S. Posse

Bio: S. Posse is an academic researcher from National Institutes of Health. The author has contributed to research in topics: In vivo magnetic resonance spectroscopy & Sequence (medicine). The author has an hindex of 9, co-authored 11 publications receiving 1311 citations. Previous affiliations of S. Posse include University of Minnesota.

Clinical Proton MR Spectroscopy in Central Nervous System Disorders

Abstract: MR spectroscopy is used worldwide as an adjunct to MR imaging in several common neurologic diseases, including brain neoplasms, inherited metabolic disorders, demyelinating disorders, and infective focal lesions.

High speed 1H spectroscopic imaging in human brain by echo planar spatial-spectral encoding.

Abstract: We introduce a fast and robust spatial-spectral encoding method, which enables acquisition of high resolution short echo time (13 ms) proton spectroscopic images from human brain with acquisition times as short as 64 s when using surface coils. The encoding scheme, which was implemented on a clinical 1.5 Tesla whole body scanner, is a modification of an echo-planar spectroscopic imaging method originally proposed by Mansfield Magn. Reson. Med. 1, 370-386 (1984), and utilizes a series of read-out gradients to simultaneously encode spatial and spectral information. Superficial lipid signals are suppressed by a novel double outer volume suppression along the contours of the brain. The spectral resolution and the signal-to-noise per unit time and unit volume from resonances such as N-acetyl aspartate, choline, creatine, and inositol are comparable with those obtained with conventional methods. The short encoding time of this technique enhances the flexibility of in vivo spectroscopic imaging by reducing motion artifacts and allowing acquisition of multiple data sets with different parameter settings.

Three-dimensional echo-planar MR spectroscopic imaging at short echo times in the human brain.

Abstract: PURPOSE: To demonstrate the feasibility of three-dimensional echo-planar spectroscopic imaging (EPSI) at short echo time (13 msec) with a conventional clinical imager in the human brain. MATERIALS AND METHODS: Periodic inversions of a readout gradient were used during data acquisition to simultaneously encode chemical shift and one spatial dimension in one excitation. Aliasing artifacts were avoided with a modified acquisition-and-processing method based on oversampling. A double outer-volume suppression technique that adapts to the ovoid brain shape was used to strongly reduce extracranial lipid resonances. RESULTS: Three-dimensional spatial encoding in vivo of eight sections with 32 x 32 voxels each (0.75 cm3) was performed in 34 minutes with four signal averages. The spectral resolution and signal-to-noise ratio (S/N) of resonances of inositol, choline, creatine, glutamate and glutamine, and N-acetyl aspartate were consistent with those previously recorded with conventional phase encoding. CONCLUSION: ...

Human brain: proton diffusion MR spectroscopy.

Abstract: Diffusion of brain metabolites was measured in 10 healthy volunteers by using localized proton diffusion magnetic resonance (MR) spectroscopy. Measurements were conducted with a clinical MR imager by using a stimulated-echo pulse sequence (3,000/60 [repetition time msec/echo time msec], 200-msec mixing time) with additional outside-volume suppression. Motion artifacts due to macroscopic brain movements were compensated by means of peripheral cardiac gating and separate collection of individual spectroscopic acquisitions into a two-dimensional data matrix. Phase errors due to macroscopic motion were subsequently corrected in individual data traces prior to spectral averaging. Mean (+/- 1 standard deviation) apparent diffusion coefficients of choline-containing compounds ([0.13 +/- 0.03] x 10(-3) mm2/sec), creatine and phosphocreatine ([0.15 +/- 0.03] x 10(-3) mm2/sec), and N-acetyl aspartate ([0.18 +/- 0.02] x 10(-3) mm2/sec) were substantially smaller than that of water and were consistent with recently published data obtained in anesthetized and paralyzed animals. Adequate diffusion sensitivity for metabolites in the human brain can be obtained with clinical whole-body imagers despite macroscopic head and brain movements.

Short echo time proton MR spectroscopic imaging.

Abstract: Proton spectroscopic imaging at short TEs (20-30 ms) in human brain requires volume preselection inside the brain to suppress overwhelming lipid and water signals from surrounding tissue. In this article we discuss limitations of conventional volume preselection using stimulated echoes that lead to spectral contamination from surrounding tissue. Improved volume preselection was obtained by adding a complete outer volume suppression (presaturation). The performance of the method is illustrated on normal volunteers and on clinical cases with brain tumors and multiple sclerosis (MS) plaques. In normal human brain, we detected resonances with short T2 values and complex J-coupling, including rather broad methyl/methylene resonances in the chemical shift range between 0 and 2 ppm. Spectroscopic images obtained on patients with intracranial tumors and on one patient with several MS plaques demonstrate the possibility of detecting regional distributions of increased methyl/methylene resonances between 0 and 2 ppm in brain lesions.

Looking into the functional architecture of the brain with diffusion MRI

Abstract: Water diffusion magnetic resonance imaging (dMRI) allows tissue structure to be probed and imaged on a microscopic scale, providing unique clues to the fine architecture of neural tissues and to changes associated with various physiological and pathological states, such as acute brain ischaemia. Because diffusion is anisotropic in brain white matter, reflecting its organization in bundles of fibres running in parallel, dMRI can also be used to map the orientation in space of the white matter tracks in the brain, opening a new window on brain connectivity and brain maturation studies. This article provides an introduction to the key physical concepts that underlie dMRI, and reviews its potential applications in the neurosciences and associated clinical fields.

