Osteoarthritis (OA) of the hip is a debilitating and painful condition. In the literature it is suggested that up to 80% of adult idiopathic hip OA cases are caused by femoroacetabular impingement (FAI) ^1,2. Standard clinical imaging techniques are unable to assess early OA in hip cartilage, but it has been shown that quantitative T1ρ MRI has the potential to detect early biochemical cartilage degeneration in OA ³. In this study we systematically investigated the analysis of hip cartilage T1ρ images in healthy as well as asymptomatic and symptomatic subjects with cam deformities.

In 57 hips (57 subjects) T1ρ mapping was performed on a 1.5T MRI scanner (Siemens Healthcare, Erlangen, Germany). 40 hips were diagnosed with cam-type deformity (18 symptomatic, 22 asymptomatic), 17 were included as controls. T1ρ images of the hip were acquired using a spin-lock preparation module combined with a TSE acquisition scheme (FOV = 180 x 180mm², number of slices = 22, slice thickness = 3mm, matrix = 384 x 384, resolution = 0.47 x 0.47mm², TR = 274ms, TE = 13ms, averages = 1). Five different spin-lock times (TSL) of 12/18/25/35/45 ms and a spin-lock field of B₁ = 400Hz were used. The total scan time for T1ρ imaging was 21 minutes. T1ρ maps were calculated offline by fitting the images pixel wise to a mono-exponential function. WOMAC was assessed in all subjects. The femoral and acetabular cartilage were analyzed parallel to the acetabular plane as a whole and divided into 3 mm thick slices (total of 9 slices). In each slice the cartilage was analyzed as a bi-layer divided into 6 zones, each a 30° wide wedge in the sagittal plane. Further, the total 54 regions (9 slices x 6 zones) were combined to six regions of interest (ROIs), following a described hip cartilage T1ρ pattern in healthy subjects ⁴. The anterior zones 1-3 were combined to the anterosuperior quadrant and the posterior zones 4-6 to the posterosuperior quadrant. The lateral slices 1-3 were combined to the lateral third, slices 4-6 to the intermediate third and slices 7-9 to the medial third.

Fig. 1 demonstrates the segmentation and post-processing of the T1ρ maps. Fig. 1a shows the division of the hip cartilage into six 30° wide zones. The axial view in Fig. 1b schematically indicates the location of the nine slices. The combination into six larger regions is presented in Fig. 1c,d: In the sagittal view the antero- and posterosuperior areas are shown (Fig. 1c) and the final six regions can be seen schematically color-coded in the axial view (Fig. 1d).

Lower T1ρ values in the anterosuperior region were found in control hips but not in hips with a cam-type deformity. Significantly prolonged T1ρ values were found in the entire hip cartilage in subjects with cam-type deformity, based on significantly prolonged T1ρ values in the lateral third of the anterosuperior quadrant as well as in the medial third in both, the anterosuperior and posterosuperior quadrants (Fig. 2). No significant difference was found between symptomatic and asymptomatic hips with a cam-type deformity. In symptomatic FAI hips a significant correlation was found between entire hip cartilage T1ρ values and WOMAC subscore pain (p<0.05).In hips with cam-type deformity, a significant T1ρ prolongation indicating loss of proteoglycan was found in the lateral third of the anterosuperior quadrant. This is where the cam-type impingement occurs and early cartilage damage is expected. Additionally, a significant T1ρ prolongation was found in the medial third in both the anterosuperior and posterosuperior quadrant. This indicates the degeneration of the cartilage occurs also without a direct contact to the deformity. The fact that no significant difference was found between symptomatic and asymptomatic hips with cam-type deformity indicates that the cartilage damage occurs in all hips with cam-type deformity. A correlation between T1ρ prolongation, indicating early cartilage damage, and the pain level was found. This correlation was strongest in the lateral third of the anterosuperior quadrant, the region of direct impact of the deformity.The study shows that in the presence of a cam deformity, T1ρ has the ability to detect proteoglycan depletion, the earliest stage of cartilage degeneration, even before to symptoms occur. Therefore T1ρ imaging may be used as a diagnostic marker in hips with cam-type deformity.