Population inversion of a selected region of a spectrum is a concept which has wide application in both NMR spectroscopy and imaging. While inversion of population at any one frequency is a trivial matter, ensuring an accurate inversion over a specified bandwidth, with negligible perturbation of the magnetization outside that bandwidth, is a major problem. However, by using as a driving function a complex radiofrequency (r.f.) pulse with an envelope of the form (sech beta t)1+5i where 1/beta is the temporal width and t is time, we have found that above a critical r.f. power threshold, magnetization is accurately inverted over a very sharply defined bandwidth, while outside that region, magnetization is returned to its initial position, and population is unaffected. Within the broad limits imposed by our equipment, we have also discovered that the phenomenon is independent of the incident r.f. power.
A theory of quadrature detection in the laboratory frame is developed. It is shown that the geometry of the two orthogonal coil systems needed for quadrature detection is radically different from that used with saddle-shaped coils, and that the homogeneity of the B1 field produced upon transmission is marginally better. The opposing quadrature phase shifts needed for transmission and reception are emphasized, and the use of a quadrature hybrid is advocated as a simple and inexpensive means of interfacing the transmitter, probes, and preamplifier. Experimental results are presented which confirm the theoretical predictions, and show that up to a 40% improvement in sensitivity and a twofold reduction in transmitter power are possible, particularly in those instances where the sample is conductive--namely, imaging of humans and in vivo spectroscopy.
The image displayed in computed tomography is a scaled representation of attenuation coefficients within the patient's body. A number of authors have presented methods by which additional information (such as electron density, effective atomic number, and extrapolated attenuation coefficients for therapy applications) can be extracted from CT scans carried out at different energies. In the present paper, the dual-energy method described by Rutherford has been used to produce complete images of effective atomic number and electron density of a known phantom (the AAPM phantom) in order to investigate the usefulness of applying this method to current commercial scanners.
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.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.