Biomedical subjects

Elfar Adalsteinsson

Publications and source records attributed to Elfar Adalsteinsson.

8 recordsLinked to original sources

PubMed2003

Quality assurance of magnetic resonance spectroscopic imaging-derived metabolic data.

PURPOSE: Spatially resolved metabolite maps, as measured by magnetic resonance spectroscopic imaging (MRSI) methods, are being increasingly used to acquire metabolic information to guide therapy, with metabolite ratio maps perhaps providing the most diagnostic information. We present a quality assurance procedure for MRSI-derived metabolic data acquired ultimately for guiding conformal radiotherapy. METHODS AND MATERIALS: An MRSI phantom filled with brain-mimicking solutions was custom-built with an insert holding eight vials containing calibration solutions of precisely varying metabolite concentrations that emulated increasing grade/density of brain tumor. Phantom metabolite ratios calculated from fully relaxed 1D, 2D, and 3D MRS data for each vial were compared with calibrated metabolite ratios acquired at 9.4 T. Additionally, 3D ratio maps were "discretized" to eight pseudoabnormality levels on a slice-by-slice basis and the accuracy of this procedure was verified. RESULTS: Regression analysis revealed expected linear relationships between experimental and calibration metabolite ratios with intercepts close to zero for the three acquisition modes. 1D MRS data agreed most with theoretical considerations (regression coefficient, b = 0.969; intercept 0.008). The 2D (b = 1.049; intercept -0.199) and 3D (correlation coefficient r(2) = 0.9978-0.7336 for five slices) MRSI indicated reduced MRS data quality in regions of degraded B(0) and B(1) homogeneity. Pseudoabnormality levels were found to be consistent with expectations within regions of adequate B(0) homogeneity. CONCLUSIONS: This simple phantom-based approach to generate baseline calibration curves for all MRS acquisition modes may be useful to identify temporal deviations from acceptable data quality in a routine clinical environment or for testing new MRS and MRSI acquisition software.

Aspartic Acid↗

PubMed2003

Replicability of diffusion tensor imaging measurements of fractional anisotropy and trace in brain.

PURPOSE: To evaluate within-scanner and between-scanner reliability of fractional anisotropy (FA) and trace (sum of the diagonal elements of the diffusion tensor) as measured by diffusion tensor imaging (DTI). MATERIALS AND METHODS: Ten young healthy adults were scanned on three separate days, on two different systems made by the same manufacturer. One scan was acquired at one site, and two scans were acquired on two different occasions on another scanner at another site. Three levels of analysis were used to compare the DTI metrics: 1) a voxel-by-voxel analysis of all supratentorial brain (gray matter + white matter + cerebrospinal fluid) and of supratentorial white matter; 2) a slice-by-slice analysis of supratentorial white matter; and 3) a single-region analysis of the corpus callosum. RESULTS: The voxel-by-voxel analysis of all supratentorial brain found that FA and trace measures and correlations were equivalently and significantly higher within than across scanners. For supratentorial white matter, FA was similar within and across scanners, whereas trace demonstrated across-scanner bias. A similar pattern was observed for the slice-by-slice comparison. For the single-region analysis of the corpus callosum, within-scanner FA and trace measures were highly reproducible for FA (CV = 1.9%) and trace (CV = 2.6%), but both DTI measures showed a systematic mean bias across scanners (CV = 4.5% for FA and CV = 7.5% for trace). CONCLUSION: These estimates of measurement variation and scanner bias can be used to predict effect sizes for longitudinal and multisite studies using diffusion tensor imaging.

Adult↗

PubMed2003

Simple analytic variable density spiral design.

Variable density spirals have been proposed in a variety of applications, including MR fluoroscopy, cardiac imaging, and MR spectroscopic imaging. In this work, a simple analytic solution for designing a flexible set of variable density spiral trajectory waveforms is presented. The method enables real-time waveform prescription on scanners with limited computational power. Both slew rate-limited and amplitude-limited regimes are incorporated in the design process.

Algorithms↗

PubMed2003

How important are brain banks for alcohol research?

This article contains the proceedings of a symposium at the 2002 RSA/ISBRA Meeting in San Francisco, organized and chaired by Clive Harper and co-chaired by Izuru Matsumoto. The presentations were (1) Introduction, by Clive Harper; (2) The quality of tissue-a critical issue, by Therese Garrick; (3) The first systematic brain tissue donor program in Japan, by Izuru Matsumoto; (4) Brain scans after death-really! by Adolf Pfefferbaum, Elfar Adalsteinsson, and Edith Sullivan; (5) Capture that (genial) expression, by Joanne Lewohl and Peter Dodd; and (6) Neurochemical/pharmacological studies: experimental design and limitations, by Roger Butterworth.

Alcohol-Induced Disorders, Nervous System↗

PubMed2002

Low N-acetyl-aspartate and high choline in the anterior cingulum of recently abstinent methamphetamine-dependent subjects: a preliminary proton MRS study. Magnetic resonance spectroscopy.

