Excitation Flip Angle Research Articles

Fast multi-planar gradient echo MR imaging: impact of variation in pulse sequence parameters on image quality and artifacts.

The purpose of this investigation was to quantitatively evaluate the practical impact of alteration of key imaging parameters on image quality and artifacts in fast multi-planar gradient echo (GRE) pulse sequences. These include multi-planar GRASS (MPGR) and fast multi-planar spoiled GRASS (FMPSPGR). We developed a composite phantom with different T(1) and T(2) values comprising the range of common biological tissues, which was also subjected to periodic motion in order to evaluate motion effects. Magnetic resonance imaging was performed on a GE Signa 1.5-T system. Experimental variations in key parameters included excitation flip angle (FL), echo time (TE), repetition time (TR), and receive bandwidth (BW). Quantitative analysis consisted of signal-to-noise-ratio (SNR) and contrast (CN), image nonuniformity (NU), full-width-at-half-maximum (FWHM) (i.e., blurring or geometric distortion), and ghosting ratio (GR). We found that flip angle, TE, and TR play particularly critical roles in determining image signal, homogeneity, and ghosting artifact with these sequences. Optimum clinical application of these pulse sequences requires careful attention to these imaging parameters and to their complex interactions.

Studies on developments of imaging and analytical methods and evaluations of properties mainly on image quality in MRI (in Japanese)

The aims of this thesis were to develop imaging and analytical methods in magnetic resonance imaging (MRI) to obtain morphological and functional information of the human body and to evaluate image quality of an MRI system. Various new methods are illustrated. A fast imaging method for obtaining a T1‐weighted image by using the rapid acquisition relaxation enhanced sequence combined with the variable flip angle excitation is described. Next, the thesis describes a method for performing a dynamic contrast‐enhanced T1‐weighted and study simultaneously to evaluate regional hemodynamics in brain tumors. This method can increase the amount of vascular information and reveals the hemodynamics of the brain tumor and surrounding regions in greater detail. The thesis describes a frequency analysis of cerebrospinal fluid (CSF) flow measured with an ECG gated phase contrast cine‐MRI to clarify the flow dynamics of intracranial CSF in normal pressure hydrocephalus (NPH). This frequency analysis can noninvasively provide a picture of the pathophysiology of NPH and of intracranial changes. Developments of the above imaging and analytical methods in MRI make it possible to obtain more detailed information on the human body. In addition, the thesis describes a new method to accurately measure the modulation transfer function and the signal‐to‐noise ratio in the positive and negative frequencies in MRI with a specially designed phantom. This evaluation method makes it possible to obtain more detail on the image quality of MRI, and is useful in various performance assessments such as the evaluation of pulse sequences.

Improved in-stent magnetic resonance angiography with high flip angle excitation.

To optimize the intraluminal signal intensity of a nitinol stent by performing contrast-enhanced three-dimensional magnetic resonance angiography (CE-MRA) with varying flip angles (FAs). Contrast-enhanced magnetic resonance angiography at 1.5 T and FAs of 30 degrees, 100 degrees, and 150 degrees was performed on five sheep with 10 iliac nitinol stents (Memotherm-FLEXX). Maximum-intensity projections (MIPs) and composite images of MIPs were performed and compared. Reconstructed MIPs at an FA of 150 degrees showed a slightly disturbed lumen visibility inside the stent accompanied by low-grade lumen visibility outside the stent and vice versa for an FA of 30 degrees. Composite images of a 30 degrees MIP added to a 150 degrees MIP resulted in improved image quality compared with the standard MIP of a single FA. Signal loss due to radiofrequency shielding inside nitinol stents imaged by CE-MRA can be reduced by applying high FAs. Composite MIP images allow simultaneous visualization of the lumen inside as well as outside the stent.

