3D diffusion tensor MRI with isotropic resolution using a steady‐state radial acquisition

Purpose

To obtain diffusion tensor images (DTI) over a large image volume rapidly with 3D isotropic spatial resolution, minimal spatial distortions, and reduced motion artifacts, a diffusion‐weighted steady‐state 3D projection (SS 3DPR) pulse sequence was developed.

Materials and Methods

A diffusion gradient was inserted in a SS 3DPR pulse sequence. The acquisition was synchronized to the cardiac cycle, linear phase errors were corrected along the readout direction, and each projection was weighted by measures of consistency with other data. A new iterative parallel imaging reconstruction method was also implemented for removing off‐resonance and undersampling artifacts simultaneously.

Results

The contrast and appearance of both the fractional anisotropy and eigenvector color maps were substantially improved after all correction techniques were applied. True 3D DTI datasets were obtained in vivo over the whole brain (240 mm field of view in all directions) with 1.87 mm isotropic spatial resolution, six diffusion encoding directions in under 19 minutes.

Conclusion

A true 3D DTI pulse sequence with high isotropic spatial resolution was developed for whole brain imaging in under 20 minutes. To minimize the effects of brain motion, a cardiac synchronized, multiecho, DW‐SSFP pulse sequence was implemented. Motion artifacts were further reduced by a combination of linear phase correction, corrupt projection detection and rejection, sampling density reweighting, and parallel imaging reconstruction. The combination of these methods greatly improved the quality of 3D DTI in the brain. J. Magn. Reson. Imaging 2009;29:1175–1184. © 2009 Wiley‐Liss, Inc.     

Impact of outliers on diffusion tensor and Q-ball imaging: clinical implications and correction strategies

Purpose:

To measure the impact of corrupted images often found to occur in diffusion‐weighted magnetic resonance imaging (DW‐MRI). To propose a robust method for the correction of outliers, applicable to diffusion tensor imaging (DTI) and q‐ball imaging (QBI).

Materials and Methods:

Monte Carlo simulations were carried out to measure the impact of outliers on DTI and QBI reconstruction in a single voxel. Methods to correct outliers based on q‐space interpolation and direction removal were then implemented and validated in real image data.

Results:

Corruption in a single voxel led to clear variations in DTI and QBI metrics. In real data, the method of q‐space interpolation was successful in identifying corrupted voxels and restoring them to values consistent with those of uncorrupted images.

Conclusion:

For images containing few gradient directions, where outlier removal was either impossible due to limited volumes or resulted in large changes in DTI/QBI metrics, q‐space interpolation proved to be the method of choice for image restoration. A simple decision support system is proposed to assist clinicians in the correction of their corrupted DW data. J. Magn. Reson. Imaging 2011;33:1491–1502. © 2011 Wiley‐Liss, Inc.     

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

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. J. Magn. Reson. Imaging 2008;28:1226–1232. © 2008 Wiley‐Liss, Inc.     

Independent slab‐phase modulation combined with parallel imaging in bilateral breast MRI

Independent slab‐phase modulation allows three‐dimensional imaging of multiple volumes without encoding the space between volumes, thus reducing scan time. Parallel imaging further accelerates data acquisition by exploiting coil sensitivity differences between volumes. This work compared bilateral breast image quality from self‐calibrated parallel imaging reconstruction methods such as modified sensitivity encoding, generalized autocalibrating partially parallel acquisitions and autocalibrated reconstruction for Cartesian sampling (ARC) for data with and without slab‐phase modulation. A study showed an improvement of image quality by incorporating slab‐phase modulation. Geometry factors measured from phantom images were more homogenous and lower on average when slab‐phase modulation was used for both mSENSE and GRAPPA reconstructions. The resulting improved signal‐to‐noise ratio (SNR) was validated for in vivo images as well using ARC instead of GRAPPA, illustrating average SNR efficiency increases in mSENSE by 5% and ARC by 8% based on region of interest analysis. Furthermore, aliasing artifacts from mSENSE reconstruction were reduced when slab‐phase modulation was used. Overall, slab‐phase modulation with parallel imaging improved image quality and efficiency for 3D bilateral breast imaging. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Diffusion imaging with prospective motion correction and reacquisition

