhibit altered RAD51 foci kinetics after irradiation exposure, indicating defective homologous recombinational repair.

DISCUSSION

The cellular features and sensitivity of MSH2-deficient primary human cells are broadly in agreement with observations of primary murine cells lacking the same gene. The data therefore support the view that the murine model recapitulates early features of MMR deficiency in humans, and implies that the variable data reported for MMR-deficient immortalised human cells may be due to further genetic or epigenetic lesions. We suggest caution in the use of radiotherapy for treatment of malignancies in individuals with functional loss of MSH2.

Time-frequency analysis of the ECG in the diagnosis of vasovagal syndrome in older people

The Smoothed Pseudo Wigner-Ville Distribution (SPWVD) is used for the time-frequency analysis of variations in RR interval. A novel technique to determine the smoothing window lengths is implemented, and a new heart rate variability (HRV) metric is developed, instantaneous center frequency variability (ICFV), which uses the time-frequency map generated by the SPWVD. The technique is then applied to 50 patients with unexplained falls and age > 60, undergoing head-upright tilt table testing (HUT). Eighteen of the patients were diagnosed with vasovagal syndrome. Attempts at syncope prediction using the new metric is an improvement on traditional techniques: an ICFV less than 0.07 Hz from 90 s to 180 s after tilt is predictive of a negative test (negative predictive value: 0.77). The comorbidity and autonomic degeneration present in elderly patients are thought to be responsible for lowering the negative predictive value.

A two-layer model of the static behaviour of blood vessel walls

A model is presented that simulates the steady state two-layer nature of blood vessel walls. The model comprises an intimal-medial layer and an adventitial layer, both of which are modeled using previously validated separate models. By including both models, different types of blood vessel can be modeled within a common framework. The necessary relationships that link the two models are presented and simulations of the complete model shown. These illustrate how different types of vessel exhibit different non-linear behaviour. The models include both mechanical and biochemical aspects of the vessel wall behaviour, which will enable the model to be used to simulate more advanced behaviour such as local autoregulatory processes.

Synchronization between arterial blood pressure and cerebral oxyhaemoglobin concentration investigated by wavelet cross-correlation

Wavelet cross-correlation (WCC) is used to analyse the relationship between low-frequency oscillations in near-infrared spectroscopy (NIRS) measured cerebral oxyhaemoglobin (O(2)Hb) and mean arterial blood pressure (MAP) in patients suffering from autonomic failure and age-matched controls. Statistically significant differences are found in the wavelet scale of maximum cross-correlation upon posture change in patients, but not in controls. We propose that WCC analysis of the relationship between O(2)Hb and MAP provides a useful method of investigating the dynamics of cerebral autoregulation using the spontaneous low-frequency oscillations that are typically observed in both variables without having to make the assumption of stationarity of the time series. It is suggested that for a short-duration clinical test previous transfer-function-based approaches to analyse this relationship may suffer due to the inherent nonstationarity of low-frequency oscillations that are observed in the resting brain.

A physiological model of the release of gas bubbles from crevices under decompression

Moving bubbles have been observed in the blood during or after decompression using ultrasonic techniques. It has been proposed that these may grow from nuclei housed on the blood vessel wall. One candidate for bubble nucleation is hydrophobic crevices. This work explores the growth of gas pockets that might exist in conical crevices and the release of bubbles from these crevices under decompression. An existing dynamic mathematical model for the stability of gas pockets in crevices [Chappell, M.A., Payne, S.J., in press. A physiological model of gas pockets in crevices and their behavior under compression. Respir. Physiol. Neurobiol.] is extended to include the behavior as the gas pocket reaches the crevice mouth and bubbles seed into the bloodstream. The behavior of the crevice bubble is explored for a single inert gas, both alone and with metabolic gases included. It was found that the presence of metabolic gases has a significant effect on the behavior under decompression and that this appears to be due to the high diffusivity of these gases.

A model of the interaction between autoregulation and neural activation in the brain

In this paper a model is proposed that predicts the response of the cerebral vasculature to changes in arterial blood pressure, arterial CO2 concentration and neural stimulation. Cerebral blood flow (CBF) is assumed to be controlled through changes in arterial compliance, and hence arterial resistance and volume, through three feedback mechanisms, which act in a linear additive manner, based on CBF, arterial CO2 and neural stimulus. Together with arterial, capillary and venous compartments, a tissue compartment is included, which contributes partly to the initial rise found in the deoxyhaemoglobin response to neural activation. Dynamic simulations of the model under different conditions show that there is significant interaction between the autoregulation and activation processes, and that the level of autoregulation has a strong influence on the CBF and deoxyhaemoglobin responses to neural activation. Overshoot in the deoxyhaemoglobin response is eliminated completely in the absence of this regulation. The feedback mechanism time constants significantly affect the CBF and deoxyhaemoglobin responses. Changes in arterial blood pressure (ABP) are found to have a strong influence on the neural activation response, with the amplitude of the response decreasing significantly at high baseline ABP. Dynamic changes in ABP also have a significant and potentially confounding impact on the measured deoxyhaemoglobin response to neural activation.

