A simple and fast flip angle calibration method
نویسندگان
چکیده
Introduction Quantitative MR techniques such as variable flip angle (VFA)[1] , DESPOT1 [2], quantitative T 2 [3] etc. rely on the accuracy with which the true flip angle is known. Although spatial inhomogeneity of flip angles is expected at high fields (>1.5T) due to B1 field inhomogeneities and uneven slice profiles, accurate estimates of true flip angle are important even at lower field strengths. Most methods relying on flip angle knowledge assume the nominal flip angle to be equal to the true flip angle for lower field strengths. For the higher field strengths, extra scans are often required to assess the flip angle calibration [4] or flip angle error [5] before useful information (eg. longitudinal relaxation, T1) can be extracted. Assessment of the true flip angle,α true , can be accomplished using a double-angle method [1,4]. This method, whether using spin-echo or spoiled gradient recalled echo (SPGR) images, relies on data acquired for a long repetition time, TR , ( 1 5T TR > ) and for at least two flip-angles (α and 2α). These requirements make the necessary scans prohibitively long. To overcome this time limitation, fast imaging techniques such as echo-planar (EPI) or spiral have been used. However, fast imaging techniques are inherently sensitive to main field inhomogeneities which can introduce another source of errors. Furthermore, there is an expected mismatch between the slice profiles associated with 2D fast imaging techniques ard the 3D VFA methods [4]. These issues result in undesirable inaccuracies associated with variable flip angle measurements. In this paper, a different approach is used to determine the flip angle calibration factor. It relies on the acquisition of several SPGR scans (which can be 2D or 3D) at short TR (TR~T1/10). This new method exploits the quasi-linear relationship between signal intensity and flip angle for large (~180°) flip angles.
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