Function Of Flip Angle Research Articles

Breast MRI at 7 Tesla with a bilateral coil and robust fat suppression.

To develop a bilateral coil and fat suppressed T1-weighted sequence for 7 Tesla (T) breast MRI. A dual-solenoid coil and three-dimensional (3D) T1w gradient echo sequence with B1+ insensitive fat suppression (FS) were developed. T1w FS image quality was characterized through image uniformity and fat-water contrast measurements in 11 subjects. Signal-to-noise ratio (SNR) and flip angle maps were acquired to assess the coil performance. Bilateral contrast-enhanced and unilateral high resolution (0.6 mm isotropic, 6.5 min acquisition time) imaging highlighted the 7T SNR advantage. Reliable and effective FS and high image quality was observed in all subjects at 7T, indicating that the custom coil and pulse sequence were insensitive to high-field obstacles such as variable tissue loading. 7T and 3T image uniformity was similar (P=0.24), indicating adequate 7T B1+ uniformity. High 7T SNR and fat-water contrast enabled 0.6 mm isotropic imaging and visualization of a high level of fibroglandular tissue detail. 7T T1w FS bilateral breast imaging is feasible with a custom radiofrequency (RF) coil and pulse sequence. Similar image uniformity was achieved at 7T and 3T, despite different RF field behavior and variable coil-tissue interaction due to anatomic differences that might be expected to alter magnetic field patterns.

Complete description of the interactions of a quadrupolar nucleus with a radiofrequency field. Implications for data fitting

We present a theory, with experimental tests, that treats exactly the effect of radiofrequency (RF) fields on quadrupolar nuclei, yet retains the symbolic expressions as much as possible. This provides a mathematical model of these interactions that can be easily connected to state-of-the-art optimization methods, so that chemically-important parameters can be extracted from fits to experimental data. Nuclei with spins >1/2 typically experience a Zeeman interaction with the (possibly anisotropic) local static field, a quadrupole interaction and are manipulated with RF fields. Since RF fields are limited by hardware, they seldom dominate the other interactions of these nuclei and so the spectra show unusual dependence on the pulse width used. The theory is tested with 23Na NMR nutation spectra of a single crystal of sodium nitrate, in which the RF is comparable with the quadrupole coupling and is not necessarily on resonance with any of the transitions. Both the intensity and phase of all three transitions are followed as a function of flip angle. This provides a more rigorous trial than a powder sample where many of the details are averaged out. The formalism is based on a symbolic approach which encompasses all the published results, yet is easily implemented numerically, since no explicit spin operators or their commutators are needed. The classic perturbation results are also easily derived. There are no restrictions or assumptions on the spin of the nucleus or the relative sizes of the interactions, so the results are completely general, going beyond the standard first-order treatments in the literature.

Fat fraction bias correction using T1 estimates and flip angle mapping

To develop a new method of reducing T1 bias in proton density fat fraction (PDFF) measured with iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL). PDFF maps reconstructed from high flip angle IDEAL measurements were simulated and acquired from phantoms and volunteer L4 vertebrae. T1 bias was corrected using a priori T1 values for water and fat, both with and without flip angle correction. Signal-to-noise ratio (SNR) maps were used to measure precision of the reconstructed PDFF maps. PDFF measurements acquired using small flip angles were then compared to both sets of corrected large flip angle measurements for accuracy and precision. Simulations show similar results in PDFF error between small flip angle measurements and corrected large flip angle measurements as long as T1 estimates were within one standard deviation from the true value. Compared to low flip angle measurements, phantom and in vivo measurements demonstrate better precision and accuracy in PDFF measurements if images were acquired at a high flip angle, with T1 bias corrected using T1 estimates and flip angle mapping. T1 bias correction of large flip angle acquisitions using estimated T1 values with flip angle mapping yields fat fraction measurements of similar accuracy and superior precision compared to low flip angle acquisitions.

Flip angle mapping with the accelerated 3D look‐locker sequence

A new approach to mapping the flip angle quickly and efficiently in 3D based on the Look-Locker technique is presented. We modified the accelerated 3D Look-Locker T1 measurement technique to allow rapid measurement of flip angle. By removing the inversion pulses and interleaving two radio frequency pulses with different amplitude, it is possible to fit directly for the true flip angle using a reduced number of parameters. This technique, non-inverted Double Angle Look-Locker, allows quick and efficient mapping of the flip angle in 3D. non-inverted Double Angle Look-Locker is validated in vitro against the actual flip angle imaging technique for a range of flip angles and T1 values. Flip angle maps produced with non-inverted Double Angle Look-Locker can be acquired in approximately 1 min, and are accurate to within 10% of the actual flip angle imaging measurement. It is shown to accurately measure the excited slab profile of several different pulses. An application to correcting in vivo DESPOT T1 data is presented. The presented technique is a rapid method for mapping flip angles across a 3D volume, capable of producing a flip angle map in approximately 1 min.

