Biomedical subjects

O Speck

Publications and source records attributed to O Speck.

At least 19 recordsLinked to original sources

PubMed2007

[Visual rating of T2'-blood-oxygen-level-dependent magnetic resonance imaging in acute stroke patients--a pilot study].

PURPOSE: Delineation of brain tissue that is at risk but not yet infarcted (penumbra) continues to be a major challenge for stroke imaging. Metabolic characterization of the penumbra might be able to be achieved using blood-oxygen-level-dependent (BOLD) imaging. MATERIALS AND METHODS: We analyzed MRI data from 20 patients within the first 6 hours after stroke onset and after 5-8 days. Among other sequences, the MRI protocol consisted of diffusion-weighted (DWI/ADC = apparent diffusion coefficient) and quantitative T2 and T2* imaging (qT2, qT2*). BOLD images (T2') were calculated using 1/T2' = 1/qT2* - 1/qT2. BOLD lesions were rated by two blinded observers. RESULTS: Based on the primary blinded reading of the BOLD images, the lesion side was rated correctly by observers 1 and 2 in 80/50 % of the cases, incorrectly in 5/40 % of the cases, and rated as not visible in 15/10 % of the cases. After unblinding the observers, the visibility was rated in 45/45 % of the cases as good, in 35/40 % of the cases as reasonable, and in 20/15 % of the cases as insufficient for diagnostic purposes. The sensitivity for subsequent infarct growth was 0.88 (95 % confidence interval, CI 0.47 to 0.99), the specificity was 0.33 (95 % CI 0.07 to 0.70), the positive predictive value (PPV) was 0.54 (95 % CI 0.25 to 0.81), and the negative predictive value (NPV) was 0.75 (95 % CI 0.19 to 0.99). The odds ratio for subsequent infarct growth was 3.5 (95 % CI 0.20 to 115.53). CONCLUSION: Hypo-intense lesions in BOLD imaging were visible and exceeded the lesion in diffusion-weighted imaging in most of the stroke patients. The encouraging results justify further testing of the hypothesis that BOLD lesions, when larger than DWI lesions, are associated with infarct growth from initial DWI to final infarct.

Acute Disease↗

PubMed2006

Prospective real-time slice-by-slice motion correction for fMRI in freely moving subjects.

Subject motion is still the major source of data quality degradation in functional magnetic resonance imaging (fMRI) studies. Established methods correct motion between successive repetitions based on the acquired imaging volumes either retrospectively or prospectively. A fast, highly accurate, and prospective real-time correction method for fMRI using external optical motion tracking has been implemented. The head position is determined by means of an optical stereoscopic tracking system. The method corrects motion during the acquisition of an fMRI time series on a slice-by-slice basis by continuously updating the imaging volume position to follow the motion of the head. This method allows the measurement of fMRI data in the presence of significant motion during the acquisition of a single volume. Even without intentional motion, fMRI signal stability is maintained and higher sensitivity to detect activation is achieved without reducing specificity. With significant motion, only the proposed approach allowed detection of brain activation. The results show that the new method is superior to image-based correction methods, which fail in the case of fast or excessive motion.

Artifacts↗

PubMed2006

Magnetic resonance imaging of freely moving objects: prospective real-time motion correction using an external optical motion tracking system.

Subject motion and associated artefacts limit the applicability of MRI and the achievable quality of the images acquired. In this paper, a fully integrated method for prospective correction of arbitrary rigid body motion employing an external motion tracking device is demonstrated for the first time. The position of the imaging volume is updated prior to every excitation of the spin system. The performance of the available tracking hardware and its connection to the MR imager is analyzed in detail. With the introduction of a novel calibration procedure the accuracy of motion correction is improved compared to previous approaches. Together with the high geometry update rate even freely moving objects can be imaged without motion related artefacts. The high performance and image quality improvement in case of subject motion are demonstrated for various imaging techniques such as gradient and spin echo, as well as echo planar imaging.

Artifacts↗

PubMed2005

Vessel size imaging in humans.

