James G. Pipe

Motion correction with PROPELLER MRI : Application to head motion and free-breathing cardiac imaging

A method for motion correction, involving both data collection and reconstruction, is presented. The PROPELLER MRI method collects data in concentric rectangular strips rotated about the k‐space origin. The central region of k‐space is sampled for every strip, which (a) allows one to correct spatial inconsistencies in position, rotation, and phase between strips, (b) allows one to reject data based on a correlation measure indicating through‐plane motion, and (c) further decreases motion artifacts through an averaging effect for low spatial frequencies. Results are shown in which PROPELLER MRI is used to correct for bulk motion in head images and respiratory motion in nongated cardiac images. Magn Reson Med 42:963–969, 1999.

Sampling density compensation in MRI: rationale and an iterative numerical solution.

Data collection of MRI which is sampled nonuniformly in k‐space is often interpolated onto a Cartesian grid for fast reconstruction. The collected data must be properly weighted before interpolation, for accurate reconstruction. We propose a criterion for choosing the weighting function necessary to compensate for nonuniform sampling density. A numerical iterative method to find a weighting function that meets that criterion is also given. This method uses only the coordinates of the sampled data; unlike previous methods, it does not require knowledge of the trajectories and can easily handle trajectories that “cross” in k‐space. Moreover, the method can handle sampling patterns that are undersampled in some regions of k‐space and does not require a post‐gridding density correction. Weighting functions for various data collection strategies are shown. Synthesized and collected in vivo data also illustrate aspects of this method.u2003Magn Reson Med 41:179–186, 1999.

Pulse waveform analysis of arterial compliance: relation to other techniques, age, and metabolic variables

To assess the physiologic and clinical relevance of newer noninvasive measures of vascular compliance, computerized arterial pulse waveform analysis (CAPWA) of the radial pulse was used to calculate two components of compliance, C1 (capacitive) and C2 (oscillatory or reflective), in 87 normotensive (N1BP, n = 20), untreated hypertensive (HiBP, n = 21), and treated hypertensive (HiBP-Rx, n = 46) subjects. These values were compared with two other indices of compliance, the ratio of stroke volume to pulse pressure (SV/PP) and magnetic resonance imaging (MRI)-based aortic distensibility; and were also correlated with demographic and biochemical values. The HiBP subjects displayed lower C1 (1.34 +/- 0.09 v. 1.70 +/- 0.11 mL/mm Hg, significance [sig] = .05) and C2 (0.031 +/- 0.003 v 0.073 +/- 0.02 mL/mm Hg, sig = .005) than N1BP subjects. This was not true for C1 (1.64 +/- 0.08 mL/mm Hg) and C2 (0.052 +/- 0.005 mL/mm Hg) values in HiBP-Rx subjects. The C1 (r = 0.917, P < .0001) and C2 (r = 0.677, P < .0001) were both closely related to SV/PP, whereas C1 (r = 0.748, P = .002), but not C2, was significantly related to MRI-determined aortic distensibility. Among other factors measured, age exerted a strong negative influence on both C1 (r = -0.696, P < .0001) and C2 (r = -0.611, P < .0001) compliance components. Positive correlations were observed between C1 (r = 0.863, P = .006), aortic distensibility (r = 0.597, P = .19) and 24-h urinary sodium excretion, and between C1- and MR spectroscopy-determined in situ skeletal muscle intracellular free magnesium (r = 0.827, P = .006), whereas C2 was inversely related to MRI-determined abdominal visceral fat area (r = -0.512, P = .042) and fasting blood glucose (r = -0.846, P = .001). Altogether, the close correspondence between CAPWA, other compliance techniques, and known cardiovascular risk factors suggests the clinical relevance of CAPWA in the assessment of altered vascular function in hypertension.

An optimized center-out k-space trajectory for multishot MRI: Comparison with spiral and projection reconstruction

A method for collecting MRI data on a set of rotated trajectories that begin at the center of k‐space is outlined. It is theoretically slightly faster, slightly less susceptible to off‐resonance and motion‐induced phase, and produces images with slightly better signal‐to‐noise ratio than methods using Archimedean spiral trajectories, particularly for a short sampling duration. It also produces more accurate images than those of projection reconstruction methods, which are significantly undersampled azimuthally. This method may be most useful when imaging areas with large inhomogeneities (e.g., near metallic implants), short T2 species, or high turbulence (e.g., gas imaging). Magn Reson Med 42:714–720, 1999.

Rapid method for deblurring spiral MR images.

A method for fast off‐resonance frequency deblurring of spiral MR images is presented. The method utilizes image‐space deconvolution. The off‐resonance phase is approximated as a separable quadratic function to allow rapid one‐dimensional deconvolution with a small compromise in accuracy. The method is used to deblur an MR angiographic image to illustrate its effectiveness. Magn Reson Med 44:491–494, 2000.

Effects of interleaf order for spiral MRI of dynamic processes.

The effects of the temporal order in which spiral interleaves are collected are discussed, in the context of artifacts from moving or changing objects. Simulations and in vivo experiments demonstrate the properties of four different ordering methods. Specific applications discussed include cardiac and interventional magnetic resonance imaging, as well as inflow and contrast‐enhanced magnetic resonance angiography. Magn Reson Med 41:417–422, 1999.

Flow effects in localized quadratic, partial Fourier MRA.

A pulse sequence for inflow‐enhanced magnetic resonance angiography, including localized quadratic encoding, partial‐Fourier slice selection, and spiral in‐plane encoding, is analyzed. The through‐plane encoding method is discussed in a space–spatial frequency context to illustrate some of its properties. This pulse sequence has the advantages of being faster and more robust to turbulent flow than conventional inflow‐enhanced methods. Simulations show the effect of different parameters on the modulation‐transfer function of the resulting images. A flow phantom is used to verify some of the simulation results.Magn Reson Med 41:309–314, 1999.