Range Of Flip Angles Research Articles

Task-based optimization of flip angle for fibrosis detection in T1-weighted MRI of liver.

Chronic liver disease is a worldwide health problem, and hepatic fibrosis (HF) is one of the hallmarks of the disease. The current reference standard for diagnosing HF is biopsy followed by pathologist examination; however, this is limited by sampling error and carries a risk of complications. Pathology diagnosis of HF is based on textural change in the liver as a lobular collagen network that develops within portal triads. The scale of collagen lobules is characteristically in the order of 1 to 5mm, which approximates the resolution limit of in vivo gadolinium-enhanced magnetic resonance imaging in the delayed phase. We use MRI of formalin-fixed human ex vivo liver samples as phantoms that mimic the textural contrast of in vivo Gd-MRI. We have developed a local texture analysis that is applied to phantom images, and the results are used to train model observers to detect HF. The performance of the observer is assessed with the area-under-the-receiver-operator-characteristic curve (AUROC) as the figure-of-merit. To optimize the MRI pulse sequence, phantoms were scanned with multiple times at a range of flip angles. The flip angle that was associated with the highest AUROC was chosen as optimal for the task of detecting HF.

Data-driven optimized flip angle selection for T1 estimation from spoiled gradient echo acquisitions.

Define criteria for selection of optimal flip angle sets for T1 estimation and evaluate effects on T1 mapping. Flip angle sets for spoiled gradient echo-based T1 mapping were selected by minimizing T1 estimate variance weighted by the joint density of M0 and T1 in an initial acquisition. The effect of optimized flip angle selection on T1 estimate error was measured using simulations and experimental data in the human and rat brain. For two-point acquisitions, optimized angle sets were similar to those proposed by other groups and, therefore, performed similarly. For multipoint acquisitions, optimal angle sets for T1 mapping in the brain consisted of a repetition of two angles. Implementation of optimal angles reduced T1 estimate variance by 30-40% compared with a multipoint acquisition using a range of angles. Performance of the optimal angle set was equivalent to that of a repetition of the two-angle set selected using criteria proposed by other researchers. Repetition of two carefully selected flip angles notably improves the precision of resulting T1 estimates compared with acquisitions using a range of flip angles. This work provides a flexible and widely applicable optimization method of particular use for those who repeatedly perform T1 estimation. Magn Reson Med 76:792-802, 2016. © 2015 Wiley Periodicals, Inc.

Flip angle optimization for quantitative phase contrast MR imaging

Background PC-MRI is a noninvasive imaging technique used to measure the velocity of flowing blood in a particular blood vessel with flexible spatial and temporal resolution [1,2]. It is considered the clinical “gold standard” for quantification of blood flow. PC-MRI boasts a variety of established applications in quantifying cardiovascular function and hemodynamics. In particular, this technique offers clinicians with a means of measuring peak velocity, mean velocity, flow rate, and total flow throughout the vasculature of the human body. One of the challenges in PC-MRI is the wide range of variables that make up a given protocol requiring careful optimization of each imaging parameter in an effort to accomplish the goals of a given scan. Surprisingly, the current PC-MRI utilizes a default flip angle, which has not been optimized. A theoretical description of the signal acquired from flowing spins in spoiled gradient-echo pulse sequences has been previously developed [3]. These efforts indicate that there exists a flip angle that provides an optimized SNR for a given tissue, echo time, repetition time, slice thickness and velocity, which leads to increased reproducibility of quantitative PC-MRI measurements. This optimization cannot be performed empirically because of the number of parameters involved, hence mathematical and computation work is required. Methods All measurements were performed on a 1.5 Tesla system (Avanto; Siemens Medical Solutions, Erlangen, Germany) using a 16-channel surface coil. Blood velocity data in the common femoral arteries were obtained in 5 healthy consenting volunteers (4 male, 1 female, age __ ± __ years). PC-MRI was performed without breathhold using retrospective ECG triggering. Imaging parameters included 340 x 212.5 mm field of view, 256 x 160 matrix, 1.3 mm isotropic resolution, 8 mm slice thickness, 5.4/3.2 msec TR/TE, 32.4 msec temporal resolution, a range of flip angles for optimization (5,10,15,20,30,40,60,75 and 90°), 888 Hz/pixel acquisition bandwidth, three views per segment and a velocity encoding strength of 75 cm/sec (53 second scan time). The imaging was repeated 5 times in each volunteer.

