Signal-to-noise ratio behavior of steady-state free precession.

Steady-state free precession (SSFP) is a rapid gradient-echo imaging technique that has recently gained popularity and is used in a variety of applications, including cardiac and real-time imaging, because of its high signal and favorable contrast between blood and myocardium. The purpose of this work was to examine the signal-to-noise ratio (SNR) behavior of images acquired with SSFP, and the dependence of SNR on imaging parameters such as TR, bandwidth, and image resolution, and the use of multi-echo sequences. In this work it is shown that the SNR of SSFP sequences is dependent only on pulse sequence efficiency, voxel dimensions, and relaxation parameters (T1 and T2). Notably, SNR is insensitive to bandwidth unless increases in bandwidth significantly decrease efficiency. Finally, we examined the relationship between pulse sequence performance (TR and efficiency) and gradient performance (maximum gradient strength and slew rate) for several imaging scenarios, including multi-echo sequences, to determine the optimum matching of maximum gradient strength and slew rate for gradient hardware designs. For standard modern gradient hardware (40 mT/m and 150 mT/m/ms), we found that the maximum gradient strength is more than adequate for the imaging resolution that is commonly encountered with rapid scouting (3 mm x 4 mm x 10 mm voxel). It is well matched for typical CINE and real-time cardiac imaging applications (1.5 mm x 2 mm x 6 mm voxel), and is inadequate for optimal matching with slew rate for high-resolution applications such as musculoskeletal imaging (0.5 x 0.8 x 3 mm voxel). For the lower-resolution methods, efficiency could be improved with higher slew rates; this provokes interest in designing methods for limiting dB/dt peripherally while achieving high switching rates in the imaging field of view. The use of multi-echo SSFP acquisitions leads to substantial improvements in sequence performance (i.e., increased efficiency and shorter TR).     

Enhanced spectral shaping in steady-state free precession imaging.

Balanced steady-state free precession (SSFP) is hindered by the inherent off-resonance sensitivity and unwanted bright fat signal. Multiple-acquisition SSFP combination methods, where multiple datasets with different fixed RF phase increments are acquired, have been used for shaping the SSFP spectrum to solve both problems. We present a new combination method (weighted-combination SSFP or WC-SSFP) that preserves SSFP contrast and enables banding-reduction and fat-water separation. Methods addressing the banding artifact have focused on either getting robust banding-reduction (complex-sum SSFP) or improved SNR efficiency (sum-of-squares SSFP). The proposed method achieves both robust banding-reduction and an SNR efficiency close to that of the sum-of-squares method. A drawback of fat suppression methods that create a broad stop-band around the fat resonance is the wedge shape of the stop-band leading to imperfect suppression. WC-SSFP improves the suppression of the stop-band without affecting the pass-band performance, and prevents fat signal from obscuring the tissues of interest in the presence of considerable resonant frequency variations. The method further facilitates the use of SSFP imaging by providing a control parameter to adjust the level of banding-reduction or fat suppression to application-specific needs.     

Flow artifacts in steady-state free precession cine imaging.

Steady-state free precession (SSFP) cardiac cine images are frequently corrupted by dark flow artifacts, which can usually be eliminated by reshimming and retuning the scanner. A theoretical explanation for these artifacts is provided in terms of spins moving through an off-resonant point in the magnetic field, and the theory is validated using phantom experiments. The artifacts can be reproduced in vivo by detuning the center frequency by an amount in the range of half the inverse repetition time (TR). Since this offset is similar in magnitude to the frequency difference between the water and lipid peaks, a likely cause of the artifacts in vivo is that the center frequency is tuned incorrectly to the lipid peak rather than the water peak.     

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.     

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.     

Three-dimensional MR of the inner ear with steady-state free precession.

PURPOSETo describe the steady-state free-precession MR sequence and its application to the study of the inner ear.METHODThe inner ear was imaged with CT and a 0.5-T MR unit in three dimension, to evaluate the various signals from the lumen of the labyrinth.RESULTSNormally, the signal from the perilymphatic and endolymphatic spaces is homogeneous. However, among our cases of neurosensory deafness, differences of signal and morphology were seen in patients with otosclerosis, ossifying labyrinthitis, and inner ear malformations.CONCLUSIONThree-dimensional MR, used together with routine two-dimensional fast spin-echo, is another diagnostic too]l that can provide new data in the evaluation of the normal and unhealthy inner ear.     

MR imaging of brain surface using steady-state free precession.

We present an imaging technique that affords direct and noninvasive visualization of brain surface structure. This technique utilizes the signal before the rf pulse in steady-state free precession. This signal highly reflects the spin-spin relaxation time T2 as was studied in our laboratory (Matsui et al. J. Magn. Reson. 62, 12, 1985). Therefore the cerebrospinal fluid (CSF), having a long T2, is depicted as high intensity. The CSF permeates cerebral sulci and fissures. The imaging time with this technique is less than 1 min.     

