A method to measure the MTF of digital x-ray systems.

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.

Noise correlations in images acquired simultaneously with a dual-energy sandwich detector.

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.

Performance characteristics of a dual-energy detector for digital scan projection radiography.

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%.

Measured performance characteristics of a solid-state linear detector array.

Experimental studies of a solid-state linear detector array developed for a prototype scanning slit digital chest radiographic unit have been completed. The detector consists of a strip of scintillating material, optically coupled to a linear silicon photodiode array. Measured performance characteristics of the detector, such as sensitivity, modulation transfer function, and detective quantum efficiency, are presented for several different scintillators. Results indicate that direct x-ray absorption events in the silicon photodiode can degrade detective quantum efficiency. Results also indicate that the inexpensive preamplifier circuits used in the digital chest prototype contribute negligible noise at diagnostic x-ray photon fluence rates.

A laser-based multiformat camera for medical imaging.

The spatial resolution and contrast resolution required of a multiformat camera (MFC) for medical imaging are discussed. A typical cathode-ray tube (CRT) MFC and a prototype laser MFC are compared based on the following measured quantities: line spread function and associated contrast transfer function, noise characteristics, intensity transfer function (dynamic range), large-area contrast, and film irradiance. The laser MFC is found to provide significantly better performance than the CRT MFC in all of these areas.

Digital chest radiography: performance evaluation of a prototype unit.

Measurements of the physical performance of a prototype digital chest unit (DCU) are presented. The parameters evaluated were entrance skin exposure, system exposure response and dynamic range, system modulation transfer function (MTF), image noise levels, detective quantum efficiency (DQE) of the detector, and scatter suppression efficiency. Compared with conventional chest imaging systems, the unit has markedly greater exposure latitude, limited spatial resolution, a lower detector DQE, and virtually scatter-free images. Routine clinical exposure levels are comparable with the 1982 national average.

Single-slit digital radiography: some practical considerations.

Clinical evaluation of an experimental digital chest unit using a solid-state linear detector array has provided information regarding the postpatient photon flux required for clinically acceptable images. Results from computer simulation show that energy subtraction imaging of the chest by the method of x-ray-tube voltage switching will be unsuccessful unless it can be demonstrated clinically that a much lower number of photons per pixel is acceptable in energy subtracted images. In addition, x-ray-tube loading limitations preclude imaging of the abdominal and pelvic regions with this technology.

A detector for scanned projection radiography.

A solid-state electronic x-ray detector, the multilinear array (MLA), was developed for general-purpose digital overhead radiography (DOR). The image sensor of the MLA consists of an x-ray phosphor affixed to approximately 29,000 photodiodes. The image sensor is rectangular (35.2 cm X 1.05 cm) and is designed for scanned projection imaging with a 1.05-cm-thick fan beam. The MLA incorporates charge-coupled device circuitry that performs time-delay integration of the photocharge. The MLA is installed in a prototype DOR system that generates a 35.2-cm X 35.8-cm image with a 2,014 X 2,048 12-bit image matrix and 0.175-mm pixels. The scan time per image is 7 seconds, and the exposure time of any given point of a patient is 0.21 seconds. Operational principles of the MLA are described, performance measurements presented, and images acquired with the prototype DOR system shown. The MLA has an exposure latitude of 333:1 10% modulation transfer function response at 1.8 cycles per millimeter, 15% detector detective quantum efficiency (DQE) at 60 keV, and 78% scatter DQE under high-scatter imaging conditions. DOR images of a contrast-detail phantom are superior to those produced by conventional medium-speed screen-film radiography.

Detector for dual-energy digital radiography.

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.

Digital radiography of the chest: design features and considerations for a prototype unit.

The general features of a prototype digital chest unit are described along with the rationale for the choice of design factors employed. It is shown that the scanning-slit, linear-detector-array approach employed can, with available x-ray tube technology, achieve a spatial resolution of 1 cy/mm and detector radiation levels comparable with those obtained with conventional screen-film systems. Also discussed are the unit's exposure latitude and its ability virtually to eliminate scatter.