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T2 relaxometry has become an important tool in quantitative MRI. Little focus has been put on the effect of the refocusing flip angle upon the  offset  parameter, which was introduced to account for a signal floor due to noise or to long T2 components. The aim of this study was to show that B1 imperfections contribute significantly to the  offset . We further introduce a simple method to reduce the systematic error in T2 by discarding the first echo and using the  offset  fitting approach.    Materials and Methods  Signal curves of T2 relaxometry were simulated based on extended phase graph theory and evaluated for 4 different methods (inclusion and exclusion of the first echo, while fitting with and without the  offset ). We further performed T2 relaxometry in a phantom at 9.4T magnetic resonance imaging scanner and used the same methods for post-processing as in the extended phase graph simulated data. Single spin echo sequences were used to determine the correct T2 time.    Results  The simulation data showed that the systematic error in T2 and the  offset  depends on the refocusing pulse, the echo spacing and the echo train length. The systematic error could be reduced by discarding the first echo. Further reduction of the systematic T2 error was reached by using the  offset  as fitting parameter. The phantom experiments confirmed these findings.    Conclusion  The fitted offset parameter in T2 relaxometry is influenced by imperfect refocusing pulses. Using the  offset  as a fitting parameter and discarding the first echo is a fast and easy method to minimize the error in T2, particularly for low to intermediate echo train length.

Mono-Exponential Fitting in T2-Relaxometry: Relevance of Offset and First Echo