Abstract

Metallic implants, such as pedicle screws, are commonly used in orthopedic surgery to fixate fractures, replace arthritic joints, and to align and immobilize vertebra. In the United States alone, there were 325,000 spinal fusions performed in 2003 and 450,000 primary or revision total knee arthroplasties performed in 2002 (1). The current standard imaging test for complications associated with metallic implants is plain radiography. For accurate diagnosis, radiography requires the X-ray beam to be oriented exactly parallel to the bone-implant interface: any obliquity of the X-ray beam can obscure the radiolucent area (2). A better substitute for two-dimensional radiograph is crosssectional imaging that images the entire bone-implant interface in three dimensions. However, the physical characteristics of metallic implants cause difficulties with crosssectional imaging techniques. Computed tomography (CT) and invasive CT myelography suffer from metal-induced streak/beam-hardening artifacts and data loss throughout the field-of-view (FOV) (3). In addition, radiography and CT are relatively insensitive to soft tissue abnormalities and bone marrow edema caused by infection, which are important complications to be evaluated in patients with implants. Magnetic resonance imaging (MRI) is potentially the best imaging modality for diagnosing patients with metallic implants because of its superior soft tissue contrast (4). However, MRI near metallic implants is hampered by severe artifacts, which stem from large metal-induced field inhomogeneities (5), local gradient-induced eddy currents on metal surfaces (6), and radiofrequency (RF) shielding effects (7). Among these, metal-induced field inhomogeneities are responsible for the most severe artifacts. It is well understood that steep field gradients near metal objects result in increased intra-voxel dephasing and a severely shortened T * 2 . As a result, MRI near metallic implants inevitably involves spin-echo (SE) sequences, which refocus the dephased spins. However, SE techniques still suffer from spatially dependent artifacts (e.g., signal voids and pile-ups), owing to a nonlinear frequencyposition mapping caused by metal-induced field inhomogeneities. In the absence of field inhomogeneities, the mapping between a spin's precession frequency and its spatial location is a linear function (dotted line in As illustrated in