Improving RF field strength and uniformity in a local region by controlling individual transmitting RF phase in a 7T tranceive array coil

To improve the radiofrequency (RF) field strength and uniformity in a local imaging region, individual RF transmitting phases were controlled in surface coil elements in a 7T transceive array coil; the RF field distribution was compared with the conventional in-phase approach at 7 T. Optimal RF transmitting phases in the individual coil elements in a four-channel transceive array coil were numerically calculated using the electromagnetic (EM) field solver to obtain uniform RF field in a local imaging region. In 7 T phantom experiments, the RF field uniformity and mean SNR were evaluated on gradient echo images. In addition, a flip angle map was obtained by the double angle method. Experimental results clearly show that the EM calculation yielded an improved RF field strength and uniformity in the specific imaging region along the center of the coil array when the RF transmitting phase offset was 180° between the left and right rows of surface coil elements. In a 7 T experiment, B1 field uniformity and mean SNR with this phase offset were better than the other cases and the FA map showed a more focused and symmetric distribution as compared to other cases. Even though the RF field distribution in the transceive array coil is still strongly affected by dielectric properties at 7 T, the 180° phase offset between the left and right rows of the coil elements gave the highest magnetic flux density in the specific imaging.

Myocardial perfusion MRI with an undersampled 3D stack‐of‐stars sequence

To determine the feasibility of three-dimensional (3D) hybrid radial (stack-of-stars) MRI with spatiotemporal total variation (TV) constrained reconstruction for dynamic contrast enhanced myocardial perfusion imaging. An ECG-triggered saturation recovery turboFLASH sequence with undersampled stack-of-stars sampling with spatiotemporal TV constrained reconstruction was developed for dynamic contrast enhanced myocardial perfusion imaging. Simulations were performed to study the dependence of the approach to steady state on flip angle and saturation recovery time for this stack-of-stars acquisition. Phantom studies were used to show the effect of the flip angle selection and imperfect spoiling on image qualities. Studies were done in three humans to test the feasibility of the approach for myocardial perfusion imaging. The simulation and phantom studies showed that imperfect spoiling and magnetization changes during the readout were a function of flip angle and nonoptimized selection of flip angle could degrade the images. Low flip angle acquisitions in the human subjects result in images with good quality similar to multislice radial 2D images. 3D stack-of-stars sampling with spatiotemporal TV constrained reconstruction provides a promising alternative for myocardial perfusion imaging.

High‐resolution mapping of human brain metabolites by free induction decay 1H MRSI at 7 T

This work describes a new approach for high-spatial-resolution (1)H MRSI of the human brain at 7 T. (1)H MRSI at 7 T using conventional approaches, such as point-resolved spectroscopy and stimulated echo acquisition mode with volume head coils, is limited by technical difficulties, including chemical shift displacement errors, B(0)/B(1) inhomogeneities, a high specific absorption rate and decreased T(2) relaxation times. The method presented here is based on free induction decay acquisition with an ultrashort acquisition delay (TE*) of 1.3 ms. This allows full signal detection with negligible T(2) decay or J-modulation. Chemical shift displacement errors were reduced to below 5% per part per million in the in-slice direction and were eliminated in-plane. The B(1) sensitivity was reduced significantly and further corrected using flip angle maps. Specific absorption rate requirements were well below the limit (~20 % = 0.7 W/kg). The suppression of subcutaneous lipid signals was achieved by substantially improving the point-spread function. High-quality metabolic mapping of five important brain metabolites was achieved with high in-plane resolution (64 × 64 matrix with a 3.4 × 3.4 × 12 mm(3) nominal voxel size) in four healthy subjects. The ultrashort TE* increased the signal-to-noise ratio of J-coupled resonances, such as glutamate and myo-inositol, several-fold to enable the mapping of even these metabolites with high resolution. Four measurement repetitions in one healthy volunteer provided proof of the good reproducibility of this method. The high spatial resolution allowed the visualization of several anatomical structures on metabolic maps. Free induction decay MRSI is insensitive to T(2) decay, J-modulation, B(1) inhomogeneities and chemical shift displacement errors, and overcomes specific absorption rate restrictions at ultrahigh magnetic fields. This makes it a promising method for high-resolution (1)H MRSI at 7 T and above.

