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Biomedical subjects

K Scheffler

Publications and source records attributed to K Scheffler.

At least 37 records · Page 2Linked to original sources
PubMed2000

Easy improvement of signal-to-noise in RARE-sequences with low refocusing flip angles. Rapid acquisition with relaxation enhancement.

It is demonstrated that the signal intensity in a RARE (TSE, FS...)-sequence with low refocusing flip angle alpha can be significantly increased by setting the flip angle of the first refocusing pulse to 90 degrees +alpha/2. In addition to the gain in signal intensity, the initial signal modulations over the first few echoes are reduced compared to a CPMG-echo train with constant alpha.

Artifacts↗
PubMed2000

Analysis of input functions from different arterial branches with gamma variate functions and cluster analysis for quantitative blood volume measurements.

Regional cerebral blood volume (rCBV) provides valuable information about the nature and progress of diseases of the central nervous system. While relative rCBV maps can be derived directly from dynamic susceptibility contrast data, the arterial input function (AIF) has to be measured for absolute rCBV quantification. For determination of the AIF pixels located completely within a feeding artery must be selected. However, by using a region-of-interest (ROI) based selection some confounding effects can occur, especially if single shot echo planar imaging (EPI) with low spatial resolution is used. In this study we analyzed the influence of partial volume effects and spatial misregistration due to frequency shifts induced by paramagnetic contrast agents. We analyzed AIFs from the internal carotid artery (ICA), the vertebral artery (VA) and the middle cerebral artery (MCA) using gamma variate function based parameterization. The concentration time curves (CTC) of several pixels which were selected on the basis of strong signal drop appeared distorted during the bolus passage. Moreover, the amplitudes of input functions derived from the MCA were smaller by a factor of three as compared to those of the ICA and VA. Simulations revealed that these effects can be attributed to a spatial shift of the vessel along phase-encoding direction during the passage of the bolus. We therefore developed a procedure for a pixel selection based on cluster analysis which classifies pixels according to the parameters of the fitted gamma variate functions. This approach accounted for misregistration of the vessel and yielded very consistent results for a group of normal subjects.

Brain↗
PubMed2000

[Detection of central auditory compensation in unilateral deafness with functional magnetic resonance tomography].

BACKGROUND: Functional magnetic resonance imaging (fMRI) is a noninvasive method to detect focal brain activity at high spatial resolution. Acoustic stimulation induces an increase of regional cerebral blood flow in the primary auditory cortex. This entails an increased concentration of diamagnetic oxyhemoglobin in the capillaries and the venous system. The resulting decrease of the local magnetic susceptibility was detected as a signal increase in T2*-weighted images. The central auditory pathways predominantly cross to the contralateral hemisphere in normally hearing subjects. The aim of the present study was to investigate the primary auditory cortex after acoustic stimulation in unilateral deaf patients using fMRI. METHODS: Magnetic resonance images were acquired on a 1.5 T Siemens Vision scanner. For fMRI, a single shot gradient recalled, echo planar imaging (EPI) sequence with decreasing excitation order was used, allowing the aquisition of 9 slices within 1.8 s. The 9 slices covered a slab of 3.6 cm in cranio-caudal extension in the region of the temporal lobes. For statistical processing of the raw image data the SPM96 software package was used. A p-value of p < 0.01 was applied to differentiate between activated and non-activated. The resulting functional activation maps were superimposed onto the EPI scan. The number of activated pixels was used to quantitate the cortical response upon acoustic stimulation. Stimulation consisted of a 1000-Hz sine tone (100 dB SPL at the distal end of the head phone, pulsed at 6 Hz) to which the patients were asked to listen passively. A piezoelectric loudspeaker was mounted on the subject table and connected to a plastic tube system leading to a combination of bilateral ear- and headphones. Auditory paradigms require disentangling experimental excitation from the scanner noise that approximates 90 dB. Headphones suppress noise by approximately 30 dB. To decrease the acoustic background-to-stimulation ratio and to keep background noise constant during stimulation and resting, we employed short scanning (1.8 s) and long resting periods (10.2 s; TR = 12 s). This acquisition mode allows sufficient recovery during off-periods and sufficient excitation during on-periods. 14 unilateral deaf patients were examined. The mean duration of deafness was 22.5 years. RESULTS: Acoustic stimulation of the deaf ear revealed only weak cortical activation which could be explained by sound transmission via bone conduction to the other ear. A significant increase of BOLD (blood oxygen level dependent)-activation in the primary auditory cortex could be demonstrated in all patients after stimulation of the hearing ear. However, remarkable individual differences were noticed concerning the absolute number of activated pixels. The lateralization ratio was calculated by the number of activated pixels on the hearing side divided by the number of activated pixels on the deaf side. A mean lateralization ratio of 0.9 (Stdv +/- 0.6) was found. The mean lateralization ratio for patients with a right deaf ear (n = 8) and those with a left deaf ear (n = 5) was 1.1 (Stdv +/- 0.7) and 0.6 (Stdv +/- 0.3) respectively. However, the difference was not significant (Wilcoxon test: p = 0.08). CONCLUSIONS: Central-auditory compensation by bilateral cortical activation was demonstrated in unilateral deaf patients. Moreover, a tendency towards a dominance of the left primary auditory cortex was found, although the difference between both hemispheres was not significant. The lateralization ratio in unilateral deaf patients is similar to findings after binaural stimulation in normally hearing subjects.

