Biomedical subjects

K Scheffler

Publications and source records attributed to K Scheffler.

48 records · Page 3Linked to original sources

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

Detection of BOLD changes by means of a frequency-sensitive trueFISP technique: preliminary results.

A new method is introduced to detect magnetic field modulation arising from brain activation-induced BOLD effects. This approach uses a two-dimensional high-bandwidth, high-resolution conventional gradient-echo steady-state imaging sequence known as TrueFISP. The ability to visualize changes in oxygen saturation comes from the fact that the method is sensitive to the local field. As is well known, as the oxygen saturation changes so does the local field associated with the venous blood. We demonstrate that it is possible to visualize not only venous blood with this approach on a macroscopic level for major veins, but also to measure conventional oscillatory like signal changes during activated and resting states. Unfortunately, the method has two major drawbacks. First, a long TR is needed to maximize signal changes and, second, the field must be very well shimmed or numerous experiments need to be run to find the activation, as the signal response is sensitive to the starting frequency in the resting state. Nevertheless, these images can be compared directly with anatomical images collected with the same method without the need for distortion correction.

Brain↗

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↗

PubMedDate not supplied

Optimization of contrast-enhanced MR angiography of the hands with a timing bolus and elliptically reordered 3D pulse sequence.

Our objective was to optimize bolus administration and sequence setting in gadolinium-enhanced magnetic resonance (MR) angiography of the hands. Elliptically reordered three-dimensional (3D) spoiled gradient-echo sequence with non-slab-selective radio frequency excitation was optimized according to the measurements of arterial and venous time-signal curves in 21 patients. Great variations in bolus arrival time and arterio-venous transit time could be observed. In most patients high-quality arterial depiction could be obtained with minor venous contamination. Contrast-to-noise, spatial resolution, and selective arterial filling is still a challenge for 3D MR angiography of the hand but can be optimized using Gadolinium-BOPTA and a dedicated pulse sequence setting with exact bolus timing.

Adolescent↗

PubMedDate not supplied

Flavin enzymes, mitochondrial radicals and reduced glutathione in daily rhythmic dependency.

The diurnal rhythms of the microsomal flavoprotein NADPH-cytochrome c reductase activity, of diaphorase and of succinic dehydrogenase are presented. Minimum levels are ascertained at 09(00), maximum levels at 21(00). The concentration of mitochondrial radicals as a function of the time of day is also demonstrated. Here too the minimum is at 09(00) and the maximum between 15(00) and 21(00). On the other hand, GSH levels are found to be high between 09(00) and 12(00) and low in the evening. Thus a causative relationship between the concentration of cellular radicals, which originate in flavin enzymes, and the concentration of the tripeptide glutathione is assumed.

Animals↗