The relation of contrast-enhanced transverse relaxation rates R2* and R2 provides in vivo mapping of the mean caliber of cerebral vessels. This technique is referred to as vessel size imaging (VSI). Here a quantitative assessment of the vessel caliber in brain tumor patients is presented. The obtained mean vessel size shows sensitivity to the tumor type. A theoretical analysis is given to elucidate the morphological information content of VSI in the context of vessel architecture. The simplification of the theory underlying the data processing results in a systematic overestimation of the vessel caliber. An increase in the magnetic susceptibility of the contrast agent allows for quantitatively more accurate measurements. Quantitative VSI must include measurements of the regional diffusion coefficient and absolute determination of the regional cerebral blood volume.

Anisotropy↗

PubMed2004

Point spread function mapping with parallel imaging techniques and high acceleration factors: fast, robust, and flexible method for echo-planar imaging distortion correction.

Echo-planar imaging (EPI) is an ultrafast magnetic resonance (MR) imaging technique prone to geometric distortions. Various correction techniques have been developed to remedy these distortions. Here improvements of the point spread function (PSF) mapping approach are presented, which enable reliable and fully automated distortion correction of echo-planar images at high field strengths. The novel method is fully compatible with EPI acquisitions using parallel imaging. The applicability of parallel imaging to further accelerate PSF acquisition is shown. The possibility of collecting PSF data sets with total acceleration factors higher than the number of coil elements is demonstrated. Additionally, a new approach to visualize and interpret distortions in the context of various imaging and reconstruction methods based on the PSF is proposed. The reliable performance of the PSF mapping technique is demonstrated on phantom and volunteer scans at field strengths of up to 4 T.

Artifacts↗

PubMed2002

Fast 31P chemical shift imaging using SSFP methods.

Steady-state free precession (SSFP) methods have been very successful due to their high signal and short imaging times. These properties make them good candidates for applications that intrinsically suffer from low signal such as low gamma nuclei imaging. A new chemical shift imaging (CSI) technique based on the SSFP signal formation has been implemented and applied to (31)P. The signal properties of the SSFP CSI method have been evaluated and the steady-state signal of (31)P has been measured in human muscles. Due to the T(2) and T(1) signal dependence of SSFP, the steady-state signal mainly consists of phosphocreatine (PCr). The technique allows fast CSI acquisitions with high SNR of the PCr signal. The SNR gain for PCr over a FLASH-based CSI method is approx. 4-5. Fast in vivo CSI of human muscle with subcentimeter resolution and high SNR is demonstrated at 2 T.

Magnetic Resonance Imaging↗

PubMed2001

Neural correlates of attention and working memory deficits in HIV patients.

OBJECTIVES: To evaluate the neural correlates of attention and working memory deficits in patients with HIV-1. METHOD: fMRI was used to evaluate brain activity in 11 patients with HIV and 11 age-, sex-, education-, and handedness-matched seronegative subjects, while performing a battery of tasks that required different levels of attention for working memory. RESULTS: Patients with HIV showed greater brain activation (blood oxygenation level dependent signal changes) in some regions compared with control subjects while performing the same tasks. For the simpler tasks, patients with HIV showed greater activation in the parietal regions. However, with more difficult tasks, patients with HIV showed greater activation additionally in the frontal lobes. Reaction times during these tasks were slower but accuracy was similar in the patients with HIV compared with control subjects. CONCLUSION: Injury to the neural substrate caused by HIV infection may necessitate greater attentional modulation of the neural circuits, hence a greater use of the brain reserve; additional activation of the frontal lobes is required to perform the more complex tasks. The task-dependent increased frontal activation in patients with HIV suggests that the neural correlate of attentional deficits may be excessive attentional modulation as a result of frontostriatal brain injury.

AIDS Dementia Complex↗

PubMed2001

Separation and quantification of perfusion and BOLD effects by simultaneous acquisition of functional I(0)- and T2(*)-parameter maps.

The nature of the coupling between neuronal activity and the hemodynamic response is the subject of intensive research. As a means to simultaneously measure parametric changes of T2(*), initial intensity (I(0)) and perfusion with high temporal resolution, a multi-image EPI technique with slice-selective inversion recovery (ssIR) for arterial spin labeling was developed and implemented. Comparative measurements with and without the preceding slice-selective inversion pulse were performed. I(0) and R2(*) changes induced by primary visual stimulation were separated. For ssIR-multi-image EPI the average change of I(0) over all 12 subjects was 3.4%, corresponding to a perfusion change of 40 ml/min/100 g, whereas only minor I(0) changes were observed without inversion. On average, the R2(*) of the activated pixels changed by -0.62 sec(-1) without inversion, while a significantly reduced average R2(*) change of -0.46 sec(-1) was calculated for ssIR-multi-image EPI due to a decreased BOLD effect contribution of the intravascular compartment.

