Aiming Lu

Rapid fat-suppressed isotropic steady-state free precession imaging using true 3D multiple-half-echo projection reconstruction.

Three‐dimensional projection reconstruction (3D PR)‐based techniques are advantageous for steady‐state free precession (SSFP) imaging for several reasons, including the capability to achieve short repetition times (TRs). In this paper, a multi‐half‐echo technique is presented that dramatically improves the data‐sampling efficiency of 3D PR sequences while it retains this short‐TR capability. The k‐space trajectory deviations are measured quickly and corrected on a per‐sample point basis. A two‐pass RF cycling technique is then applied to the dual‐half‐echo implementation to generate fat/water‐separated images. The resultant improvement in the signal‐to‐noise ratio (SNR) and contrast‐to‐noise ratio (CNR) was demonstrated in volunteer studies. Volumetric images with excellent spatial resolution, coverage, and contrast were obtained with high speed. The non‐contrast‐enhanced SSFP studies show that this technique has promising potential for MR angiography (MRA). Magn Reson Med 53:692–699, 2005.

Contrast-enhanced peripheral magnetic resonance angiography using time-resolved vastly undersampled isotropic projection reconstruction

To investigate the application of time‐resolved vastly undersampled isotropic projection reconstruction (VIPR) in contrast‐enhanced magnetic resonance angiography of the distal extremity (single station), and peripheral run‐off vasculature in the abdomen, thigh, and calf (three stations).

Improved slice selection for R2* mapping during cryoablation with eddy current compensation.

To improve the slice profile and image quality of R2* mapping in the iceball during cryoablation with ultrashort echo time (UTE) imaging by compensating for eddy currents induced by the selective gradient when half‐pulse radiofrequency (RF) excitation is employed to achieve UTEs.

Generation and visualization of four-dimensional MR angiography data using an undersampled 3-D projection trajectory

Time-resolved contrast-enhanced magnetic resonance (MR) angiography (CE-MRA) has gained in popularity relative to X-ray Digital Subtraction Angiography because it provides three-dimensional (3-D) spatial resolution and it is less invasive. We have previously presented methods that improve temporal resolution in CE-MRA while providing high spatial resolution by employing an undersampled 3-D projection (3D PR) trajectory. The increased coverage and isotropic resolution of the 3D PR acquisition simplify visualization of the vasculature from any perspective. We present a new algorithm to develop a set of time-resolved 3-D image volumes by preferentially weighting the 3D PR data according to its acquisition time. An iterative algorithm computes a series of density compensation functions for a regridding reconstruction, one for each time frame, that exploit the variable sampling density in 3D PR. The iterative weighting procedure simplifies the calculation of appropriate density compensation for arbitrary sampling patterns, which improve sampling efficiency and, thus, signal-to-noise ratio and contrast-to-noise ratio, since it is does not require a closed-form calculation based on geometry. Current medical workstations can display these large four-dimensional studies, however, interactive cine animation of the data is only possible at significantly degraded resolution. Therefore, we also present a method for interactive visualization using powerful graphics cards and distributed processing. Results from volunteer and patient studies demonstrate the advantages of dynamic imaging with high spatial resolution.

Evaluation of the articular cartilage of the knee joint with vastly undersampled isotropic projection reconstruction steady-state free precession imaging.

To determine the feasibility of the vastly undersampled isotropic projection reconstruction steady‐state free precession (VIPR‐SSFP) sequence for evaluating the articular cartilage of the knee joint.

Improved spectral selectivity and reduced susceptibility in SSFP using a near zero TE undersampled three-dimensional PR sequence

To improve the performance of fat/water separation and reduce the sensitivity to susceptibility variation in balanced SSFP sequences.

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

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.

Fat/water separation in single acquisition steady-state free precession using multiple echo radial trajectories.

Phase detection in fully refocused SSFP imaging has recently allowed fat/water separation without preparing the magnetization or using multiple acquisitions. Instead, it exploits the phase difference between fat and water at an echo time at the midpoint of the TR. To minimize the TR for improved robustness to B0 inhomogeneity, a 3D projection acquisition collecting two half echoes at the beginning and end of each excitation was previously implemented. Since echoes are not formed at the midpoint of the TR, this method still requires two passes of k‐space for fat/water separation. A new method is presented to linearly combine the half echoes to separate fat and water in a single acquisition. Separation using phase detection provides superior contrast between fat and water voxels. Results from high resolution angiography and musculoskeletal studies with improved robustness to inhomogeneity and a 50% scan time reduction compared to the two pass method are presented. Magn Reson Med, 2005.

Consistency of signal intensity and T2* in frozen ex vivo heart muscle, kidney, and liver tissue.

To investigate tissue dependence of the MRI‐based thermometry in frozen tissue by quantification and comparison of signal intensity and T2* of ex vivo frozen tissue of three different types: heart muscle, kidney, and liver.

Time-resolved undersampled projection reconstruction magnetic resonance imaging of the peripheral vessels using multi-echo acquisition

The hybrid projection reconstruction (PR) imaging provides high temporal resolution through an undersampled PR acquisition for the in‐plane dimensions and Cartesian slice encoding for the through‐plane dimension. The undersampling of projection data introduces streak artifact, which may severely compromise image quality. This study reports on a combination of multi‐echo acquisition with time‐resolved undersampled PR imaging and its application to peripheral magnetic resonance angiography. Multi‐echo acquisition improved imaging speed effectively, thereby reducing the undersampling streak artifact and improving the temporal resolution. The gradient distortion was reduced through gradient calibration and accurate k‐space trajectory measurement. Magn Reson Med 53:730–734, 2005.