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Klaus Scheffler

Publications and source records attributed to Klaus Scheffler.

At least 19 recordsLinked to original sources

A computational ontology framework for the synthesis of multi-level pathology reports from brain MRI scans.

BackgroundConvolutional neural network (CNN) based volumetry of MRI data can help differentiate Alzheimer's disease (AD) and the behavioral variant of frontotemporal dementia (bvFTD) as causes of cognitive decline and dementia. However, existing CNN-based MRI volumetry tools lack a structured hierarchical representation of brain anatomy, which would allow for aggregating regional pathological information and automated computational inference.ObjectiveDevelop a computational ontology pipeline for quantifying hierarchical pathological abnormalities and visualize summary charts for brain atrophy findings, aiding differential diagnosis.MethodsUsing FastSurfer, we segmented brain regions and measured volume and cortical thickness from MRI scans pooled across multiple cohorts (N = 3433; ADNI, AIBL, DELCODE, DESCRIBE, EDSD, and NIFD), including healthy controls, prodromal and clinical AD cases, and bvFTD cases. Employing the Web Ontology Language (OWL), we built a semantic model encoding hierarchical anatomical information. Additionally, we created summary visualizations based on sunburst plots for visual inspection of the information stored in the ontology.ResultsOur computational framework dynamically estimated and aggregated regional pathological deviations across different levels of neuroanatomy abstraction. The disease similarity index derived from the volumetric and cortical thickness deviations achieved an AUC of 0.88 for separating AD and bvFTD, which was also reflected by distinct atrophy profile visualizations.ConclusionsThe proposed automated pipeline facilitates visual comparison of atrophy profiles across various disease types and stages. It provides a generalizable computational framework for summarizing pathologic findings, potentially enhancing the physicians' ability to evaluate brain pathologies robustly and interpretably.

Humans↗

Calculation of flip angles for echo trains with predefined amplitudes with the extended phase graph (EPG)-algorithm: principles and applications to hyperecho and TRAPS sequences.

The article presents an algorithm for calculation of flip angles in multiecho experiments to generate echoes with predefined amplitudes based on the extended phase graph algorithm. The algorithm can be used to optimize the echo envelope and thus the point spread function (PSF) in hyperecho and TRAPS (transition into the pseudosteady state) experiments while minimizing the total RF power. Implementations at 3 T using echo trains with Gaussian and Lorentzian PSF demonstrate a reduction in RF power by a factor of 3-5 while maintaining high image quality.

Algorithms↗

Fast frequency mapping with balanced SSFP: theory and application to proton-resonance frequency shift thermometry.

A method is presented for the rapid acquisition of frequency maps based on multiecho balanced steady-state free precession (balanced SSFP, fast imaging with steady precession (True FISP), fast imaging employing steady-state excitation (FIESTA), or balanced fast field echo (FFE)). This technique was applied to measure temperature changes within a gel phantom based on the temperature-sensitive water proton-resonance frequency. The frequency was determined as the slope of a linear fit of the phases measured at different TEs along the echo train. The signal-to-noise ratio (SNR) of multiecho SSFP was analyzed for different parameters, such as relaxation times and flip angle, as well as for different local field inhomogeneities. The theoretical and experimental results were compared with results from the established multiecho fast low-angle shot (FLASH) method. Depending on the given tissue parameters, a significant increase in the accuracy of the frequency/temperature shift estimation compared to FLASH was observed. Furthermore, balanced SSFP can be used with very short TRs without generating a saturation-related signal loss, which is essential for real-time temperature mapping.

Magnetic Resonance Imaging↗

TRIM: TR independent multislice imaging.

