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.     

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.     

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.     

Fast CPMG‐based Bloch‐Siegert B1+ mapping

A novel method for B mapping based on the Bloch‐Siegert (BS) shift was recently presented. This method applies off‐resonant pulses before signal acquisition to encode B₁ information into the signal phase. BS‐based methods possess significant advantages in measurement time and accuracy compared to magnitude‐based B methods. This study extends the idea of BS B mapping to Carr, Purcell, Meiboom, Gill (CPMG)‐based multi‐spin‐echo (BS‐CPMG‐MSE) and turbo‐spin‐echo (BS‐CPMG‐TSE) imaging. Compared to BS‐based spin echo imaging (BS‐SE), faster acquisition of the B information was possible using the BS‐CPMG‐TSE sequence. Furthermore, signal loss by T₂* effects could be minimized using these spin echo‐based techniques. These effects are critical for gradient echo‐based BS methods at high field strengths. However, multi‐spin‐echo‐based BS B₁ methods inherently possess high specific absorption rates. Thus, the relative specific absorption rate of BS‐CPMG‐TSE sequences was estimated and compared with the specific absorption rate produced by BS‐SE sequences. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Slice profile correction for transmit sensitivity mapping using actual flip angle imaging

To enable clinical use of parallel transmission technology, it is necessary to rapidly produce transmit sensitivity (σ) maps. Actual flip angle imaging is an efficient mapping technique, which is accurate when used with 3D encoding and nonselective RF pulses. Mapping single slices is quicker, but 2D encoding leads to systematic errors due to slice profile effects. By simulating steady‐state slice profiles, we computed the relationship between σ and the signals received from the actual flip angle imaging sequence for arbitrarily chosen slice selective RF pulses. Pulse specific lookup tables were then used for reconstruction. The resulting σ‐maps are sensitive to T₁ in a manner that depends strongly on the specific pulse, for example a precision of ±3% can be achieved by using a 3‐lobe sinc pulse. The method is applicable to any RF pulse; simulations must be performed once and thereafter fast reconstruction of σ‐maps is possible. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

A phase‐sensitive method of flip angle mapping

A radiofrequency (RF) excitation scheme is presented in which flip angle is encoded in the phase of the resulting excitation. This excitation is implemented with nonselective hard pulses, and is used to give flip angle maps over three‐dimensional volumes. This phase‐sensitive B1 mapping excitation can be combined with various acquisition methods such as gradient recalled echo (GRE) and echo‐planar (EP) readouts. Imaging time depends primarily on the readout method, and is roughly equivalent to the imaging time of conventional double‐angle techniques for three‐dimensional acquisition. The phase‐sensitive method allows imaging over a much wider range of flip angles than double‐angle methods. Phantom and in vivo results are presented comparing the phase‐sensitive method with the conventional double‐angle method, demonstrating the ability of the phase‐sensitive method to measure a wider range of flip angles than double‐angle methods. Magn Reson Med 60:889–894, 2008. © 2008 Wiley‐Liss, 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.     

Optimization and validation of methods for mapping of the radiofrequency transmit field at 3T

MRI techniques such as quantitative imaging and parallel transmit require precise knowledge of the radio‐frequency transmit field (B). Three published methods were optimized for robust B mapping at 3T in the human brain: three‐dimensional (3D) actual flip angle imaging (AFI), 3D echo‐planar imaging (EPI), and two‐dimensional (2D) stimulated echo acquisition mode (STEAM). We performed a comprehensive comparison of the methods, focusing on artifacts, reproducibility, and accuracy compared to a reference 2D double angle method. For the 3D AFI method, the addition of flow‐compensated gradients for diffusion damping reduced the level of physiological artifacts and improved spoiling of transverse coherences. Correction of susceptibility‐induced artifacts alleviated image distortions and improved the accuracy of the 3D EPI imaging method. For the 2D STEAM method, averaging over multiple acquisitions reduced the impact of physiological noise and a new calibration method enhanced the accuracy of the B maps. After optimization, all methods yielded low noise B maps (below 2 percentage units), of the nominal flip angle value (p.u.) with a systematic bias less than 5 p.u. units. Full brain coverage was obtained in less than 5 min. The 3D AFI method required minimal postprocessing and showed little sensitivity to off‐resonance and physiological effects. The 3D EPI method showed the highest level of reproducibility. The 2D STEAM method was the most time‐efficient technique. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Rapid methods for concurrent measurement of the RF‐pulse flip angle and the longitudinal relaxation time

Measuring both the flip angle (FA) and the longitudinal relaxation time T₁ is essential in quantitative and longitudinal studies because the signal amplitude is dependent on these quantities. Conventional methods can only measure one of them at a time and require long scan times. In this work, two mutually consistent methods are developed; each can acquire multislice data for determining both the FA and T₁ in a scan time about half the time needed for a conventional FA measurement. On the basis of a recent development of longitudinal‐relaxation measurement (Hsu and Lowe, J Magn Reson 2004;169:270–278; Hsu and Glover, J Magn Reson 2006;181:98–106), one of the methods uses RF pulse trains of two FAs whereas the other uses pulse trains of different pulse spacing. When only the FA or T₁ is needed, the present methods can still be faster than conventional methods for the needed quantity. In benchmarking with a uniform‐density sample, both methods generate precise T₁ values independent of the FA chosen (except at and near 90°). In the demonstration with three normal volunteers at 3 T, the T₁ values of frontal and occipital white matter, putamen, and caudate are compared; the T₁ values are in agreement with literature values and the intrasubject deviation is 0.2%–2.8%. Magn Reson Med, 61:1319–1325, 2009. © 2009 Wiley‐Liss, Inc.     