K‐space filter deconvolution and flip angle self‐calibration in 2D radial hyperpolarised 3He lung MRI

In hyperpolarised (3)He lung MRI with constant flip angles, the transverse magnetisation decays with each RF excitation imposing a k-space filter on the acquired data. For radial data acquired in an angularly-sequential order, this filter causes streaking, angular shading and loss of spatial resolution in the images. The main aim of this work was to reduce the effects of the RF depletion k-space filter in radial acquisitions. Two approaches are presented; (i) retrospective deconvolution of the k-space filter for sequentially-acquired data and (ii) golden angle acquisition order. Radial trajectories sample the centre of k-space with every projection, thereby self-tracking signal decay. The inverse of the signal decay function was used to retrospectively deconvolve RF depolarisation k-space filter effects and the method was demonstrated in 2D radial imaging in phantoms and human lungs. A golden angle radial acquisition was shown to effectively suppress artefacts caused by the RF depletion k-space filter. In addition, the average flip angle per slice was calculated from the signal decay and the values were found to correspond with conventional flip angle maps, providing a means of flip angle self-calibration.

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 (B(1) ) 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 B(1) 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 B(1) field within 1 min using FAs no larger than 60°. We show that it is robust against T(1), small chemical shift, and mild background inhomogeneity. This method may especially be suitable for B(1) mapping in situations (e.g., long-T(1) 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.

B1+/actual flip angle and reception sensitivity mapping methods: simulation and comparison

Knowledge of the spatial distribution of transmission field B 1 + and reception sensitivity maps is important in high-field (≥3 T) human magnetic resonance (MR) imaging for several reasons: these include post-acquisition correction of intensity inhomogeneities, which may affect the quality of images; modeling and design of radiofrequency (RF) coils and pulses; validating theoretical models for electromagnetic field calculations; testing the compatibility with MR environment of biomedical implants. Moreover, inhomogeneities in the RF field are an essential source of error for quantitative MR spectroscopy. Recent studies have also shown that B 1 + and reception sensitivity maps can be used for direct calculation of tissue electrical parameters and for estimating the local specific absorption rate (SAR) in vivo. Several B 1 + mapping techniques have been introduced in the past few years based on actual flip angle (FA) mapping, but, to date, none has emerged as a standard. For reception sensitivity calculation, the signal intensity equation can be used where the nominal FA distribution must be replaced with the actual FA distribution calculated by one of the B 1 + mapping techniques. This study introduces a quantitative comparison between two known methods for B 1 +/actual FA and reception sensitivity mapping: the double-angle method (DAM) and the fitting (FIT) method. Experimental data obtained using DAM and FIT methods are also compared with numerical simulation results.

An analysis of the accuracy of magnetic resonance flip angle measurement methods

Several methods of flip angle mapping for magnetic resonance imaging have been proposed. We evaluated the accuracy of five methods of flip angle measurement in the presence of measurement noise. Our analysis was performed in a closed form by propagation of probability density functions (PDFs). The flip angle mapping methods compared were (1) the phase-sensitive method, (2) the dual-angle method using gradient recalled echoes (GRE), (3) an extended version of the GRE dual-angle method incorporating phase information, (4) the AFI method and (5) an extended version of the AFI method incorporating phase information. Our analysis took into account differences in required imaging time for these methods in the comparison of noise efficiency. PDFs of the flip angle estimate for each method for each value of true flip angle were calculated. These PDFs completely characterize the performance of each method. Mean bias and standard deviation were computed from these PDFs to more simply quantify the relative accuracy of each method over its range of measurable flip angles. We demonstrate that the phase-sensitive method provides the lowest mean bias and standard deviation of flip angle estimate of the five methods evaluated over a wide range of flip angles.

