Slice-selective RF waveforms that mitigate severe B 1 ؉ inhomogeneity at 7 Tesla using parallel excitation were designed and validated in a water phantom and human studies on six subjects using a 16-element degenerate stripline array coil driven with a butler matrix to utilize the eight most favorable birdcage modes. The parallel RF waveform design applied magnitude least-squares (MLS) criteria with an optimized k-space excitation trajectory to significantly improve profile uniformity compared to conventional least-squares (LS) designs. Parallel excitation RF pulses designed to excite a uniform in-plane flip angle (FA) with slice selection in the z-direction were demonstrated and compared with conventional sinc-pulse excitation and RF shimming. In all cases, the parallel RF excitation significantly mitigated the effects of inhomogeneous B 1 ؉ on the excitation FA. The optimized parallel RF pulses for human B 1 ؉ mitigation were only 67% longer than a conventional sinc-based excitation, but significantly outperformed RF shimming. Key words: parallel excitation; slice-selective excitation; RF inhomogeneity mitigation; multidimensional RF pulse; RF coil array Slice-selective excitation plays a crucial role in MRI. With the push toward higher magnetic field strength, dramatic B 1 ϩ inhomogeneity for human imaging has become a serious issue, causing inhomogeneous flip-angle (FA) distribution in-plane for slice-selective excitations and detrimental nonuniformity for both signal-to-noise ratio (SNR) and image contrast. Several RF design approaches have been suggested to compensate for this inhomogeneity, including adiabatic pulses (1,2), RF-shimming (3-6), and spatially tailored excitation designs (7-11).For relatively mild B 1 ϩ inhomogeneity, using the low-FA approximation (12) with appropriate echo-volumnar k-space trajectories (9 -11), termed either "fast-k z " or "spokes" excitation trajectories, the within-slice FA inhomogeneity can be corrected. With these pulses, slice selection is achieved with a conventional sinc-like RF pulse during each k z traversal (a spoke), and in-plane FA inhomogeneity is mitigated by the appropriate choice of the complex-valued amplitude that modulates the RF waveform of each spoke. Nonetheless, if the transmit (Tx) B 1 ϩ field is rapidly varying with position, a large number of spokes will be required at correspondingly high k x and k y locations, rendering the RF pulse too lengthy for practical use.With the introduction of parallel excitation systems (13-16), the k-space trajectory can be undersampled significantly to accelerate the RF pulse and reduce its duration. A number of successful demonstrations of this concept have been reported (e.g., . For example, it has been demonstrated at 3T (18,21) and 4.7T (17) that a parallel RF design method using low-FA approximation with spokebased excitation trajectories can produce highly uniform slice-selective excitation with reasonable excitation durations.In this work we use spoke-based excitation in combination with magnitude least-squar...
Spatially selective RF waveforms were designed and demonstrated for parallel excitation with a dedicated eight-coil transmit array on a modified 3T human MRI scanner. Measured excitation profiles of individual coils in the array were used in a low-flip-angle pulse design to achieve desired spatial target profiles with two-(2D) and three-dimensional (3D) k-space excitation with simultaneous transmission of RF on eight channels. The 2D pulse excited a high-resolution spatial pattern in-plane, while the 3D trajectory produced high-quality slice selection with a uniform in-plane excitation despite the highly nonuniform individual spatial profiles of the coil array. Radiofrequency (RF) excitation in the presence of timevarying gradients for multidimensional selective excitation (1) has several interesting applications, including flexibly shaped excitation volumes and spatial modulation of the B 1 excitation profile to mitigate RF field inhomogeneity at high field (2-4). However, due to limitations in gradient hardware and RF power, such pulse designs can result in long waveforms that may limit their performance and practical applicability. With the use of parallel excitation design in conjunction with coil arrays that are capable of simultaneous, independent RF transmission, one can shorten the pulse duration by taking advantage of variations in spatial excitation profiles among coils in the array (5-12).Various methods have been proposed for parallel RF excitation design, all of which (to date) are based on the small-tip-angle approximation (1). This simplification greatly reduces computation and can provide attractive intuition about design trade-offs, since the RF design problem is reduced to a linear system. In the method presented by Katscher et al. (7), a system of linear equations is solved in the excitation Fourier space (k-space). In contrast, the scheme developed by Zhu et al. (11) Several authors have demonstrated the concept of parallel RF by performing excitation with a single coil at a time and combining the received data with offline postprocessing to simulate parallel excitation (5-8,11). Recently, however, three-channel parallel excitation was performed by Ullman et al. (12), who demonstrated the feasibility of accelerating parallel excitation by factors of 2 and 2.67 using a 2D spiral k-space trajectory, with a design based on Grissom et al.'s (9) formulation. In addition to successfully achieving reduced pulse lengths without degradation in performance, they noted that compared to the unaccelerated excitation, the accelerated excitation was less prone to error from off-resonance effects because of its shorter duration compared to the unaccelerated version.Here we describe the design and implementation of parallel RF excitation on a 3T human scanner with an eight-channel array. Two types of excitation k-space trajectories were used for the design of the parallel RF pulses: 1) a 2D spiral excitation with integer acceleration factors of 2-8, and 2) a series of modulated sinc pulses in k z for slice-s...
