Elsevier

Magnetic Resonance Imaging

Volume 22, Issue 8, October 2004, Pages 1097-1103
Magnetic Resonance Imaging

Original contribution
MRI of hip prostheses using single-point methods: In vitro studies towards the artifact-free imaging of individuals with metal implants

Partof this work was presented at the 20th Annual Meeting of the ESMRMB,Rotterdam (The Netherlands), September 2003.
https://doi.org/10.1016/j.mri.2004.01.061Get rights and content

Abstract

Use of magnetic resonance imaging (MRI) in individuals with orthopedic implants is limited because of the large distortions caused by metallic components. As a possible solution for this problem, we suggest the use of single-point imaging (SPI) methods, which are immune to the susceptibility artifacts observed with conventional MRI methods. A further advantage of SPI, based on the fact that signal encoding is achieved in ultra-short times (as short as tens of microseconds), is that they enable the direct visualization of the polymeric elements of the implants, allowing the detection of possible implant failures. We present in vitro SPI images of polymeric sockets of two hip prostheses together with artifact-free images of gelatin phantoms containing their respective metallic stems. These data underscore the great potential of the SPI technique for obtaining artifact-free images of individuals with large metal implants.

Introduction

Replacement of destroyed or seriously damaged parts of the human body by prostheses has been a common practice since ancient times. Design of prostheses and selection of their constituent materials is usually based on factors like mimicry, functionality and durability. Nowadays, it is usual to employ polymers and ceramics to build implants, but metals still are used in most of them. This presence of metal pieces is the main reason for the limited use of magnetic resonance imaging (MRI) on individuals with prostheses, because of the distortions they cause in the local magnetic field, yielding strong image artifacts. Since those distortions are proportional to the size of the metal piece, this problem is especially acute in individuals with a hip prosthesis (one of the largest implants used in orthopedics). On the other hand, total hip arthroplasty (THA) is the most practiced orthopedic surgery procedure in elderly patients, with more than 800,000 THA performances every year around the world [1].

THA involves the replacement of the head of the femur by a metallic sphere attached to a stem (Fig. 1a,b) and, simultaneously, the insertion of a polymeric (or metallic) socket (Fig. 1c,d) in the acetabulum of the hip to encourage mobility and reduce friction. It is common practice to fix both elements of the prosthesis to the bone surfaces by using a polymeric bone cement.

Occasionally, THA patients suffer from continuous and increasing pain on the hip. In that case, it is essential to know whether this problem is related to alterations in the structure of the implant (e.g., dislocation, loosening or irregular wear) or to pathological processes in the surrounding tissues (e.g., infection, edema, or bone sclerosis) [2]. Although MRI could, in principle, contribute to the diagnosis of these complications, its use has been very limited because the severe distortions in conventional MRI images caused by the metal parts of the implant can hide abnormalities in the adjacent tissues. In spite of the importance of this problem, it is generally assumed that MRI is not suitable for patients with large metal implants. To the best of our knowledge, only few MRI studies have addressed this issue (for a review see [1]). These authors have shown that by using strong gradients, reduced slice thickness, and positioning the prosthesis longitudinally to the axis of the main magnetic field, it is possible to obtain in vitro and in vivo images with reasonable quality where metal artifacts are reduced but not completely eliminated. Similar results have been reported recently by Guermazi et al. [3]. Viano et al. [4] proposed the use of tilted readouts (aligning them with the longest axis of the implant) instead of physically orienting the patient inside the magnet.

We suggest a different approach to the problem, involving the use of the nonconventional single-point imaging (SPI) technique [5]. SPI is a solid-state MRI methodology with the main advantage over other solid-state techniques that is implementable on standard scanners. The usefulness of SPI methods has been demonstrated in solid and solid-like materials such as cement, food, polymers and hard tissues like bone or teeth (for a recent review see [6]). The physical principles on which SPI is based make it immune to most miss-registration artifacts, including distortions arising from susceptibility effects [7], [8], [9]. It is this feature of SPI techniques that we explore in this work, reporting for the first time images of large metal orthopedic implants completely free from distortions due to susceptibility effects, despite the 4.7-Tesla magnetic field strength used.

Section snippets

Single-point imaging methods

A detailed description of SPI methods can be found elsewhere [10], [11] and only some important aspects are described here.

Conventional MRI methods, which require several milliseconds for signal preparation and acquisition, fail to image solid materials because NMR signal from these systems vanishes before it can be completely recorded (ultra-short T2 values, ranging from tens to hundreds of μs). In SPI methods, this problem is solved by acquiring only one point of the free induction decay

Determination of T2* values of polymeric elements

Our first experiment consisted of measuring T2* values of the polymeric sockets (Fig. 1) by acquiring a series of 20 images using encoding times ranging from 40 μs (shortest echo-time [TE] achievable in our system) to 90 μs in steps of 2.5 μs. Representative 2D slices were selected for each socket and mean pixel intensities of two different regions of interest (PE and PMMA) were plotted versus the encoding time (Fig. 2). The decay curves were fitted using a mono-exponential model. The PE

Conclusions

It was shown that even in the presence of very large metal pieces and air pockets, SPI methods provide images completely free of distortions caused by susceptibility effects. The ultra-short encoding times required in these techniques also allow the direct visualization of polymeric materials like the ones used in the construction of prostheses. When a system with long T2 values (e.g., tissues in humans) is also present in the FOV, the intense signal produced by such elements will obscure the

Acknowledgements

We are grateful to Dr. Manuel Ramos Vivero (Servicio de Traumatologı́a, Complexo Hospitalario Xeral-Calde, Lugo, Spain) for sharing his knowledge on hip prostheses. This work has been supported by BTS Dutch program (Dutch Ministry of Economical affairs). Project BTS00103.

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