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Excitation Pulses (excitation + pulse)
Selected AbstractsImprovement of vessel visibility in time-of-flight MR angiography of the brainJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 6 2008Takaharu Shonai MD Abstract Purpose To improve vessel visibility in time-of-flight MR angiography (TOF-MRA) by careful consideration of coil choice, coil position, and frequency offset and profile of the nonspatially selective chemical shift selective (CHESS) presaturation pulse. Materials and Methods The effects of both the CHESS and the excitation radiofrequency (RF) pulses on flow signal and signals from stationary substances were evaluated by changing the spatial area where RF pulses were applied to upstream flow in a flow phantom and in human subjects. The difference between the eight-channel phased-array receive-only coil and the transmit,receive coil was evaluated. Results The CHESS pulse suppresses the flow signal over a wider frequency range than the signals from stationary substances, especially when using the body coil for transmission. Even without presaturation pulse, the excitation pulse slightly suppressed the flow signal. Adjusting the position of the transmit,receive coil relative to the head improved these TOF-MRA images. The results were better than those obtained with the eight-channel coil. Conclusion The excitation and the nonspatially selective CHESS pulses degraded the flow signal. Our results suggest that reduced spatial extent of RF pulse application to upstream flow can improve image quality of TOF-MRA. This result can be implemented on conventional scanners. J. Magn. Reson. Imaging 2008;27:1362,1370. © 2008 Wiley-Liss, Inc. [source] Real-time MR temperature mapping of rabbit liver in vivo during thermal ablationMAGNETIC RESONANCE IN MEDICINE, Issue 2 2003Claudia Weidensteiner Abstract It has been shown that quantitative MRI thermometry using the proton resonance frequency (PRF) method can be used to noninvasively monitor the evolution of tissue temperature, and to guide minimally-invasive tumor ablation based on local hyperthermia. Although hepatic tumors are among the main targets for thermal ablation, PRF-based temperature MRI of the liver is difficult to perform because of motion artifacts, fat content, and low T. In this study the stability of real-time thermometry was tested on a clinical 1.5 T scanner for rabbit liver in vivo. The fast segmented EPI principle was used together with respiratory gating to limit respiratory motion artifacts. Lipid signal suppression was achieved with a binomial excitation pulse. Saturation slabs were applied to suppress artifacts due to flowing blood. The respiratory-gated MR thermometry in the rabbit liver in vivo showed a standard deviation (SD) of 1,3°C with a temporal resolution of 3 s per slice and 1.4 mm × 1.9 mm spatial resolution in plane (slice thickness = 5 mm). The method was used to guide thermal ablation experiments with a clinical infrared laser. The estimated size of the necrotic area, based on the thermal dose calculated from MR temperature maps, corresponded well with the actual lesion size determined by histology and conventional MR images obtained 5 days posttreatment. These results show that quantitative MR temperature mapping can be obtained in the liver in vivo, and can be used for real-time control of thermal ablation and for lesion size prediction. Magn Reson Med 50:322,330, 2003. © 2003 Wiley-Liss, Inc. [source] On the transient phase of balanced SSFP sequencesMAGNETIC RESONANCE IN MEDICINE, Issue 4 2003Klaus Scheffler Abstract The signal intensity of balanced steady-state free precession (SSFP) imaging is a function of the proton density, T1, T2, flip angle (,), and repetition time (TR). The steady-state signal intensity that is established after about 5*T1/TR can be described analytically. The transient phase or the approach of the echo amplitudes to the steady state is an exponential decay from the initial amplitude after the first excitation pulse to the steady-state signal. An analytical expression of the decay rate of this transient phase is presented that is based on a simple analysis derived from the Bloch equations. The decay rate is a weighted average of the T1 and T2 relaxation times, where the weighting is determined by the flip angle of the excitation pulses. Thus, balanced SSFP imaging during the transient phase can provide various contrasts depending on the flip angle and the number of excitation pulses applied before the acquisition of the central k -space line. In addition, transient imaging of hyperpolarized nuclei, such as 3He, 129Xe, or 13C, can be optimized according to their T1 and T2 relaxation times. Magn Reson Med 49:781,783, 2003. © 2003 Wiley-Liss, Inc. [source] Non-invasive temperature imaging with thulium 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetramethyl-1,4,7,10-tetraacetic acid (TmDOTMA,)NMR IN BIOMEDICINE, Issue 1 2006Sait Kubilay Pakin Abstract Non-invasive thermometry using hyperfine-shifted MR signals from paramagnetic lanthanide complexes has attracted attention recently because the chemical shifts of these complexes are many times more sensitive to temperature than the water 1H signal. Among all the lanthanide complexes examined thus far, thulium tetramethyl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (TmDOTMA,) appears to be the most suitable for MR thermometry. In this paper, the feasibility of imaging the methyl 1H signal from TmDOTMA, using a frequency-selective radiofrequency excitation pulse and chemical shift-selective (CHESS) water suppression is demonstrated. A temperature