Studies based on animal models report that methamphetamine (MA) abuse diminishes dopamine (DA) and serotonin innervation in frontal brain regions. In this in vivo human study, we used proton magnetic resonance spectroscopy (MRS), which yields measures of N-acetyl-aspartate (NAA), a marker of living neurons, to examine frontal brain regions possibly affected by methamphetamine dependence (MD). We tested the hypothesis that MD subjects would exhibit abnormally low levels of NAA, referenced to creatine (Cr), in anterior cingulate gray matter. We further hypothesized that the primary visual cortex, which receives relatively less DA innervation than the frontal brain regions, would show normal NAA/Cr ratios in MD subjects. Subjects included nine MD men (mean+/-standard deviation (S.D.)=32.5+/-6.4 years) and nine age-matched control men (mean+/-S.D.=32.7+/-6.8 years). The MD subjects were MA-free for 4-13 weeks. Proton MRS metabolites were expressed as ratios of creatine; the absolute values of which did not distinguish controls and MD subjects. With regard to metabolite ratios, the MD men had significantly lower NAA/Cr in the cingulum (mean+/-standard error (S.E.): control=1.46+/-0.03; MD=1.30+/-0.03; Mann-Whitney P=0.01) but not in the visual cortex (mean+/-S.E.: control=1.64+/-0.06; MD=1.69+/-11; Mann-Whitney P=0.52) relative to controls. These results provide evidence for NAA/Cr deficit that is selective to the anterior cingulum, at least with respect to visual cortex, in MD subjects. The neuronal compromise that these changes reflect may contribute to the attentional deficits and dampened reward system in MD.

Adult↗

PubMed2002

Inverse planning for functional image-guided intensity-modulated radiation therapy.

Radiation therapy is an image-guided process whose success critically depends on the imaging modality used for treatment planning and the level of integration of the available imaging information. In this work, we establish a dose optimization framework for incorporating metabolic information from functional imaging modalities into the intensity-modulated radiation therapy (IMRT) inverse planning process and to demonstrate the technical feasibility of planning deliberately non-uniform dose distributions in accordance with functional imaging data. For this purpose, a metabolic map from functional images is discretized into a number of abnormality levels (ALs) and then fused with CT images. To escalate dose to the metabolically abnormal regions, we assume, for a given spatial point, a linear relation between the AL and the prescribed dose. But the formalism developed here is independent of the assumption and any other relation between AL and prescription is applicable. For a given AL and prescription relation, it is only necessary to prescribe the dose to the lowest AL in the target and the desired doses to other regions with higher AL values are scaled accordingly. To accomplish differential sparing of a sensitive structure when its functional importance (FI) distribution is known, we individualize the tolerance doses of the voxels within the structure according to their Fl levels. An iterative inverse planning algorithm in voxel domain is used to optimize the system with in homogeneous dose prescription. To model intra-structural trade-off, a mechanism is introduced through the use of voxel-dependent weighting factors, in addition to the conventional structure specific weighting factors which model the inter-structural trade-off. The system is used to plan a phantom case with a few hypothetical functional distributions and a brain tumour treatment with incorporation of magnetic resonance spectroscopic imaging data. The results indicated that it is technically feasible to produce deliberately non-uniform dose distributions according to the functional imaging requirements. Integration of functional imaging information into radiation therapy dose optimization allows for consideration of patient-specific biologic information and provides a significant opportunity to truly individualize radiation treatment. This should enhance our capability to safely and intelligently escalate dose and lays the technical foundation for future clinical studies of the efficacy of functional imaging-guided IMRT.

Algorithms↗

PubMed2002

Regularized higher-order in vivo shimming.

A regularized algorithm is presented for localized in vivo shimming. The technique uses first- (X,Y,Z), second- (Z(2), ZX, ZY, X(2)-Y(2), XY), and third-order (Z(3)) shim coils, and is robust when applied to arbitrarily-shaped, as well as off-center, regions of interest (ROIs). A single-shot spiral pulse sequence is used for rapid field map acquisition, and a least-squares calculation of the shim currents is performed to minimize the root-mean-square (RMS) value of the B(0) inhomogeneity over a user-selected ROI. The use of a singular value decomposition (SVD) in combination with a regularization algorithm significantly improves the numerical stability of the least-squares fitting procedure. The fully automated shimming package is implemented on a 3 T GE Signa system and its robust performance is demonstrated in phantom and in vivo studies.

Algorithms↗

PubMedDate not supplied

Gray matter N-acetyl aspartate deficits in secondary progressive but not relapsing-remitting multiple sclerosis.

BACKGROUND AND PURPOSE: Spectroscopic examination of multiple sclerosis (MS) patients has revealed abnormally low N-acetyl-aspartate (NAA) signal intensity, even in brain tissue that appears normal on high-resolution structural MR images but has yielded inconclusive evidence to distinguish the well-documented clinical differences between MS subtypes. This study used proton MR spectroscopic imaging (MRSI) and high-resolution MR imaging to characterize metabolite profiles in normal-appearing brain tissue of relapsing-remitting multiple sclerosis (RRMS) and secondary progressive (SP) MS. METHODS: Volumetric spiral MRSI was used together with high-resolution MR imaging to derive absolute measures of metabolite concentrations separately in normal-appearing supratentorial cerebral gray matter and white matter in five RRMS patients, five SPMS patients, and nine age-matched controls. Structural MR images were segmented into compartments of gray matter, white matter, CSF, and lesions, and metabolite signals per unit of tissue volume were calculated for gray matter and white matter separately. RESULTS: Only the SPMS group had significantly lower NAA concentrations in normal-appearing gray matter compared with concentrations in controls. NAA in normal-appearing white matter was equally reduced in RRMS and SPMS patients. The functional relevance of this brain metabolite measure was suggested by the observed but statistically nonsignificant correlation between higher disability scores on the Expanded Disability Status Scale and lower gray matter NAA concentrations. CONCLUSION: The otherwise occult abnormality in supratentorial gray matter in SPMS but not RRMS may explain the more severe physical and cognitive impairments afflicting patients with SPMS that do not correlate well with visible lesion burden.

Adult↗