Radiofrequency Power Requirements Research Articles

A Numerical Analysis of Radio-Frequency Power Requirements in Magnetic Resonance Imaging Experiment

In this paper, a numerical analysis of the radio-frequency (RF) power requirements in magnetic resonance imaging (MRI) is presented at frequencies that span 200-362 MHz. This was performed utilizing an anatomically detailed human head model and a high-frequency RF coil utilized in high-field MRI systems. For axial slices through the brain region, it is demonstrated that the power required in order to obtain an average flip angle across the slice increases with frequency plateauing at a certain value, and then dropping as the frequency increases. The results demonstrate the significance of the electromagnetic interactions between the load and coil and their effects on the so important issue of power-frequency dependence in MRI.

MARS—a project of the diffraction-limited fourth generation X-ray source based on supermicrotron

The new approach for the fourth generation X-ray source—Multiturn Accelerator–Recuperator Source (MARS)—was proposed recently. The installation consists of the radiofrequency (RF) multiturn accelerator (similar to the race-track microtron) and long undulator(s). After passing through the undulator(s) the electron beam is decelerated in the same RF accelerating structure. Such energy recovery reduces dramatically the radiation hazard and decreases the required RF power. In this paper we present a more detail explanation of this scheme, and specify further the parameter limitations and requirements for the accelerator.

Imaging of water and fat fractions in high-field MRI with multiple slice chemical shift-selective inversion recovery.

Fat and water fractions were quantified at high field using a chemical shift-selective inversion recovery (CSS-IR) sequence to address the major difficulties encountered at high field by phase-sensitive techniques used for fat/water discrimination. Water- and fat-suppressed images were perfectly registered, which is a prerequisite for quantification. Immunity of the inversion pulse to B(1) field modulations and off-resonance effects was tested for two adiabatic inversion pulses, with hyperbolic secant and asymmetric hyper-pulse waveforms. Taking into account adiabaticity, immunity to off-resonance, radiofrequency power requirements, and specific absorption rate, the former was chosen. A close correlation was found between fat content measured by CSS-IR at 4.7 T (R(2)= 0.97, P < 0.001) and 9.4 T (R(2)= 0.99, P < 0.05) and that measured by Soxhlet extraction. Different sources of bias (low signal-to-noise ratio, magnetization transfer effect) are discussed. J. Magn. Reson. Imaging 2000;12:488-496.

Radio frequency integrated circuit technology for low-power wireless communications

The radio frequency portion of a handheld wireless communications device represents a significant fraction of the battery drain of the overall unit. As a result, an intensive effort is underway on a worldwide basis to minimize the overall radio frequency power requirements. This effort has proceeded on many fronts, with the best work drawing on the multidisciplinary efforts of researchers in the communications systems, circuit design, and semiconductor technology areas. This article summarizes the significant technological challenges to the realization of low-power radio frequency circuits, and follows with a discussion of the research programs that have been put in place to address these challenges. RF transceiver architectures are discussed in particular together with performance aspects.

On self-refocusing pulses for rapid gradient-echo imaging.

Self-refocusing slice selective radio frequency pulses could potentially be used to reduce imaging times in rapid gradient-echo sequences. However, in practice, the improvement is likely to be no greater than about 20%, and at the expense of much increased radio frequency power requirements.

Cross polarization using a time-averaged precession frequency. A simple technique to reduce radiofrequency power requirements for magnetization transfer experiments in solids

