Magnetic Resonance in Medicine

A conventional 3D phase contrast acquisition generates images with good spatial resolution, but often gives rise to artifacts due to pulsatile flow. 2D cine phase contrast, on the other hand, can register dynamic flow, but has a poor spatial resolution perpendicular to the imaging plane. A combination of both high spatial and temporal resolution may be advantageous in some cases, both in quantitative flow measurements and in MR angiography. The described 3D cine phase contrast pulse sequence creates a temporally resolved series of 3D data sets with velocity encoded data.

This work details the observation of non‐Gaussian apparent diffusion coefficient (ADC) profiles in multi‐direction, diffusion‐weighted MR data acquired with easily achievable imaging parameters (b≈ 1000 s/mm²). A technique is described for modeling the profile of the ADC over the sphere, which can capture non‐Gaussian effects that can occur at, for example, intersections of different tissue types or white matter fiber tracts. When these effects are significant, the common diffusion tensor model is inappropriate, since it is based on the assumption of a simple underlying diffusion process, which can be described by a Gaussian probability density function. A sequence of models of increasing complexity is obtained by truncating the spherical harmonic (SH) expansion of the ADC measurements at several orders. Further, a method is described for selection of the most appropriate of these models, in order to describe the data adequately but without overfitting. The combined procedure is used to classify the profile at each voxel as isotropic, anisotropic Gaussian, or non‐Gaussian, each with reference to the underlying probability density function of displacement of water molecules. We use it to show that non‐Gaussian profiles arise consistently in various regions of the human brain where complex tissue structure is known to exist, and can be observed in data typical of clinical scanners. The performance of the procedure developed is characterized using synthetic data in order to demonstrate that the observed effects are genuine. This characterization validates the use of our method as an indicator of pathology that affects tissue structure, which will tend to reduce the complexity of the selected model. Magn Reson Med 48:331–340, 2002. © 2002 Wiley‐Liss, Inc.

Image distortion due to field gradient eddy currents can create image artifacts in diffusion‐weighted MR images. These images, acquired by measuring the attenuation of NMR signal due to directionally dependent diffusion, have recently been shown to be useful in the diagnosis and assessment of acute stroke and in mapping of tissue structure. This work presents an improvement on the spin‐echo (SE) diffusion sequence that displays less distortion and consequently improves image quality. Adding a second refocusing pulse provides better image quality with less distortion at no cost in scanning efficiency or effectiveness, and allows more flexible diffusion gradient timing. By adjusting the timing of the diffusion gradients, eddy currents with a single exponential decay constant can be nulled, and eddy currents with similar decay constants can be greatly reduced. This new sequence is demonstrated in phantom measurements and in diffusion anisotropy images of normal human brain. Magn Reson Med 49:177–182, 2003. © 2003 Wiley‐Liss, Inc.

The image intensity in magnetic resonance magnitude images in the presence of noise is shown to be governed by a Rician distribution. Low signal intensities (SNR < 2) are therefore biased due to the noise. it is shown how the underlying noise can be estimated from the images and a simple correction scheme is provided to reduce the bias. the noise characteristics in phase images are also studied and shown to be very different from those of the magnitude images. Common to both, however, is that the noise distributions are nearly Gaussian for SNR larger than two.

The typical functional magnetic resonance (fMRI) study presents a formidable problem of multiple statistical comparisons (i.e, > 10,000 in a 128 x 128 image). To protect against false positives, investigators have typically relied on decreasing the per pixel false positive probability. This approach incurs an inevitable loss of power to detect statistically significant activity. An alternative approach, which relies on the assumption that areas of true neural activity will tend to stimulate signal changes over contiguous pixels, is presented. If one knows the probability distribution of such cluster sizes as a function of per pixel false positive probability, one can use cluster‐size thresholds independently to reject false positives. Both Monte Carlo simulations and fMRI studies of human subjects have been used to verify that this approach can improve statistical power by as much as fivefold over techniques that rely solely on adjusting per pixel false positive probabilities.

The purpose of this study was to determine cerebral myo‐inositol (ml) in adults with Down syndrome (DS), and to trace the chronobiology of DS to Alzheimer disease (AD). AD has characteristic neuropathology of neurofibrillary plaques and tangles; indirect evidence links this to earlier deposition of β‐amyloid. Elevated ml, which distinguishes AD from other common dementias, is also elevated in 23 young patients who have DS without dementia. In one patient who has DS with dementia, ml is elevated and N‐acetylaspartate (NAA) decreased. The similarity to AD is striking and may suggest a progressive neurochemical disorder in which elevation of ml precede loss of NAA in both AD and DS.

The NMR relaxation rates ofa class of hydrated polymers (hydrogels) have been investigated over a wide range of frequencies (0.01 to 60 MHz) and compositions. The gels comprise long chains of polytetramethylene oxide diol crosslinked by copolymers of dimethylacrylamide and methylmethacrylate. The longitudinal relaxation rate dispersion curves of these materials are very similar to those shown by human tissues and they can be altered in flexible fashion by changing the polymer content. By substitution of the water by deuterium oxide, the contribution of intermolecular cross‐relaxation effects has been shown to account for two‐thirds the total water relaxation rate. With their close similarity to tissue relaxation behavior, hydrogels of differing compositions are particularly well‐suited to MRI phantoms and test objects. © 1989 Academic press, Inc.

IVIM MR imaging is a method which generates images of diffusion and perfusion in vivo. Until now, intravoxel incoherent motion (IVIM) images have been obtained using spin‐echo sequences with extragradient pulses, resulting in long acquisition times (typically 2 × 8 min 32 s). A new method is proposed here, using steady‐state free precession (SSFP), which allows IVIM images to be obtained in a couple of minutes. Phantom studies showed that the sensitivity of SSFP to IVIMs is much greater than that of spin echoes. In vivo images are shown. © 1988 Academic Press, Inc.

A theoretical analysis of the effects of diffusion and perfusion in steady‐state free precession (SSFP) imaging sequences sensitized to intravoxel incoherent motions by magnetic field gradients is presented and supported by phantom studies. The capability of such sequences to image diffusion and perfusion quickly was recently demonstrated. The possible residual effects of T₁ and T₂ in diffusion measurements are evaluated, as are the effects of the sequence design and the acquisition parameters (repetition time, flip angle, gradient pulses). It is shown theoretically and confirmed by experiments on phantoms that diffusion coefficients can be directly measured from SSFP images when large enough diffusion gradient pulses are used. © 1989 Academic Press, Inc.

The polyacrylamide gel is proposed here as a phantom material for NMR imaging. This substance has electrical and NMR relaxation characteristics very similar to those of biological tissues. The thermal and time stabilities also make this material a convenient standard for MRI. © 1987 Academic Press, Inc.