Difftrain: A Novel Approach to a True Spectroscopic Single-Scan Diffusion Measurement

Diffusion-ordered spectroscopy (DOSY) has gained considerable attention over the past decade as a useful tool for calculating diffusion-related parameters or in the analysis of complex (reaction) mixtures. A major drawback of the established methods are the relatively long recording times needed to acquire the spectra, excluding the monitoring of rapidly progressing reactions and (hence) measurements of less stable components. In order to overcome these shortcomings a new pulse sequence, Difftrain, has been developed. The sequence involves stimulated echo attenuation, multilow flip angle excitation, and multiple sampling of the FID during the longitudinal storage. The calculated diffusion parameters obtained by Difftrain were compared with those obtained by the established sequence BPPSTE (bipolar pulse pair stimulated echo) and were in good agreement. For systems with moderate to good signal-to-noise ratios the Difftrain building block yields significant saving in recording time (single-shot acquisition instead of acquiring n-different gradients strengths), thus opening up new applications in nonequilibrium systems and screening of compositions and/or interactions of (larger) compound arrays.

Field mapping without reference scan using asymmetric echo-planar techniques

Improvements in B0 mapping and shimming were achieved by measuring the static field information in multiple subsequent echoes generated by an asymmetric echo-planar readout gradient train. With careful compensation, eddy current effects were shown to affect the adjustment of the shim coils minimally. In addition to reducing the time required for field mapping by two-fold, the sensitivity was simultaneously optimized irrespective of the prevalent T*2 present, thereby minimizing the error of the static field measurement to below 0.1 Hz. With adiabatic low flip-angle excitation, the time required for field mapping was below 1 second. Magn Reson Med 43:319–323, 2000. © 2000 Wiley-Liss, Inc.

An amplitude optimized single-shot hybrid QUEST technique

Rapid MR imaging techniques either deposit high amounts of radio frequency power or require powerful gradient systems with high slew rates, which might not be available on conventional scanners. QUEST provides a fast imaging method with scan times of the order of hundreds of milliseconds and avoids these problems at the cost of low signal-to-noise ratios (SNR). In this work, QUEST was optimized with regard to image quality and measuring time. With the use of a Hybrid QUEST technique, that refocuses the image echoes several times, a spatial resolution of 1.9 mm × 1.6 mm × 5 mm was achieved. By acquiring both the necessary correction data and the image information in a single echo train, the Hybrid QUEST technique was implemented as a true single-shot measurement with a total scan time of 190 ms. Optimization of the excitation flip angles and the amplitude and phase correction methods for image reconstruction resulted in an improved SNR of 53.7 in the white matter of the human head for a 10 mm slice thickness at 1.5 T. In contrast to echo planar imaging techniques, no image distortions were observed with Hybrid QUEST in anatomic regions with many tissue interfaces.

Signal Intensities in FLASH-EPI-Hybrid Sequences

Theoretical considerations on the signal-to-noise ratio (SNR) in FLASH-EPI-Hybrid imaging were published previously. The purpose of this work was to investigate in vivo the signal intensities in Hybrid images as a function of sequence specific parameters. In detail, the SNR as a function of the number of echoes m per RF excitation, the excitation flip angle α, and the dependence on the tissue relaxation times T1 and T2* were studied. In eight healthy subjects brain and abdominal Hybrid images were acquired where m and α were changed independently. Signal intensities in human brain, liver, and kidney were evaluated for each Hybrid experiment. Additionally, T1 and T2* values of these tissue types were quantified to allow for a comparison with the theory. An excellent agreement between calculated and measured signal behavior was found. The theory was therefore validated in vivo and can thus be used to optimize the signal-to-noise in Hybrid experiments.

Two-dimensional multishot echo-planar coronary MR angiography.

This work presents a two-dimensional (2D) multishot echo-planar imaging (EPI) technique for magnetic resonance angiography (MRA) of individual coronary arteries in a 17-heartbeat breath-hold. Conventional 2D and 3D segmented gradient-echo (GRE) coronary MRA requires repetitive excitation of the same slice or slab within each cardiac cycle, which can result in reduced blood signal and in motion artifacts. Two-dimensional multishot EPI can address these limitations by eliminating multiple excitations per cardiac cycle, using large flip-angle excitations, markedly reducing the data acquisition window, and performing oblique multislice 2D imaging. The goal of this study was to assess the feasibility of breath-hold 2D multishot EPI for multislice coronary MRA and to demonstrate its reliability by consistently acquiring high-quality images of the coronary arteries in a series of 16 volunteers.

Single-shot, variable flip-angle slice-selective excitation with four gradient-modulated adiabatic half-passage segments.