Synthesis methods of multiple phase-cycled SSFP images to reduce the band artifact and noise more reliably

The band artifact in steady-state free precession can be reduced by synthesizing the multiple images obtained through different phase increments of successive radiofrequency pulses. Even though the complex summation method was reported to be effective in reducing the band artifact, it has the pitfalls of intensity abnormality and sensitivity to the phase abnormality. Two new methods have been developed for more reliable reduction of the band artifact than the complex summation method. One method is to sum the complex images partially and to take the maximum intensity of the partially summed images. The other method is to sum the free induction decay (FID) and primary echo components of the Fourier series that are obtained through Fourier analysis of the complex base images. Both proposed methods were compared with other magnitude (maximum intensity projection, spectrally decomposed synthesis, sum-of-squares, nonlinear averaging) and complex-based (complex summation, magnitude-weighted complex summation) methods experimentally at 3 T for the phantom and volunteer's head imaging. Both proposed methods were confirmed to maintain the advantage of the complex summation in reducing both the dark and bright band artifacts while reducing the intensity abnormality and sensitivity to the phase abnormality from that of the complex summation method over a wide range of flip angles and relaxation times.

Measurement and correction of transmitter and receiver induced nonuniformities in vivo

Signal intensity nonuniformities in high field MR imaging limit the ability of MRI to provide quantitative information and can negatively impact diagnostic scan quality. In this paper, a simple method is described for correcting these effects based on in vivo measurement of the transmission field B1+ and reception sensitivity maps. These maps can be obtained in vivo with either gradient echo (GE) or spin echo (SE) imaging sequences, but the SE approach exhibits an advantage over the GE approach for correcting images over a range of flip angles. In a uniform phantom, this approach reduced the ratio of the signal SD to its mean from around 30% before correction to approximately 6% for the SE approach and 9% for the GE approach after correction. The application of the SE approach for correcting intensity nonuniformities is demonstrated in vivo with human brain images obtained using a conventional spin echo sequence at 3.0 T. Furthermore, it is also shown that this in vivo B1+ and reception sensitivity mapping can be performed using segmented echo planar imaging sequences providing acquisition times of less than 2 min. Although the correction presented here is demonstrated with a simultaneous transmit and receive volume coil, it can be extended to the case of separate transmission and reception coils, including surface and phase array coils.

On neglecting chemical exchange effects when correcting in vivo (31)P MRS data for partial saturation.

Signal acquisition in most MRS experiments requires a correction for partial saturation that is commonly based on a single exponential model for T(1) that ignores effects of chemical exchange. We evaluated the errors in (31)P MRS measurements introduced by this approximation in two-, three-, and four-site chemical exchange models under a range of flip-angles and pulse sequence repetition times (T(R)) that provide near-optimum signal-to-noise ratio (SNR). In two-site exchange, such as the creatine-kinase reaction involving phosphocreatine (PCr) and gamma-ATP in human skeletal and cardiac muscle, errors in saturation factors were determined for the progressive saturation method and the dual-angle method of measuring T(1). The analysis shows that these errors are negligible for the progressive saturation method if the observed T(1) is derived from a three-parameter fit of the data. When T(1) is measured with the dual-angle method, errors in saturation factors are less than 5% for all conceivable values of the chemical exchange rate and flip-angles that deliver useful SNR per unit time over the range T(1)/5 < or = T(R) < or = 2T(1). Errors are also less than 5% for three- and four-site exchange when T(R) > or = T(1)(*)/2, the so-called "intrinsic" T(1)'s of the metabolites. The effect of changing metabolite concentrations and chemical exchange rates on observed T(1)'s and saturation corrections was also examined with a three-site chemical exchange model involving ATP, PCr, and inorganic phosphate in skeletal muscle undergoing up to 95% PCr depletion. Although the observed T(1)'s were dependent on metabolite concentrations, errors in saturation corrections for T(R) = 2 s could be kept within 5% for all exchanging metabolites using a simple interpolation of two dual-angle T(1) measurements performed at the start and end of the experiment. Thus, the single-exponential model appears to be reasonably accurate for correcting (31)P MRS data for partial saturation in the presence of chemical exchange. Even in systems where metabolite concentrations change, accurate saturation corrections are possible without much loss in SNR.