An eight-rung, 3T degenerate birdcage coil (DBC) was constructed and evaluated for accelerated parallel excitation of the head with eight independent excitation channels. Two mode configurations were tested. In the first, each of the eight loops formed by the birdcage was individually excited, producing an excitation pattern similar to a loop coil array. In the second configuration a Butler matrix transformed this "loop coil" basis set into a basis set representing the orthogonal modes of the birdcage coil. In this case the rung currents vary sinusoidally around the coil and only four of the eight modes have significant excitation capability (the other four produce anticircularly polarized (ACP) fields). The lowest useful mode produces the familiar uniform B 1 field pattern, and the higher-order modes produce center magnitude nulls and azimuthal phase variations. The measured magnitude and phase excitation profiles of the individual modes were used to generate one-, four-, six-, and eightfold-accelerated spatially tailored RF excitations with 2D and 3D k-space excitation trajectories. Transmit accelerations of up to six-fold were possible with acceptable levels of spatial artifact. The orthogonal basis set provided by the Butler matrix was found to be advantageous when an orthogonal subset of these modes was used to mitigate B 1 transmit inhomogeneities using parallel excitation. The many positive benefits of high-field MRI are accompanied by destructive interference of the transmit RF fields within a typical volume excitation coil (1,2). This effect arises when the wavelength of the electromagnetic fields in the body approaches the dimension of the human head or body. In this case the RF fields generated by different parts of the coil can destructively interfere at some locations. For cylindrically symmetric coils, such as conventional birdcage designs, the center of the object is equidistant from all the rungs in the coil, which ensures an equal phase shift for the fields generated from each rung. For the phase relationship of the standard uniform mode of the birdcage, this leads to constructive interference at this location. In the periphery of the object, fields produced from different rungs travel unequal distances and can destructively interfere. The net effect is the center-brightening phenomena that is common in uniform mode birdcage coils at 3T and 7T. Although the high dielectric constant of water is, in practice, critical for shortening the wavelength, Collins et al. (2) have pointed out that the phenomenon does not require a dielectric media, and the phenomenon is not a dielectric resonance effect.The B 1 excitation field inhomogeneity in the transmit coil leads to unwanted spatial variations in the tissue contrast and image intensity for most pulse sequences. The severity of the effect depends on the flip-angle dependence of the sequence, and since the problem arises during excitation, it is not easily dealt with in postprocessing. Where the intrinsic contrast information is not present locally, ima...
Chemical shift imaging benefits from signal-to-noise ratio (SNR) and chemical shift dispersion increases at stronger main field such as 7 Tesla, but the associated shorter radiofrequency (RF) wavelengths encountered require B 1 ؉ mitigation over both the spatial field of view (FOV) and a specified spectral bandwidth. The bandwidth constraint presents a challenge for previously proposed spatially tailored B 1 ؉ mitigation methods, which are based on a type of echovolumnar trajectory referred to as "spokes" or "fast-k z ". Although such pulses, in conjunction with parallel excitation methodology, can efficiently mitigate large B 1 ؉ inhomogeneities and achieve relatively short pulse durations with slice-selective excitations, they exhibit a narrow-band off-resonance response and may not be suitable for applications that require B 1 ؉ mitigation over a large spectral bandwidth. This work outlines a design method for a general parallel spectral-spatial excitation that achieves a target-error minimization simultaneously over a bandwidth of frequencies Key words: parallel excitation; spectral-spatial excitation; RF inhomogeneity mitigation; multidimensional RF pulse Parallel excitation offers the means to accelerate radiofrequency (RF) waveforms for complicated two-dimensional (2D) and three-dimensional (3D) spatially tailored excitations, resulting in shorter excitation duration when compared to the single-channel case (1-4). A number of successful demonstrations of this concept on a multichannel transmit system have been reported such previously shown (5-8). Accelerations of four-to sixfold have been achieved via an eight-channel transmit system (6), potentially enabling several important applications, including flexibly shaped excitation volumes, and mitigation of RF field inhomogeneity at high field for slice or slab-selective pulses.Nonetheless, previous work on parallel excitation methods have been limited to the design of excitation profiles at a single frequency, resulting in RF pulses with a narrow bandwidth characteristic. The off-resonance behavior stems from the lengthy echoplanar, spiral, or echovolumnar excitation k-space trajectories, which incur off-resonance effects analogous to the image encoding case. This narrow-band property was apparent in Setsompop et al. (9), where it was shown that B 0 inhomogeneity in standard in vivo imaging condition at 3T can have a detrimental effect on the excitation profile if a field map information was not incorporated into the RF design.The narrow-band nature of the spatially tailored excitation pulses particularly complicates their use in high field chemical shift imaging. Chemical shift imaging (CSI) benefits from high main magnetic field strength (such as 7T) through higher signal-to-noise ratio (SNR), increased chemical shift dispersion, and weaker spin coupling. However, the detrimental effects of B 1 ϩ inhomogeneities at high-field pose significant problems, and to realize the benefits of high field to CSI, broadband RF excitations that mitigate B 1 ϩ inhomoge...
At high magnetic field, B1+ non-uniformity causes undesired inhomogeneity in SNR and image contrast. Parallel RF transmission using tailored 3D k-space trajectory design has been shown to correct for this problem and produce highly uniform in-plane magnetization with good slice selection profile within a relatively short excitation duration. However, at large flip angles the excitation k-space based design method fails. Consequently, several large-flip-angle parallel transmission designs have recently been suggested. In this work, we propose and demonstrate a large-flip-angle parallel excitation design for 90° and 180° spin-echo slice-selective excitations that mitigate severe B1+ inhomogeneity. The method was validated on an 8-channel transmit array at 7T using a water phantom with B1+ inhomogeneity similar to that seen in human brain in vivo. Slice-selective excitations with parallel RF systems offer means to implement conventional high-flip excitation sequences without a severe pulse-duration penalty, even at very high B0 field strengths where large B1+ inhomogeneity is present.
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