A Simple Method to Visualize Image Quality Degradation for Off-Center Regions in Open-Configuration MR Systems

Abstract

INTRODUCTION For studies in obese patients, openconfiguration MR units have become more attractive than the restricted tunnel-type systems commonly used at clinical sites for diagnosis because of their easy access and nonclaustrophobic system configuration [1]. In addition, open-type systems allow the use of additional equipment inpatient positioning andmonitoring, and dedicated magnet designs are available that provide a significant cost advantage in musculoskeletal MRI, MR-guided biopsy, and therapeutic applications [2-4]. However, image degradation, such as geometric distortion and the associated nonuniform intensity caused by field inhomogeneity and the nonlinearity of imaging gradients, hampers the clinical application of open systems [5]. Such image degradation leads to erroneous measurement of tissues of interest in obese patients and in musculoskeletal imaging. Spatial distortion also presents a serious obstacle to the use of MRI as a reliable tool for biopsy and therapeutic applications. Mainfield homogeneity and gradientfield linearity are guaranteed at or near the isocenter relative to points far from the center because MRI systems are typically designed with homogeneous magnetic fields near the isocenter [6]. Magnetic-field inhomogeneity and gradient nonlinearity, however, gradually increase toward the edge of the main magnetic field. In particular, open MR scanners that use vertical magnets and flatdesign gradient coils have larger distortion and greater nonuniformity