Andreas Ebel

Advances in neuroimaging of traumatic brain injury and posttraumatic stress disorder

Improved diagnosis and treatment of traumatic brain injury (TBI) and posttraumatic stress disorder (PTSD) are needed for our military and veterans, their families, and society at large. Advances in brain imaging offer important biomarkers of structural, functional, and metabolic information concerning the brain. This article reviews the application of various imaging techniques to the clinical problems of TBI and PTSD. For TBI, we focus on findings and advances in neuroimaging that hold promise for better detection, characterization, and monitoring of objective brain changes in symptomatic patients with combat-related, closed-head brain injuries not readily apparent by standard computed tomography or conventional magnetic resonance imaging techniques.

Assessment of 3D proton MR echo‐planar spectroscopic imaging using automated spectral analysis

For many clinical applications of proton MR spectroscopic imaging (MRSI) of the brain, diagnostic assessment is limited by insufficient coverage provided by single‐ or multislice acquisition methods as well as by the use of volume preselection methods. Additionally, traditional spectral analysis methods may limit the operator to detailed analysis of only a few selected brain regions. It is therefore highly desirable to use a fully 3D approach, combined with spectral analysis procedures that enable automated assessment of 3D metabolite distributions over the whole brain. In this study, a 3D echo‐planar MRSI technique has been implemented without volume preselection to provide sufficient spatial resolution with maximum coverage of the brain. Using MRSI acquisitions in normal subjects at 1.5T and a fully automated spectral analysis procedure, an assessment of the resultant spectral quality and the extent of viable data in human brain was carried out. The analysis found that 69% of brain voxels were obtained with acceptable spectral quality at TE = 135 ms, and 52% at TE = 25 ms. Most of the rejected voxels were located near the sinuses or temporal bones and demonstrated poor B0 homogeneity and additional regions were affected by stronger lipid contamination at TE = 25 ms. Magn Reson Med 46:1072–1078, 2001.

Improved spectral quality for 3D MR spectroscopic imaging using a high spatial resolution acquisition strategy

Spectral quality in (1)H MR spectroscopic imaging (MRSI) of the brain is often significantly degraded in regions subject to local magnetic susceptibility variations, which results in broadened and distorted spectral lineshapes. In this report, a modified acquisition strategy for volumetric echo-planar spectroscopic imaging (3D EPSI) is presented that extends the region of the brain that can be observed. The data are sampled at higher spatial resolution, then corrected for local B(0) shifts and reconstructed such that the final spatial resolution matches that of 3D EPSI data acquired with the conventional lower spatial resolution. Comparison of in vivo data obtained at 1.5 T with these two acquisition schemes shows that the high spatial resolution acquisition provides considerable reduction of spectral linewidths in many problematic brain regions, though with a reduction in signal-to-noise ratio by a factor of approximately 1.4 to 1.6 for the matrix sizes used in this study. However, the effect of the increased noise was largely offset by the improved spectral quality, leading to an overall improvement of the metabolite image quality obtained using automated spectral analysis.

Comparison of inversion recovery preparation schemes for lipid suppression in 1H MRSI of human brain

To reduce contamination from subcutaneous lipid regions in MR spectroscopic imaging (MRSI) of whole brain, lipid signals are often suppressed using T1 nulling methods. If a range of lipid T1 values is present, the suppression efficiency will be improved using multiple inversion recovery (MIR) preparation. This study compared single IR (SIR) and double IR (DIR) applied with a volumetric MRSI sequence at 1.5 T based on experimental measurement of lipid T1 and T2 relaxation rates. At short and medium echo times (TEs), an approximately 28–47% improvement in lipid suppression was achieved with DIR compared to SIR. However, it also led to a loss of 37–43% in signal‐to‐noise ratio (SNR) for metabolites. Thus, SIR appears to be the better choice for suppressing lipid signals and maintaining metabolite sensitivity. Magn Reson Med 49:903–908, 2003.

Improved Model-Based Magnetic Resonance Spectroscopic Imaging

Model-based techniques have the potential to reduce the artifacts and improve resolution in magnetic resonance spectroscopic imaging, without sacrificing the signal-to-noise ratio. However, the current approaches have a few drawbacks that limit their performance in practical applications. Specifically, the classical schemes use less flexible image models that lead to model misfit, thus resulting in artifacts. Moreover, the performance of the current approaches is negatively affected by the magnetic field inhomogeneity and spatial mismatch between the anatomical references and spectroscopic imaging data. In this paper, we propose efficient solutions to overcome these problems. We introduce a more flexible image model that represents the signal as a linear combination of compartmental and local basis functions. The former set represents the signal variations within the compartments, while the latter captures the local perturbations resulting from lesions or segmentation errors. Since the combined set is redundant, we obtain the reconstructions using sparsity penalized optimization. To compensate for the artifacts resulting from field inhomogeneity, we estimate the field map using alternate scans and use it in the reconstruction. We model the spatial mismatch as an affine transformation, whose parameters are estimated from the spectroscopy data.

