Steady-state Free Precession Signal Research Articles

Steady-State Free Precession sequences for high and low field NMR spectroscopy in solution: Challenges and opportunities

The receptivity of NMR spectroscopy is low when compared to other techniques. Historically, increasing the strength of the static magnetic field has been the major approach to increase NMR sensitivity. In recent years several polarization transfer protocols have been used to enhance the signal-to-noise ratio (SNR), although they require special accessories and/or sample preparation. In this paper, we consider both the challenges and opportunities of steady-state free precession (SSFP) pulse sequences as a simple and efficient alternative to enhance SNR, in standard high-resolution and benchtop low-resolution NMR spectrometers. The maximum gain in these sequences is obtained with the shortest time between the pulses (Tp). However, when Tp<T2, the SSFP signal contains FID and echo components which lead to phase, intensity, and truncation artifacts on spectra obtained by Fast Fourier transform (FT). Several phase alternation SSFP sequences were used to cancel the echo component and minimize these problems in the FT spectra. Krylov base diagonalization method (KBDM) was used to eliminate the phase and truncation problems in spectra acquired by SSFP pulse sequences and can be a viable alternative to FT. The experiments were performed in high and low resolution (bench top) NMR spectrometers and significant enhancements in SNR of low receptivity nuclei such as 13C and 15N could be achieved. The SSFP sequences were also shown to enhance SNR in nuclei with high receptivity such as 19F and 31P, in very dilute samples, as is common in environmental and biological samples.

Lineshape of Magnetic Resonance and its Effects on Free Induction Decay and Steady-State Free Precession Signal Formation

Magnetic resonance imaging based on steady-state free precision (SSFP) sequences is a fast method to acquire T1, T2, and T2∗-weighted images. In inhomogeneous tissues such as lung tissue or blood vessel networks, however, microscopic field inhomogeneities cause a nonexponential free induction decay and a non-Lorentzian lineshape. In this work, the SSFP signal is analyzed for different prominent tissue models. Neglecting the effect of non-Lorentzian lineshapes can easily result in large errors of the determined relaxation times. Moreover, sequence parameters of SSFP measurements can be optimized for the nonexponential signal decay in many tissue structures.

Diffusion effect on T2 relaxometry in triple-echo steady state free precession sequence

PurposeThe purpose of this study is to evaluate the effect of diffusion on SSFP (Steady-state Free Precession) signals in triple-echo steady state (TESS) sequence and ultimately on the accuracy of T2 relaxometry. MethodsThe extended phase graph (EPG) algorithm was used to study the effect of diffusion on SSFP signals and T2 relaxometry. The simulation results were verified by a commercial phantom and in vivo studies. Based on the simulation results, a correction scheme was proposed to correct the estimated T2 values. ResultsT2 underestimation in TESS was evident in case of small flip angle and large unbalanced gradient moment on objects with large T2 and D values. The T2 underestimation mainly originated from the diffusion sensitivity of SSFP-echo. It was also observed that SSFP-FID (Free Induction Decay) signals increased with increasing diffusion weighting under some specific conditions. The proposed correction scheme corrected the T2 underestimation, which verified that the underestimation was due to the neglect of diffusion effect. For clinical practice of TESS in tissues with short T2 such as cartilage and muscle, the diffusion effect of TESS is negligible. ConclusionThe effect of diffusion cannot be neglected during TESS T2 quantification as it is the main source of T2 underestimation when small flip angle and large unbalanced gradient moment is used, especially for objects with large T2 and D values.

