Comparison of k‐t SENSE/k‐t BLAST with conventional SENSE applied to BOLD fMRI

Purpose:

To compare k‐t BLAST (broad‐use linear‐acquisition speedup technique)/k‐t SENSE (sensitivity encoding) with conventional SENSE applied to a simple fMRI paradigm.

Materials and Methods:

Blood oxygen level‐dependent (BOLD) functional magnetic resonance imaging (fMRI) was performed at 3 T using a displaced ultra‐fast low‐angle refocused echo (UFLARE) pulse sequence with a visual stimulus in a block paradigm. Conventional SENSE and k‐t BLAST/k‐t SENSE data were acquired. Also, k‐t BLAST/k‐t SENSE was simulated at different undersampling factors from fully sampled data after removal of lines of k‐space data. Analysis was performed using SPM5.

Results:

Sensitivity to the BOLD response in k‐t BLAST/k‐t SENSE was comparable with that of SENSE in images acquired at an undersampling factor of 2.3. Simulated k‐t BLAST/k‐t SENSE yielded reliable detection of activation‐induced BOLD contrast at undersampling factors of 5 or less. Sensitivity increased significantly when training data were included in k‐space before Fourier transformation (known as “plug‐in”).

Conclusion:

k‐t BLAST/k‐t SENSE performs at least as well as conventional SENSE for BOLD fMRI at a modest undersampling factor. Results suggest that sufficient sensitivity to BOLD contrast may be achievable at higher undersampling factors with k‐t BLAST/k‐t SENSE than with conventional parallel imaging approaches, offering particular advantages at the highest magnetic field strengths. J. Magn. Reson. Imaging 2010;32:235–241. © 2010 Wiley‐Liss, Inc.     

Accelerated time-resolved three-dimensional MR velocity mapping of blood flow patterns in the aorta using SENSE and k-t BLAST

Purpose

To assess the feasibility and potential limitations of the acceleration techniques SENSE and k-t BLAST for time-resolved three-dimensional (3D) velocity mapping of aortic blood flow. Furthermore, to quantify differences in peak velocity versus heart phase curves.

Materials and methods

Time-resolved 3D blood flow patterns were investigated in eleven volunteers and two patients suffering from aortic diseases with accelerated PC-MR sequences either in combination with SENSE (R = 2) or k-t BLAST (6-fold). Both sequences showed similar data acquisition times and hence acceleration efficiency. Flow-field streamlines were calculated and visualized using the GTFlow software tool in order to reconstruct 3D aortic blood flow patterns. Differences between the peak velocities from single-slice PC-MRI experiments using SENSE 2 and k-t BLAST 6 were calculated for the whole cardiac cycle and averaged for all volunteers.

Results

Reconstruction of 3D flow patterns in volunteers revealed attenuations in blood flow dynamics for k-t BLAST 6 compared to SENSE 2 in terms of 3D streamlines showing fewer and less distinct vortices and reduction in peak velocity, which is caused by temporal blurring. Solely by time-resolved 3D MR velocity mapping in combination with SENSE detected pathologic blood flow patterns in patients with aortic diseases. For volunteers, we found a broadening and flattering of the peak velocity versus heart phase diagram between the two acceleration techniques, which is an evidence for the temporal blurring of the k-t BLAST approach.

Conclusion

We demonstrated the feasibility of SENSE and detected potential limitations of k-t BLAST when used for time-resolved 3D velocity mapping. The effects of higher k-t BLAST acceleration factors have to be considered for application in 3D velocity mapping.     

Accelerated cardiac perfusion imaging using k‐t SENSE with SENSE training

In k‐t sensitivity encoding (SENSE), MR data acquisition performed in parallel by multiple coils is accelerated by sparsely sampling the k‐space over time. The resulting aliasing is resolved by exploiting spatiotemporal correlations inherent in dynamic images of natural objects. In this article, a modified k‐t SENSE reconstruction approach is presented, which aims at improving the temporal fidelity of first‐pass, contrast‐enhanced myocardial perfusion images at high accelerations. The proposed technique is based on applying parallel imaging on the training data in order to increase their spatial resolution. At a net acceleration of 5.8 (k‐t factor = 8, training profiles = 11) accurate representations of dynamic signal‐intensities were achieved. The efficacy of this approach as well as limitations due to noise amplification were investigated in computer simulations and in vivo experiments. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

k‐t PCA: Temporally constrained k‐t BLAST reconstruction using principal component analysis

