Alessandro Sbrizzi

Fast design of local N-gram-specific absorption rate-optimized radiofrequency pulses for parallel transmit systems.

Designing multidimensional radiofrequency pulses for clinical application must take into account the local specific absorption rate (SAR) as controlling the global SAR does not guarantee suppression of hot spots. The maximum peak SAR, averaged over an N grams cube (local NgSAR), must be kept under certain safety limits. Computing the SAR over a three‐dimensional domain can require several minutes and implementing this computation in a radiofrequency pulse design algorithm could slow down prohibitively the numerical process. In this article, a fast optimization algorithm is designed acting on a limited number of control points, which are strategically selected locations from the entire domain. The selection is performed by comparing the largest eigenvalues and the corresponding eigenvectors of the matrices which locally describe the tissues amount of heating. The computation complexity is dramatically reduced. An additional critical step to accelerate the computations is to apply a multi shift conjugate gradient algorithm. Two transmit array setups are studied: a two channel 3 T birdcage body coil and a 12‐channel 7 T transverse electromagnetic (TEM) head coil. In comparison with minimum power radiofrequency pulses, it is shown that a reduction of 36.5% and 35%, respectively, in the local NgSAR can be achieved within short, clinically feasible, computation times. Magn Reson Med, 2012.

Time efficient design of multi dimensional RF pulses: application of a multi shift CGLS algorithm.

Designing multi dimensional ratio frequency excitation pulses in the small flip angle regime commonly reduces to the solution of a least squares problem, which requires regularization to be solved numerically. Usually, regularization is carried out by the introduction of a penalty, λ, on the solution norm. In most cases, the optimal regularization parameter is not known a priori and the problem needs to be solved for several values of λ. The optimal value can be selected, typically by plotting the L‐curve. In this article, a conjugate gradients‐based algorithm is applied to design ratio frequency pulses in a time‐efficient way without a priori knowledge of the optimal regularization parameter. The computation time is reduced considerably (by a factor 10 in a typical set up) with respect to the standard conjugate gradients for least square since just one run of the algorithm is required. Simulations are shown and the performance is compared to that of conjugate gradients for least square. Magn Reson Med, 2011.

Optimal control design of turbo spin-echo sequences with applications to parallel-transmit systems

The design of turbo spin‐echo sequences is modeled as a dynamic optimization problem which includes the case of inhomogeneous transmit radiofrequency fields. This problem is efficiently solved by optimal control techniques making it possible to design patient‐specific sequences online.

Robust reconstruction of B1+ maps by projection into a spherical functions space

Several parallel transmit MRI techniques require knowledge of the transmit radiofrequency field profiles (B1+). During the past years, various methods have been developed to acquire this information. Often, these methods suffer from long measurement times and produce maps exhibiting regions with poor signal‐to‐noise ratio and artifacts. In this article, a model‐based reconstruction procedure is introduced that improves the robustness of B1+ mapping.

Transmit and receive RF fields determination from a single low-tip-angle gradient-echo scan by scaling of SVD data

A new method, called Transmit and Receive Patterns from Low‐Tip‐angle gradient‐Echo Images (TRIPLET), is described which simultaneously maps the B1+ and B1− fields of a transmit/receive radiofrequency coil array. The input data are low‐tip‐angle gradient‐echo images, which can be acquired in a relatively short scanning time.

Fast 3D isotropic imaging of the aortic vessel wall by application of 2D spatially selective excitation and a new way of inversion recovery for black blood imaging

Aortic vessel wall imaging requires large coverage and a high spatial resolution, which makes it prohibitively time‐consuming for clinical use. This work explores the feasibility of imaging the descending aorta in acceptable scan time, using two‐dimensional (2D) spatially selective excitation and a new way of inversion recovery for black blood imaging.

Improving the arterial input function in dynamic contrast enhanced MRI by fitting the signal in the complex plane

Dynamic contrast enhanced (DCE) imaging is a widely used technique in oncologic imaging. An essential prerequisite for obtaining quantitative values from DCE‐MRI is the determination of the arterial input function (AIF). However, it is very challenging to accurately estimate the AIF using MR. A comprehensive model, which uses complex data instead of either magnitude or phase, was developed to improve AIF estimation.

RF peak power reduction in CAIPIRINHA excitation by interslice phase optimization

The purpose of this work was to show that the overall peak power of RF pulses for CAIPIRINHA excitation can be substantially reduced by applying interslice phase relaxation. The optimal phases are scan dependent and can be quickly calculated by the proposed method.

Thermal noise variance of a receive radiofrequency coil as a respiratory motion sensor.

Development of a passive respiratory motion sensor based on the noise variance of the receive coil array.

Combining a reduced field of excitation with SENSE-based parallel imaging for maximum imaging efficiency

To show that a combination of parallel imaging using sensitivity encoding (SENSE) and inner volume imaging (IVI) combines the known benefits of both techniques. SENSE with a reduced field of excitation (rFOX) is termed rSENSE.