Visibility of epidermoid tumors on steady-state free precession images.

PURPOSETo determine whether steady-state free precession sequences improve the MR visibility of epidermoid tumors in comparison with spin-echo images.METHODSPatients were four women and three men with epidermoid tumors in the subarachnoid spaces. MR was performed with a 1.5-T superconductive unit. For steady-state free precession imaging, three-dimensional Fourier transform fast imaging with steady-state free precession (FISP) images were used (20-40/7 [repetition time/echo time], flip angle of 25 degrees). The visualization and contrast-to-noise ratio were compared in FISP images and spin-echo images. In one case, the images of FISP and fast low-angle shot were obtained with variable repetition times and flip angles to evaluate the best pulse sequences for the visualization of epidermoid tumors.RESULTSThe contrast-to-noise ratios between tumors and cerebrospinal fluid ranged from 7.9 to 17.5 (average was 12.9) on FISP images. The average of contrast-to-noise ratios on T1, T2, and proton density-weighted spin-echo images were 1.6, 2.0, and 4.2, respectively. Three-dimensional Fourier transform FISP images best showed central nervous system and demonstrated epidermoid tumors excellently.CONCLUSIONSEpidermoid tumors in the subarachnoid spaces were better demonstrated on steady-state free precession (three-dimensional Fourier transform FISP) than on conventional spin-echo images.     

Off-resonance-dependent slice profile effects in balanced steady-state free precession imaging.

The purpose of this study was the simulation and measurement of balanced steady-state free precession (bSSFP) slice profiles for a detailed analysis of the influence of off-resonance effects on slice profile shape and bSSFP signal intensity. Due to the frequency response function of the bSSFP sequence, measurements that are not on-resonance result in broadened effective slice profiles with different off-resonance-dependent shapes and signal intensities. In this study, bSSFP slice profile effects and their dependence on off-resonance were investigated based on bSSFP signal simulations of phantom data as well as blood and tissue. For a better assessment of the similarity of measured and simulated slice profiles the field map was integrated in the slice profile simulations. The results demonstrate that simulations can accurately predict bSSFP slice profiles. Both measurements and simulations indicate that there is a substantial increase in signal intensity close to the banding artifacts, i.e., at spatial locations with off-resonance frequencies corresponding to a dephasing/TR = +/- pi resulting in signal void (bands). For routine bSSFP imaging, off-resonance-dependent slice broadening may thus result in a substantial difference between nominal and true slice thickness and lead to spatially varying slice thickness and signal intensities across the imaging slice.     

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.     

Cyclic CINE-balanced steady-state free precession image intensity variations: implications for the detection of myocardial edema

Purpose

To evaluate the dependence of CINE‐balanced steady‐state free precession (bSSFP) image intensities on spatial location, cardiac phase, and disease state.

Materials and Methods

Eight subjects with recent myocardial infarctions and eight age‐ and sex‐matched normal volunteers were studied using CINE‐bSSFP imaging to describe cyclic image intensity variations as a function of the cardiac cycle and to optimize and assess the ability of CINE‐bSSFP imaging to depict myocardial edema. Signal intensities of the left ventricular (LV) bloodpool and myocardium were measured using region‐of‐interest analysis across the cardiac cycle. The magnitude and time course of the cyclic variations were evaluated. Mixed‐model analysis of variance was used to examine the influence of physical location, cardiac phase, and presence of myocardial infarction.

Results

The LV bloodpool and myocardial CINE‐bSSFP signal intensities varied significantly with spatial location, cardiac phase, and disease (P < 0.001). Cardiac phase had a significant effect on the signal intensities after adjustments for spatial location. The LV bloodpool signal decreased slowly during systole and rose sharply during LV filling. There were two distinct myocardial intensity peaks, one occurring at peak systole and the other at the end of the LV rapid inflow phase. Myocardial edema was seen as a hyperintense region. Image contrast with adjacent myocardium was the greatest at the end of systole.

Conclusion

Detection of myocardial edema using the conventional CINE‐bSSFP technique is feasible, but is complicated by normal cyclic changes in myocardial image intensities during the cardiac cycle. J. Magn. Reson. Imaging 2011;33:573–581. © 2011 Wiley‐Liss, Inc.     

Fast imaging of CSF flow/motion patterns using steady-state free precession (SSFP)

Using a rapid Fourier SSFP imaging technique, which is sensitive to slow flow (approximately 1 mm/sec) in the plane of the image, we obtained 135 brain MRI examinations. The CSF flow/motion patterns were mapped by two images with orthogonal in plane flow sensitivity directions. Analysis showed significant deviations from the "normal" pattern in ventricular enlargements because of obstruction (no evidence of CSF flow/motion) or in normal pressure hydrocephalus (complex, intensive flow pattern in lateral ventricles) suggesting a diagnostic potential for this fast imaging technique.     

