Analytical solution to the transient phase of steady‐state free precession sequences

The transient phase of short–TR steady‐state free precession (SSFP) sequences exhibits an often striking complexity and is not only important for nonequilibrium applications (e.g., rapid T₁–measurements), but can also cause severe artifacts in conventional imaging. In both cases, balanced SSFP sequences are practically (with regard to preparation efficiency) and conceptually (concerning the theoretical understanding of the decay) easier to handle their unbalanced counterparts, for which currently no theory is available. Based on a decomposition of coherence pathways into irreducible subpaths, an exact mathematical solution to the transient phase of unbalanced SSFP sequences is presented in this article, which also includes the known results for balanced SSFP and the steady state of arbitrary SSFP sequences as special cases. As an application, it is shown that the familiar Look–Locker expression for the accelerated magnetization recovery in RF‐spoiled sequences is only valid for T₂ → 0. In addition to oscillatory perturbations, systematic deviations from the monoexponential decay are observed for T₂ 0 as a consequence of memory effects. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Comparison of wideband steady‐state free precession and T2‐weighted fast spin echo in spine disorder assessment at 1.5 and 3 T

Wideband steady‐state free precession (WB‐SSFP) is a modification of balanced steady‐state free precession utilizing alternating repetition times to reduce susceptibility‐induced balanced steady‐state free precession limitations, allowing its use for high‐resolution myelographic‐contrast spinal imaging. Intertissue contrast and spatial resolution of complete‐spine‐coverage 3D WB‐SSFP were compared with those of 2D T₂‐weighted fast spin echo, currently the standard for spine T₂‐imaging. Six normal subjects were imaged at 1.5 and 3 T. The signal‐to‐noise ratio efficiency (SNR per unit‐time and unit‐volume) of several tissues was measured, along with four intertissue contrast‐to‐noise ratios; nerve‐ganglia:fat, intradural‐nerves:cerebrospinal fluid, nerve‐ganglia:muscle, and muscle:fat. Patients with degenerative and traumatic spine disorders were imaged at both MRI fields to demonstrate WB‐SSFP clinical advantages and disadvantages. At 3 T, WB‐SSFP provided spinal contrast‐to‐noise ratios 3.7–5.2 times that of fast spin echo. At 1.5 T, WB‐SSFP contrast‐to‐noise ratio was 3–3.5 times that of fast spin echo, excluding a 1.7 ratio for intradural‐nerves:cerebrospinal fluid. WB‐SSFP signal‐to‐noise ratio efficiency was also higher. Three‐dimensional WB‐SSFP disadvantages relative to 2D fast spin echo are reduced edema hyperintensity, reduced muscle signal, and higher motion sensitivity. WB‐SSFP's high resolution and contrast‐to‐noise ratio improved visualization of intradural nerve bundles, foraminal nerve roots, and extradural nerve bundles, improving detection of nerve compression in radiculopathy and spinal‐stenosis. WB‐SSFP's high resolution permitted reformatting into orthogonal planes, providing distinct advantages in gauging fine spine pathology. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

In vivo single scan detection of both iron-labeled cells and breast cancer metastases in the mouse brain using balanced steady-state free precession imaging at 1.5 T

Purpose:

To simultaneously detect iron‐labeled cancer cells and brain tumors in vivo in one scan, the balanced steady‐state free precession (b‐SSFP) imaging sequence was optimized at 1.5 T on mice developing brain metastases subsequent to the injection of micron‐sized iron oxide particle‐labeled human breast cancer cells.

Materials and Methods:

b‐SSFP sequence parameters (repetition time, flip angle, and receiver bandwidth) were varied and the signal‐to‐noise ratio, contrast between the brain and tumors, and the number of detected iron‐labeled cells were evaluated.

Results:

Optimal b‐SSFP images were acquired with a 26 msec repetition time, 35° flip angle, and bandwidth of ±21 kHz. b‐SSFP images were compared with T₂‐weighted 2D fast spin echo (FSE) and 3D spoiled gradient recalled echo (SPGR) images. The mean tumor‐brain contrast‐to‐noise ratio and the ability to detect iron‐labeled cells were the highest in the b‐SSFP images.

