Dwight G. Nishimura

Selection of a convolution function for Fourier inversion using gridding (computerised tomography application)

In the technique known as gridding, the data samples are weighted for sampling density and convolved with a finite kernel, then resampled on a grid preparatory to a fast Fourier transform. The authors compare the artifact introduced into the image for various convolving functions of different sizes, including the Kaiser-Bessel window and the zero-order prolate spheroidal wave function (PSWF). They also show a convolving function that improves upon the PSWF in some circumstances.

Homodyne detection in magnetic resonance imaging

Magnetic detection of complex images in magnetic resonance imaging (MRI) is immune to the effects of incidental phase variations, although in some applications information is lost or images are degraded. It is suggested that synchronous detection or demodulation can be used in MRI systems in place of magnitude detection to provide complete suppression of undesired quadrature components, to preserve polarity and phase information, and to eliminate the biases and reduction in signal-to-noise ratio (SNR) and contrast in low SNR images. The incidental phase variations in an image are removed through the use of a homodyne demodulation reference, which is derived from the image or the object itself. Synchronous homodyne detection has been applied to the detection of low SNR images, the reconstruction of partial k-space images, the simultaneous detection of water and lipid signals in quadrature, and the preservation of polarity in inversion-recovery images.

A k-space analysis of small-tip-angle excitation

Abstract We present here a method for analyzing selective excitation in terms of spatial frequency ( k ) space. Using this analysis we show how to design inherently refocused selective excitation pulses in one and two dimensions. The analysis is based on a small-tip model, but holds well for 90° tip angles.

Variable-rate selective excitation

Abstract A procedure is introduced for refabricating any spatially selective excitation pulse to reduce its SAR while preserving its duration and slice profile. Called variable-rate selective excitation, the procedure allows for a variable trade-off of RF amplitude for duration at each sample of the pulse. SAR reduction of 50% is possible with only a mild smearing of the off resonance slice profile. Experimental slice profiles verify the principle.

A homogeneity correction method for magnetic resonance imaging with time-varying gradients

When time-varying gradients are used for imaging, the off-resonance behavior does not just cause geometric distortion as is the case with spin-warp imaging, but changes the shape of the impulse response and causes blurring. This effect is well known for projection reconstruction and spiral k-space scanning sequences. The authors introduce a reconstruction and homogeneity correction method to correct for the zeroth order effects of inhomogeneity using prior knowledge of the inhomogeneity. In this method, the data are segmented according to collection time, reconstructed using some fast, linear algorithm, correlated for inhomogeneity, and then superimposed to yield a homogeneity corrected image. This segmented method is compared to a conjugate phase reconstruction in terms of degree of correction and execution time. The authors apply this method to in vivo images using projection-reconstruction and spiral-scan sequences.

Linear combination steady-state free precession MRI.

A new, fast, spectrally selective steady‐state free precession (SSFP) imaging method is presented. Combining k‐space data from SSFP sequences with certain phase schedules of radiofrequency excitation pulses permits manipulation of the spectral selectivity of the image. For example, lipid and water can be resolved. The contrast of each image depends on both T1 and T2, and the relative contribution of the two relaxation mechanisms to image contrast can be controlled by adjusting the flip angle. Several potential applications of the technique, referred to as linear combination steady‐state free precession (LCSSFP), are demonstrated: fast musculoskeletal, abdominal, angiographic, and brain imaging. Magn Reson Med 43:82–90, 2000.

Rapid gridding reconstruction with a minimal oversampling ratio

Reconstruction of magnetic resonance images from data not falling on a Cartesian grid is a Fourier inversion problem typically solved using convolution interpolation, also known as gridding. Gridding is simple and robust and has parameters, the grid oversampling ratio and the kernel width, that can be used to trade accuracy for computational memory and time reductions. We have found that significant reductions in computation memory and time can be obtained while maintaining high accuracy by using a minimal oversampling ratio, from 1.125 to 1.375, instead of the typically employed grid oversampling ratio of two. When using a minimal oversampling ratio, appropriate design of the convolution kernel is important for maintaining high accuracy. We derive a simple equation for choosing the optimal Kaiser-Bessel convolution kernel for a given oversampling ratio and kernel width. As well, we evaluate the effect of presampling the kernel, a common technique used to reduce the computation time, and find that using linear interpolation between samples adds negligible error with far less samples than is necessary with nearest-neighbor interpolation. We also develop a new method for choosing the optimal presampled kernel. Using a minimal oversampling ratio and presampled kernel, we are able to perform a three-dimensional (3-D) reconstruction in one-eighth the time and requiring one-third the computer memory versus using an oversampling ratio of two and a Kaiser-Bessel convolution kernel, while maintaining the same level of accuracy.

Reduced aliasing artifacts using variable‐density k‐space sampling trajectories

A variable‐density k‐space sampling method is proposed to reduce aliasing artifacts in MR images. Because most of the energy of an image is concentrated around the k‐space center, aliasing artifacts will contain mostly low‐frequency components if the k‐space is uniformly undersampled. On the other hand, because the outer k‐space region contains little energy, undersampling that region will not contribute severe aliasing artifacts. Therefore, a variable‐density trajectory may sufficiently sample the central k‐space region to reduce low‐frequency aliasing artifacts and may undersample the outer k‐space region to reduce scan time and to increase resolution. In this paper, the variable‐density sampling method was implemented for both spiral imaging and two‐dimensional Fourier transform (2DFT) imaging. Simulations, phantom images and in vivo cardiac images show that this method can significantly reduce the total energy of aliasing artifacts. In general, this method can be applied to all types of k‐space sampling trajectories. Magn Reson Med 43:452–458, 2000.

A linear class of large-tip-angle selective excitation pulses

Abstract The design of large-tip-angle selective excitation pulses is in general a nonlinear problem. In this paper we present a class of selective excitation pulses that can be designed using a linear Fourier transform analysis, even at large tip angles. This method is most useful for designing two-dimensional pulses, although it is valid in general. It may be used to design selective excitation, inversion, and spin-echo (time-reversal) pulses.

Characterization and reduction of the transient response in steady-state MR imaging.

Refocused steady‐state free precession (SSFP) imaging sequences have recently regained popularity as faster gradient hardware has allowed shorter repetition times, thereby reducing SSFPs sensitivity to off‐resonance effects. Although these sequences offer fast scanning with good signal‐to‐noise efficiency, the “transient response,” or time taken to reach a steady‐state, can be long compared with the total imaging time, particularly when using 2D sequences. This results in lost imaging time and has made SSFP difficult to use for real‐time and cardiac‐gated applications. A linear‐systems analysis of the steady‐state and transient response for general periodic sequences is shown. The analysis is applied to refocused‐SSFP sequences to generate a two‐stage method of “catalyzing,” or speeding up the progression to steady‐state by first scaling, then directing the magnetization. This catalyzing method is compared with previous methods in simulations and experimentally. Although the second stage of the method exhibits some sensitivity to B1 variations, our results show that the transient time can be significantly reduced, allowing imaging in a shorter total scan time. Magn Reson Med 46:149–158, 2001.