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Dirk Mayer

Publications and source records attributed to Dirk Mayer.

10 recordsLinked to original sources
PubMed2006

In vivo quantification of ethanol kinetics in rat brain.

Proton magnetic resonance spectroscopy was used at 3T to measure the uptake and clearance of brain ethanol in rats after bolus intraperitoneal (i.p.) or intragastric (i.g.) alcohol injection, and to estimate the effects of acute alcohol on brain metabolites. The observation duration was 1-1.5 h with temporal resolution of alcohol sampling ranging from 4 s-4 min. The observed time course of alcohol brain concentration followed a consistent pattern characterized by a rapid absorption, an intermediate distribution, and a slower clearance that approached a linear decay. In a sample of eight healthy Wistar rats, the intercept of the linear clearance term, extrapolated back to the time of injection, correlated well with the administered dose per unit of lean body mass. Alcohol concentration estimation based on spectroscopically measured clearance was compared with blood alcohol levels from blood samples at the end of observation, and were in good agreement with the administered dose. Serial proton spectroscopy measurements provide a valid in vivo method for quantifying brain alcohol uptake and elimination kinetics in real time.

Alcohol-Induced Disorders, Nervous System↗
PubMed2006

Fast CT-PRESS-based spiral chemical shift imaging at 3 Tesla.

A new sequence is presented that combines constant-time point-resolved spectroscopy (CT-PRESS) with fast spiral chemical shift imaging. It allows the acquisition of multivoxel spectra without line splitting with a minimum total measurement time of less than 5 min for a field of view of 24 cm and a nominal 1.5x1.5-cm2 in-plane resolution. Measurements were performed with 17 CS encoding steps in t1 (Deltat1=12.8 ms) and an average echo time of 151 ms, which was determined by simulating the CT-PRESS experiment for the spin systems of glutamate (Glu) and myo-inositol (mI). Signals from N-acetyl-aspartate, total creatine, choline-containing compounds (Cho), Glu, and mI were detected in a healthy volunteer with no or only minor baseline distortions within 14 min on a 3 T MR scanner.

Algorithms↗
PubMed2006

Fast metabolic imaging of systems with sparse spectra: application for hyperpolarized 13C imaging.

A fast spiral chemical shift imaging (spCSI) sequence was developed for application to hyperpolarized (13)C imaging. The sequence exploits sparse spectra, which can occur in such applications, and prior knowledge of resonance frequencies to reduce the measurement time by undersampling the data in the spectral domain. As a consequence, multiple reconstructions of a given data set have to be computed in which only components with frequencies within a certain bandwidth are reconstructed "in focus" while others are severely blurred ("spectral tomosynthesis"). The sequence was tested at 3 T on a phantom containing approximately 1.5-M solutions of alanine (Ala), lactate (Lac), and pyruvate-pyruvate hydrate C1-C2 ester (with two resonances, PPE1 and PPE2) at thermal equilibrium polarization, all enriched to 99% (13)C in the C1 carbonyl positions. Results from spCSI with a single spatial interleaf (single-shot spCSI) and three interleaves (three-shot spCSI) were compared with those obtained by phase-encoded free induction decay CSI (FIDCSI). The metabolic maps of all four resonances for three-shot spCSI, and of PPE1 and PPE2 for single-shot spCSI demonstrate resolution and localization properties similar to those of the FIDCSI images. The metabolic maps of Ala and Lac for single-shot spCSI contain minor artifacts due to signal overlap of aliased resonances.

Alanine↗
PubMed2006

Longitudinal brain magnetic resonance imaging study of the alcohol-preferring rat. Part I: adult brain growth.

