Single-shot echo-planar imaging (EPI) is well established as the method of choice for clinical, diffusion-weighted imaging with MRI because of its low sensitivity to the motion-induced phase errors that occur during diffusion sensitization of the MR signal. However, the method is prone to artifacts due to susceptibility changes at tissue interfaces and has a limited spatial resolution. The introduction of parallel imaging techniques, such as GRAPPA (GeneRalized Autocalibrating Partially Parallel Acquisitions), has reduced these problems, but there are still significant limitations, particularly at higher field strengths, such as 3 Tesla (T), which are increasingly being used for routine clinical imaging. This study describes how the combination of readoutsegmented EPI and parallel imaging can be used to address these issues by generating high-resolution, diffusion-weighted images at 1.5T and 3T with a significant reduction in susceptibility artifact compared with the single-shot case. The technique uses data from a 2D navigator acquisition to perform a nonlinear phase correction and to control the real-time reacquisition of unusable data that cannot be corrected. Measurements on healthy volunteers demonstrate that this approach provides a robust correction for motion-induced phase artifact and allows scan times that are suitable for routine clinical application. Magn Reson Med 62:468 -475, 2009.

We recently reported on three young patients with severe impairments of episodic memory resulting from brain injury sustained early in life. These findings have led us to hypothesize that such impairments might be a previously unrecognized consequence of perinatal hypoxic-ischaemic injury. Neuropsychological and quantitative magnetic resonance investigations were carried out on five young patients, all of whom had suffered hypoxic-ischaemic episodes at or shortly after birth. All five patients showed severe impairments of episodic memory (memory for events), with relative preservation of semantic memory (memory for facts). However, none had any of the major neurological deficits that are typically associated with hypoxic-ischaemic injury, and all attended mainstream schools. Quantitative magnetic resonance investigations revealed severe bilateral hippocampal atrophy in all cases. As a group, the patients also showed bilateral reductions in grey matter in the regions of the putamen and the ventral part of the thalamus. On the basis of their clinical histories and the pattern of magnetic resonance findings, we attribute the patients' pathology and associated memory impairments primarily to hypoxic-ischaemic episodes sustained very early in life. We suggest that the degree of hypoxia-ischaemia was sufficient to produce selective damage to particularly vulnerable regions of the brain, notably the hippocampi, but was not sufficient to result in the more severe neurological and cognitive deficits that can follow hypoxic-ischaemic injury. The impairments in episodic memory may be difficult to recognize, particularly in early childhood, but this developmental amnesia can have debilitating consequences, both at home and at school, and may preclude independent life in adulthood.

In polymer gel dosimetry using magnetic resonance imaging, the uncertainty in absorbed dose is dependent on the experimental determination of T2. The concept of dose resolution (Dpdelta) of polymer gel dosimeters is developed and applied to the uncertainty in dose related to the uncertainty in T2 from a range of T4 encountered in polymer gel dosimetry. Dpdelta is defined as the minimal separation between two absorbed doses such that they may be distinguished with a given level of confidence, p. The minimum detectable dose (MDD) is Dpdelta as the dose approaches zero. Dpdelta and the minimum detectable dose both give a quantifiable indication of the likely practical limitations and usefulness of the dosimeter. Dpdelta of a polyacrylamide polymer gel dosimeter is presented for customized 32-echo and standard multiple-spin-echo sequences on a clinical MRI scanner. In evaluating uncertainties in T2, a parameter of particular significance in the pulse sequence is the echo spacing (ES). For optimal results, ES should be selected to minimize Dpdelta over a range of doses of interest in polymer gel dosimetry.

