Abstract
The knowledge of relaxation times is essential for understanding the biophysical mechanisms underlying contrast in magnetic resonance imaging. Quantitative experiments, while offering major advantages in terms of reproducibility, may benefit from simultaneous acquisitions. In this work, we demonstrate the possibility of simultaneously recording relaxation-time and susceptibility maps with a prototype Multi-Echo (ME) Magnetization-Prepared 2 RApid Gradient Echoes (MP2RAGE) sequence. T1 maps can be obtained using the MP2RAGE sequence, which is relatively insensitive to inhomogeneities of the radio-frequency transmit field, . As an extension, multiple gradient echoes can be acquired in each of the MP2RAGE readout blocks, which permits the calculation of and susceptibility maps. We used computer simulations to explore the effects of the parameters on the precision and accuracy of the mapping. In vivo parameter maps up to 0.6 mm nominal resolution were acquired at 7 T in 19 healthy volunteers. Voxel-by-voxel correlations and the test-retest reproducibility were used to assess the reliability of the results. When using optimized paramenters, T1 maps obtained with ME-MP2RAGE and standard MP2RAGE showed excellent agreement for the whole range of values found in brain tissues. Simultaneously obtained and susceptibility maps were of comparable quality as Fast Low-Angle SHot (FLASH) results. The acquisition times were more favorable for the ME-MP2RAGE (≈ 19 min) sequence as opposed to the sum of MP2RAGE (≈ 12 min) and FLASH (≈ 10 min) acquisitions. Without relevant sacrifice in accuracy, precision or flexibility, the multi-echo version may yield advantages in terms of reduced acquisition time and intrinsic co-registration, provided that an appropriate optimization of the acquisition parameters is performed.
Highlights
Quantitative mapping of Magnetic Resonance (MR) relaxation times has become a useful tool in brain research as well as for clinical applications due to the possibility of directly comparing results across subjects and sites
Note that these parameters are quite sensitive to outliers and may vary considerably depending on the efficacy of some of the procedures used in this work
The results obtained by relaxometry simulations indicate that, for a given Signal-to-Noise Ratio (SNR), the number of echoes and their distribution is crucial for T2Ã mapping, while the specific choice of inversion times is less critical for T1 mapping
Summary
Quantitative mapping of Magnetic Resonance (MR) relaxation times has become a useful tool in brain research as well as for clinical applications due to the possibility of directly comparing results across subjects and sites. The correlation between relaxation times and brain tissue composition has been highlighted [1, 2]. T1 maps can be obtained, for example, with ‘Fast Low-Angle SHot’ (FLASH) [7, 8] employing different flip angles [9] or with inversion-recovery sequences [10]. The ‘Magnetization-Prepared 2 RApid Gradient Echoes’ (MP2RAGE) sequence has been proposed for an efficient 3D mapping of T1 [5, 13]. It consists of two Gradient-Recalled Echo (GRE) image volumes acquired within a single repetition at two different inversion times, TI, and TI,. Assuming that the recovery can be characterized by a single exponential, accurate T1 estimates are obtained as long as the acquisition parameters are properly chosen, allowing for sufficient insensitivity to inhomogeneities of the Radio-Frequency (RF) transmit magnetic field amplitude, Bþ1
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