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Biomedical subjects

S Duewell

Publications and source records attributed to S Duewell.

31 records · Page 2Linked to original sources
PubMed1991

Nonionic polyethylene glycol-ferrioxamine as a renal magnetic resonance contrast agent.

The MR relaxation properties of ferrioxamine-B, a chelate of iron, were investigated in vitro and in vivo to establish the potential use of the compound as a paramagnetic contrast agent. Whereas the paramagnetic relaxivity of ferrioxamine-B is such that, compared to gadolinium-DTPA (Gd-DTPA), two to three times higher concentrations are necessary to produce the same relaxation effects, the toxicity of the iron ion should be much lower because of the availability of physiological metabolic pathways. Preliminary experiments in three dogs under invasive cardiovascular monitoring demonstrated that high-dose bolus application (0.1-0.3 mmol/kg body weight) of ferrioxamine-B leads to a precipitous blood pressure drop to almost zero, lasting for several minutes. This reaction seems most likely the result of a negative inotropic effect of ferrioxamine-B. In order to reduce these side effects ferrioxamine was modified to a nonionic derivative, PEG-ferrioxamine-B. In vivo experiments with this compound did not demonstrate any substantial change in blood pressure. Dynamic MR imaging of the kidneys and the liver was performed after bolus injection of the compound in six dogs. The results indicate that PEG-ferrioxamine-B produces effects very similar to Gd-DTPA, resulting in T1-mediated signal intensity increases in the liver and in the early stages of passage through the kidneys. During the phase of medullary concentration, T2 effects seem to dominate visualization of the renal medulla. The nonionic PEG-ferrioxamine-B derivative appears to offer an alternative to gadolinium-containing chelates as an MR contrast agent.

Animals↗
PubMed1990

MRI signal loss due to microcirculation: phantom studies.

In order to study perfusion effects in MRI under different conditions we developed two different kinds of phantoms. The first phantom exhibits linear capillary flow with several capillaries within one voxel. The second consists of an anion exchange resin with beads forcing the spins to change directions such that they undergo accelerations and decelerations. Both phantoms were imaged with standard spin-echo sequences and signal intensities were quantified at various echo times. Qualitative and quantitative agreements of the data with the results obtained by computations of signal loss due to spin-phase phenomena are excellent, thus suggesting that perfusion effects can be fully understood using these phenomena. It is argued that the phantoms used in conjunction with conventional spin-echo sequences represent a realistic model for studying true capillary networks in conjunction with specialized perfusion sequences.

Humans↗
PubMed1990

[MRT and CT in alveolar echinococcosis of the liver].

To compare the value of MRI and CT in evaluating hepatic alveolar echinococcosis a study was conducted on 30 patients. The liver was initially affected in all patients. At the time of examination, 15 patients had undergone partial liver resection. MRI showed no advantage over CT in demonstrating an echinococcal mass lesion. MRI was superior in identifying concomitant pathological changes of the intrahepatic and extrahepatic venous system due to the intrinsic contrast of vascular structures. However, CT was superior in identifying calcifications, an important attribute of the disease. MRI should only be used in imaging alveolar echinococcosis of the liver if diagnostic questions remain open after CT, in particular questions concerning venous pathology. MRI may also be used to replace CT in patients with a contraindication to urographic contrast material.

Adult↗
PubMed1990

A pharmacokinetic and MRI study of unilamellar gadolinium-, manganese-, and iron-DTPA-stearate liposomes as organ-specific contrast agents.

