Laurance D. Hall

A STUDY OF ROTATIONALLY INVARIANT AND SYMMETRIC INDICES OF DIFFUSION ANISOTROPY

This study investigated the properties of a class of rotationally invariant and symmetric (relative to the principal diffusivities) indices of the anisotropy of water self-diffusion, namely fractional anisotropy (FA), relative anisotropy (RA), and volume ratio (VR), with particular emphasis to their measurement in brain tissues. A simplified theoretical analysis predicted significant differences in the sensitivities of the anisotropy indices (AI) over the distribution of the principal diffusivities. Computer simulations were used to investigate the effects on AI image quality of three magnetic resonance (MR) diffusion tensor imaging (DTI) acquisition schemes, one being novel: the schemes were simulated on cerebral model fibres varying in shape and spatial orientation. The theoretical predictions and the results of the simulations were corroborated by experimentally determined spatial maps of the AI in a normal feline brain in vivo. We found that FA mapped diffusion anisotropy with the greatest detail and SNR whereas VR provided the strongest contrast between low- and high-anisotropy areas at the expense of increased noise contamination and decreased resolution in anisotropic regions. RA proved intermediate in quality. By sampling the space of the effective diffusion ellipsoid more densely and uniformly and requiring the same total imaging time as the published schemes, the novel DTI scheme achieved greater rotational invariance than the published schemes, with improved noise characteristics, resulting in improved image quality of the AI examined. Our findings suggest that significant improvements in diffusion anisotropy mapping are possible and provide criteria for the selection of the most appropriate AI for a particular application.

Minimal gradient encoding for robust estimation of diffusion anisotropy

This study has investigated the relationship between the noise sensitivity of measurement by magnetic resonance imaging (MRI) of the diffusion tensor (D) of water and the number N of diffusion-weighting (DW) gradient directions, using computer simulations of strongly anisotropic fibers with variable orientation. The DW directions uniformly sampled the diffusion ellipsoid surface. It is shown that the variation of the signal-to-noise ratio (SNR) of three ideally rotationally invariant scalars of D due to variable fiber orientation provides an objective quantitative measure for the diffusion ellipsoid sampling efficiency, which is independent of the SNR value of the baseline signal obtained without DW; the SNR variation decreased asymptotically with increasing N. The minimum number N(0) of DW directions, which minimized the SNR variation of the three scalars of D was determined, thereby achieving the most efficient ellipsoid sampling. The resulting time efficient diffusion tensor imaging (DTI) protocols provide robust estimation of diffusion anisotropy in the presence of noise and can improve the repeatability/reliability of DTI experiments when there is high variability in the orientation of similar anisotropic structures, as for example, in studies which require repeated measurement of one individual, intersubject comparisons or multicenter studies.

MEASUREMENT OF LOCALIZED CARTILAGE VOLUME AND THICKNESS OF HUMAN KNEE JOINTS BY COMPUTER ANALYSIS OF THREE-DIMENSIONAL MAGNETIC RESONANCE IMAGES

RATIONALE AND OBJECTIVES This work demonstrates a new method for computerized measurement of the dimensions (thickness and volume) of articular cartilage for any specified region of the human knee joint. Three-dimensional magnetic resonance (MR) images optimized for cartilage contrast have been analyzed using computerized edge-detection techniques, and the reproducibility of articular cartilage thickness and volume measurements is assessed. METHODS A fat-suppressed, three-dimensional SPoiled GRass MR sequence (45/7.5/30 degrees) with total scan time of approximately 12 minutes was used to acquire volume images of human knee joints at spatial resolution of 0.6 x 1.2 x 1.2 mm. Measurements were made using six repeated scans for three healthy volunteers over a period of 2 months. The subsequent semi-automated image processing to establish total cartilage volume and cartilage thickness maps for the femur required approximately 60 minutes of operator time. RESULTS The mean coefficient of variation for total cartilage volume for the six repeated scans for the three volunteers was 3.8%, and the average coefficient of variation for the user-selected cartilage plugs was 2.0%. The cartilage thickness maps from the repeated scans of the same knee were similar. CONCLUSIONS Standard resolution MR images with fat-suppressed contrast lead to an objective and reproducible measurement of spatial dimensions of articular cartilage when analyzed semi-automatically using computerized edge-detection methods.