A major source of artifacts in diffusion‐weighted imaging is subject motion. Slow bulk subject motion causes misalignment of data when more than one average or diffusion gradient direction is acquired. Fast bulk subject motion can cause signal dropout artifacts in diffusion‐weighted images and results in erroneous derived maps, e.g., fractional anisotropy maps. To address both types of artifacts, a fully automatic method is presented that combines prospective motion correction with a reacquisition scheme. Motion correction is based on the prospective acquisition correction method modified to work with diffusion‐weighted data. The images to reacquire are determined automatically during the acquisition from the imaging data, i.e., no extra reference scan, navigators, or external devices are necessary. The number of reacquired images, i.e., the additional scan duration can be adjusted freely. Diffusion‐weighted prospective acquisition correction corrects slow bulk motion well and reduces misalignment artifacts like image blurring. Mean absolute residual values for translation and rotation were <0.6 mm and 0.5°. Reacquisition of images affected by signal dropout artifacts results in diffusion maps and fiber tracking free of artifacts. The presented method allows the reduction of two types of common motion related artifacts at the cost of slightly increased acquisition time. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Halving imaging time of whole brain diffusion spectrum imaging and diffusion tractography using simultaneous image refocusing in EPI

Purpose

To increase the efficiency of densely encoded diffusion imaging of the brain, such as diffusion spectrum imaging (DSI), we time‐multiplex multiple slices within the same readout using simultaneous image refocusing echo‐planar imaging (SIR‐EPI).

Materials and Methods

Inefficiency in total scan time results from the long time of diffusion encoding gradient pulses which must be repeated for each and every image. We present a highly efficient multiplexing method, simultaneous image refocusing (SIR), for reducing the total scan time of diffusion imaging by nearly one‐half. SIR DSI is performed in 10 minutes rather than 21 minutes, acceptable for routine clinical application.

Results

Two identical studies were completed, comparing conventional single‐slice EPI DSI and SIR‐EPI DSI, showing equal signal‐to‐noise ratio (SNR) and contrast and small differences in registration, likely due to typical subject motion. Comparison of DSI and DTI tractographs showed matching quality and detection of white matter tracts.

Conclusion

The net reduction to nearly half the number of diffusion encoding gradient pulses in SIR‐EPI significantly reduces acquisition times of DSI and DTI. J. Magn. Reson. Imaging 2009;29:517–522. © 2009 Wiley‐Liss, Inc.     

k-t-Space accelerated myocardial perfusion

Purpose

To investigate the performance of the recently introduced spatiotemporal parallel imaging technique called parallel MRI with extended and averaged generalized autocalibrating partially parallel acquisitions (GRAPPA) kernels (PEAK‐GRAPPA) for myocardial perfusion measurements.

Materials and Methods

A study with 11 patients with myocardial infarction was performed to compare nonaccelerated perfusion imaging, i.e., fully acquired k‐space data, with the results of conventional GRAPPA and PEAK‐GRAPPA with a net acceleration factor of 2.4 to 3.4. Signal time courses reflecting the passage of the contrast agent bolus in different regions of the heart were evaluated for these different reconstruction methods.

Results

Reconstruction with PEAK‐GRAPPA demonstrated considerably improved image quality compared to conventional GRAPPA. In addition, signal time courses for PEAK‐GRAPPA demonstrated an excellent agreement compared to full k‐space data, which is necessary for an accurate qualitative and quantitative assessment of myocardial perfusion.

Conclusion

Qualitative and quantitative results of patient measurements illustrate that the temporal fidelity of nonperiodic processes such as myocardial perfusion are preserved with PEAK‐GRAPPA up to net acceleration factors of more than 3 while showing a superior image quality compared to conventional GRAPPA and a sliding‐window reconstruction. J. Magn. Reson. Imaging 2008;28:1080–1085. © 2008 Wiley‐Liss, Inc.     

