To evaluate spatiotemporal parallel imaging with R = 5 in comparison to conventional parallel imaging with R = 2 applied to phase contrast (PC) magnetic resonance imaging (MRI). This was motivated by the fact that scan times for PC imaging often exceed breath‐hold capabilities of patients even with standard parallel imaging using typical reduction factors of R = 2.

MR-PET Respiration Compensation Using Self-Gated Motion Modeling Robert Grimm, Sebastian Fürst, Isabel Dregely, Stephan Nekolla, Sibylle Ziegler, Simon Bauer, Dominik Nickel, Berthold Kiefer, Joachim Hornegger, Markus Schwaiger, and Tobias Block Pattern Recognition Lab, FAU Erlangen-Nuremberg, Erlangen, Germany, Department of Nuclear Medicine, Technische Universität München, Munich, Germany, Siemens Healthcare, Erlangen, Germany, Department of Radiology, NYU Langone Medical Center, New York City, NY, United States

Background— Hemodynamics may play a role contributing to the progression of bicuspid aortic valve (BAV) aortopathy. This study measured the impact of BAV on the distribution of regional aortic wall shear stress (WSS) compared with control cohorts. Methods and Results— Local WSS distribution was measured in the thoracic aorta of 60 subjects using 4-dimensional (4D) flow-sensitive magnetic resonance imaging. WSS analysis included 15 BAV patients: 12 with fusion of the right-left coronary cusp (6 stenotic) and 3 with fusion of the right and noncoronary cusp. The right-left BAV cohort was compared with healthy subjects (n=15), age-appropriate subjects (n=15), and age-/aorta size–controlled subjects (n=15). Compared with the age-appropriate and age-/aorta size–matched controls, WSS patterns in the right-left BAV ascending aorta were significantly elevated, independent of stenosis severity (peak WSS=0.9±0.3 N/m 2 compared with 0.4±0.3 N/m 2 in age-/aorta size–controlled subjects; P <0.001). Time-resolved (cine) 2D images of the bicuspid valves were coregistered with 4D flow data, directly linking cusp fusion pattern to a distinct ascending aortic flow jet pattern. The observation of right-anterior ascending aorta wall/jet impingement in right-left BAV patients corresponded to regions with statistically elevated WSS. Alternative jetting patterns were observed in the right and noncoronary cusp fusion patients. Conclusions— The results of this study demonstrate that bicuspid valves induced significantly altered ascending aorta hemodynamics compared with age- and size-matched controls with tricuspid valves. Specifically, the expression of increased and asymmetric WSS at the aorta wall was related to ascending aortic flow jet patterns, which were influenced by the BAV fusion pattern.

INTRODUCTION: Motion due to respiration poses a significant challenge for MR imaging of the abdomen and thorax. However, it is possible to compensate for the motion, e.g. by accepting data from only a certain respiration phase, when the temporal phases during the acquisition are known. Traditionally, the respiration phases are obtained using respiration bellows, but the clinical acceptance of such devices is very low. Self-gating techniques determine this information directly from the measurement data, in contrast to navigator techniques that perform additional intermediate navigation scans. Because the navigation scans interrupt the steady state of the magnetization and because navigators prolong the overall scan time, self-gating is favorable for T1-weighted gradient-echo (GRE) sequences. Radial k-space trajectories are particularly well suited because they continuously sample the center of k-space, which allows for detection of respiratory motion. Several algorithms have been proposed for the signal extraction where in most methods a suitable coil element has to be selected manually to get reasonable results. This is time-consuming for modern MRI systems with high number of receive channels, and infeasible for clinical routine applications. Here, we propose a fully automated procedure to generate proper self-gating signals for 3D stack-of-stars GRE sequences with golden-angle sampling, which are comparable to the signal from a respiratory bellows.

Introduction & Purpose: Optimized, single-slab, 3D fast/turbo spin-echo (FSE/TSE) imaging (e.g., CUBE [GE], SPACE [Siemens], VISTA [Philips]) has recently gained popularity for a variety of clinical applications. While currently-available commercial implementations use conventional Cartesian (rectilinear) sampling of k space, certain applications may benefit from the favorable properties of non-Cartesian k-space trajectories. Several previous research studies using FSE/TSE have shown potential benefits for radial or spiral trajectories, particularly for reducing sensitivity to motion [e.g., 1,2]. The purpose of our current work was to incorporate radial k-space sampling into optimized, single-slab, 3D FSE/TSE imaging, and to perform a preliminary evaluation of imaging performance. Our long-term goal is to develop a single-slab 3D FSE/TSE method that has substantially reduced sensitivity to motion relative to existing rectilinear-based techniques. Compared to previous research in this area, an open question is whether acceptable image quality can be achieved when radial k-space trajectories are combined with the very long spin-echo trains and variable-flip-angle refocusing RF pulses used in state-of-the-art single-slab 3D FSE/TSE techniques.