The introduction of the cross-polarization technique made it possible to overcome the low sensitivity of dilute spins, and provided a means of observing the NMR signals of those nuclei in solids (I). However, this method cannot readily be applied to nuclei with low gyromagnetic ratios, such as, 57Fe, ‘03Rh, ‘*‘OS, etc., because to achieve the Hartmann-Hahn condition (2) with those nuclei, very high values of the rf power are required. Recently, Bax et al. (3) have proposed an interesting new technique utilizing off-resonance effects (4) which effectively reduce the rf power requirements for crosspolarization. While Bax et al. demonstrated the successful application of their method for a static sample of 15N-enriched glycine, they noted that there are several difficulties with their technique. In that method, rf inhomogeneity can easily cause a mismatch of the Hartmann-Hahn condition, and the technique is experimentally difficult to apply. Another method of cross-polarization using multiple-pulse irradiation on the Z spins has been proposed by Quiroga and Virlet (5). In this method, an HW-8 multipulse sequence (6) was employed to perform two conflicting demands at the same time: (i) to suppress the homonuclear dipolar interaction, and (ii) to provide a spin-locking field for the Z spins. Since a perfectly tuned WAHUHA-4 (7) type homonucleardecoupling pulse sequence cannot provide the Z spin-locking field, one has to introduce the pulse length “error” into the pulse sequence without destroying the homonucleardecoupling effect. Therefore, in the Quiroga and Virlet method, it is difficult to control the Z spin-locking field strength provided by the pulse length “error.” We present an alternative method to reduce the rf power requirements for the cross-polarization of nuclei with small gyromagnetic ratios, which we find to be easy to apply, and which is based on the time average of the precession frequency. Recently, we have shown that by alternating the phase of the Z spin-locking rf field, the precession of an Z spin isochromat around the effective field is time reversed (8). In a tilted rotating frame, the time-evolution operator describing the overall precession of the Z spin isochromat around the effective field after a one cycle time tc = t, + t2 of the phase-alternating spin-locking pulse sequence (Fig. 1) can be written as

CONFINEMENT OF LASER‐PRODUCED PLASMA IN RESONANT CAVITIES BY RF ELECTROMAGNETIC FIELDS

Pressure and energy balance calculations are described for such a system. Radiofrequency power requirements and maintenance of a superconductor near a thermonuclear plasma are discussed. Some requirements for an actual experiment are given. (MOW)

The electron accelerator Desy

Before giving a short report of the present status of the electron accelerator at Hamburg, let me describe very briefly the principal features of the design. The electron accelerator is a strong focusing synchrotron for accelerating electrons up to about 6 GeV. Such a machine differs in two essential features from a proton synchrotron: (i) At an injection energy of 40 MeV, the velocity of the electrons is practically equal to that of light, so no frequency modulation of the R. F. accelerating units is necessary. (ii) A very intense synchrotron radiation is emitted when an electron moves in a transverse magnetic field. Since the radiated power is inversely proportional to the fourth power of the rest mass of the accelerated particles, the radiation from electrons is about 10 13 times more intense than that from protons with the same energy. The radiation loss at 6 GeV amounts to 3.6 MeV per turn; so the radio-frequency power requirements are greatly increased. In addition, the motion of the particles is strongly influenced by radiation forces, and while the vertical betatron oscillations are damped, the radial betatron oscillations are rather strongly antidamped. Figure 102 demonstrates this effect. Further, particles can be lost from the stable region of the synchrotron oscillations by too small a radio-frequency amplitude of the accelerator units. Since the radiation loss per turn is inversely proportional to the radius, it was necessary to choose a rather large radius of curvature for the accelerator; namely 31.7 m. One accelerates 50 times per second to keep small the excursions of the particles from the mean orbit. The acceleration time is therefore about 10 ms. Figure 103 shows the dependence with energy of the oscillation amplitudes. One sees that the amplitudes reach a minimum in the region of 2 to 3 GeV, due to the adiabatic damping of the synchro­tron and betatron oscillations, and afterwards they increase due to the radiation effects.

The feasibility of a superconducting proton linear accelerator

The use of superconducting resonators for a proton linear accelerator is shown to reduce greatly the radio-frequency power requirements and, in principle, to make a c.w. high intensity machine practicable. Rough estimates of costs do not preclude the proposal on economic grounds. Some of the technical problems involved are also discussed.