Evidence for attachment of magnetic starch microspheres to Kupffer cells receptors in excised and perfused rat liver.
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Animals↗
Biomedical subjects
R N Muller
Publications and source records attributed to R N Muller.
In vivo field dependence of proton relaxation times in human brain, liver and skeletal muscle: a multicenter study.
T1 and T2 relaxation times are fundamental parameters for signal contrast behaviour in MRI. A number of ex vivo relaxometry studies have dealt with the magnetic field dispersion of T1. By means of multicenter study within the frame of the COMAC BME Concerted Action on Tissue Characterization by MRI and MRS, the in vivo field dispersion of T1 and T2 has been measured in order to evaluate whether ex vivo data are representative for the in vivo situation. Brain, skeletal muscle, and liver of healthy human volunteers were studied. Fifteen MR units with a field strength ranging from 0.08 T to 1.5 T took part in the trial, which comprised 218 volunteers. All the MR systems were tested for measurement accuracy using the Eurospin TO5 test object. The measured relaxation data were subsequently corrected according to the obtained calibration curves. The results showed a clear field dispersion of T1, whereas no significant variations were seen for T2. Our in vivo data were generally in reasonable agreement with proposed models based on ex vivo measurements.
Adult↗
Optimization schemes for selective excitations: application to the DIGGER pulses.
In this work, two distinct approaches to the tailoring of selective radiofrequency (RF) pulses were applied to a cos-sinc pulse of the DIGGER sequence used in localized spectroscopy. Ideally, three such pulses combined with a gradient in each direction destroy the Mz component of the magnetization everywhere but in the volume of interest. In the first approach, the optimal RF amplitude modulation is found by minimizing the difference between the ideal and the computed Mz profiles. In the second strategy, the RF pulse envelope is adjusted by optimization of the tip angle profile. Computed slice profiles optimized by each of these methods have been compared to experimental results. Simulations as well as experiments show that the second approach, which optimizes tip angles, is the most efficient.
Magnetic Resonance Spectroscopy↗
Transverse relaxivity of particulate MRI contrast media: from theories to experiments.
Computer simulations and experimental approach have been used to characterize the properties of particulate MRI contrast agents with special attention paid to the influence of particle size. Up to approximately 50 nm, an increase of the particle diameter leads to a strong enhancement of the transverse magnetization decay rate. For larger grains or aggregates, the decay rate measured without spin-echo formation reaches a plateau. When observed through a spin-echo sequence, the transverse magnetization decay rate becomes slower on increasing the particle size or on shortening the echo time. For these large particles, multiexponential decay rates are observed. Definition and measurement of relaxivity in such systems is discussed.
Computer Simulation↗
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↗
Nuclear relaxation of human brain gray and white matter: analysis of field dependence and implications for MRI.
The dependence of 1/T1 on the magnetic field strength (the relaxation dispersion) has been measured at 37 degrees C on autopsy samples of human brain gray and white matter at field strengths corresponding to proton Larmor frequencies between 10 kHz and 50 MHz (0.0002-1.2 T). Additional measurements of 1/T1 and 1/T2 have been performed at 200 MHz (4.7 T) and 20 MHz (0.47 T), respectively. Absolute signal amplitudes are found to be proportional to the sample water content, not to the "proton density," and it is concluded that the myelin lipids do not contribute to the signal. Transverse magnetization decay data can be fitted with a triple exponential function, giving characteristic results for each tissue type, and are insensitive to variations of the pulse spacing interval. The longitudinal relaxation dispersion curves show characteristic shapes for each tissue type. The most striking difference is a large dispersion for white matter at very high fields. As a consequence, the relative difference in 1/T1 between gray and white matter shows a marked maximum around 10 MHz. Possible implications for MRI are discussed. A weighted least-squares fit of the dispersions has been performed using a four-parameter function of the form 1/T1 = 1/T1,w + D + A/(1 + (f/fc)beta'). The quality of the fit is superior to that of other functions proposed previously. The results of these fits are used to predict image contrast between gray and white matter at different field strengths.
Brain↗
Recent developments in design, characterization, and understanding of MRI and MRS contrast media.
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Contrast Media↗
Amiodarone pretreatment effects on ischemic isovolumic rat hearts: a P-31 nuclear magnetic resonance study of intracellular pH and high-energy phosphates contents evolutions.
Effects of amiodarone injected intravenously (i.v.) at two doses (10 and 20 mg/kg) on perfused isovolumic rat hearts were assessed by P-31 nuclear magnetic resonance (NMR). P-31 NMR is used to measure intracellular myocardial pH, phosphocreatine (PCr), and ATP contents time evolutions. Myocardial mechanical function is estimated by heart rate (HR), left ventricular developed pressure (LVP), and coronary flow (CF). In experimental procedure A (2-h retrograde perfusion), drug injection induced a dose-dependent bradycardia (10-20%) and a slight decrease in LVP but did not affect CF, pH, PCr, or ATP contents. Experimental procedure B consisted of 30-min stabilization, 18-min ischemia, and 72-min reperfusion. During ischemia, amiodarone did not preserve ATP and PCr pools and did not alleviate acidosis. ATP decreased to 30% of its control values, whereas the PCr peak was hardly detectable after 12 min of ischemia. After 24 min of reflow, HR, PCr, and pH of treated hearts recovered. LVP recovered after 36 min, whereas for control hearts, HR, PCr, and pH recovered after 42 min and LVP did not reach its control values at the end of reperfusion time. Faster pH recovery is explained by a preservation of Na+/K+ ATPase due to the influence of amiodarone on membrane lipid dynamics.
Adenosine Triphosphate↗
The effect of aging and storage conditions on excised tissues as monitored by longitudinal relaxation dispersion profiles.