This article introduces a novel concept to overcome the dependence of image contrast on spatial positioning parameters such as the number of slices and slice separation in multislice measurements: TR-independent multislice (TRIM) acquisition allows the number of slices in a single measurement to remain independent of the repetition time TR. Ramped TRIM (rTRIM) allows the distance between the sections excited in each repetition to remain independent of the distance between the reconstructed slices. Even images from overlapping slices can be acquired without crosstalk between the images of adjacent slices due to spatially overlapping excitation profiles. This concept is based on a special reordering scheme: Within a single TR acquisition, steps are only taken from a fraction of all slices. This necessitates attribution of different phase-encoding steps to different slices within each repetition cycle. The reordering scheme can be derived by the use of a design matrix. The imaging properties of the technique are discussed theoretically and illustrated by a point spread function analysis based on simulations and phantom measurements. Potential sources of artifacts are identified and methods for their prevention are developed. Optimized implementations with different T(1)-weighted sequences such as spin echo (SE), turbo spin echo (TSE), and spoiled gradient echo acquisitions are shown on normal volunteers with imaging parameters used in routine diagnosis.

Humans↗

Principles and applications of balanced SSFP techniques.

During the past 5 years balanced steady-state free precession (SSFP) has become increasingly important for diagnostic and functional imaging. Balanced SSFP is characterized by two unique features: it offers a very high signal-to noise ratio and a T2/T1-weighted image contrast. This article focuses on the physical principles, on the signal formation, and on the resulting properties of balanced SSFP. Mechanisms for contrast modification, recent clinical application, and potential extensions of this technique are discussed.

Angiography↗

Is TrueFISP a gradient-echo or a spin-echo sequence?

It is commonly accepted that TrueFISP (balanced FFE, FIESTA) belongs to the class of gradient-echo (GRE) sequences. GRE sequences are sensitive to dephasing effects of the transverse magnetization between the excitation pulse and echo acquisition, and phase coherence is only established directly after and before excitation pulses. However, an analysis of the phase evolution of transverse magnetization in a TrueFISP experiment shows very close similarities to the echo formation of a spin-echo (SE) experiment. If dephasing between excitation pulses is below +/-pi, TrueFISP exhibits a nearly complete refocusing of transverse magnetization at TE = TR/2. Only signals acquired before and after TR/2 show an additional T*2 sensitivity.

Magnetic Resonance Imaging↗

Multiecho sequences with variable refocusing flip angles: optimization of signal behavior using smooth transitions between pseudo steady states (TRAPS).

A variation of the rapid acquisition with relaxation enhancement (RARE) sequence (also called turbo spin-echo (TSE) or fast spin-echo (FSE)) is presented. This technique uses variable flip angles along the echo train such that magnetization is initially prepared into the static pseudo steady state (PSS) for a low refocusing flip angle (alpha < 180 degrees ). It is shown that after such a preparation, magnetization will always stay very close to the static PSS even after significant variation of the subsequent refocusing flip angles. This allows the design of TSE sequences in which high refocusing flip angles yielding 100% of the attainable signal are applied only for the important echoes encoding for the center of k-space. It is demonstrated that a reduction of the RF power (RFP) by a factor of 2.5-6 can be achieved without any loss in signal intensity. The contribution of stimulated-echo pathways leads to a reduction of the effective TE by a factor f(t), which for typical implementations is on the order of 0.5-0.8. This allows the use of longer echo readout times, and thus longer echo trains, for acquiring images with a given T(2) contrast.

Computer Simulation↗

On the transient phase of balanced SSFP sequences.

The signal intensity of balanced steady-state free precession (SSFP) imaging is a function of the proton density, T(1), T(2), flip angle (alpha), and repetition time (TR). The steady-state signal intensity that is established after about 5*T(1)/TR can be described analytically. The transient phase or the approach of the echo amplitudes to the steady state is an exponential decay from the initial amplitude after the first excitation pulse to the steady-state signal. An analytical expression of the decay rate of this transient phase is presented that is based on a simple analysis derived from the Bloch equations. The decay rate is a weighted average of the T(1) and T(2) relaxation times, where the weighting is determined by the flip angle of the excitation pulses. Thus, balanced SSFP imaging during the transient phase can provide various contrasts depending on the flip angle and the number of excitation pulses applied before the acquisition of the central k-space line. In addition, transient imaging of hyperpolarized nuclei, such as (3)He, (129)Xe, or (13)C, can be optimized according to their T(1) and T(2) relaxation times.

