A quality control (QC) test suitable for routine daily use has been developed for video based electronic portal imaging devices. It provides an objective and quantitative test for acceptable image quality on the basis of the high contrast spatial resolution and the contrast-to-noise ratio (CNR). The test uses a phantom consisting of five sets of high-contrast rectangular bar patterns with spatial frequencies of 0.1, 0.2, 0.25, 0.4, and 0.75 lp/mm. Data obtained during a one month calibration period were used to determine a critical frequency fc for the relative square wave modulation transfer function and a critical contrast-to-noise ratio (CNRc). Subsequent measurements indicating significant deviations from these critical values result in warning messages to the operator indicating potential problems in system performance. Measurements over a period of two years show that the QC test provides a sensitive indication of imaging performance.

The physics of imaging with metal/phosphor (Gd2O2S:Tb on brass) screens at megavoltage energies has been investigated using Monte Carlo simulation. It has been found that pair production is a significant contributor to energy deposition for Bremsstrahlung beams with energies greater than 6 MV. The effects of different thicknesses of phosphor and metal have been studied, and it is shown that the metal plays a significant role in establishing electronic equilibrium in the phosphor. The transport of optical photons through the phosphor has been modeled, and was found that only 10% to 20% of the light created in the phosphor escapes from the surface, with much of the loss being due to total internal reflection at the surface. Calculated results have been compared with experimental measurements of screen brightness for different phosphor and metal thicknesses. The SNR of a video electronic portal imaging device (VEPID) has been calculated as a function of x-ray and optical photon detection efficiency. The non-Poisson distribution of energy deposition in the phosphor is an important contributor to the SNR. The results of this paper should serve as a useful guide to the engineering design of future electronic portal imaging systems.

The dosimetric characteristics of a scanning liquid-filled ionization chamber (SLIC) electronic portal imaging device have been investigated. To assess the system's response in relation to incident radiation beam intensity, a series of characteristic curves are obtained for various field sizes and nominal energies of 6 and 10 MV photons. The response of the imaging system is dependent on incident radiation intensity and can be described to within 1% accuracy on central axis using a square root function. Portal dose measurements with the SLIC at the plane of the detector, on central axis of the beam using homogeneous attenuating phantom materials show that the imaging system is capable of measuring the portal (transmission) dose to within 3% of the ionization chamber results for homogeneous material. For two-dimensional dosimetry applications, the system is calibrated with a 10 cm Perspex block used as beam flattening material on the detector cassette to correct for variations in individual ion chamber sensitivity and the effect of nonuniform beam profiles produced by the flattening filter. Open and wedged dose profiles measured with the SLIC agreed with ion chamber measured profiles to within 3.5% accuracy.

In Medical Imaging Physics, the authors cover the spectrum from radiation physics to computed tomography to radiobiology, while including some basic statistics and image analysis, in a style that would be comfortable for resident physicians in radiology, radiological technologists, and students beginning a career in medical physics and radiation biology. The math requirements are minimal, but sufficient for the material presented. In this book, the authors bring together a number of areas that one would normally find only in several separate books. The authors introduce the reader to ͑and explain at the appropriate level of complex-ity͒ basics of radiation ͑its production, its interactions, and its detection͒, types of imaging ͑nuclear medicine, radiography, CT, MR, ultrasound͒, image quality ͑resolution, noise, visual perception͒, and radiobiology. The chapters on radiation provide a good presentation of radioactive decay and interactions with matter, with various levels of depth being provided by the text and the side-bars. The discussion of x-ray imaging was comparable to that seen in other texts. Image quality was covered with a comprehensiveness that was welcome. The chapters on ultrasound and magnetic resonance are very nice introductions to these technologies, written at a level that nonpractitioners can readily grasp. In addition to the theory, the authors discuss the technology used in the various fields: nuclear detectors and instrumentation, ultrasound transducers and instrumentation, and MRI spectroscopy and instrumentation, as well as new developments in imaging. The radiobiology chapters could be especially useful since these issues are now in the public consciousness and would help address questions that can arise during in-services.

Studies were conducted to determine the optimal metal/phosphor screen for on-line video verification of radiation treatment portals. Screens were evaluated for luminance and spatial resolution as a function of composition and thickness at 6- and 23-MV x-ray energies. A new video technique was used to determine modulation transfer functions. Gd2O2S was found to be the most efficient (brightest) phosphor for this application. Luminance was found to vary linearly with phosphor thickness up to a thickness of 500 mg/cm2. Metal plates made of iron, brass, copper, lead, and sintered tungsten of various thicknesses were also tested for luminance and resolution with Gd2O2S phosphor. Brightness peaked at about 2-mm thickness for most metals. Significant contributions to the brightness were found to come from x rays interacting with the phosphor itself.