Widespread extrahippocampal NAA/(Cr+Cho) abnormalities in TLE with and without mesial temporal sclerosis

MR spectroscopy has demonstrated extrahippocampal NAA/(Cr+Cho) reductions in medial temporal lobe epilepsy with (TLE-MTS) and without (TLE-no) mesial temporal sclerosis. Because of the limited brain coverage of those previous studies, it was, however, not possible to assess differences in the distribution and extent of these abnormalities between TLE-MTS and TLE-no. This study used a 3D whole brain echoplanar spectroscopic imaging (EPSI) sequence to address the following questions: (1) Do TLE-MTS and TLE-no differ regarding severity and distribution of extrahippocampal NAA/(Cr+Cho) reductions? (2) Do extrahippocampal NAA/(Cr+Cho) reductions provide additional information for focus lateralization? Forty-three subjects (12 TLE-MTS, 13 TLE-no, 18 controls) were studied with 3D EPSI. Statistical parametric mapping (SPM2) was used to identify regions of significantly decreased NAA/(Cr+Cho) in TLE groups and in individual patients. TLE-MTS and TLE-no had widespread extrahippocampal NAA/(Cr+Cho) reductions. NAA/(Cr+Cho) reductions had a bilateral fronto-temporal distribution in TLE-MTS and a more diffuse, less well defined distribution in TLE-no. Extrahippocampal NAA/(Cr+Cho) decreases in the single subject analysis showed a large inter-individual variability and did not provide additional focus lateralizing information. Extrahippocampal NAA/(Cr+Cho) reductions in TLE-MTS and TLE-no are neither focal nor homogeneous. This reduces their value for focus lateralization and suggests a heterogeneous etiology of extrahippocampal spectroscopic metabolic abnormalities in TLE.

Spectral phase-corrected GRAPPA reconstruction of three-dimensional echo-planar spectroscopic imaging (3D-EPSI).

MR spectroscopic (MRS) images from a large volume of brain can be obtained using a 3D echo‐planar spectroscopic imaging (3D‐EPSI) sequence. However, routine applications of 3D‐EPSI are still limited by a long scan time. In this communication, a new approach termed “spectral phase‐corrected generalized autocalibrating partially parallel acquisitions” (SPC‐GRAPPA) is introduced for the reconstruction of 3D‐EPSI data to accelerate data acquisition while preserving the accuracy of quantitation of brain metabolites. In SPC‐GRAPPA, voxel‐by‐voxel spectral phase alignment between metabolite 3D‐EPSI from individual coil elements is performed in the frequency domain, utilizing the whole spectrum from interleaved water reference 3D‐EPSI for robust estimation of the zero‐order phase correction. The performance of SPC‐GRAPPA was compared with that of fully encoded 3D‐EPSI and conventional GRAPPA. Analysis of whole‐brain 3D‐EPSI data reconstructed by SPC‐GRAPPA demonstrates that SPC‐GRAPPA with an acceleration factor of 1.5 yields results very similar to those obtained by fully encoded 3D‐EPSI, and is more accurate than conventional GRAPPA. Magn Reson Med 57:815–820, 2007.

Sensitive and fast T1 mapping based on two inversion recovery images and a reference image

We developed a fast method to obtain T1 relaxation maps in magnetic resonance imaging (MRI) based on two inversion recovery acquisitions and a reference acquisition, while maintaining high sensitivity by utilizing the full dynamic range of the MRI signal. Optimal inversion times for estimating T1 in the human brain were predicted using standard error propagation theory. In vivo measurements on nine healthy volunteers yielded T1 values of 1094±18ms in gray matter and 746±40ms in white matter, in reasonable agreement with literature values using conventional approaches. The proposed method should be useful for clinical studies because the T1 maps can be obtained within a few seconds.

Comparison of methods for reduction of lipid contamination for in vivo proton MR spectroscopic imaging of the brain

In vivo proton MR spectroscopic imaging (MRSI) of human brain is complicated by the presence of a strong signal from subcutaneous lipids, which may result in signal contamination in metabolite images obtained following Fourier‐transform reconstruction. In this study, two approaches for reduction of lipid contamination—using postprocessing and additional data acquisition—are compared. The first uses extrapolation of k‐space information for subcutaneous lipid, which has been applied to data obtained using conventional fully phase‐encoded MRSI with circularly sampled k‐space or echo‐planar spectroscopic imaging (EPSI). The second uses a dual EPSI technique that combines multiple‐averaged central k‐space data with a single EPSI acquisition of additional information that is used for improved lipid reconstruction. Comparisons are carried out with data obtained from human brain in vivo at 1.5 T with short and medium TEs. Results demonstrate that the performance of both methods for reducing the effects of lipid contamination is similar, and that both are limited by the effects of instrumental instabilities and subject motion, which also depend on the acquisition method used. Magn Reson Med 46:706–712, 2001.

Achieving Sufficient Spectral Bandwidth for Volumetric 1H Echo-Planar Spectroscopic Imaging at 4 Tesla

Complete coverage of the in vivo proton metabolite spectrum, including downfield resonances, requires a spectral bandwidth of approximately 9 ppm. Spectral bandwidth of in vivo echo‐planar spectroscopic imaging (EPSI) is primarily limited by gradient strength of the oscillating readout gradient, gradient slew rate, and limits on peripheral nerve stimulation for human subjects. Furthermore, conventional EPSI reconstruction, which utilizes even and odd readout echoes separately, makes use of only half the spectral bandwidth. In order to regain full spectral bandwidth in EPSI, it has previously been suggested to apply an interlaced Fourier transform (iFT), which uses even and odd echoes simultaneously. However, this method has not been thoroughly analyzed regarding its usefulness for in vivo 3D EPSI. In this Note, limitations of the iFT method are discussed and an alternative, cyclic spectral unwrapping, is proposed, which is based on prior knowledge of typical in vivo spectral patterns. Magn Reson Med, 2005.