Results 1 - 10 of 9558
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[en] The description is given of an irradiator for the line by line examination of slices of a body to be examined. This appliance includes a radioactive source housed in a protection block absorbing the ionizing radiations, except along a direction defining each line of examination. This protection block which for this purpose comprises an internal collimation aperture is characterized in that the radioactive source is housed in a source holder located in the protection block and can be moved in the protection block by means of controls so as to have at least two positions: a so called ''active'' first position in which the source is aligned with the internal collimation aperture of the protection block and a so called ''obturated'' second position where the source is no longer aligned with the internal collimation aperture
[fr]On decrit un irradiateur pour l'examen ligne par ligne, de sections d'un corps a examiner qui comporte une source radioactive logee dans un bloc de protection absorbant les rayonnements ionisants, sauf le long d'une direction definissant chaque ligne d'examen, le bloc de protection qui, comporte pour cela, un trou de collimation interne, est caracterise en ce que la source radioactive est logee dans un porte-source dispose dans le bloc de protection, ce porte-source etant susceptible de se deplacer dans le bloc de protection sous l'action de moyens de commandes de maniere a avoir au moins deux positions: une premiere position dite ''active'' dans laquelle la source est alignee avec le trou de collimation interne du bloc de protection, et une deuxieme position dite ''obturee'' dans laquelle la source n'est plus alignee avec le trou de collimation interne
[en] In radiotherapy apparatus using a multileaf collimator, the adjustment positions of the individual leaves can be determined optically by means of a video camera which observes the leaves via a radiation transparent mirror in the beam path. In order to overcome problems of low contrast and varying object brightness, the improvement comprises adding retroreflectors to the collimator leaves whose positions are known relative to the inner edge of the respective leaf. The retroreflectors can extend along the length of the leaf or they can be small. For setting up, corresponding manually adjustable optical diaphragm leaves can be used to project an optical simulation of the treatment area onto the patient, retroreflectors being similarly located relative to the shadow-casting edge of the leaves. (author)
[en] A collimator for a shielded effective point source of penetrating radiation, e.g. a radiotherapy machine which includes, after a primary collimator, a succession of a multileaf collimator, and a first and a second pair of independently adjustable block diaphragm leaves at right angles to each other. By using rectilinear displacement of the multileaf collimator leaves and the block diaphragm leaves a compact arrangement can be provided which fits into a standard RT collimator head. By curving the inner edges of the leaves to be tangential to the boundary ray an optimally reduced penumbra can be achieved. The block diaphragms enable the radiation leakage found in prior multileaf collimators to be minimised for small and off centre irradiation patterns. (author)
[en] This invention provides a highly collimated radiation beam capable of being effectively transmitted a large distance to a detector and a method and apparatus using such a beam and detector
[en] A filtered backprojection algorithm is developed for single photon emission computed tomography (SPECT) imaging with an astigmatic collimator having a displaced center of rotation. The astigmatic collimator has two perpendicular focal lines, one that is parallel to the axis of rotation of the gamma camera and one that is perpendicular to this axis. Using SPECT simulations of projection data from a hot rod phantom and point source arrays, it is found that a lack of incorporation of the mechanical shift in the reconstruction algorithm causes errors and artifacts in reconstructed SPECT images. The collimator and acquisition parameters in the astigmatic reconstruction formula, which include focal lengths, radius of rotation, and mechanical shifts, are often partly unknown and can be determined using the projections of a point source at various projection angles. The accurate determination of these parameters by a least squares fitting technique using projection data from numerically simulated SPECT acquisitions is studied. These studies show that the accuracy of parameter determination is improved as the distance between the point source and the axis of rotation of the gamma camera is increased. The focal length to the focal line perpendicular to the axis of rotation is determined more accurately than the focal length to the focal line parallel to this axis. copyright 1998 American Association of Physicists in Medicine
[en] Using the electromagnetic simulation code ECHO, we have found a simple phenomenological formula that accurately describes the loss factor for short bunches traversing an axisymmetric tapered collimator. In this paper, we consider tapered collimators with rectangular cross-section and use the GdfidL code to calculate the loss factor dependence on the geometric parameters for short bunches. The results for both axisymmetric and rectangular collimators are discussed. The behaviour of the impedance of tapered structures for very short bunches in the optical regime has been determined in refs. (10,11). Here, for the loss factors for two particular geometries, we have studied the departure from the optical regime behaviour as bunch length is increased. In both cases, the ratio of the loss factor for the tapered collimator to the loss factor in the optical regime is a function only of the scaling parameter σL/d2. The fact that the bunch length a and the taper length L appear as a product is consistent with the recent scaling derived by Stupakov in ref. (12), since there is only a weak dependence on g. One noteworthy fact that is not a priori expected is that only the larger radius or vertical half-aperture d appears. The reduction factor is independent of b. Moreover, it is striking that the specific form involving the arctan given in Eq. (5) holds for both geometries, with only the coefficient μ differing by a factor of ∼2 for flat vs round. This suggests that there may be a useful phenomenological form for more general geometries which may follow from natural extensions of Eq. (5). This possibility is presently being investigated.
[en] Accurate determination of collimator output factors is important for Leksell Gamma Knife radiosurgery. The new Leksell Gamma Knife (registered) Perfexion(TM) system has a completely redesigned collimator system and the collimator output factors are different from the other Leksell Gamma Knife (registered) models. In this study, a simple method was developed to validate the collimator output factors specifically for Leksell Gamma Knife (registered) Perfexion(TM). The method uses double-shot exposures on a single film to eliminate repeated setups and the necessity to acquire dose calibration curves required for the traditional film exposure method. Using the method, the collimator output factors with respect to the 16 mm collimator were measured to be 0.929 ± 0.009 and 0.817 ± 0.012 for the 8 mm and the 4 mm collimator, respectively. These values are in agreement (within 2%) with the default values of 0.924 and 0.805 in the Leksell Gamma Plan (registered) treatment planning system. These values also agree with recently published results of 0.917 (8 mm collimator) and 0.818 (4 mm collimator) obtained from the traditional methods. Given the efficiency of the method, measurement and validation of the collimator output factors can be readily adopted in commissioning and quality assurance of a Leksell Gamma Knife (registered) Perfexion(TM) system.
[en] Described herein is a method for making a low energy gamma ray collimator which involves corrugating lead foil strips by passing them through pinion wire rollers and gluing corrugated strips between straight strips using an adhesive such as epoxy to build up a honeycomb-like structure. A thin aluminum sheet is glued to both edges of the strips to protect them and to provide a more rigid assembly which may be sawed to a desired shape. (Patent Office Record)