New insights into the mechanisms of signal formation in RF-spoiled gradient echo sequences.

RF spoiling is a well established method to produce T(1)-weighted images with short repetition-time gradient-echo sequences, by eliminating coherent transverse magnetization with appropriate RF phase modulation. This paper presents 2 novel approaches to describe signal formation in such sequences. Both methods rely on the formulation of RF spoiling as a linear increase of the precession angle between RF pulses, which is an alternative to the commonly used quadratic pulse phase scheme. The first technique demonstrates that a steady state signal can be obtained by integrating over all precession angles within the voxel, in spite of the lack of a genuine steady-state for separate isochromats. This clear mathematical framework allows a straightforward incorporation of off-resonance effects and detector phase settings. Moreover, it naturally introduces the need for a large net gradient area per repetition interval. In the second step, a modified partition method including RF spoiling is developed to obtain explicit expressions for all signal components. This provides a physical interpretation of the deviations from ideal spoiling behavior in FLASH and echo-shifted sequences. The results of the partition method in the small flip angle regime are compared with numerical simulations based on a Fourier decomposition of magnetization states. Measurements performed with in vitro solutions were in good agreement with numerical simulations at short relaxation times (T(1)/TR = 32 and T(2)/TR = 4); larger deviations occurred at long relaxation times (T(1)/TR = 114 and T(2)/TR = 82).     

Superbalanced steady state free precession

Steady state free precession (SSFP) signal theory is commonly derived in the limit of quasi-instantaneous radiofrequency (RF) excitation. SSFP imaging protocols, however, are frequently set up with minimal pulse repetition times and RF pulses can thus constitute a considerable amount to the actual pulse repetition time. As a result, finite RF pulse effects can lead to 10-20% signal deviation from common SSFP theory in the transient and in the steady state which may impair the accuracy of SSFP-based quantitative imaging techniques. In this article, a new and generic approach for intrinsic compensation of finite RF pulse effects is introduced. Compensation is based on balancing relaxation effects during finite RF excitation, similar to flow or motion compensation of gradient moments. RF pulse balancing, in addition to the refocusing of gradient moments with balanced SSFP, results in a superbalanced SSFP sequence free of finite RF pulse effects in the transient and in the steady state; irrespective of the RF pulse duration, flip angles, relaxation times, or off-resonances. Superbalancing of SSFP sequences can be used with all quantitative SSFP techniques where finite RF pulse effects are expected or where elongated RF pulses are used.     

Steady state of echo-shifted sequences with radiofrequency phase cycling.

Due to a delayed echo-formation, echo-shifted gradient echo sequences (ES-GRE) allow for an enhanced T(2)*-weighting at short repetition times. While they are in use with and without RF spoiling, analytical solutions are only known for the latter. The signal formation in the former could only be assessed in approximative form, so far. In this article an exact analytical solution is presented for TR-periodic ES-GRE sequences with RF phase cycling. Besides providing a better numerical performance, it should be useful for systematic sequence development. The relation to recent approximative solutions is discussed.     

Steady state of gradient echo sequences with radiofrequency phase cycling: Analytical solution,contrast enhancement with partial spoiling

Spoiled gradient echo sequences can only reach a homogeneous steady state if sufficiently strong crusher gradients are used in combination with RF phase cycling (RF spoiling). However, the signal depends quite sensitively on the chosen phase increment ? and—lacking analytical solutions—numerical simulations must be used to study the transient and steady‐state magnetization. For the steady state an exact analytical solution is derived, which holds for arbitrary sequence and tissue parameters. Besides a considerably improved computation performance, the analytical approach enables a better understanding of the complicated dependence on ?. For short repetition times (TR) the regime of small ? turns out to be particularly interesting: It is shown that the typical ?_c, where RF spoiling starts to become effective, is essentially inversely proportional to T₂. This tissue dependence implies that contrasts can be considerably larger with partial spoiling (? ≈ ?_c) than with conventional RF spoiling (? ? ?_c). As an example, the uptake of contrast agents in tissues is investigated. For typical parameters a considerably improved contrast enhancement can be obtained, both theoretically and experimentally. Magn Reson Med, 2006. © 2005 Wiley‐Liss, Inc.     

Optimization of signal behavior in the transition to driven equilibrium in steady-state free precession sequences.

