Dual‐echo Dixon imaging with flexible choice of echo times

In this work, a new two‐point method for water–fat imaging is described and explored. It generalizes existing two‐point methods by eliminating some of the restrictions that these methods impose on the choice of echo times. Thus, the new two‐point method promises to provide more freedom in the selection of protocol parameters and to reach higher scan efficiency. Its performance was studied theoretically and was evaluated experimentally in abdominal imaging with a multigradient‐echo sequence. While depending on the choice of echo times, it is generally found to be favorable compared to existing two‐point methods. Notably, water images with higher spatial resolution and better signal‐to‐noise ratio were attained with it in single breathholds at 3.0 T and 1.5 T, respectively. The use of more accurate spectral models of fat is shown to substantially reduce observed variations in the extent of fat suppression. The acquisition of in‐ and opposed‐phase images is demonstrated to be replaceable by a synthesis from water and fat images. The new two‐point method is finally also applied to autocalibrate a multidimensional eddy current correction and to enhance the fat suppression achieved with three‐point methods in this way, especially toward the edges of larger field of views. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

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.     

Water-fat separation with IDEAL gradient-echo imaging

Purpose

To combine gradient‐echo (GRE) imaging with a multipoint water–fat separation method known as “iterative decomposition of water and fat with echo asymmetry and least squares estimation” (IDEAL) for uniform water–fat separation. Robust fat suppression is necessary for many GRE imaging applications; unfortunately, uniform fat suppression is challenging in the presence of B₀ inhomogeneities. These challenges are addressed with the IDEAL technique.

Materials and Methods

Echo shifts for three‐point IDEAL were chosen to optimize noise performance of the water–fat estimation, which is dependent on the relative proportion of water and fat within a voxel. Phantom experiments were performed to validate theoretical SNR predictions. Theoretical echo combinations that maximize noise performance are discussed, and examples of clinical applications at 1.5T and 3.0T are shown.

Results

The measured SNR performance validated theoretical predictions and demonstrated improved image quality compared to unoptimized echo combinations. Clinical examples of the liver, breast, heart, knee, and ankle are shown, including the combination of IDEAL with parallel imaging. Excellent water–fat separation was achieved in all cases. The utility of recombining water and fat images into “in‐phase,” “out‐of‐phase,” and “fat signal fraction” images is also discussed.

Conclusion

IDEAL‐SPGR provides robust water–fat separation with optimized SNR performance at both 1.5T and 3.0T with multicoil acquisitions and parallel imaging in multiple regions of the body. J. Magn. Reson. Imaging 2007;25:644–652. © 2007 Wiley‐Liss, Inc.     

Noise analysis for 3‐point chemical shift‐based water‐fat separation with spectral modeling of fat

Purpose:

To model the theoretical signal‐to‐noise ratio (SNR) behavior of 3‐point chemical shift‐based water‐fat separation, using spectral modeling of fat, with experimental validation for spin‐echo and gradient‐echo imaging. The echo combination that achieves the best SNR performance for a given spectral model of fat was also investigated.

Materials and Methods:

Cramér‐Rao bound analysis was used to calculate the best possible SNR performance for a given echo combination. Experimental validation in a fat‐water phantom was performed and compared with theory. In vivo scans were performed to compare fat separation with and with out spectral modeling of fat.

Results:

Theoretical SNR calculations for methods that include spectral modeling of fat agree closely with experimental SNR measurements. Spectral modeling of fat more accurately separates fat and water signals, with only a slight decrease in the SNR performance of the water‐only image, although with a relatively large decrease in the fat SNR performance.

Conclusion:

The optimal echo combination that provides the best SNR performance for water using spectral modeling of fat is very similar to previous optimizations that modeled fat as a single peak. Therefore, the optimal echo spacing commonly used for single fat peak models is adequate for most applications that use spectral modeling of fat. J. Magn. Reson. Imaging 2010;32:493–500. © 2010 Wiley‐Liss, Inc.     

