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. Measurement of flip angle can be used to correct image intensity variation caused by B 1 inhomogeneity (1-4). Measurement of flip angle is also useful to obtain correct results in T 1 mapping with multiple-flip-angle techniques (5). Measurement of both flip angle and relative phase of the B 1 field is also necessary for the design of RF waveforms for multiple simultaneous transmitter systems (6,7).Mapping of the B 1 field has been performed by double angle techniques (8 -11). In these methods, two spin-echo or gradient recalled echo (GRE) acquisitions are performed. In the spin-echo double angle method, the first acquisition M 1 is performed with an ␣-2␣ sequence, and the second acquisition M 2 is performed with a 2␣-4␣ sequence. Dividing the magnitude of the first acquisition by the magnitude of the second acquisition gives a function of flip angle ␣:from which the flip angle ␣ is calculated asIn GRE double angle methods, two GRE acquisitions are performed with flip angles ␣ and 2␣. The ratio of signal intensities of the two acquisitions is thenEquations [1] and [3] assume that TR 3 ϱ. Other B 1 mapping methods which derive estimates of B 1 from signal intensities have been developed. One method uses two acquisitions with the same flip angle but alternating variable short sequence repetition time (TR) (12). Another method uses three GRE acquisitions with flip angle distributed around 180°and fits the observed signal intensity to the expected 180°signal null (13). In another technique, flip angle is calculated from the ratio of a spin echo and a stimulated echo, similar to double angle methods but with both echoes obtained during the same sequence repetition (14).A unique approach to B 1 mapping uses a series of RF pulses to produce a transverse magnetization whose phase is a function of flip angle (15,16). This method, with some refinements, is presented in this study as the phase-sensitive B 1 mapping method. THEORYThe phase-sensitive B 1 -mapping method is performed by the application of a nonselective 180°pulse about the x axis followed immediately by a nonselective 90°flip about the y axis. B 1 inhomogeneity results in the actual flip angles of the two nonselective pulses deviating from 180°a nd 90°to give unknown angles 2␣ and ␣.In the absence of main field (B 0 ) inhomogeneity, the initial 2␣ pulse about the x axis rotates the initial longitudinal magnetization M 0 into the y-z plane. The subsequent ␣ flip about the y axis br...
Propagation of errors, in conjunction with the theoretical signal equation for spoiled gradient echo pulse sequences, is used to derive a theoretical expression for uncertainty in quantitative variable flip angle T(1) mapping using two flip angles. This expression is then minimized to derive a rigorous expression for optimal flip angles that elucidates a commonly used empirical result. The theoretical expressions for uncertainty and optimal flip angles are combined to derive a lower bound on the achievable uncertainty for a given set of pulse sequence parameters and signal-to-noise ratio (SNR). These results provide a means of quantitatively determining the effect of changing acquisition parameters on T(1) uncertainty.
in the AE/RE group demonstrated additional improvements in thigh lean tissue and BMI. Improvements in thigh lean tissue may be important in this population as a means to increase resting metabolic rate, protein reserve, exercise tolerance, and functional mobility.
Dynamic shimming has been implemented in three pulse sequences on a commercial GE Signa 1.5-T imaging system. Multi-slice field maps are acquired before the imaging sequence, and linear shim terms and center frequencies are calculated for each slice. During the imaging scan, the linear shim terms and center frequency are set before each pulse sequence repetition according to the current slice. Acquisition of multi-slice field maps and calculation of shim terms and center frequency for each slice are accomplished in a matter of seconds. Pulse sequences require only minimal modification to add dynamic shimming capability. Results are shown for a fat saturation spin-echo sequence, a single-shot echo-planar gradient-recalled echo sequence, and a spiral acquisition gradient-recalled echo sequence. In all cases, dynamic shimming with shim currents and center frequency optimized for each slice is shown to give better results than constant shim currents and a single center frequency optimized for the entire volume of interest.
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