Julia Velikina

Highly constrained backprojection for time‐resolved MRI

Recent work in k‐t BLAST and undersampled projection angiography has emphasized the value of using training data sets obtained during the acquisition of a series of images. These techniques have used iterative algorithms guided by the training set information to reconstruct time frames sampled at well below the Nyquist limit. We present here a simple non‐iterative unfiltered backprojection algorithm that incorporates the idea of a composite image consisting of portions or all of the acquired data to constrain the backprojection process. This significantly reduces streak artifacts and increases the overall SNR, permitting decreased numbers of projections to be used when acquiring each image in the image time series. For undersampled 2D projection imaging applications, such as cine phase contrast (PC) angiography, our results suggest that the angular undersampling factor, relative to Nyquist requirements, can be increased from the present factor of 4 to about 100 while increasing SNR per individual time frame. Results are presented for a contrast‐enhanced PR HYPR TRICKS acquisition in a volunteer using an angular undersampling factor of 75 and a TRICKS temporal undersampling factor of 3 for an overall undersampling factor of 225. Magn Reson Med, 2006.

Improved Waveform Fidelity Using Local HYPR Reconstruction (HYPR LR)

The recently introduced HYPR (HighlY constrained backPRojection) method allows reconstruction of serial images from highly undersampled data. In HYPR, individual timeframes are obtained via unfiltered backprojections of normalized sinograms using anatomical constraints provided by a composite image. Here we develop the idea of constraining the backprojected data further to a series of local regions of interest in order to decrease the corruption of local information by distant signals. HYPR LR (local reconstruction) permits the use of a longer temporal window in the formation of the composite image, resulting in increased signal‐to‐noise ratio and quantitative reconstruction accuracy. Unlike HYPR, the new HYPR LR method can be applied to images acquired with arbitrary k‐space trajectories. It is suitable for a broad range of medical imaging applications involving serial changes in image sequence, offering exciting new opportunities in the future. Magn Reson Med 59:456–462, 2008.

Accelerating MR parameter mapping using sparsity-promoting regularization in parametric dimension

MR parameter mapping requires sampling along additional (parametric) dimension, which often limits its clinical appeal due to a several‐fold increase in scan times compared to conventional anatomic imaging. Data undersampling combined with parallel imaging is an attractive way to reduce scan time in such applications. However, inherent SNR penalties of parallel MRI due to noise amplification often limit its utility even at moderate acceleration factors, requiring regularization by prior knowledge. In this work, we propose a novel regularization strategy, which uses smoothness of signal evolution in the parametric dimension within compressed sensing framework (p‐CS) to provide accurate and precise estimation of parametric maps from undersampled data. The performance of the method was demonstrated with variable flip angle T1 mapping and compared favorably to two representative reconstruction approaches, image space‐based total variation regularization and an analytical model‐based reconstruction. The proposed p‐CS regularization was found to provide efficient suppression of noise amplification and preservation of parameter mapping accuracy without explicit utilization of analytical signal models. The developed method may facilitate acceleration of quantitative MRI techniques that are not suitable to model‐based reconstruction because of complex signal models or when signal deviations from the expected analytical model exist. Magn Reson Med 70:1263–1273, 2013.

PC HYPR flow: A technique for rapid imaging of contrast dynamics

To improve spatial and temporal resolution and signal‐to‐noise ratio (SNR) in three‐dimensional (3D) radial contrast‐enhanced (CE) time‐resolved MR angiography by means of a novel hybrid phase contrast (PC) and CE MRA acquisition and HYPR reconstruction (PC HYPR Flow).

Limited view angle tomographic image reconstruction via total variation minimization

In tomosynthesis, cone-beam projection data are acquired from a few of view angles, which are not sufficient for an exact reconstruction of an image object using state-of-the-art image reconstruction algorithms. In the case of parallel-beam projections, the well-known projection-slice theorem may be utilized to transform the parallel-beam projections into the Fourier space of an image object. Due to the limited range of view angles, the available projection data can only populate a portion of Fourier space. Moreover, the angular sampling rate of the populated portion of the Fourier space may not satisfy the Nyquist criterion. Thus, reconstructed images using direct Fourier inversion contain severe streaking and distortion artifacts. In this paper, we present a novel image reconstruction method via minimizing the total variation (TV) of the reconstructed image for limited view angle X-ray computed tomography. Specifically, the missing data points in Fourier space, due to either the limited range or undersampling of view angles, are iteratively filled using the following two constraint conditions: (1) the total variation of the reconstructed image is minimized and (2) reconstructed image maintains fidelity to the sampled data in the Fourier space. Using analytical phantoms, numerical simulations were conducted to validate the new image reconstruction method. Images are compared with two other image reconstruction methods in terms of image artifact level and noise properties. Numerical results demonstrated that the new image reconstruction algorithm is superior to direct Fourier inversion reconstruction algorithm and the projection onto convex sets (POCS) image reconstruction algorithm.