A new technique to avoid the initial signal fluctuations in steady-state free precession (SSFP)-sequences, such as trueFISP, FIESTA, and refocused FFE, is presented. The "transition into driven equilibrium" (TIDE) sequence uses modified flip angles over the initialization phase of a SSFP experiment, which not only avoids image artifacts but also improves the signal-to-noise ratio (SNR) and contrast behavior compared to conventional approaches. TIDE is demonstrated to be robust against variations of T(1) and T(2), and leads to a monotonous signal evolution for off-resonance spins. The basic principles can also be applied repetitively to optimize continuous 3D acquisitions.     

Increasing spoiling efficiency in RF‐spoiled gradient echo sequences by averaging of RF phase‐cycle‐adapted k‐spaces

A method is presented that improves the effectiveness of RF‐spoiling whenever an image consists of the averaging of several identical single acquisitions. Using this method, it becomes possible to reduce the necessary moment of the spoiler gradient by a factor equal to the number of averages. The idea is to perform the collection of k‐space lines such that the averaging takes over a part of the spoiling process. The number of sequence cycles played out between two acquisitions of a certain k‐space line is adjusted based on both the number of total averages of this line and the phase‐cycling pattern of the RF‐pulse train of a RF‐spoiled gradient echo sequence. In this way, ghost artifacts visible in the single images add to zero in the averaged image while the depiction of the object experiences normal averaging with enhanced signal‐to‐noise ratio. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Interleaved balanced SSFP imaging: Artifact reduction using gradient waveform grouping

Purpose

To analyze steady‐state signal distortions in interleaved balanced steady‐state free precession (bSSFP) caused by slightly unbalanced eddy‐current fields and develop a general strategy for mitigating these artifacts.

Materials and Methods

We considered bSSFP sequences in which two gradient waveforms are interleaved in a “groupwise” fashion, ie, each waveform is executed consecutively two or more times before switching to the other waveform (we let “N” count the number of times each waveform is executed consecutively). The steady‐state signal profile over the bSSFP passband was calculated using numerical Bloch simulations and measured experimentally in a uniform phantom. The proposed “grouped” interleaved bSSFP strategy was applied to cardiac velocity mapping using interleaved phase‐contrast imaging with N = 2 and N = 6 in one healthy volunteer.

Results

Simulation and phantom measurements show that signal distortions are systematically reduced with increasing grouping number N. For most tissues, significant suppression was achieved with N = 4, and increasing N beyond this value produced only marginal gains. However, signal distortions for blood remain relatively high even for N > 4. In vivo cardiac velocity mapping using interleaved phase‐contrast imaging with N = 6 demonstrated reduced image artifact levels compared to the N = 2 acquisition.

Conclusion

Gradient waveform “grouping” offers a simple and general strategy for mitigating steady‐state eddy‐current distortions in bSSFP sequences that interleave two different gradients. Blood exhibits significant distortion even with “grouping,” which is a major obstacle for cardiovascular bSSFP approaches that interleave multiple gradient waveforms. The grouping concept may also benefit applications that acquire images during the transient approach to steady state. J. Magn. Reson. Imaging 2009;29:745–750. © 2009 Wiley‐Liss, Inc.     

Balanced alternating steady-state elastography.

A conventional balanced steady-state free precession (b-SSFP) sequence scheme was modified such that the dynamic equilibrium becomes very sensitive to small cyclic displacements, generating two distinct and alternating steady states. This novel technique is proposed for the visualization of propagating transverse acoustic shear waves, as used in MR elastography (MRE) to determine the mechanical properties of materials or in vivo soft tissue. Experiments with tissue-like agarose gel phantoms and simulations demonstrate that the novel sequence offers an increase in phase sensitivity by about one order in magnitude compared to standard motion-encoding methods. In addition, the new method benefits from the very short acquisition times achieved by b-SSFP protocols.     

Moment and direction of the spoiler gradient for effective artifact suppression in RF-spoiled gradient echo imaging.

Radiofrequency (RF) -spoiled gradient echo sequences were developed with the aim to produce images with T(1) weighted contrast within short acquisition time. Over the past two decades, this type of sequence has proven to be a robust technique and represents a reliable workhorse in clinical MRI. This study presents an analysis of ghost artifacts, which appear occasionally in RF-spoiled gradient echo images. It is demonstrated that the artifacts result from intrinsically emerging signal oscillations, which can be damped down by the application of spoiler gradients.     