Effects of intravoxel incoherent motions (IVIM) in steady-state free precession (SSFP) imaging: application to molecular diffusion imaging

A theoretical analysis of the effects of diffusion and perfusion in steady-state free precession (SSFP) imaging sequences sensitized to intravoxel incoherent motions by magnetic field gradients is presented and supported by phantom studies. The capability of such sequences to image diffusion and perfusion quickly was recently demonstrated. The possible residual effects of T1 and T2 in diffusion measurements are evaluated, as are the effects of the sequence design and the acquisition parameters (repetition time, flip angle, gradient pulses). It is shown theoretically and confirmed by experiments on phantoms that diffusion coefficients can be directly measured from SSFP images when large enough diffusion gradient pulses are used.     

Fat and water separation in balanced steady-state free precession using the Dixon method.

In this work the feasibility of separating fat and water signals using the balanced steady-state free precession (SSFP) technique is demonstrated. The technique is based on the observation (Scheffler and Hennig, Magnetic Resonance in Medicine 2003;49:395-397) that at the nominal values of TE = TR/2 in SSFP imaging, phase coherence can be achieved at essentially only two orientations (0 degrees and 180 degrees ) relative to the RF pulses in the rotating frame, under the assumption of TR < T2, and independently of the SSFP angle. This property allows in-phase and out-of-phase SSFP images to be obtained by proper choices of the center frequency offset, and thus allows the Dixon subtraction method to be utilized for effective fat-water separation. The TR and frequency offset for optimal fat-water separation are derived from theories. Experimental results from healthy subjects, using a 3.0 Tesla system, show that nearly complete fat suppression can be accomplished.     

Detecting microcirculatory changes in blood oxygen state with steady-state free precession imaging.

Recently, it has been demonstrated that oxygen-weighted images of whole blood can be obtained with steady-state methods. In this article, based on computational and experimental models, we investigate the potential for employing this technique to monitor oxygen changes in microcirculation. Results show that oxygen-sensitive images of rabbit kidney and muscle may be obtained at high signal-to-noise ratio within a few seconds. The results also show that in steady-state free precession imaging, in addition to the exchange mechanism that generates oxygen contrast in blood, there are additional mechanisms that provide oxygen-sensitive contrast in microcirculation.     

The application of steady-state free precession to the study of very slow fluid flow

An NMR imaging technique sensitive to slow flow (approximately 1 mm/s) using a conventional imaging gradient strength (0.025 mT/cm) is described. Two projections with different spatial magnetic periodicity (determined by the SSFP pulse interval), and thus with different flow sensitivities, are subtracted to give signal from flows in a velocity window.     

High-resolution MRI of cardiac function with projection reconstruction and steady-state free precession.

The purpose of this study was to investigate the trabecular structure of the endocardial wall of the living human heart, and the effect of that structure on the measurement of myocardial function using MRI. High-resolution MR images (0.8 x 0.8 x 8 mm voxels) of cardiac function were obtained in five volunteers using a combination of undersampled projection reconstruction (PR) and steady-state free precession (SSFP) contrast in ECG-gated breath-held scans. These images provide movies of cardiac function with new levels of endocardial detail. The trabecular-papillary muscle complex, consisting of a mixture of blood and endocardial structures, is measured to constitute as much as 50% of the myocardial wall in some sectors. Myocardial wall strain measurements derived from tagged MR images show correlation between regions of trabeculae and papillary muscles and regions of high strain, leading to an overestimation of function in the lateral wall.     

Balanced phase-contrast steady-state free precession (PC-SSFP): a novel technique for velocity encoding by gradient inversion.

A technique for measuring velocity is presented that combines cine phase contrast (PC) MRI and balanced steady-state free precession (SSFP) imaging, and is thus termed PC-SSFP. Flow encoding was performed without the introduction of additional velocity encoding gradients in order to keep the repetition time (TR) as short as in typical SSFP imaging sequences. Sensitivity to through-plane velocities was instead established by inverting (i.e., negating) all gradients along the slice-select direction. Velocity sensitivity (VENC) could be adjusted by altering the first moments of the slice-select gradients. Disturbances of the SSFP steady state were avoided by acquiring different flow echoes in consecutively (i.e., sequentially) executed scans, each over several cardiac cycles, using separate steady-state preparation periods. A comparison of phantom measurements with those from established 2D-cine-PC MRI demonstrated excellent correlation between both modalities. In examinations of volunteers, PC-SSFP exhibited a higher intrinsic signal-to-noise ratio (SNR) and consequently low phase noise in measured velocities compared to conventional PC scans. An additional benefit of PC-SSFP is that it relies less on in-flow-dependent signal enhancement, and thus yields more uniform SNRs and better depictions of vessel geometry throughout the whole cardiac cycle in structures with slow and/or pulsatile flow.     

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.