B1 mapping by Bloch‐Siegert shift

A novel method for amplitude of radiofrequency field (B) 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. 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. 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 mapping in a clinically acceptable time frame. Magn Reson Med 63:1315–1322, 2010. © 2010 Wiley‐Liss, Inc.     

Improved encoding strategy for CPMG-based Bloch-Siegert B(1)(+) mapping

Bloch-Siegert (BS) based B(1)(+) mapping methods use off-resonant pulses to encode quantitative B(1)(+) information into the signal phase. It was recently shown that the principle behind BS-based B(1)(+) mapping can be expanded from spin echo (BS-SE) and gradient-echo (BS-FLASH) based BS B(1)(+) mapping to methods such as Carr, Purcell, Meiboom, Gill (CPMG)-based turbo-spin echo (BS-CPMG-TSE) and multi-spin echo (BS-CPMG-MSE) imaging. If CPMG conditions are preserved, BS-CPMG-TSE allows fast acquisition of the B(1)(+) information and BS-CPMG-MSE enables simultaneous mapping of B(1)(+), M(0), and T(2). To date, however, two separate MRI experiments must be performed to enable the calculation of B(1)(+) maps. This study investigated a modified encoding strategy for CPMG BS-based methods to overcome this limitation. By applying a "bipolar" off-resonant BS pulse before the refocusing pulse train, the needed phase information was able to be encoded into different echo images of one echo train. Thus, this technique allowed simultaneous B(1)(+) and T(2) mapping in a single BS-CPMG-MSE experiment. To allow single-shot B(1)(+) mapping, this method was also applied to turbo-spin echo imaging. Furthermore, the presented modification intrinsically minimizes phase-based image artifacts in BS-CPMG-TSE experiments.     

Adiabatic RF pulse design for Bloch‐Siegert B mapping

The Bloch–Siegert (B–S) mapping method has been shown to be fast and accurate, yet it suffers from high Specific Absorption Rate (SAR) and moderately long echo time. An adiabatic RF pulse design is introduced here for optimizing the off‐resonant B–S RF pulse to achieve more B–S measurement sensitivity for a given pulse width. The extra sensitivity can be used for higher angle‐to‐noise ratio maps or traded off for faster scans. Using numerical simulations and phantom experiments, it is shown that a numerically optimized 2‐ms adiabatic B‐S pulse is 2.5 times more efficient than a conventional 6‐ms Fermi‐shaped B–S pulse. The adiabatic B–S pulse performance is validated in a phantom, and in vivo brain mapping at 3T and 7T are shown. Magn Reson Med 70:829–835, 2013. © 2012 Wiley Periodicals, Inc.     

RF pulse optimization for Bloch‐Siegert B mapping

The Bloch–Siegert (B–S) method of B mapping has been shown to be fast and accurate, yet has high SAR and moderately long TE. These limitations can lengthen scan times and incur signal loss due to B₀ inhomogeneity, particularly at high field. The B–S method relies on applying a band‐limited off‐resonant B–S radiofrequency pulse to induce a B‐dependent frequency‐shift for resonant spins. A method for optimizing the B–S radiofrequency pulse is presented here, which maximizes B–S B measurement sensitivity for a given SAR and T₂. A 4‐ms optimized pulse is shown to have 35% less SAR compared with the conventional 6‐ms Fermi pulse while still improving B map angle‐to‐noise ratio by 22%. The optimized pulse performance is validated both in phantom and in vivo brain imaging at 7 T. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Integrated Bloch‐Siegert B1 mapping and multislice imaging of hyperpolarized 13C pyruvate and bicarbonate in the heart