Intrascanner and interscanner variability of magnetization transfer‐sensitized balanced steady‐state free precession imaging

Recently, a new and fast three‐dimensional imaging technique for magnetization transfer ratio (MTR) imaging has been proposed based on a balanced steady‐state free precession protocol with modified radiofrequency pulses. In this study, optimal balanced steady‐state free precession MTR protocol parameters were derived for maximum stability and reproducibility. Variability between scans was assessed within white and gray matter for nine healthy volunteers using two different 1.5 T clinical systems at six different sites. Intrascanner and interscanner MTR measurements were well reproducible (coefficient of variation: c_v < 0.012 and c_v < 0.015, respectively) and results indicate a high stability across sites (c_v < 0.017) for optimal flip angle settings. This study demonstrates that balanced steady‐state free precession MTR not only benefits from short acquisition time and high signal‐to‐noise ratio but also offers excellent reproducibility and low variability, and it is thus proposed for clinical MTR scans at individual sites as well as for multicenter studies. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Static susceptibility effects in balanced SSFP sequences.

The steady state of balanced steady-state free precession (b-SSFP) sequences in presence of field inhomogeneities can be expressed in terms of a rapidly convergent series, which contains the characteristic function of the susceptibility related phase variations. As long as the intravoxel standard deviation sigma of the accumulated phase per repetition time (TR) is of order 1 (radians) or lower, the signal at TE=TR/2 behaves like a spin echo (SE), in accordance with recent observations. The signal decays quadratically around the maximum near TR/2 with a curvature, which solely depends on sigma and not on T'2. For larger sigma, a qualitative change toward a gradient echo (GRE) occurs and the signal shows characteristics of an usual FID. With increasing off-resonance the width of the SE regime decreases.     

Quantitative in vivo diffusion imaging of cartilage using double echo steady-state free precession

Single-shot echo-planar imaging techniques are commonly used for diffusion-weighted imaging (DWI) but offer rather poor spatial resolution and field-of-view coverage for species with short T(2) . In contrast, steady-state free precession (SSFP) has shown promising results for DWI of the musculoskeletal system, but quantification is generally hampered by its prominent sensitivity on relaxation times. In this work, a new and truly diffusion-weighted (that is relaxation time independent) SSFP DWI technique is introduced using a double-echo steady-state approach. Within this framework (and this is in contrast to common SSFP DWI techniques using SSFP-Echo) both primary echo paths of nonbalanced SSFP are acquired, namely the FID and the Echo. Simulations and in vitro measurements reveal that the ratio of the Echo/FID signal ratios of two double-echo steady-state scans acquired with and without diffusion sensitizing dephasing moments provides a highly relaxation independent quantity for diffusion quantification. As a result, relaxation-independent high-resolution (0.4 × 0.4 - 0.6 × 0.6 mm(2) in-plane resolution) quantitative in vivo SSFP DWI is demonstrated for human articular cartilage using diffusion-weighted double-echo steady-state scans in the knee and ankle joint at 3.0 T. The derived diffusion coefficients for cartilage (D ～ 1.0-1.5 μm(2) /ms) and synovial fluid (D ～ 2.6 μm(2) /ms) are in agreement with previous work.     

Practical applications of balanced steady‐state free‐precession (bSSFP) imaging in the abdomen and pelvis

Balanced steady‐state free‐precession (bSSFP) is an important pulse sequence that may be underutilized in abdominal and pelvic magnetic resonance imaging (MRI). bSSFP offers several advantages for abdominal and pelvic MRI that include: bright blood effects, a relative insensitivity to the dephasing effects which occur in structures with linear movement, low specific absorption rate (SAR), high signal‐to‐noise ratio (SNR), high spatial resolution, and rapid acquisition times. Bright blood effects can be exploited to diagnose or confirm vascular pathologies when gadolinium‐enhanced imaging cannot be performed, is indeterminate, or is degraded by artifact. The relative insensitivity to dephasing artifact in areas of linear movement is useful when imaging the biliary, urinary, and gastrointestinal tracts where dephasing artifacts may mimic filling defects such as calculi or polyps. Low SAR imaging is important in pediatric and pregnant patients and may be useful in patients with medical devices that restrict SAR levels. Rapid acquisition times and high SNR are extremely valuable assets in abdominal and pelvic MRI and bSSFP (which can be performed as static or cine acquisitions) and can be added to most existing abdominal and pelvic protocols when deemed suitable without significantly prolonging examination times. This article reviews the fundamentals of bSSFP imaging, presents vascular and nonvascular applications of bSSFP in abdominal and pelvic MRI, and discusses potential limitations (including imaging artifacts) of bSSFP. Level of Evidence: 5 J. Magn. Reson. Imaging 2017;45:11–20.     

