A novel method for amplitude of radiofrequency field (B þ 1 ) mapping based on the Bloch-Siegert shift is presented. Unlike conventionally applied double-angle or other signal magnitude-based methods, it encodes the B 1 information into signal phase, resulting in important advantages in terms of acquisition speed, accuracy, and robustness. The Bloch-Siegert frequency shift is caused by irradiating with an off-resonance radiofrequency pulse following conventional spin excitation. When applying the off-resonance radiofrequency in the kilohertz range, spin nutation can be neglected and the primarily observed effect is a spin precession frequency shift. This shift is proportional to the square of the magnitude of B 2 1 . Adding gradient image encoding following the off-resonance pulse allows one to acquire spatially resolved B 1 maps. The frequency shift from the Bloch-Siegert effect gives a phase shift in the image that is proportional to B 2 1 . The phase difference of two acquisitions, with the radiofrequency pulse applied at two frequencies symmetrically around the water resonance, is used to eliminate undesired off-resonance effects due to amplitude of static field inhomogeneity and chemical shift. In vivo Bloch-Siegert B 1 mapping with 25 sec/ slice is demonstrated to be quantitatively comparable to a 21-min double-angle map. As such, this method enables robust, high-resolution B A wide variety of amplitude of radiofrequency (RF) field (B 1 ) mapping methods has been developed to date; however, no single one has emerged yet in widespread application. B 1 mapping is used in diverse applications in MR, including transmit gain adjustment to produce specific flip-angle RF pulses, design of multitransmit channel RF pulses (1-3), T 1 mapping and other quantitative MRI (4), and chemical shift imaging.Generally, B 1 mapping methods fall into two classes: signal magnitude or signal phase based. The majority of B 1 mapping methods depend on changes in signal magnitude based on RF flip angle. Existing methods in this category include fitting progressively increasing flip angles (5), stimulated echoes (6), image signal ratios (7-10), signal null at certain flip angles (11), and comparison of spin echo and stimulated echo signals (12).These methods suffer from combinations of the following problems: T 1 dependence; long acquisition times, mainly from acquiring many images and/or a long pulse repetition time (TR) to mitigate the T 1 dependence; inability to use some of these methods with slice selection or in a multislice acquisition; inaccuracy over a large range of B 1 , especially at low flip angles or flip angles close to 90 or 180 ; and large RF power deposition in the case of B 1 mapping sequences based on large flip angles or reset pulses to mitigate the T 1 dependence.There are far fewer phase-based B 1 mapping methods. One method by Morrell (13) uses the phase accrued from a 2a-a flip angle sequence to determine B 1 . This method has the same long TR requirement as the signal magnitude-based sequences, although it ...

PD-weighted ZT imaging provides robust and efficient depiction of bone structures in the head, with an excellent contrast between air, soft-tissue, and bone. Besides structural bone imaging, the presented method is expected to be of relevance for attenuation correction in positron emission tomography (PET)/MR and MR-based radiation therapy planning.

Current spokes pulse design methods can be grouped into methods based either on sparse approximation or on iterative local (gradient descent-based) optimization of the transverse-plane spatial frequency locations visited by the spokes. These two classes of methods have complementary strengths and weaknesses: sparse approximation-based methods perform an efficient search over a large swath of candidate spatial frequency locations but most are incompatible with off-resonance compensation, multifrequency designs, and target phase relaxation, while local methods can accommodate off-resonance and target phase relaxation but are sensitive to initialization and suboptimal local cost function minima. This article introduces a method that interleaves local iterations, which optimize the radiofrequency pulses, target phase patterns, and spatial frequency locations, with a greedy method to choose new locations. Simulations and experiments at 3 and 7 T show that the method consistently produces single- and multifrequency spokes pulses with lower flip angle inhomogeneity compared to current methods.

MR-based attenuation correction is instrumental for integrated PET/ MR imaging. It is generally achieved by segmenting MR images into a set of tissue classes with known attenuation properties (e.g., air, lung, bone, fat, soft tissue). Bone identification with MR imaging is, however, quite challenging, because of the low proton density and fast decay time of bone tissue. The clinical evaluation of a novel, recently published method for zero-echo-time (ZTE)-based MR bone depiction and segmentation in the head is presented here. Methods: A new paradigm for MR imaging bone segmentation, based on proton density-weighted ZTE imaging, was disclosed earlier in 2014. In this study, we reviewed the bone maps obtained with this method on 15 clinical datasets acquired with a PET/CT/MR trimodality setup. The CT scans acquired for PET attenuation-correction purposes were used as reference for the evaluation. Quantitative measurements based on the Jaccard distance between ZTE and CT bone masks and qualitative scoring of anatomic accuracy by an experienced radiologist and nuclear medicine physician were performed. Results: The average Jaccard distance between ZTE and CT bone masks evaluated over the entire head was 52% ± 6% (range, 38%-63%). When only the cranium was considered, the distance was 39% ± 4% (range, 32%-49%). These results surpass previously reported attempts with dualecho ultrashort echo time, for which the Jaccard distance was in the 47%-79% range (parietal and nasal regions, respectively). Anatomically, the calvaria is consistently well segmented, with frequent but isolated voxel misclassifications. Air cavity walls and bone/fluid interfaces with high anatomic detail, such as the inner ear, remain a challenge. Conclusion: This is the first, to our knowledge, clinical evaluation of skull bone identification based on a ZTE sequence. The results suggest that proton density-weighted ZTE imaging is an efficient means of obtaining high-resolution maps of bone tissue with sufficient anatomic accuracy for, for example, PET attenuation correction.

Transmit gain (B 1+) calibration is necessary for the adjustment of radiofrequency (RF) power levels to the desired flip angles. In proton MRI, this is generally an automated process before the actual scan without any user interaction. For other nuclei, it is usually time consuming and difficult, especially in the case of hyperpolarised MR. In the current work, transmit gain calibration was implemented on the basis of the Bloch-Siegert phase shift. From the same data, the centre frequency, line broadening and SNR could also be determined. The T(1) and B(0) insensitivity, and the wide range of B 1+ over which this technique is effective, make it well suited for nonproton applications. Examples are shown for hyperpolarised (13)C and (3)He applications.