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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 radiofrequency (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, that is, 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 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 echo planar imaging and compared to literature values. A multi-echo readout was used to increase signal-to-noise ratio, 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 magnetization transfer., Conclusion: When performing DFA-based T 1 -mapping one should (a) limit the higher flip angle to avoid incomplete spoiling, (b) use a RF pulse shape insensitive to B 0 inhomogeneities and (c) apply a constant RF pulse duration, balanced to minimize incidental magnetization transfer., (© 2020 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine.)

Despite the constant improvement of algorithms for automated brain tissue classification, the accurate delineation of subcortical structures using magnetic resonance images (MRI) data remains challenging. The main difficulties arise from the low gray-white matter contrast of iron rich areas in T1-weighted (T1w) MRI data and from the lack of adequate priors for basal ganglia and thalamus. The most recent attempts to obtain such priors were based on cohorts with limited size that included subjects in a narrow age range, failing to account for age-related gray-white matter contrast changes. Aiming to improve the anatomical plausibility of automated brain tissue classification from T1w data, we have created new tissue probability maps for subcortical gray matter regions. Supported by atlas-derived spatial information, raters manually labeled subcortical structures in a cohort of healthy subjects using magnetization transfer saturation and R2* MRI maps, which feature optimal gray-white matter contrast in these areas. After assessment of inter-rater variability, the new tissue priors were tested on T1w data within the framework of voxel-based morphometry. The automated detection of gray matter in subcortical areas with our new probability maps was more anatomically plausible compared to the one derived with currently available priors. We provide evidence that the improved delineation compensates age-related bias in the segmentation of iron rich subcortical regions. The new tissue priors, allowing robust detection of basal ganglia and thalamus, have the potential to enhance the sensitivity of voxel-based morphometry in both healthy and diseased brains., (Copyright © 2016 The Authors. Published by Elsevier Inc. All rights reserved.)

Purpose: The longitudinal relaxation rate (R1 ) measured in vivo depends on the local microstructural properties of the tissue, such as macromolecular, iron, and water content. Here, we use whole brain multiparametric in vivo data and a general linear relaxometry model to describe the dependence of R1 on these components. We explore a) the validity of having a single fixed set of model coefficients for the whole brain and b) the stability of the model coefficients in a large cohort., Methods: Maps of magnetization transfer (MT) and effective transverse relaxation rate (R2 *) were used as surrogates for macromolecular and iron content, respectively. Spatial variations in these parameters reflected variations in underlying tissue microstructure. A linear model was applied to the whole brain, including gray/white matter and deep brain structures, to determine the global model coefficients. Synthetic R1 values were then calculated using these coefficients and compared with the measured R1 maps., Results: The model's validity was demonstrated by correspondence between the synthetic and measured R1 values and by high stability of the model coefficients across a large cohort., Conclusion: A single set of global coefficients can be used to relate R1 , MT, and R2 * across the whole brain. Our population study demonstrates the robustness and stability of the model., (© 2014 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc.)

Background and Purpose: The differential diagnosis of Parkinson syndromes remains a major challenge. Quantitative MR imaging can aid in this classification, but it is unclear which of the proposed techniques is best suited for this task. We, therefore, conducted a head-to-head study with different quantitative MR imaging measurements in patients with IPS, MSA-type Parkinson, PSP, and healthy elderly controls., Materials and Methods: Thirty-one patients and 13 controls underwent a comprehensive quantitative MR imaging protocol including R2*-, R2- and R1-mapping, magnetization transfer, and DTI with manual region-of-interest measurements in basal ganglia regions. Group differences were assessed with a post hoc ANOVA with a Bonferroni error correction and an ROC., Results: The best separation of MSA from IPS in patients and controls could be achieved with R2*-mapping in the PU, with an ROC AUC of ≤0.96, resulting in a sensitivity of 77.8% (with a specificity 100%). MD was increased in patients with PSP compared with controls and to a lesser extent compared with those with IPS and MSA in the SN., Conclusions: Among the applied quantitative MR imaging methods, R2*-mapping seems to have the best predictive power to separate patients with MSA from those with IPS, and DTI for identifying PSP.

