Magnetic resonance phase difference techniques are commonly used to study flow velocities in the human body. Acceleration is often present, either in the form of pulsatile flow, or in the form of convective acceleration. Questions have arisen about the exact time point at which the velocity is encoded, and also about the sensitivity to (convective) acceleration and higher order motion derivatives. It has become common practice to interpret the net phase shifts measured with a phase difference velocity technique as being the velocity at a certain (Taylor) expansion time point, chosen somewhere between the RF excitation and the echo readout. However, phase shifts are developed over the duration of the encoding magnetic field gradient wave form, and should therefore be interpreted as a more or less time-averaged velocity. It will be shown that the phase shift as measured with a phase difference velocity technique represents the velocity at the "gravity" center of the encoding bipolar gradient (difference) function, without acceleration contribution. Any attempt to interpret the measured phase shift in terms of velocity on any other time point than the gradient gravity point will automatically introduce acceleration sensitivity.
This paper describes an implementation of a quadrature cross-coupled relaxation oscillator to be used in an OFDM RF frontend transceiver. A prototype of the oscillator was realized in a Si-Ge BiCMOS technology, and an oscillator frequency of 5.8 GHz was obtained, which is 1/6 of the maximum f/sub T/ of the bipolar transistors. The circuit uses two first order relaxation oscillators to which a level control stage has been added to make the circuit parameters less interdependent and to allow a higher frequency, without the need to further reduce the passive components values. The circuit performance is evaluated by simulation and by experiment.
Purpose To develop a free‐breathing (FB) 2D radial balanced steady‐state free precession cine cardiac MRI method with 100% respiratory gating efficiency using respiratory auto‐calibrated motion correction (RAMCO) based on a motion‐sensing camera. Methods The signal from a respiratory motion‐sensing camera was recorded during a FB retrospectively electrocardiogram triggered 2D radial balanced steady‐state free precession acquisition using pseudo–tiny‐golden‐angle ordering. With RAMCO, for each acquisition the respiratory signal was retrospectively auto‐calibrated by applying different linear translations, using the resulting in‐plane image sharpness as a criterium. The auto‐calibration determines the optimal magnitude of the linear translations for each of the in‐plane directions to minimize motion blurring caused by bulk respiratory motion. Additionally, motion‐weighted density compensation was applied during radial gridding to minimize through‐plane and non‐bulk motion blurring. Left ventricular functional parameters and sharpness scores of FB radial cine were compared with and without RAMCO, and additionally with conventional breath‐hold Cartesian cine on 9 volunteers. Results FB radial cine with RAMCO had similar sharpness scores as conventional breath‐hold Cartesian cine and the left ventricular functional parameters agreed. For FB radial cine, RAMCO reduced respiratory motion artifacts with a statistically significant difference in sharpness scores ( P < 0.05) compared to reconstructions without motion correction. Conclusion 2D radial cine imaging with RAMCO allows evaluation of left ventricular functional parameters in FB with 100% respiratory efficiency. It eliminates the need for breath‐holds, which is especially valuable for patients with no or impaired breath‐holding capacity. Validation of the proposed method on patients is warranted.
Beim oben genannten Beitrag ist ein Fehler in der Institutszuordnung der Autoren korrigiert worden. D. Thomas ist der Radiologische Universitätsklinik, Rheinische Friedrich-Wilhelms Universität Bonn zuzuordnen.
To evaluate the effect of dual-source parallel radiofrequency (RF) transmission with patient-adaptive local RF shimming on image quality, image contrast, and diagnostic confidence at routine clinical cardiac magnetic resonance (MR) imaging with use of a 3.0-T dual-channel transmit whole-body MR system.Written informed consent was obtained from all patients, and the study protocol was approved by the local institutional review board. Cardiac MR imaging was performed in 28 patients by using a 3.0-T MR unit equipped with a dual-source RF transmission system. The effect of conventional versus dual-source RF transmission on steady-state free precession (SSFP) cine sequences and turbo spin-echo (TSE) black-blood (BB) sequences was evaluated. The homogeneity of the B1 field and contrast-to-noise ratios (CNRs) were measured and tested for statistical significance with the paired t test. Images were analyzed qualitatively for homogeneity, the presence of off-resonance artifacts, and diagnostic confidence independently by two readers. Statistical significance was assessed with the Wilcoxon signed rank test. Inter- and intraobserver agreement was assessed with κ statistics.Quantitative image analysis revealed that B1 homogeneity and CNR were significantly improved for images acquired with dual-source RF transmission compared with conventional RF transmission (P = .005). The quality of SSFP and TSE BB images of the left and the right ventricles showed a significant improvement with respect to image homogeneity and diagnostic confidence as evaluated by the readers (P = .0001, κ > 0.74). As a side effect, off-resonance artifacts were significantly reduced on SSFP images (P = .0001, κ > 0.76).Dual-source parallel RF transmission significantly improves image homogeneity, image contrast, and diagnostic confidence compared with conventional RF transmission of cardiac SSFP and TSE BB sequences.
A low-power transadmittance amplifier is presented, that suppresses EMI by suppression of the detection mechanism as well as a compensation technique. The detection is suppressed by cancellation of the even harmonics. Compensation is achieved with the help of a dummy amplifier that reproduces the disturbance. The presented measurement results show that especially the compensation suppresses the EMI quite well.
To describe the effects of noise in translinear filters, large-signal equations have to be used, due to the internal non-linearities and the nonstationary nature of the noise sources. In this paper, a noise analysis method is proposed, which takes into account the non-linear and non-stationary aspects. As an example, the signal/spl times/noise intermodulation is calculated for a class A and a class AB operated log-domain filter.
Current noise analysis techniques for translinear and other companding filters are unsuitable for circuits that contain intentional dynamic nonlinearities. The authors present a method for treating dynamic nonlinearities in a straightforward manner using stochastic differential equations.
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