Highly k‐t‐space–accelerated phase‐contrast MRI

The purpose of this study was to combine a recently introduced spatiotemporal parallel imaging technique, PEAK‐GRAPPA (parallel MRI with extended and averaged generalized autocalibrating partially parallel acquisition), with two‐dimensional (2D) cine phase‐contrast velocity mapping. Phase‐contrast MRI was applied to measure the blood flow in the thoracic aorta and the myocardial motion of the left ventricle. To evaluate the performance of different reconstruction methods, fully acquired k‐space data sets were used to compare conventional parallel imaging using GRAPPA with reduction factors of R = 2–6 and PEAK‐GRAPPA as well as sliding window reconstruction with reduction factors R = 2–12 (net acceleration factors up to 5.2). PEAK‐GRAPPA reconstruction resulted in improved image quality with considerably reduced artifacts, which was also supported by error analysis. To analyze potential blurring or low‐pass filtering effects of spatiotemporal PEAK‐GRAPPA, the velocity time courses of aortic flow and myocardial tissue motion were evaluated and compared with conventional image reconstructions. Quantitative comparisons of blood flow velocities and pixel‐wise correlation analysis of velocities highlight the potential of PEAK‐GRAPPA for highly accelerated dynamic phase‐contrast velocity mapping. Magn Reson Med 60:1169–1177, 2008. © 2008 Wiley‐Liss, Inc.     

Artifact reduction in moving-table acquisitions using parallel imaging and multiple averages.

Two-dimensional (2D) axial continuously-moving-table imaging has to deal with artifacts due to gradient nonlinearity and breathing motion, and has to provide the highest scan efficiency. Parallel imaging techniques (e.g., generalized autocalibrating partially parallel acquisition GRAPPA)) are used to reduce such artifacts and avoid ghosting artifacts. The latter occur in T(2)-weighted multi-spin-echo (SE) acquisitions that omit an additional excitation prior to imaging scans for presaturation purposes. Multiple images are reconstructed from subdivisions of a fully sampled k-space data set, each of which is acquired in a single SE train. These images are then averaged. GRAPPA coil weights are estimated without additional measurements. Compared to conventional image reconstruction, inconsistencies between different subsets of k-space induce less artifacts when each k-space part is reconstructed separately and the multiple images are averaged afterwards. These inconsistencies may lead to inaccurate GRAPPA coil weights using the proposed intrinsic GRAPPA calibration. It is shown that aliasing artifacts in single images are canceled out after averaging. Phantom and in vivo studies demonstrate the benefit of the proposed reconstruction scheme for free-breathing axial continuously-moving-table imaging using fast multi-SE sequences.     

Variation of noise in multi-run functional MRI using generalized autocalibrating partially parallel acquisition (GRAPPA)

Purpose:

To investigate the noise variation in multi‐run functional MRI (fMRI) scans using generalized autocalibrating partially parallel acquisition (GRAPPA), with a focus on the cause of this variation.

Materials and Methods:

A phantom was continuously scanned for 10 runs using echo‐planar imaging (EPI) combined with GRAPPA to simulate a multi‐run fMRI exam. The variation of noise between runs was examined for different GRAPPA acceleration factors. The noise variation was also evaluated in a real fMRI experiment with human subjects at an acceleration factor of two. The cause of noise variation was explored by offline reconstruction using different GRAPPA weights and numerical simulation of GRAPPA reference scans.

Results:

It was found that the noise distribution in the image is stable within a run but may vary randomly from run to run. The variation of noise was also observed in fMRI experiments with human subjects. The variation can be significantly reduced if all the images from individual runs are reconstructed using the same reference scan data.

Conclusion:

Both phantom experiments and human data showed that the noise pattern may change in different fMRI runs. The variation is mainly caused by the random noise in separate reference scans for GRAPPA in each run. J. Magn. Reson. Imaging 2012;462‐470. © 2011 Wiley Periodicals, Inc.     