imaging method using a phase-sensitive spin-echo imaging sequence was validated in phantom experiments. A comparison of regional temperature changes measured with fiber-optic probes and the temperatures calculated from the phase shift near each probe showed that the accuracy of imaging the temperature with TmDOTMA, is at least 0.1,0.2°C. The feasibility of imaging temperature changes in an intact rat at 0.5,0.6,mmol/kg dose in only a few minutes is demonstrated. Similar to commonly used MRI contrast agents, the lanthanide complex does not cross the blood,brain barrier. TmDOTMA, may prove useful for temperature imaging in many biomedical applications but further studies relating to acceptable dose and signal-to-noise ratio are necessary before clinical applications. Copyright © 2006 John Wiley & Sons, Ltd. [source] Piezoelectric effects in sidewall quantum wires grown on patterned (311)A GaAs substratePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 5 2003D. Alderighi Abstract Time-resolved photoluminescence measurements have been performed on sidewall InGaAs/AlGaAs quantum wires and quantum wells. Experimental data show a band filling of quantum wells and quantum wires that is dominant in the first 400 ps after the excitation pulse, then a dynamical screening of the built-in piezoelectric field (Fp) by means of fast injection of photo-generated charges is observed allowing an efficient radiative recombination. At longer time delay, during the regime when the Fp unscreened value is recovered, a strong quantum confined Stark effect is observed. A good agreement is obtained for the energy shift and the overlap integrals of electrons and heavy holes by means of discrete element calculations. [source] Assessing arterial blood flow and vessel area variations using real-time zonal phase-contrast MRIJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 3 2006Markus Oelhafen PhD Abstract Purpose To measure peripheral artery function using a real-time phase-contrast (PC)-MRI sequence with tailored image-processing algorithms for flow computation. Materials and Methods An approach to real-time flow measurements was developed based on two-dimensional spatially selective excitation pulses and consecutive tailored processing of the data to derive blood flow and vessel area variations. The data acquisition strategy allows for flow measurements at high spatial and temporal resolutions of 1 mm2 and 50 msec, respectively. In postprocessing the vessel area is automatically extracted using correlation measures in conjunction with morphological image operators. By means of in vitro and in vivo validations, it is shown that the current methods provide accurate and reproducible measurements of flow and vessel area variations. Results In vitro the comparison between the lumen area measured with the presented method and the values obtained by calliper gauge measurement showed a difference of 3.4% ± 3.4% (mean ± 2 SD). Similarly, the comparison between the stroke volumes determined with the presented method and by stopwatch and bucket measurements yielded a difference of 6.1% ± 2.1%. In vivo the results from the real-time measurements for lumen area and stroke volume were compared with those from a gated PC-MRI technique with differences of 4.8% ± 14% and 3.0% ± 24.7%, respectively. Conclusion The presented method constitutes a reliable tool set for quantifying the variations of blood flow and lumen area in the superficial femoral artery during reactive hyperemia and for studying their correlation with cardiovascular risk factors. J. Magn. Reson. Imaging 2006. © 2006 Wiley-Liss, Inc. [source] On the transient phase of balanced SSFP sequencesMAGNETIC RESONANCE IN MEDICINE, Issue 4 2003Klaus Scheffler Abstract The signal intensity of balanced steady-state free precession (SSFP) imaging is a function of the proton density, T1, T2, flip angle (,), and repetition time (TR). The steady-state signal intensity that is established after about 5*T1/TR can be described analytically. The transient phase or the approach of the echo amplitudes to the steady state is an exponential decay from the initial amplitude after the first excitation pulse to the steady-state signal. An analytical expression of the decay rate of this transient phase is presented that is based on a simple analysis derived from the Bloch equations. The decay rate is a weighted average of the T1 and T2 relaxation times, where the weighting is determined by the flip angle of the excitation pulses. Thus, balanced SSFP imaging during the transient phase can provide various contrasts depending on the flip angle and the number of excitation pulses applied before the acquisition of the central k -space line. In addition, transient imaging of hyperpolarized nuclei, such as 3He, 129Xe, or 13C, can be optimized according to their T1 and T2 relaxation times. Magn Reson Med 49:781,783, 2003. © 2003 Wiley-Liss, Inc. [source] Volume tracking cardiac 31P spectroscopyMAGNETIC RESONANCE IN MEDICINE, Issue 2 2002Sebastian Kozerke Abstract The limited reliability and accuracy of cardiac spectroscopy have been partly attributed to effects from respiratory motion. In this work, we developed a prospective volume tracking method for respiratory motion compensation based on multiple navigator echoes and demonstrated its application in cardiac 31P spectroscopy. The sequence consists of two 2D selective excitation pulses preceding the spectroscopic experiment to sample respiratory motion components. The navigator information is evaluated in real-time to calculate the shift of the heart from respiration. Based on the displacement information, the spectroscopic