A physiological model of gas pockets in crevices and their behavior under compression

The formation of bubbles in the body from dissolved gases during decompression causes a range of symptoms, often referred to as Decompression Sickness. It is likely that these bubbles grow from pre-existing nuclei. It has been proposed that such nuclei are pockets of gas that are stabilized against collapse under raised pressure, such as that experienced by deep-sea divers. This work explores the stabilization of gas pockets in crevices, in its application to the nucleation of bubbles in blood vessels. A model has been derived to describe the dynamics of a bubble in a crevice, assuming that gases diffuse in and out of the bubble via the crevice wall from the body tissues surrounding the crevice. The time-varying behavior of the bubble has been examined and it has been shown that a crevice bubble can survive compression by temporary variations in the gas-liquid interface curvature or by shrinking, its behavior being found to be strongly dependent upon the model parameters.

Combined transfer function analysis and modelling of cerebral autoregulation

The clinical importance of cerebral autoregulation has resulted in a significant body of literature that attempts both to model the underlying physiological processes and to estimate the mathematical relationships between clinically measurable variables, the most common of which are Arterial Blood Pressure and Cerebral Blood Flow Velocity. These approaches have, however, rarely been used together to interpret clinical data. A simple model of cerebral autoregulation is thus proposed here, based on a flow dependent feedback mechanism with gain and time constant that adjusts arterial compliance. Analysis of this model shows that it closely approximates a second order system for typical values of physiological parameters. The model parameters can be optimally estimated from available experimental data for the Impulse Response (IR), yielding physiologically reasonable values, although there is one free parameter that must be fixed. The effects of changes in feedback gain and time constant are found to be significant on the predicted IR and can thus be estimated robustly from experimental data. The effects of elevated baseline Intracranial Pressure (ICP) are found to be exactly equivalent to a reduced feedback gain, although the solution is much less sensitive to the former effect. A transfer function approach can be used to estimate autoregulation status clinically using a physiologically-based model, thus providing greater insight into the processes that govern cerebral autoregulation.

A physiological model of the interaction between tissue bubbles and the formation of blood-borne bubbles under decompression

Under decompression, bubbles can form in the human body, and these can be found both within the body tissues and the bloodstream. Mathematical models for the growth of both types of bubbles have previously been presented, but they have not been coupled together. This work thus explores the interaction between the growth of tissue and blood-borne bubbles under decompression, specifically looking at the extent to which they compete for the common resource of inert gas held in solution in the tissues. The influence of tissue bubbles is found to be significant for densities as low as 10 ml(-1) for tissues which are poorly perfused. However, the effects of formation of bubbles in the blood are not found until the density of bubble production sites reaches 10(6) ml(-1). From comparison of the model predictions with experimental evidence for bubbles produced in animals and man under decompression, it is concluded that the density of tissue bubbles is likely to have a significant effect on the number of bubbles produced in the blood. However, the density of nucleation sites in the blood is unlikely to be sufficiently high in humans for the formation of bubbles in the blood to have a significant impact on the growth of the bubbles in the tissue.

Epidermal naevus in Proteus syndrome showing loss of heterozygosity for an inherited PTEN mutation

A 3-year-old boy with Proteus syndrome has a novel germline p.Y68D mutation of the PTEN gene inherited from his mother who has Cowden syndrome. In addition, DNA extracted from curettings of his widespread epidermal naevus shows loss of heterozygosity for this mutation. To our knowledge, this has not been described before.

A method for the automated detection of venous gas bubbles in humans using empirical mode decomposition

Doppler ultrasound signals are widely used to grade the quantity of circulating venous bubbles in divers. Current techniques rely on trained observers, making the grading process both time-consuming and subjective. The automated detection of bubbles, however, is confounded by the presence of other signals, primarily those arising from blood motion. Empirical Mode Decomposition was used here to calculate the intrinsic mode functions (IMFs) of a number of Doppler ultrasound signals from recreational divers, post-decompression. The IMFs provide a basis set for signal decomposition, each IMF corresponding to a different timescale in the signal. Each signal was found to comprise approximately 20 IMFs: the precise number being dependent upon the nature of the signal. A method is presented to detect bubbles using the IMF; features are first identified in the individual heart cycles, these having been previously determined using a robust peak detection method, by examining deviations from the ensemble averaged IMF. Bubbles are then identified as features appearing in more than one IMF, with significant energy in the original signal. This method has been applied to a subset of the available database and appears to perform with good sensitivity even when the signal has variable signal strength.