High resolution diffusion‐weighted imaging using readout‐segmented echo‐planar imaging,parallel imaging and a two‐dimensional navigator‐based reacquisition

Single‐shot echo‐planar imaging (EPI) is well established as the method of choice for clinical, diffusion‐weighted imaging with MRI because of its low sensitivity to the motion‐induced phase errors that occur during diffusion sensitization of the MR signal. However, the method is prone to artifacts due to susceptibility changes at tissue interfaces and has a limited spatial resolution. The introduction of parallel imaging techniques, such as GRAPPA (GeneRalized Autocalibrating Partially Parallel Acquisitions), has reduced these problems, but there are still significant limitations, particularly at higher field strengths, such as 3 Tesla (T), which are increasingly being used for routine clinical imaging. This study describes how the combination of readout‐segmented EPI and parallel imaging can be used to address these issues by generating high‐resolution, diffusion‐weighted images at 1.5T and 3T with a significant reduction in susceptibility artifact compared with the single‐shot case. The technique uses data from a 2D navigator acquisition to perform a nonlinear phase correction and to control the real‐time reacquisition of unusable data that cannot be corrected. Measurements on healthy volunteers demonstrate that this approach provides a robust correction for motion‐induced phase artifact and allows scan times that are suitable for routine clinical application. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Improving quality of arterial spin labeling MR imaging at 3 Tesla with a 32-channel coil and parallel imaging

Purpose:

To compare 12‐channel and 32‐channel phased‐array coils and to determine the optimal parallel imaging (PI) technique and factor for brain perfusion imaging using Pulsed Arterial Spin labeling (PASL) at 3 Tesla (T).

Materials and Methods:

Twenty‐seven healthy volunteers underwent 10 different PASL perfusion PICORE Q2TIPS scans at 3T using 12‐channel and 32‐channel coils without PI and with GRAPPA or mSENSE using factor 2. PI with factor 3 and 4 were used only with the 32‐channel coil. Visual quality was assessed using four parameters. Quantitative analyses were performed using temporal noise, contrast‐to‐noise and signal‐to‐noise ratios (CNR, SNR).

Results:

Compared with 12‐channel acquisition, the scores for 32‐channel acquisition were significantly higher for overall visual quality, lower for noise and higher for SNR and CNR. With the 32‐channel coil, artifact compromise achieved the best score with PI factor 2. Noise increased, SNR and CNR decreased with PI factor. However mSENSE 2 scores were not always significantly different from acquisition without PI.

Conclusion:

For PASL at 3T, the 32‐channel coil at 3T provided better quality than the 12‐channel coil. With the 32‐channel coil, mSENSE 2 seemed to offer the best compromise for decreasing artifacts without significantly reducing SNR, CNR. J. Magn. Reson. Imaging 2012;35:1233‐1239. © 2012 Wiley Periodicals, Inc.     

Cardiac motion in diffusion-weighted MRI of the liver: artifact and a method of correction

Purpose:

To characterize cardiac motion artifacts in the liver and assess the use of a postprocessing method to mitigate these artifacts in repeat measurements.

Materials and Methods:

Three subjects underwent breathhold diffusion‐weighted (DW) scans consisting of 25 repetitions for three b‐values (0, 500, 1000 sec/mm²). Statistical maps computed from these repetitions were used to assess the distribution and behavior of cardiac motion artifacts in the liver. An objective postprocessing method to reduce the artifacts was compared with radiologist‐defined gold standards.

Results:

Signal dropout is pronounced in areas proximal to the heart, such as the left lobe, but also present in the right lobe and in distal liver segments. The dropout worsens with b‐value and leads to overestimation of the diffusivity. By reference to a radiologist‐defined gold standard, a postprocessing correction method is shown to reduce cardiac motion artifact.

Conclusion:

Cardiac motion leads to significant artifacts in liver DW imaging; we propose a postprocessing method that may be used to mitigate the artifact and is advantageous to standard signal averaging in acquisitions with multiple repetitions. J. Magn. Reson. Imaging 2012;318‐327. © 2011 Wiley Periodicals, Inc.     