Estimating T1 from multichannel variable flip angle SPGR sequences

Quantitative estimation of T₁ is a challenging but important task inherent to many clinical applications. The most commonly used paradigm for estimating T₁ in vivo involves performing a sequence of spoiled gradient‐recalled echo acquisitions at different flip angles, followed by fitting of an exponential model to the data. Although there has been substantial work comparing different fitting methods, there has been little discussion on how these methods should be applied for data acquired using multichannel receivers. In this note, we demonstrate that the manner in which multichannel data is handled can have a substantial impact on T₁ estimation performance and should be considered equally as important as choice of flip angles or fitting strategy. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Local flip angle correction for improved volume T1‐quantification in three‐dimensional dGEMRIC using the look‐locker technique

Purpose

To present an evaluation method for three‐dimensional Look‐Locker (3D‐LL) based T1 quantification, calculating correct T1 values independent of local flip angle (FA) variations. The method was evaluated both in phantoms and in vivo in a delayed Gadolinium Enhanced MRI of Cartilage (dGEMRIC) study with 33 subjects.

Materials and Methods

T1 was measured with 3D‐LL, using both local FA correction and a precalculated FA slice profile, and compared with standard constant FA correction, for all slices in phantoms and in both femur condyles in vivo. T1 measured using two‐dimensional Inversion Recovery (2D‐IR) was used as gold standard.

Results

Due to the FA being slice dependent, the standard constant FA correction results in erroneous T1 (systematic error = 109.1 ms in vivo), especially in the outer slices. With local FA correction, the calculated T1 is excellent for all slices in phantoms (<5% deviation from 2D‐IR). In vivo the performance is lower (systematic error = ?57.5 ms), probably due to imperfect inversion. With precalculated FA correction the performance is very good also in vivo (systematic error = 13.3 ms).

Conclusion

With the precalculated FA correction method, the 3D‐LL sequence is robust enough for in vivo dGEMRIC, even outside the centermost slices. J. Magn. Reson. Imaging 2009;30:834–841. © 2009 Wiley‐Liss, Inc.

     

Rapid B1 mapping using orthogonal,equal‐amplitude radio‐frequency pulses

We present a new phase‐based method for mapping the amplitude of the radio‐frequency field (B₁) of a transmitter coil in three‐dimension. This method exploits the noncommutation relation between rotations about orthogonal axes. Our implementation of this principle in the current work results in a simple relation between the phase of the final magnetization and the flip angle (FA). In this study, we focus on FAs less than 90°. Our method is rapid and easy to implement compared with the existing B₁ mapping schemes. The mapping sequence can be simply obtained by adding to a regular three‐dimensional gradient‐echo sequence a magnetization preparation radio‐frequency pulse of the same FA but orthogonal in phase to the excitation radio‐frequency pulse. This method is demonstrated capable of generating reliable maps of the B₁ field within 1 min using FAs no larger than 60°. We show that it is robust against T₁, small chemical shift, and mild background inhomogeneity. This method may especially be suitable for B₁ mapping in situations (e.g., long‐T₁ and hyperpolarized‐gas imaging) where magnitude‐based methods are not readily applicable. A noise calculation of the FA map using this method is also presented. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Simultaneous variable flip angle-actual flip angle imaging method for improved accuracy and precision of three-dimensional T1 and B1 measurements

A new time-efficient and accurate technique for simultaneous mapping of T(1) and B(1) is proposed based on a combination of the actual flip angle (FA) imaging and variable FA methods. Variable FA-actual FA imaging utilizes a single actual FA imaging and one or more spoiled gradient-echo acquisitions with a simultaneous nonlinear fitting procedure to yield accurate T(1)/B(1) maps. The advantage of variable FA-actual FA imaging is high accuracy at either short T(1) times or long repetition times in the actual FA imaging sequence. Simulations show this method is accurate to 0.03% in FA and 0.07% in T(1) for ratios of repetition time to T1 time over the range of 0.01-0.45. We show for the case of brain imaging that it is sufficient to use only one small FA spoiled gradient-echo acquisition, which results in reduced spoiling requirements and a significant scan time reduction compared to the original variable FA method. In vivo validation yielded high-quality 3D T(1) maps and T(1) measurements within 10% of previously published values and within a clinically acceptable scan time. The variable FA-actual FA imaging method will increase the accuracy and clinical feasibility of many quantitative MRI methods requiring T(1)/B(1) mapping such as dynamic contrast enhanced perfusion and quantitative magnetization transfer imaging.