Humans↗

Detection of the non-steroidal anti-inflammatory drug niflumic acid in humans: a combined 19F-MRS in vivo and in vitro study.

This study describes for the first time results of a (19)F-MRS study on humans exposed to the fluorinated non-steroidal anti-inflammatory drug niflumic acid. The accumulation and elimination of this commercially available selective prostaglandin synthase inhibitor is studied after an oral bolus in the human liver, in blood plasma and in urine samples. The in vivo spectra of the liver display two resonances with a similar increase in signal intensity during the investigation period of 240 min. One resonance refers to the parent compound niflumic acid (P), whereas the second resonance corresponds to a metabolite (M1) formed by the biotransformation by liver enzymes. The spectroscopic comparison with model compounds suggests 4'-hydroxyniflumic acid as the metabolite. During the entire experiment the concentration ratios of these resonances (P/M1) ranged between 0.7 and 0.9, indicating a high metabolite concentration most probably due to an efficient first pass metabolism. Both resonances (P, M1) were observed in the in vitro study of the blood plasma samples after plasma protein denaturation. However, in comparison to the liver spectra, the amount of the metabolite M1 is very small with a P/M1-ratio of 36.6 after 90 min and 16.1 after the end of measurement. This finding suggests an efficient biliary excretion of the metabolite M1, which bypasses the blood circulation system. Both resonances are also identified in the native urine samples. The signal intensity of the parent compound dominates the spectra of all urine samples, whereas the signal intensity of M1 increases slowly reaching a similar value to the parent compound P at the end of the measurement. This observation demonstrates an effective renal elimination of niflumic acid and suggests the existence of an enterohepatic circuit with a re-entry mechanism for the biliary excreted metabolite M1. In the urine spectra, an additional metabolite M2 is found. This resonance exhibits a low but constant signal intensity. The chemical origin of this metabolite is unclear.

Administration, Oral↗

fMRI of the auditory system: understanding the neural basis of auditory gestalt.

Functional magnetic resonance imaging (fMRI) has rapidly become the most widely used imaging method for studying brain functions in humans. This is a result of its extreme flexibility of use and of the astonishingly detailed spatial and temporal information it provides. Nevertheless, until very recently, the study of the auditory system has progressed at a considerably slower pace compared to other functional systems. Several factors have limited fMRI research in the auditory field, including some intrinsic features of auditory functional anatomy and some peculiar interactions between fMRI technique and audition. A well known difficulty arises from the high intensity acoustic noise produced by gradient switching in echo-planar imaging (EPI), as well as in other fMRI sequences more similar to conventional MR sequences. The acoustic noise interacts in an unpredictable way with the experimental stimuli both from a perceptual point of view and in the evoked hemodynamics. To overcome this problem, different approaches have been proposed recently that generally require careful tailoring of the experimental design and the fMRI methodology to the specific requirements posed by the auditory research. The novel methodological approaches can make the fMRI exploration of auditory processing much easier and more reliable, and thus may permit filling the gap with other fields of neuroscience research. As a result, some fundamental neural underpinnings of audition are being clarified, and the way sound stimuli are integrated in the auditory gestalt are beginning to be understood.

Auditory Cortex↗

Invasive and non-invasive evaluation of spontaneous arteriogenesis in a novel porcine model for peripheral arterial obstructive disease.