Parallel acquisition techniques in cardiac cine magnetic resonance imaging using TrueFISP sequences: comparison of image quality and artifacts

PURPOSE: To compare image quality, artifacts, and signal-to-noise ratio (SNR) in cardiac cine TrueFISP magnetic resonance imaging (MRI) with and without parallel acquisition techniques (PAT). MATERIALS AND METHODS: MRI was performed in 16 subjects with a TrueFISP sequence (1.5 T; Magnetom Sonata, Siemens): TR, 3.0 msec; TE, 1.5 msec; flip angle (FA), 60 degrees. Three axes were scanned without PAT (no PAT) and using the generalized autocalibrating partially parallel acquisition (GRAPPA) and modified sensitivity encoding (mSENSE) reconstruction algorithms with an autocalibration mode to reduce scan time. A conventional spine array and a body flex array were used. Artifacts, image noise, and overall image quality were classified on a 4-point scale by an observer blinded to the implemented technique; for quantitative comparison, SNR was measured. RESULTS: With a PAT factor of two, acquisition time could be reduced by 39%. No PAT did not show artifacts, and GRAPPA revealed fewer artifacts than mSENSE. PAT provided inferior-quality scores concerning image noise and overall image quality. In quantitative measurements, GRAPPA and mSENSE (20.1 +/- 6.2 and 15.6 +/- 6.2, respectively) yielded lower SNR than no PAT (30.6 +/- 20.1; P < 0.05) and P < 0.001). CONCLUSION: Time savings in PAT are accompanied by artifacts and an increase in image noise. The GRAPPA algorithm was superior to mSENSE concerning image quality, noise, and SNR.     

Rapid PROPELLER‐MRI: A combination of iterative reconstruction and under‐sampling

Purpose:

To develop a technique that is able to reduce acquisition time and remove uneven blurring in reconstructed image for PROPELLER MRI. By using under‐sampling and iterative reconstruction, this proposed technique will be less sensitive to subject motion.

Materials and Methods:

Numerical simulations, as well as experiments on a phantom and healthy human subjects were performed to demonstrate advantages of a combination of under‐sampled acquisition and iterative reconstruction. Method of motion correction was modified to increase accuracy of motion correction for the under‐sampled PROPELLER acquisition.

Results:

It was demonstrated that the proposed approach achieved substantial acceleration of PROPELLER acquisition while maintaining its motion correction advantage.

Conclusion:

An effective method for reducing imaging time in PROPELLER was introduced in this study, which minimizes typical under‐sampling artifacts without uneven spatial resolution and maintains the ability of motion correction. J. Magn. Reson. Imaging 2012;36:1241–1247. © 2012 Wiley Periodicals, Inc.     

Diffusion-tensor MRI reveals the complex muscle architecture of the human forearm

Purpose:

To design a time‐efficient patient‐friendly clinical diffusion tensor MRI protocol and postprocessing tool to study the complex muscle architecture of the human forearm.

Materials and Methods:

The 15‐minute examination was done using a 3 T system and consisted of: T₁‐weighted imaging, dual echo gradient echo imaging, single‐shot spin‐echo echo‐planar imaging (EPI) diffusion tensor MRI. Postprocessing comprised of signal‐to‐noise improvement by a Rician noise suppression algorithm, image registration to correct for motion and eddy currents, and correction of susceptibility‐induced deformations using magnetic field inhomogeneity maps. Per muscle one to five regions of interest were used for fiber tractography seeding. To validate our approach, the reconstructions of individual muscles from the in vivo scans were compared to photographs of those dissected from a human cadaver forearm.

Results:

Postprocessing proved essential to allow muscle segmentation based on combined T₁‐weighted and diffusion tensor data. The protocol can be applied more generally to study human muscle architecture in other parts of the body.

Conclusion:

The proposed protocol was able to visualize the muscle architecture of the human forearm in great detail and showed excellent agreement with the dissected cadaver muscles. J. Magn. Reson. Imaging 2012;36:237–248. © 2012 Wiley Periodicals, Inc.     

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.     

