Model-based Acceleration of Parameter mapping (MAP) for saturation prepared radially acquired data

A reconstruction technique called Model-based Acceleration of Parameter mapping (MAP) is presented allowing for quantification of longitudinal relaxation time and proton density from radial single-shot measurements after saturation recovery magnetization preparation. Using a mono-exponential model in image space, an iterative fitting algorithm is used to reconstruct one well resolved and consistent image for each of the projections acquired during the saturation recovery relaxation process. The functionality of the algorithm is examined in numerical simulations, phantom experiments, and in-vivo studies. MAP reconstructions of single-shot acquisitions feature the same image quality and resolution as fully sampled reference images in phantom and in-vivo studies. The longitudinal relaxation times obtained from the MAP reconstructions are in very good agreement with the reference values in numerical simulations as well as phantom and in-vivo measurements. Compared to available contrast manipulation techniques, no averaging of projections acquired at different time points of the relaxation process is required in MAP imaging. The proposed technique offers new ways of extracting quantitative information from single-shot measurements acquired after magnetization preparation. The reconstruction simultaneously yields images with high spatiotemporal resolution fully consistent with the acquired data as well as maps of the effective longitudinal relaxation parameter and the relative proton density. Magn Reson Med 70:1524-1534, 2013. © 2013 Wiley Periodicals, Inc. Copyrigh

ECG triggering in ultra-high-field cardiovascular MRI

Cardiac magnetic resonance imaging at ultra-high field (B0 ≥ 7 T) potentially provides improved resolution and new opportunities for tissue characterization. Although an accurate synchronization of the acquisition to the cardiac cycle is essential, electrocardiogram (ECG) triggering at ultra-high field can be significantly impacted by the magnetohydrodynamic (MHD) effect. Blood flow within a static magnetic field induces a voltage, which superimposes the ECG and often affects the recognition of the R-wave. The MHD effect scales with B0 and is particularly pronounced at ultra-high field creating triggering-related image artifacts. Here, we investigated the performance of a conventional 3-lead ECG trigger device and a state-of-the-art trigger algorithm for cardiac ECG synchronization at 7 T. We show that by appropriate subject preparation and by including a learning phase for the R-wave detection outside of the magnetic field, reliable ECG triggering is feasible in healthy subjects at 7 T without additional equipment. Ultra-high field cardiac imaging was performed with the ECG signal and the trigger events recorded in 8 healthy subjects. Despite severe ECG signal distortions, synchronized imaging was successfully performed. Recorded ECG signals, vectorcardiograms, and large consistency in trigger event spacing indicate high accuracy for R-wave detection

Density weighted turbo spin echo imaging

Purpose To optimize the spatial response function (SRF) while maintaining optimal signal to noise ratio (SNR) in T weighted turbo spin echo (TSE) imaging by prospective density weighting. Materials and Methods Density weighting optimizes the SRF by sampling the k-space with variable density without the need of retrospective filtering, which would typically result in nonoptimal SNR. For TSE, the T decay needs to be considered when calculating an optimized sampling pattern. Simulations were carried out and T weighted in vivo TSE measurements were performed on a 3 Tesla MRI system. To evaluate the SNR, reversed centric density weighted and retrospectively filtered Cartesian acquisitions with identical measurement parameters and SRFs were compared with TE = 90 ms and a density weighted k-space sampling optimized to yield a Kaiser function for SRF side lobe suppression for white matter. Results Density weighting of a reversed centric reordering scheme resulted in an SNR increase of (43 ± 13)% compared with the Cartesian acquisition with retrospective filtering while maintaining comparable contrast behavior. Conclusion Density weighting is applicable to TSE imaging and results in significantly increased SNR. The gain can be used to shorten the measurement time, which suggests applying density weighting in both time and SNR constrained MRI

High resolution myocardial first-pass perfusion imaging with extended anatomic coverage

Purpose To evaluate and to compare Parallel Imaging and Compressed Sensing acquisition and reconstruction frameworks based on simultaneous multislice excitation for high resolution contrast-enhanced myocardial first-pass perfusion imaging with extended anatomic coverage. Materials and Methods The simultaneous multislice imaging technique MS-CAIPIRINHA facilitates imaging with significantly extended anatomic coverage. For additional resolution improvement, equidistant or random undersampling schemes, associated with corresponding reconstruction frameworks, namely Parallel Imaging and Compressed Sensing can be used. By means of simulations and in vivo measurements, the two approaches were compared in terms of reconstruction accuracy. Comprehensive quality metrics were used, identifying statistical and systematic reconstruction errors. Results The quality measures applied allow for an objective comparison of the frameworks. Both approaches provide good reconstruction accuracy. While low to moderate noise enhancement is observed for the Parallel Imaging approach, the Compressed Sensing framework is subject to systematic errors and reconstruction induced spatiotemporal blurring. Conclusion Both techniques allow for perfusion measurements with a resolution of 2.0 × 2.0 mm and coverage of six slices every heartbeat. Being not affected by systematic deviations, the Parallel Imaging approach is considered to be superior for clinical studies

Self-gated non-contrast-enhanced functional lung MR imaging for quantitative ventilation assessment in patients with cystic fibrosis

Purpose: To assess the clinical feasibility of self-gated non-contrast-enhanced functional lung (SENCEFUL) magnetic resonance (MR) imaging for quantitative ventilation (QV) imaging in patients with cystic fibrosis (CF). Materials and Methods: Twenty patients with CF and 20 matched healthy volunteers underwent functional 1.5-T lung MR imaging with the SENCEFUL imaging approach, in which a two-dimensional fast low-angle shot sequence is used with quasi-random sampling. The lungs were manually segmented on the ventilation-weighted images to obtain QV measurements, which were compared between groups. QV values of the patients were correlated with results of pulmonary function testing. Three radiologists rated the images for presence of ventilation deficits by means of visual inspection. Mann-Whitney U tests, receiver operating characteristic analyses, Spearman correlations, and Gwet agreement coefficient analyses were used for statistical analysis. Results: QV of the entire lungs was lower for patients with CF than for control subjects (mean ± standard deviation, 0.09 mL/mL ± 0.03 vs 0.11 mL/mL ± 0.03, respectively; P = .007). QV ratios of upper to lower lung halves were lower in patients with CF than in control subjects (right, 0.84 ± 0.2 vs 1.16 ± 0.2, respectively [P < .001]; left, 0.88 ± 0.3 vs 1.11 ± 0.1, respectively [P = .017]). Accordingly, ventilation differences between the groups were larger in the upper halves (Δ = 0.04 mL/mL, P ≤ .001-.002). QV values of patients with CF correlated with forced vital capacity (r = 0.7; 95% confidence interval [CI]: 0.21, 0.91), residual volume (static hyperinflation, r = -0.8; 95% CI: -0.94, 0.42), and forced expiratory volume in 1 second (airway obstruction, r = 0.7; 95% CI: 0.21, 0.91). Disseminated small ventilation deficits were the most frequent involvement pattern, present in 40% of the functional maps in CF versus 8% in the control subjects (P < .001). Conclusion: SENCEFUL MR imaging is feasible for QV assessment. Less QV, especially in upper lung parts, and correlation to vital capacity and to markers for hyperinflation and airway obstruction were found in patients with CF