Small unilamellar liposomes that contain the lipophilic chelate DTPA-stearate (DTPASA) were used as carriers for the paramagnetic metal ions gadolinium, manganese, and iron. The iron liposomes were unstable in vitro and thus not studied further. The natural targeting properties of these liposomes to the reticuloendothelial system was used in rats and dogs for the imaging of liver and spleen. In vitro incubations with human plasma, followed by high-pressure liquid chromatography (HPLC) separation of the Gd-DTPASA and Mn-DTPASA liposomes showed that after an incubation period of 24 hours, only 4% of the gadolinium was bound to the plasma proteins, whereas, with the Mn-DTPASA liposomes, a transfer of 40% manganese was seen. These results indicate that the Mn-DTPASA complex is not stable. On T1-weighted images, both liposome preparations gave a strong signal enhancement of the organs of the mononuclear phagocyte system (MPS). Gadolinium liposomes accumulated in the liver of rats at a peak concentration 4 hours after application and at a higher concentration compared with the manganese liposomes. Gd-DTPASA liposomes had an elimination half-time from the liver of 61 hours. Manganese liposomes produced stronger contrast at lower concentrations and had faster elimination kinetics from the liver, with a major elimination half-time of 10 hours. Both chelate complexes were eliminated predominantly by the hepatobiliary route. Thus, liposomal Gd-DTPASA appears to be a stable, efficient, and specific magnetic resonance imaging (MRI) contrast agent for the upper abdomen.

Animals↗
PubMed1988

[Diagnostic imaging in the evaluation of heart transplant recipients].

Cardiac transplantation has become an important treatment option in terminal cardiac disease since the introduction of cyclosporin A for immunosuppression. Imaging methods play an important role in the diagnosis of all complications occurring after heart transplantation except for the transplant rejection reaction; endomyocardial biopsy is in continued use as none of the diagnostic imaging methods is able at present to diagnose rejection with sufficient accuracy. However, new imaging modalities that are only just starting to be widely used may change this in the foreseeable future.

Echocardiography↗
PubMed1986

Uptake of a monoclonal antibody against CEA (Tumak 431/31) in a human colon tumor (Co-112) xenografted in the nude mouse. Dependence on tumor size and injected dose.

A monoclonal antibody (Tumak) against carcinoembryonic antigen (CEA) was injected into nude mice bearing a human colon carcinoma (Co-112). The tumor uptake was found to be dependent on the size of the tumors: relative uptake (percentage of the injected dose/gram tumor (% i.d./g) decreased for tumors with weights up to approximately 1 g, although the absolute uptake (% i.d./tumor) still increased over the same weight range. In the constant region (greater than or equal to 1 g) mean relative tumor uptake was approximately 4% i.d./g. The same tumor size dependence was found for the relative Tumak uptake in the other mouse organs studied (e.g., blood, liver, spleen and muscle). Consequently tumor/organ ratios were found to be independent of tumor size. Tumor uptake was also studied for various doses of Tumak (0.07-120 micrograms) in tumors of approximately 1 g. Evidence was found for a threshold dose of 0.1 microgram under which no serious tumor uptake appeared. From 1 to 120 micrograms no further dependence of Tumak distribution on applied dose was found: the relative uptake of all organs remained the same but the absolute uptake increased with dose.

Animals↗
PubMedDate not supplied

Quantitative MR signal behavior in cardiac cine loops as a function of heart rate.

To evaluate the heart cycle-dependent signal intensity changes in the cardiac chambers, the aorta, and the pulmonary artery, five healthy volunteers were studied with gradient-echo magnetic resonance cine loops at different heart rates. Quantitative evaluation of signal intensity on each side of the cardiac valves showed that there were changes in signal intensity due to section-entry and spin-phase phenomena but none due to the increase in heart rate. The authors conclude that there is no heart rate-dependent signal loss in healthy persons that simulates valvular dysfunction, thus suggesting that signal intensity change can be used as an indicator for this disease, independent of heart rate.

Adult↗
PubMedDate not supplied

Single-shot versus interleaved echo-planar MR imaging: application to visualization of cardiac valve leaflets.