Compressed sensing for chemical shift‐based water–fat separation

Multi echo chemical shift‐based water–fat separation methods allow for uniform fat suppression in the presence of main field inhomogeneities. However, these methods require additional scan time for chemical shift encoding. This work presents a method for water–fat separation from undersampled data (CS‐WF), which combines compressed sensing and chemical shift‐based water–fat separation. Undersampling was applied in the k‐space and in the chemical shift encoding dimension to reduce the total scanning time. The method can reconstruct high quality water and fat images in 2D and 3D applications from undersampled data. As an extension, multipeak fat spectral models were incorporated into the CS‐WF reconstruction to improve the water–fat separation quality. In 3D MRI, reduction factors of above three can be achieved, thus fully compensating the additional time needed in three‐echo water–fat imaging. The method is demonstrated on knee and abdominal in vivo data. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Three‐point dixon method enables whole‐body water and fat imaging of obese subjects

Dixon imaging techniques derive chemical shift‐separated water and fat images, enabling the quantification of fat content and forming an alternative to fat suppression. Whole‐body Dixon imaging is of interest in studies of obesity and the metabolic syndrome, and possibly in oncology. A three‐point Dixon method is proposed where two solutions are found analytically in each voxel. The true solution is identified by a multiseed three‐dimensional region‐growing scheme with a dynamic path, allowing confident regions to be solved before unconfident regions, such as background noise. 2π‐Phase unwrapping is not required. Whole‐body datasets (256 × 184 × 252 voxels) were collected from 39 subjects (body mass index 19.8‐45.4 kg/m²), in a mean scan time of 5 min 15 sec. Water and fat images were reconstructed offline, using the proposed method and two reference methods. The resulting images were subjectively graded on a four‐grade scale by two radiologists, blinded to the method used. The proposed method was found superior to the reference methods. It exclusively received the two highest grades, implying that only mild reconstruction failures were found. The computation time for a whole‐body dataset was 1 min 51.5 sec ± 3.0 sec. It was concluded that whole‐body water and fat imaging is feasible even for obese subjects, using the proposed method. Magn Reson Med 63:1659–1668, 2010. © 2010 Wiley‐Liss, Inc.     

Removal of olefinic fat chemical shift artifact in diffusion MRI

Diffusion‐weighted (DW) MRI has emerged as a key tool for assessing the microstructure of tissues in healthy and diseased states. Because of its rapid acquisition speed and insensitivity to motion, single‐shot echo‐planar imaging is the most common DW imaging technique. However, the presence of fat signal can severely affect DW‐echo planar imaging acquisitions because of the chemical shift artifact. Fat suppression is usually achieved through some form of chemical shift‐based fat saturation. Such methods effectively suppress the signal originating from aliphatic fat protons, but fail to suppress the signal from olefinic protons. Olefinic fat signal may result in significant distortions in the DW images, which bias the subsequently estimated diffusion parameters. This article introduces a method for removing olefinic fat signal from DW images, based on an echo time‐shifted acquisition. The method is developed and analyzed specifically in the context of single‐shot DW‐echo‐planar imaging, where image phase is generally unreliable. The proposed method is tested with phantom and in vivo datasets, and compared with a standard acquisition to demonstrate its performance. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Phase‐contrast velocimetry with simultaneous fat/water separation

Phase‐contrast MRI can provide high‐resolution angiographic velocity images, especially in conjunction with non‐Cartesian k‐space sampling. However, acquisitions can be sensitive to errors from artifacts from main magnetic field inhomogeneities and chemical shift from fat. Particularly in body imaging, fat content can cause degraded image quality, create errors in the velocity measurements, and prevent the use of self‐calibrated amplitude of static field heterogeneity corrections. To reduce the influence of fat and facilitate self‐calibrated amplitude of static field heterogeneity corrections, a combination of chemical shift imaging with phase‐contrast velocimetry with nonlinear least‐squares estimation of velocity, fat, and water signals is proposed. A chemical shift and first‐moment symmetric dual‐echo sequence is proposed to minimize the scan time penalty, and initial investigations are performed in phantoms and volunteers that show reduced influence from fat in velocity images. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Spiral water-fat imaging with integrated off-resonance correction on a clinical scanner

Purpose

To integrate water‐fat–resolved spiral gradient‐echo imaging with off‐resonance correction into a clinical MR scanner and to evaluate its basic feasibility and performance.

Materials and Methods

Three‐point chemical shift imaging was implemented with forward and strongly T₂*‐weighted reverse spiral sampling and with off‐resonance correction after water–fat separation. It was applied in a volunteer study on single breathhold abdominal imaging, which included a brief comparison with Cartesian sampling.

