ORIGINAL CONTRIBUTIONSImproved MR imaging for patients with metallic implants☆☆
Introduction
Metallic implants cause significant image distortion in magnetic resonance (MR) imaging. Metal-induced artifacts impede accurate identification of recurrent tumor, fracture, infection in the region of an implant, or loosening of the implant. At times, metal artifacts reduce the usefulness of MR imaging so severely that only conventional x-ray methods can be used, seriously limiting the quality and quantity of information available for diagnosis and evaluation.
Metal-induced image artifacts arise from the large differences between the magnetic properties of human tissues and those of metals. Many authors have investigated various aspects of imaging metallic objects, such as the role of implant composition [1] and mechanical deformation [2]. Several post-processing techniques for reducing large magnetic field inhomogeneities such as those created by bulky metal prostheses have been reported [3], [4], [5], [6], [7], [8]. But these methods require additional image processing time and are, therefore, not suitable for clinical imaging. Changes that can be made at the time of image acquisition or to the MR pulse sequence itself are more useful clinically. The long axis of an implant is the direction of the most serious artifact,[9] and image quality can be improved by ensuring that the read-out direction is parallel to the long axis of the implant [10], [11]. Fast spin echo pulse sequences show less metal artifact than conventional spin echo sequences.[11] Increased readout bandwidth and multiple refocusing also reduce metal artifacts [10]. A combination of these approaches improves imaging for patients with implanted metallic screws [11]. However, to successfully image patients with severe metal artifacts from large bulky implants (prostheses), more aggressive approaches are needed.
Our goal was to develop a routine and easily implemented imaging procedure to improve MR image quality for pediatric oncology patients with large metal implants. We developed two new imaging schemes and evaluated them in patients. Our first method, a “tilted view-angle”, was proposed by Cho et al. [12] as a means of reducing chemical shift artifacts and small susceptibility artifacts, and was recently investigated for reducing metal-induced artifacts [13], [14], [15]. Our second approach used a combination of high readout bandwidth, short TE, and multiple (turbo) spin echoes for both T1-weighted (T1W) turbo spin echo and turbo short-tau inversion recovery (STIR) imaging.
Section snippets
Tilted view-angle technique
The tilted view-angle method uses a normal spin echo pulse sequence with only one modification: an additional “tilting” gradient in the direction of slice selection during data acquisition. This gradient (dashed line in Fig. 1) has the magnitude of the normal slice selection gradient, but its timing is the same as the read gradient. A pre-dephase lobe is added to ensure that the total phase accumulated at the echo time TE remains zero. The inhomogeneity correction arises from the additional
Tilted view-angle method
The size and shape of the stainless steel thigh plate in a patient’s left femur is apparent in the plain film x-ray (Fig. 2a). However, in the conventional T1W image, the implant is seen as a larger area of signal void with hypointense arcs and streaks inside and surrounding it (Fig. 2b). A circular void extends well outside the region of the proximal end of the implant, and bright arcs are seen at the bend in the implant. The characteristic arrowhead or cloverleaf shape is evident at the
Discussion
Images obtained with the tilted view angle method are somewhat blurred at interfaces, as has been noted in earlier studies [12], [13]. The amount of blur varies with the tilt angle, which is determined by the gradient values. The best artifact correction occurs when the magnitudes of the tilting gradient and the slice select gradient are equal.
The tilting gradient also causes a field of view shift. Corrections can be made for this shift, but they involve pre-scan calculations which are not
Conclusion
In the past, there has been no satisfactory imaging method to detect tumor recurrence in cancer patients near metal implants. Our metal artifact reduction techniques have brought us closer to adequate post-operative imaging.
Acknowledgements
The authors are grateful to Dr. Sue Kaste for her assistance in the construction of an MR implant phantom.
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☆Supported in part by Grants P30 CA-21765 and P01 CA-20180 from the National Cancer Institute and by the American Lebanese Syrian Associated Charities (ALSAC).