Cardiac cine MRI: Quantification of the relationship between fast gradient echo and steady-state free precession for determination of myocardial mass and volumes

PURPOSE: To determine the correlation function between the steady-state free precession (SSFP) and fast gradient echo (FGRE) cine MRI pulse sequences for measuring the myocardial mass and volumes. MATERIALS AND METHODS: Cardiac cine MRI examinations were acquired in 50 individuals (female: 35, male: 15, mean age 64.1 +/- 9.1 years, range 48-83) using SSFP and FGRE cardiac pulse sequences. RESULTS: The mean (standard deviation [SD]) left ventricular end diastolic volume measured by SSFP was significantly larger (4.5%) than by FGRE (P < 0.001); this was also the case for end systolic volume (15.0%, P < 0.001). The relationship between SSFP and FGRE measures were linear and highly correlated (P < 0.001) for both left ventricular end diastolic and end systolic volumes (r(2) = 0.90 vs. 0.91, respectively). We determined linear regression models to estimate the SSFP values based on the FGRE measures. Slope (intercept) for ejection fraction, stroke volume, and cardiac output were 0.99 (-2.79), 0.77 (17.5), and 0.76 (1.29), respectively. CONCLUSION: Linear relationships exist for key LV function parameters when comparing SSFP and FGRE cine MRI. These results indicate that existing databases and normal values for FGRE LV function may be converted to corresponding LV function values for SSFP MRI.     

Is TrueFISP a gradient‐echo or a spin‐echo sequence?

It is commonly accepted that TrueFISP (balanced FFE, FIESTA) belongs to the class of gradient-echo (GRE) sequences. GRE sequences are sensitive to dephasing effects of the transverse magnetization between the excitation pulse and echo acquisition, and phase coherence is only established directly after and before excitation pulses. However, an analysis of the phase evolution of transverse magnetization in a TrueFISP experiment shows very close similarities to the echo formation of a spin-echo (SE) experiment. If dephasing between excitation pulses is below +/-pi, TrueFISP exhibits a nearly complete refocusing of transverse magnetization at TE = TR/2. Only signals acquired before and after TR/2 show an additional T*2 sensitivity.     

Intrinsic fat suppression in TIDE balanced steady-state free precession imaging.

A novel fat-suppressed balanced steady-state free precession (b-SSFP) imaging method based on the transition into driven equilibrium (TIDE) sequence with variable flip angles is presented. The new method, called fat-saturated (FS)-TIDE, exploits the special behavior of TIDE signals from off-resonance spins during the flip angle ramp. As shown by simulations and experimental data, the TIDE signal evolution for off-resonant isochromats during the transition from turbo spin-echo (TSE)-like behavior to the true fast imaging with steady precession (TrueFISP) mode undergoes a zero crossing. The resulting signal notch for off-resonant spins is then used for fat suppression. The efficiency of FS-TIDE is demonstrated in phantoms and healthy volunteers on a 1.5T system. The resulting images are compared with standard TrueFISP data with and without fat suppression. It is demonstrated that FS-TIDE provides a fast and stable means for homogenous fat suppression in abdominal imaging while maintaining balanced SSFP-like image contrast and signal-to-noise ratio (SNR). The scan time of FS-TIDE is not increased compared to normal TrueFISP imaging without fat suppression and identical k-space trajectories. Because of the intrinsic fat suppression, no additional preparation is needed. Possible repetition times (TRs) are not firmly limited to special values and are nearly arbitrary.     

Steady-state preparation for spoiled gradient echo imaging.

While spoiled gradient echo sequences provide a rapid means of acquiring T(1)-weighted images, it is often desirable that the magnetization be in the steady state to avoid artifacts. For some applications, this requires many "dummy" repetitions of the pulse sequence prior to data collection, delaying image acquisition. A method is presented in which a saturation pulse, followed by a prescribed recovery period, places longitudinal magnetization levels of all materials near steady state, ready for data acquisition much sooner than when employing only dummy repetitions to achieve steady state. Effects of transverse coherences are studied using configuration theory. The method is shown to be effective in both phantom studies and in vivo applications, including real-time imaging, multiphase cardiac imaging, and triggered contrast-enhanced angiography. Magn Reson Med 45:653-661, 2001.