Purpose: To aid in discussion about the mechanism for central brightening in high field magnetic resonance imaging (MRI), especially regarding the appropriateness of using the term dielectric resonance to describe the central brightening seen in images of the human head.
Materials and Methods:We present both numerical calculations and experimental images at 3 T of a 35-cm-diameter spherical phantom of varying salinity both with one surface coil and with two surface coils on opposite sides, and further numerical calculations at frequencies corresponding to dielectric resonances for the sphere. STRONG CENTRAL BRIGHTENING has been observed in magnetic resonance imaging (MRI) of the human head using volume coils at high B 0 field strength and high B 1 field frequency (1-3). Recently there has been some discussion in the literature about the appropriateness of attributing this brightening to dielectric resonance (4 -8), as it has been previously (1,2). While it is certain that B 1 wavelengths are on the order of dimensions of the human body at these frequencies, relatively high tissue conductivity inhibits the creation of strong resonances (4 -8), and alternative explanations for the observed central brightening are warranted.
ResultsA true dielectric resonance is characterized by relatively large oscillating electromagnetic fields in and around an object with a frequency of oscillation near a natural frequency for the object, and elicited by a relatively small stimulus near that natural frequency. The natural resonant frequencies for an object are determined by the geometry and electrical properties of the object, as well as the electrical properties of the surrounding medium. A single object typically has several different resonant modes at several different frequencies, each with a characteristic electromagnetic field pattern (8 -11). For some simple shapes, these frequencies and the characteristic field patterns can be calculated with analytical methods (11). But classically, the term resonance refers to field intensity as a function of frequency more than as a function of position.Although other authors have pointed out the lack of strong true resonances in biological tissues, they have not offered an easily understandable alternative explanation for observed central brightening. An enhancement of magnetic field strength near the center of an object can be created by a number of ways, and is not by itself evidence of a dielectric resonance. For example, midway between sources of traveling waves with currents in opposing directions (or 180°out of phase) is a location of constructive magnetic field interference at any frequency, provided the waves from each source travel through the same media for the same distance. This can occur even in an empty coil if the frequency is high enough for the wavelength to be on the order of the coil dimensions. We suggest that it is the combination of multiple current-carrying elements in the coil and wavelength effects in the sample leading to constructive and destructive interfe...
Es wird eine Einrichtung zur direkten Beobachtung der Aggregatbildung in geriihrtenLosungen beschrieben.The formation of aggregates in technical crystallizers is an important element in technical crystallization, but the investigation is very difficult. A device for direct observation of aggregation in stirred solutions is published.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.