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.     

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.     

Three‐dimensional mapping of the B1 field using an optimized phase‐based method: Application to hyperpolarized 3He in lungs

A novel method is presented for the three‐dimensional mapping of the B₁‐field of a transmit radio‐frequency MR coil. The method is based on the acquisition of phase images, where the effective flip angle is encoded in the phase of the nonselective hard pulse excitation. The method involves the application of a rectangular composite pulse as excitation in a three‐dimensional gradient recall echo to produce measurable phase angle variation. However, such a pulse may significantly increase the radio‐frequency power deposition in excess of the standard acceptable SAR limits, imposing extremely long TRs (>100 msec), which would result in acquisition times significantly greater than a single breath‐hold. In this study, the phases of the radio‐frequency excitation are modified, resulting in a different pulse sequence scheme. It is shown that the new method increases sensitivity with respect to radio‐frequency inhomogeneities by up to 10 times, and reduces the total duration of the pulse so that three‐dimensional B₁ mapping is possible with ³He in lungs within a single breath‐hold. Computer simulations demonstrate the increase in sensitivity. Phantom results with ¹H MRI are used for validation. In vivo results are presented with hyperpolarized ³He in human lungs at 1.5T. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

T1 corrected B1 mapping using multi‐TR gradient echo sequences

This work presents a new approach toward a fast, simultaneous amplitude of radiofrequency field (B₁) and T₁ mapping technique. The new method is based on the “actual flip angle imaging” (AFI) sequence. However, the single pulse repetition time (TR) pair used in the standard AFI sequence is replaced by multiple pulse repetition time sets. The resulting method was called “multiple TR B₁/T₁ mapping” (MTM). In this study, MTM was investigated and compared to standard AFI in simulations and experiments. Feasibility and reliability of MTM were proven in phantom and in vivo experiments. Error propagation theory was applied to identify optimal sequence parameters and to facilitate a systematic noise comparison to standard AFI. In terms of accuracy and signal‐to‐noise ratio, the presented method outperforms standard AFI B₁ mapping over a wide range of T₁. Finally, the capability of MTM to determine T₁ was analyzed qualitatively and quantitatively, yielding good agreement with reference measurements. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

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.     

B1‐based specific energy absorption rate determination for nonquadrature radiofrequency excitation

The current gold standard to estimate local and global specific energy absorption rate for MRI involves numerically modeling the patient and the transmit radiofrequency coil. Recently, a patient‐individual method was presented, which estimated specific energy absorption rate from individually measured B₁ maps. This method, however, was restricted to quadrature volume coils due to difficulties distinguishing phase contributions from radiofrequency transmission and reception. In this study, a method separating these two phase contributions by comparing the electric conductivity reconstructed from different transmit channels of a parallel radiofrequency transmission system is presented. This enables specific energy absorption rate estimation not only for quadrature excitation but also for the nonquadrature excitation of the single elements of the transmit array. Though the contributions of the different phases are known, unknown magnetic field components and tissue boundary artifacts limit the technique. Nevertheless, the high agreement between simulated and experimental results found in this study is promising. B₁‐based specific energy absorption rate determination might become possible for arbitrary radiofrequency excitation on a patient‐individual basis. Magn Reson Med, 2012. © 2012 Wiley Periodicals, 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.     

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.