BACKGROUND: The alcohol-preferring (P) rat, a Wistar strain selectively bred to consume large amounts of alcohol voluntarily, has been used as an animal model of human alcoholism for 3 decades. Heretofore, knowledge about brain morphology has been confined to postmortem examination. Quantitative neuroimaging procedures make it feasible to examine the potential longitudinal effects of alcohol exposure in vivo, while controlling modifying factors, such as age, nutrition, and exercise. To date, few imaging studies have considered what morphological changes occur with age in the rodent brain, and none has systematically applied quantitative neuroimaging approaches to measure volume changes in regional brain structures over extended periods in the adult rat. METHODS: We used structural magnetic resonance imaging (MRI) in a longitudinal design to examine 2 cohorts of adult P rats, never exposed to alcohol: Cohort A included 8 rats, 7 of which survived the entire study (578 days) and 4 MRI sessions; Cohort B included 9 rats, all of which survived the study (452 days) and 5 MRI sessions. RESULTS: Growth in whole-brain volume reached maximal levels by about 450 days of age, whereas body weight continued its gain without asymptote. Growth was not uniform across the brain structures measured. Over the initial 12 months of the study, the corpus callosum area expanded 36%, cerebellum 17%, and hippocampus 10%, whereas ventricle size was unchanged. Factors affecting growth rate estimates included litter effects, MR image signal-to-noise ratio, and measurement error. CONCLUSION: Unlike longitudinal human reports of regional volume declines in aging brain tissue, several brain structures in adult rats continued growing, and some growth patterns were litter-dependent. Determining normal regional growth patterns of brain and of the substantial variance exerted by litter differences, even in selectively bred rats, is essential for establishing baselines against which normal and aberrant dynamic changes can be detected in animal models of aging and disease.

Aging↗
PubMed2006

Longitudinal brain magnetic resonance imaging study of the alcohol-preferring rat. Part II: effects of voluntary chronic alcohol consumption.

BACKGROUND: Tracking the dynamic course of human alcoholism brain pathology can be accomplished only through naturalistic study and without opportunity for experimental manipulation. Development of an animal model of alcohol-induced brain damage, in which animals consume large amounts of alcohol following cycles of alcohol access and deprivation and are examined regularly with neuroimaging methods, would enable hypothesis testing focused on the degree, nature, and factors resulting in alcohol-induced brain damage and the prospects for recovery or relapse. METHODS: We report the results of longitudinal magnetic resonance imaging (MRI) studies of the effects of free-choice chronic alcohol intake on the brains of 2 cohorts of selectively bred alcohol-preferring (P) rats. In the companion paper, we described the MRI acquisition and analysis methods, delineation of brain regions, and growth patterns in total brain and selective structures of the control rats in the present study. Both cohorts were studied as adults for about 1 year and consumed high doses of alcohol for most of the study duration. The paradigm involved a 3-bottle choice with 0, 15 (or 20%), and 30% (or 40%) alcohol available in several different exposure schemes: continuous exposure, cycles of 2 weeks on followed by 2 weeks off alcohol, and binge drinking in the dark. RESULTS: Brain structures of the adult P rats in both the alcohol-exposed and the water control conditions showed significant growth, which was attenuated in a few measures in the alcohol-exposed groups. The region with the greatest demonstrable effect was the corpus callosum, measured on midsagittal images. CONCLUSION: The P rats showed an age-alcohol interaction different from humans, in that normal growth in selective brain regions that continues in adult rats was retarded.

Animals↗
PubMed2005

Scanning probe microscopic studies of the oriented attachment and membrane reconstitution of cytochrome C oxidase to a gold electrode.

Scanning probe microscopy was used to monitor the resulting surface of the oriented incorporation of cytochrome c oxidase into electrode supported lipid bilayer at four crucial stages with molecular resolution. We were able to reveal the formation of a densely packed monolayer of the active ester dithio(succiniimidylepropionate) (DTSP) and the covalent linkage of the nitrilotriacetic acid (NTA) to the thiol anchored DTSP by scanning tunneling microscopy. Atomic force microscopy investigations showed that the detergent solubilized oxidase is immobilized as monomers and small aggregates via histidine residues. Finally, the reconstitution of the proteins within the supported membrane was verified. The amount of oxidase immobilized within the solid supported membrane was estimated.

