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.
The present method of steroe shift for three-dimensional (3-D) reconstruction of radium implants has been found to produce displacement errors. Analysis shows that the maximum displacement error decreases as the shift (S) increases with fixed focal spot-film distance (H), while conversely for a fixed shift distance the maximum displacement error with increased focal spot-film distance. The authors suggest that sufficiently accurate 3-D reconstruction may be achieved when H greater than or equal to 100 cm and H/S less than 2.
A germanium (Ge) camera with laminar collimator-detector geometry, ROLEC, simulating a 195 X 195 mm2 detection area, has been constructed and tested. The detector consists of five separate Ge blocks grooved into distinct electrical channels. Results show that ROLEC measuring times have to be about 3 X greater than Anger camera measuring times to achieve a 5% signal-to-noise ratio with each device. Both spatial and energy resolution of the ROLEC exceed that of the Anger camera but sensitivity variations along the detector strips pose a significant problem in development of a clinically useful device.
Experimental results of a prototype rotating laminar emission camera (ROLEC) for nuclear medicine imaging are reported. A 11.5 mm thick, 45 x 45 mm high-purity germanium detector is segmented into 30 1.47 mm wide parallel channels and collimated with 39 mm high parallel plates. Projection data acquired at multiple angular orientations as the detector-collimator assembly is rotated about its center are mathematically reconstructed to image the activity distribution. The spatial resolution of the ROLEC is at least twice as good, at all distances, as that of gamma cameras with high resolution collimators. The better energy resolution of the germanium enhances the detection and resolution of the ROLEC in comparison to gamma cameras with NaI(T1) crystals, the relative superiority increasing with greater volumes and with greater depths. Adequate sensitivity is maintained while achieving these improvements in spatial resolution and in practice; ROLEC images are acquired in less time than pinhole collimator images with gamma cameras.
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