Acoustic Stimulation↗
PubMed1999

Titration of the BOLD effect: separation and quantitation of blood volume and oxygenation changes in the human cerebral cortex during neuronal activation and ferumoxide infusion.

Most functional magnetic resonance imaging (fMRI) techniques are sensitive to susceptibility variations and rely on the change in blood oxygenation level in response to neuronal activation (BOLD). The BOLD effect is accompanied by a change in cerebral blood flow (rCBF) and cerebral blood volume (rCBV). Intravascular contrast agents, such as magnetite nanoparticles, can be used to measure changes in rCBV. A new measuring protocol has been developed that enables the separate quantification of changes in blood volume and oxygenation levels. A combination of alternating acoustic stimulation blocks and infusion of a superparamagnetic contrast agent offers the possibility to disentangle the competing influences of oxygenation and blood volume changes. Serial blood sampling during infusion was used to assess the actual contrast agent concentration during infusion in order to calculate absolute blood volume changes during neuronal resting and activation states. Magn Reson Med 42:829-836, 1999.

Acoustic Stimulation↗
PubMed1998

Motor, somatosensory and auditory cortex localization by fMRI and MEG.

Functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) were performed in six subjects during self-paced finger movement performance, tactile somatosensory stimulation and binaural auditory stimulation using identical stimulation paradigms. Both functional imaging modalities localized brain activity in adjacent areas of anatomically correct cortex. The mean distances measured between fMRI activity and the corresponding MEG dipoles were 10.1 mm (motor), 10.7 mm (somatosensory), 13.5 mm (auditory right hemisphere) and 14.3 mm (auditory left hemisphere). The distances found may reflect the correlation between electrophysiological and hemodynamic responses due to the different underlying substrates of neurophysiology measured by fMRI and MEG: BOLD contrast vs neuronal biomagnetic activity.

Adult↗
PubMed1998

29Si imaging of silicone breast implants and intraocular silicone oil.

Silicon-29 (29Si) imaging was investigated as a potential imaging modality for monitoring silicone prostheses in humans. The 29Si relaxation times of several silicone gels were measured and found to average T1 = 21.2 +/- 1.5 s and T2 = 207 +/- 40 ms, with no significant difference between virgin and explanted gels. A single-shot half-Fourier rapid acquisition with relaxation enhancement (RARE) and a refocused gradient-echo sequence were used for acquiring 29Si images with 5 x 5 mm2 resolution and no slice selection. Three volunteers with silicone-gel-filled breast implants and one subject with an intraocular silicone oil injection were thus examined in a total acquisition time of 10-15 min per image. On all 29Si images, the shape of the silicone object was well depicted. Although at present, conventional proton images are superior in resolution and signal-to-noise ratio, 29Si imaging has the advantage of optimal specificity, since only the silicone itself is visible.

Biocompatible Materials↗
PubMed1998

Reduced circular field-of-view imaging.

A rapid dynamic imaging technique based on polar k-space sampling is presented. A gain in temporal resolution is achieved by angular undersampling. A detailed analysis of the point spread function of angular undersampled polar imaging reveals a reduced diameter of the corresponding circular field of view. Under the assumption that dynamic changes are restricted to a local circular field of view, angular undersampled dynamic imaging allows the recording of rapid changes at high temporal and spatial resolution. The theoretical and experimental details of the technique are presented.