A comparison of the early development of ischaemic damage following permanent middle cerebral artery occlusion in rats as assessed using magnetic resonance imaging and histology

Recent developments in diffusion-weighted imaging (DWI) have enabled the pathological changes that occur during cerebral ischaemia to be studied. The present studies utilised DWI to investigate the development of early ischaemic changes following permanent middle cerebral artery (MCA) occlusion in the rat, which represents a model of stroke. An increased DWI signal was seen in the region of the occluded MCA and this was detectable as early as 1 h postocclusion. DWI images were obtained at nine stereotactic levels throughout the brain, providing a quantifiable measure of the volume of increased signal intensity in each animal. At 1 h post-MCA occlusion the hyperintense areas were seen in the frontoparietal cortex and lateral caudate nucleus; these areas represent the core of the infarct and no protection is seen with any compounds in these areas. There was a progressive increase in the area of hyperintensity up to 4 h post-MCA occlusion, and at this time point the hyperintensity was seen in the dorsolateral cortex and caudate nucleus. At 4 h post-MCA occlusion there was a significant correlation between the volume of hemispheric and cortical ischaemic damage measured using DWI and histology. Thus, it appears that the increased DWI signal seen during the early time points after MCA occlusion was demarcating tissue that was destined for infarction. The area beyond the hyperintense region at 1 h represents the so-called “penumbral” region, because with increasing time (post-MCA occlusion) this area became incorporated into the infarct. There was also a slight increase in infarct size between 4 and 24 h, when assessed using DWI or histology, although two groups of animals were being compared, as opposed to the time-course study, in which just one group of animals was used. At 24 h post-MCA occlusion there was a good correlation between DWI, histology, and conventional T2 weighted imaging. There was no further increase in size of the infarct between 24 h and 7 days as assessed using histology and T2-weighted imaging. DWI could not be used to quantify infarct volume at 7 days because there was no uniform signal in the damaged area. At 7 days the area of infarction actually appeared to be darker in the diffusion-weighted images. The hyperintensity seen in diffusion-weighted images appears to decrease some time between 24 h and 7 days. These data provide evidence that DWI can be used to detect early ischaemic changes and it appears that the hyperintense areas seen at 1 h delineates tissue that is destined to become the core of the infarct. Thus, DWI could be used to identify the so-called penumbral areas, which are salvageable if treated within the therapeutic window of 2–3 h and may therefore have an important role in detecting such changes in the clinical setting.

Cartilage swelling and loss in a spontaneous model of osteoarthritis visualized by magnetic resonance imaging

The objective of this study was to investigate whether the rate of change in cartilage pathology could be effectively monitored by magnetic resonance imaging (MRI) as part of a longitudinal investigation of an osteoarthritis model in vivo, and to define the minimum requirements necessary to establish disease progression. Magnetic resonance images of the knee of eight male Dunkin-Hartley guinea-pigs were obtained at 8, 12, 18, 24, 30, 36 and 52 weeks of age using a two-dimension spin-echo sequence with a TR of 1500 ms and TE of 40 ms. The total thickness of the femoral and tibial cartilage was measured from those images. Over the same time course, sets of spin-spin relaxation-weighted images were acquired from two additional animals of the same age, from which the T2 relaxation times of water in the articular cartilage were estimated and compared with those of muscle and adipose tissue within the same joint. The cartilage thickness of all the animals increased during the first 6 months, then either stayed thicker (4/8) or became progressively thinner (4/8). Up to 18 weeks of age, the cartilage T2 value was between 23-24 ms but became elevated by 30 weeks and the mean value was more than 40 ms at the end of the study, T2 values for the muscle and adipose tissue remained within the range 30-33 ms, or 47 ms, respectively. We concluded that in this model, cartilage thickness measurements from an magnetic resonance image would not provide a reliable marker to stage osteoarthritis progression partly because the cartilage was so thin in a joint of this size, but also because the changes with time were not linear but biphasic. However, quantitation of the T2 relaxation values may provide a more predictable indicator of cartilage pathology for longitudinal studies because the changes were monotonic and independent of cartilage thickness.

Nuclear magnetic resonance imaging in the solid state

During the last two decades much research effort has been invested in the development of techniques for nuclear magnetic resonance (NMR) imaging. Because of the difficulties associated with imaging of solids, the majority of the research carried out to date has been directed towards the liquid state, and applications have been principally directed to the imaging of free water distribution in biological tissues. Recently, however, a number of techniques have been described which are designed to overcome the linewidth problems associated with solid-state imaging

Evolution of photochemically induced focal cerebral ischemia in the rat. Magnetic resonance imaging and histology.