The k‐t broad‐use linear acquisition speed‐up technique (BLAST) has become widespread for reducing image acquisition time in dynamic MRI. In its basic form k‐t BLAST speeds up the data acquisition by undersampling k‐space over time (referred to as k‐t space). The resulting aliasing is resolved in the Fourier reciprocal x‐f space (x = spatial position, f = temporal frequency) using an adaptive filter derived from a low‐resolution estimate of the signal covariance. However, this filtering process tends to increase the reconstruction error or lower the achievable acceleration factor. This is problematic in applications exhibiting a broad range of temporal frequencies such as free‐breathing myocardial perfusion imaging. We show that temporal basis functions calculated by subjecting the training data to principal component analysis (PCA) can be used to constrain the reconstruction such that the temporal resolution is improved. The presented method is called k‐t PCA. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Accelerating cine phase-contrast flow measurements using k-t BLAST and k-t SENSE.

Conventional phase-contrast velocity mapping in the ascending aorta was combined with k-t BLAST and k-t SENSE. Up to 5.3-fold net acceleration was achieved, enabling single breath-hold acquisitions. A standard phase-contrast (PC) sequence with interleaved acquisition of the velocity-encoded segments was modified to collect data in 2 stages, a high-resolution under sampled and a low-resolution fully sampled training stage. In addition, a modification of the k-t reconstruction strategy was tested. This strategy, denoted as "plug-in," incorporates data acquired in the training stage into the final reconstruction for improved data consistency, similar to conventional keyhole. "k-t SENSE plug-in" was found to provide best image quality and most accurate flow quantification. For this strategy, at least 10 training profiles are required to yield accurate stroke volumes (relative deviation <5%) and good image quality. In vivo 2D cine velocity mapping was performed in 6 healthy volunteers with 30-32 cardiac phases (spatial resolution 1.3 x 1.3 x 8-10 mm(3), temporal resolution of 18-38 ms), yielding relative stroke volumes of 106 +/- 18% (mean +/- 2*SD) and 112 +/- 15% for 3.8 x and 5.3 x net accelerations, respectively. In summary, k-t BLAST and k-t SENSE are promising approaches that permit significant scan-time reduction in PC velocity mapping, thus making high-resolution breath-held flow quantification possible.     

Breathheld autocalibrated phase‐contrast imaging

Purpose:

To compare generalized autocalibrating partially parallel acquisitions (GRAPPA), modified sensitivity encoding (mSENSE), and SENSE in phase‐contrast magnetic resonance imaging (PC‐MRI) applications.

Materials and Methods:

Aliasing of the torso can occur in PC‐MRI applications. If the data are further undersampled for parallel imaging, SENSE can be problematic in correctly unaliasing signals due to coil sensitivity maps that do not match that of the aliased volume. Here, a method for estimating coil sensitivities in flow applications is described. Normal volunteers (n = 5) were scanned on a 1.5 T MRI scanner and underwent PC‐MRI scans using GRAPPA, mSENSE, SENSE, and conventional PC‐MRI acquisitions. Peak velocity and flow through the aorta and pulmonary artery were evaluated.

Results:

Bland–Altman statistics for flow in the aorta and pulmonary artery acquired with mSENSE and GRAPPA methods (R = 2 and R = 3 cases) have comparable mean differences to flow acquired with conventional PC‐MRI. GRAPPA and mSENSE PC‐MRI have more robust measurements than SENSE when there is aliasing artifact caused by insufficient coil sensitivity maps. For peak velocity, there are no considerable differences among the mSENSE, GRAPPA, and SENSE reconstructions and are comparable to conventional PC‐MRI.

Conclusion:

Flow measurements of images reconstructed with autocalibration techniques have comparable agreement with conventional PC‐MRI and provide robust measurements in the presence of wraparound. J. Magn. Reson. Imaging 2010;31:1004–1014. ©2010 Wiley‐Liss, Inc.     

