The relationship between blood oxygenation level-dependent (BOLD) MRI signals, cerebral blood flow (CBF), and oxygen consumption (CMR O 2 ) in the physiological steady state was investigated. A quantitative model, based on flow-dependent dilution of metabolically generated deoxyhemoglobin, was validated by measuring BOLD signals and relative CBF simultaneously in the primary visual cortex (V1) of human subjects (N ‫؍‬ 12) during graded hypercapnia at different levels of visual stimulation. BOLD and CBF responses to specific conditions were averaged across subjects and plotted as points in the BOLD-CBF plane, tracing out lines of constant CMR O 2 . The quantitative deoxyhemoglobin dilution model could be fit to these measured iso-CMR O 2 contours without significant (P I 0.05) residual error and yielded MRI-based CMR O 2 measurements that were in agreement with PET results for equivalent stimuli. BOLD and CBF data acquired during graded visual stimulation were then substituted into the model with constant parameters varied over plausible ranges. Relative changes in CBF and CMR O 2 appeared to be coupled in an approximate ratio of D2:1 for all realistic parameter settings. Magn Reson Med 42:849

While holding vast potential, diffusion tensor imaging (DTI) with single-excitation protocols still faces serious challenges. Limited spatial resolution, susceptibility to magnetic field inhomogeneity, and low signal-to-noise ratio (SNR) may be considered the most prominent limitations. It is demonstrated that all of these shortcomings can be effectively mitigated by the transition to parallel imaging technology and high magnetic field strength. Using the sensitivity encoding (SENSE) technique at 3 T, brain DTI was performed in nine healthy volunteers. Despite enhanced field inhomogeneity, parallel acquisition permitted both controlling geometric distortions and enhancing spatial resolution up to 0.8 mm in-plane. Heightened SNR requirements were met in part by high base sensitivity at 3 T. Diffusion tensor imaging (1,2) is a promising noninvasive method for studying white matter structure of the human brain in vivo. Based on the concept of anisotropic water diffusion across tissue, the measurement of 3D diffusion properties, as described by a local diffusion tensor, allows the characterization of the axonal architecture of white matter networks. For that purpose, a 3D tracking of axonal projections, known as fiber tracking (3-7), is required. However, the low SNR and the limited spatial resolution (8 -10) of the method severely impair its application. A serious resolution limit stems from the strong link between voxel size and SNR, the latter being inherently low due to diffusion weighting. Only improving the SNR of the initial diffusion-weighted (DW) images will enable better spatial resolution. Therefore, the use of high magnetic fields and the related SNR gain could considerably enhance the performance of DTI and fiber tracking.The calculation of the local diffusion tensor requires a set of DW images, acquired with diffusion gradients applied in at least six noncollinear directions, plus a reference image without diffusion weighting. The sequence most commonly used for DTI is spin-echo single-shot EPI (SE-sshEPI). It allows for whole brain coverage in an acceptable scan time and is insensitive to bulk motion due to its speed. Critical shortcomings of sshEPI are image blurring due to T* 2 decay during the EPI readout interval and off-resonance effects, caused by the long EPI echo train (11,12). Both effects scale with the main magnetic field B 0 , making the transition to higher field strength challenging. At 3 T, signal alteration and geometric distortion due to static resonance offset effects, e.g., in the vicinity of airtissue interfaces, are a serious problem when using sshEPIbased protocols.Recently, the potential of parallel imaging techniques, such as simultaneous acquisition of spatial harmonics (SMASH) (13) and sensitivity encoding (SENSE) (14), has been demonstrated for sshEPI in general (15), as well as for diffusion-weighted MRI (DWI) (16 -18) and DTI (19) at 1.5 T. Parallel imaging techniques were shown to significantly reduce EPI-related artifacts as a result of shortening the echo train by facto...

