Xeromammographic automatic exposure termination

A method of automatic exposure termination (AET) for xeromammography has been devised, significantly reducing the rate of repeat exposures due to poor choice of manual exposure factors. AET images are of good quality and are reliably produced. The concept of AET is based on the existence of an optimal transmitted exposure to the selenium plate, which is easily determined experimentally. In routine clinical xeromammography, a repeat rate of 20% was eliminated by the use of AET.     

The control of stable postures in the multijoint arm

The stiffness that is measured at the hand of a multijoint arm emerges from the combined effects of the elastic properties of the muscles and joints, the geometry of the linkages and muscle attachments, and the neural control circuits that act on the arm. The effective stiffness of a nonlinear linkage such as a two-joint arm depends on the force acting on the system as well as the intrinsic stiffness of the actuators. This paper presents an analysis of the factors that affect limb stiffness, including the effects of external forces. Three potential strategies for controlling the stability of the limb are proposed and demonstrated by computer simulations. The predictions from the simulations are then compared experimentally with measured stiffness values for human subjects working against an external force. These experiments were directed toward understanding what strategies are used by the CNS to control limb stiffness and stability. The experimental evidence showed that human subjects must increase the stiffness at the joints in order to maintain limb stability in the presence of applied external forces at the hand. In the process we identified a precise role for muscles which span two or more joints in the control of overall limb stiffness. A local strategy may be used to achieve limb stability, in which the muscle stiffness increases with muscle force. Multijoint muscles are shown to provide mechanical couplings which are necessary for the maintenance of stability. By utilizing these muscles, the neuro-musculo-skeletal system can control a global property of the system (stability) with a passive local strategy.     

Stability constraints for the distributed control of motor behavior

We have investigated the relation between static stability of a limb, equilibrium-point control, and the ill-posed problem of coordinating a redundant ensemble of muscles. Geometrically, equilibrium-point control is equivalent to establishing a mapping between the command signals delivered to the muscles and the equilibrium configurations of a limb. A necessary condition for the existence of such mapping is that the limb be stable across the workspace. We analyzed how this condition may be translated into precise biomechanical constraints for single-and multi-joint limbs. The satisfaction of these constraints is necessary for the equilibrium-point hypothesis to be a viable control paradigm. We show how these same constraints are sufficient to insure the successful operation of a distributed algorithm based upon minimization of potential energy that computes the muscle-control signals corresponding to a desired time sequence of equilibrium points.     

Choroidal melanoma: A review of the experience of the Sydney Eye Hospital Professorial Unit 1979–1995

Purpose: This study examines 90 patients presenting with choroidal or ciliochoroidal melanoma to the Professorial Unit at the Sydney Eye Hospital. The indications for treatment, and the outcome for the eye and vision are presented together with an account of mortality and the incidence of metastases. Methods: A retrospective analysis of 90 choroidal melanoma patients managed by one surgeon over a 16-year period was undertaken. Initial findings, investigations performed, incidence of metastatic disease, treatment received and complication rates and mortality, where applicable, were recorded. Results: The group was followed for an average of 64 months (range, 5–172 months). Primary treatment was with either Iodine¹²⁵ (¹²⁵1) brachytherapy, local excision or enucleation. Radiation retinopathy was prominent in ¹²⁵1 cases resulting in poor visual acuity when the tumour resided in the posterior pole. Local excision even of large tumours was effective particularly if peripheral. Overall metastatic disease was seen in 11% with 5-year survival rates for the metastatic group being 10%. Prognosis after diagnosis of metastases was poor. Conclusions: Specific therapy for choroidal melanoma must relate to the size and location of the tumour at the time of diagnosis. Visual outcome relates directly to the proximity of the tumour to the optic nerve and fovea. Metastatic disease latency can be prolonged; therefore caution about prognosis is required long after therapy is given. The 5-year survival is encouraging with all forms of therapy. However, as the natural history of ocular melanoma is variable and not fully delineated it is important to monitor the effects of conservative therapy. Further long-term survival data is required to distinguish whether one form of treatment is advantageous over the others, although case-control studies are difficult for ethical and practical reasons. In this regard the Collaborative Ocular Melanoma Study (COMS) will provide further evidence for the safety and efficacy of conservative therapy with brachytherapy compared to enucleation.