Best fast MRI protocols for the knee: advantages and limitations

Knee MRI plays a central role in musculoskeletal diagnostics but has traditionally been associated with relatively long acquisition times. Recent technological advances have fundamentally changed this paradigm. Parallel imaging (PI), simultaneous multi-slice acquisition (SMS), compressed sensing (CS), and combinations thereof have substantially reduced scan times without compromising diagnostic image quality. The introduction of deep learning (DL)-based reconstruction further elevates this transformative breakthrough, as it can reconstruct high-quality diagnostic MR images at higher acceleration factors, where conventional image reconstruction methods have traditionally struggled to succeed. Sixfold PIxSMS-accelerated DL protocols have demonstrated excellent diagnostic performance and image quality, allowing comprehensive knee MRI examinations to be completed in under five minutes. Accelerated three-dimensional (3D) TSE techniques, such as CAIPIRINHA-accelerated SPACE sequences, further expand the potential of knee MRI by enabling high-resolution isotropic 3D imaging at acquisition times that are increasingly practical for routine clinical use. Ongoing improvements in DL-based reconstruction and denoising may soon bridge the remaining gap, promising to enable the acquisition of isotropic 3D datasets with multiple contrasts within minutes. Beyond technical acceleration, the successful implementation of fast MRI requires careful workflow optimization and consideration of architectural and economic factors. This review outlines the technical principles underlying modern acceleration strategies, summarizes evidence from validation studies, discusses practical aspects of clinical implementation and protocol optimization, and highlights future opportunities and challenges.