A dosimetry system for computed tomography (CT) x-ray scanners has been developed featuring X-Y as well as Z-axis directional exposure measurement capabilities. The device is unique because it uses an array of closely spaced silicon p-i-n diodes as radiation detectors. This arrangement allows detailed mapping of dose levels along the length of the detector module.Computed tomography scanners are a new generation of diagnostic radiology devices using highly collimated x-ray beams that pass through a body section axially.The photons not absorbed are detected (depend-ing on the manufacturer's model) by scintillating crystals, ionization chambers, or semiconductor materials, and their number is a function of the density of the material they must traverse. The x-ray tube and detectors generally rotate in unison at opposite sides around the patient while collecting absorption data for later reconstruction by computer and display.viiiPrior to the development of this dosimeter, no simple and convenient method was available for performing exposure distribution measurements on CT scanners. Previous measurement methods have included the use of photographic film, pencil-type ionization chambers, or an array of as many as 300 to 500 thermoluminescent dosimeter chips.Silicon diodes are superior to these methods because of their email size, higher photon stopping power, linear energy response, and minimalx-ray beam directional dependence.The system described features 25 evenly spaced silicon diode detectors each with its own current-to-voltage converter. The amplified output from each diode is transferred to the data handling module that integrates the signal and provides permanent storage via a thermal printer.Calibration factors can be applied allowing the readout to correspond to the radiation exposure in Roentgens received by the diodes during the course of a CT scan. The dynamic range of the instrument enables it to measure exposures in fields as high as 100 R/sec and to present the integrated exposure information in three ranges of 0-1, 10, and 100 R.
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