Automated classification and analysis of the calcium response of single T lymphocytes using a neural network approach

The gene activities in T lymphocytes that regulate immune responses are influenced by Ca2+ ([Ca2+]i). The intracellular calcium signals are highly heterogeneous and vitally important in determining the immune outcome. The signals in individual cells can be measured using fluorescence microscopy but to group the cells into classes with similar signal kinetics is currently laborious. Here, we demonstrate a method for the automated classification of the responses into four categories formerly identified by an expert's inspection. This method comprises characterising the response by a second-order model, performing frequency analysis, and using derived features as inputs to two multilayer perceptron neural networks (NNs). We compare the algorithm's performance on an example data set against the human classification: it was found to classify identically more than 70% of the data, despite small sample sizes in two categories and significant overlap between the other two classes. The group characterized by an oscillating signal showed the presence of a number of frequencies, which may be important in determining gene activation. A classification threshold enables the automatic identification of patterns with a low-classification certainty. Future refinement of the algorithm may allow the identification of more classes, which may be important in different immune responses associated with disease.

The prevalence and clinical significance of autoantibodies to plasminogen activator inhibitor 1 in systemic lupus erythematosus

We have recently described the novel autoantigen plasminogen activator inhibitor (PAI-1) in systemic lupus erythematosus (SLE). The aim of this study was to determine the prevalence and clinical significance of anti-PAI-1 autoantibodies in patients with SLE. Autoantibodies to recombinant PAI-1 were measured in retrospective sera of 48 lupus patients by immunoassay in order to assess their clinical significance. This showed that 71% of sera from 48 lupus patients had significantly elevated anti-PAI-1 autoantibodies as compared with normal control subjects (P < 0.0001). There was a weak but significant (P < 0.043) correlation with anti-dsDNA autoantibodies. In longitudinal studies, autoantibodies against PAI-1 correlated with clinical parameters measured by the BILAG disease activity index including global clinical score. Our study demonstrates the high frequency of novel autoantibodies to PAI-1 in patients with lupus. The serial clinical correlations with anti-PAI-1 autoantibodies also support the hypothesis that these autoantibodies may play a pathogenic role in lupus.

Analysis of the effects of gravity and wall thickness in a model of blood flow through axisymmetric vessels

The effects of gravitational forces and wall thickness on the behaviour of a model of blood flow through axisymmetric vessels were studied. The governing fluid dynamic equations were derived from the Navier-Stokes equations for an incompressible fluid and linked to a simple model of the vessel wall. A closed form of the hyperbolic partial differential equations was found, including a significant source term from the gravitational forces. The inclination of the vessel is modelled using a slope parameter that varied between -1 and 1. The wave speed was shown to be related to the wall thickness, and the time to first shock formation was shown to be directly proportional to this thickness. Two non-dimensional parameters were derived for the ratio of gravitational forces to viscous and momentum forces, respectively, and their values were calculated for the different types of vessel found in the human vasculature, showing that gravitational forces were significant in comparison with either viscous or momentum forces for every type of vessel. The steady-state solution of the governing equations showed that gravitational forces cause an increase in area of approximately 5% per metre per unit slope. Numerical simulations of the flow field in the aorta showed that a positive slope causes a velocity pulse to change in amplitude approximately linearly with distance: -4% per metre and +5% per metre for vessels inclined vertically upwards and downwards, respectively, in comparison with only +0.5% for a horizontal vessel. These simulations also showed that the change relative to the zero slope condition in the maximum rate of change of area with distance, which was taken to be a measure of the rate of shock formation, is proportional to both the slope and the wall thickness-to-inner radius ratio, with a constant of proportionality of 1.2. At a ratio of 0.25, typical of that found in human arteries, the distance to shock formation is thus decreased and increased by 30% for vessels inclined vertically downwards and upwards, respectively. Gravity and wall thickness thus have a significant impact on a number of aspects of the fluid and wall behaviour, despite conventionally being neglected.

Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) as a tool for differentiation of P2Y2-receptor-mediated vasorelaxation in guinea pig aorta

The action of the putative P2Y1-receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) was studied in guinea pig aorta for potential use in the differentiation between P2Y1 and P2Y2 purine receptors. Concentration-effect curves to 2-methylthioadenosine triphosphate (2MeSATP) and uridine triphosphate (UTP), agonists at P2Y1- and P2Y2-receptor sites, respectively, and the common agonist adenosine triphosphate (ATP) were constructed in guinea pig aortic ring preparations with the tone raised by 5 microM of noradrenaline. A ranked order of agonist potency of 2MeSATP > ATP > UTP resulted from the construction of concentration response curves to vasorelaxation of the agonists. Deendothelialization virtually abolished vasorelaxation to UTP but made no significant difference to 2MeSATP-induced responses. PPADS exhibited noncompetitive inhibition at P2Y1-receptor sites by reducing the maximal response to 2MeSATP, although a trend towards a right shift of the concentration-effect curves was observed. In total contrast, PPADS enhanced P2Y2-mediated vasorelaxation to UTP by shifting the concentration-effect curves to the left and increasing maximal responses. Thus, PPADS is a noncompetitive antagonist at P2Y1 receptors but enhances responses, at least to UTP, at P2Y2 receptors in guinea pig aorta via a hitherto unknown mechanism. Thus, PPADS is a potentially useful substance that may be used for differentiation between P2Y1 and P2Y2 receptors in the guinea pig aorta.

Interleaving reading and acting while following procedural instructions

Memory for an interactive procedure acquired from written instructions is improved if the procedure can be carried out while the instructions are being read. The size of the read-act cycle was manipulated in Experiments 1 and 2 by comparing chunked instruction-following, in which 3 or 4 steps are read then performed with single-step conditions. In both experiments, enforced chunking improved subsequent unaided performance of the procedure. In Experiment 3, participants were allowed to manage the interleaving of reading and acting. The imposition of a small behavioral cost (a single mouse point-and-click operation) on the switch between instructions and device encouraged more chunking and better subsequent test performance. The authors concluded that the interleaving of reading and acting is an important practical concern in the design of interactive procedures and that more effective chunk-based strategies can quite readily be encouraged.

Incidental memory and navigation in panoramic virtual reality for electronic commerce

Recently much effort has been dedicated to designing and implementing World Wide Web sites for virtual shopping and e-commerce. Despite this effort, relatively little empirical work has been done to determine the effectiveness with which different site designs sell products. We report three experiments in which participants were asked to search for products in various experimental e-commerce sites. Across the experiments participants were asked to search in either QTVR (QuickTime Virtual Reality), hypertext, or pictorially rich hypertext environments; they were then tested for their ability to recall the products seen and to recognize product locations. The experiments demonstrated that when using QTVR (Experiments 1, 2, and 3) or pictorial environments (Experiment 2), participants retained more information about products that were incidental to their goals. In two of the experiments it was shown that participants navigated more efficiently when using a QTVR environment. The costs and benefits of using 3D virtual environments for on-line shops are discussed. Actual or potential applications of this research include support for the development of e-commerce design guidelines.

Strategic use of familiarity in display-based problem solving

The authors report 4 experiments that investigated the role of recognition memory and plausibility in a display-based problem-solving task (computer menu search). It was found that both the familiarity of options and their plausibility played a role in determining choices when the correct options could not be recollected. The use of familiarity was adaptive: Participants relied less on familiarity when it was a less valid guide to correct choices. The implications of these findings for theories of problem solving and learning are discussed.

Long-term memory for spatial and temporal mental models includes construction processes and model structure

There is a strong case that people construct and manipulate mental models in working memory but relatively little evidence that mental models are preserved in long-term memory. Instead people may remember an episodic construction trace: a record of the operations used to construct a mental model (Payne, 1993). Experiment 1 investigated memory for determinate spatial descriptions (which describe a single configuration of objects) and indeterminate spatial descriptions (which describe two equally plausible configurations). Recognition performance was impaired when the overlap between the episodic construction trace of a description at learning and at test was disrupted by reordering the sentences within a description. Participants were better at remembering the gist of determinate descriptions than that of indeterminate descriptions. For indeterminate descriptions, provided differences in gist recognition were controlled, participants showed better memory for the original description. Experiment 2 showed a similar pattern of results with temporal descriptions. A third experiment manipulated the similarity between foils in the recognition test and the original descriptions to provide further evidence for both episodic construction trace and remembered mental models. In combination, these results favour a hybrid account of memory for mental models, which includes information about both construction processes and model structure.