Steady‐state free precession signals of arbitrary dephasing order and their sensitivity to T2∗

AbstractA simple approach how to calculate analytical expressions for unbalanced steady‐state free precession (ubSSFP) signals of arbitrary dephasing order is presented. Dephasing order is the number of effective gradient dephasing cycles that magnetization immediately after an RF‐pulse has experienced during the ubSSFP sequence. Based on the obtained equations, which are in accordance with existing literature, the sensitivity of ubSSFP signals to T2∗ is derived under the assumption of a Lorentzian frequency distribution resulting from static field inhomogeneities. Further, the phases of all ubSSFP signals are calculated and a general expression of how to use them for B0‐fieldmapping is given. The derivation is supported by the extended phase graph (EPG) model, and as such the work also acts as a comprehensive introduction to the formal description of SSFP. In addition, the balanced SSFP (bSSFP) sequence is explored. The connection of bSSFP to ubSSFP is shown, and potential T2∗‐sensitivity of bSSFP is examined based on numerical simulations. It is shown that ubSSFP signals with negative dephasing order have a refocusing character and behave similar to spin‐echo signals. Conversely, ubSSFP signals with zero or positive dephasing order can be regarded as T2∗‐weighted. The behavior of bSSFP depends largely on the exact distribution of frequencies. For instance, for a narrow spherical distribution, bSSFP acts like a spin‐echo sequence, while for a Lorentzian distribution a refocusing behavior does not occur.

Rapid High-resolution, Self-registered, Dual Lumen-contrast MRI Method for Vessel-wall Assessment in Peripheral Artery Disease:: A Preliminary Investigation

Contrast-enhanced angiographic evaluation by magnetic resonance imaging (MRI) and computed tomography (CT) is the reference standard for assessing peripheral artery disease (PAD). However, because PAD and diabetes often coexist, the prevalence of renal insufficiency is a major challenge to contrast-based angiography. The objective of this work is to describe and demonstrate a new application of three-dimensional double-echo steady-state (3D DESS) as a noncontrast vascular MRI method for evaluating peripheral atherosclerosis at 3 Tesla (3T). A water-selective 3D DESS pulse sequence was designed to simultaneously collect two steady-state free-precession signals (free induction decay and Echo) yielding "black blood" (BB) and "gray blood" (GB) images. For completeness Bloch equation, simulations were performed to characterize DESS signals of various tissues including blood at different velocities and to assess two healthy subjects for the purpose of pulse sequence optimization. Exploratory studies were performed as an add-on protocol to an existing study involving patients with PAD. To evaluate the method's specificity for detecting calcification, images from select patients were compared against CT angiography. Simulations agreed qualitatively with in vivo images supporting DESS' potential for generating distinct lumen contrast (GB vs BB). Lesions representing calcium were easily identifiable on the basis of signal void occurring on both image types and were confirmed by CT angiography. Further, BB allowed visualization of stent restenosis, and data suggest its ability to visualize acute thrombus by virtue of T2 weighting. Preliminary investigation and results suggest noncontrast 3D DESS to have the potential to improve diagnosis of PAD patients by providing detailed structural assessment of vessel-wall architecture.

High-resolution mapping of neuronal activation with balanced SSFP at 9.4 tesla.

This work investigates the feasibility of high-resolution functional imaging of the human brain using passband balanced steady state free precession (SSFP) at 9.4 Tesla (T). To this end, the temporal signal stability, blood-oxygen-level-dependent (BOLD)-related signal changes and sensitivity to frequency offsets were evaluated. Three-dimensional slab selective and nonselective balanced SSFP have been implemented with minimized repetition time and high temporal resolution using parallel imaging, partial Fourier acquisition and elliptical scanning. Using a volume repetition time of approximately 3 s, a visual checker board stimulation was applied for 6 min. Temporal signal stability of balanced SSFP and BOLD response-related signal changes and sensitivity to frequency changes were evaluated. Activation could be detected in all volunteers with BOLD-related signal changes from 3% to 6%. At 1 mm isotropic resolution, the thermal noise SNR0 was 67 and the total temporal noise variation tSNR was 45 supporting a very high signal stability of balanced SSFP. No significant changes of activation at different offresonance frequencies were detected. High spatial and temporal resolution balanced SSFP at 9.4T to detect functional activation is feasible. Activation patterns and signal changes are stable and reproducible across subjects within the visual cortex, and comparable to reported values of SE-EPI at 7T and 9.4T. Magn Reson Med 76:163-171, 2016. © 2015 Wiley Periodicals, Inc.

Utilization of a balanced steady state free precession signal model for improved fat/water decomposition.