Sparsity transform k‐t principal component analysis for accelerating cine three‐dimensional flow measurements

Time‐resolved three‐dimensional flow measurements are limited by long acquisition times. Among the various acceleration techniques available, k‐t methods have shown potential as they permit significant scan time reduction even with a single receive coil by exploiting spatiotemporal correlations. In this work, an extension of k‐t principal component analysis is proposed utilizing signal differences between the velocity encodings of three‐directional flow measurements to further compact the signal representation and hence improve reconstruction accuracy. The effect of sparsity transform in k‐t principal component analysis is demonstrated using simulated and measured data of the carotid bifurcation. Deploying sparsity transform for 8‐fold undersampled simulated data, velocity root‐mean‐square errors were found to decrease by 52 ± 14%, 59 ± 11%, and 16 ± 32% in the common, external, and internal carotid artery, respectively. In vivo, errors were reduced by 15 ± 17% in the common carotid artery with sparsity transform. Based on these findings, spatial resolution of three‐dimensional flow measurements was increased to 0.8 mm isotropic resolution with prospective 8‐fold undersampling and sparsity transform k‐t principal component analysis reconstruction. Volumetric data were acquired in 6 min. Pathline visualization revealed details of helical flow patterns partially hidden at lower spatial resolution. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Highly undersampled phase‐contrast flow measurements using compartment‐based k–t principal component analysis

The applicability of cine blood flow measurements in a clinical setting is often compromised by the long scan times associated with phase‐contrast imaging. In this work, we propose an extension to the k–t principal component analysis method and demonstrate that by definition of spatial compartment‐dependent temporal basis functions, significant improvements in reconstruction accuracy can be achieved relative to the original k–t principal component analysis and k–t SENSE formulations. Using this method, it is shown that prospective nominal undersampling of up to 16 corresponding to a net acceleration factor of 8 including training data acquisition can be realized while keeping the error in stroke volume below 5%. As a practical application, the acquisition of cine flow data in the aorta is demonstrated permitting assessment of two‐dimensional velocity images and pulse wave velocities at 100 frames per second in a single breathhold per slice. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Flow and peak velocity measurements in patients with aortic valve stenosis using phase contrast MR accelerated with k-t BLAST

Objective

To investigate the accuracy of velocity measurements in patients with aortic valve stenosis using phase contrast (PC) imaging accelerated with SENSE (Sensitivity Encoding) and k-t BLAST (Broad-use Linear Acquisition Speed-up Technique).

Methods

Accelerated quantitative breath hold PC measurements, using SENSE and k-t BLAST, were performed in twelve patients whose aortic valve stenosis had been initially diagnosed using echocardiography. Stroke volume (SV) and peak velocity measurements were performed on each subject in three adjacent slices using both accelerating methods.

Results

The peak velocities measured with PC MRI using SENSE were −8.0 ± 9.5% lower (p < 0.01) compared to the peak velocities measured with k-t BLAST and the correlation was r = 0.83. The stroke volumes when using SENSE were slightly higher 0.4 ± 17.1 ml compared to the SV obtained using k-t BLAST but the difference was not significant (p > 0.05).

Conclusions

In this study higher peak velocities were measured in patients with aortic stenosis when combining k-t BLAST with PC MRI compared to PC MRI using SENSE. A probable explanation of this difference is the higher temporal resolution achieved in the k-t BLAST measurement. There was, however, no significant difference between calculated SV based on PC MRI using SENSE and k-t BLAST, respectively.     

