Optimized balanced steady-state free precession magnetization transfer imaging.

Balanced steady-state free precession (bSSFP) suffers from a considerable signal loss in tissues. This apparent signal reduction originates from magnetization transfer (MT) and may be reduced by an increase in repetition time or by a reduction in flip angle. In this work, MT effects in bSSFP are modulated by a modification of the bSSFP sequence scheme. Strong signal attenuations are achieved with short radio frequency (RF) pulses in combination with short repetition times, whereas near full, i.e., MT-free, bSSFP signal is obtained by a considerable prolongation of the RF pulse duration. Similar to standard methods, the MT ratio (MTR) in bSSFP depends on several sequence parameters. Optimized bSSFP protocol settings are derived that can be applied to various tissues yielding maximal sensitivity to MT while minimizing contribution from other impurities, such as off-resonances. Evaluation of MT in human brain using such optimized bSSFP protocols shows high correlation with MTR values from commonly used gradient echo (GRE) sequences. In summary, a novel method to generate MTR maps using bSSFP image acquisitions is presented and factors that optimize and influence this contrast are discussed.     

Concomitant gradient field effects in balanced steady-state free precession.

Linear magnetic field gradients spatially encode the image information in MRI. Concomitant gradients are undesired magnetic fields that accompany the desired gradients and occur as an unavoidable consequence of Maxwell's equations. These concomitant gradients result in undesired phase accumulation during MRI scans. Balanced steady-state free precession (bSSFP) is a rapid imaging method that is known to suffer from signal dropout from off-resonance phase accrual. In this work it is shown that concomitant gradient phase accrual can induce signal dropout in bSSFP. The spatial variation of the concomitant phase is explored and shown to be a function of gradient strength, slice orientation, phase-encoding (PE) direction, distance from isocenter, and main field strength. The effect on the imaging signal level was simulated and then verified in phantom and in vivo experiments. The nearest signal-loss artifacts occurred in scans that were offset from isocenter along the z direction with a transverse readout. Methods for eliminating these artifacts, such as applying compensatory frequency or shim offsets, are demonstrated. Concomitant gradient artifacts can occur at 1.5T, particularly in high-resolution scans or with additional main field inhomogeneity. These artifacts will occur closer to isocenter at field strengths below 1.5T because concomitant gradients are inversely proportional to the main field strength.     

Fat-suppressed steady-state free precession imaging using phase detection.

Fully refocused steady-state free precession (SSFP) is a rapid, efficient imaging sequence that can provide diagnostically useful image contrast. In SSFP, the signal is refocused midway between excitation pulses, much like in a spin-echo experiment. However, in SSFP, the phase of the refocused spins alternates for each resonant frequency interval equal to the reciprocal of the sequence repetition time (TR). Appropriate selection of the TR results in a 180 degrees phase difference between lipid and water signals. This phase difference can be used for fat-water separation in SSFP without any increase in scan time. The technique is shown to produce excellent non-contrast-enhanced, flow-independent angiograms of the peripheral vasculature.     

Double average parallel steady-state free precession imaging: optimized eddy current and transient oscillation compensation.

Balanced steady-state free precession (SSFP) imaging is sensitive to off-resonance effects, which can lead to considerable artifacts during a transient phase following magnetization preparation or steady-state interruption. In addition, nonlinear k-space encoding is required if contrast-relevant k-space regions need to be acquired at specific delays following magnetization preparation or for transient artifact reduction in cardiac-gated k-space segmented CINE imaging. Such trajectories are problematic for balanced SSFP imaging due to nonconstant eddy current effects and resulting disruption of the steady state.In this work, a novel acquisition strategy for balanced SSFP imaging is presented that utilizes scan time reduction by parallel imaging for optimized "double average" eddy current compensation and artifact reduction during the transient phase following steady-state storage and magnetization preparation. Double average parallel SSFP imaging was applied to k-space segmented CINE SSFP tagging as well as nongated centrically encoded SSFP imaging. Phantom and human studies exhibit substantial reduction in steady-state storage and eddy current artifacts while maintaining spatial resolution, signal-to-noise ratio, and similar total scan time of a standard SSFP acquisition. The proposed technique can easily be extended to other acquisition schemes that would benefit from nonlinear reordering schemes and/or rely on interruption of the balanced SSFP steady state.     

