[Show abstract][Hide abstract] ABSTRACT: The theoretical shapes of nuclear spin-noise spectra in NMR are derived by
considering a receiver circuit with finite, preamplifier input impedance and a
transmission line between the preamplifier and the probe. Using this model, it
becomes possible to reproduce all observed experimental features: variation of
the NMR resonance linewidth as a function of the transmission line phase,
nuclear spin-noise signals appearing as a "bump" or as a "dip" superimposed on
the average electronic noise level even for a spin system and probe at the same
temperature, pure in-phase Lorentzian spin-noise signals exhibiting
non-vanishing frequency shifts. Extensive comparison to experimental
measurements validate the model predictions, and define the conditions for
obtaining pure in-phase Lorentzian-shape nuclear spin noise with a vanishing
frequency shift, in other words, the conditions for simultaneously obtaining
the Spin-Noise and Frequency-Shift Tuning Optima.
The Journal of Chemical Physics 07/2015; 143(9). DOI:10.1063/1.4929783 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Transmit arrays have been developed to mitigate the RF field inhomogeneity commonly observed in high field Magnetic Resonance Imaging (MRI), typically above 3 T for a human chest and 7 T for a human brain. To this end, the knowledge of the RF complex-valued transmit-sensitivities of each independent radiating element has become essential. This article details a method to speed up a currently available -calibration method. The principle relies on slice undersampling, slice and channel interleaving and kriging, an interpolation method developed in geostatistics and applicable in many domains. It has been demonstrated that, under certain conditions, kriging gives the best estimator of a field in a region of interest. The resulting accelerated sequence allows mapping a complete set of 8 volumetric field maps of the human head in about one minute. For validation, the accuracy of kriging is first evaluated against a well-known interpolation technique based on Fourier transform as well as to a -maps interpolation method presented in the literature. This analysis is carried out on simulated maps and on a decimated set of experimental maps measured with a standard sequence. Finally, the new accelerated sequence is compared to the standard sequence on a phantom and a volunteer. While mapping is becoming a routine for transmitting array coils, the new sequence will provide maps at least three times faster with a loss of accuracy limited potentially to about 5%.
[Show abstract][Hide abstract] ABSTRACT: A new set-up for exposure of human cells in vitro at 37°C to pulse-modulated 300 MHz and 500 MHz signals of future magnetic resonance imaging (MRI) systems is designed, built up, and characterized. Two dipole antennas, specifically designed for ultra high field MRI, are used as radiating structures. The electromagnetic (EM) field distribution inside the incubator containing the cells is computed, and it is shown to be in a good agreement with measurements. The electric field at the cell level is quantified numerically. Local, 1g average, and averaged over the culture medium volume SAR are provided along with the standard deviation values for each well. Temperature increments are measured inside the culture medium during the exposure using an optical fiber thermometer. Then we identify the pulse parameters corresponding to the thermal threshold of 1°C, usually considered as a threshold for thermally-induced biological effects. For these parameters, the induction of heat shock proteins is assessed to biologically verify a potential thermal response of cells. The data demonstrate that, under the considered experimental conditions, exposure to pulsemodulated radiations emulating typical ultra high field MRI signals, corresponding to temperature increments below 1°C, does not trigger any heat shock response in human brain cells.
[Show abstract][Hide abstract] ABSTRACT: To investigate, via numerical simulations, the compliance of the specific absorption rate (SAR) versus temperature guidelines for the human head in magnetic resonance imaging procedures utilizing parallel transmission at high field.
A combination of finite element and finite-difference time-domain methods was used to calculate the evolution of the temperature distribution in the human head for a large number of parallel transmission scenarios. The computations were performed on a new model containing 20 anatomical structures.
Among all the radiofrequency field exposure schemes simulated, the recommended 39°C maximum local temperature was never exceeded when the local 10-g average SAR threshold was reached. On the other hand, the maximum temperature barely complied with its guideline when the global SAR reached 3.2 W/kg. The maximal temperature in the eye could very well rise by more than 1°C in both cases.
Considering parallel transmission, the recommended values of local 10-g SAR may remain a relevant metric to ensure that the local temperature inside the human head never exceeds 39°C, although it can lead to rises larger than 1°C in the eye. Monitoring temperature instead of SAR can provide increased flexibility in pulse design for parallel transmission.
Journal of Magnetic Resonance Imaging 06/2012; 35(6):1312-21. DOI:10.1002/jmri.23542 · 3.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: One of the promises of Ultra High Field (UHF) MRI scanners is to bring finer spatial resolution in the human brain images due to an increased signal to noise ratio. However, at such field strengths, the spatial non-uniformity of the Radio Frequency (RF) transmit profiles challenges the applicability of most MRI sequences, where the signal and contrast levels strongly depend on the flip angle (FA) homogeneity. In particular, the MP-RAGE sequence, one of the most commonly employed 3D sequences to obtain T1-weighted anatomical images of the brain, is highly sensitive to these spatial variations. These cause deterioration in image quality and complicate subsequent image post-processing such as automated tissue segmentation at UHF.
[Show abstract][Hide abstract] ABSTRACT: Transmit arrays have been developed to compensate for radiofrequency inhomogeneities in high-field MRI using different excitation schemes. They can be classified into static or dynamic shimmings depending on the target: homogenizing the radiofrequency field directly or homogenizing the flip angle distribution using the Bloch equation. We have developed an intermediate solution to compare shimming performances between different transmit arrays. This solution, called generalized double-acquisition imaging, is easier to implement than most dynamic shimming methods and offers more degrees of freedom than static shimmings. It uses two acquisitions so that the second acquisition complements the excitation of the first one to obtain by superposition an image that minimizes radiofrequency artefacts. For validation, the method is demonstrated experimentally for a gradient echo sequence on a spherical homogeneous phantom and by simulation on a human head model.
Magnetic Resonance in Medicine 01/2012; 67(1):175-82. DOI:10.1002/mrm.23006 · 3.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: With Transmit SENSE, we demonstrate the feasibility of uniformly exciting a volume such as the human brain at 7T through the use of an original minimalist transmit k-space coverage, referred to as "k(T) -points." Radio-frequency energy is deposited only at a limited number of k-space locations in the vicinity of the center to counteract transmit sensitivity inhomogeneities. The resulting nonselective pulses are short and need little energy compared to adiabatic or other B 1+-robust pulses available in the literature, making them good candidates for short-repetition time 3D sequences at high field. Experimental verification was performed on three human volunteers at 7T by means of an 8-channel transmit array system. On average, whereas the standard circularly polarized excitation resulted in a 33%-flip angle spread (standard deviation over mean) throughout the brain, and a static radio-frequency shim showed flip angle variations of 17% and up, application of k(T) -point-based excitations demonstrated excellent flip angle uniformity (8%) for a small target flip angle and with sub-millisecond durations.
Magnetic Resonance in Medicine 01/2012; 67(1):72-80. DOI:10.1002/mrm.22978 · 3.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Introduction: The possibility of high local SAR values can be a limiting factor to in-vivo transmit-SENSE applications at high field [1,2]. In this work we introduce a novel method to reduce the local SAR and demonstrate its application based on simulations.