Function Of Flip Angle Research Articles

Rapid determination of the RF pulse flip angle and spin–lattice relaxation time for materials imaging

For samples with T 1s longer than 10 s, calibration of the RF probe and a measurement of T 1 can be very time-consuming. A technique is proposed for use in imaging applications where one wishes to rapidly obtain information about the RF flip angle and sample T 1 prior to imaging. The flip angle measurement time is less than 1 s for a single scan. Prior knowledge of the RF flip angle is not required for the measurement of T 1. The resulting time savings in measuring the values of flip angle and T 1 are particularly significant in the case of samples with very long T 1 and short T 2 ∗ . An imaging extension of the technique provides RF flip angle mapping without the need for incrementing the pulse duration, i.e., RF mapping can be performed at fixed RF amplifier output.

Hyperpolarized 3He lung ventilation imaging with B 1-inhomogeneity correction in a single breath-hold scan

The signal-to-noise ratio (SNR) of hyperpolarized noble gas MR images is sensitive to the flip angle used. Variations in flip angle due to B1-inhomogeneity of the RF coil cause intensity variation artifacts in lung ventilation images which may mask or mimic disease. We show these artifacts can be minimized by using the optimal flip angle and corrected if the local flip angle is known. Hyperpolarized 3He lung images were obtained in ten healthy subjects using both a conventional gradient-echo sequence and a new hybrid pulse sequence designed to simultaneously acquire lung ventilation images and corresponding flip-angle maps in comparable imaging time. Flip-angle maps and corrected images were calculated from the hybrid scan and compared with conventional images. The qualitative theoretical dependence of flip angle on SNR was verified. Ventilation images and flip-angle maps were successfully obtained with the hybrid sequence. Corrections to image intensity calculated from the flip-angle maps appeared reasonable for images acquired using an average flip angle near optimal. Use of the optimal flip angle is crucial to the quality of lung ventilation images. Artifactual intensity variations due to RF-coil inhomogeneity may be identified and potentially corrected using our hybrid sequence.

MR microimaging of the lung using volume projection encoding.

Radial acquisition (RA) techniques have been extended to produce isotropic, three-dimensional images of lung in live laboratory animals at spatial resolution down to 0.013 mm3 with a signal-to-noise ratio of 30:1. The pulse sequence and reconstruction algorithm have been adapted to allow acquisition of image matrices of up to 256(3) in less than 15 min. Scan-synchronous ventilation has been incorporated to limit breathing motion artifacts. The imaging sequence permits randomizing and/or discarding selected views to minimize the consequences of breathing motion. The signal in lung parenchyma was measured as a function of flip angle (alpha) for different repetition times and found to follow the predictions for which there is an optimum excitation (Ernst) angle. A single T1 relaxation value of 780 +/- 54 ms fits all data from six guinea pigs at 2.0 T. This T1 value parameterizes the signal and allows for a priori optimization, such as calculation of the Ernst angle appropriate for lung imaging.

Radio frequency field intensity mapping using a composite spin-echo sequence

A novel radio frequency (RF) field intensity mapping or imaging method using a composite NMR spin-echo sequence is proposed. A composite spin-echo RF pulse with 90° y -180° x -90° y sequence makes phase change in the final image depending on the RF field intensity on the object. The resultant phase change or phase map can be used to obtain the actual RF flip-angle map for a given condition which includes the status of tuning and RF inhomogeneity, etc. Bloch equation has been solved numerically to obtain the effects of the RF field intensity as well as the main magnetic field inhomogeneity and the results are used for the mapping (imaging) of the RF field intensity. Phantom studies have been performed using a 1.5 Tesla whole body MRI system and the results are presented.

Signal-to-noise optimization and observed volume localization with circular surface coils

Sensitivity optimization in surface coil NMR is complicated by the inhomogeneity (or shape) of the B 1 field produced by the surface coil antenna. The sensitivity and the size and location of the observed volume are dependent upon experimental conditions. Therefore, we have derived three-dimensional signal intensity maps for a single-turn, flat, circular coil operating on resonance in the single-coil mode to determine the effect of experimental conditions on the sensitivity and spatial characteristics of the observed volume. The results indicate that rapid repetition rates ( T ⪡ T 1) provide optimum sensitivity and contribute significant signal intensity from regions not immediately adjacent to the coil. Intermediate repetition rates ( T ∼ T 1) provide a significant sensitivity increase over slow repetition rates ( T ⩾ 5 T 1) while localizing the volume observed primarily to regions adjacent to the coil. For homogeneous samples further separated from the coil (0.2–0.4 radii), longer pulse widths provide signal from a large region of the sample with a minimum loss in sensitivity. This analysis is verified by agreement of calculated and experimental total intensities as a function of flip angle (pulse width), relaxation time ( T 1), and repetition rate. Furthermore, the results from 2H (30 MHz) and 19F (188 MHz) NMR experiments with ionic and nonionic solutions indicate that rf attenuation problems associated with conductive samples are not likely to be significant in surface coil NMR experiments that utilize small diameter coils at high fields.

FT NMR of nonequilibrium states of complex spin systems. II. Weakly coupled systems

Since the Fourier transform NMR spectrum of a nonequilibrium state may be different from the slow-passage spectrum, a theory is developed which describes the phenomenon in any weakly coupled system, including those with magnetic equivalence. Liouville space techniques are used to derive simple expressions for the line intensities as a function of flip angle, in terms of Legendre polynomials. The theory uses F 2 representations directly and is applicable to any weakly coupled system. The results are tabulated for A m X n systems, m, n = 1, 2, 3, and 6. The theory is still applicable when. the weak coupling approximation starts to break down and line intensities are perturbed. It is shown that if the nonequilibrium intensities are corrected by a simple factor, as in the equilibrium case, these systems may be treated as if they were weakly coupled. In practice, the theory may be used to extrapolate to the slow-passage, zero flip angle spectrum. Furthermore, different parts of a degenerate line may be separated by means of the flip angle dependence. As an illustration, a nonequilibrium state was created among the protons in ethyl acetate. The experimental flip angle dependence of its spectra is well represented by the theory.

FT NMR of nonequilibrium states of complex spin systems. I. A Lionville space description

We present a theoretical description of pulse Fourier transform NMR experiments which is well adapted to multispin systems. It uses Liouville space with a spherical tensor basis. Liouville space lets us work directly with the density matrix and the use of spherical tensors greatly simplifies the calculations. The superoperators associated with the Hamiltonian and the pulses act in Liouville space like familiar angular momentum operators. Their matrix elements are easily calculated and the blocking due to symmetry can be exploited. We use this method to describe the effect of an arbitrary pulse on an AB spin system. If the system is not in equilibrium then its FT spectrum may depend on flip angle. The rotation associated with the pulse is described completely by three Wigner matrix elements. We derive explicit expressions for the line intensities of the AB system as a function of flip angle.