Recognition memory for sentences from spatial descriptions: a test of the episodic construction trace hypothesis

Many researchers believe that when people read spatial descriptions, they construct mental models of the configurations described. Payne (1993) proposed that reading a spatial description produces a memory of the operations used to construct a mental model, an episodic construction trace. The episodic construction trace hypothesis predicts that memory for a spatial description will be influenced by the degree of overlap between the construction processes required by the original description and the construction processes prompted by an item in a recognition test. The two experiments reported here show that readers of spatial descriptions are more likely to accept sentences in a recognition test that are consistent with the operations used to construct a mental model than to accept sentences that are inconsistent. Consistency with the episodic construction trace leads to both correct recognition of verbatim sentences from the original description and false recognition of sentences that were not present in the original descriptions.

A rapid, PCR based test for differential molecular diagnosis of Prader-Willi and Angelman syndromes

Approximately 98% of Prader-Willi syndrome (PWS) and 80% of Angelman syndrome (AS) cases have deletions at a common region in chromosome 15q11-13, uniparental disomy for chromosomes 15 (UPD15), or mutations affecting gene expression in this region. The resulting clinical phenotype (PWS or AS) in each class of mutation depends upon the parent of origin. Both disorders are characterised at the molecular level by abnormal methylation of imprinted genes at 15q11-q13 including the small nuclear ribonucleoprotein N gene (SNRPN). Current diagnostic strategies include high resolution cytogenetics, fluorescence in situ hybridisation (FISH), Southern blot hybridisation, or microsatellite typing. We have developed a novel and rapid diagnostic test for PWS and AS based on differential digestion of expressed (paternally imprinted) SNRPN sequences by the methylation sensitive endonuclease NotI or repressed (maternally imprinted) SNRPN sequences by the methylation requiring nuclease McrBC, followed by PCR amplification of the SNRPN promoter. We have evaluated this test by blinded analysis of 60 characterised DNA samples (20 PWS, 20 AS, and 20 unaffected controls). SNRPN sequences could not be amplified from PWS patient DNA which had been digested with McrBC, nor from AS patient DNA which had been digested with NotI. We were able to make a correct diagnosis of PWS, AS, or unaffected in all 60 samples tested. This novel test is rapid and has a high specificity and sensitivity for deletion and UPD15 cases. These features make this new test suitable as the initial step in a molecular diagnostic strategy for PWS/AS.

The effects of operator implementation cost on planfulness of problem solving and learning

This paper explores the idea that problem solving search strategies are chosen so as to optimize performance within the constraints of a particular situation. Four experiments are reported that examine the hypothesis that the cost of performing an operation affects "planfulness"--the level of planning during problem solving. The first experiment investigated problem solving with the 8-puzzle and compared strategies adopted when there was a high versus a low cost of making a move, manipulating cost in terms of command length. The second experiment used protocol analysis to provide more direct evidence for increased planning. The third and fourth experiments looked at the effects of these different strategies on learning: the third examined how problem solving performance on a direct manipulation interface is affected by prior problem solving experience in the same domain with either a high cost or low cost command-driven interface, demonstrating improved performance as a result of training on an interface with high cost operations; the fourth experiment, like the third, examined the effects of prior problem solving experience with either a high cost or low cost interface on subsequent problem solving performance in a different domain using a direct manipulation interface showing no effect to training interface on subsequent performance.

Female germline mosaicism in tuberous sclerosis confirmed by molecular genetic analysis

We have investigated a family in which three siblings with the autosomal dominant disorder tuberous sclerosis had unaffected parents. The family were typed for polymorphic markers spanning the two genes known to cause tuberous sclerosis located at 9q34 (TSC1) and 16p13.3 (TSC2). TSC1 markers showed different maternal and paternal haplotypes in affected children, excluding a mutation in TSC1 as the cause of the disease. For the TSC2 markers all the affected children had the same maternal and paternal haplotypes, as did three of their unaffected siblings. Mutation screening by RT-PCR and direct sequencing of the TSC2 gene identified a 4 bp insertion TACT following nucleotide 2077 in exon 18 which was present in the three affected children but not in five unaffected siblings or the parents. This mutation would cause a frameshift and premature termination at codon 703. Absence of the mutation in lymphocyte DNA from the parents was consistent with germline mosaicism and this was confirmed by our finding of identical chromosome 16 haplotypes in affected and unaffected siblings, providing unequivocal evidence of two different cell lines in the gametes. Molecular analysis of the TSC2 alleles present in the affected subjects showed that the mutation had been inherited from the mother. This is the first case of germline mosaicism in tuberous sclerosis proven by molecular genetic analysis and also the first example of female germline mosaicism for a characterized autosomal dominant gene mutation apparently not associated with somatic mosaicism.