No abstract

Quantitative T2 mapping seems to reflect differences in repair tissues formed after two surgical cartilage repair procedures. (c) RSNA, 2008.

Cartilage injuries are common, especially in athletes. Because these injuries frequently affect young patients, and they have the potential to progress to osteoarthritis, treatment to alleviate symptoms and delay joint degeneration is warranted. A number of surgical techniques are available to treat focal chondral defects, including marrow stimulation, osteochondral auto- and allografting, and autologous chondrocyte implantation. Although arthroscopy is considered the standard of reference for the evaluation of cartilage before and after repair, it is invasive with associated morbidity and cannot adequately depict the deep cartilage layer and underlying bone. Magnetic resonance (MR) imaging provides unparalleled noninvasive assessment of the repair site and all other joint tissues. MR observation of cartilage repair tissue is a well-established semiquantitative scoring system for repair tissue that has primarily been used in clinical research studies. The cartilage repair osteoarthritis knee score (CROAKS) optimizes comprehensive morphologic assessment of the knee joint after cartilage repair. Furthermore, quantitative, compositional MR imaging measurements (eg, T2, T2*, T1ρ), delayed gadolinium-enhanced MR imaging of cartilage (dGEMRIC), and sodium imaging are available for biochemical assessment. These quantitative MR imaging techniques help assess collagen content and orientation, water content, and glycosaminoglycan and/or proteoglycan content both in the repair tissue as it matures and in the "native" cartilage. In this review, the authors discuss the principles of state-of-the-art morphologic and compositional MR imaging techniques for imaging of cartilage repair and their application to longitudinal studies.

Purpose:To use a 3D gradient-echo (GRE) sequence with two flip angles for delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) to evaluate relative glycosaminoglycan content of repair tissue after matrix-associated autologous chondrocyte transplantation (MACT). Materials and Methods:In a phantom study, T1-mapping based on a 3D-GRE sequence with different flip angle combinations was compared with a standard inversion recovery (IR) sequence at 3.0T. Fifteen patients were examined after MACT in the knee at "3-13 months" (group I) and "19 -42 months" (group II). The ␦ relaxation rate (␦R1) calculated for repair tissue and normal hyaline cartilage was measured and mean values were compared in different postoperative intervals using analysis of variance. Results:The flip angle combination 35/10°provided the best agreement with IR sequence for short and long T1 values. The mean ␦R1 for repair tissue was 2.49 versus 1.04 at the intact control site in group I and 1.90 compared with 0.81 in group II. Differences from repair tissue to control sites showed statistically significance for both groups; no significant difference was found between groups. Conclusion:The 3D dual flip angle dGEMRIC technique optimized for cartilage imaging is comparable to standard T1 IR technique for T1 mapping. Furthermore, the preliminary in vivo study demonstrates the feasibility of the technique in the evaluation of MACT patients.

Within this feasibility study, both T2 and T2* relaxation parameters demonstrated a similar response in the assessment of articular cartilage and cartilage repair tissue. The potential advantages of T2*-mapping of cartilage include faster imaging times and the opportunity for 3D acquisitions, thereby providing greater spatial resolution and complete coverage of the articular surface.

The double-echo-steady-state (DESS) sequence generates two signal echoes that are characterized by a different contrast behavior. Based on these two contrasts, the underlying T2 can be calculated. For a flip-angle of 90°, the calculated T2 becomes independent of T1, but with very low signal-to-noise ratio. In the present study, the estimation of cartilage T2, based on DESS with a reduced flip-angle, was investigated, with the goal of optimizing SNR, and simultaneously minimizing the error in T2. This approach was validated in phantoms and on volunteers. T2 estimations based on DESS at different flipangles were compared with standard multiecho, spin-echo T2. Furthermore, DESS-T2 estimations were used in a volunteer and in an initial study on patients after cartilage repair of the knee. A flip-angle of 33°was the best compromise for the combination of DESS-T2 mapping and morphological imaging.

Strong evidence to determine whether morphological MRI is reliable in predicting clinical outcome after cartilage repair is lacking. Future research aiming specifically at clinical sensitivity of advanced morphological and biochemical MRI techniques after articular cartilage repair could be of great importance to the field.

Using T2-mapping and DWI, additional information could be gained compared to clinical scoring or morphological MRI. In combination clinical, MR-morphological and MR-biochemical parameters can be seen as a promising multimodal tool in the follow-up of cartilage repair.