Chemical shift based fat/water decomposition methods such as IDEAL are frequently used in challenging imaging environments with large B0 inhomogeneity. However, they do not account for the signal modulations introduced by a balanced steady state free precession (bSSFP) acquisition. Here we demonstrate improved performance when the bSSFP frequency response is properly incorporated into the multipeak spectral fat model used in the decomposition process. Balanced SSFP allows for rapid imaging but also introduces a characteristic frequency response featuring periodic nulls and pass bands. Fat spectral components in adjacent pass bands will experience bulk phase offsets and magnitude modulations that change the expected constructive and destructive interference between the fat spectral components. A bSSFP signal model was incorporated into the fat/water decomposition process and used to generate images of a fat phantom, and bilateral breast and knee images in four normal volunteers at 1.5 Tesla. Incorporation of the bSSFP signal model into the decomposition process improved the performance of the fat/water decomposition. Incorporation of this model allows rapid bSSFP imaging sequences to use robust fat/water decomposition methods such as IDEAL. While only one set of imaging parameters were presented, the method is compatible with any field strength or repetition time.

Fast 3D T2 -weighted imaging using variable flip angle transition into driven equilibrium (3D T2 -TIDE) balanced SSFP for prostate imaging at 3T.

Three-dimensional (3D) T2 -weighted fast spin echo (FSE) imaging of the prostate currently requires long acquisition times. Our objective was to develop a fast 3D T2 -weighted sequence for prostate imaging at 3T using a variable flip angle transition into driven equilibrium (T2 -TIDE) scheme. 3D T2 -TIDE uses interleaved spiral-out phase encode ordering to efficiently sample the ky -kz phase encodes and also uses the transient balanced steady-state free precession signal to acquire the center of k-space for T2 -weighted imaging. Bloch simulations and images from 10 healthy subjects were acquired to evaluate the performance of 3D T2 -TIDE compared to 3D FSE. 3D T2 -TIDE images were acquired in 2:54 minutes compared to 7:02 minutes for 3D FSE with identical imaging parameters. The signal-to-noise ratio (SNR) efficiency was significantly higher for 3D T2 -TIDE compared to 3D FSE in nearly all tissues, including periprostatic fat (45 ± 12 vs. 31 ± 7, P < 0.01), gluteal fat (48 ± 8 vs. 41 ± 10, P = 0.12), right peripheral zone (20 ± 4 vs. 16 ± 8, P = 0.12), left peripheral zone (17 ± 2 vs. 12 ± 3, P < 0.01), and anterior fibromuscular stroma (12 ± 4 vs. 4 ± 2, P < 0.01). 3D T2 -TIDE images of the prostate can be acquired quickly with SNR efficiency that exceeds that of 3D FSE.

‐mapping with the transient phase of unbalanced steady‐state free precession

A novel B1+-mapping technique (B1-TRAP) is presented, which derives the actual flip angle from the frequency of signal oscillations, observed in the transient phase of unbalanced steady-state free precession sequences. For short repetition times (TR), the angular frequency of distinct oscillations in the transient phase of steady-state free precession sequences is proven to be approximately proportional to the actual flip angle: ω⋅TR≈α. The result is not influenced by off-resonance and it can be shown that deviations are only of second order in the small parameter TR/T2. B1-TRAP makes use of this effect through a frequency analysis of the transient phase of a train of steady-state free precession signals. In terms of reliability and time efficiency, a two-dimensional multislice implementation was found to be optimal. Unlike many steady-state B1+-mapping methods, the accuracy of B1-TRAP was not impaired by imperfect slice profiles. Simulations, phantom, and in vivo measurements showed that B1-TRAP offers a good compromise with respect to speed, robustness, and accuracy.