Split‐acquisition real‐time CINE phase‐contrast MR flow measurements

The temporal and spatial resolution of real‐time phase‐contrast magnetic resonance (PCMR) is restricted by the need to acquire two interleaved phase images. In this article, we propose a split‐acquisition real‐time CINE PCMR technique, where the acquisition of flow‐encoded and flow‐compensated data is divided into separate blocks. By comparing magnitude images, automatic matching of data in cardio‐respiratory space allows subtraction of background phase offsets. Thus, the data is acquired in real‐time but with phase correction originating from a different heart beat. This effectively doubles the frame rate, allowing either higher temporal or spatial resolution. Two split‐acquisition sequences were tested: one with high‐temporal resolution and one with high‐spatial resolution. Both sequences showed excellent agreement in stroke volumes in 20 adults when validated against cardiac‐gated PCMR and interleaved real‐time PCMR (cardiac gated: 95.2 ± 20.0 mL, interleaved real‐time: 96.2 ± 20.7 mL, high‐temporal resolution: 95.6 ± 20.1 mL, high‐spatial resolution: 95.5 ± 20.4 mL). In six children, the high‐spatial resolution sequence provided more accurate flow measurements than interleaved real‐time PCMR, when compared with cardiac‐gated PCMR (cardiac gated: 20.6 ± 7.6 mL, interleaved real‐time: 24.3 ± 9.2 mL, high‐spatial resolution: 20.8 ± 7.8 mL), due to the increased spatial resolution. The matching technique is shown to be accurate (truth: 94.6 ± 21.8, split‐acquisition: 95.0 ± 21.9 mL) and quantitative image quality (signal‐to‐noise ratio, velocity‐to‐noise ratio and edge sharpness) is acceptable. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

k-t BLAST and k-t SENSE: dynamic MRI with high frame rate exploiting spatiotemporal correlations.

Dynamic images of natural objects exhibit significant correlations in k-space and time. Thus, it is feasible to acquire only a reduced amount of data and recover the missing portion afterwards. This leads to an improved temporal resolution, or an improved spatial resolution for a given amount of acquisition. Based on this approach, two methods were developed to significantly improve the performance of dynamic imaging, named k-t BLAST (Broad-use Linear Acquisition Speed-up Technique) and k-t SENSE (SENSitivity Encoding) for use with a single or multiple receiver coils, respectively. Signal correlations were learned from a small set of training data and the missing data were recovered using all available information in a consistent and integral manner. The general theory of k-t BLAST and k-t SENSE is applicable to arbitrary k-space trajectories, time-varying coil sensitivities, and under- and overdetermined reconstruction problems. Examples from ungated cardiac imaging demonstrate a 4-fold acceleration (voxel size 2.42 x 2.52 mm(2), 38.4 fps) with either one or six receiver coils. k-t BLAST and k-t SENSE are applicable to many areas, especially those exhibiting quasiperiodic motion, such as imaging of the heart, the lungs, the abdomen, and the brain under periodic stimulation.     

k‐t FOCUSS: A general compressed sensing framework for high resolution dynamic MRI

A model‐based dynamic MRI called k‐t BLAST/SENSE has drawn significant attention from the MR imaging community because of its improved spatio‐temporal resolution. Recently, we showed that the k‐t BLAST/SENSE corresponds to the special case of a new dynamic MRI algorithm called k‐t FOCUSS that is optimal from a compressed sensing perspective. The main contribution of this article is an extension of k‐t FOCUSS to a more general framework with prediction and residual encoding, where the prediction provides an initial estimate and the residual encoding takes care of the remaining residual signals. Two prediction methods, RIGR and motion estimation/compensation scheme, are proposed, which significantly sparsify the residual signals. Then, using a more sophisticated random sampling pattern and optimized temporal transform, the residual signal can be effectively estimated from a very small number of k‐t samples. Experimental results show that excellent reconstruction can be achieved even from severely limited k‐t samples without aliasing artifacts. Magn Reson Med 61:103–116, 2009. © 2008 Wiley‐Liss, Inc.     

Analysis and correction of background velocity offsets in phase‐contrast flow measurements using magnetic field monitoring

The value of phase‐contrast magnetic resonance imaging for quantifying tissue motion and blood flow has been long recognized. However, the sensitivity of the method to system imperfections can lead to inaccuracies limiting its clinical acceptance. A key source of error relates to eddy current‐induced phase fluctuations, which can offset the measured object velocity significantly. A higher‐order dynamic field camera was used to study the spatiotemporal evolution of background phases in cine phase‐contrast measurements. It is demonstrated that eddy current‐induced offsets in phase‐difference data are present up to the second spatial order. Oscillatory temporal behaviors of offsets in the kHz range suggest mechanical resonances of the MR system to be non‐negligible in phase‐contrast imaging. By careful selection of the echo time, their impact can be significantly reduced. When applying field monitoring data for correcting eddy current and mechanically induced velocity offsets, errors decrease to less than 0.5% of the maximum velocity for various sequence settings proving the robustness of the correction approach. In vivo feasibility is demonstrated for aortic and pulmonary flow measurements in five healthy subjects. Using field monitoring data, mean error in stroke volume was reduced from 10% to below 3%. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Reconstruction of divergence‐free velocity fields from cine 3D phase‐contrast flow measurements

