Presence Of B1 Inhomogeneities Research Articles

A novel method for T2 quantification in presence of B1 inhomogeneities

IntroductionT2 relaxometry as a quantitative MRI technique offers a more precise characterization of tissue, compared with qualitative T2 weighted imaging. The issue, however, is that in typically used T2 relaxometry sequences such as a multi-spin echo sequences, incorrect refocussing flip angles and B1 inhomogeneities cause inaccuracies in T2 determination. The work presented here within presents a new technique for modelling T2 decay curves and thereby correcting T2 times altered by inhomogeneities. Materials and methodsThe proposed technique models the signal decay acquired by multi-spin echo sequences based on extended phase graph theory. We have then put this new correction technique to test under both noise free and noise affected simulated signal and compared it to other previously introduced correction methods. To compare the efficacy of the T2 correction techniques, it was applied to MRI data acquired at 9.4Tesla. Results and conclusionThe results show that the proposed technique is able to minimize the systematic error in T2 relaxometry for a wide range of T2 times. Beyond that, the technique reveals the actual refocussing pulse angle in the respective voxel and reduces the error in So, which is mandatory if one aims for quantification of proton density.

Correction for the T1 effect incorporating flip angle estimated by Kalman filter in cardiac‐gated functional MRI

To develop an improved and generalized technique for correcting T1-related signal fluctuations (T1 effect) in cardiac-gated functional magnetic resonance imaging (fMRI) data with flip angle estimation. Spatial maps of flip angle and T1 are jointly estimated from cardiac-gated time series using a Kalman filter. These maps are subsequently used for removing the T1 effect in the presence of B1 inhomogeneity. The new technique was compared with a prior technique that uses T1 only while assuming a homogeneous flip angle of 90°. The robustness of the new technique is demonstrated with simulated and experimental data. Simulation results revealed that the new method led to increased temporal signal-to-noise ratio across a large range of flip angles, T1s, and stimulus onset asynchrony means compared to the T1 only approach. With the experimental data, the new approach resulted in higher average gray matter temporal signal-to-noise ratio of seven subjects (84 vs. 48). The new approach also led to a higher statistical score of activation in the lateral geniculate nucleus (P < 0.002). The new technique is able to remove the T1 effect robustly and is a promising tool for improving the ability to map activation in fMRI, especially in subcortical regions.

Myocardial tagging with B1 insensitive adiabatic DANTE inversion sequences.

A new technique, based on adiabatic delays alternating with mutations for tailored excitation (DANTE) inversion sequences, is presented for generating uniform contrast tags across the myocardial wall even in the presence of B1 inhomogeneities. The utility of this pulse was demonstrated using a surface coil for both transmission and signal reception in phantom and animal heart tagging studies. The experimental data demonstrated uniform grid contrast over a sixfold variation of B1 magnitude, sharp tagging profiles, and the ability to follow the cardiac wall motion through the deformation of the fine rectangular tagging grid at different phases throughout the cardiac cycle.