Conclusion:

A single b‐SSFP scan can be used to visualize both iron‐labeled cells and brain metastases. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Fast diffusion‐weighted steady state free precession imaging of in vivo knee cartilage

Quantification of molecular diffusion with steady state free precession (SSFP) is complicated by the fact that diffusion effects accumulate over several repetition times (TR) leading to complex signal dependencies on transverse and longitudinal magnetization paths. This issue is commonly addressed by setting TR > T₂, yielding strong attenuation of all higher modes, except of the shortest ones. As a result, signal attenuation from diffusion becomes T₂ independent but signal‐to‐noise ratio (SNR) and sequence efficiency are remarkably poor. In this work, we present a new approach for fast in vivo steady state free precession diffusion‐weighted imaging of cartilage with TR << T₂ offering a considerable increase in signal‐to‐noise ratio and sequence efficiency. At a first glance, prominent coupling between magnetization paths seems to complicate quantification issues in this limit, however, it is observed that diffusion effects become rather T₂(ΔD ～ 1/10 ΔT₂) but not T₁ independent (ΔD ～ 1/2 ΔT₁) for low flip angles α ～ 10 ? 15°. As a result, fast high‐resolution (0.35 × 0.35 ? 0.50 × 0.50 mm² in‐plane resolution) quantitative diffusion‐weighted imaging of human articular cartilage is demonstrated at 3.0 T in a clinical setup using estimated T₁ and T₂ or a combination of measured T₁ and estimated T₂ values. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

T2‐weighted 3D fMRI using S2‐SSFP at 7 tesla

In this study, the sensitivity of the S₂‐steady‐state free precession (SSFP) signal for functional MRI at 7 T was investigated. In order to achieve the necessary temporal resolution, a three‐dimensional acquisition scheme with acceleration along two spatial axes was employed. Activation maps based on S₂‐steady‐state free precession data showed similar spatial localization of activation and sensitivity as spin‐echo echo‐planar imaging (SE‐EPI), but data can be acquired with substantially lower power deposition. The functional sensitivity estimated by the average z‐values was not significantly different for SE‐EPI compared to the S₂‐signal but was slightly lower for the S₂‐signal (6.74 ± 0.32 for the TR = 15 ms protocol and 7.51 ± 0.78 for the TR = 27 ms protocol) compared to SE‐EPI (7.49 ± 1.44 and 8.05 ± 1.67) using the same activated voxels, respectively. The relative signal changes in these voxels upon activation were slightly lower for SE‐EPI (2.37% ± 0.18%) compared to the TR = 15 ms S₂‐SSFP protocol (2.75% ± 0.53%) and significantly lower than the TR = 27 ms protocol (5.38% ± 1.28%), in line with simulations results. The large relative signal change for the long TR SSFP protocol can be explained by contributions from multiple coherence pathways and the low intrinsic intensity of the S₂ signal. In conclusion, whole‐brain T₂‐weighted functional MRI with negligible image distortion at 7 T is feasible using the S₂‐SSFP sequence and partially parallel imaging. Magn Reson Med 63:1015–1020, 2010. © 2010 Wiley‐Liss, Inc.     

Flip angle profile correction for T1 and T2 quantification with look‐locker inversion recovery 2D steady‐state free precession imaging

Fast methods using balanced steady‐state free precession have been developed to reduce the scan time of T₁ and T₂ mapping. However, flip angle (FA) profiles created by the short radiofrequency pulses used in steady‐state free precession deviate substantially from the ideal rectangular profile, causing T₁ and T₂ mapping errors. The purpose of this study was to develop a FA profile correction for T₁ and T₂ mapping with Look‐Locker 2D inversion recovery steady‐state free precession and to validate this method using 2D spin echo as a reference standard. Phantom studies showed consistent improvement in T₁ and T₂ accuracy using profile correction at multiple FAs. Over six human calves, profile correction provided muscle T₁ estimates with mean error ranging from excellent (?0.6%) at repetition time/FA = 18 ms/60° to acceptable (6.8%) at repetition time/FA = 4.9 ms/30°, while muscle T₂ estimates were less accurate with mean errors of 31.2% and 47.9%, respectively. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Fundamentals of balanced steady state free precession MRI

Balanced steady state free precession (balanced SSFP) has become increasingly popular for research and clinical applications, offering a very high signal‐to‐noise ratio and a T₂/T₁‐weighted image contrast. This review article gives an overview on the basic principles of this fast imaging technique as well as possibilities for contrast modification. The first part focuses on the fundamental principles of balanced SSFP signal formation in the transient phase and in the steady state. In the second part, balanced SSFP imaging, contrast, and basic mechanisms for contrast modification are revisited and contemporary clinical applications are discussed. J. Magn. Reson. Imaging 2013;38:2–11. © 2013 Wiley Periodicals, Inc.     