Results

Water‐fat–resolved, off‐resonance–corrected forward and reverse three‐dimensional interleaved spiral imaging was found to be feasible on a clinical MR scanner with only minor changes to the existing data acquisition and reconstruction, and to provide good image quality. Three‐point chemical shift encoded data thus support both, water–fat separation and off‐resonance correction with high accuracy.

Conclusion

The combination of chemical shift encoding and appropriate postprocessing could pave the way for water‐fat–resolved spiral imaging in clinical applications. J. Magn. Reson. Imaging 2010;32:1262–1267. © 2010 Wiley‐Liss, Inc.     

A simple low‐SAR technique for chemical‐shift selection with high‐field spin‐echo imaging

We have discovered a simple and highly robust method for removal of chemical shift artifact in spin‐echo MR images, which simultaneously decreases the radiofrequency power deposition (specific absorption rate). The method is demonstrated in spin‐echo echo‐planar imaging brain images acquired at 7 T, with complete suppression of scalp fat signal. When excitation and refocusing pulses are sufficiently different in duration, and thus also different in the amplitude of their slice‐select gradients, a spatial mismatch is produced between the fat slices excited and refocused, with no overlap. Because no additional radiofrequency pulse is used to suppress fat, the specific absorption rate is significantly reduced compared with conventional approaches. This enables greater volume coverage per unit time, well suited for functional and diffusion studies using spin‐echo echo‐planar imaging. Moreover, the method can be generally applied to any sequence involving slice‐selective excitation and at least one slice‐selective refocusing pulse at high magnetic field strengths. The method is more efficient than gradient reversal methods and more robust against inhomogeneities of the static (polarizing) field (B₀). Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Improved fat suppression using multipeak reconstruction for IDEAL chemical shift fat‐water separation: Application with fast spin echo imaging

Purpose

To evaluate and quantify improvements in the quality of fat suppression for fast spin‐echo imaging of the knee using multipeak fat spectral modeling and IDEAL fat‐water separation.

Materials and Methods

T₁‐weighted and T₂‐weighted fast spin‐echo sequences with IDEAL fat‐water separation and two frequency‐selective fat‐saturation methods (fat‐selective saturation and fat‐selective partial inversion) were performed on 10 knees of five asymptomatic volunteers. The IDEAL images were reconstructed using a conventional single‐peak method and precalibrated and self‐calibrated multipeak methods that more accurately model the NMR spectrum of fat. The signal‐to‐noise ratio (SNR) was measured in various tissues for all sequences. Student t‐tests were used to compare SNR values.

Results

Precalibrated and self‐calibrated multipeak IDEAL had significantly greater suppression of signal (P < 0.05) within subcutaneous fat and bone marrow than fat‐selective saturation, fat‐selective partial inversion, and single‐peak IDEAL for both T₁‐weighted and T₂‐weighted fast spin‐echo sequences. For T₁‐weighted fast spin‐echo sequences, the improvement in the suppression of signal within subcutaneous fat and bone marrow for multipeak IDEAL ranged between 65% when compared to fat‐selective partial inversion to 86% when compared to fat‐selectivesaturation. For T2‐weighted fast spin‐echo sequences, the improvement for multipeak IDEAL ranged between 21% when compared to fat‐selective partial inversion to 81% when compared to fat‐selective saturation.

Conclusion

Multipeak IDEAL fat‐water separation provides improved fat suppression for T₁‐weighted and T₂‐weighted fast spin‐echo imaging of the knee when compared to single‐peak IDEAL and two widely used frequency‐selected fat‐saturation methods. J. Magn. Reson. Imaging 2009;29:436–442. © 2009 Wiley‐Liss, Inc.     

Fat‐water separated delayed hyperenhanced myocardial infarct imaging

Fat deposition associated with myocardial infarction (MI) has been reported as a commonly occurring phenomenon. Magnetic resonance imaging (MRI) has the ability to efficiently detect MI using T₁‐sensitive contrast‐enhanced sequences and fat via its resonant frequency shift. In this work, the feasibility of fat‐water separation applied to the conventional delayed hyperenhanced (DHE) MI imaging technique is demonstrated. A three‐point Dixon acquisition and reconstruction was combined with an inversion recovery gradient‐echo pulse sequence. This allowed fat‐water separation along with T₁ sensitive imaging after injection of a gadolinium contrast agent. The technique is demonstrated in phantom experiments and three subjects with chronic MI. Areas of infarction were well defined as conventional hyperenhancement in water images. In two cases, fatty deposition was detected in fat images and confirmed by precontrast opposed‐phase imaging. Magn Reson Med 60:503–509, 2008. © 2008 Wiley‐Liss, Inc.     