Our current knowledge regarding the efficacy of factors stimulating collateral artery growth in the peripheral circulation primarily stems from models in small animals. However, experimental models in large sized animals are a prerequisite for extrapolation of growth factor therapy to patients with peripheral atherosclerotic obstructive disease. Therefore, we have developed a novel porcine femoral artery ligation model using non-invasive and invasive evaluation techniques. In 12 young farm pigs and nine older minipigs, a ligation of the superficial femoral artery was performed. Using an intra-arterial catheter, phosphate buffered saline (PBS) was administered with a first-pass over the collateral vascular bed. Directly after ligation as well as after 2 weeks of continuous infusion of PBS, perfusion of the leg was measured using various flow and pressure parameters. Using a pump driven extracorporal system, collateral conductance was determined under maximal vasodilatation. Conductance decreased after acute ligation to similar levels in both young farm pigs as well as the older minipigs (both 9.3% of normal perfusion) and recovered after 2 weeks to a higher value in farm pigs compared with minipigs (22.4 vs. 12.7% of normal; P<0.05). Angiography using both X-ray and magnetic resonance imaging was performed to visualize the formed collateral arteries. To the best of our knowledge this is the first in vivo pig model for hemodynamic assessment of growth of collateral arteries in the peripheral circulation, that is suitable for evaluation of arteriogenic effects of growth factors or genes.

Angiography↗

Temporal integration of sequential auditory events: silent period in sound pattern activates human planum temporale.

Temporal integration is a fundamental process that the brain carries out to construct coherent percepts from serial sensory events. This process critically depends on the formation of memory traces reconciling past with present events and is particularly important in the auditory domain where sensory information is received both serially and in parallel. It has been suggested that buffers for transient auditory memory traces reside in the auditory cortex. However, previous studies investigating "echoic memory" did not distinguish between brain response to novel auditory stimulus characteristics on the level of basic sound processing and a higher level involving matching of present with stored information. Here we used functional magnetic resonance imaging in combination with a regular pattern of sounds repeated every 100 ms and deviant interspersed stimuli of 100-ms duration, which were either brief presentations of louder sounds or brief periods of silence, to probe the formation of auditory memory traces. To avoid interaction with scanner noise, the auditory stimulation sequence was implemented into the image acquisition scheme. Compared to increased loudness events, silent periods produced specific neural activation in the right planum temporale and temporoparietal junction. Our findings suggest that this area posterior to the auditory cortex plays a critical role in integrating sequential auditory events and is involved in the formation of short-term auditory memory traces. This function of the planum temporale appears to be fundamental in the segregation of simultaneous sound sources.

Acoustic Stimulation↗

Sustained blood oxygenation and volume response to repetition rate-modulated sound in human auditory cortex.

The blood oxygen level-dependent (BOLD) signal time course in the auditory cortex is characterized by two components, an initial transient peak and a subsequent sustained plateau with smaller amplitude. Because the T(2)(*) signal detected by functional magnetic resonance imaging (fMRI) depends on at least two counteracting factors, blood oxygenation and volume, we examined whether the reduction in the sustained BOLD signal results from decreased levels of oxygenation or from increased levels of blood volume. We used conventional fMRI to quantify the BOLD signal and fMRI in combination with superparamagnetic contrast agent to quantify blood volume and employed repetition rate-modulated sounds in a silent background to manipulate the response amplitude in the auditory cortex. In the BOLD signal, the initial peak reached 3.3% with pulsed sound and 1.9% with continuous sound, whereas the sustained BOLD signal fell to 2.2% with pulsed sound and to 0.5% with continuous sound, respectively. The repetition rate-dependent reduction in the sustained BOLD amplitude was accompanied by concordant changes in sustained blood volume levels, which, compared to silence, increased by approximately 30% with pulsed and by approximately 10% with continuous sound. Thus, our data suggest that the reduced amplitude of the sustained BOLD signal reflects stimulus-dependent modulation of blood oxygenation rather than blood volume-related effects.

Acoustic Stimulation↗

Neural processing of auditory looming in the human brain.

