Separation of water and fat MR images in a single scan at .35 T using „sandwich”︁ echoes

A method was developed for separation of water and fat MR images in a single scan with correction of static field inhomogeneity. The imaging sequence uses a single radiofrequency (RF) echo that is ?sandwiched”? between two gradient echoes. The gradient echoes are used to determine the B_o distribution and to produce out-of-phase images after phase correction using the field map. An algorithm was developed to unwrap the phase images for quantitating the B_o inhomogeneity. To account for differences in geometric distortion between the RF echo image and the gradient echo images due to the reversal of the read gradients, methods were developed to correct the images before the calculation of the final water and fat images. The proposed technique was implemented at .35 T. Both phantom and human images were acquired using the method. It is shown that water- and fat-separated images can be obtained in a single scan using the ?sandwich”? echoes in the presence of a relatively large B_o inhomogeneity.     

Dual half‐echo phase correction for implementation of 3D radial SSFP at 3.0 T

Fat/water separation methods such as fluctuating equilibrium magnetic resonance and linear combination steady‐state free precession have not yet been successfully implemented at 3.0 T due to extreme limitations on the time available for spatial encoding with the increase in magnetic field strength. We present a method to utilize a three‐dimensional radial sequence combined with linear combination steady‐state free precession at 3.0 T to take advantage of the increased signal levels over 1.5 T and demonstrate high spatial resolution compared to Cartesian techniques. We exploit information from the two half‐echoes within each pulse repetition time to correct the accumulated phase on a point‐by‐point basis, thereby fully aligning the phase of both half‐echoes. The correction provides reduced sensitivity to static field (B₀) inhomogeneity and robust fat/water separation. Resultant images in the knee joint demonstrate the necessity of such a correction, as well as the increased isotropic spatial resolution attainable at 3.0 T. Results of a clinical study comparing this sequence to conventional joint imaging sequences are included. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

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.     

Homogeneous preparation encoding (HoPE) in multislice imaging.

Fast magnetization preparation techniques acquire a series of echoes after a single magnetization preparation. If these echoes are acquired from different slices using a multislice technique the change in the preparation state of the echoes due to relaxation effects leads to different contrast modification for each slice. Encoding different preparation states along the phase-encoding direction of each slice instead of acquiring each slice in a different preparation state is introduced as a general concept to obtain images of identical contrast and point-spread function. This can be realized either by cycling the slice excitation order several times over the total number of repetitions or by moving the point of time at which the preparation is applied within each repetition. One possible application of this method is chemical shift selective fat saturation imaging. A homogeneous fat suppression across a multislice volume could be achieved using a FLASH sequence at a repetition time of TR = 145 ms, including a single fat saturation preparation. Conventional fat saturated spin-echo imaging at any TR can be accelerated significantly by reducing the number of applied preparations per repetition. A further application of the homogeneous preparation encoding (HoPE) method is described that encodes the spatial self-saturation of the multislice excitation order homogeneously in all slices. Only a reduced number of slices of the total volume are excited in each repetition and the slice excitation order is continuously moved along the imaging volume. This method is applied for time of flight (TOF) imaging. Using a TONE-like series of flip angles for the slice excitations of each repetition homogeneous TOF images can be obtained on the basis of a multislice acquisition.     

T2‐weighted 3D fast spin echo imaging with water–fat separation in a single acquisition

Purpose:

To develop a robust 3D fast spin echo (FSE) T₂‐weighted imaging method with uniform water and fat separation in a single acquisition, amenable to high‐quality multiplanar reformations.

Materials and Methods:

The Iterative Decomposition of water and fat with Echo Asymmetry and Least squares estimation (IDEAL) method was integrated with modulated refocusing flip angle 3D‐FSE. Echoes required for IDEAL processing were acquired by shifting the readout gradient with respect to the Carr‐Purcell‐Meiboom‐Gill echo. To reduce the scan time, an alternative data acquisition using two gradient echoes per repetition was implemented. Using the latter approach, a total of four gradient echoes were acquired in two repetitions and used in the modified IDEAL reconstruction.