Background Parallel imaging can reduce scan time with typical reduction factors of ~2, often not sufficient to reduce scan time to breath hold duration. Spatiotemporal parallel imaging such as k-t-GRAPPA allows for a significantly higher speed-up in data acquisition, but previous studies mostly acquired full k-space data while removing data retrospectively. Here, aortic flow scans were acquired during breath-hold using kt-GRAPPA based reconstruction [1,2] and compared to conventional protocols using GRAPPA and navigator respiration gating.

OBJECTIVES:To evaluate the feasibility of free-breathing, dynamic contrast-enhanced (DCE) MRI of the abdomen and thorax using the radial-gradient-echo sequence with k-space weighted image contrast (KWIC) reconstruction.METHODS:Institutional review board approval was obtained. Fourteen patients underwent free-breathing radial DCE-MRI. Radial MRI yielded full-frame images by gridding all k-space data and time-resolved subframe images by using KWIC reconstruction technique. Using subframe KWIC images, voxel-wise perfusion maps were created. For comparison, the breath-hold conventional Cartesian 3D-gradient-echo sequence (VIBE) was also performed during the equilibrium phase. The image qualities of radial and conventional VIBE images were compared quantitatively and qualitatively.RESULTS:Radial DCE-MRI provided high spatial resolution (1.4 × 1.4 mm) and temporal resolution (4.1 s for subframe images) allowing voxel-wise perfusion mapping with negligible motion or streaking artefacts. There were no significant differences in SNR between full-frame radial images and conventional VIBE images (79.08 vs 74.80, P > 0.05). Overall image quality score of full-frame radial images was slightly lower than that of conventional VIBE images (3.88 ± 0.59 vs. 4.31 ± 0.97, P < 0.05), but provided clinically useful images.CONCLUSIONS:The free-breathing radial DCE-MRI can provide high spatial and temporal resolution while maintaining reasonably high image quality and thus is a feasible technique for DCE-MRI in the abdomen and thorax.KEY POINTS:• Dynamic contrast-enhanced magnetic resonance imaging (DCE) MRI is important in oncological imaging • Radial MRI with k-space weighted image contrast (KWIC) reconstruction offers potential improvements • Radial DCE-MRI provides good image quality, reduced artefacts and high spatial/temporal resolution.

The purpose of this study was to explore how to optimally undersample and reconstruct time‐resolved 3D data using a k‐t‐space‐based GRAPPA technique. The performance of different reconstruction strategies was evaluated using data sets with different ratios of phase (Ny) and partition (Nz) encoding lines (Ny × Nz = 64–128 × 40–64) acquired in a moving phantom. Image reconstruction was performed for different kernel configurations and different reduction factors (R = 5, 6, 8, and 10) and was evaluated using regional error quantification and SNR analysis. To analyze the temporal fidelity of the different kernel configurations in vivo, time‐resolved 3D phase contrast data were acquired in the thoracic aorta of two healthy volunteers. Results demonstrated that kernel configurations with a small kernel extension yielded superior results especially for more asymmetric data matrices as typically used in clinical applications. The application of k‐t‐GRAPPA to in vivo data demonstrated the feasibility of undersampling of time‐resolved 3D phase contrast data set with a nominal reduction factors of up to Rnet = 8, while maintaining the temporal fidelity of the measured velocity field. Extended GRAPPA‐based parallel imaging with optimized multidimensional reconstruction kernels has the potential to substantially accelerate data acquisitions in time‐resolved 3D MRI. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.

An optimized acceleration encoded phase contrast method termed "acceleration phase mapping" for the assessment of regional myocardial function is presented. Based on an efficient gradient waveform design using two-sided encoding for in vivo three-directional acceleration mapping, echo and repetition times TE = 12-14 ms and TR = 15-17 ms for low accelerations sensitivity aenc = 5-8 m/s(2) were achieved. In addition to phantom validation, the technique was applied in a study with 10 healthy volunteers at 1.5T and 3T to evaluate its feasibility to assess regional myocardial acceleration at 1.5T and 3T. Results of the acceleration measurements were compared with the temporal derivative of myocardial velocities from three-directional velocity encoded standard phase contrast MRI in the same volunteers. The feasibility to assess myocardial acceleration along the radial, circumferential, and longitudinal direction of the left ventricle was demonstrated. Despite improved signal-to-noise-ratio at 3T (34% increase compared with 1.5T), image quality with respect to susceptibility artifacts was better 1.5T compared with 3T. Analysis of global and regional left ventricular acceleration showed characteristic patterns of systolic and diastolic acceleration and deceleration. Comparisons of directly measured and derived myocardial acceleration dynamics over the cardiac cycle revealed good correlation (r = 0.45-0.68, P < 0.01) between both methods.

