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.
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 ).
|Parameters||Axial FSE/TSE |
|Coronal IR |
|Coronal FSE/TSE |
|Axial FSE/TSE |
|Sagittal FSE/TSE |
|TE (msec)||28-34||30||TR (msec)||28-34||28-34|
|Matrix||384 × 256||256 × 192||384 × 300||384 × 256||384 × 300|
|Slice thickness (mm)||5||5||4||4||4|
Imaging for Postoperative Complications
After total hip arthroplasty (THA), repetitive articular motion exerts substantial pressure on the high-molecular-weight polyethylene insert of the acetabular component and on the metallic femoral head. Over time, millions of microscopic polyethylene and metal particles are shed from the prosthesis into the joint. They are phagocytized by macrophages within the joint and induce an inflammatory response. Eventually, the engorged, foamy macrophages die and rupture, releasing a host of cytokines and enzymes into the joint space. The resulting autoimmune response is known as particle disease . Particle disease can result in proliferative synovitis, pseudotumors (e.g., aseptic lymphocytic vasculitis-associated lesion [ALVAL]), aggressive bone resorption around the prosthesis (i.e., osteolysis), and loosening.
Particle disease initially results in a thickened synovium, with additional foci of nodular proliferation appearing as intermediate signal intensity on proton density images. This can occur with or without associated joint effusion or intraarticular debris ( Fig. 40.2 ). The complex synovial process should be differentiated from simple postoperative fluid collections, which are commonly associated with posterior joint pseudocapsule dehiscence ( Fig. 40.3 ). These postoperative fluid collections are often incidental findings and reported to be symptomatic only when greater than 4 cm in the largest dimension.
A variant manifesting as a more solid lesion is ALVAL. It is a complex periprosthetic collection or pseudotumor resulting from a hypersensitivity reaction with a perivascular infiltrate ( Fig. 40.4 ).