Multiplex RARE: A simultaneous multislice spin‐echo sequence that fulfils CPMG conditions

This work presents a new imaging sequence in which multiple slices are simultaneously excited and refocused in a spin‐echo train. The multiple spin‐echo trains are interleaved in such a manner that (i) the Carr‐Purcell‐Meiboom‐Gill conditions are fulfilled at all times, and (ii) the signals from slices can be separated, preventing aliasing. This paper also demonstrates how the sequence may be used in a novel fat‐water Dixon method that enables fast volume coverage. The technique is demonstrated in phantoms and in vivo. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

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.     

In vivo high‐resolution magnetic resonance skin imaging at 1.5 T and 3 T

As a noninvasive modality, MR is attractive for in vivo skin imaging. Its unique soft tissue contrast makes it an ideal imaging modality to study the skin water content and to resolve the different skin layers. In this work, the challenges of in vivo high‐resolution skin imaging are addressed. Three 3D Cartesian sequences are customized to achieve high‐resolution imaging and their respective performance is evaluated. The balanced steady‐state free precession (bSSFP) and gradient echo (GRE) sequences are fast but can be sensitive to off‐resonance artifacts. The fast large‐angle spin echo (FLASE) sequence provides a sharp depiction of the hypodermis structures but results in more specific absorption rate (SAR). The effect of increasing the field strength is assessed. As compared to 1.5 T, signal‐to‐noise ratio at 3 T slightly increases in the hypodermis and almost doubles in the dermis. The need for fat/water separation is acknowledged and a solution using an interleaved three‐point Dixon method and an iterative reconstruction is shown to be effective. The effects of motion are analyzed and two techniques to prevent motion and correct for it are evaluated. Images with 117 × 117 × 500 μm³ resolution are obtained in imaging times under 6 min. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Fast spin-echo triple echo dixon: Initial clinical experience with a novel pulse sequence for simultaneous fat-suppressed and nonfat-suppressed T2-weighted spine magnetic resonance imaging

Purpose

To evaluate a prototype fast spin‐echo (FSE) triple‐echo Dixon (FTED) technique for T2‐weighted spine imaging with and without fat suppression compared to conventional T2‐weighted fast recovery (FR) FSE and short‐tau inversion recovery (STIR) imaging.

Materials and Methods

Sixty‐one patients were referred for spine magnetic resonance imaging (MRI) including sagittal FTED (time 2:26), STIR (time 2:42), and T2 FRFSE (time 2:55). Two observers compared STIR and FTED water images and T2 FRFSE and FTED T2 images for overall image quality, fat suppression, anatomic sharpness, motion, cerebrospinal fluid (CSF) flow artifact, susceptibility, and disease depiction.

Results

On FTED images water and fat separation was perfect in 58 (.95) patients. Compared to STIR, the FTED water images demonstrated less motion in 57 (.93) of 61 patients (P < 0.05), better anatomic sharpness in 51 (.84) and patients (P < 0.05), and less CSF flow artifact in 7 (.11) P < 0.05) patients. There was no difference in fat suppression or chemical shift artifact. T2 FRFSE and FTED T2 images showed equivalent motion, CSF flow, and chemical shift artifact. Lesion depiction was equivalent on FTED water and STIR images and FTED T2 and T2 FRFSE images.

Conclusion

FTED efficiently provides both fat‐suppressed and nonfat‐suppressed T2‐weighted spine images with excellent image quality, equal disease depiction, and 56% reduction in scan time compared to conventional STIR and T2 FRFSE. J. Magn. Reson. Imaging 2011;33:390–400. © 2011 Wiley‐Liss, Inc.     