Three‐dimensional phase‐contrast velocity vector field mapping shows great potential for clinical applications; however measurement inaccuracies may limit the utility and robustness of the technique. While parts of the error in the measured velocity fields can be minimized by background phase estimation in static tissue and magnetic field monitoring, considerable inaccuracies remain. The present work introduces divergence‐reduction processing of 3D phase‐contrast flow data based on a synergistic combination of normalized convolution and divergence‐free radial basis functions. It is demonstrated that this approach effectively addresses erroneous flow for image reconstructions from both fully sampled and undersampled data. Using computer simulations and in vivo data acquired in the aorta of healthy subjects and a stenotic valve patient it is shown that divergence arising from measurement imperfections can be reduced by up to 87% resulting in improved vector field representations. Based on the results obtained it is concluded that integration of the divergence‐free condition into postprocessing of vector fields presents an efficient approach to addressing flow field inaccuracies. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Optimizing spatiotemporal sampling for k-t BLAST and k-t SENSE: application to high-resolution real-time cardiac steady-state free precession.

In k-t BLAST and k-t SENSE, data acquisition is accelerated by sparsely sampling k-space over time. This undersampling in k-t space causes the object signals to be convolved with a point spread function in x-f space (x = spatial position, f = temporal frequency). The resulting aliasing is resolved by exploiting spatiotemporal correlations within the data. In general, reconstruction accuracy can be improved by controlling the k-t sampling pattern to minimize signal overlap in x-f space. In this work, we describe an approach to obtain generally favorable patterns for typical image series without specific knowledge of the image series itself. These optimized sampling patterns were applied to free-breathing, untriggered (i.e., real-time) cardiac imaging with steady-state free precession (SSFP). Eddy-current artifacts, which are otherwise increased drastically in SSFP by the undersampling, were minimized using alternating k-space sweeps. With the synergistic combination of the k-t approach with optimized sampling and SSFP with alternating k-space sweeps, it was possible to achieve a high signal-to-noise ratio, high contrast, and high spatiotemporal resolutions, while achieving substantial immunity against eddy currents. Cardiac images are shown, demonstrating excellent image quality and an in-plane resolution of approximately 2.0 mm at >25 frames/s, using one or more receiver coils.     

Effect of improving spatial or temporal resolution on image quality and quantitative perfusion assessment with k‐t SENSE acceleration in first‐pass CMR myocardial perfusion imaging

k‐t Sensitivity‐encoded (k‐t SENSE) acceleration has been used to improve spatial resolution, temporal resolution, and slice coverage in first‐pass cardiac magnetic resonance myocardial perfusion imaging. This study compares the effect of investing the speed‐up afforded by k‐t SENSE acceleration in spatial or temporal resolution. Ten healthy volunteers underwent adenosine stress myocardial perfusion imaging using four saturation‐recovery gradient echo perfusion sequences: a reference sequence accelerated by sensitivity encoding (SENSE), and three k‐t SENSE–accelerated sequences with higher spatial resolution (“k‐t High”), shorter acquisition window (“k‐t Fast”), or a shared increase in both parameters (“k‐t Hybrid”) relative to the reference. Dark‐rim artifacts and image quality were analyzed. Semiquantitative myocardial perfusion reserve index (MPRI) and Fermi‐derived quantitative MPR were also calculated. The k‐t Hybrid sequence produced highest image quality scores at rest (P = 0.015). Rim artifact thickness and extent were lowest using k‐t High and k‐t Hybrid sequences (P < 0.001). There were no significant differences in MPRI and MPR values derived by each sequence. Maximizing spatial resolution by k‐t SENSE acceleration produces the greatest reduction in dark rim artifact. There is good agreement between k‐t SENSE and standard acquisition methods for semiquantitative and fully quantitative myocardial perfusion analysis. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.