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.     

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.     

Rapid MR imaging of articular cartilage with steady-state free precession and multipoint fat-water separation

OBJECTIVE: To obtain high-quality high-resolution images of articular cartilage with reduced imaging time, we combined a novel technique of generalized multipoint fat-water separation with three-dimensional (3D) steady-state free precession (SSFP) imaging. SUBJECTS AND METHODS: The cartilage of 10 knees in five healthy volunteers was imaged with 3D SSFP imaging and a multipoint fat-water separation method capable of separating fat and water with short TE increments. Fat-saturated 3D spoiled gradient-echo (SPGR) images were obtained for comparison. RESULTS: High-quality images of the knee with excellent fat-water separation were obtained with 3D SSFP imaging. Total imaging time required was 58% less than that required for 3D SPGR imaging with a comparable cartilage signal-to-noise ratio and spatial resolution. Unlike 3D SPGR images, 3D SSFP images exhibited bright synovial fluid, providing a potential arthrographic effect. CONCLUSION: High-quality high-resolution images of articular cartilage with improved fat-water separation, bright synovial fluid, and markedly reduced acquisition times can be obtained with 3D SSFP imaging combined with a fat-water separation technique.     

Improved spectral selectivity and reduced susceptibility in SSFP using a near zero TE undersampled three-dimensional PR sequence

PURPOSE: To improve the performance of fat/water separation and reduce the sensitivity to susceptibility variation in balanced SSFP sequences. MATERIALS AND METHODS: Decreasing the repetition time (TR) reduces susceptibility artifacts in SSFP imaging. A shorter TR may also improve the spectral selectivity obtained when linearly combining data acquired using different radiofrequency phase cycling schedules. The desired short TR is achieved by using an angularly undersampled three-dimensional radial acquisition sequence that achieves a near zero echo time (TE) and also a short TR. RESULTS: Images from human volunteers demonstrate broad coverage of the cervical spine and knee with isotropic resolution. Excellent fat/water separation is achieved in these studies. CONCLUSION: The short TR capability of the proposed sequence greatly improves the fat suppression in SSFP imaging. High-resolution volumetric T2-like contrast imaged with reduced susceptibility artifacts can be obtained from a single acquisition using this technique.     

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.     

Multicoil Dixon chemical species separation with an iterative least-squares estimation method.

This work describes a new approach to multipoint Dixon fat-water separation that is amenable to pulse sequences that require short echo time (TE) increments, such as steady-state free precession (SSFP) and fast spin-echo (FSE) imaging. Using an iterative linear least-squares method that decomposes water and fat images from source images acquired at short TE increments, images with a high signal-to-noise ratio (SNR) and uniform separation of water and fat are obtained. This algorithm extends to multicoil reconstruction with minimal additional complexity. Examples of single- and multicoil fat-water decompositions are shown from source images acquired at both 1.5T and 3.0T. Examples in the knee, ankle, pelvis, abdomen, and heart are shown, using FSE, SSFP, and spoiled gradient-echo (SPGR) pulse sequences. The algorithm was applied to systems with multiple chemical species, and an example of water-fat-silicone separation is shown. An analysis of the noise performance of this method is described, and methods to improve noise performance through multicoil acquisition and field map smoothing are discussed.     

Fat‐water separation with alternating repetition time balanced SSFP

Balanced SSFP achieves high SNR efficiency, but suffers from bright fat signal. In this work, a multiple‐acquisition fat‐water separation technique using alternating repetition time (ATR) balanced SSFP is proposed. The SSFP profile can be modified using alternating repetition times and appropriate phase cycling to yield two spectra where fat and water are in‐phase and out‐of‐phase, respectively. The signal homogeneity and the broad width of the created in‐phase and out‐of‐phase profiles lead to signal cancellation over a broad stop‐band. The stop‐band suppression is achieved for a wide range of flip angles and tissue parameters. This property, coupled with the inherent flexibility of ATR SSFP in repetition time selection, makes the method a good candidate for fat‐suppressed SSFP imaging. The proposed method can be tailored to achieve a smaller residual stop‐band signal or a decreased sensitivity to field inhomogeneity depending on application‐specific needs. Magn Reson Med 60:479–484, 2008. © 2008 Wiley‐Liss, Inc.     