BACKGROUND AND PURPOSE Magnetic resonance imaging (MRI) is increasingly used to study the pathophysiological evolution of cerebral ischemia in humans and animals. We have investigated photochemically induced (rose bengal) focal cerebral ischemia, a relatively noninvasive, reproducible model for stroke, and compared the evolution of the ischemic response in vivo and postmortem with MRI and histology, respectively. METHODS MR images weighted for T2, diffusion, and T2* and parallel histological sections stained with cresyl fast violet (CFV) and for glial fibrillary acid protein were obtained from 34 adult male Hooded Lister rats at seven time points (3.75 to 196 hours) after bilateral ischemia induction. From CFV histology, lesion volumes and cell counts were calculated; from diffusion-weighted and T2-weighted images, apparent diffusion coefficients and lesion volumes were determined. RESULTS Both MRI and histology revealed a well-defined lesion at 3.75 hours after irradiation and a consistent pattern of temporal evolution; lesion apparent diffusion coefficients decreased significantly by 3.75 hours, increased significantly by day 2, and correlated strikingly with the decline in lesion CFV-positive cell numbers. After day 2, astrocytes and connective tissue cells invaded the infarct. Throughout the time course, lesion volumes determined in vivo and postmortem (after shrinkage correction) agreed well. CONCLUSIONS MRI changes quantitatively reflect histopathology, revealing reproducible primary and secondary damage characteristics noninvasively. These changes essentially replicate those reported for other animal stroke models and clinically, emphasizing the value both of MRI and the photochemically induced focal cerebral ischemia model in stroke research.

Cortical spreading depression in the gyrencephalic feline brain studied by magnetic resonance imaging

1 Time‐lapse diffusion‐weighted magnetic resonance imaging (DWI) was used to detect and characterize complex waves of cortical spreading depression (CSD) evoked with KCl placed upon the suprasylvian gyrus of anaesthetized cats. 2 The time‐lapse representations successfully demonstrated primary CSD waves that propagated with elliptical wavefronts selectively over the ipsilateral cerebral hemispheres with a velocity of 3.8 ± 0.70 mm min−1 (mean ± s.e.m. of 5 experiments). 3 In contrast, the succeeding secondary waves often remained within the originating gyrus, were slower (velocity 2.0 ± 0.18 mm min−1), more fragmented and varied in number. 4 Computed traces of the apparent diffusion coefficients (ADCs) showed negative deflections followed by monotonic decays (amplitudes: primary wave, ‐19.9 ± 2.8 %; subsequent waves, ‐13.6 ± 1.9 %; duration at half‐maximal decay, 150‐200 s) when determined from regions of interest (ROIs) through which both primary and succeeding CSD waves propagated. 5 The passage of both the primary and the succeeding waves often correlated with transient DC potential deflections recorded from the suprasylvian gyrus. 6 The detailed waveforms of the ADC and the T2*‐weighted (blood oxygenation level‐dependent: BOLD) traces showed a clear reciprocal correlation. These imaging features that reflect disturbances in cellular water balance agree closely with BOLD measurements that followed the propagation velocities of the first and subsequent CSD events. They also provide a close physiological correlate for clinical observations of cortical blood flow disturbances associated with human migraine.

Gradient preemphasis calibration in diffusion-weighted echo-planar imaging.

This article describes a method which enables fast and objective pulse‐sequence‐specific preemphasis calibration, using standard pulse sequences and system hardware. The method is based on a k‐space measurement technique, and has been applied to single‐shot, diffusion‐weighted, spin‐echo, echo‐planar imaging (DW‐SE‐EPI), which is particularly sensitive to eddy‐current‐induced image distortions. The efficiency of the technique was demonstrated not only by the reduction of eddy‐current fields to a negligible level using full preemphasis compensation, but also by the fact that adjustment of the slow time‐base alone sufficed for the practical elimination of image distortions in the DW‐SE‐EPI images and the subsequent diffusion tensor maps (in a phantom and a human brain). By seeking to eliminate directly the effect of eddy‐current‐induced phase shifts during the EPI data collection, the method is free of the complications and restrictions associated with other eddy‐current correction techniques for DW‐SE‐EPI (such as acquisition of additional calibration scans, intense postprocessing, extensive pulse‐sequence modifications), making their use redundant. Magn Reson Med 44:616–624, 2000.

The gelation of sodium alginate with calcium ions studied by magnetic resonance imaging (MRI)

Abstract The displacement of the sol/gel interface during the gelation of sodium alginate by calcium ions can be tracked in both one and two dimensions using magnetic resonance imaging. In the one-dimensional gelation experiments, the distance moved by the sol/gel interface was proportional to (time 1 2 ), which implies that the gelation process is diffusion-limited. Using the gelation model previously proposed by Smidsrod and co-workers, together with the initial reaction conditions and assuming that the diffusion coefficient of the alginate molecules is extremely small, it was found that the diffusion of calcium ions through the gel network is dependent on the initial concentration of the calcium, the ionic strength of the alginate solution, and the size of pores in the gel which is formed.