Advanced Imaging in Femoroacetabular Impingement: Current State and Future Prospects

Abstract

Symptomatic femoroacetabular impingement (FAI) is now a known precursor of early osteoarthritis (OA) of the hip. In terms of clinical intervention, the decision between joint preservation and joint replacement hinges on the severity of articular cartilage degeneration. The exact threshold during the course of disease progression when the cartilage damage is irreparable remains elusive. The intention behind radiographic imaging is to accurately identify the morphology of osseous structural abnormalities and to accurately characterize the chondrolabral damage as much as possible. However, both plain radiographs and computed tomography (CT) are insensitive for articular cartilage anatomy and pathology. Advanced magnetic resonance imaging (MRI) techniques include magnetic resonance arthrography and biochemically sensitive techniques of delayed gadolinium-enhanced MRI of cartilage (dGEMRIC), T1rho (T1ρ), T2/T2* mapping, and several others. The diagnostic performance of these techniques to evaluate cartilage degeneration could improve the ability to predict an individual patient-specific outcome with non-surgical and surgical care. This review discusses the facts and current applications of biochemical MRI for hip joint cartilage assessment covering the roles of dGEMRIC, T2/T2*, and T1ρ mapping. The basics of each technique and their specific role in FAI assessment are outlined. Current limitations and potential pitfalls as well as future directions of biochemical imaging are also outlined.

Keywords: MRI; T1rho mapping; T2 mapping; T2* mapping; cartilage; dGEMRIC; femoroacetabular impingement; hip.

Figure 1

Radial double-echo steady state (DESS) reformat depicting the superior zone (12 o’clock position) in a cam-type FAI hip. Note the aspherical femoral head and the corresponding labral tear with intraosseous and extraosseous extravasation of synovial fluid arising from the torn labrum and peripheral acetabular cartilage abrasion.

Figure 2

Two-dimensional proton-density (PD) – weighted MR image of a pincer-type FAI patient depicting an increased signal within the center of the labrum that does not extend to the labral margin reflecting intra-labral degeneration. Note that the saturation effect (band of low signal in the center of acetabulum and femoral neck) is constantly present in 2D radial MR imaging.

Figure 3

Intra-operative photographs made with an arthroscopic surgery camera after surgical hip dislocation demonstrating a bump deformity at the femoral head–neck junction (A), the femoral head–neck osteochondroplasty to improve the femoral head–neck offset (B), and acetabular evaluation under full direct visualization revealing full-thickness chondral damage at the anterior–superior aspect of the acetabulum (white arrow) and an extended torn labrum that was re-attached to the acetabular rim with five suture anchors (C) in a 56-year-old with impingement.

Figure 4

Multi-planar-reconstruction of the three-dimensional (3D) T1_Gd data set including plane adjustment through the center of the femoral head and perpendicular to the femoral neck within the sagittal oblique view and the coronal oblique view to create radial T1_Gd planes throughout the hip joint. T1_Gd values are visualized in a color scale. Note the aspherical nature of the femoral head of this asymptomatic volunteer yet without a decrease in the T1_Gd values indicating a normal GAG content within cartilage.

Figure 5

Schematic drawing demonstrating the T1_Gd decrease in various regions of hip joint cartilage of cam- (A) and pincer- (B) FAI patients. The percentage values refer to the T1_Gd average in corresponding hip joint regions of asymptomatic controls. Note that, particularly in the anterior aspect of the joint, the cam-FAI group exhibited not only a peripheral but also a central cartilage T1_Gd decrease (A), whereas the pincer-FAI cohort demonstrated a rather global T1_Gd decrease for all areas of the hip (B). Figure reprinted with permission (40).

Figure 6

T1rho (A–C) and T2 (D–F) relaxation times in bi-layered (A,D), femoral (B,E), and acetabular (C,F) sub-regions of hip joint cartilage in 12 healthy volunteers (controls) and 9 FAI patients. The segmented regions of interest (ROIs) were automatically divided radially into 12 equal sub-regions (30° intervals) based on the fitted center of the femur head in which R2 represents the posterior–inferior region (5–4 o’clock in a clockwise system, right hip), R5 the posterior–superior (2–3 o’clock), and R8 the anterior–superior (1–2 o’clock) region. Error bars represent SD. *represents a significant difference between relaxation times of controls and FAI subjects. Figure reprinted with permission (54).

Figure 7

T2 mean values in various of sections (regions) and layers (zones) of femoral and acetabular cartilage. The bar indicates the SE of the mean. Note the topographic variation in the T2 values of hip joint cartilage probably because of factors including cartilage matrix composition and magic angle effect that need to be considered when interpreting and evaluating T2 values in hip joint cartilage. Figure reprinted with permission (70).

Figure 8

Double-echo steady state (DESS; A,C) and corresponding T2* reformat (B,D) of an asymptomatic volunteer. Sufficient image resolution could be achieved to delineate the cartilage layers of the acetabulum and the femoral head for ROI analysis within peripheral acetabular cartilage, central acetabular cartilage, peripheral femoral cartilage, and central femoral cartilage. The DESS reformats (A,C) served as reference for accurate placement of the ROI squares within cartilage. T2* values are illustrated in a color scale (B,D). Figure reprinted with permission (80).