Effects Of Radiofrequency Pulses Research Articles

Analysis of Electromagnetic Interference Effect of the Pulse Interference on the Navigation Receiver

Aiming at the problem that satellite navigation signals are easily interfered by the radio frequency (RF) pulse signal, the electromagnetic interference effect of RF pulse on the navigation receiver is studied in this paper. A mathematical model of the pulse interference signal is established, and we choose the bit error rate (BER) as an indicator of the quality of the BDS signal. It is found that the BER is proportional to the duty cycle of the pulse signal and inversely proportional to the equivalent carrier-to-noise ratio (C/N0) through simulation. Then, an experiment of electromagnetic injection on the BDS receiver has been carried out, which studied the influence of the pulse interference parameters, such as repetition frequency and duty cycle on the C/N0 of the BDS signal and the electromagnetic sensitivity threshold of the receiver. The comparative experiment between the pulse interference and the single-frequency continuous wave (CW) interference was also carried out, and we found that the effect of the pulse interference is better than that of single-frequency CW interference. The former is the correlation interference, and the latter is the blocking interference. Combined with the experiment phenomenon, the interference mechanism was further analyzed according to the relationship between the pulse period and the ranging code period of the navigation signal.

Theory of coherent averaging in magnetic resonance using effective Hamiltonians.

A perturbative approach based on multimode Floquet theory is proposed to explain the coherent averaging effects of radio frequency pulses on nuclear spins in magnetic resonance experiments. Employing effective Hamiltonians, a uniform description of the time evolution of spins under arbitrary multiple pulse schemes is presented. The choice of interaction frames and transformation functions desired for faster convergence of the perturbation series is identified based on the experimental conditions. We believe that the methodology outlined would be beneficial in the design and optimization of experiments beyond existing strategies.

A simple 5-DoF MR-compatible motion signal measurement system

The purpose of this study was to develop a simple motion measurement system with magnetic resonance (MR) compatibility and safety. The motion measurement system proposed here can measure 5-DoF motion signals without deteriorating the MR images, and it has no effect on the intense and homogeneous main magnetic field, the temporal-gradient magnetic field (which varies rapidly with time), the transceiver radio frequency (RF) coil, and the RF pulse during MR data acquisition. A three-axis accelerometer and a two-axis gyroscope were used to measure 5-DoF motion signals, and Velcro was used to attach a sensor module to a finger or wrist. To minimize the interference between the MR imaging system and the motion measurement system, nonmagnetic materials were used for all electric circuit components in an MR shield room. To remove the effect of RF pulse, an amplifier, modulation circuit, and power supply were located in a shielded case, which was made of copper and aluminum. The motion signal was modulated to an optic signal using pulse width modulation, and the modulated optic signal was transmitted outside the MR shield room using a high-intensity light-emitting diode and an optic cable. The motion signal was recorded on a PC by demodulating the transmitted optic signal into an electric signal. Various kinematic variables, such as angle, acceleration, velocity, and jerk, can be measured or calculated by using the motion measurement system developed here. This system also enables motion tracking by extracting the position information from the motion signals. It was verified that MR images and motion signals could reliably be measured simultaneously.

NMR spectroscopy explained: simplified theory, applications and examples for organic chemistry and structural biology

NMR spectroscopy explained: simplified theory, applications and examples for organic chemistry and structural biology

Use of computer algebra for the study of quadrupole spin systems

Analytical computation of the effect of radiofrequency pulses and free evolution between pulses are studied in a spin system dominated by the nuclear electric quadrupole interaction using ‘symbolic computer algebra’. The program MAPLE is used to compute spin density matrices for single spin systems with spin magnitude I = 1, 3/2, 2 and 5/2. In all cases except I = 5/2, the pure nuclear electric quadrupole Hamiltonian with arbitrary asymmetry parameter η is used to obtain results for a single pulse and an evolution period following the pulse. In the spin 5/2 system η is set to zero. The calculations are done using a simple matrix representation of the density operator and the Baker-Campbell-Hausdorff formula. In addition, the validity of dropping non-secular (i.e., time-dependent) terms from the Hamiltonian for the pulse in the quadrupole interaction frame is examined for the spin 1 case using a truncated Magnus expansion.

Theory of pulses in nuclear quadrupole resonance spectroscopy

The effect of radiofrequency pulses in nuclear quadrupole resonance (NQR) spectroscopy is examined. The description is different from that needed in nuclear magnetic resonance (NMR) spectroscopy. In particular, both rotating and counter-rotating radiofrequency field components are retained. Pulses in NQR spectroscopy are selective and cause transitions between two pairs of levels (±M) → ±(M + 1), other transitions are not normally excited. The formulation of pulses is then described for any spin I by two 2 × 2 rotation matrices, one causing an M → M + 1 transition and the other causing a - (M + 1) → -M transition. Calculations on resonance for spins with an axially symmetric nuclear quadrupole for up to three pulses are presented for spins I = 1 and I = 3/2. The results agree with the work of Reddy, R. and Narasimhan, P. T., 1991, Molec. Phys., 72, 491, in the appropriate limits.