Incorporating reversible and irreversible transverse relaxation effects into Steady State Free Precession (SSFP) signal intensity expressions for fMRI considerations

Among the multiple sequences available for functional magnetic resonance imaging (fMRI), the Steady State Free Precession (SSFP) sequence offers the highest signal-to-noise ratio (SNR) per unit time as well as distortion free images not feasible with the more commonly employed single-shot echo planar imaging (EPI) approaches. Signal changes occurring with activation in SSFP sequences reflect underlying changes in both irreversible and reversible transverse relaxation processes. The latter are characterized by changes in the central frequencies and widths of the inherent frequency distribution present within a voxel. In this work, the well-known frequency response of the SSFP signal intensity is generalized to include the widths and central frequencies of some common frequency distributions on SSFP signal intensities. The approach, using a previously unnoted series expansion, allows for a separation of reversible from irreversible transverse relaxation effects on SSFP signal intensity changes. The formalism described here should prove useful for identifying and modeling mechanisms associated with SSFP signal changes accompanying neural activation.

Qualitative agreement of aortic regurgitation assessed by steady state free precession signal void as compared to doppler echocardiography

Introduction Previous studies demonstrated correlation in assessment of severity of AR between spoiled gradient-echo CMR and phase contrast color Doppler echocardiography in AR patients as well as normal subjects (Pflugfelder, et al. Am J Roentgenol 1989; 152:729-35). SSFP gradient-echo sequence is now a mainstay in evaluating cardiac function, as well as valvular structures. Aortic regurgitation creates flow disturbances that manifests as signal void from signal dephasing. This study compares the qualitative assessment of AR using SSFP CMR versus Doppler echocardiography.

SSFP signal with finite RF pulses

The theoretical description of steady state free precession (SSFP) sequences is generally well accepted and unquestioned, although it is based on instantaneously acting radiofrequency (RF) pulses. In practice, however, all excitation processes are finite, thereby questioning the overall validity of the common SSFP signal description for use with finite RF pulses. In this paper, finite RF pulse effects on balanced SSFP signal formation are analyzed as a function of the RF time, the pulse repetition time, the flip angle (alpha) and relaxation times (T(1,2)). The observed signal modulations from finite RF pulses (compared to infinitesimal ones) can range from only a few percent (for RF time/pulse repetition time << 1, alpha << 90 degrees, T(2)/T(1) approximately 1) to over 10% (for RF time/pulse repetition time << 1, alpha approximately 90 degrees, T(2)/T(1) << 1) and may even exceed 100% in the limit of RF time/pulse repetition time --> 1 (for alpha approximately 90 degrees, T(2)/T(1) << 1). As a result, a revision of SSFP signal theory is indicated not only for reasons of completeness but also seems advisable, e.g., for all quantitative SSFP methods. A simple modification for the common balanced SSFP equation is derived that provides an accurate framework for SSFP signal description over a wide variety of practical and physiologic parameters.

Investigating exchange and multicomponent relaxation in fully‐balanced steady‐state free precession imaging

To investigate the effect of chemical exchange and multicomponent relaxation on the rapid T(2) mapping method, DESPOT2 (driven equilibrium single pulse observation of T(2)) and the steady-state free precession (SSFP) sequence upon which it is based. Although capable of rapid T(2) determination, an assumption implicit of the method is single-component relaxation. In many biological tissues (such as white and gray matter), it is well established that the T(2) decay curve is more accurately described by the summation of more than one relaxation species. The effects of exchange were first incorporated into the general SSFP magnetization expressions and its effect on the measured SSFP signal investigated using Bloch-McConnell simulations. Corresponding imaging experiments were performed to support the presented theory. Simulations show the measured multicomponent SSFP signal may be expressed as a linear summation of signal from each species under usual imaging conditions where the repetition time is much less than T(2). Imaging experiments performed using dairy cream demonstrate strong agreement with the presented theory. Finally, using a dairy cream model, we demonstrate quantification of multicomponent relaxation from multiangle SSFP data for the first time, showing good agreement with reference spin-echo values. SSFP and DESPOT2 may provide a new method for investigating multicomponent systems, such as human brain, and disease processes, such as multiple sclerosis.

