However, at high static field strengths (> 1.5 T), the radio-frequency (RF) transmit field (here, also called B1 + field) is significantly distorted leading to systematic deviations of the local excitation flip angle across the brain ( Lutti et al., 2010a) and hence inaccurate R1 values. Quantitative R1 mapping has been successfully used to study the morphology and the microstructure of brain tissue ( Tofts, 2003).Ī method based on acquisitions of 3D FLASH with variable excitation flip angles (VFA) is popular due to its speed, high signal-to-noise ratio (SNR) and therefore its ability to provide R1 maps at high resolution with high precision ( Deoni et al., 2003 Deoni, 2007 Helms et al., 2008a).

The latter exhibits a contrast depending primarily, but not solely on T1 relaxation and does not directly provide absolute values of the T1 time. Thus, quantitative magnetic resonance imaging (MRI) improves the comparability and interpretability of results in comparison to the widely used T1-weighted (T1w) MRI ( Ashburner et al., 2003). Quantitative mapping of the longitudinal relaxation provides absolute values of the longitudinal relaxation time T1 and longitudinal relaxation rate R1 (= 1/T1), which makes the results system independent and separates different sources of contrast ( Tofts, 2003). UNICORT is easy to apply, as it is computationally efficient and its basic framework is implemented as part of the tissue segmentation in SPM8. The results of different groups of subjects with a wide age range and different levels of atypical brain anatomy further suggest that the method is robust and generalizes well to wider populations. Extensive cross-validation with the established standard using measured B1 + maps shows that UNICORT yields accurate B1 + and R1 maps with a mean deviation from the standard of less than 4.3% and 5%, respectively. The framework incorporates a physically informed generative model of smooth B1 + inhomogeneities and their multiplicative effect on R1 estimates. It employs a probabilistic framework for unified segmentation based correction of R1 maps for B1 + inhomogeneities (UNICORT). We propose a technique that simultaneously estimates the B1 + inhomogeneities and R1 values from the uncorrected R1 maps in the human brain without need for B1 + mapping. However, these R1 maps need to be corrected for bias due to RF transmit field (B1 +) inhomogeneities, requiring additional B1 + mapping which is usually time consuming and difficult to implement. R1 maps are now commonly estimated from two fast high-resolution 3D FLASH acquisitions with variable excitation flip angles, because this approach is fast and does not rely on special acquisition techniques. Quantitative mapping of the longitudinal relaxation rate (R1 = 1/T1) in the human brain enables the investigation of tissue microstructure and macroscopic morphology which are becoming increasingly important for clinical and neuroimaging applications.

Roi transmit