volume and/or grid position is prospectively corrected to track the volume of interest. The method was validated with a moving compartment phantom simulating in vivo respiratory motion. With volume tracking, no signal contamination was apparent. Spectra obtained in 14 healthy volunteers were evaluated using time-domain fitting procedures. The fitting accuracy improved consistently with volume tracking compared to data from non-navigated reference acquisitions. Compared to other gating approaches available for spectroscopy, the current technique does not degrade the scan efficiency, thus allowing effective use of scan time. Magn Reson Med 48:380,384, 2002. © 2002 Wiley-Liss, Inc. [source] T1 quantification with inversion recovery TrueFISPMAGNETIC RESONANCE IN MEDICINE, Issue 4 2001Klaus Scheffler Abstract A snapshot FLASH sequence can be used to acquire the time course of longitudinal magnetization during its recovery after a single inversion pulse. However, excitation pulses disturb the exponential recovery of longitudinal magnetization and may produce systematic errors in T1 estimations. In this context the possibility of using the TrueFISP sequence to detect the recovery of longitudinal magnetization for quantitative T1 measurements was examined. Experiments were performed on different Gd-doped water phantoms and on humans. T1 values derived from inversion recovery TrueFISP were in excellent agreement with the single-point method even for flip angles up to 50°. In terms of T1 accuracy and SNR, the proposed method seems to be superior to the conventional inversion recovery snapshot FLASH technique. Magn Reson Med 45:720,723, 2001. © 2001 Wiley-Liss, Inc. [source] The design of excitation pulses for spin systems using optimal control theory: With application to NMR spectroscopyOPTIMAL CONTROL APPLICATIONS AND METHODS, Issue 5 2009Naum I. Gershenzon Abstract This paper considers the use of optimal control theory in designing radio frequency excitation pulses for magnetic spin systems satisfying Bloch dynamics. Such pulses are required in applications of nuclear magnetic resonance to initially transfer sample magnetization vectors to the transverse plane. Once transferred, signals released by nuclei as they respond to a static magnetic field normal to the transverse plane are then analyzed and interpreted. Continuous time deterministic optimal control theory is employed to determine time-dependent pulse amplitudes and frequencies that minimize the distance between final magnetization vectors and a chosen target vector. Pulses are designed to excite a range of resonant frequencies and to tolerate miscalibration errors in applied fields. The model presented permits a unified treatment of the control problem as considered by a variety of authors, and a thorough mathematical analysis of the existence, and characteristics of, optimal excitation pulses. Practical numerical algorithms for designing optimal pulses are given, and the effectiveness of the algorithms is illustrated by comparing the pulses that they generate with those commonly used in high-resolution spectroscopy. Copyright © 2008 John Wiley & Sons, Ltd. [source] Simultaneous optical coherent control of excitonic and biexcitonic polarization in a ZnSe quantum wellPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 5 2003Tobias Voss Abstract The optical coherent-control technique is used to study biexcitonic effects in the four-wave-mixing signal of a ZnSe single quantum well. The signal is analyzed in both directions 2k1 , k2 and 2k2 , k1 which are not equivalent if a pulse pair is applied from direction k1 to achieve coherent control of the induced polarization. It is shown that the coherent control enables a selective enhancement or suppression of the contribution at the exciton and biexciton resonance to the signal, respectively, but only for certain sequences of the excitation pulses. Further, the suppression of exciton-biexciton beats in the signal as a function of tdel by a selective destruction of the biexciton polarization is demonstrated. [source] `Making the molecular movie': first framesACTA CRYSTALLOGRAPHICA SECTION A, Issue 2 2010R. J. Dwayne Miller Recent advances in high-intensity electron and X-ray pulsed sources now make it possible to directly observe atomic motions as they occur in barrier-crossing processes. These rare events require the structural dynamics to be triggered by femtosecond excitation pulses that prepare the system above the barrier or access new potential energy surfaces that drive the structural changes. In general, the sampling process modifies the system such that the structural probes should ideally have sufficient intensity to fully resolve structures near the single-shot limit for a given time point. New developments in both source intensity and temporal characterization of the pulsed sampling mode have made it possible to make so-called `molecular movies', i.e. measure relative atomic motions faster than collisions can blur information on correlations. Strongly driven phase transitions from thermally propagated melting to optically modified potential energy surfaces leading to ballistic phase transitions and bond stiffening are given as examples of the new insights that can be gained from an atomic level perspective of structural dynamics. The most important impact will likely be made in the fields of chemistry and biology where the central unifying concept of the transition state will come under direct observation and enable a reduction of high-dimensional complex reaction surfaces to the key reactive modes, as long mastered by Mother Nature. [source] | ||||||||||