Hyperpolarization of ¹³C labeled substrates via dynamic nuclear polarization has been used as a method to noninvasively study real‐time metabolic processes occurring in vivo. In these studies, proper calibration of radiofrequency transmit power is required to efficiently observe rapidly decaying magnetization. Conventional transmit radiofrequency field $(B_1^{+})$ mapping methods rely on placing magnetization in a fixed, known state prior to imaging, making them unsuitable for imaging of hyperpolarized magnetization. Recently, a phase‐based B₁ mapping method based on the Bloch‐Siegert shift has been reported. This method uses a B₁‐dependent shift in the resonance frequency of nuclei in the presence of an off‐resonance radiofrequency pulse. In this article, we investigate the feasibility of Bloch‐Siegert B₁ mapping and observation of metabolism of hyperpolarized $[1{-}^{13}{\rm C}]$ pyruvate in vivo, in a single injection. The technique is demonstrated with phantom experiments, and in normal rat and pigs in vivo. This method is anticipated to improve quantitative measurements of hyperpolarized ¹³C metabolism in vivo by enabling accurate flip‐angle corrections. This work demonstrates the use of Bloch‐Siegert B₁ mapping under challenging out‐of‐equilibrium imaging conditions. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Efficient bloch‐siegert B1+ mapping using spiral and echo‐planar readouts

The Bloch‐Siegert (B‐S) B₁⁺ mapping technique is a fast, phase‐based method that is highly SAR limited especially at 7T, necessitating the use of long repetition times. Spiral and echo‐planar readouts were incorporated in a gradient‐echo based B‐S sequence to reduce specific absoprtion rate (SAR) and improve its scan efficiency. A novel, numerically optimized 4 ms B‐S off‐resonant pulse at + 1960 Hz was used to increase sensitivity and further reduce SAR compared with the conventional 6 ms Fermi B‐S pulse. Using echo‐planar and spiral readouts, scan time reductions of 8–16 were achieved. By reducing the B‐S pulse width by a factor of 1.5, SAR was reduced by a factor of 1.5 and overall sensitivity was increased by a factor of 1.33 due to the nearly halved resonance offset of the new B‐S pulse. This was validated on phantoms and volunteers at 7 T. Magn Reson Med 70:1669–1673, 2013. © 2013 Wiley Periodicals, Inc.     

Fast radiofrequency flip angle calibration by Bloch–Siegert shift

In a recent work, we presented a novel method for B 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. © 2011 Wiley Periodicals, Inc.     

SAR-reduced spin-echo-based Bloch-Siegert B(1)(+) mapping: BS-SE-BURST

Fast and accurate B(1)(+) mapping is possible using phase-based Bloch-Siegert (BS) methods. Importantly, the off-resonant pulses needed for BS B(1)(+) mapping methods can easily be implemented in multiple MR sequences. BS-based B(1)(+) mapping has thus been introduced for gradient echo (BS-FLASH), spin-echo (BS-SE), and Carr, Purcell, Meiboom, Gill (CPMG)-based multi-SE and turbo-SE sequences. When using SE and multi-SE/turbo-SE-based BS sequences, however, the high intrinsic specific absorption rates must be considered in clinical situations. This study introduces a fast BS B(1)(+) mapping method based on a SE-BURST sequence (BS-SE-BURST). With SE-BURST sequences, multiple low-magnitude excitation pulses are applied prior to the refocusing pulse. Thus, multiple and different phase-encoded echoes can be acquired per excitation cycle. Compared with a SE sequence, this excitation strategy results in a similar signal-to-noise ratio (SNR) per unit time but with reduced specific absorption rate. The proposed BS-SE-BURST sequence was implemented on a conventional 3 T whole body MRI scanner and applied successfully.     

Phase‐sensitive sodium B1 mapping

Quantitative sodium MRI requires accurate knowledge of factors affecting the sodium signal. One important determinant of sodium signal level is the transmit B₁ field strength. However, the low signal‐to‐noise ratio typical of sodium MRI makes accurate B₁ mapping in reasonable scan times challenging. A new phase‐sensitive B₁ mapping technique has recently been shown to work better than the widely used dual‐angle method in low‐signal‐to‐noise ratio situations and over a broader range of flip angles. In this work, the phase‐sensitive B₁ mapping technique is applied to sodium, and its performance compared to the dual‐angle method through both simulation and phantom studies. The phase‐sensitive method is shown to yield higher quality B₁ maps at low signal‐to‐noise ratio and greater consistency of measurement than the dual‐angle method. An in vivo sodium B₁ map of the human breast is also shown, demonstrating the phase‐sensitive method's feasibility for human studies. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.