As with other digital imaging systems in heavy medical use, it is desirable with magnetic resonance imaging (MRI) to obtain extensive, rigorous system performance measures from a small set of images of one or two relatively simple test objects. Digital analysis of the parallel square rod (PSR) test object introduces digital image system self‐evaluation to MRI and extends automated image evaluation to include rigorous measures throughout the imaging volume rather than just average measures over the image. Precise comparisons with theory and between systems can be performed as well as quality control and corrections for nonuniformities. The PSR test object consists of an 18×18×36 cm rectangular acrylic container enclosing 60 parallel square acrylic rods running the entire length. The inter‐rod space is filled with a liquid or gel that produces strong, tissuelike signals in MRI and high contrast relative to the rods for computed tomography (CT). For profiles of slice thickness and separation, the rods are tilted in the test object to intersect the image plane at a 45° angle when the test object sides are parallel and perpendicular to the image plane. The test object itself is rotated 6–12° about its major axis so that the sides of the rods make a small angle to the rows and columns of pixels. This allows digital sampling at finer spacing than the pixels for determination of edge response functions. Over the 25–49 blocks in each slice of the imaged volume, maxima, minima, mean values, variances, and ratios currently are reported for the following variables: signal‐to‐noise ratio and sensitivity, linear and nonlinear image distortion, full width at half maximum (FWHM) resolution of the point spread function (PSF), slice separation, and slice thickness. These performance values at each rod or edge are displayed as gray scale functional images. Individual rod values are recorded and plotted as histograms and profiles. Results of the automated analysis for MRI system examples are in good agreement with expectations from theory and more manual tests.
Abstract Bill Gray of Project Pluto brought to our attention an error of 0.03° in the listed latitude of our Kitt Peak telescope. While correcting the table where this occurred, we also take the opportunity to update the instrument properties and weather statistics of our remote telescopes.
A three-dimensional simulation method has been developed for evaluating the potential to quantify myocardial function with PET (positron emission tomography) using a three-dimensional beating heart phantom. Expected tracer kinetics are assigned regionally throughout the heart phantom and tomographic simulation methods are utilized to form a dynamic sequence of images. Analysis of the simulated data using standard kinetic evaluation tools provides estimates of bias and variance in the kinetic parameters as a function of emission and transmission scan statistics, image scatter fraction, and placement of regions of interest. Comparison of the simulation results with measured PET data demonstrates the validity of the simulation approach and its application in the evaluation of quantitative myocardial PET studies.< >
Using an image subtraction method we have searched for variable stars in the globular cluster M14. We confirmed 62 previously known catalogued variables. In addition to the previoulsy known variables we have identified 71 new variables. We have confirmed the periods of most of the cataloged variables with just a few exceptions. Of the the total number of confirmed variables, we found a total of 112 RR Lyrae stars, several of which exhibited the Blazhko Effect. Of the total we classified 55 RR0, 57 RR1, 19 variables with periods greater than 2 days, a W UMa contact binary, and an SX Phe star. We present the periods of previously found variables as well as the periods, classification, and lightcurves of the newly discovered variables.