Simultaneous in vivo spectral editing and water suppression

Abstract: Water suppression is typically performed in vivo by exciting the longitudinal magnetization in combination with dephasing, or by using frequency-selective coherence generation. MEGA, a frequency-selective refocusing technique, can be placed into any pulse sequence element designed to generate a Hahn spin-echo or stimulated echo, to dephase transverse water coherences with minimal spectral distortions. Water suppression performance was verified in vivo using stimulated echo acquisition mode (STEAM) localization, which provided water suppression comparable with that achieved with four selective pulses in 3,1-DRYSTEAM. The advantage of the proposed method was exploited for editing J-coupled resonances. Using a double-banded pulse that selectively inverts a J-coupling partner and simultaneously suppresses water, efficient metabolite editing was achieved in the point resolved spectroscopy (PRESS) and STEAM sequences in which MEGA was incorporated. To illustrate the efficiency of the method, the detection of gamma-aminobutyric acid (GABA) was demonstrated, with minimal contributions from macromolecules and overlying singlet peaks at 4 T. The estimated occipital GABA concentration was consistent with previous reports, suggesting that editing for GABA is efficient when based on MEGA at high field strengths.

NMR relaxation times in the human brain at 3.0 tesla.

Abstract: Relaxation time measurements at 3.0 T are reported for both gray and white matter in normal human brain. Measurements were made using a 3.0 T Bruker Biospec magnetic resonance imaging (MRI) scanner in normal adults with no clinical evidence of neurological disease. Nineteen subjects, 8 female and 11 male, were studied for T1 and T2 measurements, and 7 males were studied for T2. Measurements were made using a saturation recovery method for T1, a multiple spin-echo experiment for T2, and a fast low-angle shot (FLASH) sequence with 14 different echo times for T2. Results of the measurements are summarized as follows. Average T1 values measured for gray matter and white matter were 1331 and 832 msec, respectively. Average T2 values measured for gray matter and white matter were 80 and 110 msec, respectively. The average T2 values for occipital and frontal gray matter were 41.6 and 51.8 msec, respectively. Average T2 values for occipital and frontal white matter were 48.4 and 44.7 msec, respectively. ANOVA tests of the measurements revealed that for both gray and white matter there were no significant differences in T1 from one location in the brain to another. T2 in occipital gray matter was significantly higher (0.0001 < P < .0375) than the rest of the gray matter, while T2 in frontal white matter was significantly lower (P < 0.0001). Statistical analysis of cerebral hemispheric differences in relaxation time measurements showed no significant differences in T1 values from the left hemisphere compared with the right, except in insular gray matter, where this difference was significant at P = 0.0320. No significant difference in T2 values existed between the left and right cerebral hemispheres. Significant differences were apparent between male and female relaxation time measurements in brain.

Anatomical accuracy of brain connections derived from diffusion MRI tractography is inherently limited

Abstract: Tractography based on diffusion-weighted MRI (DWI) is widely used for mapping the structural connections of the human brain. Its accuracy is known to be limited by technical factors affecting in vivo data acquisition, such as noise, artifacts, and data undersampling resulting from scan time constraints. It generally is assumed that improvements in data quality and implementation of sophisticated tractography methods will lead to increasingly accurate maps of human anatomical connections. However, assessing the anatomical accuracy of DWI tractography is difficult because of the lack of independent knowledge of the true anatomical connections in humans. Here we investigate the future prospects of DWI-based connectional imaging by applying advanced tractography methods to an ex vivo DWI dataset of the macaque brain. The results of different tractography methods were compared with maps of known axonal projections from previous tracer studies in the macaque. Despite the exceptional quality of the DWI data, none of the methods demonstrated high anatomical accuracy. The methods that showed the highest sensitivity showed the lowest specificity, and vice versa. Additionally, anatomical accuracy was highly dependent upon parameters of the tractography algorithm, with different optimal values for mapping different pathways. These results suggest that there is an inherent limitation in determining long-range anatomical projections based on voxel-averaged estimates of local fiber orientation obtained from DWI data that is unlikely to be overcome by improvements in data acquisition and analysis alone.

Magnetic Resonance Imaging of Acute Stroke

Abstract: In the investigation of ischemic stroke, conventional structural magnetic resonance (MR) techniques (eg, T1-weighted imaging, T2-weighted imaging, and proton density-weighted imaging) are valuable for the assessment of infarct extent and location beyond the first 12 to 24 hours after onset, and can be combined with MR angiography to noninvasively assess the intracranial and extracranial vasculature However, during the critical first 6 to 12 hours, the probable period of greatest therapeutic opportunity, these methods do not adequately assess the extent and severity of ischemia Recent developments in functional MR imaging are showing great promise for the detection of developing focal cerebral ischemic lesions within the first hours These include (1) diffusion-weighted imaging, which provides physiologic information about the self-diffusion of water, thereby detecting one of the first elements in the pathophysiologic cascade leading to ischemic injury; and (2) perfusion imaging The detection of acute intraparenchymal hemorrhagic stroke by susceptibility weighted MR has also been reported In combination with MR angiography, these methods may allow the detection of the site, extent, mechanism, and tissue viability of acute stroke lesions in one imaging study Imaging of cerebral metabolites with MR spectroscopy along with diffusion-weighted imaging and perfusion imaging may also provide new insights into ischemic stroke pathophysiology In light of these advances in structural and functional MR, their potential uses in the study of the cerebral ischemic pathophysiology and in clinical practice are described, along with their advantages and limitations