T2* maps obtained by the processing of multiecho MR sequences can be useful in several clinical applications. T2* map generation procedures should join a processing time compatible with on-line image analysis with a good precision in the entire T2* range of clinical interest. Fast generation of T2* maps can be achieved by the estimation of the T2* values by the weighted linear fitting of the logarithm of the signal (WLSL) method. This approach fails if the signal decay diverges from a pure exponential decay, as happens at low T2* values where the rapid decay in the signal intensity leads to a plateau in the later echo times (TE). The proposed method implements the automatic truncation of the signal decay curves to be fitted in order to compensate for the signal collapse at low T2* values, allowing the extension of the WLSL method through the entire clinical range of T2* values. Validation was performed on synthetic images and on 60 thalassemia major patients with different levels of myocardial iron overload. Phantom experiments showed that a 5% fitting error threshold represented the best compromise between T2* value measurement precision and processing time. A good agreement was found between T2* map pixel-wise measurements and ROI-based measurements performed by expert readers (CoV=1.84% in global heart T2*, CoV=5.8% in segmental analysis). In conclusion, the developed procedure was effective in generating correct T2* maps for the entire T2* clinical range.
Keywords: Iron overload; Magnetic resonance imaging; Relaxometry; T2(⁎) mapping; Thalassemia.
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