Biomedical subjects

K Scheffler

Publications and source records attributed to K Scheffler.

At least 37 records · Page 2Linked to original sources

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↗

PubMed1997

[Conventional magnetic resonance angiography and contrast enhanced magnetic resonance angiography of extracranial blood vessel segments].

The introduction of fast gradient systems allows a reliable visualization of the extracranial carotid vessels by the magnetic resonance angiography (MRA) which meanwhile is implemented into clinical routine. By the mainly applied time-of-flight (TOF) technique, vessels can be imaged without contrast agent (CA). Due to the application of ultra-fast gradient-echo-sequences, the first-pass evaluation of an intravenous bolus-injection of Gadolinium in the carotids from the aortic arch up to the skull base can be performed in less than 30 s. In this study. Advantages and disadvantages of both techniques are discussed. For a qualitatively optimal contrast enhanced MRA (CE-MRA) timing parameters like injection delay, flow rate and the adjustment of sequence parameters have to be considered in relation to the fast venous return from the sinus to the jugular veins. First, the optimal time point of the data acquisition have been determined at a model and with a computer simulation in reference to the presence of CA in the arteries. As a result, 90% of the contrast contribution is defined by 16% of the symmetrically acquired central k-space lines. A measuring protocol for clinical use was obtained by a gradual variation of spacial resolution, measuring time and CA-injection parameters and was proved in normal volunteers and patients. An exact determination of the bolus-arrival-time by means of a test-bolus injection was acquired. The best qualitative results were achieved by a double-dose- injection at 2 ml/s injection rate. The temporal reserves of ultra-fast sequences should be invested in the improvement of the spatial resolution. To date, further investigations related to the problem of optimal CA-application may improve the potentials of CE-MRA procedures.

Carotid Arteries↗

PubMed1997

[MR angiography of thoracic blood vessels].

Through the introduction of newly invented high-performance gradient systems to MRI, which enable for echoplanar imaging (EPI), also magnetic resonance angiography (MRA) has gained an entirely new field of applications and techniques. Ultrafast imaging techniques in MRA allow the investigation of larger vascular areas within a single breath-hold-period. Artifacts like motion induced signal misregistrations, dephasing or saturation of the vascular signal are minimized by extremely short echo times. The technique thus requires the intravenous application of a contrast media bolus, usually a gadolinium compound, which is in standard clinical use. Coordination of the bolus injection and the timing of the data acquisition is crucial for optimal results. The first pass evaluation of the contrast media resembles CTA to a certain extend. Due to the fast measurement and the high contrast in contrast-enhanced MRA (CE-MRA) new applications and indications are developed like MRA of the pulmonary vessels. The paper offers considerations and trials for optimization of thoracical CE-MRA. Besides parameter constellation also bolus-optimization is described with respect to the dedicated anatomical premises. Investigations on volunteers and on patients build a basis for suggestions of optimized CE-MRA procedures. To date, a final estimation of the clinical value of the new technique cannot be given since ongoing improvements change the optimal protocol frequently and the potential of further developments is high.

Adolescent↗

PubMed1997

Application of homonuclear 3D NMR experiments and 1D analogs to study the conformation of sialyl Lewis(x) bound to E-selectin.

The conformation of the sialyl Lewis(x) tetrasaccharide bound to E-selectin was previously determined from transfer NOE (trNOE) experiments in conjunction with a distance-geometry analysis. However, the orientation of the tetrasaccharide ligand in the binding site of E-selectin is still unknown. It can be predicted that the accurate quantitative analysis of all trNOEs, including those originating from spin diffusion, is one key to analyze the orientation of sialyl Lewis(x) in the binding pocket of E-selectin. Therefore, we applied homonuclear 3D NMR experiments and 1D analogs to obtain trNOEs that could not unambiguously be assigned from previous 2D trNOESY spectra, due to severe resonance-signal overlap. A 3D TOCSY-trNOESY experiment, a 1D TOCSY-trNOESY experiment, and a 1D trNOESY-TOCSY experiment of the sialyl Lewis(x)/E-selectin complex furnished new interglycosidic trNOEs and provided additional information for the interpretation of trNOEs that have been described before. A 2D trROESY spectrum of the sialyl Lewis(x)/E-selectin complex allowed one to identify the amount of spin-diffusion contributions to trNOEs. Finally, an unambiguous assignment of all trNOEs, and an analysis of spin-diffusion pathways, was obtained, creating a basis for a quantitative analysis of trNOEs in the sialyl Lewis(x)/E-selectin complex.

Carbohydrate Sequence↗

PubMed1996

Frequency resolved single-shot MR imaging using stochastic k-space trajectories.

A new single-shot stochastic imaging technique with a random k-space path that provides very selective filtering with respect to chemical shift or off-resonance signals of the investigated tissue is proposed. It is demonstrated that in stochastic imaging only on-resonance compartments are visible whereas frequency shifted compartments cancel to noise that is distributed over the whole image. This method can be used as a single-shot chemical shift selective imaging technique and allows to calculate frequency resolved spectra for each spatial position of the image based on a single signal acquisition. The single-shot stochastic imaging sequence makes high demands on the gradient system and the theoretical k-space trajectory is distorted by imperfect gradient performance. Therefore an additional k-space guided imaging technique that uses the true, measured k-space trajectory to correct artifacts generated by eddy currents and delay times of the rapid switched gradients is presented. In vitro and in vivo measurements demonstrate the successful implementation of single-shot stochastic imaging on a conventional MR scanner with unshielded gradient systems.