As a consequence of the Maxwell equations, linear field gradients are accompanied by additional spatially dependent field components. A description of the Maxwell field terms is presented which explicitly takes into account the asymmetry of the gradient coil. It is shown both theoretically and experimentally that, in contrast to symmetric coils, an asymmetric coil generates concomitant field terms of zeroth and first order in space. Artifacts induced by concomitant fields can be much more pronounced for asymmetric coil designs than for symmetric ones. For the strong gradient amplitudes available on modern MR systems the effect of these concomitant magnetic fields on the evolution of magnetization has to be taken into consideration in a variety of NMR acquisition techniques. The formalism is used experimentally to compensate for artifacts ob- In MRI, linear magnetic field gradients are used to encode spatial information with the requirement that the gradient coils produce a linear field over the volume of interest. However, this requirement cannot be completely fulfilled as it contradicts the general principle of electromagnetism expressed by Maxwell's equations. For a source-free region they require a zero divergence (div B ϭ 0) and a zero rotation (rot B ϭ 0) of the magnetic flux. Consequently, the linear magnetic fields are accompanied by secondary field terms, which are typically referred to as Maxwell or concomitant fields.Concomitant fields and their effects on NMR images were first described in 1985 (1), but remained mostly unrecognized in routine imaging. Since these early observations, improvement in gradient coil technology has made it possible to significantly increase the amplitude of the applied magnetic field gradients used in NMR imaging studies. This has amplified the influence of the secondary fields and has lead to a new appreciation of their effects. They have subsequently been studied extensively in a series of publications (2-7), which have provided a mathematical description of the concomitant fields and an understanding of some of the associated image artifacts. These studies have focused on the case of cylindrical symmetry, which is appropriate for the symmetrical gradient coils used by most MR systems.However, some systems use an asymmetric coil design, which can be advantageous in some cases. The MRI system used in this work is a human head-only scanner equipped with a short-axis gradient coil of a reduced inner diameter of 36 cm. This compact gradient design increases efficiency and minimizes coil inductance. It reduces the noise level due to a smaller number of coil turns and a shortened length. It also reduces the physiological stimulation dB/dt due to a smaller gradient region size and has a smaller heat generation (see Ref. 8 and references therein). As a result, it provides a better performance for high-speed MRI than coils of a whole body imaging system. Because the patient's head access into the coil is limited by the shoulders, the MRI system uses an asymmetric transverse coil w...

Anatomical MRI studies at 7T have demonstrated the ability to provide high-quality images of human tissue in vivo. However, diffusion-weighted imaging at 7T is limited by the increased level of artifact associated with standard, singleshot, echo-planar imaging, even when parallel imaging techniques such as generalized autocalibrating partially parallel acquisitions (GRAPPA) are used to reduce the effective echo spacing. Readout-segmented echo-planar imaging in conjunction with parallel imaging has the potential to reduce these artifacts by allowing a further reduction in effective echo spacing during the echo-planar imaging readout. This study demonstrates that this approach does indeed provide a substantial improvement in image quality by reducing image blurring and susceptibility-based distortions, as well as by allowing the acquisition of diffusion-weighted images with a high spatial resolution. A preliminary application of the technique to highresolution diffusion tensor imaging provided a high level of neuroanatomical detail, which should prove valuable in a wide range of applications. Magn Reson Med 64:9-14, 2010. V C 2010 Wiley-Liss, Inc. Key words: readout-segmented EPI; parallel imaging; 2D navigator; multi-shot diffusion; DTI MRI of humans at 7T can provide highly detailed anatomical brain images and perform localized functional MRI with very high spatial resolution. However, it remains a challenge to acquire good-quality diffusionweighted (DW) images at ultra-high field strengths. Single-shot echo-planar imaging (ss-EPI) is well established as the method of choice for DW imaging and therefore for diffusion tensor imaging (DTI). This is due to its low sensitivity to the motion-induced phase errors that occur during diffusion sensitization of the MR signal. However, ss-EPI is prone to artifacts related to susceptibility changes at tissue interfaces and has a limited spatial resolution due to T 2 * relaxation. Parallel imaging can be used to improve the image quality of ss-EPI acquisitions (1), but there are still significant limitations, particularly at higher field strength. Susceptibility effects and T 2 * blurring increase with field strength, and therefore parallel imaging techniques with high acceleration factors (greater than 4) are necessary to address these problems for high-resolution DW imaging and DTI (2). The shorter T 2 of brain tissue at 7T presents further difficulties by limiting the acquisition of data at long echo times (TEs). At lower field strengths, improved image quality has been demonstrated in several studies using a variety of multi-shot DW sequences, which correct for the shot-toshot, spatially varying phase errors caused by cardiac pulsation during diffusion preparation (3-6). An additional technique that can be used with nonlinear phase correction is readout-segmented echo-planar imaging (rs-EPI). Following an initial description by Robson et al. (7), the method was further developed in subsequent work to include two-dimensional (2D) navigator correction, parallel imaging, an...