Velocity Measurements in the Middle Cerebral Arteries of Healthy Volunteers Using 3D Radial Phase-Contrast HYPRFlow: Comparison with Transcranial Doppler Sonography and 2D Phase-Contrast MR Imaging

BACKGROUND AND PURPOSE: We have developed PC HYPRFlow, a comprehensive MRA technique that includes a whole-brain CE dynamic series followed by PC velocity-encoding, yielding a time series of high-resolution morphologic angiograms with associated velocity information. In this study, we present velocity data acquired by using the PC component of PC HYPRFlow (PC-VIPR). MATERIALS AND METHODS: Ten healthy volunteers (6 women, 4 men) were scanned by using PC HYPRFlow and 2D-PC imaging, immediately followed by velocity measurements by using TCD. Velocity measurements were made in the M1 segments of the MCAs from the PC-VIPR, 2D-PC, and TCD examinations. RESULTS: PC-VIPR showed approximately 30% lower mean velocity compared with TCD, consistent with other comparisons of TCD with PC-MRA. The correlation with TCD was r = 0.793, and the correlation of PC-VIPR with 2D-PC was r = 0.723. CONCLUSIONS: PC-VIPR is a technique capable of acquiring high-resolution MRA of diagnostic quality with velocity data comparable with TCD and 2D-PC. The combination of velocity information and fast high-resolution whole-brain morphologic angiograms makes PC HYPRFlow an attractive alternative to current MRA methods.

Three-dimensional imaging of ventilation dynamics in asthmatics using multiecho projection acquisition with constrained reconstruction.

The purpose of this work is to detect dynamic gas trapping in three dimensions during forced exhalation at isotropic high spatial resolution and high temporal resolution using hyperpolarized helium‐3 MRI. Ten subjects underwent hyperpolarized helium‐3 MRI and multidetector CT. MRI was performed throughout inspiration, breath‐hold, and forced expiration. A multiecho three‐dimensional projection acquisition was used to improve data collection efficiency and an iterative constrained reconstruction was implemented to improve signal to noise ratio (SNR) and increase robustness to motion. Two radiologists evaluated the dynamic MRI and breath‐held multidetector CT data for gas and air trapping, respectively. Phantom studies showed the proposed technique significantly improved depiction of moving objects compared to view‐sharing methods. Gas trapping was detected using MRI in five of the six asthmatic subjects who displayed air trapping with multidetector CT. Locations in disagreement were found to represent small to moderate regions of air trapping. The proposed technique provides whole‐lung three‐dimensional imaging of respiration dynamics at high spatial and temporal resolution and compares well to the current standard, multidetector CT. While multidetector CT can provide information about static regional air trapping, it is unable to depict dynamics in a setting more comparable to a spirometry maneuver and explore the longitudinal time evolution of the trapped regions. Magn Reson Med, 2009.

Fast Whole-Brain 4D Contrast-Enhanced MR Angiography with Velocity Encoding Using Undersampled Radial Acquisition and Highly Constrained Projection Reconstruction: Image-Quality Assessment in Volunteer Subjects

SUMMARY: We report on the image quality obtained by using fast contrast-enhanced whole-brain 4D radial MRA with 0.75-second temporal resolution, isotropic submillimeter spatial resolution, and velocity encoding (HYPRFlow). Images generated by HYPR-LR by using the velocity-encoded data as the constraining image were of diagnostic quality. In addition, we demonstrate that measurements of shear stress within the middle cerebral artery can be derived from the high-resolution 3D velocity data.

Reconstruction of dynamic image series from undersampled MRI data using data-driven model consistency condition (MOCCO)

To accelerate dynamic MR imaging through development of a novel image reconstruction technique using low‐rank temporal signal models preestimated from training data.

Simulation of relative temporal resolution of time-resolved MRA sequences

Time‐resolved MRA sequences are typically characterized by the display frame rate. However, true temporal resolution should be defined in a manner analogous to spatial resolution; it is not the ability of a sequence to update rapidly but rather the ability to discern changes that occur within a small time that should characterize temporal resolution. For view‐shared methods like Keyhole and time‐resolved imaging of contrast kinetics (TRICKS), regions of k‐space from multiple time frames are combined to form a single reconstructed time frame. This often causes the time needed to acquire all k‐space data points to be significantly longer than the displayed frame time, resulting in a poor frequency response. Simulated here are the temporal impulse response and temporal frequency response (TFR) curves of three time‐resolved MRA methods, including the recently introduced highly‐constrained backprojection local reconstruction (HYPR LR) method. It is found that the HYPR LR reconstruction method exhibits a better TFR for a larger spectrum of temporal and spatial frequencies than the Keyhole and TRICKS methods. Magn Reson Med 60:398–404, 2008.