Adiabatic pulses, although useful in generating uniform spin nutation in the presence of inhomogeneous B1 fields, are limited for NMR imaging applications due to the lack of slice-selective excitation capability. Selective excitation techniques using gradient modulation have been introduced; however, present methods require either a minimum of two excitations or eight adiabatic segments. Here, a scheme is presented that allows single-shot, arbitrary flip-angle, and slice-selective excitation with only four adiabatic half-passage segments. The technique is demonstrated via computer simulation and experimental tests on a phantom. Furthermore, issues associated with the implementation of these gradient-modulated adiabatic pulses are discussed.

Fuzzy clustering of gradient‐echo functional MRI in the human visual cortex. Part II: Quantification

Fuzzy cluster analysis (FCA) is a new exploratory method for analyzing fMRI data. Using simulated functional MRI (fMRI) data, the performance of FCA, as implemented in the software package Evident, was tested and a quantitative comparison with correlation analysis is presented. Furthermore, the fMRI model fit allows separation and quantification of flow and blood oxygen level dependent (BOLD) contributions in the human visual cortex. In gradient-recalled echo fMRI at 1.5 T (TR = 60 ms, TE = 42 ms, radiofrequency excitation flip angle [theta] = 10 degrees-60 degrees) total signal enhancement in the human visual cortex, ie, flow-enhanced BOLD plus inflow contributions, on average varies from 5% to 10% in or close to the visual cortex (average cerebral blood volume [CBV] = 4%) and from 100% to 20% in areas containing medium-sized vessels (ie, average CBV = 12% per voxel), respectively. Inflow enhancement, however, is restricted to intravascular space (= CBV) and increases with increasing radiofrequency (RF) flip angle, whereas BOLD contributions may be obtained from a region up to three times larger and, applying an unspoiled gradient-echo (GRE) sequence, also show a flip angle dependency with a minimum at approximately 30 degrees. This result suggests that a localized hemodynamic response from the microvasculature at 1.5 T may be extracted via fuzzy clustering. In summary, fuzzy clustering of fMRI data, as realized in the Evident software, is a robust and efficient method to (a) separate functional brain activation from noise or other sources resulting in time-dependent signal changes as proven by simulated fMRI data analysis and in vivo data from the visual cortex, and (b) allows separation of different levels of activation even if the temporal pattern is indistinguishable. Combining fuzzy cluster separation of brain activation with appropriate model calculations allows quantification of flow and (flow-enhanced) BOLD contributions in areas with different vascularization.

Intracranial arteries: prospective blinded comparative study of MR angiography and DSA in 50 patients.

To compare the reliability of magnetic resonance (MR) angiography performed with magnetization transfer suppression and variable flip angle excitation with that of intraarterial digital subtraction angiography (DSA) in imaging of the cerebral arteries. Fifty nonconsecutive patients referred for intraarterial DSA gave informed consent to also undergo MR angiography of the intracranial arteries. MR angiography had a sensitivity of 100% and a specificity of 95% for detection of vessel occlusion (10 abnormalities). For detection of substantial vessel stenosis (seven abnormalities), sensitivity and specificity were both 86%; for detection of aneurysm (six abnormalities), sensitivity was 83% and specificity was 98%. Sensitivity and specificity were 100% for detection of arteriovenous malformation (three abnormalities), vessel displacement (three abnormalities), and extracranial-intracranial bypass (two abnormalities). MR angiography did not reliably demonstrate small feeding vessels of a tumor or faint residual perfusion of a thrombosed aneurysm. MR angiographic diagnosis of disease of the intracranial vasculature has high sensitivity and specificity but still is limited in comparison with intraarterial DSA.

Large angle spin-echo imaging

A study was undertaken to assess the use of excitation flip angles greater than 90 degrees for T1 weighted spin-echo (SE) imaging with a single 180 degrees refocusing pulse and short TR values. Theoretical predictions of signal intensity for SE images with excitation pulse angles of 90-180 degrees were calculated based on the Bloch equations and then measured experimentally from MR images of MnCl2 phantoms of various concentrations. Liver signal-to-noise ratios (SNR) and liver-spleen contrast-to-noise ratios (CNR) were measured from breathhold MR images of the upper abdomen in 16 patients using 90 and 110 degrees excitation flip angles. The theoretical predictions showed significant improvements in SNR with excitation flip angles > 90 degrees, which were more pronounced at small TR values. The phantom studies showed reasonably good agreement with the theoretical predictions in correlating the excitation pulse angle with signal intensity. In the human imaging studies, the 110 degrees excitation pulse angle resulted in a 7.4% (p < .01) increase in liver SNR and an 8.2% (p = .2) increase in liver-spleen CNR compared to the 90 degrees pulse angle at TR = 275 ms. Increased signal intensity resulting from the use of large flip angle excitation pulses with a single echo SE pulse sequence was predicted and confirmed experimentally in phantoms and humans.