Our purpose was to correlate atherogenic low wall shear stress (WSS) and high oscillatory shear index (OSI) with the localization of aortic plaques. Flow‐sensitive four‐dimensional MRI was used to acquire three‐dimensional blood flow in the aorta of 62 patients with proven aortic atherosclerosis and 31 healthy volunteers. Multiplanar data analysis of WSS magnitude and OSI in 12 wall segments was performed in analysis planes distributed along the aorta. Disturbed WSS and OSI were defined as areas exposed to low WSS magnitude and high OSI beyond individual 15% thresholds. Planewise analysis revealed a good correlation (r = 0.85) of individual low WSS magnitude but not of high OSI with plaque distribution. Although plaques occurred only rarely in the ascending aorta, the incidence of low WSS magnitude and high OSI was similar to findings in other aortic segments where plaques occurred more frequently. Case‐by‐case comparisons of plaque location and critical wall parameters revealed a shift of atherogenic WSS magnitude (78% of all cases) and OSI (91%) to wall segments adjacent to the atheroma. Our results indicate that the predictive value of WSS for plaque existence depends on the aortic segment and that locations of critical wall parameters move to neighboring segments of regions affected by atherosclerosis. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.

PURPOSE:To evaluate the feasibility of time-resolved flow-sensitive MRI for the three-dimensional (3D) visualization and quantification of normal and pathological portal venous (PV) hemodynamics.MATERIALS AND METHODS:Portal venous hemodynamics were evaluated in 18 healthy volunteers and 5 patients with liver cirrhosis. ECG- and adaptive respiratory navigator gated flow-sensitive 4D MRI (time-resolved 3D MRI with three-directional velocity encoding) was performed on a 3 Tesla MR system (TRIO, Siemens, Germany). Qualitative flow analysis was achieved using 3D streamlines and time-resolved particle traces originating from seven emitter planes precisely placed at anatomical landmarks in the PV system. Quantitative analysis included retrospective extraction of regional peak and mean velocities and vessel area. Results were compared with standard 2D flow-sensitive MRI and to the reference standard Doppler ultrasound.RESULTS:Qualitative flow analysis was successfully used in the entire PV system. Venous hemodynamics in all major branches in 17 of 18 volunteers and 3 of 5 patients were reliably depicted with good interobserver agreement (kappa = 0.62). Quantitative analysis revealed no significant differences and moderate agreement for peak velocities between 3D MR and 2D MRI (r = 0.46) and Doppler ultrasound (US) (r = 0.35) and for mean velocities between 3D and 2D MRI (r = 0.41). The PV area was significantly (P < 0.01) higher in 3D and 2D MRI compared with US.CONCLUSION:We successfully applied 3D MR velocity mapping in the PV system, providing a detailed qualitative and quantitative analysis of normal and pathological hemodynamics.

Phase contrast MRI with multidirectional velocity encoding requires multiple acquisitions of the same k-space lines to encode the underlying velocities, which can considerably lengthen the total scan time. To reduce scan time, parallel imaging is often applied. In dynamic phase contrast MRI using standard generalized autocalibrating partially parallel acquisitions (GRAPPA), several central k-spaces for autocalibration of the reconstruction (autocalibrating signal lines (ACS)) are typically acquired, separately for each velocity direction and each cardiac timeframe, for calculating the reconstruction weights. To further accelerate data acquisition, we developed two methods, which calculated weights with a substantially reduced number of ACSl lines. The effects on image quality and flow quantification were compared to fully sampled data, standard GRAPPA, and time-interleaved sampling scheme in combination with generalized autocalibrating partially parallel acquisitions (TGRAPPA). The results show that the two proposed methods can clearly improve scan efficiency while maintaining image quality and accuracy of measured flow or myocardial tissue velocities. Compared to TGRAPPA, the proposed methods were more accurate in evaluating flow velocity. In conclusion, the proposed reconstruction strategies are promising for dynamic multidirectionally encoded acquisitions and can easily be implemented using the standard GRAPPA reconstruction algorithm.