ToF‐SWI: Simultaneous time of flight and fully flow compensated susceptibility weighted imaging

To perform systematic investigations on parameter selection of a dual-echo sequence (ToF-SWI) for combined 3D time-of-flight (ToF) angiography and susceptibility weighted imaging (SWI). ToF-SWI was implemented on 1.5 T and 3 T MR scanners with complete 3D first-order flow compensation of the second echo. The efficiency of flow compensating the SWI echo was studied based on phantom and in vivo examinations. Arterial and venous contrasts were examined in volunteers as a function of flip angle and compared with additionally acquired single-echo ToF and single-echo SWI data. Complete flow compensation is required to reduce arterial contamination in the SWI part caused by signal voids. A ramped flip angle of 20 degrees depicted arteries best while venous contrast was preserved. Comparing ToF-SWI with single-echo ToF demonstrated arteries with similar quality and delineated all major arteries equally well. Venous delineation was degraded due to lower SNR associated with the thinner slabs used with ToF-SWI compared to single-echo SWI acquisition. A dual-echo sequence (ToF-SWI) with full flow compensation of the second echo in a single scan is feasible. This sequence allows simultaneous visualization of intrinsically coregistered arteries and veins without spatial mis-registration of vessels caused by oblique flow and with minimal signal loss in arteries.

On the use of steady-state signal equations for 2D TrueFISP imaging

To explain the signal behavior in 2D-TrueFISP imaging, a slice excitation profile should be considered that describes a variation of effective flip angles and magnetization phases after excitation. These parameters can be incorporated into steady-state equations to predict the final signal within a pixel. The use of steady-state equations assumes that excitation occurs instantaneously, although in reality this is a nonlinear process. In addition, often the flip angle variation within the slice excitation profile is solely considered when using steady-state equations, while TrueFISP is especially known for its sensitivity to phase variations. The purpose of this study was therefore to evaluate the precision of steady-state equations in calculating signal intensities in 2D TrueFISP imaging. To that end, steady-state slice profiles and corresponding signal intensities were calculated as function of flip angle, RF phase advance and pulse shape. More complex Bloch simulations were considered as a gold standard, which described every excitation within the sequence until steady state was reached. They were used to analyze two different methods based on steady-state equations. In addition, measurements on phantoms were done with corresponding imaging parameters. Although the Bloch simulations described the steady-state slice profile formation better than methods based on steady-state equations, the latter performed well in predicting the steady-state signal resulting from it. In certain cases the phase variation within the slice excitation profile did not even have to be taken into account.

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.

Comparison of three commercially available radio frequency coils for human brain imaging at 3 Tesla

To evaluate a transverse electromagnetic (TEM), a circularly polarized (CP) (birdcage), and a 12-channel phased array head coil at the clinical field strength of B0 = 3T in terms of signal-to-noise ratio (SNR), signal homogeneity, and maps of the effective flip angle alpha. SNR measurements were performed on low flip angle gradient echo images. In addition, flip angle maps were generated for alpha(nominal) = 30 degrees using the double angle method. These evaluation steps were performed on phantom and human brain data acquired with each coil. Moreover, the signal intensity variation was computed for phantom data using five different regions of interest. In terms of SNR, the TEM coil performs slightly better than the CP coil, but is second to the smaller 12-channel coil for human data. As expected, both the TEM and the CP coils show superior image intensity homogeneity than the 12-channel coil, and achieve larger mean effective flip angles than the combination of body and 12-channel coil with reduced radio frequency power deposition. At 3T the benefits of TEM coil design over conventional lumped element(s) coil design start to emerge, though the phased array coil retains an advantage with respect to SNR performance.

Fast, accurate, and precise mapping of the RF field in vivo using the 180° signal null

RF field B1 nonuniformity is the largest cause of error in the quantitative measurement of many clinically relevant parameters in MR images and spectra. Knowledge of the absolute flip angles at every region will improve the accuracy and precision of such parameters. This method uses the 180 degrees signal null to construct a flip angle map of the entire brain in less than 4 min, independent of T1, T2, and proton density. Three spoiled gradient echo volume acquisitions of the whole brain were made with three different flip angles. The optimum choice of flip angles was determined to be 145 degrees, 180 degrees, and 215 degrees. Linear regression analysis was used to determine the nominal (system calibrated) flip angle required for a signal null at every pixel and thence determine the absolute flip angle at that location. The experiment utilizes an existing MR sequence supplied by the scanner manufacturer. The technique is validated experimentally and a theoretical investigation into the optimum experimental parameters is presented.