Parallel and partial Fourier imaging with prospective motion correction

Subject motion during scan is a major source of artifacts in MR examinations. Prospective motion correction is a promising technique that tracks subject motion and adjusts the imaging volume in real time; however, additional retrospective correction may be necessary to achieve robust image quality and compatibility with other imaging options. Real‐time realignment of the imaging volume by prospective motion correction changes the coil sensitivity weighting and the field inhomogeneity relative to the imaging volume. This can pose image reconstruction problems with parallel imaging and partial Fourier imaging, which rely on coil sensitivity and image phase information, respectively. This work presents a practical method for reconstructing images acquired using prospective motion correction with parallel imaging and/or partial Fourier imaging. Our proposed approach is data driven and noniterative; data are binned into several position bins based on motion measurements made during the prospective motion correction acquisition and the data in each bin are processed through intrabin operations such as parallel imaging reconstruction (in case of undersampling), phase correction, and coil combination before combination of the position bins. We demonstrate the effectiveness of our technique through simulation studies and in vivo experiments using a prospectively motion‐corrected three‐dimensional fast spin echo sequence. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Half‐fourier‐acquisition single‐shot turbo spin‐echo (HASTE) MRI of the lung at 3 Tesla using parallel imaging with 32‐receiver channel technology

Purpose

To assess the feasibility of half‐Fourier‐acquisition single‐shot turbo spin‐echo (HASTE) of the lung at 3 Tesla (T) using parallel imaging with a prototype of a 32‐channel torso array coil, and to determine the optimum acceleration factor for the delineation of intrapulmonary anatomy.

Materials and Methods

Nine volunteers were examined on a 32‐channel 3T MRI system using a prototype 32‐channel‐torso‐array‐coil. HASTE‐MRI of the lung was acquired at both, end‐inspiratory and end‐expiratory breathhold with parallel imaging (Generalized autocalibrating partially parallel acquisitions = GRAPPA) using acceleration factors ranging between R = 1 (TE = 42 ms) and R = 6 (TE = 16 ms). The image quality of intrapulmonary anatomy and subjectively perceived noise level was analyzed by two radiologists in consensus. In addition quantitative measurements of the signal‐to‐noise ratio (SNR) of HASTE with different acceleration factors were assessed in phantom measurements.

Results

Using an acceleration factor of R = 4 image blurring was substantially reduced compared with lower acceleration factors resulting in sharp delineation of intrapulmonary structures in expiratory scans. For inspiratory scans an acceleration factor of 2 provided the best image quality. Expiratory scans had a higher subjectively perceived SNR than inspiratory scans.

Conclusion

Using optimized multi‐element coil geometry HASTE‐MRI of the lung is feasible at 3T with acceleration factors up to 4. Compared with nonaccelerated acquisitions, shorter echo times and reduced image blurring are achieved. Expiratory scanning may be favorable to compensate for susceptibility associated signal loss at 3T. J. Magn. Reson. Imaging 2009;30:541–546. © 2009 Wiley‐Liss, Inc.     

Self-calibrated multiple-echo acquisition with radial trajectories using the conjugate gradient method (SMART-CG)

Purpose:

To remove phase inconsistencies between multiple echoes, an algorithm using a radial acquisition to provide inherent phase and magnitude information for self correction was developed. The information also allows simultaneous support for parallel imaging for multiple coil acquisitions.

Materials and Methods:

Without a separate field map acquisition, a phase estimate from each echo in multiple echo train was generated. When using a multiple channel coil, magnitude and phase estimates from each echo provide in vivo coil sensitivities. An algorithm based on the conjugate gradient method uses these estimates to simultaneously remove phase inconsistencies between echoes, and in the case of multiple coil acquisition, simultaneously provides parallel imaging benefits. The algorithm is demonstrated on single channel, multiple channel, and undersampled data.

Results:

Substantial image quality improvements were demonstrated. Signal dropouts were completely removed and undersampling artifacts were well suppressed.

Conclusion:

The suggested algorithm is able to remove phase cancellation and undersampling artifacts simultaneously and to improve image quality of multiecho radial imaging, the important technique for fast three‐dimensional MRI data acquisition. J. Magn. Reson. Imaging 2011;33:980–987. © 2011 Wiley‐Liss, Inc.