Results:

3D‐FSE T₂‐weighted images with uniform water–fat separation were successfully acquired in various anatomies including breast, abdomen, knee, and ankle in clinically feasible scan times, ranging from 5:30–8:30 minutes. Using water‐only and fat‐only images, in‐phase and out‐of‐phase images were reconstructed.

Conclusion:

3D‐FSE‐IDEAL provides volumetric T₂‐weighted images with uniform water and fat separation in a single acquisition. High‐resolution images with multiple contrasts can be reformatted to any orientation from a single acquisition. This could potentially replace 2D‐FSE acquisitions with and without fat suppression and in multiple planes, thus improving overall imaging efficiency. J. Magn. Reson. Imaging 2010;32:745–751. © 2010 Wiley‐Liss, Inc.     

Two‐point dixon method with flexible echo times

The two‐point Dixon method is a proton chemical shift imaging technique that produces separated water‐only and fat‐only images from a dual‐echo acquisition. It is shown how this can be achieved without the usual constraints on the echo times. A signal model considering spectral broadening of the fat peak is proposed for improved water/fat separation. Phase errors, mostly due to static field inhomogeneity, must be removed prior to least‐squares estimation of water and fat. To resolve ambiguity of the phase errors, a corresponding global optimization problem is formulated and solved using a message‐passing algorithm. It is shown that the noise in the water and fat estimates matches the Cramér‐Rao bounds, and feasibility is demonstrated for in vivo abdominal breath‐hold imaging. The water‐only images were found to offer superior fat suppression compared with conventional spectrally fat suppressed images. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Single-acquisition chemical-shift imaging of a binary system with use of stimulated echoes

A method for separating binary chemical-shift components with a single image data acquisition by means of stimulated echoes is demonstrated. With a strategy analogous to the modified Dixon method, three stimulated echoes were acquired to form three complex images. In each of the images, the complex pixel intensities were imparted, by design of the pulse sequence, with a phase factor carrying chemical-shift or field inhomogeneity information. With these three images, true fat/water separation can be obtained in biologic tissues. Studies at high field strength (4.7 T) on a toluene phantom, a pseudo-binary chemical-shift system, were used to evaluate the applicability of the method. Its clinical feasibility was demonstrated on a healthy human subject in a 0.6-T whole-body imaging system.     

Proton MR chemical shift imaging using double and triple phase contrast acquisition methods

Conventional chemical shift magnetic resonance (MR) imaging with the phase contrast technique has a number of limitations with respect to quantitative accuracy. The hypothesis of this study is that the accuracy of phase contrast chemical shift MR may be improved by increasing the number of basis images from two to three. Water and fat images were obtained from phantoms and volunteers with a 1.5 T MR system using double and triple acquisition phase contrast chemical shift methods. Longitudinal relaxation time and relative water and fat content were calculated from these basis images. The T1 relaxation times of the aqueous component of composite phantoms were determined more accurately using the triple acquisition method than with the double acquisition method. In vivo studies demonstrate that the triple acquisition method separated fat and water signals more accurately and showed less field inhomogeneity dependence than the modified double acquisition method. The new method also provided a map of static field magnetic inhomogeneity and tissue magnetic susceptibility. The triple acquisition phase contrast chemical shift imaging technique should improve the prospect for quantitative tissue characterization in clinical MR.     

A three-point dixon method for water and fat separation using 2D and 3D gradient-echo techniques

A water and fat separation technique based on a three-point Dixon method using two-dimensional (2D) and three-dimensional (3D) gradient-echo sequences and a new phase-unwrapping approach is presented. Using velocity-compensated sequences, three images, with water and fat protons in phase, opposed phase, and in phase, were acquired in an interleaved fashion for each phase-encoding line of the image. A fast 2D scan permitted acquisition of images within a single breathhold, eliminating respiratory motion artifacts. The 3D sequence allowed coverage of a large region of interest with thin, contiguous slices. To correct field inhomogeneity effects, phase unwrapping was required. This was accomplished by comparing the corresponding pixels in the two water and fat in-phase images on a pixel-by-pixel basis. This phase-unwrapping approach is faster, more reliable, and requires less user interfacing than other methods. The water and fat separation method performed well in various parts of the body.     