Artifacts↗
PubMed1998

The MR tomograph as a sound generator: fMRI tool for the investigation of the auditory cortex.

An automatic fMRI protocol for the investigation of the human auditory cortex has been developed. For the activation of the auditory system, the scanner noise itself is the source of sound. Rapid switching of B0 gradients allows the generation of tones of defined frequencies and amplitude on a conventional scanner without hardware change. No additional device for sound generation or sound delivery is needed. The proposed technique allows functional MRI of the auditory cortex in a clinical routine session.

Acoustic Stimulation↗
PubMed1998

Effect of eye movements on the magnitude of functional magnetic resonance imaging responses in extrastriate cortex during visual motion perception.

We have studied the effects of pursuit eye movements on the functional magnetic resonance imaging (fMRI) responses in extrastriate visual areas during visual motion perception. Echoplanar imaging of 10-12 image planes through visual cortex was acquired in nine subjects while they viewed sequences of random-dot motion. Images obtained during stimulation periods were compared with baseline images, where subjects viewed a blank field. In a subsidiary experiment, responses to moving dots, viewed under conditions of fixation or pursuit, were compared with those evoked by static dots. Eye movements were recorded with MR-compatible electro-oculographic (EOG) electrodes. Our findings show an enhanced level of activation (as indexed by blood-oxygen level-dependent contrast) during pursuit compared with fixation in two extrastriate areas. The results support earlier findings on a motion-specific area in lateral occipitotemporal cortex (human V5). They also point to a further site of activation in a region approximately 12 mm dorsal of V5. The fMRI response in V5 during pursuit is significantly enhanced. This increased response may represent additional processing demands required for the control of eye movements.

Adult↗
PubMed1998

Pulmonary circulation.

Evaluation of the pulmonary vasculature is mainly indicated in patients with suspected pulmonary thromboembolism. The routine procedure so far is ventilation-perfusion scintigraphy alone or in combination with diagnostic assessment of the legs to rule out deep venous thrombosis. The results are still not reliable for the majority of patients. In the case of equivocal diagnosis, invasive conventional angiography is considered the gold standard. With steady improvements in tomographic imaging techniques, such as computed tomography (CT) or magnetic resonance imaging (MRI), non-invasive alternatives to the routine diagnostic work-up are given. Helical CT and CTA techniques are already in clinical use and estimated to sufficiently serve the demands for detection/exclusion of pulmonary thromboembolism. The disadvantages mainly concern peripheral disease and reconstruction artifacts. MRI and MR angiography have been implemented in the diagnosis of pulmonary vascular disease since the introduction of contrast-enhanced MRA. In breath-hold techniques, the entire lung vascularization can be delineated and thromboemboli can be detected. The clinical experience in this field is limited, but MRI has the potential to demonstrate its superiority over CT due to its improved delineation of the vascular periphery and the more comprehensive three-dimensional reconstruction.

Aged↗
PubMed1998

Tonotopic organization of the human auditory cortex as detected by BOLD-FMRI.

Functional magnetic resonance imaging is a noninvasive and nonradioactive method for the detection of focal brain activity. In the present study the auditory cortex was investigated in nine normal subjects who were binaurally stimulated using pulsed sine tones of 500 Hz and 4000 Hz. The BOLD (blood oxygenation level dependent) signal change coincided with the stimulation paradigm and was detected in the plane of the superior temporal gyrus. The comparison of the spatial distribution of activated areas revealed a different behavior for the two frequencies. The present findings underline the existence of a frequency specific organization in the medio-lateral, fronto-occipital and cranio-caudal extension in both hemispheres of the auditory cortex in human. The activated areas for the high tone were found more frontally and medially orientated than the low tone stimulated areas. Furthermore, a slight cranio-caudal shift was observed for the higher frequency, more pronounced in the right than in the left temporal lobe. Finally, for most of the subjects investigated the BOLD activation area of the 500 Hz sine tone was larger than that of the 4000 Hz stimulation. Both frequencies showed a lateralization of signal response to the left temporal lobe.