Dixon techniques for water and fat imaging

In 1984, Dixon published a first paper on a simple spectroscopic imaging technique for water and fat separation. The technique acquires two separate images with a modified spin echo pulse sequence. One is a conventional spin echo image with water and fat signals in-phase and the other is acquired with the readout gradient slightly shifted so that the water and fat signals are 180 degrees out-of-phase. Dixon showed that from these two images, a water-only image and a fat-only image can be generated. The water-only image by the Dixon's technique can serve the purpose of fat suppression, an important and widely used imaging option for clinical MRI. Additionally, the availability of both the water-only and fat-only images allows direct image-based water and fat quantitation. These applications, as well as the potential that the technique can be made highly insensitive to magnetic field inhomogeneity, have generated substantial research interests and efforts from many investigators. As a result, significant improvement to the original technique has been made in the last 2 decades. The following article reviews the underlying physical principles and describes some major technical aspects in the development of these Dixon techniques.     

Combination of complex‐based and magnitude‐based multiecho water‐fat separation for accurate quantification of fat‐fraction

Multipoint water–fat separation techniques rely on different water–fat phase shifts generated at multiple echo times to decompose water and fat. Therefore, these methods require complex source images and allow unambiguous separation of water and fat signals. However, complex‐based water–fat separation methods are sensitive to phase errors in the source images, which may lead to clinically important errors. An alternative approach to quantify fat is through “magnitude‐based” methods that acquire multiecho magnitude images. Magnitude‐based methods are insensitive to phase errors, but cannot estimate fat‐fraction greater than 50%. In this work, we introduce a water–fat separation approach that combines the strengths of both complex and magnitude reconstruction algorithms. A magnitude‐based reconstruction is applied after complex‐based water–fat separation to removes the effect of phase errors. The results from the two reconstructions are then combined. We demonstrate that using this hybrid method, 0–100% fat‐fraction can be estimated with improved accuracy at low fat‐fractions. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Incidental magnetization transfer contrast by fat saturation preparation pulses in multislice look‐locker echo planar imaging

In this study, it is demonstrated that fat saturation (FS) preparation (prep) pulses generate incidental magnetization transfer contrast (MTC) in multislice Look‐Locker (LL) imaging. It is shown that frequency‐selective FS prep pulses can invoke MTC through the exchange between free and motion‐restricted protons. Simulation reveals that the fractional signal loss by these MTC effects is more severe for smaller flip angles (FAs), shorter repetition times (TRs), and greater number of slices (SN). These incidental MTC effects result in a signal attenuation at a steady state (up to 30%) and a T₁ measurement bias (up to 20%) when using inversion recovery (IR) LL echo‐planar imaging (EPI) sequences. Furthermore, it is shown that water‐selective MRI using binomial pulses has the potential to minimize the signal attenuation and provide unbiased T₁ measurement without fat artifacts in MR images. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Improved fat water separation with water selective inversion pulse for inversion recovery imaging in cardiac MRI

Purpose:

To develop an improved chemical shift‐based water‐fat separation sequence using a water‐selective inversion pulse for inversion recovery 3D contrast‐enhanced cardiac magnetic resonance imaging (MRI).

Materials and Methods:

In inversion recovery sequences the fat signal is substantially reduced due to the application of a nonselective inversion pulse. Therefore, for simultaneous visualization of water, fat, and myocardial enhancement in inversion recovery‐based sequences such as late gadolinium enhancement imaging, two separate scans are used. To overcome this, the nonselective inversion pulse is replaced with a water‐selective inversion pulse. Imaging was performed in phantoms, nine healthy subjects, and nine patients with suspected arrhythmogenic right ventricular cardiomyopathy plus one patient for tumor/mass imaging. In patients, images with conventional turbo‐spin echo (TSE) with and without fat saturation were acquired prior to contrast injection for fat assessment. Subjective image scores (1 = poor, 4 = excellent) were used for image assessment.

Results:

Phantom experiments showed a fat signal‐to‐noise ratio (SNR) increase between 1.7 to 5.9 times for inversion times of 150 and 300 msec, respectively. The water‐selective inversion pulse retains the fat signal in contrast‐enhanced cardiac MR, allowing improved visualization of fat in the water‐fat separated images of healthy subjects with a score of 3.7 ± 0.6. Patient images acquired with the proposed sequence were scored higher when compared with a TSE sequence (3.5 ± 0.7 vs. 2.2 ± 0.5, P < 0.05).

Conclusion:

The water‐selective inversion pulse retains the fat signal in inversion recovery‐based contrast‐enhanced cardiac MR, allowing simultaneous visualization of water and fat. J. Magn. Reson. Imaging 2013;37:484–490. © 2012 Wiley Periodicals, Inc.