Brain activation during motor imagery has been the subject of a large number of studies in healthy subjects, leading to divergent interpretations with respect to the role of descending pathways and kinesthetic feedback on the mental rehearsal of movements. We investigated patients with complete spinal cord injury (SCI) to find out how the complete disruption of motor efferents and sensory afferents influences brain activation during motor imagery of the disconnected feet. Eight SCI patients underwent behavioral assessment and functional magnetic resonance imaging. When compared to a healthy population, stronger activity was detected in primary and all non-primary motor cortical areas and subcortical regions. In paraplegic patients the primary motor cortex was consistently activated, even to the same degree as during movement execution in the controls. Motor imagery in SCI patients activated in parallel both the motor execution and motor imagery networks of healthy subjects. In paraplegics the extent of activation in the primary motor cortex and in mesial non-primary motor areas was significantly correlated with the vividness of movement imagery, as assessed by an interview. The present findings provide new insights on the neuroanatomy of motor imagery and the possible role of kinesthetic feedback in the suppression of cortical motor output required during covert movements.

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Purpose:To further validate the quantitative use of flowsensitive four-dimensional velocity encoded cine magnetic resonance imaging (4D VEC MRI) for simultaneously acquired venous and arterial blood flow in healthy volunteers and for abnormal flow in patients with congenital heart disease.Materials and Methods: Stroke volumes (SV) obtained in arterial and venous thoracic vessels were compared between standard two-dimensional (2D), 4D VEC MRI with and without respiratory navigator gating (gated/nongated) in volunteers (n ¼ 7). In addition, SV and regurgitation fractions (RF) measured in aorta or pulmonary trunk of patients with malformed and/or insufficient valves (n ¼ 10) were compared between 2D and nongated 4D VEC MRI methods. Results:In volunteers and patients, Bland-Altman tests showed excellent agreement between 2D, gated, and nongated 4D VEC MRI obtained quantitative blood flow measurements. The bias between 2D and gated 4D VEC MRI was <0.5 mL for SV; between 2D and nongated 4D VEC MRI the bias was <0.7 mL for SV and <1% for RF.Conclusion: Blood flow can be quantified accurately in arterial, venous, and pathological flow conditions using 4D VEC MRI. Nongated 4D VEC MRI has the potential to be suited for clinical use in patients with congenital heart disease who require flow acquisitions in multiple vessels.

Phantom limbs are traditionally conceptualized as the phenomenal persistence of a body part after deafferentation. Previous clinical observations of subjects with phantoms of congenitally absent limbs are not compatible with this view, but, in the absence of experimental work, the neural basis of such ''aplasic phantoms'' has remained enigmatic. In this paper, we report a series of behavioral, imaging, and neurophysiological experiments with a university-educated woman born without forearms and legs, who experiences vivid phantom sensations of all four limbs. Visuokinesthetic integration of tachistoscopically presented drawings of hands and feet indicated an intact somatic representation of these body parts. Functional magnetic resonance imaging of phantom hand movements showed no activation of primary sensorimotor areas, but of premotor and parietal cortex bilaterally. Movements of the existing upper arms produced activation expanding into the hand territories deprived of afferences and efferences. Transcranial magnetic stimulation of the sensorimotor cortex consistently elicited phantom sensations in the contralateral fingers and hand. In addition, premotor and parietal stimulation evoked similar phantom sensations, albeit in the absence of motor evoked potentials in the stump. These data indicate that body parts that have never been physically developed can be represented in sensory and motor cortical areas. Both genetic and epigenetic factors, such as the habitual observation of other people moving their limbs, may contribute to the conscious experience of aplasic phantoms.

4D PC MR imaging allows for a detailed assessment of CSF flow dynamics helping to distinguish physiological from complex pathological flow patterns at the craniocervical junction and the cervical spine.

4D-PC flow imaging allowed comprehensive analysis of CSF flow in patients with Chiari I malformation. Alterations of CSF hydrodynamics were most pronounced in patients with syringomyelia.