Brain↗

PubMed2001

Biexponential modeling of multigradient-echo MRI data of the brain.

Functional MRI (fMRI) using fast multigradient-echo acquisition methods allows the quantitative determination of the relevant parameter T2*. Previously, the TE-dependent signal decay has been modeled with a monoexponential function despite the complex composition of the brain. In this study, biexponential modeling was used to evaluate the relaxation of brain parenchyma and blood separate from that of cerebrospinal fluid. Single-shot multigradient-echo data acquired with spiral or EPI techniques were analyzed. In phantom experiments the biexponential method proved to be accurate. Compared to the biexponential procedure, the monoexponential model overestimated T2* (72.2 msec vs. 65.3 msec) and underestimated DeltaT2* (2.96 msec vs. 3.19 msec) during visual stimulation. The biexponential method may allow intrinsic correction for partial volume effects due to cerebrospinal fluid. The activation-induced parameter changes are detected with a sensitivity equal to that of a monoexponential method. The resulting T2* and DeltaT2* values describe the experimental data more accurately.

Brain↗

PubMed2001

Motion correction of parametric fMRI data from multi-slice single-shot multi-echo acquisitions.

Fast parametric imaging using multi-echo techniques has been proven to yield quantitative parameter maps with high stability for functional MRI (fMRI). Due to the different contrasts and signal-to-noise ratios (SNRs) in the various images, motion correction of the echo images or the resulting parameter maps is not a straightforward process. 3D motion correction of parametric imaging data has not yet been examined thoroughly. However, motion correction is an essential step in fMRI data processing. In this study several possible motion detection methods were tested and compared. Motion parameters can be estimated from the different echo images as well as from the parameter maps. The accuracy of the different methods was examined in simulations and in in vivo experiments. Motion parameters should be estimated from the I(0)-parameter maps and subsequently applied to the T(*)(2)-parameter maps.

Brain↗

PubMed2001

HIV-2 infection with cerebral toxoplasmosis and lymphomatoid granulomatosis.

A Nigerian man had acute onset of headache and vertigo due to a cerebellar mass. A brain biopsy of the mass revealed toxoplasmosis despite repeated negative HIV-1 serology. The presence of an opportunistic infection and his country of origin raised the suspicion for HIV-2; this was confirmed by positive HIV-2 serology. Despite his preliminary pathological diagnosis, results of physiological magnetic resonance imaging (MRI) (perfusion MRI and proton magnetic resonance spectroscopy) were not typical for toxoplasmosis. The lesion showed a biochemical and perfusion pattern that was intermediate for infectious and neoplastic processes. Further neuropathology confirmed a secondary diagnosis of lymphomatoid granulomatosis.

AIDS-Related Opportunistic Infections↗

PubMed2000

Cerebral perfusion abnormalities in abstinent cocaine abusers: a perfusion MRI and SPECT study.

Nuclear medicine studies found decreased regional cerebral blood flow (rCBF) in the cortex and deep gray matter of cocaine users. Perfusion magnetic resonance imaging (MRI), a non-radioactive technique, has not been applied to evaluate persistent rCBF abnormalities. Twenty-five abstinent cocaine users and 15 healthy subjects without a history of drug use were examined with perfusion MRI, using dynamic bolus-tracking, and single photon emission computed tomography (SPECT), using 133Xe-calibrated 99mTc-HMPAO. After coregistration of SPECT with MRI, the relative rCBF (from perfusion MRI and SPECT) and absolute rCBF (from SPECT) were determined in 10 brain regions in each hemisphere. There was a statistically significant interaction between drug use and brain region on SPECT alone (relative and absolute rCBF), and on SPECT and perfusion MRI combined, but not on perfusion MRI alone. There also was a significant interaction among gender, drug use, and brain region. Compared to the control subjects, cocaine users showed increased rCBF in the frontal white matter (+8.6%, P=0.02) and in the globus pallidus (+6.3%, P=0.05), and decreased rCBF in the putamen (-3.9%, P=0.04) and the temporal cortex (-2.4%, P=0.02). SPECT and perfusion MRI detect a regional pattern of rCBF abnormalities in cocaine users that is consistent across the two methods. The hypoperfusion in the cortex and deep gray matter of the cocaine users is consistent with previous results. The increased rCBF in the white matter of cocaine users may be due to the presence of reactive glia.