Spin‐locked balanced steady‐state free‐precession (slSSFP)

A spin‐locked balanced steady‐state free‐precession (slSSFP) pulse sequence is described that combines a balanced gradient‐echo acquisition with an off‐resonance spin‐lock pulse for fast MRI. The transient and steady‐state magnetization trajectory was solved numerically using the Bloch equations and was shown to be similar to balanced steady‐state free‐precession (bSSFP) for a range of T₂/T₁ and flip angles, although the slSSFP steady‐state could be maintained with considerably lower radio frequency (RF) power. In both simulations and brain scans performed at 7T, slSSFP was shown to exhibit similar contrast and signal‐to‐noise ratio (SNR) efficiency to bSSFP, but with significantly lower power. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

3D flow‐independent peripheral vessel wall imaging using T2‐prepared phase‐sensitive inversion‐recovery steady‐state free precession

Purpose:

To develop a 3D flow‐independent peripheral vessel wall imaging method using T₂‐prepared phase‐sensitive inversion‐recovery (T₂PSIR) steady‐state free precession (SSFP).

Materials and Methods:

A 3D T₂‐prepared and nonselective inversion‐recovery SSFP sequence was designed to achieve flow‐independent blood suppression for vessel wall imaging based on T₁ and T₂ properties of the vessel wall and blood. To maximize image contrast and reduce its dependence on the inversion time (TI), phase‐sensitive reconstruction was used to restore the true signal difference between vessel wall and blood. The feasibility of this technique for peripheral artery wall imaging was tested in 13 healthy subjects. Image signal‐to‐noise ratio (SNR), wall/lumen contrast‐to‐noise ratio (CNR), and scan efficiency were compared between this technique and conventional 2D double inversion recovery – turbo spin echo (DIR‐TSE) in eight subjects.

Results:

3D T₂PSIR SSFP provided more efficient data acquisition (32 slices and 64 mm in 4 minutes, 7.5 seconds per slice) than 2D DIR‐TSE (2–3 minutes per slice). SNR of the vessel wall and CNR between vessel wall and lumen were significantly increased as compared to those of DIR‐TSE (P < 0.001). Vessel wall and lumen areas of the two techniques are strongly correlated (intraclass correlation coefficients: 0.975 and 0.937, respectively; P < 0.001 for both). The lumen area of T₂PSIR SSFP is slightly larger than that of DIR‐TSE (P = 0.008). The difference in vessel wall area between the two techniques is not statistically significant.

Conclusion:

T₂PSIR SSFP is a promising technique for peripheral vessel wall imaging. It provides excellent blood signal suppression and vessel wall/lumen contrast. It can cover a 3D volume efficiently and is flow‐ and TI‐independent. J. Magn. Reson. Imaging 2010;32:399–408. © 2010 Wiley‐Liss, Inc.     

Quantification of fat infiltration in oculopharyngeal muscular dystrophy: comparison of three MR imaging methods

Purpose

To analyze and compare three quantitative MRI methods to determine the degree of muscle involvement in oculopharyngeal muscular dystrophy (OPMD).

Materials and Methods

Muscle fat content (MFC) was determined based on water–fat quantification using a 2‐point Dixon (2PD) method and on a histogram analysis of the free induction decay (FID) signal of a gradient‐spoiled steady‐state free precession (SSFP) sequence. In addition, transverse relaxation times (T₂) of muscle tissue were calculated using a monoexponential decay model.

Results

We observed an increased mean MFC in OPMD patients as compared to healthy controls with the adductor magnus and soleus muscles being the most involved muscles in the thigh and calf, respectively. Furthermore, strong correlations (0.78 < R² < 0.94) between different quantitative MR methods were observed. Fewer outliers, however, were obtained by the 2PD method and T₂ measurements, suggesting these methods being superior to the SSFP‐FID method.

Conclusion

Quantitative MR techniques, such as fast multiecho Dixon methods and T₂ imaging, can reliably differentiate between healthy and dystrophic muscles in OPMD, even if muscles are only marginally affected. Quantitative methods thus represent a promising tool that may be able to monitor more objectively the individual disease progression and treatment response in future clinical trials in muscular dystrophies. J. Magn. Reson. Imaging 2011;33:203–210. © 2010 Wiley‐Liss, Inc.     

