A formalism is presented that concisely describes the magnetization of a sample subjected to a periodic series of RF pulses. In this formalism, the steady state of the magnetization is shown to be a sum of magnetic substates, each with unique contrast characteristics. When more than one substate contributes to a given image, the substates interfere with each other, producing ghosts and other artifacts. Properly designed gradient protocols can image single substates, producing ghost-free images. The contrast of the image depends largely on the choice of the imaged substate. Analytic solutions for unspoiled, RF spoiled, and gradient spoiled magnetizations are presented.
The effect of x-ray tube potential and prepatient and interdetector filtration in single exposure dual energy chest imaging has been studied employing a carefully benchmarked model. The analysis utilized published methodology. Noise in simulated lung and mediastinum fields of the aluminum (bone) and Lucite (soft tissue) images were studied at fixed entrance skin exposure (ESE) for commonly employed sandwich detector and sandwich imaging plate configurations. Our results indicate noise in the lung increases slowly with tube potential above 120 kVp, while noise in the mediastinum decreases rapidly. Also, at high tube potential (> or = 120 kVp) adding moderate amounts of prepatient K-edge filtration (approximately equal to 100 mg/cm2) while optimizing imaging conditions for the lung tends to decrease noise in the lungs by approximately equal to 30% while increasing noise in the mediastinum by a similar amount. Without K-edge prepatient filtration, noise in the lung is minimized with Cu interdetector filter weights near 400 mg/cm2. In the mediastinum noise is minimized with heavier interdetector filter and prepatient K-edge filter weights. Prepatient K-edge filter weights that minimize image noise in either field can increase the tube loading by factors ranging from 10 to 10(10). Systems designed with sandwich detectors using commercially available phosphors and coating weights can produce contrast-to-noise ratios (CNRs) as high as 50% of the theoretical limit (defined as an optimized system with a totally absorbing rear detector).
Linear focused grids are commonly used in general radiography and mammography to control scatter. In these applications, if lines would be visible when the grid was stationary, then the grid is moved during the x-ray exposure to blur out grid lines. Presented is a theoretical framework for estimating grid line artifact magnitude and evaluating artifact suppression techniques. The framework takes as parameters the grid pitch, septum thickness, and exposure time, and allows for a variation in grid velocity and in x-ray tube output during the exposure. Grid line artifacts are evaluated for a variety of conditions. These include a stationary grid, a grid moving at a constant velocity with no kV ripple, a grid moving at a constant velocity with large kV ripple, and a grid moving with decreasing velocity and no kV ripple. Also evaluated are grid line artifacts for a novel suppression technique in which the grid moves at a constant velocity and the x-ray exposure waveform is "feathered," i.e., when the x-ray exposure waveform has a soft start and stop. Of practical interest is that it is possible to effectively eliminate grid line artifacts when the grid moves only a short distance with an appropriately "feathered" exposure waveform. This capability permits one to design efficient and compact coarse strip density grid systems.
It has been established that coarse strip density, air-interspace grid systems can suppress scatter in general radiography and in mammography more effectively than conventional high strip density grids. However, such systems have never gained clinical acceptance due to the large distance the grid needs to move to suppress gridline artifacts and due to their corresponding bulk. We present a novel technique for suppressing grid lines using an x-ray exposure wave form with a soft start and soft stop. The wave form is achieved by varying the x-ray tube current during the exposure. We derive the conditions that the time dependence of the x-ray exposure output needs to meet to suppress gridline artifacts with only a modest grid movement. The technique allows for the design of compact coarse strip density grid systems. We present experimental results that demonstrate the feasibility of the technique.
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