A detection scheme is described that allows one to accomplish dual-energy scanned projection digital radiography without switching the x-ray tube voltage. The method employs a high/low atomic number detector sandwich that simultaneously separates the x-ray beam transmitted by the patient into low and high energy components. To test the method, the response of a scanning linear array of energy-sensitive detectors was simulated, and bone and soft tissue images of an anthropomorphic chest phantom were obtained at 140 kVp. These were compared with similar images obtained by switching the x-ray tube voltage from 80 kVp to a heavily filtered 140 kVp. For comparable entrance skin exposures, the dual-energy detector images required a lower tube load and resulted in higher noise levels. The latter is attributable to the fact that the separation in energy between the high and low energy components is smaller with the dual-energy detector than with the voltage switching technique, and to misregistration problems associated with the simulation methodology. A detector design is also discussed that would result in improved energy separation and lower noise levels. In view of this possibility and the tube loading advantage, the method looks promising for digital scanned projection radiography.
A method has been devised to accurately measure the modulation transfer function (MTF) of digital x-ray systems up to and, for undersampled systems, beyond the pixel Nyquist frequency (fN). A phantom consisting of an array of parallel tungsten or similar wires is imaged, and discrete Fourier transforms of rows of pixel values are computed. Under suitable conditions of phantom orientation, wire diameter, wire spacing, and image magnification, the envelope of the modulus of the mean Fourier transform represents the system MTF. Experimental results extending beyond fN are presented for an undersampled prototype digital chest x-ray system and shown to be in reasonable agreement with predicted values. Employment of the method with other digital imaging modalities [i.e., computerized tomography (CT) scanners and nuclear magnetic resonance (NMR) units] is also discussed as well as error considerations and practical problems in implementing the method.
An energy discriminating detector for dual-energy radiography can be configured as a two-layer sandwich, where the mean energy of photons detected by the two layers differs. To characterize the quantum noise of such a detector, the noise covariance between the two layers must be known in addition to the noise variance in each layer. A theory is presented which permits the calculation of the noise covariance, and it is found to be negligibly small. Experimental results, based on measurements with a Na1 sandwich detector and an isotope gamma ray source, are reported and shown to confirm the theory. The quantum noise in each layer is independent and Poisson.
We calculate the lamellar period L and interphase thickness a of an incompressible melt of symmetric diblock copolymers from the onset of phase segregation (weak segregation limit) to the limit where segregation is almost complete (strong segregation limit) by numerically solving a mean field lattice model, where the lattice spacing is taken small enough to approximate a continuum. Our results for L and a agree with previously derived theoretical formulas in both limits. Based on the first few terms of a Fourier series expansion of the density profile, we show analytically that L = 0.844R(xN)o m at the weak segregation limit, where R is the unperturbed molecular radius of gyration, the Flory interaction parameter, and N the number of statistical segments per molecule; the compressibility of the melt-even in the incompressible limit-must be taken into consideration to get the correct dependence of L on , and the Fourier series approximation turns out to be accurate over a very small range of .
An energy discriminating x-ray detector has been developed for dual-energy, scan projection digital radiography. The detector is comprised of a pair of x-ray intensifying screen/linear photodiode arrays, aligned one behind the other. Energy discrimination is achieved by employing a low atomic number phosphor in the front screen and a high atomic number phosphor in the back screen. The x-ray response, modulation transfer function, and defective quantum efficiency of the detector are reported along with the experimental methodology utilized for the measurements. Also presented is an analysis which indicates that in a typical patient's lung field, the detector can resolve the projected density (g/cm2) of a 3-mm-thick, 1-cm2 area of bone to better than 1.5%.
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