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Zusammenfassung:
PURPOSE: Quantitative magnetic resonance imaging (qMRI) helps reveal the biophysical properties governing
MRI contrast. By eliminating instrumental biases and other contrast mechanisms influencing the signal
amplitude, quantitative parameter maps can be derived and ultimately serve as in vivo biomarkers 1 . Biases in
proton density (PD) map estimation include radio-frequency transmit (B 1 + ) and receive (B 1 - ) fields and T 2 *
weighting 2-5 . We focus on the T 2 * bias in multi-echo fast low angle shot (FLASH) protocols, where the T 2 * signal
dependence is often neglected 5,6 . Although often pointed out as a potential limitation especially in high iron content areas 5,7,8 , the extent and severity of this bias and the evaluation of correction strategies have not yet been fully reported.
RESULTS: Simulated FLASH multiparameter mapping datasets with increasing noise levels were analysed with the hMRI toolbox and various processing strategies for PD estimation. Without T 2 * bias correction and with calibration to PD=69% in the WM, PD values were overestimated in the cortex (since T 2 * GM >T 2 * WM ) and
strongly underestimated in high iron content areas (globus pallidus, red nuclei, substantia nigra).
CONCLUSIONS: T 2 * bias correction is necessary to increase the sensitivity and specificity of qMRI in these areas. All methods taking T 2 * weighting bias into account are effective. However, method (2) shows lower SNR (relies on a single echo), while methods (1) (with T 2 * correction) and (3) perform similarly.
