An assessment of two methods for generating automatic regions of interest.

Two fully automatic methods for generating regions of interest (ROIs) for nuclear medicine images are described and assessed. One of these, involving registration of a previously defined ROI onto a new image, uses spatial information and is appropriate for two- and three-dimensional images which may be static or dynamic. The other method is based on artificial neural networks and uses temporal information. It is appropriate for dynamic images only. The registration method has been tested using 10 pairs of stress and redistribution images obtained from cardiac perfusion SPET. Regions of interest of the left ventricular muscle, defined on the stress images, were registered onto the redistribution images, where they were compared with reproducibility of manually drawn ROIs. Both methods were tested on 17 99Tcm-MAG3 kidney dynamic studies, where the original ROIs corresponding to both kidneys and the bladder were defined using the COST B2 hybrid phantom. Our results indicate that neither method is as reliable as having ROIs redrawn by the operator, although there are indications that an artificial neural network which combines the use of the spatial and temporal information could prove useful for dynamic studies.

The electrotonic location of low-resistance intercellular junctions between a pair of giant neurones in the snail Lymnaea.

The passive electrotonic properties of neurones VD1 and RPD2 in the brain of the snail Lymnaea can be represented by a soma-finite cable model with closed-circuit axon termination. There is a considerable individual variation in input resistance, membrane time constant, electrotonic length and axon-soma conductance ratio, but the average values for these parameters are similar in the two neurones. The cells are tightly coupled by an electrotonic synapse giving an average steady-state coupling coefficient of 0.68 and an average resistance measured between recording sites in the cell bodies of 20 M omega. Calculations using a model consisting of a symmetrical pair of cells with standard values for the electrotonic parameters show that in this system, for a soma-soma resistance of 20 M omega, the junction cannot be more than 0.16 length constants from the cell bodies. Reduction in coupling due to membrane current losses in such short proximal axon segments is insignificant. Intra-axonal recordings indicate that most of the coupling resistance is located at the junction between VD1 and RPD2, which must therefore be closer to the cell bodies than the limiting value of 0.16 length constants assuming an electrical equivalent model which includes the standard electrotonic parameters. If all the soma-soma resistance is located at the junction, then it could be physically a single array of gap-junction particles. Despite its low conductance (1/20 M omega = 50 nS) and possibly small physical dimensions, the electrotonic synapse is more than sufficient to ensure spike synchrony in the two cells.

Experimental coupling of crab (Carcinus maenas) second maxilla neural motor to an alternating current.

Movements of the crab gill bailer were entrained to an alternating current applied to the thoracic ganglion. There was little distortion of the muscle recruitment cycle, both absolute and relative coordination were observed, and the phase of the driven system in the driving cycle was a function of the difference between free running and driving frequencies.