Factors influencing flip angle mapping in MRI: RF pulse shape, slice‐select gradients, off‐resonance excitation, and B0 inhomogeneities

To understand the various effects that influence actual flip angles, and correct for these effects, it is important to precisely quantify the MRI parameters (such as T1, T2, and perfusion). In this paper actual flip angle maps are calculated using a conventional gradient-echo (GRE) sequence with different radiofrequency (RF) pulse shapes (Gaussian, sinc, and truncated-sinc), slice-selection gradients, off-resonance excitations, and B0 field inhomogeneities. The experimental results demonstrate that RF pulse shapes significantly affect the flip angle distribution and calibration factors. Off-resonance RF excitations, B0 nonuniformities, and slice-selection gradients can lead to degradations in the signal intensities of the images used to map the flip angle, and potentially introduce a bias and increased variance in the measured flip angles.

Evaluation of the RF field uniformity of a double‐tuned 31P/1H birdcage RF coil for spin‐echo MRI/MRS of the diabetic foot

To evaluate the B1 field uniformity of a double-tuned birdcage coil designed for (31)P/(1)H MRI/MRS spin-echo (SE) imaging of the metatarsal head region of the foot in neuropathic diabetic patients. A low-pass double-tuned (31)P/(1)H RF birdcage coil was constructed to fit over the adult forefoot. Flip angle (FA) maps were created from B1 data acquired at the 3T (31)P (four normal subjects) and (1)H (five normal subjects) frequencies. T2-weighted (T2-W) (1)H images, (31)P rapid acquisition with relaxation enhancement (RARE) images, and composite SE pulse CSI data were acquired to demonstrate the uniformity of the resulting images and data. The means and standard deviations (SDs) of the range of FAs across the feet of the volunteer subjects indicated good uniformity (the maximum coefficients of variation (CVs) for all of the (31)P and (1)H FA maps were 7.6% and 7.3%, respectively). The FA values across the metatarsal head region indicated a maximum signal intensity variation of +/-3% in a RARE image acquired using an echo train length of 32. A (31)P/(1)H birdcage coil constructed for MRI/MRS studies of the human forefoot provided sufficient signal uniformity of SE data to facilitate accurate (31)P concentration measurements in muscle.

Fast high‐resolution T1 mapping of the human brain

A sequence for the acquisition of high-resolution T1 maps, based on magnetization-prepared multislice fast low-angle shot (FLASH) imaging, is presented. In contrast to similar methods, no saturation pulses are used, resulting in an increased dynamic range of the relaxation process. Furthermore, it is possible to acquire data during all relaxation delays because only slice-selective radiofrequency (RF) pulses are used for inversion and excitation. This allows for a reduction of the total acquisition time, or scanning with a reduced bandwidth, which improves the signal-to-noise ratio (SNR). The method generates quantitative T1 maps with an in-plane resolution of 1 mm, slice thickness of 4 mm, and whole-brain coverage in a clinically acceptable imaging time of about 19 s per slice. It is shown that the use of off-center RF pulses does not result in imperfect inversion or magnetization transfer (MT) effects. In addition, an improved fitting algorithm based on smoothed flip angle maps is presented and tested successfully.

肝臓ダイナミックMRIにおける3D-VIBE法の至適フリップ角の検討

The aim of this study was to determine a moderate flip angle (FA) for dynamic liver MR imaging (MRI) with the three-dimensional volumetric interpolated breath-hold examination (3D-VIBE) technique. Images of phantoms with various T(1) values (44-560 msec) were acquired with the 3D-VIBE sequence (TR/TE=5.2/2.6 msec) using different FA (5-50 degree). We estimated signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR), considered to indicate tumor-to-liver contrast, as a function of FA. In phantoms, in which T(1) values (44-191 msec) were assumed to be shortened by the effect of Gd-DTPA in hepatocellular carcinoma (HCC), the highest SNR in each phantom was observed at FA ranging from 15 to 30 degrees. SNRs in other phantoms, in which T(1) values (298-560 msec) were assumed to be normal liver-tissue pre- or post-enhancement, were high with FA of 10-12 degrees, and were remarkably decreased with FA of more than 30 degrees. CNR increased as FA increased in every phantom, especially in the phantom with shortened T(1) values (44-191 msec), suggesting that enlarging FA improved the tumor-to-liver contrast. Taking both results of SNR and CNR into account, we concluded that a moderate FA was approximately 25 degrees. The advantage with an FA of 25 degrees was confirmed in a clinical study of a patient with hypervascular HCC, in which we could observe coronal enhancement surrounding the lesion in the late phase of the double arterial phase by dynamic MRI using the 3D-VIBE technique.