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.     

Wideband SSFP: alternating repetition time balanced steady state free precession with increased band spacing.

Balanced steady-state free precession (SSFP) imaging is limited by off-resonance banding artifacts, which occur with periodicity 1/TR in the frequency spectrum. A novel balanced SSFP technique for widening the band spacing in the frequency response is described. This method, called wideband SSFP, utilizes two alternating repetition times with alternating RF phase, and maintains high SNR and T(2)/T(1) contrast. For a fixed band spacing, this method can enable improvements in spatial resolution compared to conventional SSFP. Alternatively, for a fixed readout duration this method can widen the band spacing, and potentially avoid the banding artifacts in conventional SSFP. The method is analyzed using simulations and phantom experiments, and is applied to the reduction of banding artifacts in cine cardiac imaging and high-resolution knee imaging at 3T.     

Deriving blood-oxygen-level-dependent contrast in MRI with T2*-weighted, T2-prepared and phase-cycled SSFP methods: theory and experiment.

The objectives of this work were: 1) to perform a comparative evaluation of the oxygen-sensitive contrast (OC) derived from the phase-cycled steady-state free precession (SSFP PC) method against T*2-weighted gradient recalled echo (GRE) and T2-prepared (T2-prep) methods with theoretical simulations and imaging studies using an ischemic leg cuff model at 1.5T and 3.0T; and 2) to investigate the dependence of SSFP PC-based OC on imaging parameters. Results showed that the SSFP PC method (repetition time (TR) = 6.3 ms; flip angle (alpha) = 90 degrees ) provides significantly higher OC compared to T2-prep (at both field strengths) and GRE (3.0T) (P < 0.05). The OC of low TR SSFP (TR = 3.5 ms at 1.5T; TR = 4.5 ms at 3.0T; alpha = 90 degrees ) was significantly lower compared to GRE (P < 0.05) at 1.5T and 3.0T and to T2-prep methods at 1.5T (P < 0.05). In summary, the findings from this study are the following: 1) SSFP-based OC is directly dependent on TR and alpha at 1.5T and 3.0T; and 2) OC derived with SSFP PC can be increased above GRE and T2-prep methods with an appropriate choice of imaging parameters.     

Free-breathing renal magnetic resonance angiography with steady-state free-precession and slab-selective spin inversion combined with radial k-space sampling and water-selective excitation.

The impact of radial k-space sampling and water-selective excitation on a novel navigator-gated cardiac-triggered slab-selective inversion prepared 3D steady-state free-precession (SSFP) renal MR angiography (MRA) sequence was investigated. Renal MRA was performed on a 1.5-T MR system using three inversion prepared SSFP approaches: Cartesian (TR/TE: 5.7/2.8 ms, FA: 85 degrees), radial (TR/TE: 5.5/2.7 ms, FA: 85 degrees) SSFP, and radial SSFP combined with water-selective excitation (TR/TE: 9.9/4.9 ms, FA: 85 degrees). Radial data acquisition lead to significantly reduced motion artifacts (P < 0.05). SNR and CNR were best using Cartesian SSFP (P < 0.05). Vessel sharpness and vessel length were comparable in all sequences. The addition of a water-selective excitation could not improve image quality. In conclusion, radial k-space sampling reduces motion artifacts significantly in slab-selective inversion prepared renal MRA, while SNR and CNR are decreased. The addition of water-selective excitation could not improve the lower CNR in radial scanning.     

Fat/water separation in single acquisition steady-state free precession using multiple echo radial trajectories.

