Simon Bauer

Three-dimensional analysis of segmental wall shear stress in the aorta by flow-sensitive four-dimensional-MRI

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.

In vivo wall shear stress distribution in the carotid artery: effect of bifurcation geometry, internal carotid artery stenosis, and recanalization therapy.

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 (β, 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 β, 0.48; tortuosity β, ≤−0.56; bifurcation angle β, ≥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.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.

In vivo assessment of wall shear stress in the atherosclerotic aorta using flow-sensitive 4D MRI

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.

Parallel MRI with extended and averaged GRAPPA kernels (PEAK-GRAPPA): Optimized spatiotemporal dynamic imaging†

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.

MR-based visualization and quantification of three-dimensional flow characteristics in the portal venous system

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.

On the undersampling strategies to accelerate time-resolved 3D imaging using k-t-GRAPPA†

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.

The effect of reconstruction and acquisition parameters for GRAPPA-based parallel imaging on the image quality

Parallel imaging based on generalized autocalibrating partially parallel acquisitions is widely used in the clinical routine. To date, no detailed analysis has been presented describing the dependence of the image quality on the reconstruction and acquisition parameters such as the number of autocalibration signal (ACS) lines NACS, the reconstruction kernel size (bx × by), and the undersampling factor R. To evaluate their influence on the performance of generalized autocalibrating partially parallel acquisitions, two phantom data sets acquired with 12‐channel and 32‐channel receive coils and three in vivo measurements were analyzed. Reconstruction parameters were systematically varied between R = 2–4, NACS = 4–64, bx = 1–9, and by = 2–10 to characterize their influence on image quality and noise. A main aspect of the analysis was to optimize the parameter set with respect to the effectively achieved net image acceleration. Selecting the undersampling factor R as small as possible for a given net acceleration yielded the best result in a clear majority of cases. For all data sets and coil geometries, the optimal kernel sizes and number of ACS lines were similar for a chosen undersampling factor R. In summary, the number of ACS lines should not be chosen below NACS = 10–16. A robust choice for the kernel size was bx = 9 and by = 2–4. Magn Reson Med, 2011.

Increasing efficiency of parallel imaging for 2D multislice acquisitions.

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 2009

K-t GRAPPA accelerated phase contrast MRI: Improved assessment of blood flow and 3-directional myocardial motion during breath-hold.

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.

Three-directional acceleration phase mapping of myocardial function†

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/s2 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. Magn Reson Med, 2011.