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Biomedical subjects

L Glaeser

Publications and source records attributed to L Glaeser.

17 recordsLinked to original sources
PubMed1999

Horizontal propagation of visual activity in the synaptic integration field of area 17 neurons.

The receptive field of a visual neuron is classically defined as the region of space (or retina) where a visual stimulus evokes a change in its firing activity. At the cortical level, a challenging issue concerns the roles of feedforward, local recurrent, intracortical, and cortico-cortical feedback connectivity in receptive field properties. Intracellular recordings in cat area 17 showed that the visually evoked synaptic integration field extends over a much larger area than that established on the basis of spike activity. Synaptic depolarizing responses to stimuli flashed at increasing distances from the center of the receptive field decreased in strength, whereas their onset latency increased. These findings suggest that subthreshold responses in the unresponsive region surrounding the classical discharge field result from the integration of visual activation waves spread by slowly conducting horizontal axons within primary visual cortex.

Action Potentials↗
PubMed1996

An intracellular study of space and time representation in primary visual cortical receptive fields.

In contrast with previous knowledge based on extracellular recordings, the recent development of intracellular techniques in vivo (sharp electrode or 'blind patch') ideally allows experimenters to analyze and dissect the contribution of feedforward and lateral connectivity in the functional expression of a synaptic 'integration field'. We will present recent data which demonstrate that the visual receptive field of cortical neurons described at the level of subthreshold synaptic events extends over much larger regions of the visual field than previously thought, and that the capacity of cells to amplify subthreshold responses depends on the immediate past history of their membrane potential. Our data suggest that visual cortical receptive fields should not be considered as a fixed entity but more as a dynamic field of integration and association. Two types of dynamics can be argued for: 1) the spatial structure of the minimal discharge field (defined by suprathreshold activation of the cell) can be profoundly reorganized at least during development and most probably during selective phases of learning under the control of activity-dependent mechanisms. Adaptive changes in visual responses are thought to reflect long-lasting potentiation and/or depression of synaptic efficacies conveying ON- and OFF-center information; and 2) during sensory processing, reconfiguration of synaptic weights may be achieved on a much faster time-scale and linked to nor-linear properties of the postsynaptic membrane as well as that of recruited networks. Association of information available in the central part of the receptive field (RF) and of input coming from the reputedly 'unresponsive' regions surrounding it, or arising simultaneously from different parts of the visual field, might be suppressive in certain cases and capable of boosting hidden responses in other cases, depending on the global stimulus configuration.

Animals↗
PubMed1986

Total body irradiation in Essen--dosimetry and physical treatment planning.

Since 1975, in Essen 109 patients received total body irradiation (TBI) prior to bone marrow transplantation. About 80 patients were treated by bilateral 5.7 MeV photon beams. Three new TBI techniques were developed providing precise, homogeneous, reliable and reasonable a. p./p. a. TBI for adults and children. Systematic TBI dosimetry and the beam-zone method enable for individual treatment planning.

Acute Disease↗
PubMed1986

Influence of lung tissue on the dose distribution of high energy photon beams.

Dose distributions in unit density (water equivalent), low density (lung equivalent) and heterogeneous (water with regions of lung equivalent) materials are described. While interest is focussed on the inhomogeneous situation the homogeneous cases will provide better understanding. In each case it is shown how primary and scattered photons as well as secondary electrons shape the depth dose curve and the lateral dose profiles. The influence of several parameters like lung thickness and density, distance from interfaces, field size, photon energy etc. is discussed. An investigation on dose calculation methods for inhomogeneous phantoms shows to which extent these are suitable for total body irradiation. Possible improvements require further systematical experiments. Because lung tissue differs from soft tissue only in its density, dose measurements in the lung can be handled in the same way as in tissue equivalent materials.

Dose-Response Relationship, Radiation↗
PubMed1982

Irregular field dose determination with the weighted beam-zone method.