Adolescent↗

PubMed2000

Gender differences in the functional organization of the brain for working memory.

Gender differences in brain activation during working memory tasks were examined with fMRI. Seventeen right-handed subjects (nine males, eight females) were studied with four different verbal working memory tasks of varying difficulty using whole brain echo-planar fMRI. Consistent with prior studies, we observed activation of the lateral prefrontal cortices (LPFC), the parietal cortices (PC), and additionally, caudate activation in both sexes. The volume of activated brain tissue increased with increasing task difficulty. For all four tasks, the male subjects showed bilateral activation or right-sided dominance (LPFC, PC and caudate), whereas females showed activation predominantly in the left hemisphere. The task performance data demonstrated higher accuracy and slightly slower reaction times for the female subjects. Our results show a highly significant (p < 0.001) gender differences in the functional organization of the brain for working memory. These gender-specific differences in functional organization of the brain may be due to gender-differences in problem solving strategies or the neurodevelopment. Therefore, gender matching or stratification is required for studies of brain function using imaging techniques.

Adult↗

PubMed2000

Evidence for long-term neurotoxicity associated with methamphetamine abuse: A 1H MRS study.

OBJECTIVE: To determine whether proton MRS (1H MRS) can detect long-term metabolite abnormalities in abstinent methamphetamine users. BACKGROUND: Methamphetamine is toxic to dopaminergic and serotonergic neurons in rodents; however, little data are available on the toxic effects of methamphetamine on the human brain. METHODS: 1H MRS was performed in 26 abstinent methamphetamine abusers with a history of methamphetamine dependence (median total cumulative lifetime exposure, 3,640 g; median recency of last methamphetamine use, 4.25 months) and 24 healthy subjects without a history of drug abuse. Cerebral metabolite concentrations on 1H MRS were measured in the frontal cortex, frontal white matter, and basal ganglia. RESULTS: The concentration of N-acetylaspartate ([NA]), a neuronal marker, was reduced significantly (-5 to -6%) in the basal ganglia and frontal white matter of methamphetamine users compared with control subjects. The frontal white matter [NA] correlated inversely with the logarithm of the lifetime methamphetamine use. The methamphetamine users also showed significantly reduced total creatine in the basal ganglia (-8%), and increased choline-containing compounds ([CHO], +13%) and myo-inositol ([MI], +11%) in the frontal grey matter. CONCLUSIONS: The reduced [NA] on 1H MRS provides evidence for long-term neuronal damage in abstinent methamphetamine users.

Adult↗

PubMed2000

Perfusion MRI detects rCBF abnormalities in early stages of HIV-cognitive motor complex.

OBJECTIVE: To evaluate patients with early HIV-cognitive motor complex (HIV-CMC) for possible regional cerebral blood flow (rCBF) abnormalities on perfusion MRI (pMRI). BACKGROUND: Nuclear medicine techniques have demonstrated global and focal cerebral perfusion abnormalities in patients with HIV dementia. Ultrafast pMRI enables the measurement of rCBF throughout the brain without the need to apply radioactive tracers or ionizing radiation. METHODS: pMRI was used to measure the rCBF in 19 patients with early stages of HIV-CMC and 15 healthy seronegative control subjects. The rCBF maps were registered to high-resolution anatomic MRI scans and transformed into Talairach space. Statistical analysis of the rCBF maps was performed with SPM96. RESULTS: Compared with the control subjects, the patients with HIV had statistically significantly decreased rCBF bilaterally in the inferior lateral frontal cortices (right: -15%, p < 0.002; left: -12%, p < 0.005) and in the inferior medial parietal brain region (-15%, p < 0.0009). In contrast, rCBF was increased bilaterally in the posterior inferior parietal white matter (right: +19%, p < 0.0001; left: + 17%, p < 0.001). Furthermore, rCBF abnormalities correlated significantly with clinical disease severity as measured by CD4 count, plasma viral load, Karnofsky score, and HIV dementia scale. DISCUSSION: Our results are consistent with previous findings from PET and SPECT studies. Furthermore, pMRI can detect rCBF abnormalities that correlate with disease severity in HIV-CMC. Because pMRI is more cost-effective, faster, and safer than nuclear medicine techniques for monitoring rCBF changes, pMRI may be more feasible for monitoring the effects of therapy for HIV-CMC.

