Magnetic Resonance Imaging of the Painful Total Hip Arthroplasty

Introduction

Magnetic resonance imaging (MRI) has proved extremely useful for evaluating the painful hip prosthesis. MRI with excellent soft tissue contrast offers significant advantages over computed tomography and plain radiography for evaluating periprosthetic soft tissues and adjacent osseous structures. MRI is more sensitive and specific for the detection of periprosthetic osteolysis, aseptic loosening, and intraarticular burden of particle disease without the harmful effects of ionizing radiation. High-resolution imaging can also detect pathology in adjacent tendons and nerves.

Imaging Techniques

Metallic components rapidly degrade image quality by many processes. Ferromagnetic metal is more easily magnetized than the surrounding soft tissues. This causes regional magnetic field inhomogeneity and degradation of the signal, resulting in signal voids accompanied by surrounding regions of bright signal. This artifact distorts and partially obscures the interface of the prosthesis and adjacent soft tissues.

The magnitude of the artifact is influenced by component shape, orientation, and composition of the metallic alloy. Titanium is less ferromagnetic than cobalt–chromium alloys and results in fewer artifacts. Careful patient positioning with the long axis of the metallic components parallel to the external magnetic field (B ₀ ) and the axis of the frequency-encoding gradients helps to diminish the artifact. This accounts for the improved visualization of the surrounding soft tissue structures around the stem of the femoral component, which is parallel to the external magnetic field, as opposed to the acetabular component, in which the screw or the cement mantle is oriented obliquely relative to the external field ( Fig. 40.1 ).

Artifact produced by the metallic implant can be overcome by optimizing the imaging techniques designed to decrease the effects of rapid dephasing adjacent to the metal implant and those of frequency shifts. Imaging protocols are widely available for all commercial closed, high-field units. The most common modifications are the use of wide receiver bandwidths and variable radiofrequency pulses, which decrease inter-echo spacing. Fast spin-echo techniques allow increased echo train lengths and numerous 180-degree refocusing pulses, which limit signal loss and thereby increase overall signal-to-noise ratio. Short tau (fast) inversion recovery sequences (STIR) are a suitable substitute for frequency-selective, fat-suppression techniques, which are not recommended because local field disturbance occurs in the presence of metal. The use of a small field of view, high-resolution matrix, thin sections, and high gradient strength can help to reduce metal-related artifacts ( Table 40.1 ).

Imaging for Postoperative Complications