Femoroacetabular impingement (FAI) is a hip pathology that leads to osteoarthritis if left untreated¹. Surgical procedures aimed at correcting the bony defects responsible for FAI are successful only in patients with limited cartilage damage². T2 relaxation time is a biomarker for early changes in the cartilage collagen structure and water content³, yet quantitative T₂ mapping is highly challenging in vivo owing to the extensive scan times of single Spin-Echo imaging protocols and the high resolution required in order to delineate the thin cartilage layers. Fast Multi Spin-Echo (MSE) protocols offers clinically feasible scan durations but, due to intrinsic scanner imperfections, their signal deviates significantly from the theoretical exponential decay S(t)=S₀exp[–t/T₂ ] and follows instead a more generalized echo modulation curve (EMC). This results in a bias in exponentially fitted T₂ values that is, moreover not constant across different T₂ distribution but rather depends on the protocol and parameter-set used. As a result, T₂ values for the same subject vary between scanners and vendors^4,5, thereby preventing the adoption of quantitative T₂ as a biomarker for cartilage damage. A recently introduced method – the EMC algorithm^6,7 – can overcomes these limitations and deliver accurate and reliable maps of the true tissue T₂ values that are independent of the scanner and protocol-implementation. In this work we hypothesize that quantitative EMC-based T₂ values which are normalized using an internal reference, can enable accurate detection of surgically confirmed hip cartilage lesions in FAI patients.

Data acquisition: retrospective analysis of 21 hips (12 left) was performed in 20 patients (9 males) diagnosed with FAI and either torn or detached labrum. Patients (38 ± 12 y/o) underwent pre-operative MRI at 3T after intravenous injection of Gd-DTPA^2- for indirect MR arthrography. MSE data were acquired for either a radial or a sagittal section depicting the anterior-superior region of the hip articular cartilage (Figure 1). See Table 1 for a list of imaging parameters.

Surgical assessment: patients underwent routine hip arthroscopy 53 ± 34 days after the MRI to correct the bone defect associated with FAI. All cartilage surfaces were evaluated, providing a ground truth for assessing the capability of the pre-operative T₂ values to detect cartilage lesions.

EMC algorithm^6,7: Bloch simulations of the MSE protocol were performed using the exact RF pulse shapes and other experimental parameters. Simulations were repeated for a range of T₂ and B₁⁺ values (T₂=1…1000ms, B₁⁺ = 50…130 % of nominal value), producing a database of EMCs, each associated with a unique [B₁⁺,T₂] value pair. Reconstruction: T₂ maps were generated using (1) standard monoexponential fit and (2) the EMC algorithm by pixel-by-pixel matching the time-series of MSE DICOMs to the EMC database via l₂-norm minimization of the difference between experimental and simulated EMCs.

Statistical analysis: Patient population was divided into lesion (any chondral defect) and non-lesion subgroups, based on surgical findings. Three regions-of-interest were segmented (Figure 2): FEM – the central portion of the femoral cartilage which is usually healthy in early-stage FAI, ACT – denoting the weight-bearing portion of the acetabular cartilage where lesions are typically found in FAI, and ALL – including both femoral and acetabular cartilage. For each section, a normalized T₂ index was defined as T₂-index=mean(T₂^ACT) / mean(T₂^FEM) in order to remove inter-patient baseline variability.

Figure 3 shows representative T₂ maps illustrating the non-uniform bias that is typical to exponential fitting and the ability of EMC to produce more faithful delineation of the cartilage and muscle tissues. This was additionally reflected in lower intra-subject and inter-scanner variability for EMC-based T₂ values. Figure 4: the lesion and non-lesion patient subgroups could be separated using both mean T₂ value within ACT (p-value=0.053) and the T₂-index (p-value<0.001). On a patient-specific level, the EMC-based T₂-index detected chondral lesions with 100% specificity, 92.3% sensitivity and 95.2% accuracy. The two subgroups were not separable using exponentially fitted T₂ values.Previous work using dGEMRIC had shown the benefit of using the central portion of the femoral cartilage as a normalization reference for quantitative T₁-based assessment of the hip cartilage in FAI⁸. This study demonstrated the ability of the EMC-reconstructed T₂-index to detect hip cartilage lesions consistently over different protocol parameters and scanners. The higher specificity of the EMC algorithm can thus be used to guide and improve the accuracy of pre-operative radiologic evaluation^9,10 and enable longitudinal, cross-platform, and multi-center clinical studies. Future work includes assessing intra- and inter-observer variability in T₂-index analysis, and a prospective study on a larger patient population to investigate detection and staging of cartilage damage.