True myocardial motion tracking

Myocardial tagging is a powerful tool for the assessment of in-plane cardiac motion. However, for previous myocardial tagging techniques, the imaged slice is fixed with respect to the magnet coordinate system. Thus, images acquired at different heart phases do not always represent the same slice of the myocardium. A new myocardial tagging technique is presented, which takes the through-plane motion into consideration. It involves tagging of the desired myocardial slice and applying a subtraction imaging technique to image just that part of the myocardium. The examination time can be reduced considerably by the acquisition of two one-dimensionally tagged images. To increase the signal-to-noise ratio especially at later heart phases, variable imaging RF excitation flip angles are applied. To reduce motion artifacts a repetitive breathhold scheme was applied. In vivo results demonstrate that the tags can be accurately tracked within the entire heart period with a temporal resolution of 35 ms, even at a top basal level of the heart and right ventricle.

Improved MR angiography: magnetization transfer suppression with variable flip angle excitation and increased resolution.

Results at conventional magnetic resonance (MR) angiography were compared with results at MR imaging with a sequence combining optimized magnetization transfer (MT) saturation and tilted optimized nonsaturating excitation (TONE). Forty images were obtained of five healthy volunteers and five patients with known intracranial vascular abnormalities (four men and six women, aged 22-72 years). Four blinded readers found improved vessel penetration, enhanced vessel-to-background contrast, the better vessel detail in the MT saturation-TONE images than in the conventional three-dimensional time-of-flight MR angiograms.

Intrapelvic two-dimensional time-of-flight magnetic resonance angiography in healthy and diseased subjects

We evaluated the use of magnetic resonance angiography (MRA) based on two-dimensional (2D) time-of-flight (TOF) technique in the pelvic vasculature. In the initial phase of this study, MRA was performed in 10 normal subjects for determination of the optimum imaging parameters. Systemically varied scan parameters included flip angle ranging from 20 degrees to 90 degrees by 10 degrees increments and repetition time (TR) of 30, 40, and 50 ms. The optimum imaging parameters for intrapelvic MR angiography were found to be flip angle of 90 degrees, pulse sequence of 30/4.9 (TR ms/TE ms). Using these imaging parameters, 70-100% of the second division branches of the internal iliac artery could be visualized in each subject. In the second phase of this study, we evaluated the use of MR angiography in patients with intrapelvic vascular disease by comparing its accuracy in evaluating the iliac arteries with that of conventional angiography. In patients with atheromatous disease, the MR angiographic determination was consistent with that of conventional angiography in 23 of 28 visualized arterial segments (82%). In eight lesions in patients with non-stenotic vascular disease or hypervascular pelvic tumours, all lesions were demonstrated clearly with MR angiography. 2D TOF technique with high flip angle excitation is suitable for the evaluation of main iliac branches and diseased vessels.

MR angiography with pulsatile flow

To achieve acceptable scan times, current multiple thin slice and 3D MR angiography (MRA) methods usually are based on continuous data acquisition, without ECG-synchronization. The purpose of this work is to study consequences of pulsatile blood flow for the 2D inflow method. Arterial blood flow and blood signal intensity versus cardiac phase were studied by a 2D phase based method with retrospective cardiac synchronization. Such studies were performed in different parts of the body and with different excitation flip angles. As expected, a clear relation between intensity enhancement and time dependent flow can be demonstrated. The raw data of these multiphase studies was used to simulate alternative inflow MRA data acquisition strategies to improve image quality, without the excessive increase in scan time implied by standard cardiac triggering. The alternatives investigated were data collection during part of the cardiac cycle and cardiac-ordered phase encoding. Simulation results indicate that the best results are obtained by a combination of both strategies. This method was implemented on Philips Gyroscan systems to compare it with standard nontriggered 2D inflow in practical NRA studies. For highly pulsatile flow, much better MR angiograms were obtained in this way.

