Stochastic dynamic causal modelling of FMRI data with multiple-model Kalman filters

INTRODUCTION

This article is part of the Focus Theme of Methods of Information in Medicine on "Biosignal Interpretation: Advanced Methods for Neural Signals and Images".

BACKGROUND

Dynamic Causal Modelling (DCM) is a generic formalism to study effective brain connectivity based on neuroimaging data, particularly functional Magnetic Resonance Imaging (fMRI). Recently, there have been attempts at modifying this model to allow for stochastic disturbances in the states of the model.

OBJECTIVES

This paper proposes the Multiple-Model Kalman Filtering (MMKF) technique as a stochastic identification model discriminating among different hypothetical connectivity structures in the DCM framework; moreover, the performance compared to a similar deterministic identification model is assessed.

METHODS

The integration of the stochastic DCM equations is first presented, and a MMKF algorithm is then developed to perform model selection based on these equations. Monte Carlo simulations are performed in order to investigate the ability of MMKF to distinguish between different connectivity structures and to estimate hidden states under both deterministic and stochastic DCM.

RESULTS

The simulations show that the proposed MMKF algorithm was able to successfully select the correct connectivity model structure from a set of pre-specified plausible alternatives. Moreover, the stochastic approach by MMKF was more effective compared to its deterministic counterpart, both in the selection of the correct connectivity structure and in the estimation of the hidden states.

CONCLUSIONS

These results demonstrate the applicability of a MMKF approach to the study of effective brain connectivity using DCM, particularly when a stochastic formulation is desirable.

[Assessment of aortic atherosclerosis with transesophageal echocardiography]

OBJECTIVE

To image the thoracic aorta by transesophageal echocardiography (TEE) and study atherosis (morphology and extent of atheroma plaques) and sclerosis (stiffness) and secondarily correlate them with serum lipid levels (cholesterol, HDL, LDL and triglycerides).

METHODS

We studied 29 patients (pts) who underwent TEE electively (male 18 pts, age 57.8 +/- 14.6 years). The parameters evaluated were: the stiffness coefficient = ln (PAsist/PAdiast)/(Dsyst/Dmin), and the morphology, location and extent of atheroma plaques. The systolic distension (Dsyst) was the difference between the maximal and the minimal dimensions (Dmin) of the aortic diameter measured by M mode. The lesions were classified in 4 degrees (0-3): 0--normal intima, 1--intimal thickening, 2--atheroma, 3--complicated lesion. Five aortic segments were studied: arch, D1-D4 (descending aorta at 5 cm intervals from the first 25 cm distal of the incisors line). We calculated the individual score = 1 x theta 1/180 + 2 x theta 2/180 + 3 x theta 3/180, theta n represents the angles occupied by the lesions and n (1-3) the severity of atherosis of each lesion. The total atherosis index (TAI) was sigma scores/(n. degree of visualized segments).

RESULTS

The arch was not visualized in 3 pts, and the segment D4 was only visualized in 3 pts. TAI mean = 0.82 +/- 0.74, stiffness coefficient mean (SC) = 9.56 +/- 15.072. There were no significant correlations between the lipid levels and the TAI or SC. The only significant positive correlations were: TAI vs age (r = 0.62, p < 0.001) and SC and diastolic blood pressure (BP) (r = 0.42, p < 0.05).

CONCLUSIONS

The best visualized segments belong to the descending aorta (25 to 40 cm from the incisors). In this group of patients the lipid levels did not seem to be a preponderant factor in aortic atherosclerosis. The most important factors were age for atherosis and BP for sclerosis.

[Changes in regional wall contractility induced by effort in women with normal coronary angiography --report of a clinical case]

The diagnosis of coronary artery disease in women has been thought to be more difficult than in men, owing to the lower overall prevalence of disease in women, as well as more subtle clinical presentations and unspecific changes in ST segment. The authors report a clinical case of a 61-year old woman, with low cardiovascular risk and history of atypical chest pain and a positive treadmill exercise test on the inferior leads. She did an exercise echocardiogram that revealed severe hypokinesis on the anterior wall and septum with late normalization. The patient was submitted to a coronary angiography that revealed normal arteries. An echocardiogram with hyperventilation was later performed and showed the same ischemic changes as exercise did, on the inferior leads but no regional wall motions abnormalities occurred. The patient is currently asymptomatic under calcium antagonist treatment.