Abstract
In MRI, at ultrahigh field, the use of parallel transmit radiofrequency (RF) arrays is very beneficial to better control spin excitation spatially. In that framework, the so-called “universal pulse” technique, proposed recently for head imaging at 7 tesla, gives access to “plug-and-play” nonadiabatic solutions exhibiting good robustness against intersubject variations in the resonant transmit fields. This new type of solution has been defined so far as the result of numerical pulse optimizations performed across a collection of RF field maps acquired on a small population sample (pulse design database). In this work, we investigate an alternative universal pulse design approach in the linear small tip angle regime whereby the database of RF field maps is first transformed into a second-order statistical model and which then exploits a statistical robust design formalism for the optimization of the RF and magnetic field gradient waveforms. Experimental validation with an eightfold transmit RF coil for 7 tesla brain imaging shows that this new approach brings some benefit in terms of computational efficiency. Hence, for a design database composed of 35 maps, the computation time initially of 50 min could be reduced down to 3 min. The proposed statistical approach thus enables integration of large databases, presumably necessary to ensure robust solutions. Finally, it provides means to compute flip angle statistics and, along with it, simple performance metrics for quality assurance (RF pulse performance) or guidance in the optimization of TX array architectures.
Highlights
In magnetic resonance imaging at high magnetic field, the apparition of standing wave effects [1,2,3] prevents uniform excitation of the spins across extended anatomical regions
To better control the actual performance of universal pulse (UP) on the entire population, we propose in this work to model the transmit RF field as a stochastic process, whereby every new subject is a realization of this process. is representation appears very useful as it allows formulating the action of a flip angle (FA) shimming pulse in statistical terms
One concrete application of this concept is the comparison of the circularly polarized (CP) mode ultimate performance (g⌣∞) with the pTX performance (g∞): for the Nova TX array, we found that the availability of eight independent TX channels to locally tune the drive mode so as to minimize the variance of the effective control allowed reducing the coefficient of variation (CV) of the flip angle by a factor of ≃ 2, while achieving close to the target on average
Summary
In magnetic resonance imaging at high magnetic field, the apparition of standing wave effects [1,2,3] prevents uniform excitation of the spins across extended anatomical regions. From the RF pulse design perspective, it provides an adequate framework to apply the socalled statistical robust design theory, proposed by Alotto et al [17], applied to the design of electromagnetic devices For the latter application manufacturing errors of the components or uncontrollable environmental conditions such as temperature or humidity can lead to uncertainties in the response of the system. We demonstrate in particular (1) the utility of statistical robust design approach to boost computational efficiency, (2) the capacity of the method to enable the integration of a large database TX field maps, and (3) the utility of a statistical framework to compute flip angle statistics and to derive simple performance metrics for quality assurance
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