Knowledge about the dependence of NMR parameters of excised tissues on time and storage conditions is important for correct interpretation of in vivo clinical results based upon in vitro measurements which in many cases are easier to perform. Literature data on this topic are scarce and sometimes contradictory. In this study, we investigated the change of the proton longitudinal relaxation rate (1/T1) of freshly excised rat tissues with storage conditions over a wide range of proton Larmor frequencies (0.01-50 MHz). Special interest was paid to long-term storage in the deep frozen state. After deep freezing of tissue samples on dry ice, no significant changes in 1/T1 over the whole frequency range could be observed for brain tissue, whereas for muscle and liver tissue characteristic alterations were detected. We conclude that for normal brain tissue this storage procedure is suitable for the prediction of in vivo 1/T1 results from in vitro studies.
Animals↗
The uncommon longitudinal relaxation dispersion of human brain white matter.
Human brain white matter exhibits an uncommonly strong dispersion of 1/T1 at high fields. An additional relaxation pathway via the myelin lipids, which are not directly visible to medical NMR due to their fast T2 decay, is proposed. Relaxation dispersion data are successfully analyzed on the basis of this model.
Body Water↗
The importance of nuclear magnetic relaxation dispersion (NMRD) profiles in MRI contrast media development.
Observation of the relaxivity of MRI contrast media over a wide range of magnetic fields is not only necessary for predicting their efficiency at any field but also compulsory for understanding and improving their mechanisms of action. The best experimental approach to this problem is the field cycling method, which allows the exploration of nuclear relaxation over a broad interval of magnetic field intensity but requires a specially dedicated instrument called a relaxometer. Particularly relevant are the relaxivity profiles of the two chelates Gd-DOTA and Gd-DTPA. Both show an important decrease from low to high fields within the current imaging range (0.02 T to 1.5 T). Although high field relaxivities of these chelates are similar, Gd-DTPA becomes less efficient in facilitating water protons relaxation at fields lower than 0.15 T. This behavior has to be related to different electronic relaxation times due to a different chelate symmetry.
Animals↗
Field-cycling relaxometry: medical applications.
Relaxometry between 10 kHz and 200 MHz (0.2 mT and 4.7 T) with a field-cycling device and a high-field-strength magnetic resonance (MR) unit permitted the determination of longitudinal relaxation rates of tissues and chemical compounds at numerous field strengths. The resulting nuclear magnetic relaxation dispersion profiles allowed the prediction of tissue contrast and efficacy of contrast agents at any field strength. Pure T1 contrast of normal brain tissue and pathologic lesions (multiple sclerosis, astrocytoma) increased from low field strengths to a maximum between 10 and 20 MHz and decreased afterward. Quadripolar dips reflecting the interaction between water and nitrogen atoms of the protein backbone appeared at 2.15 and 2.8 MHz, reducing T1 and opening the possibility of shorter imaging times and better tissue discrimination at these field strengths. Furthermore, it was shown that zero T1 contrast between normal and pathologic tissue samples may exist at certain field strengths. Gadolinium diethylenetriaminepentaacetic acid and gadolinium tetraazacyclododecanetetraacetic acid provided different contrast enhancement depending on the field strength.
Animals↗
[Abridged lexicon of terms in nuclear magnetic resonance].
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Humans↗
[Field and temperature dependence of contrast in magnetic resonance imaging].
Whereas a change of temperature within the physiological range does not lead to significant contrast alterations in MRI, changing magnetic field strength can lead to drastic alterations in tissue contrast caused by incalculable changes of T1 relaxation. At medium and high fields, longitudinal relaxation is mainly influenced by water content of tissues, at low and ultra-low fields macromolecular structures gain additional influence. Combination of relaxometric measurements with a field cycling device (between 0.0002 and 1.2 T, i.e. 10 kHz and 50 MHz) and imaging experiments at ultra-low, low, and medium fields (between 0.02 and 0.6 T) showed field depending inherent contrast changes with a T1 contrast maximum at medium fields. Induced contrast alterations by application of contrast changing substances in medium and high fields are unpredictable because there is no simple interrelationship between field strength and T1. Furthermore, it could be shown that T1 of pathologically altered tissue is not higher than T1 of normal tissue in any case but can be lower or equal below a certain field strength what contradicts the hypothesis that pathological tissue has a higher T1 than normal tissue.
Animals↗
[Contrast in magnetic resonance imaging].
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Humans↗
NMR approach of the periventricular white matter.
From a series of 117 neurological patients presenting a pathological periventricular white matter signal on NMR, the authors discuss the differential diagnosis possibilities based on the configuration of the lesions, on their localization in the brain, and on the calculated apparent T2 (T2**) values achieved with the single multi-echos technique.
Acquired Immunodeficiency Syndrome↗
[NMR-whole body tomography: a new imaging method].
NMR tomography (zeugmatography) can provide 2D and true 3D images of the human body. Advantages of this new imaging method in medicine are the possibilities to get both anatomic and functional images at one time without any invasive procedures. Furthermore, NMR has no ionizing radiation hazards. The spacial resolution of NMR tomography ranges between 0.5 and 3.0 mm. Within the next years, NMR tomography will replace several conventional X-ray, CAT scanning and nuclear medicine techniques. This article gives a brief survey of NMR basics and its medical applications.
Abdominal Neoplasms↗
[NMR whole-body tomography. Requirements for equipment and space].
First general considerations of the new NMR technique are given. Then disturbing influences on the magnet coming from outside as well as influences upon the environs coming from the magnet are pointed out. Next the inside arrangements of examination rooms are discussed. Finally accessory and emergency equipment for the examination rooms are mentioned.
Computers↗