Finite RF pulse correction on DESPOT2

Magnetization transfer and finite radiofrequency (RF) pulses affect the steady state of balanced steady state free precession. As quantification of transverse relaxation (T₂) with driven equilibrium single pulse observation of T₂ is based on two balanced steady state free precession acquisitions, both effects can influence the outcome of this method: a short RF pulse per repetition time (T_RF/TR ? 1) leads to considerable magnetization transfer effects, whereas prolonged RF pulses (T_RF/TR > 0.2) minimize magnetization transfer effects, but lead to increased finite pulse effects. A correction for finite pulse effects is thus implemented in the driven equilibrium single pulse observation of T₂ theory to compensate for reduced transverse relaxation effects during excitation. It is shown that the correction successfully removes the driven equilibrium single pulse observation of T₂ dependency on the RF pulse duration. A reduction of the variation in obtained T₂ from over 50% to less than 10% is achieved. We hereby provide a means of acquiring magnetization transfer‐free balanced steady state free precession images to yield accurate T₂ values using elongated RF pulses. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Imaging iron‐loaded mouse glioma tumors with bSSFP at 3 T

The poor prognosis for patients with high‐grade glioma is partly due to the invasion of tumor cells into surrounding brain tissue. The goal of the present work was to develop a mouse model of glioma that included the potential to track cell invasion using MRI by labeling GL261 cells with iron oxide contrast agents prior to intracranial injection. Two types of agents were compared with several labeling schemes to balance between labeling with sufficient iron to curb the dilution effect of cell division while avoiding overwhelming signal loss that could prevent adequate visualization of tumor boundaries. The balanced steady‐state free precession (bSSFP) pulse sequence was evaluated for its suitability for imaging glioma tumors and compared to T₂‐weighted two‐dimensional fast spin echo (FSE) and T₁‐weighted spoiled gradient recalled echo (SPGR) at 3 T in terms of signal‐to‐noise ratio and contrast‐to‐noise ratio efficiencies. Ultimately, a three‐dimensional bSSFP protocol consisting of a set of two images with complementary contrasts was developed, allowing excellent tumor visualization with minimal iron contrast when using pulse repetition time = 6 ms and α = 40°, and extremely high sensitivity to iron when using pulse repetition time = 22 ms and α = 20°. Quantitative histologic analysis validated that the strong signal loss seen in balanced steady state free precession pulse sequence images of iron‐loaded tumors correlated well with the presence of iron. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Fast three‐dimensional proton spectroscopic imaging of the human brain at 3 T by combining spectroscopic missing pulse steady‐state free precession and echo planar spectroscopic imaging

The combination of the principles of two fast spectroscopic imaging (SI) methods, spectroscopic missing pulse steady‐state free precession and echo planar SI (EPSI) is described as an approach toward fast 3D SI. This method, termed missing pulse steady‐state free precession echo planar SI, exhibits a considerably reduced minimum total measurement time T_min, allowing a higher temporal resolution, a larger spatial matrix size, and the use of k‐space weighted averaging and phase cycling, while maintaining all advantages of the original spectroscopic missing pulse steady‐state free precession sequence. The minor signal‐to‐noise ratio loss caused by using oscillating read gradients can be compensated by applying k‐space weighted averaging. The missing pulse steady‐state free precession echo planar SI sequence was implemented on a 3 T head scanner, tested on phantoms and applied to healthy volunteers. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Corticomedullary differentiation of the kidney: Evaluation with noncontrast‐enhanced steady‐state free precession (SSFP) MRI with time‐spatial labeling inversion pulse (time‐SLIP)

Purpose:

To assess whether noncontrast‐enhanced steady‐state free precession (SSFP) magnetic resonance imaging (MRI) with time‐spatial labeling inversion pulse (Time‐SLIP) can improve the visibility of corticomedullary differentiation of the normal kidney.

Materials and Methods:

A series of noncontrast‐enhanced SSFP MRI with Time‐SLIP were performed in 20 patients by using various inversion times (TIs); 500–1800 msec in increments of 100 msec. In‐phase (IP) and opposed‐phase (OP) MR images were also obtained. The signal intensity (SI) of the renal cortex and medulla was measured to calculate corticomedullary contrast ratio (SI of cortex/medulla). Additionally, the visibility of corticomedullary differentiation was visually categorized using a four‐point scale.