Morphing steady‐state free precession

A novel concept for visualization of positive contrast originating from susceptibility-related magnetic field distortions is presented. In unbalanced steady-state free precession (SSFP) the generic, gradient-induced dephasing competes with local gradient fields generated by paramagnetic materials. Thus, within the same image, SSFP may morph its own appearance from unbalanced to balanced SSFP (bSSFP) as a result of local gradient compensation. In combination with low to very low flip angles, unbalanced SSFP signals are heavily suppressed, whereas bSSFP locally produces very high steady-state amplitudes at certain frequency offsets. As a result, bSSFP signals appear hyperintense on an almost completely dark background. In this study, the conceptual issues of local gradient compensation and frequency matching, as well as the feasibility of proper detection of marker materials for interventional MRI from hyperintense pixels locations, are evaluated both in vitro and in vivo. Signal dependencies of morphing SSFP on sequence parameters such as flip angle or repetition time are investigated theoretically and experimentally. In addition to passive tracking of interventional devices, morphing SSFP might also be a promising new concept for the generation of positive contrast from super-paramagnetic iron oxide (SPIO) particles in contrast-enhanced MRI as well as for particle tracking.

Dephased FLAPS for improved visualization of susceptibility-shifted passive devices for real-time interventional MRI

Recently, balanced steady-state free precession (SSFP) magnetic resonance imaging at small flip angles (i.e. FLAPS imaging) has been shown to preferentially enhance the visibility of spins residing near susceptibility-shifted media by exploiting the unique spectral response of SSFP signals. Nevertheless, the use of FLAPS imaging to visualize passive, susceptibility-shifted devices within thick slices is complicated by partial volume effects that reduce the conspicuity of the devices. In this work, dephased FLAPS (dFLAPS), a variation of the FLAPS method with dephasing gradients applied in the slice direction, is presented as a method for improving the contrast of susceptibility-shifted interventional devices in thick-slice magnetic resonance images by suppressing signal from on-resonant spins. In comparison to FLAPS imaging, results demonstrate that dFLAPS imaging of thick, projection-like slices improves the contrast of susceptibility-shifted devices relative to background signal with no increase in acquisition time.

Rapid dark‐blood carotid vessel‐wall imaging with random bipolar gradients in a radial SSFP acquisition

To investigate and evaluate a new rapid dark-blood vessel-wall imaging method using random bipolar gradients with a radial steady-state free precession (SSFP) acquisition in carotid applications. The carotid artery bifurcations of four asymptomatic volunteers (28-37 years old, mean age = 31 years) were included in this study. Dark-blood contrast was achieved through the use of random bipolar gradients applied prior to the signal acquisition of each radial projection in a balanced SSFP acquisition. The resulting phase variation for moving spins established significant destructive interference in the low-frequency region of k-space. This phase variation resulted in a net nulling of the signal from flowing spins, while the bipolar gradients had a minimal effect on the static spins. The net effect was that the regular SSFP signal amplitude (SA) in stationary tissues was preserved while dark-blood contrast was achieved for moving spins. In this implementation, application of the random bipolar gradient pulses along all three spatial directions nulled the signal from both in-plane and through-plane flow in phantom and in vivo studies. In vivo imaging trials confirmed that dark-blood contrast can be achieved with the radial random bipolar SSFP method, thereby substantially reversing the vessel-to-lumen contrast-to-noise ratio (CNR) of a conventional rectilinear SSFP "bright-blood" acquisition from bright blood to dark blood with only a modest increase in TR (approximately 4 msec) to accommodate the additional bipolar gradients. Overall, this sequence offers a simple and effective dark-blood contrast mechanism for high-SNR SSFP acquisitions in vessel wall imaging within a short acquisition time.

