Laura I. Sacolick

B1 Mapping by Bloch-Siegert Shift

A novel method for amplitude of radiofrequency field (B  1+ ) mapping based on the Bloch‐Siegert shift is presented. Unlike conventionally applied double‐angle or other signal magnitude–based methods, it encodes the B1 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  12 . Adding gradient image encoding following the off‐resonance pulse allows one to acquire spatially resolved B1 maps. The frequency shift from the Bloch‐Siegert effect gives a phase shift in the image that is proportional to B  12 . 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 B1 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  1+ mapping in a clinically acceptable time frame. Magn Reson Med 63:1315–1322, 2010.

Zero TE MR bone imaging in the head

To investigate proton density (PD)‐weighted zero TE (ZT) imaging for morphological depiction and segmentation of cranial bone structures.

Anatomic Evaluation of 3-Dimensional Ultrashort-Echo-Time Bone Maps for PET/MR Attenuation Correction

Ultrashort-echo-time (UTE) sequences have been proposed in the past for MR-based attenuation correction of PET data, because of their ability to image cortical bone. In the present work we assessed the limitations of dual-echo UTE imaging for bone segmentation in head and neck imaging. Sequentially acquired MR and PET/CT clinical data were used for this purpose. Methods: Twenty patients referred for a clinical oncology examination were scanned using a trimodality setup. Among the MR sequences, a dual-echo UTE acquisition of the head was acquired and used to create tissue R2 maps. The different undesired structures present in these maps were identified by an experienced radiologist. Global and local measurements of the overlap between R2-based and CT-based bone masks were computed. Results: UTE R2 maps displayed a nonfunctional relation with CT data. The obtained bone masks showed acceptable overlap with the corresponding CT data, in the case of the skull itself (e.g., 47% mismatch for the parietal region), with decreased performance in the base of the skull and in the neck (e.g., 78% for the maxillary region). Unwanted structures were detected, both anatomic (e.g., sternocleidomastoid, temporal, and masseter muscles) and artifactual (e.g., dental implants and air–tissue interfaces). Conclusion: It is indeed possible to estimate the anatomic location of bone tissue using UTE sequences. However, using pure parametric maps for attenuation correction may lead to bias close to certain anatomic structures and areas of high magnetic field inhomogeneity. More sophisticated approaches are necessary to compensate for these effects.

Clinical Evaluation of Zero-Echo-Time MR Imaging for the Segmentation of the Skull

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 dual-echo 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.

Small-tip-angle spokes pulse design using interleaved greedy and local optimization methods.

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. Magn Reson Med, 2012.

Transmit gain calibration for nonproton MR using the Bloch–Siegert shift

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 T1 and B0 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 13C and 3He applications. Copyright

Fast radiofrequency flip angle calibration by Bloch–Siegert shift

In a recent work, we presented a novel method for B  1+ field mapping based on the Bloch–Siegert shift. Here, we apply this method to automated fast radiofrequency transmit gain calibration. Two off‐resonance radiofrequency pulses were added to a slice‐selective spin echo sequence. The off‐resonance pulses induce a Bloch–Siegert phase shift in the acquired signal that is proportional to the square of the radiofrequency field magnitude B12. The signal is further spatially localized by a readout gradient, and the signal‐weighted average B1 field is calculated. This calibration from starting system transmit gain to average flip angle is used to calculate the transmit gain setting needed to produce a desired imaging sequence flip angle. A robust implementation is demonstrated with a scan time of 3 s. The Bloch–Siegert‐based calibration was used to predict the transmit gain for a 90° radiofrequency pulse and gave a flip angle of 88.6 ± 3.42° when tested in vivo in 32 volunteers. Magn Reson Med, 2011.

Quiet and distortion-free, whole brain BOLD fMRI using T2-prepared RUFIS

To develop and evaluate a novel MR method that addresses some of the most eminent technical challenges of current BOLD‐based fMRI in terms of 1) acoustic noise and 2) geometric distortions and signal dropouts.

Field shaping arrays: A means to address shading in high field breast MRI

To develop a simple correction approach to mitigate shading in 3 Tesla (T) breast MRI.