Measurement of radiation isocenter is a fundamental part of commissioning and quality assurance (QA) for a linear accelerator (linac). In this work we present an automated procedure for the analysis of the stars-shots employed in the radiation isocenter determination. Once the star-shot film has been developed and digitized, the resulting image is analyzed by scanning concentric circles centered around the intersection of the lasers that had been previously marked on the film. The center and the radius of the minimum circle intersecting the central rays are determined with an accuracy and precision better than 1% of the pixel size. The procedure is applied to the position and size determination of the radiation isocenter by means of the analysis of star-shots, placed in different planes with respect to the gantry, couch and collimator rotation axes.
This paper describes the study of the effects of radiation damage on the quality of data collected from a protein crystal at 100 K. It is shown that radiation damage causes measurable effects in the diffraction pattern. This implies that, even at liquid nitrogen temperatures, there is a limit to the size of a crystal from which a complete data set can be collected.
Charge-coupled device (CCD)-based X-ray detectors allow data to be collected much more quickly (--,10 times) than with current on-line imaging-plate systems. At the ESRF, X-ray image intensifier/CCD detector systems have been developed. These have great potential as fast read-out detectors for macromolecular and other forms of crystallography. They are relatively large sensitive X-ray detectors but have two inherent weaknesses: convex detection surfaces leading to spatial distortion and non-uniformity of intensity response, and susceptibility to small changes in magnetic fields. A large improvement has been made to the accuracy obtained by non-uniformity of response calibration and correction, using fluorescence from doped lithium borate glasses. Monochromatic macromolecular crystallography demonstration experiments with external user groups have shown that high-quality results may be obtained under real experimental conditions.
A novel intensity uniformity calibration method for area X-ray detectors is described. In diffraction experiments, amorphous lithium glass plates, containing doping elements chosen for their K edges just below the energy of the main beam, replace the crystallographic samples for the calibration measurement. The fluorescent emission excited by the X-ray beam is almost isotropic. It has exactly the same geometry as the diffracted radiation, and can be obtained at the same wavelength by proper selection of the element and excitation energy. A sample 2theta scan allows the emission distribution as a function of angle to be characterized with an accuracy of a fraction of a percent. This allows a flat-field correction of similar accuracy. The quality of crystallographic data collected with an X-ray image intensifier/CCD detector was significantly improved by flat-field correction using an Sr-doped lithium tetraborate glass. This technique can be applied to X-ray energies from 5 to 50 KeV; the calibration sample is small, stable and easily handled.
A model for the description of the near-field dose deposition from a 32p impregnated stent in an arterial system consisting of soft tissue and dense plaque is presented. The model is based on the scaling property of the dose-point-kernel (DPK) function which is extended to a heterogeneous medium consisting of a series of layers of different materials. It is shown that, for each point source originating from the stent surface, the DPK function for water can be scaled consistently along the path through the different layers of material to predict the dose at a given point in the heterogeneous medium. Radiochromic film dosimetry on actual 32p stents is used to test the new model. The experimental setup consists of a water-equivalent phantom in which a stent is deployed and on which a thin layer of polytetrafluoroethylene (PTFE) is deposited to simulate the presence of plaque. Layers of radiochromic films stacked over the phantom are used to measure the dose at distances varying from approximately 0.1 mm to approximately 3 mm from the stent surface with and without PTFE. It is shown that the proposed new DPK model for a heterogeneous medium agrees very well with the experimental data and that it compares favorably to the usual homogeneous DPK model. These results indicate that the new model can be used with confidence to predict the dose in a realistic artery in the presence of plaque.
The BaFBrI:Eu2+ storage phosphor plate (SPP) is a reusable radiation image detector, widely used in diagnostic computed radiography, x-ray crystallography and radioactive tracer studies. When exposed to ionizing radiation, the SPP stores a latent image until it is scanned with a red reading laser which causes blue photostimulated luminescent (PSL) photons to be emitted. The mechanism of formation of the latent image is still poorly understood, especially for megavoltage photon beams. In order to gain insight into this mechanism and aid applications to high-energy beam dosimetry, the authors have directly determined the SPP generation efficiency, W, the energy required to produce one quantum of emitted PSL when it is irradiated by 60Co and 6 MV photon beams. This was done in four steps: 1. The SPP, in a water-equivalent plastic (WEP) phantom, was exposed to a 60Co or 6 MV beam, which had been calibrated to give a known absorbed dose to water in a water phantom at the position of the sensitive layer of the SPP. 2. Monte Carlo simulations were used to calculate the ratio of the dose to the sensitive layer in the WEP phantom to the dose to water at the same position in a water phantom. 3. A bleaching experiment was used to determine the number of photons emitted by a plate given a known dose. 4. The generation efficiency was calculated from the number of photons and the dose. This method is much more direct than previous calculations for kilovoltage x-ray beams based on quantum noise analysis. W was found, within experimental uncertainty, to be 190 eV for 60Co and 160 eV for 6 MV, independent of dose. The values for kilovoltage x-ray beams determined previously agree, within their large uncertainty, with these values for megavoltage beams.
We report a case of gamma knife pallidotomy resulting in a permanent contralateral homonymous hemianopsia and transient contralateral hemiparesis with some improvement in contralateral parkinsonian symptoms. This case illustrates the risk of gamma knife pallidotomy which precludes physiologic target localization and can subject structures surrounding the target to a significant radiosurgical dose. Until noninvasive physiologic target localization is available gamma knife pallidotomy and thalamotomy should be limited to patients with an unacceptably high risk for stereotactic percutaneous thermocoagulation.
Large area flat crystal x-ray spectrometer with high integrated intensity for an electron beam ion trap Rev. Sci. Instrum. 71, 4065 (2000)
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