Respiration effects and cardiac pulsatility can induce signal modulations in functional MR image time series that increase noise and degrade the statistical significance of activation signals. A simple image-based correction method is described that does not have the limitations of k-space methods that preclude high spatial frequency correction. Low-order Fourier series are fit to the image data based on time of each image acquisition relative to the phase of the cardiac and respiratory cycles, monitored using a photoplethysmograph and pneumatic belt, respectively. The RETROICOR method is demonstrated using resting-state experiments on three subjects and compared with the k-space method. The method is found to perform well for both respiration-and cardiac-induced noise without imposing spatial filtering on the correction. Magn Reson Med 44:162-167, 2000.

Phase images in susceptibility-weighted MRI provide excellent contrast. However, the phase is affected by tissue geometry and orientation relative to the main magnetic field (B0) and phase changes extend beyond areas of altered susceptibility. Magnetic susceptibility, on the other hand, is an intrinsic tissue property, closely reflecting tissue composition. Therefore, recently developed inverse Fourier-based methods were applied to calculate susceptibility maps from high-resolution phase images acquired at a single orientation at 7 Tesla in the human brain (in vivo and fixed) and at 11.7 Tesla in fixed marmoset brain. In susceptibility images, the contrast of cortical layers was more consistent than in phase images and was independent of the structures’ orientation relative to B0. The contrast of iron-rich deep-brain structures (red nucleus and substantia nigra) in susceptibility images agreed more closely with iron-dominated R2* images than the phase image contrast which extended outside the structures. The mean susceptibility in these regions was significantly correlated with their estimated iron content. Susceptibility maps calculated using this method overcome the orientation-dependence and non-locality of phase image contrast and seem to reflect underlying tissue composition. Susceptibility images should be easier to interpret than phase images and could improve our understanding of the sources of susceptibility contrast.

Recent advances in high-field MRI have dramatically improved the visualization of human brain anatomy in vivo. Most notably, in cortical gray matter, strong contrast variations have been observed that appear to reflect the local laminar architecture. This contrast has been attributed to subtle variations in the magnetic properties of brain tissue, possibly reflecting varying iron and myelin content. To establish the origin of this contrast, MRI data from postmortem brain samples were compared with electron microscopy and histological staining for iron and myelin. The results show that iron is distributed over laminae in a pattern that is suggestive of each region's myeloarchitecture and forms the dominant source of the observed MRI contrast. myelin | ferritin | laminar variation | brain structure | magnetic susceptibility M uch of the human cerebral cortex is organized in a network of functionally specialized regions. This understanding forms the basis of studies that attempt to expose the brain's inner workings using modern functional imaging techniques such as PET and MRI (1, 2). Some of this functional specialization is reflected in the local cortical architecture, and may be observed in postmortem brain samples from the laminar and columnar variation in a number of tissue properties including cell shape (3), myelin content (4), metabolic state (5), neurochemical profile (6), and receptor density (7).One of the most striking examples of this structure-function relationship is in the primary visual cortex (V1, or Brodmann area 17), which is involved in the early stages of visual information processing. Visual area V1 can be readily identified with postmortem tissue analysis based on the prominence of myelinated fibers in layer IVb, known as the line of Gennari. Many other brain areas show characteristic myelination patterns (8, 9), and systematic analyses of this type of structural information may provide important clues about the brain's functional organization. Whole-brain postmortem studies of cellular distributions and myelin content in human brain sections at high resolution have previously been described (8, 10).A number of attempts have been made to reveal laminar cortical structure in vivo with structural MRI, by exploiting contrast based on the altered density and NMR relaxation times of water protons in myelin-rich environments (11)(12)(13)(14)(15). Although these studies have had some success, their impact has not been widespread as a result of the rather limited contrast and resolution available with conventional MRI. Nevertheless, much progress has been made over the last few years. Recent developments in high-field MRI (≥7 T) have increased spatial resolution to less than 300 μm (16-18), and improved the ability to detect subtle variations in the magnetic susceptibility of tissue. These variations are reflected in a number of intrinsic MRI parameters, including the longitudinal relaxation rate R 1 , the transverse relaxation rates R 2 and R 2 *, and the NMR resonance frequency. R 2 * indicates th...