Small tip angle three‐dimensional tailored radiofrequency slab‐select pulse for reduced B₁ inhomogeneity at 3 T

Abstract

This work presents a small tip-angle 3D tailored RF slab-select pulse for reducing the B 1 field inhomogeneity at 3T. The compensated slice profile was determined from a B 1 inhomogeneity map. SNR improvement and degree of artifact reduction were evaluated in a NiCl 2 doped phantom and human brains. The technique was found to reduce inhomogeneities as large as 30% of the peak image magnitude in all three spatial directions in the brain using a standard head coil. (4,5). This is a result of the decreased RF wavelength, coupled with the dielectric and conductive properties of the body, producing spatially dependent flip angles and receiver sensitivities. Methods have been proposed to mitigate B 1 inhomogeneity including coil designs (6,7), adiabatic pulses (8), active transmit power modulation (9), and impulse 2D pulses (10). Although improved coils are necessary, they alone may not be able to remove all inhomogeneity because the effect also depends on sample geometry and its physical properties. Adiabatic pulses are problematic because of increased SAR. Active transmit power modulation works well if coil geometry is the dominant source of inhomogeneity. Impulse 2D pulses are also effective; however, they are not slab selective. Furthermore, a 3D correction may be useful because the B 1 variation has been shown to be larger than 10% over a 10-cm diameter in a birdcage coil at 3 T (11). We propose using 3D tailored RF (TRF) pulses (12-15) to reduce B 1 inhomogeneity at 3T. The pulses use a map of the inhomogeneity and are implemented for slab selection in 3D imaging. We demonstrate the technique using a NiCl 2 doped phantom and normal human brains. THEORYIgnoring relaxation and off-resonance, the signal s(t) at time t for a gradient echo and small tip angles ␣(r) can be written as[1]Here V is the coil volume, G acq (s) are the acquisition gradients, and C(r) is the receive sensitivity. The spatial dependencies of ␣(r) and C(r) reflect the effect of an inhomogeneous B 1 field and will be present in the reconstructed image I(r) of the spin density (r):The small tip approximation (16) equates the pulse waveform P(t) to the Fourier transform of the slice profile, W(k(t)), weighted by the k-space velocity produced by the excitation gradients G(t):This equation is valid for tip angles less than 30°. In the above equation, W(k(t)) is defined as W͑k͑t͒͒ ϭ ͐W͑r͒e ik͑t͒⅐r dr, where k͑t͒ ϭ Ϫ ␥ ͵ t T G͑s͒ds. [4]The trajectory k(t) is equal to the area remaining under the gradients at time t until the end of the pulse T. The term ⌬(k(t)) is one over the sampling density (17 In this equation W 0 (r) is the desired uniform 3D slice profile.

Investigating the effect of exchange and multicomponent T1 relaxation on the short repetition time spoiled steady‐state signal and the DESPOT1 T1 quantification method

“…The use of a fully 3D sequence in combination with optimized Shinnar-Le Roux RF pulse designs (34) minimizes slab profile errors, particularly within the center portion of the 3D slab. B 1 inhomogeneities effects may be minimized through the use of adiabatic or composite RF pulses (35) or through calibration via B 1 field mapping. Within the context of the presented work, incorrect knowledge of the transmitted flip angle does not change the basic premise that the measured signal may be modeled using the linear summation model.…”

Spatial domain method for the design of RF pulses in multicoil parallel excitation

“…Neglecting off-resonance and T1 and T2 decay, the signal received during readout subsequent to small-tip parallel excitation is given by a substitution of Eq. [2] into the MR signal equation: [7] FIG. 2. a: Field map used in the simulation of off-resonance correction.…”

“…In a manner analogous to parallel imaging methods, such as sensitivity encoding (SENSE) (4) and generalized autocalibrating partially parallel acquisition (GRAPPA) (5), a reduced excitation k-space trajectory (6) can be used to achieve a desired excitation pattern by exploiting the blurring behavior of coil sensitivity patterns in the excitation k-space domain to deposit RF energy in regions that are not traversed by the trajectory. Accelerated selective excitation is useful for reducing specific absorption rate (SAR) (2), and shortening multidimensional RF pulses in such applications as compensation for B 1 and B 0 inhomogeneity (7)(8)(9)(10). The feasibility of parallel excitation was also recently verified experimentally (11,12).…”

Basis function cross‐correlations for Robust k‐space sample density compensation, with application to the design of radiofrequency excitations

“…Multidimensional RF excitation (1,10,11) has many applications, such as inner volume imaging, navigator acquisition, and accounting for field inhomogeneities (10,(12)(13)(14)(15)(16)(17)(18)(19). Conversely, non-Cartesian sampling (20) has applications in fast imaging (21).…”