Iterative decomposition of water and fat with echo asymmetry and least‐squares estimation (IDEAL): Application with fast spin‐echo imaging

Chemical shift based methods are often used to achieve uniform water–fat separation that is insensitive to B_o inhomogeneities. Many spin‐echo (SE) or fast SE (FSE) approaches acquire three echoes shifted symmetrically about the SE, creating time‐dependent phase shifts caused by water–fat chemical shift. This work demonstrates that symmetrically acquired echoes cause artifacts that degrade image quality. According to theory, the noise performance of any water–fat separation method is dependent on the proportion of water and fat within a voxel, and the position of echoes relative to the SE. To address this problem, we propose a method termed “iterative decomposition of water and fat with echo asymmetric and least‐squares estimation” (IDEAL). This technique combines asymmetrically acquired echoes with an iterative least‐squares decomposition algorithm to maximize noise performance. Theoretical calculations predict that the optimal echo combination occurs when the relative phase of the echoes is separated by 2π/3, with the middle echo centered at π/2+πk (k = any integer), i.e., (–π/6+πk, π/2+πk, 7π/6+πk). Only with these echo combinations can noise performance reach the maximum possible and be independent of the proportion of water and fat. Close agreement between theoretical and experimental results obtained from an oil–water phantom was observed, demonstrating that the iterative least‐squares decomposition method is an efficient estimator. Magn Reson Med, 2005. © 2005 Wiley‐Liss, Inc.     

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.     

On the application of chemical shift-based multipoint water-fat separation methods in balanced SSFP imaging.

Chemical shift-based multipoint water-fat separation methods have been applied in balanced steady-state free precession (bSSFP) sequences because of the high signal-to-noise-ratio (SNR) attainable. In this approach the echo formation is approximated to occur concurrently for both water and fat at an echo time (TE) equal to half the repetition time (TR/2 approximation). However, the degree to which the imaging conditions underlying the TR/2 approximation are satisfied can significantly vary in vivo depending upon the imaging region of interest (ROI) and the pixels across a field of view (FOV). The consequence of the TR/2 approximation on chemical shift-based multipoint water-fat separation was investigated. The influence of a mismatch between the pass-band profiles of water and fat (pass-band mismatch) on fat quantification was also examined. Theoretical and experimental results demonstrate that the TR/2 approximation can result in spatially dependent noise performance of multipoint water-fat separation methods, and the pass-band mismatch can render the precision of fat quantification spatially dependent. Given that local tissue characteristics in affected liver can be substantially variable, this study is of particular importance in liver imaging.     

Fast spin-echo triple-echo dixon (fTED) technique for efficient T2-weighted water and fat imaging.

Previously published fast spin-echo (FSE) implementations of a Dixon method for water and fat separation all require multiple scans and thus a relatively long scan time. Further, the minimum echo spacing (esp), a time critical for FSE image quality and scan efficiency, often needs to be increased in order to bring about the required phase shift between the water and fat signals. This work proposes and implements a novel FSE triple-echo Dixon (fTED) technique that can address these limitations. In the new technique, three raw images are acquired in a single FSE scan by replacing each frequency-encoding gradient in a conventional FSE with three consecutive gradients of alternating polarity. The timing of the three gradients is adjusted by selecting an appropriate receiver bandwidth (RBW) so that the water and fat signals for the three corresponding echoes have a relative phase shift of -180 degrees , 0 degrees , and 180 degrees , respectively. A fully automated postprocessing algorithm is then used to generate separate water-only and fat-only images for each slice. The technique was implemented with and without parallel imaging. We demonstrate that the new fTED technique enables both uniform water/fat separation and fast scanning with uncompromised scan parameters, including applications such as T(2)-weighted separate water and fat imaging of the abdomen during breath-holding.     

