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.

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This study defines the feasibility of utilizing three-dimensional (3D) gradient-echo (GRE) MRI at 1.5T for T* 2 mapping to assess hip joint cartilage degenerative changes using standard morphological MR grading while comparing it to delayed gadolinium-enhanced MRI of cartilage (dGEMRIC). MRI was obtained from 10 asymptomatic young adult volunteers and 33 patients with symptomatic femoroacetabular impingement (FAI). The protocol included T* 2 mapping without gadolinium-enhancement utilizing a 3D-GRE sequence with six echoes, and after gadolinium injection, routine hip sequences, and a dual-flipangle 3D-GRE sequence for dGEMRIC T 1 mapping. Cartilage was classified as normal, with mild changes, or with severe degenerative changes based on morphological MRI. T 1 and T* 2 findings were subsequently correlated. There were significant differences between volunteers and patients in normally-rated cartilage only for T 1 values. Both T 1 and T* 2 values decreased significantly with the various grades of cartilage damage. There was a statistically significant correlation between standard MRI and T* 2 (T 1 ) (P < 0.05). High intraclass correlation was noted for both T 1 and T* 2 . Correlation factor was 0.860 to 0.954 (T* 2 -T 1 intraobserver) and 0.826 to 0.867 (T* 2 -T 1 interobserver). It is feasible to gather further information about cartilage status within the hip joint using GRE T* 2 mapping at 1.5T. Magn Reson Med 62:896 -901, 2009.

Traditional MRI with or without arthrography is unable to discriminate between morphologically normal cartilage and cartilage having diminished functional capacity in early osteoarthritis (OA). The delayed Gadolinium Enhanced MRI of Cartilage (dGEMERIC) is a functional MRI technique that was designed to detect the loss of the charged extracellular matrix in cartilage in early OA (1,2). Cartilage fixed-negative-charge density is composed of negatively charged glycosaminoglycans. It is this fixed charge density that provides much of the mechanical stiffness of articular cartilage. In dGEMRIC, the anionic contrast agent gadopentetate (Gd-DTPA 2Ϫ ) is allowed to distribute into the tissue. The Gd-DTPA 2Ϫ should distribute into cartilage in a manner inversely proportional to the concentration of negatively charged Glycosaminoglycan (GAG). The concentration of Gd-DTPA 2Ϫ will be relatively low in normal cartilage with abundant GAG and will be relatively high in degraded cartilage. MRI is then used to measure the change in T1 relaxation time of cartilage, which is a reflection of the underlying tissue charge density (3).This technique has been validated in vitro and in vivo and has been demonstrated to be useful in clinical research studies (4 -11). In hip dysplasia, dGEMRIC has been shown to correlate with patient symptoms and the severity of dysplasia (12). Additionally, dGEMRIC has been shown to be a good predictor of outcome after pelvic osteotomy for hip dysplasia (13).Despite the usefulness of this technique in clinical research studies, the current implementation of this technique is limited by the long scan times and postprocessing necessary to obtain the T1 map. At present, the standard dGEMRIC technique uses either a two-dimensional (2D) or 3D spin-echo inversion recovery or saturation recovery sequence with varying inversion time (TI) or repetition time (TR). These series of images are then post-processed to obtain the T1 maps (14). Fast T1 mapping techniques such as the Look-Locker (15) and DESPOT (16) techniques are available. In this study, we have used a fast 3D sequence for dGEMRIC, based on a previously reported T1 mapping technique using 3D gradient echo (GRE) sequence with two different flip angles (17). Our objective was to validate this technique against the standard 2D inversion recovery (IR) sequence and to characterize the accuracy of the 2 flip angle technique when applied to a range of T1 values of interest for dGEMRIC of hip cartilage. Our goal is to make this technique widely available for clinical use. MATERIALS AND METHODSFirst, a phantom study was performed to experimentally determine the optimal flip angles for the Fast 2 angle T1 mapping (F2T1) technique for the range of T1 values relevant for clinical studies. Second, patient scans were obtained using both the F2T1 technique and an Inversion Recovery T1 (IRT1) mapping technique and the results compared. Both the phantom and clinical studies were performed on the same 1.5 Tesla (T) scanner (Magnetom Avanto, Siemens Erlangen). Phantom St...