Electrodes↗
PubMed2005

Detection of glutamate in the human brain at 3 T using optimized constant time point resolved spectroscopy.

A CT-PRESS sequence was implemented on a 3-T MR scanner and optimized for the detection of the C4 resonance of glutamate. By simulating the sequence using the full density matrix it was found that 121 chemical shift encoding steps in t1 with an increment delta t1 = 1.6 ms were sufficient to separate the glutamate C4 resonance. The simulations also showed that the highest signal-to-noise ratio was achieved at an average echo time of 131 ms. When using an eightfold undersampling scheme in f1 in order to reduce the minimum total measurement time, the average echo time was 139 ms with 17 encoding steps (delta t1 = 12.8 ms). The sequence was tested on phantoms containing solutions of various brain metabolites and on healthy human volunteers. Besides resolving glutamate, other resonances detected in vivo comprised N-acetyl aspartate, total creatine, choline containing compounds, and myo-inositol. However, glutamine resonances could not be resolved due to severe signal overlap from glutamate and N-acetyl aspartate.

Brain↗
PubMed2004

Hybrid segmentation and virtual bronchoscopy based on CT images.

RATIONALE AND OBJECTIVES: Introduction of combination of the segmentation tool SegoMeTex and the virtual endoscopy system VIVENDI to perform virtual endoscopic inspections of the human lung. This virtual bronchoscopy system enables visualization of the tracheobronchial tree down to seventh generation. Furthermore, the modified virtual system visualizes hidden structures such as segmented vascular system or tumors. MATERIALS AND METHODS: The segmentation is based on image data acquired by a multislice computed tomography scanner. SegoMeTex is used to segment the tracheobronchial tree by a hybrid system with minimal user action. Similarly, the complementary pulmonary arterial can be segmented, whereas additional structures such as tumors are marked manually. On this dataset, subsequently, data structures of the inner surface for virtual endoscopy are generated. Finally, the dataset can be explored by a virtual bronchoscopy procedure using the VIVENDI system. RESULTS: The segmentation method was successfully tested on 22 patients. The hybrid segmentation system identified bronchi up to the sixth generation with a sensitivity of more than 58%, and a positive predictive value of more than 90%. After the segmentation, the datasets are explored interactively (>30 fps on a standard personal computer platform in real-time rendering) using the virtual endoscopy software. The exploration exposed a high-quality reconstruction, even of small structures throughout the dataset. CONCLUSION: Virtual bronchoscopy in combining with a highly sensitive segmentation is a valuable tool for the localization and measurement of stenosis for resection planning.

Bronchial Diseases↗
PubMed2003

Fast U-FLARE-based correlation-peak imaging with complete effective homonuclear decoupling.

A new fast correlation-peak imaging technique is presented that combines 2D correlation spectroscopy with an ultrafast low-angle rapid acquisition with relaxation enhancement (U-FLARE) imaging module. Constant time chemical shift (CS) encoding is used in both time dimensions to achieve effective homonuclear decoupling in both frequency dimensions. The intervals between excitation and mixing (t(c1)) and between mixing and start of the MRI module (t(c2)) can be optimized to maximize the coherence transfer for a particular metabolite. Experiments were performed with evolution times t(c1) and t(c2) of 100 ms and 50 ms, respectively, which were determined by simulation of the spectroscopic part of the sequence for the spin systems of myo-inositol (Ins) and taurine (Tau). The use of a circularly reduced CS-encoding scheme shortened the minimum total measurement time to 35 min. The sequence was implemented on a 4.7 T imaging system and tested on a spherical phantom filled with a solution of Ins. The in vivo application of this method on healthy rat brain demonstrates its improved spectral resolution, as cross-peak signals from both Ins and Tau can be separated clearly.

Animals↗