Acoustic intensity change, along with interaural, spectral, and reverberation information, is an important cue for the perception of auditory motion. Approaching sound sources produce increases in intensity, and receding sound sources produce corresponding decreases. Human listeners typically overestimate increasing compared to equivalent decreasing sound intensity and underestimate the time to contact of approaching sound sources. These characteristics could provide a selective advantage by increasing the margin of safety for response to looming objects. Here, we used dynamic intensity and functional magnetic resonance imaging to examine the neural underpinnings of the perceptual priority for rising intensity. We found that, consistent with activation by horizontal and vertical auditory apparent motion paradigms, rising and falling intensity activated the right temporal plane more than constant intensity. Rising compared to falling intensity activated a distributed neural network subserving space recognition, auditory motion perception, and attention and comprising the superior temporal sulci and the middle temporal gyri, the right temporoparietal junction, the right motor and premotor cortices, the left cerebellar cortex, and a circumscribed region in the midbrain. This anisotropic processing of acoustic intensity change may reflect the salience of rising intensity produced by looming sources in natural environments.

Adult↗

Dynamic contrast enhancement of paragangliomas of the head and neck: evaluation with time-resolved 2D MR projection angiography.

Our objective was to evaluate tumor hemodynamics of paragangliomas of the head and neck. A 2D magnetic resonance projection angiography (MRPA) protocol was developed. Six patients with histologically proven paragangliomas were included in the study. The sequence protocol and data post-processing procedure were optimized in view of a high temporal resolution and maximum contrast-to-noise ratio. Image-time series of two freely orientated slabs could be obtained. Correlation analysis was performed to generate selective arterial and venous weighted angiographic images. Glomus tumors showed a rapid and intense homogeneous enhancement following the intravenous administration of contrast material. The smallest tumors investigated measured 10 mm and could be visualized clearly. Time-resolved analysis of the tumor enhancement revealed an early bolus arrival within the vascular bed of the tumor and prolonged incomplete washout indicating blood pooling. The dynamic contrast enhancement of glomus tumors can be evaluated non-invasively with MRPA providing a high temporal resolution and high image quality. The characteristic contrast enhancement of glomus tumors can be helpful in the diagnostic workup of lesions that may mimic glomus tumors.

Carotid Body Tumor↗

Spatiotemporal pattern of neural processing in the human auditory cortex.

The principles that the auditory cortex uses to decipher a stream of acoustic information have remained elusive. Neural responses in the animal auditory cortex can be broadly classified into transient and sustained activity. We examined the existence of similar principles in the human brain. Sound-evoked, blood oxygen level-dependent signal response was decomposed temporally into independent transient and sustained constituents, which predominated in different portions-core and belt-of the auditory cortex. Converging with unit recordings, our data suggest that this spatiotemporal pattern in the auditory cortex may represent a fundamental principle of analyzing sound information.

Acoustic Stimulation↗

fMRI of the auditory cortex in patients with unilateral carotid artery steno-occlusive disease.

PURPOSE: To examine whether an internal carotid artery (ICA) steno-occlusive disease leads to a reduced blood oxygenation level dependent (BOLD)-signal change due to a decreased vasodilatory reserve capacity. MATERIALS AND METHODS: Patients suffering from unilateral ICA stenosis or occlusion were examined using functional magnetic resonance imaging (fMRI) of the auditory cortex with a bilateral stimulation applying a pulsed 1000 Hz sine-tone. RESULTS: Compared to control subjects, who showed symmetric bilateral BOLD-responses within the auditory cortex, the ICA patients revealed either a normal bilateral cortical activation pattern or a reduced cortical activation on the steno-occluded side. This latter decrease of BOLD-signal change might indicate a depressed vasomotor reserve capacity. The observed coincidence between this asymmetric reduction in BOLD-signal and ischemic borderzone lesions on the affected side in this subgroup of patients strongly supports this assumption. CONCLUSION: This study shows that fMRI of the auditory cortex appears to have clinical merit in the workup of cerebrovascular conditions.

Acoustic Stimulation↗