Polymeric microcomponents are widely used in microelectromechanical systems (MEMS) and lab-on-a-chip devices, but they suffer from the lack of complex motion, effective addressability and precise shape control. To address these needs, we fabricated polymeric nanocomposite microactuators driven by programmable heterogeneous magnetic anisotropy. Spatially modulated photopatterning was applied in a shape-independent manner to microactuator components by successive confinement of self-assembled magnetic nanoparticles in a fixed polymer matrix. By freely programming the rotational axis of each component, we demonstrate that the polymeric microactuators can undergo predesigned, complex two- and three-dimensional motion.

SUMMARY:In this article, the underlying theory of clinical diffusion MR imaging, including diffusion tensor imaging (DTI) and fiber tractography, is reviewed. First, a brief explanation of the basic physics of diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) mapping is provided. This is followed by an overview of the additional information that can be derived from the diffusion tensor, including diffusion anisotropy, color-encoded fiber orientation maps, and 3D fiber tractography. This article provides the requisite background for the second article in this 2-part review to appear next month, which covers the major technical factors that affect image quality in diffusion MR imaging, including the acquisition sequence, magnet field strength, gradient amplitude and slew rate, and multichannel radio-frequency coils and parallel imaging. The emphasis is on optimizing these factors for state-of-the-art DWI and DTI based on the best available evidence in the literature. Diffusion MR imaging of the brain was first adopted for use in clinical neuroradiology during the early 1990s and was found to have immediate utility for the evaluation of suspected acute ischemic stroke. Since that time, enormous strides forward in the technology of diffusion imaging have greatly improved image quality and enabled many new clinical applications. These include the diagnosis of intracranial pyogenic infections, masses, trauma, and vasogenic-versus-cytotoxic edema. Furthermore, the advent of diffusion tensor imaging (DTI) and fiber tractography has opened an entirely new noninvasive window on the white matter connectivity of the human brain. DTI and fiber tractography have already advanced the scientific understanding of many neurologic and psychiatric disorders and have been applied clinically for the presurgical mapping of eloquent white matter tracts before intracranial mass resections.This 2-part review explores the current state of the art for the acquisition and analysis of clinical diffusion imaging, including DTI and fiber tractography. This first article begins with an explanation of the basic physics and underlying theory of diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) mapping. This is followed by an overview of the additional information that can be derived from the diffusion tensor, including diffusion anisotropy, color-encoded fiber-orientation maps, and 3D fiber tractography. This article provides the necessary background for the second article, which focuses on the major technical factors that affect image quality in diffusion imaging, with an emphasis on optimizing these factors for state-of-the-art DWI and DTI based on the best available evidence in the literature. Theoretic Underpinnings of Diffusion ImagingBrownian Motion and Tissue Ultrastructure Diffusion, also known as "Brownian motion," refers to constant random microscopic molecular motion due to heat. At a fixed temperature, the rate of diffusion can be described by the Einstein equation 1 :where Ͻr 2 Ͼ refers...

SUMMARY:This second article of the 2-part review builds on the theoretic background provided by the first article to cover the major technical factors that affect image quality in diffusion imaging, including the acquisition sequence, magnet field strength, gradient amplitude, and slew rate as well as multichannel radio-frequency coils and parallel imaging. The sources of many common diffusion image artifacts are also explored in detail. The emphasis is on optimizing these technical factors for stateof-the-art diffusion-weighted imaging and diffusion tensor imaging (DTI) based on the best available evidence in the literature. An overview of current methods for quantitative analysis of DTI data and fiber tractography in clinical research is also provided. In this article, the major technical factors that affect image quality in diffusion MR imaging are evaluated in detail. The first half focuses on diffusion-weighted imaging (DWI). The strengths and weaknesses of single-shot echo-planar imaging, by far the most popular sequence for brain DWI, are considered, and alternative sequences are presented for special purpose applications. The effect of hardware and software variables such as magnetic field strengths, gradient amplitudes and slew rates, radio-frequency coils, and parallel imaging reconstruction methods is reviewed. The causes of common DWI artifacts are explained, and strategies are provided for minimizing artifacts and optimizing image quality.The second half of the article focuses on technical considerations specific to diffusion tensor imaging (DTI) and fiber tractography, including optimizing the b-values, the number and orientations of diffusion-weighted acquisitions, as well as the fiber tracking parameters. The undesirable effects of common problems such as low signal intensity-to-noise ratio (SNR) and pulsation artifact are reviewed. An overview is provided of current methods for analyzing quantitative DTI data for clinical research, including the reproducibility of DTI measurements. Throughout this review, the emphasis is on optimizing the many technical factors needed for state-of-the-art DWI and DTI based on the best available evidence in the literature. Technical Considerations for State-of-the-Art DWI Echo-Planar DWIAdvantages of Single-Shot Echo-Planar DWI. Because even minimal bulk patient motion during acquisition of DWIs can obscure the effects of the much smaller microscopic water motion due to diffusion, ultrafast imaging sequences are necessary for successful clinical DWI. Most commonly, diffusion imaging is performed by using spin-echo single-shot echoplanar imaging (SS-EPI) techniques. The term "single shot" means that an entire 2D image is formed from a single radiofrequency excitation pulse. Images can be acquired in a fraction of a second; therefore, artifact from physiologic cardiac and respiratory pulsatility and from patient motion is greatly reduced, including motion between acquisitions with different orientations of the diffusion-sensitizing gradients. Another advantage of S...

Fluidic self-assembly is a promising pathway for parallel fabrication of devices made up of many small components. Here, we introduce 'railed microfluidics' as an agile method to guide and assemble microstructures inside fluidic channels. The guided movement of microstructures in microfluidic channels was achieved by fabricating grooves ('rails') on the top surface of the channels and also creating complementary polymeric microstructures that fit with the grooves. Using the rails as a guiding mechanism, we built complex one- and two-dimensional microsystems in which all the microstructures initially involved in the fabrication method were incorporated as components in the final product. Complex structures composed of more than 50 microstructures (each sized smaller than 50 microm) were fluidically self-assembled with zero error. Furthermore, we were able to use the rails to guide microstructures through different fluid solutions, successfully overcoming strong interfacial tension between solutions. On the basis of rail-guided self-assembly and cross-solution movement, we demonstrated heterogeneous fluidic self-assembly of polymeric microstructures and living cells. In addition to such assembly of in situ polymerized structures, we also guided and assembled externally fabricated silicon chips-demonstrating the feasible application of railed microfluidics to other materials systems.

Fiber tracks delineated using DT imaging can be used to identify the motor tract in deep white matter and define a safety margin around the tract.