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.     

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.     

Simultaneous fat suppression and band reduction with large-angle multiple-acquisition balanced steady-state free precession

Balanced steady-state free precession (bSSFP) MRI is a rapid and signal-to-noise ratio-efficient imaging method, but suffers from characteristic bands of signal loss in regions of large field inhomogeneity. Several methods have been developed to reduce the severity of these banding artifacts, typically involving the acquisition of multiple bSSFP datasets (and the accompanying increase in scan time). Fat suppression with bSSFP is also challenging; most existing methods require an additional increase in scan time, and some are incompatible with bSSFP band-reduction techniques. This work was motivated by the need for both robust fat suppression and band reduction in the presence of field inhomogeneity when using bSSFP for flow-independent peripheral angiography. The large flip angles used in this application to improve vessel conspicuity and contrast lead to specific absorption rate considerations, longer repetition times, and increased severity of banding artifacts. In this work, a novel method that simultaneously suppresses fat and reduces bSSFP banding artifact with the acquisition of only two phase-cycled bSSFP datasets is presented. A weighted sum of the two bSSFP acquisitions is taken on a voxel-by-voxel basis, effectively synthesizing an off-resonance profile at each voxel that puts fat in the stop band while keeping water in the pass band. The technique exploits the near-sinusoidal shape of the bSSFP off-resonance spectrum for many tissues at large (>50°) flip angles.     

Spiral balanced steady-state free precession cardiac imaging.

Balanced steady-state free precession (SSFP) sequences are useful in cardiac imaging because they achieve high signal efficiency and excellent blood-myocardium contrast. Spiral imaging enables the efficient acquisition of cardiac images with reduced flow and motion artifacts. Balanced SSFP has been combined with spiral imaging for real-time interactive cardiac MRI. New features of this method to enable scanning in a clinical setting include short, first-moment nulled spiral trajectories and interactive control over the spatial location of banding artifacts (SSFP-specific signal variations). The feasibility of spiral balanced SSFP cardiac imaging at 1.5 T is demonstrated. In observations from over 40 volunteer and patient studies, spiral balanced SSFP imaging shows significantly improved contrast compared to spiral gradient-spoiled imaging, producing better visualization of cardiac function, improved localization, and reduced flow artifacts from blood.     

Alternating repetition time balanced steady state free precession.

A novel balanced SSFP technique for the separation or suppression of different resonance frequencies (e.g., fat suppression) is presented. The method is based on applying two alternating and different repetition times, TR(1) and TR(2). This RF scheme manipulates the sensitivity of balanced SSFP to off-resonance effects by a modification of the frequency response profile. Starting from a general approach, an optimally broadened stopband within the frequency response function is designed. This is achieved with a TR(2) being one third of TR(1) and an RF-pulse phase increment of 90 degrees . With this approach TR(2) is too short ( approximately 1 ms) to switch imaging gradients and is only used to change the frequency sensitivity. Without a significant change of the spectral position of the stopband, TR(1) can be varied over a range of values ( approximately 2.5-4.5 ms) while TR(2) and phase cycling is kept constant. On-resonance spins show a magnetization behavior similar to balanced SSFP, but with maximal magnetization at flip angles about 10 degrees lower than in balanced SSFP. The total scan time is increased by about 30% compared to conventional balanced SSFP. The new technique was applied on phantoms and volunteers to produce rapid, fat suppressed images.     

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.     

Navigator-gated three-dimensional MR angiography of the pulmonary arteries using steady-state free precession

PURPOSE: To assess the quality of a navigator-gated, free breathing, steady-state free precession (SSFP) technique in comparison to a single breathhold for pulmonary artery imaging in normal volunteers. MATERIALS AND METHODS: Sagittal sections of the left pulmonary arteries of 10 volunteers were obtained with a three-dimensional SSFP sequence using both a single breathhold of 30 seconds and a navigator-gated version of the same sequence. The images were compared and rated by a blinded cardiovascular radiologist for image quality, sharpness, and artifact. RESULTS: On a scale ranging from -2 to 2, in which positive numbers denote that the navigator method was favorable compared to the single breathhold method, image quality was rated 0.7+/-1.4, sharpness 0.6+/-1.5, and artifact 0.1+/-1.4. Thus, there was no statistical difference between the two methods. CONCLUSION: The navigator-gated SSFP sequence is able to acquire images equal in quality to the breathhold sequence. This may be of clinical importance for pulmonary imaging in patients who are unable to sustain a long breathhold.     

