MR Orthopedics ~ Clinical Cases

Dr. Hollis Potter, Hospital for Special Surgery, New York City, New York.

When placed in a large magnetic field such as the main magnetic field in MR imaging, different materials become magnetized to different extents, depending on their chemical composition. Magnetized objects create their own magnetic fields, and can distort the main magnetic field in their vicinity. The induced magnetic fields created around these objects, while small compared to the main applied field, can cause several different types of image artifacts. Ferromagnetic materials become strongly magnetized when placed in an external field. Orthopedic instrumentation made of stainless steel becomes magnetized to a higher extent than titanium and is more prone to creating image artifacts.

In addition to chemical composition, geometry and orientation of the object are also important determinants for the size and extent of the induced artifacts. A cylindrical object oriented perpendicular to Bo will create artifacts that are more severe than those created when the object is oriented parallel to Bo.

Metal artifacts can be reduced substantially by using a protocol that is tailored for imaging orthopedic hardware. Hospital for Special Surgery has optimized FSE-XL to image orthopedic hardware by incorporating the following principles to reduce artifact: (a) Use of Fast Spin Echo technique; (b) Increased Receive Bandwidth, (c) Reduced inter-echo spacing by use of FSE-XL and Tailored RF (d) Use of a longer ETL to allow for multiple NEX to increase SNR.

Case C:

40 year-old male who has undergone an anterior cruciate ligament reconstruction, a medial collateral ligament reconstruction as well as a high tibial osteotomy. The instrumentation is stainless steel, and the lag screw from the osteotomy runs perpendicular to Bo. Note that the meniscus and cartilage are well visualized despite the presence of extensive instrumentation.

The axial (C1), sagittal (C2) and coronal (C3) images show the lag screws from the osteotomy. Staples from the medial collateral ligament repair are also visualized on the coronal image.

Case D:

22 year-old male who has undergone a calcaneal osteotomy transfixed by multiple stainless steel lag screws, as well as a medial malleolar osteotomy with replacement of bone graft for an osteochondral lesion over the talar dome. A sagittal image (D1) shows extensive stainless steel instrumentation running in multiple obliquities relative to Bo. By use of the optimized hardware protocol, the presence of the instrumentation does not preclude evaluation of the articular cartilage of the ankle or hindfoot.