Background—The purpose of this study was to analyze the in vivo distribution of absolute wall shear stress (WSSabs) and oscillatory shear index (OSI) in the carotid bifurcation and to evaluate its dependence on bifurcation geometry, the presence of internal carotid artery (ICA) stenosis, and recanalization therapy. Methods and Results—Time-resolved 3D blood flow was acquired with flow-sensitive 4D MRI in 64 normal carotid bifurcations and 17 carotid arteries with moderate ICA stenosis (48±6%) or after surgical recanalization. Among 64 normal arteries, atherogenic wall parameters were consistently concentrated in proximal bulb regions of the common (CCA) and internal (ICA) carotid arteries. The fraction of the carotid bulb exposed to atherosclerosis-prone wall parameters (low WSSabs below and high OSI above group-defined 20% and 10% thresholds) was correlated with the individual bifurcation geometry. Multiple regressions revealed significant (P<0.01) relationships (&bgr;, 0.44 to 0.48) between the areas with atherosclerosis-prone wall parameters and the dICA/dCCA diameter ratio. The size of regions exposed to high OSI demonstrated highly significant (P⩽0.01) relationships with all analyzed geometry parameters (dICA/dCCA &bgr;, 0.48; tortuosity &bgr;, ⩽−0.56; bifurcation angle &bgr;, ≥0.47). Moderate ICA stenosis altered the distribution of wall parameters (45%/61% reduction of individually low WSSabs/high OSI in the proximal ICA), which were relocated to segments distal to the arterial stenosis. WSSabs/OSI topology after recanalization was similar compared with the normal wall parameter distribution. Conclusions—Flow-sensitive 4D MRI identified alterations in the segmental in vivo WSS distribution associated with atherosclerotic disease, surgical therapy, and individual bifurcation geometry and could be a valuable technique to assess the individual risk of flow-mediated atherosclerosis and carotid plaque progression.

Parallel imaging algorithms require precise knowledge about the spatial sensitivity variation of the receiver coils to reconstruct images with full field of view (FOV) from undersampled Fourier encoded data. Sensitivity information must either be given a priori, or estimated from calibration data acquired along with the actual image data. In this study, two approaches are presented, which require very little or no additional data at all for calibration in two-dimensional multislice acquisitions. Instead of additional data, information from spatially adjacent slices is used to estimate coil sensitivity information, thereby increasing the efficiency of parallel imaging. The proposed approaches rely on the assumption that over a small range of slices, coil sensitivities vary smoothly in slice direction. Both methods are implemented as variants of the GRAPPA algorithm. For a given effective acceleration, superior image quality is achieved compared to the conventional GRAPPA method. For the latter calibration lines for coil weight computation must be acquired in addition to the undersampled k-spaces for coil weight computation, thus requiring higher k-space undersampling, that is, a higher reduction factor to achieve the same effective acceleration. The proposed methods are particularly suitable to speed up parallel imaging for clinical applications where the reduction factor is limited to two or three. Magn Reson Med 61:1459-1470, 2009. (C) 2009 Wiley-Liss, Inc.

Purpose: To assess the distribution and regional differences of flow and vessel wall parameters such as wall shear stress (WSS) and oscillatory shear index (OSI) in the entire thoracic aorta.Materials and Methods: Thirty-one healthy volunteers (mean age = 23.7 +/- 3.3 years) were examined by flow-sensitive four-dimensional (4D)-MRI at 3T. For eight retrospectively positioned 2D analysis planes distributed along the thoracic aorta, flow parameters and vectorial WSS and OSI were assessed in 12 segments along the vascular circumference.Results: Mean absolute time-averaged WSS ranged between 0.25 +/- 0.04 N/m(2) and 0.33 +/- 0.07 N/m(2) and incorporated a substantial circumferential component (-0.05 +/- 0.04 to 0.07 +/- 0.02 N/m(2)). For each analysis plane, a segment with lowest absolute WSS and highest OSI was identified which differed significantly from mean values within the plane (P < 0.05). The distribution of atherogenic low WSS and high OSI closely resembled typical locations of atherosclerotic lesions at the inner aortic curvature and supraaortic branches.Conclusion: The normal distribution of vectorial WSS and OSI in the entire thoracic aorta derived from flow-sensitive 4D-MRI data provides a reference constituting an important perquisite for the examination of patients with aortic disease. Marked regional differences in absolute WSS and OSI may help explaining why atherosclerotic lesions predominantly develop and progress at specific locations in the aorta.

PURPOSE:To evaluate an optimized k-t-space related reconstruction method for dynamic magnetic resonance imaging (MRI), a method called PEAK-GRAPPA (Parallel MRI with Extended and Averaged GRAPPA Kernels) is presented which is based on an extended spatiotemporal GRAPPA kernel in combination with temporal averaging of coil weights. MATERIALS AND METHODS:The PEAK-GRAPPA kernel consists of a uniform geometry with several spatial and temporal source points from acquired k-space lines and several target points from missing k-space lines. In order to improve the quality of coil weight estimation sets of coil weights are averaged over the temporal dimension. RESULTS:The kernel geometry leads to strongly decreased reconstruction times compared to the recently introduced k-t-GRAPPA using different kernel geometries with only one target point per kernel to fit. Improved results were obtained in terms of the root mean square error and the signal-to-noise ratio as demonstrated by in vivo cardiac imaging. CONCLUSION:Using a uniform kernel geometry for weight estimation with the properties of uncorrelated noise of different acquired timeframes, optimized results were achieved in terms of error level, signal-to-noise ratio, and reconstruction time.