Purpose: To address the systematic bias in whole-brain dual flip angle (DFA) T 1 -mapping at 7T by optimizing the flip angle pair and carefully selecting RF pulse shape and duration.Theory and Methods: Spoiled gradient echoes can be used to estimate whole-brain maps of T 1 . This can be accomplished by using only two acquisitions with different flip angles, i.e., a DFA-based approach. Although DFA-based T 1 -mapping is seemingly straightforward to implement, it is sensitive to bias caused by incomplete spoiling and incidental magnetization transfer (MT) effects. Further bias is introduced by the increased B 0 and B 1 + inhomogeneities at 7T. Experiments were performed to determine the optimal flip angle pair and appropriate RF pulse shape and duration. Obtained T 1 estimates were validated using inversion recovery prepared EPI and compared to literature values. A multi-echo readout was used to increase SNR, enabling quantification of R 2 * and susceptibility, χ. Results: Incomplete spoiling was observed above a local flip angle of approximately 20°. An asymmetric gauss-filtered sinc pulse with a constant duration of 700 μs showed a sufficiently flat frequency response profile to avoid incomplete excitation in areas with high B 0 offsets. A pulse duration of 700 μs minimized effects from incidental MT. Conclusion: When performing DFA-based T 1 -mapping one should (i) limit the higher flip angle to avoid incomplete spoiling, (ii) use a RF pulse shape insensitive to B 0 inhomogeneities and (iii) apply a constant RF pulse duration, balanced to minimize incidental MT.
Purpose To optimize a whole‐brain magnetization transfer saturation (MTsat) protocol at 7T, focusing on maximizing obtainable MTsat under the constraints of specific absorption rate (SAR) and transmit field inhomogeneity, while avoiding bias and keeping scan time short. Theory and Methods MTsat is a semi‐quantitative metric, obtained by spoiled gradient‐echo MRI in the imaging steady‐state. Optimization was based on an established 7T dual flip angle protocol, and focused on MT pulse, readout flip angle, repetition time (TR), offset frequency (Δ), and correction of residual effects from transmit field inhomogeneities by separate flip angle mapping. Results A 100% SAR level was reached at a 180° MT pulse flip angle, using a compact sinc main lobe (4 ms duration) and minimum TR = 26.5 ms. The use of Δ = +2.0 kHz caused no discernible direct saturation, while Δ = −2.0 kHz resulted in 45% higher MTsat in white matter (WM) compared to Δ = +2.0 kHz. A 4° readout flip angle eliminated bias while yielding a good signal‐to‐noise ratio. Increased TR yielded only a little increase in MTsat, and TR = 26.5 ms (scan time 04:58 min) was thus selected. Post hoc transmit field correction clearly improved homogeneity, especially in WM. Conclusions The range of MTsat is limited at 7T, and this can partly be overcome by the exploitation of the asymmetry of the macromolecular lineshape through the sign of Δ. To reduce scan time, a compact MT pulse with a sufficiently narrow frequency response should be used. TR and readout flip angle should be kept short/small. Transmit field correction through separate flip angle mapping is required.
Purpose To map T1 and the local flip angle (B1+) in human brain using a single MP3RAGE sequence with 3 rapid acquisitions of gradient echoes (RAGEs). Theory and methods A third RAGE with a relatively high flip angle was appended to an MP2RAGE sequence. Through curve fitting and a rational approximation for small flip angles and short TR, closed form solutions for T1 and B1+ were derived. The influence of different k‐space encoding schemes on precision and whether edge enhancement artifacts could be reduced with a saturation pulse applied prior to the third RAGE were explored. Validation of T1 estimates was performed using single‐slice inversion recovery (IR) and a subsequent region‐of‐interest–based comparison, whereas validation of B1+ was performed using a whole brain pixelwise comparison to a DREAM flip angle mapping protocol. Lastly, MP3RAGE was compared to T1‐mapping by MP2RAGE with separate B1+ correction. Results Whole brain maps of T1 and B1+ at 1 mm isotropic resolution were obtained with MP3RAGE in 06:37 min. A linear–reverse centric–reverse centric phase‐encoding order of the 3 RAGEs improved precision, and artifacts were successfully reduced with the saturation pulse. Estimations of T1 and B1+ deviated +2.5 ± 3.1% and −1.7 ± 8.6% from their respective references. Conclusion T1 and B1+ can be mapped simultaneously using MP3RAGE. The approach can be thought of as combining MP2RAGE with a dual flip angle T1‐mapping protocol. Both maps can be solved for analytically and will be inherently co‐registered at the high resolution associated with MPRAGE.
Background and Purpose: Cerebral tissue oxygenation is a critical brain viability parameter, and the magnetic properties of hemoglobin offer the opportunity to noninvasively quantify oxygen extraction fraction (OEF) by magnetic resonance imaging (MRI).Ultrahigh-field MRI shows advantages such as increased sensitivity to magnetic susceptibility differences and improved signal-to-noise ratio that can be translated into smaller voxel size, but also increased sensitivity to static and B1 field inhomogeneities. The aim was to produce a systematic comparison of three MRI-based methods for estimation of OEF.Methods: OEF estimates in 16 healthy subjects were obtained at 7T utilizing susceptometry-based oximetry (SBO), quantitative susceptibility mapping (QSM), and transverse relaxation rate (R2*). Two major draining veins, that is, the superior sagittal sinus (SSS) and the straight sinus (SS), were investigated, including mutual agreement between the methods in each of the two different vessels, agreement between vessels as well as potential vessel angle and vessel size dependences.Results: Very good correlation (r = .88) was found between SBO-based and QSM-based OEF estimates in SSS. Only QSM showed a moderate correlation (r = .61) between corresponding OEF estimates in SSS and SS. For SBO, a trend of increasing OEF estimates was observed as the SS vessel angle relative to the main magnetic field increased. No obvious size dependence could be established for any method. The R2*-based OEF estimates were reasonable (35%-36%), but the observed range was somewhat low. Conclusion:The results indicate that QSM is a promising candidate for assessment of OEF estimates, for example, providing reasonably robust estimates across a wide range of vessel orientations.
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