Acoustic Stimulation↗
PubMed1998

Conformational analysis of a Chlamydia-specific disaccharide alpha-Kdo-(2-->8)-alpha-Kdo-(2-->O)-allyl in aqueous solution and bound to a monoclonal antibody: observation of intermolecular transfer NOEs.

The disaccharide alpha-Kdo-(2-->8)-alpha-Kdo (Kdo: 3-deoxy-D-manno-oct-2-ulosonic acid) represents a genus-specific epitope of the lipopolysaccharide of the obligate intracellular human pathogen Chlamydia. The conformation of the synthetically derived disaccharide alpha-Kdo-(2-->8)-alpha-Kdo-(2-->O)-allyl was studied in aqueous solution, and complexed to a monoclonal antibody S25-2. Various NMR experiments based on the detection of NOEs (or transfer NOEs) and ROEs (or transfer ROEs) were performed. A major problem was the extensive overlap of almost all 1H NMR signals of alpha-Kdo-(2-->8)-alpha-Kdo-(2-->O)-allyl. To overcome this difficulty, HMQC-NOESY and HMQC-trNOESY experiments were employed. Spin diffusion effects were identified using trROESY experiments, QUIET-trNOESY experiments and MINSY experiments. It was found that protein protons contribute to the observed spin diffusion effects. At 800 MHz, intermolecular trNOEs were observed between ligand protons and aromatic protons in the antibody binding site. From NMR experiments and Metropolis Monte Carlo simulations, it was concluded that alpha-Kdo-(2-->8)-alpha-Kdo-(2-->O)-allyl in aqueous solution exists as a complex conformational mixture. Upon binding to the monoclonal antibody S25-2, only a limited range of conformations is available to alpha-Kdo-(2-->8)-alpha-Kdo-(2-->O)-allyl. These possible bound conformations were derived from a distance geometry analysis using transfer NOEs as experimental constraints. It is clear that a conformation is selected which lies within a part of the conformational space that is highly populated in solution. This conformational space also includes the conformation found in the crystal structure. Our results provide a basis for modeling studies of the antibody-disaccharide complex.

Antibodies, Monoclonal↗
PubMed1998

[Visualization of central auditory processes with functional magnetic resonance tomography].

BACKGROUND: Central auditory processes can be visualized using functional MRI in a non-invasive manner and at high spatial resolution. Acoustic stimulation leads to an increase of blood flow of activated areas in the plane of the superior temporal gyrus. Radiologically, this may be visualized based on the long T2-relaxation time of oxyhemoglobin. PATIENTS: Ten normal-hearing subjects with ages between 28 and 38 years took part in the investigations. They received binaural, monaural right, and monaural left stimulation with pulsed sine tones of 1000 Hz at a pulse rate of 6 Hz and a sound pressure level of 100 dB SPL. Tonotopic organization of the auditory cortex was visualized using stimulation by pulsed sine tones of 500 Hz and 4000 Hz. RESULTS: Following monaural acoustic stimulation, increased activity of the contralateral auditory cortex could be demonstrated in 9 subjects. In one subject, bilateral activity was noted. Concerning the tonotopic organization of the auditory cortex, we could show that the higher frequencies were localized more medially and anteriorly; the lower frequencies were localized more laterally and posteriorly in the superior temporal gyrus. However, considerable overlap was noted. CONCLUSIONS: The overlap of the different frequencies could explain the controversial discussion of the tonotopic organization of the auditory cortex. The results of the monaural acoustic stimulation show clearly the predominant signal increase of contralateral areas in the primary auditory cortex. These results confirm the opinion of the current textbooks that the fiber of the auditory pathways mostly cross. Further investigations using functional MRI are necessary for better understanding of physiological and pathophysiological central-auditory processes.

Acoustic Stimulation↗
PubMed1998

Auditory cortical responses in hearing subjects and unilateral deaf patients as detected by functional magnetic resonance imaging.