High‐resolution 3D radial bSSFP with IDEAL

Radial trajectories facilitate high‐resolution balanced steady state free precession (bSSFP) because the efficient gradients provide more time to extend the trajectory in k‐space. A number of radial bSSFP methods that support fat–water separation have been developed; however, most of these methods require an environment with limited B₀ inhomogeneity. In this work, high‐resolution bSSFP with fat–water separation is achieved in more challenging B₀ environments by combining a 3D radial trajectory with the IDEAL chemical species separation method. A method to maintain very high resolution within the timing constraints of bSSFP and IDEAL is described using a dual‐pass pulse sequence. The sampling of a unique set of radial lines at each echo time is investigated as a means to circumvent the longer scan time that IDEAL incurs as a multiecho acquisition. The manifestation of undersampling artifacts in this trajectory and their effect on chemical species separation are investigated in comparison to the case in which each echo samples the same set of radial lines. This new bSSFP method achieves 0.63 mm isotropic resolution in a 5‐min scan and is demonstrated in difficult in vivo imaging environments, including the breast and a knee with ACL reconstruction hardware at 1.5 T. Magn Reson Med 71:95–104, 2014. © 2013 Wiley Periodicals, Inc.     

Method for efficient fast spin echo Dixon imaging.

In order to satisfy the Carr-Purcell-Meiboom-Gill (CPMG) condition, echo shift as dictated in fast-spin-echo (FSE)-based Dixon imaging was previously achieved by applying a time shift to the readout gradient and the data acquisition window. Accordingly, interecho spacing is increased, which entails increased image blurring and, in multislice imaging, a significant reduction in the slice coverage for a given imaging time. In this work, a new method is developed by which the echo shift is induced by "sandwiching" in time the readout gradient with a pair of small gradients of equal area and of opposite polarity. While data with non-zero phase shifts between water and fat signals are collected as fractional echoes, no increase in echo spacing is necessary with the modified acquisition strategy, and increased time efficiency is therefore achieved. In order to generate separate water-only and fat-only images in data processing, a set of low-resolution images are first reconstructed from the central symmetric portion (either 128 x 128 or 64 x 64) of the acquired multipoint Dixon data. High-resolution images using all the acquired data, including some partial Fourier-reconstructed images, are then phase demodulated using the phase errors determined from the low-resolution images. The feasibility of the technique is demonstrated using a water and fat phantom as well as in clinical patient imaging.     

Radial alternating TE sequence for faster fat suppression.

This study describes a steady-state sequence that uses a radial k-space trajectory and alternating echo times (TEs) between even and odd k-space views. The sequence generated a single data set that was used to reconstruct images with inherent fat suppression. This fat suppression results from the fat phase variation in alternate echoes giving rise to cancellation in the central portion of k-space. This new fat-suppression method provides inherent fat suppression in half the acquisition time relative to the radial two-point Dixon method. The improvement in k-space sampling efficiency is demonstrated in phantom and clinical images, and through measured point-spread functions (PSFs). As a result, the radial alternating TE sequence offers improved temporal resolution over a radial version of the two-point Dixon sequence by requiring fewer total projections to obtain the same effective resolution in water-based tissues.     