Visualization of the cardiac valves with standard magnetic resonance (MR) imaging is not adequate because of long acquisition times. Echo-planar imaging (EPI) can, however, be performed with a temporal resolution (30-50 msec) comparable to that of echocardiography. The authors evaluated the feasibility of real-time imaging of cardiac valve motion with ultrafast MR techniques. Eight healthy volunteers and three patients with mitral stenosis and regurgitation were studied with a 1.5-T whole-body imager. Two different EPI sequences were assessed: a standard single-shot gradient-echo EPI (GEPI) sequence and a fast imaging technique based on multiple-shot EPI with interleaved k-space acquisition (IGEPI). Fat-suppressed images with an in-plane resolution of 3.7 x 3.7 mm were obtained equally spaced through the cardiac cycle. Half-k-space acquisition was used. Morphologic evaluation was superior with IGEPI, owing to the better intracavitary signal homogeneity (P < .01), and the mitral valve leaflets were easier to identify on systolic images. IGEPI provided adequate valve visibility in all three patients.

Adult↗
PubMedDate not supplied

Radiofrequency shielding of surface coils at 4.0 T.

Because radiation loss associated with a radiofrequency (RF) coil increases as roughly the fourth power of the frequency, this loss mechanism may become important in high-field studies above 2.0 T. In this study, the contribution of radiation losses at 4.0 T were determined in a rectangular surface coil using an RF shield to modify the radiation losses. The effect of this shield was determined on coil Q, B1 distribution, and signal to noise as a function of distance between the coil and the shield. Phantoms and human tissue were evaluated to characterize the loss mechanisms. The results demonstrate a large radiation loss in the unshielded surface coil. However, the radiation losses in vivo were not dominant owing to a large inductive loss occurring from dielectric currents in the body at 170 MHz.

Artifacts↗
PubMedDate not supplied

MRI of the foot and ankle: diagnostic performance and patient acceptance of a dedicated low field MR scanner.

The objective of this study was to compare image quality and patient acceptance of a dedicated .2-T MR system and a 1.0-T whole body system. Forty-one consecutive patients referred for MRI of the foot or ankle were prospectively examined with a dedicated .2-T low field system and a 1.0-T whole body system. Images were evaluated qualitatively by two observers and quantitatively using signal-difference-to-noise ratios. The patients were interviewed with respect to positioning, examination time, noise, claustrophobia, confidence in the diagnosis, and willingness to repeat the examination, using a questionnaire. The qualitative score was significantly higher for the 1.0-T system (2.6 vs 2.2 for reader 1 [P = .008] and 2.6 vs 1.7 for reader 2 [P < .0001]), respectively). The signal-difference-to-noise ratios were also superior for the 1.0-T MR system (2.96 vs .88, P < .0001). However, 96% of the lesions visualized at 1.0 T were also detected with the low field system. Patient acceptance was significantly better for the 1.0-T MR scanner (48.6 vs 43.9, P = .007). Image quality of the dedicated low field system was inferior to the 1.0-T system using objective parameters, and patients did not prefer the low field system. Although only 4% of lesions were missed in this series, the low field MR system can only be recommended when funding is limited and the available space is limited.

Adult↗
PubMedDate not supplied

Semiautomated ROI analysis in dynamic MR studies. Part II: Application to renal function examination.

Fast MR techniques and the application of water-soluble contrast agents allow the simultaneous examination of renal morphology and the functional aspects of glomerular filtration using bolus injections of Gd-DTPA. Spatial resolution is sufficient to resolve individual renal pyramids, but the quantitative examination of regions of interest (ROIs) is severely impeded by organ movements due to variations of the end-inspiratory position. A new image-processing scheme has been used and tested in 23 normal volunteers and patients. This scheme replaces a tedious frame-by-frame ROI analysis by positional correction of renal regions of all frames of the sequence such that the definition of the regions has to be performed only once. The signal intensities (SIs) of the local regions in each frame are used to compute statistics and to generate curves representing local temporal SI changes due to contrast agent excretion. The success rate of the procedure depends largely on the image quality and on the adherence to a proved acquisition protocol. The present article shows that the combination of MR and robust and reliable image-processing methods can be important for the highly automated analysis of a large number of images acquired as dynamic studies.

Adult↗