Seven T 2 maps (multiecho (ME) sequence: 3000 ms, eight echoes with 13.2 ms of echo spacing, 20 sections) and T 1 -weighted (T 1 -w) fast low-angle shot (FLASH-water excitation (WE)) data sets from four imaging sessions (right patellae of 10 healthy volunteers) were obtained. A segmentation of cartilage (WE sequence) was overlaid on the ME data and T 2 values were calculated for total cartilage, three layers, three facets (global), and 240 ROIs (regional). Reproducibility (precision error) was calculated as the root mean square average (RMSA) of the individual coefficients of variation (COVs, %) and standard deviations (SDs, ms) for intra-and intersession reproducibility. The precision error was 3-7% and 6 -29% for global and regional T 2 , respectively. There was no difference between intraand intersession reproducibility, but there was worse reproducibility in the superficial layers compared to the deeper layers. Peripheral ROI reproducibility (mean ‫؍‬ 13%) was worse than in the central portions (mean ‫؍‬ 11%), but omission of the periphery did not positively affect the globally calculated T 2 reproducibility. The precision errors were small compared to reported changes in diseased cartilage, suggesting good discriminatory power of the technique. Our data provide a first estimate of global and regional reproducibility errors of T 2 in healthy cartilage, and may serve as a basis for sample size calculations and aid study designs for longitudinal and cross-sectional trials in osteoarthritis (OA). Magn Reson Med 56:527-534, 2006.

At 3.0 T, the 3D-FLASH sequence showed tendency to be slightly superior to the 3D-TrueFISP sequence considering robust and valid assessment of quantitative cartilage parameters in young healthy adults, although there was found no significant statistical difference between both imaging techniques. However, in patients suffering from osteoarthritis (OA), the 3D-TrueFISP sequence might prove advantageous for monitoring of disease progression and evaluation of therapy success, particularly because the substantially higher signal to noise ratio/contrast to noise ratio values might allow for higher spatial resolution and hence for improvement of the accuracy of segmentation process especially at the articular surface.

Our data provides an estimation of the global and regional reproducibility errors of T2 in healthy cartilage. In the analysis of small subregions, an increase in the regional reproducibility error must be accepted. The data may serve as a basis for sample size calculations of study populations and may contribute to the decision regarding the level of detail of an evaluation of study data.

Our results revealed a clear dependence of apparent T2 relaxation times on the pulse sequence design, emphasizing that the "true" T2 is hard to find. In addition, the effect on the apparent T2 values resulting from the specific modification of any sequence variant varied according to the respective tissue's properties. Therefore, the acquisition technique in conjunction with the specific tissue on which T2 mapping is performed need to be reported in detail and should kept consistent to allow large-scale comparisons and monitoring of treatment strategies, e.g., in osteoarthritis.

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