Phase detection in fully refocused SSFP imaging has recently allowed fat/water separation without preparing the magnetization or using multiple acquisitions. Instead, it exploits the phase difference between fat and water at an echo time at the midpoint of the TR. To minimize the TR for improved robustness to B0 inhomogeneity, a 3D projection acquisition collecting two half echoes at the beginning and end of each excitation was previously implemented. Since echoes are not formed at the midpoint of the TR, this method still requires two passes of k-space for fat/water separation. A new method is presented to linearly combine the half echoes to separate fat and water in a single acquisition. Separation using phase detection provides superior contrast between fat and water voxels. Results from high resolution angiography and musculoskeletal studies with improved robustness to inhomogeneity and a 50% scan time reduction compared to the two pass method are presented.     

Pilot study of improved lesion characterization in breast MRI using a 3D radial balanced SSFP technique with isotropic resolution and efficient fat‐water separation

Purpose

To assess a 3D radial balanced steady‐state free precession (SSFP) technique that provides submillimeter isotropic resolution and inherently registered fat and water image volumes in comparison to conventional T2‐weighted RARE imaging for lesion characterization in breast magnetic resonance imaging (MRI).

Materials and Methods

3D projection SSFP (3DPR‐SSFP) combines a dual half‐echo radial k‐space trajectory with a linear combination fat/water separation technique (linear combination SSFP). A pilot study was performed in 20 patients to assess fat suppression and depiction of lesion morphology using 3DPR‐SSFP. For all patients fat suppression was measured for the 3DPR‐SSFP image volumes and depiction of lesion morphology was compared against corresponding T2‐weighted fast spin echo (FSE) datasets for 15 lesions in 11 patients.

Results

The isotropic 0.63 mm resolution of the 3DPR‐SSFP sequence demonstrated improved depiction of lesion morphology in comparison to FSE. The 3DPR‐SSFP fat and water datasets were available in a 5‐minute scan time while average fat suppression with 3DPR‐SSFP was 71% across all 20 patients.

Conclusion

3DPR‐SSFP has the potential to improve the lesion characterization information available in breast MRI, particularly in comparison to conventional FSE. A larger study is warranted to quantify the effect of 3DPR‐SSFP on specificity. J. Magn. Reson. Imaging 2009;30:135–144. © 2009 Wiley‐Liss, Inc.     

Fast phase-contrast velocity measurement in the steady state.

A new method of encoding flow velocity as image phase in a refocused steady-state free precession (SSFP) sequence, called steady-state phase contrast (SSPC), can be used to generate velocity images rapidly while retaining high signal. Magnitude images with refocused-SSFP contrast are simultaneously acquired. This technique is compared with the standard method of RF-spoiled phase contrast (PC), and is found to have more than double the phase-signal to phase-noise ratio (PNR) when compared with standard PC at reasonable repetition intervals (TRs). As TR decreases, this advantage increases exponentially, facilitating rapid scans with high PNR efficiency. Rapid switching between the two necessary steady states can be accomplished by the insertion of a single TR interval with no flow-encoding gradient. The technique is implemented in a 2DFT sequence and validated in a phantom study. Preliminary results indicate that further TR reduction may be necessary for high-quality cardiac images; however, images in more stationary structures, such as the descending aorta and carotid bifurcation, exhibit good signal-to-noise ratio (SNR) and PNR. Comparisons with standard-PC images verify the PNR advantage predicted by theory.     

Comparison of fat quantification methods: a phantom study at 3.0T

PURPOSE: To compare different imaging methods with single-voxel MR spectroscopy (MRS) for the quantification of fat content in phantoms at 3.0T. MATERIALS AND METHODS: Imaging and spectroscopy was performed on a GE Signa system. Eleven novel homogeneous fat-water phantoms were constructed with variation in fat content from 0% to 100%. These were imaged using three techniques and compared with single-voxel non-water-suppressed MRS. Pixel-by-pixel maps of fat fraction were produced and mean values compared to MRS-determined measurements. Preliminary in vivo examinations were subsequently performed in the breast and spine to compare the best imaging technique with MRS. RESULTS: All imaging methods significantly correlated with MRS (P < 0.001): IDEAL (r(2) = 0.985), IOP (r(2) = 0.888), WS (r(2) = 0.939), and FS (r(2) = 0.938). In addition, IDEAL provided artifact-free maps of fat fraction with superior uniformity. In vivo results using IDEAL produced values that were between 9% to 16% of the corresponding MRS values. CONCLUSION: This work demonstrates that imaging may be utilized as a high-resolution alternative to MRS for the quantification of fat content. In the future we intend to replace MRS with IDEAL in our clinical studies involving fat measurement.     