Functional magnetic resonance imaging is a non-invasive method for the detection of focal brain activity at high spatial resolution. Acoustic stimulation leads to a blood oxygenation level dependent signal change in the plane of the superior temporal gyrus. The dependence of this response in the auditory cortex on binaural, monaural left and monaural right acoustic stimulation for 10 healthy subjects and five monaural deaf patients is described. Acoustic stimulation consists of 1000 Hz pulsed sine tones at a pulse rate of 6 Hz and a sound pressure level of 95 dB. For monaural stimulation, normal-hearing subjects revealed a strong lateralization of cortical response towards the contralateral hemisphere. The lateralization ratios between left and right hemispheric response areas were 3.4-5.2 for monaural stimulation and nearly balanced for binaural stimulation. Additionally, the sum of cortical activation volumes induced by monaural left and right stimulation was approximately 30% smaller than for binaural stimulation, indicating either inhibitory mechanisms or neuronal facilitation within the auditory pathways. For monaural deaf subjects the lateralization ratio between left to right response was just 1.3 towards the contralateral hemisphere of the healthy ear, which is comparable to binaural responses of normal-hearing subjects. This observation seems to indicate a plasticity or a reorganization of auditory pathways of monaural deaf patients.

Adult↗
PubMed1998

Contrast-enhanced magnetic resonance angiography of peripheral vessels. Different contrast agent applications and sequence strategies: a review.

In this article the relation between contrast medium (CM) application and sequence parameters will be discussed with respect to clinical use of the contrast-enhanced magnetic resonance angiography (CE-MRA) in the peripheral vessel region. The adjustment of the sequence parameters, the CM application timing and the bolus geometry is necessary for an effective use of CE-MRA. Investigation protocols for several vascular regions differ mainly corresponding to varying fields of view and slab thickness. Restrictions of increasing the measurement time are expected in peripherally localized vessels if fast arteriovenous transit time occurs. The vessel contrast depends from (1) optimal CM bolus timing and (2) bolus geometry defined by the parameters of the intravenous bolus injection (flow rate, dose and NaCl flush volume). Our study results have shown that the bolus remains compact but also shorter if a higher flow rate is being applied at equal dose. The enlargement of the NaCl flush volume has evidently caused an increased intraarterial CM concentration and a slightly bolus lengthening. The exact timing regimen requires an automated mechanical CM injection pump. In most countries, a total dose of 0.3 mmol/kg Gd is allowed for application during one investigation. Therefore, obtaining an angiogram of the entire iliac and leg region this total dose must be separated. 0.1 mmol/kg for each of the three measurements can be recommended. Otherwise, using this lower CM dose results in less spatial resolution. At least a dosage of 0.2 mmol/kg Gd is necessary to achieve a higher spatial resolution. The calculation of CM dosage should be also related to the dedicated vessel region of interest than to the body weight only.

Arterial Occlusive Diseases↗
PubMed1997

Time-resolved projection angiography after bolus injection of contrast agent.

A method for MR angiography after bolus injection of a normal dose (0.1 mmol/kg) of contrast agent is presented. Projection angiograms are acquired with a non-slice selective Snapshot FLASH sequence with a time resolution of 1 s per image or better. Typically 40 to 60 images are acquired consecutively after bolus injection of a contrast agent. The signal from vessels can be separated from background by postprocessing based on the observed temporal evolution of the signal intensities during bolus passage. The subsecond projection MR-DSA is a reliable and robust technique to produce high resolution anatomical images of the vascular system avoiding the necessity of exact timing of the contrast agent bolus. It also supplies functional information about the hemodynamics in the observed region including perfusion.

Contrast Media↗
PubMed1997

Clinical utility of contrast-enhanced MR angiography.

MR angiography (MRA) is a technique under ongoing discussion. Its non-invasiveness and sensitivity to flow irregularities make it an investigational technique which is easy to apply but which does not always lead to comprehensive results. It requires special skill to perform and also experience for correct interpretation of the results. The lengthiness of the procedure combined with certain physical properties tends to limit its use to mostly neurovascular applications. With the introduction of ultrafast MRA in conjunction with peripheral bolus-injection of extracellular contrast media, a new approach to the imaging of such regions as the thoracic and abdominal vasculature not to mention other vascular territories has become possible. In this paper, considerations of bolus and measurement optimization and timing protocols for dedicated indications are presented together with an overview on the experience acquired to date for CE-MRA of the carotid artery, mediastinal and pulmonary vessels, abdominal vasculature, and peripheral vessels. The main advantage of ultrafast CE-MRA is that patients are subjected to much more tolerable breath-hold investigations with the result that physiological motion such as peristalsis or major pulsation is minimised.

Adolescent↗