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.     

3D non-contrast-enhanced MR angiography with balanced steady-state free precession Dixon method.

Balanced steady-state free precession (bSSFP) is capable of producing ample fat-water separation. In the case of the bSSFP Dixon method, the phase between fat and water can be manipulated by setting repetition time (TR) to an odd-half-multiple of the cycle time and adjusting the center frequency to acquire fat-water in in-phase and opposed-phase images. Adding an image collected when fat and water are in-phase to an image in which fat and water are opposed-phase produces a water-only image. Of the water signals, arterial blood has the highest T(2)/T(1) contrast, making the arterial signal appear brighter than both venous blood and muscle in the final image. In this study, the bSSFP Dixon method was used to collect coronal water-only three-dimensional (3D) volumes at multiple anatomical stations in the legs of five healthy volunteers. The image quality was quantified by region-of-interest (ROI) analysis of signal intensities between arterial blood, venous blood, muscle, and fat. The images were also assessed for diagnostic quality by a trained radiologist. The bSSFP Dixon method was successful in producing non-contrast-enhanced (NCE) images of the blood vessels in the lower limbs. The work presented here is a proof-of-concept for the use of the bSSFP Dixon method for 3D peripheral angiography.     

S5FP: spectrally selective suppression with steady state free precession.

A method is presented that employs the inherent spectral selectivity of the Steady-State Free Precession (SSFP) pulse sequence to provide a spectral band of suppression. At TE = TR/2, SSFP partitions the magnetization into two phase-opposed spectral components. Z-storing one of these components simultaneously further excites the other, which is then suppressed by gradient crushing and RF spoiling. The Spectrally Selective Suppression with SSFP (S(5)FP) method is shown to provide significant attenuation of fat signals, while the water signals are essentially unaffected and provide the normal SSFP contrast. Fat suppression is achieved with relatively little temporal overhead (less than 10% reduction in temporal resolution). S(5)FP was validated using simulations, phantoms, and human studies.     

On multiple alternating steady states induced by periodic spin phase perturbation waveforms

Direct measurement of neural currents by means of MRI can potentially open a high temporal resolution (10–100 ms) window applicable for monitoring dynamics of neuronal activity without loss of the high spatial resolution afforded by MRI. Previously, we have shown that the alternating balanced steady state imaging affords high sensitivity to weak periodic currents owing to its amplification of periodic spin phase perturbations. This technique, however, requires precise synchronization of such perturbations to the radiofrequency pulses. Herein, we extend alternating balanced steady state imaging to multiple balanced alternating steady states for estimation of neural current waveforms. Simulations and phantom experiments show that the off‐resonance profile of the multiple alternating steady state signal carries information about the frequency content of driving waveforms. In addition, the method is less sensitive than alternating balanced steady state to precise waveform timing relative to radiofrequency pulses. Thus, multiple alternating steady state technique is potentially applicable to MR imaging of the waveforms of periodic neuronal activity. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

MR angiography using steady-state free precession.

Contrast-enhanced MR angiography (CE-MRA) using steady-state free precession (SSFP) pulse sequences is described. Using SSFP, vascular structures can be visualized with high signal-to-noise ratio (SNR) at a substantial (delay) time after the initial arterial pass of contrast media. The peak blood SSFP signal was diminished by <20% 30 min after the initial administration of 0.2 mmol/kg of Gd-chelate. The proposed method allows a second opportunity to study arterial or venous structures with high image SNR and high spatial resolution. A mask subtraction scheme using spin echo SSFP-S(-) acquisition is also described to reduce stationary background signal from the delayed SSFP angiography images.