Optimization of a dual echo in the steady state (DESS) free-precession sequence for imaging cartilage

Three-dimensional (3D) MR imaging of the knee is useful to detect cartilage abnormalities, although the tissue contrast in 3D gradient-recalled echo (GRE) sequences such as gradient-recalled acquisition in the steady state (GRASS) or fast low-angle shot (FLASH) is poor. T2 contrast can be added to a GRASS sequence by combining the signals from the first and second gradient echoes, which form immediately after and immediately before each radiofrequency (RF) pulse in a 3D GRE sequence. We have optimized a 3D dual echo in the steady state (DESS) sequence, which produces one averaged image from the two echoes, for use in the detection of articular cartilage abnormalities. In the optimization process, we examined the imaging parameters of flip angle (α), repetition time (TR), echo time (TE), and bandwidth to maximize the contrast between cartilage and joint fluid. A theoretical simulation of the sequence was confirmed with experiments conducted on phantoms with known T1 and T2. On the basis of theoretical predictions and experiments using healthy volunteers, we determined that an optimized sequence with a bandwidth of 98 Hz per pixel, a TR of 30 msec, a TE of 7.1 msec, and an α of 60° produced the highest contrast between cartilage and fluid within a defined acquisition time of 6 minutes. Additional contrast was obtained by filtering the second-echo image to eliminate noise before adding it to the first-echo image.     

Three-dimensional time-of-flight MR angiography with variable TE (VARIETE) for fat signal reduction

Uniform fat saturation over a large region of interest remains a problem in time-of-flight (TOF) magnetic resonance angiography applications. We demonstrate that a variable echo time with an opposed phase value at low spatial slice select frequencies can effectively reduce most of the fat signal in an otherwise standard three-dimensional TOF acquisition. We evaluated this method at 1.5 T using a short TE = 5.3 ms and a long TE = 6.75 ms for different values of the slice encoding gradient (i.e., different k₂ values). Shorter echo time (TE = 5.3 msec) was used at higher spatial slice select frequencies, but all echoes have the same gradient structures. By keeping the number of slice encoding steps with longer echoes to a minimum, field inhomogeneity effects on flow compensation remained small. A magnetization transfer saturation pulse was used to suppress signal of brain parenchyma. Overall, highly uniform and selective fat signal reduction was obtained while maintaining superior flow compensation in all volunteer studies.     

Gradient and stimulated echo (GRASTE) imaging.

As a modification of single-shot stimulated echo acquisition mode (STEAM) MRI, a gradient and stimulated echo (GRASTE) sequence is presented that acquires multiple gradient echoes in addition to each stimulated echo. While "contiguous" GRASTE exploits all stimulated echoes for the central part of k-space and the gradient echoes for outer lines, "interleaved" GRASTE assigns all echoes of a particular readout interval to directly neighboring lines. Phase distortions may be corrected by the reference signals of a single readout interval without phase encoding. Experimental results obtained for the human brain demonstrate that contiguous GRASTE yields up to 30% better SNR per acquisition time than conventional single-shot STEAM due to a better efficiency and maintains most of its robustness. Interleaved GRASTE can improve the SNR by a factor of 2 because of the possibility of using larger flip angles in the readout interval. However, its more pronounced sensitivity to off-resonance effects requires short echo trains.     

Motion-insensitive, steady-state free precession imaging

Steady-state free precession (SSFP) pulse sequences employing gradient reversal echoes and short repetition time (TR) between successive rf excitation pulses offer high signal-to-noise ratio per unit time. However, SSFP sequences are very sensitive to motion. A new SSFP method is presented which avoids the image artifacts and loss of signal intensity due to motion. The pulse sequence is designed so that the time integral of each of the three gradients is zero over each TR time interval. The signal then consists of numerous echoes which are superimposed. These echoes are isolated by combining the data from N different scans. In each scan a specific phase shift is added during every TR interval. Each of these N isolated echoes produces a motion-insensitive, artifact-free image. Because all the echoes are sampled simultaneously, the signal-to-noise ratio per unit time in this SSFP method is higher than in existing SSFP techniques which sample only one echo at a time. The new method was implemented and used to produce both two- and three-dimensional images of the head and cervical spin of a human patient. In these images the high signal intensity of cerebrospinal fluid is preserved regardless of its motion. Further work is required to evaluate the imaging parameters (TR, TE, rf tip angle) so as to give optimal tissue contrast for the various echoes.