ECG-gated multiecho Dixon fat-water separation in cardiac MRI: advantages over conventional fat-saturated imaging

OBJECTIVE: The purpose of this pictorial essay is to explore the advantages of multiecho Dixon fat-water separation techniques in cardiac MRI. The clinical indications, potential artifacts, and imaging findings with this technique are reviewed. CONCLUSION: Multiecho Dixon fat-water separation can be used to help characterize cardiac masses, evaluate for myocardial lipomatous infiltration, and diagnose pericarditis. Advantages over conventional fat-saturation techniques include fewer artifacts from background inhomogeneity, improved contrast of microscopic fat, and capability for use in combination with cine and contrast-enhanced imaging.     

Two-point Dixon fat-water separation: improving reliability and accuracy in phase correction algorithms

PURPOSE: To propose an advanced phase-correction region-growing algorithm for two-point fat-water separation suitable for parotid assessment, and to evaluate the general performance of phase-correction algorithms. MATERIALS AND METHODS: Two region-growing algorithms were evaluated in test objects and in head images: the original phase-correction algorithm (OPC) and the advanced phase-correction algorithm with voxel size manipulation (VSM) which includes: 1) starting the region-growing process from images of lower resolution and subsequently stepping toward the original matrix size, and 2) limiting the use of low-pass filters to fat-water interfaces with partial volume effects RESULTS: Fundamental problems relate to biological tissue spectrum being poorly approximated by two discrete peaks for fat and water. The VSM algorithm was shown to be less noise-sensitive, faster, and to produce a better approximation for the field inhomogeneity map. In head images (6 volunteers, 10 slices each) 43 errors were found with the OPC algorithm and only 6 errors with the VSM algorithm. Only the OPC algorithm produced errors surrounding the parotids (10 errors). CONCLUSION: The VSM algorithm provides a more accurate and less noise-sensitive fat-water separation. This highly significant performance improvement allows the application of phase-correction algorithms to a wider range of clinical applications.     

Unenhanced MR angiography of the renal arteries with balanced steady-state free precession dixon method

OBJECTIVE: The purpose of this study was to evaluate the feasibility of a novel technique for fat-water separation to image the renal arteries without using a contrast agent. CONCLUSION: Five healthy volunteers were imaged on a 3-T clinical MR scanner using the balanced steady-state free precession (SSFP) Dixon method. We were able to image the proximal renal arteries with high conspicuity within a 3-minute overall scanning time. The balanced-SSFP Dixon method shows potential for unenhanced MR angiography of the proximal renal arteries.     

Phase contrast using multiecho steady-state free precession.

Conventional phase-contrast (PC) MRI is limited in the temporal resolution (typically 50 ms) that can be achieved, due to the need to implement bipolar velocity encoding gradients. PC using steady-state free precession (SSFP) has recently been developed to acquire PC data at higher rates without sacrificing contrast-to-noise ratio (CNR). This work presents two multiecho SSFP PC implementations that can be used to increase the time efficiency of PCSSFP. Both approaches (extrinsic and intrinsic) enable reference image lines to be acquired within the same TR as the flow-encoded lines, thus minimizing the scan time and permitting TR-equivalent temporal resolutions. Both approaches have been implemented and tested successfully on human volunteers at 1.5T and 3T. While the intrinsic approach is useful for encoding higher velocity flows in-plane, the extrinsic implementation can be used for studying a wider range of encoding velocities for flow in the imaging plane and through the imaging plane.