Biomedical subjects

P Cofrancesco

Publications and source records attributed to P Cofrancesco.

3 recordsLinked to original sources

PubMed1999

Evolution strategy optimization for adiabatic pulses in MRI.

We propose a new type of adiabatic pulses for uniform inversion of the magnetization in magnetic resonance imaging. We produced these pulses with an evolution strategy optimization, by which the search of the "best solution" has been made more efficient than by deterministic algorithms. The pulse parametrization takes into account an "offset-independent adiabaticity condition," which guarantees insensitivity to RF inhomogeneities. The RF pulse power (both peak and mean) contributes to the cost to be minimized, as well as the error function does: in this way we obtain solutions that require lower energy than the well-known hyperbolic-secant pulse, with no loss of quality in the response profile.

Algorithms↗

PubMed1999

New adiabatic inversion pulses for magnetic resonance imaging.

We present a comparison between our strategy of computing new adiabatic pulses and the best results given so far in the literature by Rosenfeld and co-workers. Our technique has been described elsewhere and is based upon a simple physical idea (the adiabatic factor is offset independent) and an evolution strategy algorithm which stochastically searches for a 'solution' with optimal inversion profile and power characteristics. As expected for all adiabatic pulses, the inversion profiles are similar for our solutions and those of the literature. On the other hand, some of our pulses offer a substantial reduction of peak and average power relative to the solutions of Rosenfeld. We discuss the properties of the families of analytical functions, where, in our opinion, it is convenient to search for a pulse shape with optimal inversion profile and power characteristics.

Algorithms↗

PubMed1998

Evolution strategy optimization for selective pulses in NMR

We present a first set of improved selective pulses, obtained with a numerical technique similar to the one proposed by Geen and Freeman. The novelty is essentially a robust and efficient "evolution strategy" which consistently leads, in a matter of minutes, to "solutions" better than those published so far. The other two ingredients are a "cost function," which includes contributions from peak and average radiofrequency power, and some understanding of the peculiar requirements of each type of pulse. For example, good solutions for self-refocusing pulses and "negative phase excitation pulses" (which yield a maximum signal well after the end of the pulse) are found, as may have been predicted, among amplitude modulated pulses with 270 degrees tip angles. Emphasis is given to the search for solutions with low RF power for selective excitation, saturation, and inversion pulses. Experimental verification of accuracy and power requirements of the pulses has been performed with a 4.7 T Sisco imager. Copyright 1998 Academic Press.

Journal Article↗