For successful radiotherapy, exact application of a certain dose to the target volume is as necessary as optimal sparing of included or neighbouring organs at risk. A new simple but versatile method for rapid dose determination at relevant reference points in target volumes or shielded organs at risk irradiated with irregularly shaped photon fields has proved suitable for clinical routine. The weighted beam-zone method does not need extensive individual dosimetrical measurements or sophisticated treatment planning systems. All essential influences on the absorbed dose, delivered to a regarded dose reference point can be derived from existing dose measurements in square fields, only. Since this method shows the way to include arbitrarily shaped fields into the principle of equivalent square fields with known dose, dose determination at free or shielded on or off-axis points is reduced to simple counting of dose contributing or non contributing weighted beam-zones. The influence of shielded areas on the dose distribution, according to their extent and distance, can obviously be seen. Instant dose estimation in shielded regions shows the efficiency of sparing organs at risk. The practical experiences allow standardization for typical shapes of target volumes, giving the mean reduction of the effective field size, the relative dose to organs at risk and the correct timing for shielding.

Humans↗
PubMed1979

Multidetector endodosimetry probe with silicon-pn-junction-diodes for in-vivo-dosimetry.

In-vivo-dosimetry is necessary to prevent overdosage during radiation treatment techniques of great risk. New special dosemeter probes with up to five Si-pn-junction-diodes in a silicone tube were developed and tested for endodosimetry. They show high efficiency and good linearity of response, without dose-rate dependence. The steep increase of the sensitivity for low energies causes a phantom depth dependence of response. A low temperature dependence but a large anisotropy of response is found for the tested diodes. These multidetector probes proved to be sufficient for direct reading endodosimetric control of dose and dose-distribution during radiotherapy.

Brachytherapy↗
PubMed1978

[Advantages and disadvantages of a 5.7 MeV linear accelerator compared to a 60Co-unit (author's transl)].

A comparison between a 60Co therapy unit and a 5.7 MeV linac shows the following advantages of the accelerator: 1. More favourable depth dose distribution with an improvement of the field homogeneity in the midplane for mantle techniques by a factor of 3 without the use of compensating filters. 2. Change of the maximum from 4 to 12 mm depth and thereby better skin sparing and greater dose tolerance of the skin and subcutaneous tissue. 3. Up to 10 times less penumbra. 4. Increased and stable dose rate of approximately 250 rd/min. At the moment the main disadvantages are the 20 to 30% higher purchase and running costs, and furthermore a downtime of at least 1 week per year for servicing and repair.

Adipose Tissue↗
PubMed1978

[Possibilities, limitations and errors with ultrasound tomography in computer-assisted treatment planning (author's transl)].

Ultrasound tomography provides two-dimensional images, true in scale, of sonographic interfaces within nearly every sectional plane desired; it has become especially important, therefore, in irradiation planning. Complementary to results from a therapy simulator, an improvement in localization of tumor and target volume or of critical organs and of tissue inhomogeneities is possible. The hard-copy of gray-scale sonotomograms furnishes all essential geometric and anatomical input data needed for electronic systems used in irradiation planning. Technical, physical and diagnostic limitations of the method are stated. The possible systematic or technical-instrumental errors in sonographic treatment planning are discussed and the necessary calibration controls specified.

Abdomen↗
PubMed1978

[Homogenization of the dose to the target volume in case of irregular body-surface by means of compensation (author's transl)].

Irregular body-surface contours in the treatment part of high-energetic photon fields are producing an inhomogeneous dose distribution within the target volume. Homogenization of the dose is necessary in order to avoid over- or underdosage. A technique of making compensators is described. These are individual compensation filters substituting tissue layers which are lacking, they are correctly scaled down and optimally corrected in view of absorption and scattering according to the actual irradiation conditions. A simple mechanical device makes possible the production of a mould for heavy-metal compensators immediately in the course of scanning of the body-surface. A universal semi-automatic version of this method is appropriate to clinical practice; it records optically, and therefore without contact and quickly, all body-contours desired. By means of the image of moiré contour-lines thus obtained can be made the mould for the compensator in a second operation using an electronically controlled copy-milling machine.

Filtration↗
PubMed1976

[Importance of sonotomography in radiation therapy (author's transl)].

Ultrasound tomography provides true scale representation of body contours and organ structures. The image supplies substantial, individual geometrical data, essential for computerized radiation treatment planning. The mehtod is described. Typical planning examples for therapy are demonstrated. The value of follow up sonograms for radiation therapy is described. The limitations of the method are pointed out.

Humans↗