AIDS Dementia Complex↗

PubMed2000

Perfusion MRI of the human brain with dynamic susceptibility contrast: gradient-echo versus spin-echo techniques.

In this study, spin-echo and gradient-echo-based perfusion magnetic resonance imaging (MRI) techniques are systematically compared with respect to their application in the human brain. Six healthy subjects were evaluated with both techniques consecutively and injected twice with a gadolinium-based contrast agent. In accordance with theoretical predictions and with previous animal experiments, the spin-echo-based technique shows a markedly reduced appearance of large vessels. The intersubject standard deviations of the two methods are similar but smaller for the spin-echo technique in small regions adjacent to large vessels. Therefore, the sensitivity of the two acquisition methods for evaluating pathologic abnormalities may be similar despite the higher contrast-to-noise ratio of the gradient-echo-based technique. The gray-to-white matter ratio of the regional cerebral blood flow of the spin-echo method is closer to previous nuclear medicine measurements than that of the gradient-echo method. Our measurements indicate that spin-echo-based perfusion MRI is more representative of capillary perfusion than gradient-echo measurements. J. Magn. Reson. Imaging 2000;12:381-387.

Adult↗

PubMed2000

Comparison of the hemodynamic response to different visual stimuli in single-event and block stimulation fMRI experiments.

Experiments with three different types of basic visual stimulation were performed to compare cortical activation in single-event and block trials. Independent of the stimulation paradigm, the single-event presentation leads to highly consistent signal responses regarding both the activated cortical areas and the dynamics of the signal time course. In contrast, signal time courses during block paradigms depend on the stimulus applied and are a complex and nonlinear function of the single-event responses. Additionally, the initial dip during the first 2 seconds after stimulus onset is consistently observed. However, the small amplitude change (-0.1% to -0.3%) requires signal averaging to establish statistical significance of the effect. Furthermore, different patterns of activation were observed within the primary visual cortex. In an anterior part of the primary visual cortex, activation was only observed at the onset and at the cessation of stimulation involving luminance changes.

Adult↗

PubMed1999

Highly active antiretroviral therapy reverses brain metabolite abnormalities in mild HIV dementia.

OBJECTIVE: To determine whether cerebral metabolite abnormalities normalize with highly active antiretroviral therapy (HAART). BACKGROUND: Patients with HIV-cognitive motor complex (HIV-CMC) show cerebral metabolite abnormalities in the early stages of dementia. METHODS: Sixteen patients with HIV-CMC were evaluated before and after HAART, and compared with 15 HIV-negative healthy volunteers. Cerebral metabolite ratios and concentrations in the frontal lobe and basal ganglia were measured using proton MRS (1H MRS). RESULTS: In 14 of 16 patients who tolerated HAART, CD4 count increased by 133+/-101 cells/mm3 (p = 0.0003), HIV Dementia Scale score increased by 1.8+/-2.4 points (p = 0.02), and AIDS dementia complex (ADC) stage decreased by 0.54+/-0.54 points (p = 0.003). The initially increased choline/creatine (CHO/CR) reversed in the midfrontal cortex (-8.0%; p = 0.02) and in the basal ganglia (-14.7%; p = 0.01). The initially elevated myoinositol (MI)/CR and myoinositol concentration [MI] in the basal ganglia also decreased (MI/CR: -14.1%; p = 0.005; [MI]: 11.8%; p = 0.02), along with normalization of [MI] in the frontal white matter (11.4%; p = 0.05). Furthermore, the change in [MI] in the frontal white matter correlated with the change in CD4 count (r = -0.67, p = 0.03) and with the change in ADC stage (p = 0.04). CONCLUSIONS: HAART improves HIV-CMC in addition to systemic measures of HIV infection. 1H MRS detects improvement of brain injury measured by cerebral metabolites, particularly the glial marker [MI], in patients with early HIV-CMC after HAART. In addition, the degree of improvement in clinical severity of HIV-CMC is related to the degree of recovery with [MI].

AIDS Dementia Complex↗