Signal Intensities andT1 Calculations in Multiple-Echo Sequences with Imperfect Pulses

Formulas are derived for longitudinal magnetization in multiple-echo inversion recovery (IR), saturation recovery, and progressive saturation pulse sequences with imperfect pulses. The effects of imperfect pulses on the signal intensities and on the calculated T1 relaxation times are discussed. Relaxation data from all three methods can be fitted with an equation of the form: Signal intensity=[a+b exp(−TX/T1)]/[1+c exp(−TX/T1)], where TX is a variable delay time and the values of T1, a, b, and c are adjusted to optimize the fit. Only three parameters need to be fitted for progressive saturation with no echoes because b=−a. For modified fast IR (with constant repetition time TR), c=0 and a three-parameter fit suffices even if several non–steady-state signals are acquired and a range of flip angles is present within the sample. The advantages of the modified fast IR method for multiple-slice measurements of T1 in magnetic resonance imaging (MRI) are reviewed. Other applications of these formulas include estimating errors in T1 calculations, finding the flip angles (“Ernst angles”), echo spacing, and TR that maximize the signal-to-noise ratio in multiple-echo sequences, optimizing contrast between tissues in MRI, and determining how quickly a steady state is approached. ©1999 John Wiley & Sons, Inc. Concepts Magn Reson 11: 29–49, 1999

Signal Intensities and T1 Calculations in Multiple‐Echo Sequences with Imperfect Pulses

Formulas are derived for longitudinal magnetization in multiple-echo inversion recovery (IR), saturation recovery, and progressive saturation pulse sequences with imperfect pulses. The effects of imperfect pulses on the signal intensities and on the calculated T1 relaxation times are discussed. Relaxation data from all three methods can be fitted with an equation of the form: Signal intensity=[a+b exp(−TX/T1)]/[1+c exp(−TX/T1)], where TX is a variable delay time and the values of T1, a, b, and c are adjusted to optimize the fit. Only three parameters need to be fitted for progressive saturation with no echoes because b=−a. For modified fast IR (with constant repetition time TR), c=0 and a three-parameter fit suffices even if several non–steady-state signals are acquired and a range of flip angles is present within the sample. The advantages of the modified fast IR method for multiple-slice measurements of T1 in magnetic resonance imaging (MRI) are reviewed. Other applications of these formulas include estimating errors in T1 calculations, finding the flip angles (“Ernst angles”), echo spacing, and TR that maximize the signal-to-noise ratio in multiple-echo sequences, optimizing contrast between tissues in MRI, and determining how quickly a steady state is approached. ©1999 John Wiley & Sons, Inc. Concepts Magn Reson 11: 29–49, 1999

INEPT-enhanced 13C spectroscopy using double-tuned surface coils

The problems of in vivo 13C spectroscopy are threefold, viz., the low sensitivity of the 13C nucleus, the low natural abundance of 13C (1. 1’S), and the low in vivo concentration of metabolites. Despite these difficulties 13C spectra of natural abundance glycogen have been obtained (I), but the experimental duration of such experiments is relatively long. This article describes an attempt to increase the sensitivity of 13C spectroscopy for in vivo applications by means of the INEPT polarization transfer technique (2). INEPT, nuclear Overhauser enhancement (NOE), and DEPT (3, 4) are established techniques for increasing the sensitivity of low-y nuclei, by a transfer of polarization from a J-coupled higher-y nucleus, normally ‘H. The in vivo application of these techniques for i3C spectroscopy is handicapped by the low intrinsic sensitivity of the experiment, which requires that signal reception be via a surface coil. However, if this is also used for transmission, then there is an extremely inhomogeneous Bi field. If signal transmission is by means of a larger coil, with a homogeneous field, then it may be necessary to decouple the coils. There is also a reduction in the effective bore size due to the presence of two coils. A number of schemes for 90” excitation, 180” refocusing, and inversion pulses have been suggested that are largely insensitive to the effects of BI inhomogeneity (5, 6). These in principle provide the requisite pulses for INEPT by surface coil excitation only. The full implementation of DEPT requires RF pulses having a range of flip angles, which is at present unrealistic for surface coil excitation, although DEPT enhancement of the signal from CH groups would be possible in these circumstances. INEPT has the advantage over NOE that the effective T1 is governed by the proton relaxation rate, not the longer ‘H-13C cross-relaxation rates, and the advantage over DEPT that CH, CH1, and CH3 groups are equally enhanced. However, the basic INEPT sequence gives spectra that are difficult to interpret, while refocused INEPT requires more RF pulses than DEPT. The basic INEPT pulse sequence is