Journal of Magnetic Resonance, Series B最新文献

An amplitude transformation of adiabatic pulses stated previously [Baumet al., Phys. Rev. A32, 3435–3447 (1985)] is generalized using the Bloch equations. Both amplitude and frequency transformations are used to create new adiabatic pulses of wide bandwidth and low RF power deposition. Several adiabatic pulses (including the tanh/tan pulse used for construction of BIR-4, SSAP, and BISEP pulses) are transformed into new pulses. These new pulses are demonstrated numerically and experimentally to operate at a significantly lower RF power level while maintaining the same performance or over a wider bandwidth while using the same RF power. They are expected to be useful forin vivoNMR experiments, especially for applications involving wide frequency dispersion and pulse sequences composed of many adiabatic half-passage pulses.

New double-quantum filtering (DQF) schemes are theoretically developed by reformulating the equations describing the double-quantum (DQ) signal. The equations describing the second- and third-rank DQ signals are simplified by restricting the RF phases as required for DQF. The equations are then factorized into two terms representing the separate contribution to the DQ signal from the RF pulses involved in the preparation and evolution times. This allows analysis of the DQ signal of a particular DQF scheme separately for each of these times in a concise manner. By use of the reformulated equations, the conventional DQF scheme is shown to be only one of four possible DQF schemes. The three new DQF schemes offer some desirable properties over the conventional DQF scheme. In the conventional DQF scheme, the third-rank DQ signal declines rapidly to null as the flip angles of the creation and readout RF pulses deviate from 90° to 54.7° or 125.3°. In addition, the second- and third-rank DQ signals in the conventional DQF scheme are opposite in their polarities, resulting in attenuation of the total DQ signal due to destructive interference between them. In one of three new DQF schemes, the DQ signal does not vanish at 54.7° and 125.3°, but varies smoothly with the same functional dependence on the RF flip angles as the second-rank DQ and triple-quantum signals. Furthermore, in two of the three new DQF schemes, the second- and third-rank DQ signals have the same polarity so that the total DQ signal may be enhanced through constructive interference between them. These features of new DQF schemes have been confirmed experimentally.

This paper presents the first simultaneous²H single- and double-quantum (SQ and DQ, respectively) diffusion study of excised brain tissue. The apparent diffusion coefficients (ADCs) of the²H SQ and DQ signals were measured at a fixed diffusion time (Δ − δ/3 = 21.3 ms) and as a function of the diffusion time to assess restricted diffusion [(Δ − δ/3) was changed from 21.3 to 271.3 ms]. As expected, the ADC of the SQ signal was higher than that of the DQ signal [0.53 ± 0.03 × 10⁻⁵(n= 3) and 0.30 ± 0.03 × 10⁻⁵cm²s⁻¹(n= 4), respectively]. When the ADCs of the SQ and DQ signals were measured as a function of the diffusion time, two components, a fast and a slow component, were observed in each case. The ADCs for the SQ signal were 1.16 ± 0.2 × 10⁻⁵and 0.35 ± 0.06 × 10⁻⁵cm²s⁻¹(n= 3) for the fast and the slow components, respectively. The ADCs for the DQ signal were 0.31 ± 0.05 × 10⁻⁵and ∼0.03 ± 0.03 × 10⁻⁵cm²s⁻¹(n= 2) with the slow component being relatively small. Interestingly, the slow-diffusion component of the SQ signal was found to have an ADC similar to that of the fast component of the DQ signal. These results suggest that brain water can be divided into at least three water populations and that the DQ signal originates from water molecules which interact with slow-diffusing structural components of the brain. The new insights that one can obtain using simultaneous SQ and DQ diffusion measurement and the ability to distinguish among water populations in biological tissues using the above approach are discussed.

The directed TOCSY pulse sequence element transfers coherence predominantly into “forward-directed” antiphase coherences while simultaneously suppressing in-phase and “backward-directed” antiphase coherences. This novel selection principle, based on the “direction” of the target coherences, provides a new approach for the simplification of crowded spectra. In this article, the theory of directed TOCSY is presented for linear spin systems that are frequently found in carbon-labeled biomolecules.

A new 3D Fourier imaging method based on sparse radial scanning (SRS-FT) ofkspace is proposed. It allows acquisition of FIDs and is therefore well suited to imaging objects with very shortT₂. Use of a Bayesian procedure allows (1) an important reduction of scan time to below that of the projection-reconstruction (PR) method by reducing the number of “Cartesian radial” encoding directions, and (2) a good image quality by estimating missing and corrupted Cartesian samples. SRS-FT images reconstructed from FIDs are compared to conventional FT and PR images.

The spin-lock saturation transfer experiment introduced by B. Adams and L. Lerner (J. Magn. Reson.96, 604–607, 1992) is analyzed in terms of the Bloch equations. It is shown that theT_1ρrelaxation of the solvent is introduced in the decay of the exchangeable protons under conditions of relatively rapid exchange. An alternative experiment is suggested that randomizes the solvent magnetization with a pulsed field gradient before the observe pulse. This gives a single exponential intensity decay for the exchanging protons at all exchange rates. In addition, efficient water suppression and an even excitation profile are obtained.