Brian M. Dale

Determining and optimizing the precision of quantitative measurements of perfusion from dynamic contrast enhanced MRI

To examine the sensitivity of quantitative dynamic contrast enhanced MRI (DCE‐MRI) perfusion maps to errors in the various source images and to determine optimal imaging parameters for reducing this sensitivity.

A rapid look-up table method for reconstructing MR images from arbitrary K-space trajectories

Look-up tables (LUTs) are a common method for increasing the speed of many algorithms. Their use can be extended to the reconstruction of nonuniformly sampled k-space data using either a discrete Fourier transform (DFT) algorithm or a convolution-based gridding algorithm. A table for the DFT would be precalculated arrays of weights describing how each data point affects all of image space. A table for a convolution-based gridding operation would be a precalculated table of weights describing how each data point affects a small k-space neighborhood. These LUT methods were implemented in C++ on a modest personal computer system; they allowed a radial k-space acquisition sequence, consisting of 180 views of 256 points each, to be gridded in 36.2 ms, or, in approximately 800 ns/point. By comparison, a similar implementation of the gridding operation, without LUTs, required 45 times longer (1639.2 ms) to grid the same data. This was possible even while using a 4/spl times/4 Kaiser-Bessel convolution kernel, which is larger than typically used. These table-based computations will allow real time reconstruction in the future and can currently be run concurrently with the acquisition allowing for completely real-time gridding.

Functional magnetic resonance imaging of the human lumbar spinal cord

To determine whether consistent regions of activity could be observed in the lumbar spinal cord of single subjects with spin‐echo functional MRI (fMRI) if several repeated experiments were performed within a single imaging session.

Block regional off-resonance correction (BRORC): a fast and effective deblurring method for spiral imaging.

One primary disadvantage of spiral imaging is blurring artifact due to off‐resonance effects. The conventional frequency segmented off‐resonance correction method that is performed over the entire image is computationally intense due to the large number of fast Fourier transforms (FFTs) required. Here, a new fast off‐resonance correction method, block regional off‐resonance correction (BRORC), is presented. In this method, off‐resonance correction proceeds block‐by‐block through the reconstructed image with FFTs performed on matrices that are smaller than the full image matrix. The BRORC algorithm is typically several times more computationally efficient than the conventional off‐resonance correction algorithm. Additional computational reductions can be expected for the BRORC if only specific image regions require deblurring. The newly proposed off‐resonance correction method offers significant speed advantages and equivalent image quality when compared to conventional off‐resonance correction methods. Magn Reson Med 50:643–648, 2003.

Blood attenuation with SSFP-compatible saturation (BASS)

To investigate a rapid flow‐suppression method for improving the contrast‐to‐noise ratio (CNR) between the vessel wall and the lumen for cardiovascular imaging applications.

Radial Alternating TE Sequence for Faster Fat Suppression

This study describes a steady‐state sequence that uses a radial k‐space trajectory and alternating echo times (TEs) between even and odd k‐space views. The sequence generated a single data set that was used to reconstruct images with inherent fat suppression. This fat suppression results from the fat phase variation in alternate echoes giving rise to cancellation in the central portion of k‐space. This new fat‐suppression method provides inherent fat suppression in half the acquisition time relative to the radial two‐point Dixon method. The improvement in k‐space sampling efficiency is demonstrated in phantom and clinical images, and through measured point‐spread functions (PSFs). As a result, the radial alternating TE sequence offers improved temporal resolution over a radial version of the two‐point Dixon sequence by requiring fewer total projections to obtain the same effective resolution in water‐based tissues. Magn Reson Med 50:1095–1099, 2003.

Optimal design of k-space trajectories using a multi-objective genetic algorithm

Spiral, radial, and other nonrectilinear k‐space trajectories are an area of active research in MRI due largely to their typically rapid acquisition times and benign artifact patterns. Trajectory design has commonly proceeded from a description of a simple shape to an investigation of its properties, because there is no general theory for the derivation of new trajectories with specific properties. Here such a generalized methodology is described. Specifically, a multi‐objective genetic algorithm (GA) is used to design trajectories with beneficial flow and off‐resonance properties. The algorithm converges to a well‐defined optimal set with standard spiral trajectories on the rapid but low‐quality end, and a new class of trajectories on the slower but high‐quality end. The new trajectories all begin with nonzero gradient amplitude at the k‐space origin, and curve gently outward relative to standard spirals. Improvements predicted in simulated imaging experiments were found to correlate well with improvements in actual experimental measures of image quality. The impact of deviations from the desired k‐space trajectory is described, as is the impact of using different phantoms. Magn Reson Med 52:831–841, 2004.

Rapid dark-blood carotid vessel-wall imaging with random bipolar gradients in a radial SSFP acquisition

To investigate and evaluate a new rapid dark‐blood vessel‐wall imaging method using random bipolar gradients with a radial steady‐state free precession (SSFP) acquisition in carotid applications.