Artifacts↗

PubMed1996

Aluminum-27 nuclear magnetic resonance spectroscopy and imaging of the human gastric lumen.

Aluminum NMR is proposed as a new imaging and spectroscopy modality. Its potential is exemplified by in vivo studies of the human stomach. The dissolution kinetics of aluminum-containing drugs at physiological doses and their removal from the human stomach have been followed by 27Al magnetic resonance spectroscopy (MRS). Aluminum concentrations as low as 0.5 mg Al3+ in the human stomach can be detected. The time course of gastric emptying has been visualized with 27Al magnetic resonance imaging (MRI) under normal conditions and in the presence of an antimuscarinic agent, which reduces the gastric motor function. 27Al MRI is the only direct method to visualize the gastric pH. 27Al MRI opens new possibilities for medical and pharmaceutical science.

Aluminum↗

PubMed1989

Oxygen dependent regulation of mammalian ribonucleotide reductase in vivo and possible significance for replicon initiation.

The intracellular concentration of the M2 specific free tyrosyl radical of ribonucleotide reductase in cultured Ehrlich ascites cells was estimated by EPR spectroscopy during imposition and after reversal of hypoxia. Under the same conditions, the intracellular intensity of CDP reduction was estimated indirectly by measuring the incorporation of radioactivity into DNA from labeled ribo- and deoxyribo-cytidine, respectively. The radical concentration distinctly decreased under hypoxia and reincreased upon reaeration. At the same time, the CDP reduction was greatly diminished and reactivated, respectively. These observations are interpreted in the sense of an O2 dependent regulation of the intracellular activity of ribonucleotide reductase. The O2 dependent deactivation and reactivation of the enzyme was temporally related to a specific suppression or a burst-like release of replicon initiations, respectively. Addition of 100 microM dCtd to hypoxic cells could substitute for reoxygenation with respect to triggering replicon initiations. A possible implication of the intracellular ribonucleotide reductase activity and the size of the dCTP pool in replicon initiation is discussed.

Binding Sites↗

PubMedDate not supplied

Hydro-MRI for the visualization of gastric wall motility using RARE magnetic resonance imaging sequences.

BACKGROUND: Although different imaging techniques such as conventional X-ray, ultrasonography, and hydro-computed tomography are available for the imaging of the stomach, none can depict this organ in full size without radiation. Therefore, the study of the entire gastric wall motility of the stomach is difficult and in principle only performable with rapid magnetic resonance imaging (MRI) techniques. T1-weighted imaging sequences have been used for the dynamic study of gastric wall motility. This technique was combined with the oral intake of para- or superparamagnetic contrast agents to achieve sufficient intraluminal contrast. The technique described in the present study is based on a different contrast mechanism. METHODS: The stomach was filled with 500 mL of 10% of aqueous dextrose solution, and a strongly T2-weighted fast rapid acquisition with relaxation enhancement (RARE) type imaging sequence was used for data acquisition. No other contrast agents were applied. An ultrafast RARE imaging sequence with an asymmetric phase-encoding scheme was developed to achieve a high temporal and spatial resolution. The scanning time per image was approximately 1 s. RESULTS: The stomach was imaged in full size. The concentric constrictor rings moved from the proximal part of the body toward the antrum. The mean duration for one contraction cycle was approximately 17.9 +/- 2.5 s, the mean contractile frequency was 3.4 +/- 0.5 s, and the mean spreading velocity was 65.5 +/- 3.6 cm/min. CONCLUSIONS: The purpose of this study was to demonstrate a new technical approach for a noninvasive dynamic study of gastric motor function with hydro-MRI. This robust method may have clinical application, e.g., in the diagnosis of gastroparesis, and may be extended to the rest of the gastrointestinal tract.

Administration, Oral↗

PubMedDate not supplied

Tomographic imaging with nonuniform angular sampling.

A new class of polar sampling patterns for projection imaging is described. In contrast to conventional techniques, a nonuniform variation of the angular increments between consecutive projections is introduced. The proposed method allows adaptation of the polar sampling pattern to different shapes of the object. Numerical simulations and MR experiments verify that angular sampling density can be varied according to the radial extension of the object, resulting in a reduction of the total number of required projections (and X-ray exposure) compared with conventional circular field-of-view imaging.

Technology, Radiologic↗

PubMedDate not supplied

Contrast-enhanced subtraction MR angiography in occlusive disease of the pelvic and lower limb arteries: results of a prospective intraindividual comparative study with digital subtraction angiography in 76 patients.

PURPOSE: The purpose of this work was to evaluate the feasibility and clinical use of MR angiography (MRA) for examining the pelvic and lower limb arteries in patients with arterial occlusive disease. METHOD: Seventy-six patients with clinical signs of peripheral arterial occlusive disease were included in the study. MRA was performed using a fast contrast-enhanced high-resolution 3D technique that covered the area from the distal abdominal aorta to the distal lower limbs in two examination steps. RESULTS: In all patients, diagnostic images comparable with those of conventional intraarterial digital subtraction angiography (DSA) could be obtained. No false-negative findings were seen in the iliac, femoral, or popliteal arteries. Ten to 16% of the mild stenoses and 6-14% of the severe stenoses, mainly in the crural vessels, were overgraded compared with intraarterial DSA. Particularly in patients with proximal severe obstructions or occlusions, the crural segments could be depicted more clearly due to decreased arterial runoff in conventional angiography. CONCLUSION: The consistency of the excellent depiction of the vascular territories of the distal aorta and the pelvic and lower limb arteries in a standardized setting suggests great potential for the use of MRA in the primary diagnosis of peripheral arterial occlusive disease.

Aged↗