NEMA MS 12

Abstract

This standards publication defines test methods for measuring the absolute spatial variation of geometric accuracy within MR images. This standard presents the absolute geometric accuracy as a map, graph, or table throughout the imaging region rather than as simple figures of merit such as average or worst case error. Specifying both the acquisition and data presentation methods is the key function of this standard because the results are not easily reduced to a few simple figures of merit; the results are spatial in nature. This standard deals exclusively with absolute error measurements because it is assumed the end user will need geometric distortion error measurements in absolute versus relative terms. <br>While the intent of this standard is to quantify equipment induced geometric errors only, the phantom used for these measurements will also introduce some geometric errors. It is not possible to remove the phantom induced errors within the scope of this standard, and this standard assumes that the measured errors are exclusively equipment errors. Therefore it is necessary for the user of this standard to be able to differentiate between geometric errors due to the MR imaging system and errors that arise from measuring geometric distortion with a test object. The user should attempt to estimate the error the phantom introduces for the specific test conditions used. <br>This standard also recognizes that these measurements are ideally performed with three dimensional acquisitions and large volume phantoms, but the cost, weight, and size of the required phantom may be prohibitive in certain situations. Therefore, this standard permits the use of a substantially two dimensional phantom in conjunction with a set of two dimensional image acquisitions in different orientations. It is recognized that the use of a two dimensional phantom will fundamentally undersample the three dimensional spatial error map. <br>These procedures could also be helpful in evaluating the impact of system changes on performance, for quality control programs that seek to continually reaffirm system performance, or in demonstrating effectiveness for FDA applications. However, this standard does not supercede MS 2, &#8220;<em>Determination of Two-Dimensional Geometric Distortion in Diagnostic Magnetic Resonance Images.&#8221;</em> Standard MS 2 is designed to produce simple figures of merit that describes basic geometric distortions, or image field of view errors, that could arise from imaging gradient amplitude scaling errors.