A Boundary value technique for solving singularly perturbed,fixed end-point optimal control problems

A method is proposed to solve fixed end-point, linear optimal control problems with quadratic cost and singularly perturbed state. After translating the problem into a two-point boundary value problem, we choose two points t₁, t₂ ? [t₀, t_f] and let τ = (t-t₀)/? and σ = (t_f-t)/?. The τ-scaled, original and σ-scaled boundary value problems are then solved on the intervals [t₀, t₁], [t₁, t₂] and [t₂, t_f] respectively. A test example is solved to illustrate the method.     

Influence of inhomogeneities in muscle tissue on single-fibre action potentials: a model study

The influence of changes in electrical conductivity, due to the muscle boundary, layers and compartments of intramuscular connective tissue and blood vessels, on computed single-muscle fibre action potentials (SFAPs) in rat hindleg muscle is calculated. The position of the active fibre is varied throughout the muscle. For fibres close to the muscle boundary, peak-to-peak voltages of SFAPs increase by up to a factor of 3 compared with the unbounded situation. For inner fibres, the presence of nearby connective tissue compartments causes an increase of up to 40%. A blood vessel in the neighbourhood of the active fibre leads to a decrease of at most 20%, for recording sites between the active fibre and the blood vessel. For recording sites beyond the blood vessel, peak-to-peak voltages increase by up to 20%.     

Influence of head model in biomagnetic source localization

Summary We evaluated the influence of the head model on biomagnetic source localization by utilizing a computer simulation. We localized the source of a magnetic field that was calculated using a realistic head model, and then evaluated the localization errors. It was seen that the sphere model adequately localized the dipole in cases near the sensor, but not in cases where the dipole was deeply situated.     

Automatic boundary identification of a region in gamma camera image

A new automatic method for the identification of the boundary of an image obtained with a gamma camera is proposed. The advantages of this method are a short calculation time and its applicability to images with a low signal-to-noise ratio. The limitations of the method are discussed.     

实验性隐睾术后大白鼠生精上皮的反应性变化——一,光镜下形态学观察

本文报道人工隐睾术后雄性成年大白鼠生精上皮的组织学变化。术后1—10天,曲细精管界膜呈波纹状收缩、增厚和断裂。支持细胞发生空泡样变。各级生精细胞的增生、分化失调、排列紊乱、游离和逐渐退化,出现双核和多核细胞。术后15—30天,上皮中只剩支持细胞和精原细胞,其它生精细胞逐渐消失。术后40—70天,形态学变化处于相对稳定状态。本文提出了实验性隐睾术后,引起生精上皮的原发性和继发性变化,生精上皮各种细胞对腹温的敏感性和耐受性的差异,初步讨论了多核细胞形成机理及生物学意义。     

Role of Cellular Orientation in Electrical Coupling Between Gastrointestinal Smooth Muscle

Electrical fields produced during depolarization as well as low resistance pathways through gap junctions have been proposed as electrical coupling mechanisms serving to coordinate electrical control activity in gastrointestinal smooth muscle. The differing orientations of the longitudinal and circular muscle layers offer many possible configurations for cells coupled by electrical fields. The boundary element method is used to investigate coupling, with respect to both gap junctions and field effects for ellipsoidal and cylindrical cells. Physiological considerations allow the possibility of aggregates of cells with coordinated electrical activity. The effect of multiple source cells on field coupling is also modeled. Results indicate that even small gap junctional conductances are effective for coupling of smooth muscle and that field coupling is most efficacious when the ellipsoidal cells are coupled side by side and when cylindrical cells are coupled end to end. © 1998 Biomedical Engineering Society. PAC98: 8722Jb, 8710+e, 0260Lj, 8750-a     

A comparison of two boundary conditions used with the bidomain model of cardiac tissue

In the bidomain model, two alternative sets of boundary conditions at the interface between cardiac tissue and a saline bath have been used. It is shown that these boundary conditions are equivalent if the length constant of the tissue in the direction transverse to the fibers is much larger than the radius of the individual cardiac cells. If this is not the case, the relative merits of the two boundary conditions are closely related to the question of the applicability of a continuum model, such as the bidomain model, to describe a discrete multicellular tissue.     

Parallel implementation of the accelerated BEM approach for EMSI of the human brain

Boundary element method (BEM) is one of the numerical methods which is commonly used to solve the forward problem (FP) of electro-magnetic source imaging with realistic head geometries. Application of BEM generates large systems of linear equations with dense matrices. Generation and solution of these matrix equations are time and memory consuming. This study presents a relatively cheap and effective solution for parallel implementation of the BEM to reduce the processing times to clinically acceptable values. This is achieved using a parallel cluster of personal computers on a local area network. We used eight workstations and implemented a parallel version of the accelerated BEM approach that distributes the computation and the BEM matrix efficiently to the processors. The performance of the solver is evaluated in terms of the CPU operations and memory usage for different number of processors. Once the transfer matrix is computed, for a 12,294 node mesh, a single FP solution takes 676 ms on a single processor and 72 ms on eight processors. It was observed that workstation clusters are cost effective tools for solving the complex BEM models in a clinically acceptable time.