Blood attenuation with SSFP‐compatible saturation (BASS)

To investigate a rapid flow-suppression method for improving the contrast-to-noise ratio (CNR) between the vessel wall and the lumen for cardiovascular imaging applications. In this study a new dark-blood steady-state free precession (SSFP) sequence utilizing two excitation pulses per TR was developed. The first pulse is applied immediately adjacent to the slice of interest, while the second is a conventional slice-selective pulse designed to excite an SSFP signal for the static spins in the slice of interest. The slice-selective pulse is followed by fully refocused gradients along all three imaging axes over each TR. The signal amplitude (SA) from the moving spins excited by the "saturation" pulse is attenuated since they are not fully refocused at the TE. This work provides confirmation, by both simulation and experiments, that modest adaptations of the basic True-FISP structure can limit unwanted "bright blood" signal within the vessels while simultaneously preserving the contrast and speed advantages of this well-established rapid imaging method. Animal imaging trials confirm that dark-blood contrast is achieved with the BASS sequence, which substantially reverses the lumen-to-muscle CNR of a conventional True-FISP "bright blood" acquisition from 14.77 (bright blood) to -13.96 (dark blood) with a modest increase (24.2% of regular TR of SSFP for this implementation) in acquisition time to accommodate the additional slab-selective excitation pulse and gradient pulses.

Frequency-shift based detection of BMS contrast agents using SSFP: potential for MRA

A novel mechanism of MRI contrast enhancement, based on the detection by a balanced steady-state free precession (SSFP) sequence of the proton resonance frequency shift induced by bulk magnetic susceptibility (BMS) contrast agents, was investigated. The potential for this contrast mechanism to image blood vessels was explored. The relaxation time and the frequency shift effects of gadolinium- and dysprosium-DOTA on SSFP signal was first simulated and evaluated on a water phantom at 1.5 T. In vitro, a 5-mM concentration in contrast agent induced a 20-Hz frequency shift, leading to a signal increase of 92% for Dy-DOTA, and a 10-Hz frequency shift, leading to a signal increase of 58% for Gd-DOTA at the reference frequency, taking into account the nonlinear SSFP signal response on frequency offset. The concept was then evaluated in vivo on anesthetized rabbits. Low doses of dysprosium-DOTA were injected in their vascular system, and imaging was performed at the level of neck vessels. Following a bolus injection, mean signal changes of 31%, 20% and 14% were observed in the carotid arteries, the vertebral veins and the jugular veins, respectively. The bolus peak times in arteries and veins were consistent with the rabbit vascular circulation. This frequency-shift based contrast mechanism presents interesting potential for contrast-enhanced MR angiography (CE-MRA) compared to usual relaxation-based contrast, but further investigations on reproducibility will be necessary.

Oxygen‐sensitive contrast in blood for steady‐state free precession imaging

Steady-state free precession (SSFP) methods have gained widespread recognition for their ability to provide fast scans at high signal-to-noise ratio. This paper demonstrates that such methods are also capable of reflecting functional information, particularly blood oxygenation state. It is well known that SSFP signals show substantial sensitivity to small off-resonance frequency variations. However, that mechanism cannot explain the oxygen-sensitive contrast in blood that was observed with steady-state methods using phase-cycled radiofrequency pulses. From theoretical and experimental models it is demonstrated that the mechanism responsible for such contrast originates from the motion of spins through local field inhomogeneities in and around deoxygenated red blood cells. In addition, this work shows that it is critical to choose the scan parameters carefully for robust oxygen-sensitive contrast. Finally, it is demonstrated that it is possible to build a quantitative model that incorporates the Luz-Meiboom model, which had been used in the past to estimate quantitative measures of vascular blood oxygen levels. It is envisioned that this method could be instrumental in real-time imaging focused on detecting diseases where the oxygen state of blood is impaired.

Understanding steady-state free precession: A geometric perspective

Steady-state free precession (SSFP) sequences are becoming more common in clinical and research studies. Though there have been mathematical treatments of SSFP signals, a simple and/or intuitive understanding of how such signals are established is lacking in the literature. In this work, the classical picture of a spin system is used to develop an analysis of the behavior of magnetization in phase space. This leads to a geometric description that demonstrates the essential features of SSFP signals. This analysis yields an intuitive understanding of SSFP signal formation and the intricate interactions between the choice of flip angle, pulse repetition time, relaxation effects, and phase-cycling patterns on the final SSFP signal. © 2005 Wiley Periodicals, Inc. Concepts Magn Reson Part A 26A: 1–10, 2005