Compressed Sensing in Metal Hip Imaging: Our Experience

For end stage hip disease, total hip arthroplasty (THA) has become an attractive management option for many patients [1, 2]. While THA offers excellent pain relief and helps a majority of patients to regain some portion of day to day mobility, it is not without complications. About 40% of patients who undergo THA report groin and thigh pain [3, 4]. Despite the development in implant design, fixation approaches, and bearing materials, most prostheses eventually fail [5]. Given this, there is an increasing demand for more accurate diagnosis and visualization prior to hip revision. Recently, magnetic resonance imaging (MRI) has become the imaging modality of choice for most clinicians to image potential THA-related complications [6, 7]. In one imaging session, MRI can provide useful information about periprosthetic fractures, and osteolysis, postoperative hematoma, disruption of the pseudocapsule, synovitis caused by polyethylene wear and adverse local tissue reactions, periprosthetic masses, bursitis, tendinopathy, and neurovascular compromise [8]. However, MRI near metal comes with a well-known challenge, the susceptibility induced blooming artifact. This artifact hinders image quality and consequently diagnostic accuracy. Magnetic susceptibility refers to the extent by which a substance is magnetised when exposed to the magnetic field. Different substances exhibit various degrees of magnetic susceptibility when exposed to a static magnetic field [9, 10]. Metallic objects have higher magnetic susceptibility than biological tissues. This induces severe spin dephasing (incoherence) around metallic implants and causes signal drop out and a form of image distortion [11]. In practice, using high bandwidth (BW), thinner slices, smaller field of view, finer matrix and imaging at lower magnetic fields are all helpful protocol adaptions to reduce the metal-induced artifact. However, these changes to the MR sequence lead to reduced signal to noise ratio (SNR) and often increased specific absorption rate (SAR). Therefore, practitioners tend to scan for longer times to mitigate the adverse effects associated with reducing the metalinduced artifact. syngo WARP is a Siemens Healthineers solution that offers techniques to reduce susceptibility-related distortions. syngo WARP comprises • Turbo Spin Echo (TSE) sequence optimized for imaging in the presence of metal implants • “View Angle Tilting” or VAT and • “Slice Encoding for Metal Artifact Correction” or SEMAC. When VAT is added to a turbo spin echo pulse sequence, an additional gradient is applied in the data readout step to correct the in-plane distortion. However, only correcting for the in-plane distortion is not enough. Hence, the SEMAC option has been introduced. SEMAC offers throughplane distortion correction, similar to 3D imaging, where additional phase-encoding steps are added in the third dimension. This provides information on how the slice profile is distorted, which is used later to correct the distortion during the image reconstruction stage. Therefore, the more additional phase-encoding steps, the richer the slice profile, which enhances the distortion correction process. However, while adding additional phase-encoding steps helps in improving the image quality, it requires longer scanning time and additional postprocessing [12, 13]. What is promising is that one can use VAT and SEMAC simultaneously. That is, concurrently correcting