Results:

In SSFP with Time‐SLIP, corticomedullary contrast ratio was highest with TI of 1200 msec in eight subjects (40%), followed by 1100 msec in seven (35%) and 1000 msec in three (15%). The corticomedullary contrast ratio in SSFP with optimal Time‐SLIP (4.93 ± 1.25) was significantly higher (P < 0.001) than those of IP (1.46 ± 0.12) and OP (1.43 ± 0.14). The visibility of corticomedullary differentiation was significantly better (P < 0.001) in SSFP images with Time‐SLIP (averaged grade = 4.0) than in IP images (averaged grade = 2.63) and OP images (averaged grade = 2.05).

Conclusion:

SSFP MRI with Time‐SLIP can improve the visibility of renal corticomedullary differentiation without using contrast agents. J. Magn. Reson. Imaging 2012;37:1178–1181. © 2012 Wiley Periodicals, Inc.     

Influence of MT effects on T2 quantification with 3D balanced steady‐state free precession imaging

Signal from balanced steady‐state free precession is affected by magnetization transfer. To investigate the possible effects on derived T₂ values using variable nutation steady‐state free precession, magnetization transfer‐effects were modulated by varying the radiofrequency pulse duration only or in combination with variable pulse repetition time. Simulations reveal a clear magnetization transfer dependency of T₂ when decreasing radiofrequency pulse duration, reaching maximal deviation of 34.6% underestimation with rectangular pulses of 300 μs duration. The observed T₂ deviation evaluated in the frontal white matter and caudate nucleus shows a larger underestimation than expected by numerical simulations. However, this observed difference between simulation and measurement is also observed in an aqueous probe and can therefore not be attributed to magnetization transfer: it is an unexpected sensitivity of derived T₂ to radiofrequency pulse modulation. As expected, the limit of sufficiently long radiofrequency pulse duration to suppress magnetization transfer‐related signal modulations allows for proper T₂ estimation with variable nutation steady‐state free precession. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Factors controlling T2 mapping from partially spoiled SSFP sequence: optimization for skeletal muscle characterization

A fast and robust methodology for in vivo T(2) mapping is presented. The approach is based on the partially spoiled steady state free precession technique recently proposed by Bieri et al. (Magn Reson Med 2011). The accuracy of this method was demonstrated in simulations and phantom experiments. Variations in skeletal muscle T(2) relaxation time have been correlated with cell damage and inflammatory response. Nonetheless, the lack of easily implementable, fast, accurate and reproducible methods has hampered the adoption of T(2) measurement as a noninvasive tool for skeletal muscle characterization. The applicability of the partially spoiled steady state free precession method for tissue characterization in muscle disease is illustrated in this work by several examples. Quantitative MRI, in particular T(2) mapping based on partially spoiled steady state free precession acquisitions, might provide objective markers of muscle damage and degenerative changes, and an alternative to serial muscle biopsies.     

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.     

Non‐contrast‐enhanced MR angiography for selective visualization of the hepatic vein and inferior vena cava with true steady‐state free‐precession sequence and time‐spatial labeling inversion pulses: Preliminary results

Purpose

To selectively visualize the hepatic vein and inferior vena cava (IVC) using three‐dimensional (3D) true steady‐state free‐precession (SSFP) MR angiography with time‐spatial labeling inversion pulse (T‐SLIP), and to optimize the acquisition protocol.

Materials and Methods

Respiratory‐gated 3D true SSFP scans were conducted in 23 subjects in combination with two different T‐SLIPs (one placed in the thorax to suppress the arterial signal and the other in the abdomen to suppress the portal venous signal). One of the most important factors was the inversion time (TI) of abdominal T‐SLIP, and the image quality was evaluated at four different TIs of 800, 1200, 1600, and 2000 msec in terms of relative signal‐to‐noise ratio (SNR), contrast‐to‐noise ratio (CNR), and mean visualization scores.

Results

No significant difference was observed in SNR and CNR between each TI. However, IVC visualization scores were better at TIs of 1600 and 2000 msec, and overall image quality was better at TIs of 1200 and 1600 msec. Therefore, the TI of 1600 msec was considered to provide the optimal balance between IVC visualization and signal suppression of the portal vein in our protocol.

Conclusion

True SSFP scan with T‐SLIPs enabled selective visualization of the hepatic vein and IVC without an exogenous contrast agent. J. Magn. Reson. Imaging 2009;29:474–479. © 2009 Wiley‐Liss, Inc.