Introduction

Magnetic Resonance Imaging (MRI) stands as a cornerstone in the evaluation of knee joint anatomy, offering unparalleled soft tissue contrast and multiplanar capabilities without ionizing radiation. The intricate anatomy of the knee comprises bones, cartilage, tendons, ligaments, muscles, and menisci, with each tissue exhibiting distinct signal characteristics on MRI. The advent of diverse MRI sequences such as T1-weighted, T2-weighted, Proton Density (PD) with fat saturation, Short Tau Inversion Recovery (STIR), and Diffusion-Weighted Imaging (DWI) has further enhanced the diagnostic accuracy by highlighting different tissue properties. This article delineates the knee anatomy as visualized on these varied MRI sequences.

Knee Anatomy MRI

T1-Weighted Imaging

T1-weighted sequences are fundamental in anatomical delineation. They provide high spatial resolution with excellent contrast between fat and water-based tissues.

Bone: Appears with low signal intensity due to its cortical composition.

Fat: Exhibits high signal intensity, serving as a natural contrast material and delineating the anatomy.

Muscles and Tendons: Show intermediate signal intensity.

Cartilage: Has a slightly higher signal than muscle, providing contrast against subchondral bone.

Fluid: Such as joint effusion, appears dark due to its low signal.

T2-Weighted Imaging and Proton Density (PD) with Fat Saturation

T2-weighted and PD fat-saturated images accentuate fluid detection and edema.

Bone: Signal varies based on marrow content; edema from stress or occult fractures enhances the signal.

Fat: Normally suppressed on fat-sat sequences, enhancing the visibility of pathology.

Muscles and Tendons: Remain intermediate but can increase in signal with pathology like tendinopathy or muscle injury.

Cartilage: Exhibits higher signal intensity on PD than T1, which may indicate cartilage degeneration when excessively bright.

Fluid and Edema: Shine bright on T2 and PD fat-sat sequences, highlighting effusions, synovitis, and bone marrow edema.

Short Tau Inversion Recovery (STIR)

STIR images provide excellent sensitivity for detecting edema and are less prone to magnetic susceptibility artifacts.

Bone: Increased signal in the presence of edema or marrow infiltration.

Fat: Signal is suppressed, similar to fat-sat techniques, emphasizing fluid and edema.

Muscles and Tendons: Abnormalities are more conspicuous as edema within these structures increases signal intensity.

Cartilage: Less well-defined compared to T1 and PD sequences.

Fluid: Bright, making STIR invaluable for detecting ligamentous, meniscal, and marrow pathologies.

Diffusion-Weighted Imaging (DWI)

DWI is not commonly used in knee MRI but can be useful for differentiating between benign and malignant bone tumors and detecting abscesses or soft tissue infections.

Bone: Usually low signal, but changes in diffusivity can indicate pathology.

Fat: Not specifically suppressed, can be problematic for image interpretation.

Muscles and Tendons: Typically low signal unless pathology is present.

Cartilage: Not typically evaluated with DWI.

Fluid: Restricted diffusion appears bright, indicating a higher cellularity, as seen in abscesses or tumors.

Clinical Implications of MRI Knee Sequences

Understanding the MRI appearance of knee anatomy across various sequences is vital for identifying pathologies such as meniscal tears, ligamentous injuries, chondral lesions, bone contusions, and synovial disorders. Meniscal tears may appear as increased signal within the meniscus on T2-weighted images that contact the meniscal surface. Anterior Cruciate Ligament (ACL) injuries demonstrate increased signal and disrupted fibers on T2/PD fat-sat sequences. Chondral damage is best assessed on T2-weighted and PD fat-sat images, as fissures and defects in the normally smooth cartilage become evident. Bone contusions or fractures are typically visible on T2 and STIR sequences due to the high signal of associated edema. Synovial disorders such as arthritis and synovial chondromatosis are characterized by synovial proliferation and effusion, which are highlighted on T2 and PD fat-sat images.

Conclusion

MRI provides a detailed assessment of knee anatomy, aiding clinicians in diagnosis and management of knee pathologies. The contrast between different tissues provided by T1, T2, PD fat-sat, STIR, and DWI sequences enables precise anatomical and pathological evaluation. It is the exquisite detail and variation in tissue signal across these sequences that allow MRI to remain at the forefront of musculoskeletal imaging.

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