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.

Here we describe the insights gained from sequencing the whole genomes of 2,636 Icelanders to a median depth of 20×. We found 20 million SNPs and 1.5 million insertions-deletions (indels). We describe the density and frequency spectra of sequence variants in relation to their functional annotation, gene position, pathway and conservation score. We demonstrate an excess of homozygosity and rare protein-coding variants in Iceland. We imputed these variants into 104,220 individuals down to a minor allele frequency of 0.1% and found a recessive frameshift mutation in MYL4 that causes early-onset atrial fibrillation, several mutations in ABCB4 that increase risk of liver diseases and an intronic variant in GNAS associating with increased thyroid-stimulating hormone levels when maternally inherited. These data provide a study design that can be used to determine how variation in the sequence of the human genome gives rise to human diversity.

The signal decay with increasing b‐factor at fixed echo time from brain tissue in vivo has been measured using a line scan Stejskal–Tanner spin echo diffusion approach in eight healthy adult volunteers. The use of a 175 ms echo time and maximum gradient strengths of 10 mT/m allowed 64 b‐factors to be sampled, ranging from 5 to 6000 s/mm2, a maximum some three times larger than that typically used for diffusion imaging. The signal decay with b‐factor over this extended range showed a decidedly non‐exponential behavior well‐suited to biexponential modeling. Statistical analyses of the fitted biexponential parameters from over 125 brain voxels (15 × 15 × 1 mm3 volume) per volunteer yielded a mean volume fraction of 0.74 which decayed with a typical apparent diffusion coefficient around 1.4 µm2/ms. The remaining fraction had an apparent diffusion coefficient of approximately 0.25 µm2/ms. Simple models which might explain the non‐exponential behavior, such as intra‐ and extracellular water compartmentation with slow exchange, appear inadequate for a complete description. For typical diffusion imaging with b‐factors below 2000 s/mm2, the standard model of monoexponential signal decay with b‐factor, apparent diffusion coefficient values around 0.7 µm2/ms, and a sensitivity to diffusion gradient direction may appear appropriate. Over a more extended but readily accessible b‐factor range, however, the complexity of brain signal decay with b‐factor increases, offering a greater parametrization of the water diffusion process for tissue characterization. Copyright © 1999 John Wiley & Sons, Ltd.

A disturbance in the frontal-striatal-thalamic circuitry has been proposed for schizophrenia, but this concept has been based primarily on indirect evidence from psychopharmacology and analogies with animal research. Diffusion tensor imaging, a new MRI technique that permits direct assessment of the large axon masses stretching from the prefrontal cortex to the striatum, was used to study white matter axon bundles. Diffusion tensor images, high-resolution structural MRI and positron emission tomography scans with 18-fluorodexoyglucose were obtained on five patients with schizophrenia and six age- and sex-matched normal controls. Significantly lower diffusion anisotropy in the white matter of the prefrontal cortex in schizophrenic patients than in normal controls was observed in statistical probability maps. Co-registered PET scans revealed significantly lower correlation coefficients between metabolic rates in the prefrontal cortex and striatum in patients than in controls. These twin findings provide convergent evidence for diminished fronto-striatal connectivity in schizophrenia.

A novel line scan diffusion imaging sequence (LSDI) is introduced. LSDI is inherently insensitive to motion artifacts and high quality diffusion maps of the brain can be obtained rapidly without the use of head restraints or cardiac gating. Results from a stroke study and abdominal diffusion images are presented. The results indicate that it is feasible to use the LSDI technique for clinical evaluation of acute ischemic stroke. In contrast to echo-planar diffusion imaging, LSDI does not require modified gradient hardware and can be implemented on conventional scanners. Thus, LSDI should dramatically increase the general availability of robust clinical diffusion imaging.

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StephanE.Maier1 Hákon Gudbjartsson1 SamuelPatz1 LianggeHsu1 Karl-OlofLovblad2 RobertR.Edelman3 StevenWarach2FerencA. Jolesz1OBJECTIVE. Ourobjective wastoevaluate a newscanning method, MR linescandiffu sion imaging, and assessthe apparent diffusion coefficient in the brains of healthy subjects and stroke patients. SUBJECTS AND METHODS. Line scandiffusionimagingwithoutcardiacgatingorheadrestraintswasimplementedon low-(0.5 T) andmedium-(1.5 T) field-strengthscanners with conventional hardware. Diffusion-weighted images were obtained in six healthy subjects and eight stroke patients. Unidirectional diffusion encoding was used for fast localization of strokelesions.For further characterization, orthogonaldiffusionencodingwas applied,and the trace of the apparent diffusion coefficient was calculated. Single-shot diffusion-weighted echoplanar imaging served as the reference standard. For healthy subjects, imaging was re peatedfour times on each scanner.Mean and relative precision of the apparentdiffusion coef ficient trace values were calculated for each pixel. In stroke lesions and adjacent normal tissue, apparent diffusion coefficient trace values were determined. RESULTS.In the108scans obtained, linescandiffusion imaging proved toberobust, virtually free of artifact(independent of slice locationand orientation),reproducible,and rapid for localization of a stroke. Scan time for 14 slices at 7-mm thickness was 8 mm at 0.5 T and 7 mm at 1.5 T. Image qualities with line scan diffusion imaging and single-shot diffusion weighted echoplanar imaging were comparable. At I .5 T, precision was essentially the same for line scan diffusion imaging (4.3%) and echoplanar imaging (4.7%). With line scan diffu sion imaging at 0.5 T and 1.5 T, normal paraventricular apparent diffusion coefficient trace values averaged 0.71 pm2/msec, and with echoplanar imaging these values averaged 0.69 J1m2/msec. In acute lesions apparent diffusion coefficient trace values were low, and in chronic lesions thesevalues were high. CONCLUSION. Line scandiffusionimagingon low-andmedium-field-strength MRscanners equippedwith conventional hardwarewasreliableandpracticalfor measuringbrain apparentdiffusionvalues,whichcanbe appliedto theearly diagnosis, andhencetimely man agement,of stroke.

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