The purpose of this experimental study was to evaluate whether the effective k-space coverage of MR images can in principle be viewed after multidimensional Fourier transform back to k-space. A water-soaked sponge phantom providing homogeneous k-space pattern was imaged with different standard MR sequences, utilizing elliptic acquisitions, partial-Fourier acquisitions and elliptic filtering as imaging examples. The resulting MR images were Fourier-transformed to the spatial frequency domain (the k-space) to visualize their effective k-space coverage. These frequency domain images are named "backtransformed k-space images." For a quantitative assessment, the sponge phantom was imaged with three-dimensional partial-Fourier sequences while varying the partial acquisition parameters in slice and phase direction. By linear regression analysis, the k-space coverage as expected from the sequence menu parameters was compared to the effective k-space coverage as observed in the backtransformed k-space images. The k-space coverage of elliptic and partial-Fourier acquisitions became visible in the backtransformed k-space images, as well as the effect of elliptic filtering. The expected and the observed k-space coverage showed a highly significant correlation (r=.99, P<.001). In conclusion, the effective k-space coverage of MR images becomes visible when Fourier-transforming MR images of a sponge phantom back to k-space. This method could be used for several purposes including sequence parameter optimization, basic imaging research, and to enhance a visual understanding of k-space, especially in three-dimensional MR imaging.

Magnetization transfer effects represent a major source of contrast in multislice turbo spin echo sequences (TSE)/fast spin echo sequences. Generally, low refocusing flip angles have become common in such MRI sequences, especially to mitigate specific absorption rate problems. Since the strength of magnetization transfer effects is related to the radiofrequency power and therefore specific absorption rate applied, magnetization transfer induced signal attenuations are investigated for a variety of TSE sequences with low constant and variable flip angles. Noticeable differences between the sequences have been observed. In particular, fewer signal attenuations are observed for TSE with low flip angles such as hyperecho-TSE and smooth transitions between pseudo steady states-TSE, leading to contrast that is less dependent on the number of slices. It is shown that the strength of the magnetization transfer-induced signal attenuations can be understood and described by a physical framework, which is based on the mean square flip angle of a given TSE sequence., (Copyright (c) 2009 Wiley-Liss, Inc.)

Purpose: To demonstrate how averaging of multiple gradient echoes can improve high-resolution FLASH (fast low angle shot) magnetic resonance imaging (MRI) of the human brain., Materials and Methods: 3D-FLASH with multiple bipolar echoes was studied by simulation and in three experiments on human brain at 3T. First, the repetition time (TR) was increased by the square of the flip angle to maintain contrast as derived by theory. Then the number of echoes was increased at constant TR with bandwidths between 110 and 1370 Hz/pixel. Finally, signals of a 12-echo acquisition train (echo times 4.9-59 msec) were averaged consecutively to study the increase in SNR., Results: At unchanged contrast, the signal increased proportionally with flip angle and sqrt(TR). Increasing the bandwidth improved delineation of the basal cortex and vessels, while most of the loss in the signal-to-noise ratio (SNR) was recovered by averaging. Consecutive averaging increased the SNR to reach maximum efficiency at an echo train length corresponding roughly to T(2) (*)., Conclusion: SNR is gained efficiently by acquiring additional echoes and increasing TR (and flip angle accordingly to maintain contrast) until the associated T(2) (*) loss in the averaged signal consumes the sqrt(TR) increase in the steady state. A bandwidth of 350 Hz/pixel or higher and echo trains shorter than T(2) (*) are recommended.

Reliable identification of the subthalamic nucleus (STN) is a critical step in deep brain stimulation for Parkinson disease but difficult on T1-weighted stereotactic MR imaging. By simultaneous imaging of multiple gradient echoes, susceptibility contrast is added to conventional T1-weighted high-resolution MR image. Thus, the visibility of the STN is enhanced on a second co-localized dataset by exploiting the sensitivity of the T2*-relaxation to local iron deposits. The feasibility is underpinned by quantitative measurements on healthy adults.

Partial volume averaging of signal from multiple sclerosis lesions influences biexponential fitting of the water T2 relaxation as used for tissue/CSF segmentation of spectroscopic volumes. Periventricular volumes-of-interest comprising CSF, lesion and normal-appearing white matter in varying proportion were studied. The relaxation of the localized water signal was measured at 12 echo times (STEAM, geometric spacing from 30 ms to 2000 ms, least-squares fit). Magnetization transfer (MT) was applied to identify contributions of tissue signal to the CSF component. The T2 of the longlived component (T2(long)=433-1782 ms) and its prolongation after MT were inversely correlated to the MT ratio. Hence, short T2(long) is associated with overestimation of CSF partial volume, and thus metabolite concentrations. A T2-correction for the CSF partial volume was suggested and applied to the quantification of MR spectra of large MS lesions. The T2 of bulk CSF (2280+/-87 ms) and the influence of the TE sampling scheme were also studied.