Coronary arteries: breath-hold MR angiography.

The authors describe a method for performance of ultrafast magnetic resonance (MR) angiography of coronary arteries with a standard clinical MR system and a body coil. Each image was obtained within a single breath hold by using an electrocardiography-gated, segmented, ultrafast, gradient-echo pulse sequence with an incremental excitation flip angle for the eight phase-encoding steps acquired per segment. By using overlapping 4-mm-thick sections, the coronary arteries were routinely depicted from the coronary ostia distally at MR in healthy subjects. Ultrafast MR angiography of the coronary arteries is feasible with use of a standard body coil. This technique offers considerable potential as an investigational tool and, with further development, may become a clinically useful imaging application.

MR image contrast at high field strength

The T1 of soft tissues increases with magnetic field strength. Some tissue contrast may be diminished on high-field-strength magnetic resonance (MR) images when conventional TRs are used, because of altered T1 effects on the MR signals. This necessitates longer TRs in techniques that use long TRs, which prolongs the examination excessively. Behavior of macroscopic magnetization is governed by the Bloch equations. Therefore, T1 contributions to the MR signal can be modulated by means of both timing intervals and radio-frequency pulses. The analytic solution to the Block equations allowed calculation of white matter/gray matter and gray matter/cerebrospinal fluid contrast in both spin-echo and inversion-recovery (IR) imaging. Rabbit brains (normal and tumor-containing) were then imaged in vivo at 1.5 and 4.7 T. In addition, MR images of a human head were obtained at 4.0 T. Experimental results supported the theoretical predictions that brain contrast on long TR spin-echo or IR images increases with field strength. However, varying the excitation flip angle allowed optimization of the T1 contribution to the MR signals, improving image contrast and/or reducing examination time. Thus, the dependence of T1 on field strength determines the optimum choice of imaging techniques and parameters in a predictable fashion.

Variable flip angle imaging and fat suppression in combined gradient and spin-echo (GREASE) techniques

Conventional “proton density” and “ T 2-weighted” spin-echo images are susceptible to motion induced artifact, which is exacerbated by lipid signals. Gradient moment nulling can reduce motion artifact but lengthens the minimum TE, degrading the “proton density” contrast. We designed a pulse sequence capable of optimizing proton density and T 2-weighted contrast while suppressing lipid signals and motion induced artifacts. Proton density weighting was obtained by rapid readout gradient reversal immediately after the excitation RF pulse, within a conventional spin-echo sequence. By analyzing the behavior of the macroscopic magnetization and optimizing excitation flip angle, we suppressed T 1 contribution to the image, thereby enhancing proton density and T 2-weighted contrast with a two- to four-fold reduction of repetition time. This permitted an increased number of averages to be used, reducing motion induced artifacts. Fat suppression in the presence of motion was investigated in two groups of 8 volunteers each by (i) modified Dixon technique, (ii) selective excitation, and (iii) hybrid of both. Elimination of fat signal by the first technique was relatively uniform across the field of view, but it did not fully suppress the ghosts originating from fat motion. Selective excitation, while sensitive to the main field inhomogeneity, largely eliminated the ghosts (0.21 ± 0.05 vs. 0.29 ± 0.06, p < 0.01). The hybrid of both techniques combined with bandwidth optimization, however, showed the best results (0.17 ± 0.04, p < 0.001). Variable flip-angle imaging allows optimization of image contrast which, along with averaging and effective fat suppression, significantly improves gradient- and spin-echo imaging, particularly in the presence of motion.

RASEE: A rapid spin-echo pulse sequence

Most fast-imaging sequences use gradient echoes and a low flip-angle excitation. The low flip angle is used because it gives increased signal when TR < T 1. However, spin-echo sequences are less productive of certain artifacts, among them flow and magnetic susceptibility artifacts. We present a modification of the spin-echo pulse sequence designed to optimize the signal-to-noise ratio for repetition times (TR) less than 100 msec while preserving good image quality. Our implementation performs a 128 × 128 image in under 7 sec (TR = 50 msec) and has been used to follow the dynamics of Gd-DTPA in the rat kidneys.