Four-dimensional flow MRI using spiral acquisition

Fat suppression is an essential part of routine MRI scanning. Multiecho chemical‐shift based water‐fat separation methods estimate and correct for Bo field inhomogeneity. However, they must contend with the intrinsic challenge of water‐fat ambiguity that can result in water‐fat swapping. This problem arises because the signals from two chemical species, when both are modeled as a single discrete spectral peak, may appear indistinguishable in the presence of Bo off‐resonance. In conventional methods, the water‐fat ambiguity is typically removed by enforcing field map smoothness using region growing based algorithms. In reality, the fat spectrum has multiple spectral peaks. Using this spectral complexity, we introduce a novel concept that identifies water and fat for multiecho acquisitions by exploiting the spectral differences between water and fat. A fat likelihood map is produced to indicate if a pixel is likely to be water‐dominant or fat‐dominant by comparing the fitting residuals of two different signal models. The fat likelihood analysis and field map smoothness provide complementary information, and we designed an algorithm (Fat Likelihood Analysis for Multiecho Signals) to exploit both mechanisms. It is demonstrated in a wide variety of data that the Fat Likelihood Analysis for Multiecho Signals algorithm offers highly robust water‐fat separation for 6‐echo acquisitions, particularly in some previously challenging applications. Magn Reson Med, 2011. © 2011 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.     

Chemical shift-based water/fat separation in the presence of susceptibility-induced fat resonance shift

Chemical shift‐based water/fat separation methods have been emerging due to the growing clinical need for fat quantification in different body organs. Accurate quantification of proton‐density fat fraction requires the assessment of many confounding factors, including the need of modeling the presence of multiple peaks in the fat spectrum. Most recent quantitative chemical shift‐based water/fat separation approaches rely on a multipeak fat spectrum with precalibrated peak locations and precalibrated or self‐calibrated peak relative amplitudes. However, water/fat susceptibility differences can induce fat spectrum resonance shifts depending on the shape and orientation of the fatty inclusions. The effect is of particular interest in the skeletal muscle due to the anisotropic arrangement of extracellular lipids. In this work, the effect of susceptibility‐induced fat resonance shift on the fat fraction is characterized in a conventional complex‐based chemical shift‐based water/fat separation approach that does not model the susceptibility‐induced fat resonance shift. A novel algorithm is then proposed to quantify the resonance shift in a complex‐based chemical shift‐based water/fat separation approach that considers the fat resonance shift in the signal model, aiming to extract information about the orientation/geometry of lipids. The technique is validated in a phantom and preliminary in vivo results are shown in the calf musculature of healthy and diabetic subjects. Magn Reson Med, 2012. © 2012 Wiley Periodicals, 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.     

Chemical shift encoding‐based water–fat separation methods

The suppression of signal from fat constitutes a basic requirement in many applications of magnetic resonance imaging. To date, this is predominantly achieved during data acquisition, using fat saturation, inversion recovery, or water excitation methods. Postponing the separation of signal from water and fat until image reconstruction holds the promise of resolving some of the problems associated with these methods, such as failure in the presence of field inhomogeneities or contrast agents. In this article, methods are reviewed that rely on the difference in chemical shift between the hydrogen atoms in water and fat to perform such a retrospective separation. The basic principle underlying these so‐called Dixon methods is introduced, and some fundamental implementations of the required chemical shift encoding in the acquisition and the subsequent water–fat separation in the reconstruction are described. Practical issues, such as the selection of key parameters and the appearance of typical artifacts, are illustrated, and a broad range of applications is demonstrated, including abdominal, cardiovascular, and musculoskeletal imaging. Finally, advantages and disadvantages of these Dixon methods are summarized, and emerging opportunities arising from the availability of information on the amount and distribution of fat are discussed. J. Magn. Reson. Imaging 2014;40:251–268 . © 2014 Wiley Periodicals, Inc .     

Fat and water magnetic resonance imaging

A wide variety of fat suppression and water–fat separation methods are used to suppress fat signal and improve visualization of abnormalities. This article reviews the most commonly used techniques for fat suppression and fat–water imaging including 1) chemically selective fat suppression pulses “FAT‐SAT”; 2) spatial‐spectral pulses (water excitation); 3) short inversion time (TI) inversion recovery (STIR) imaging; 4) chemical shift based water–fat separation methods; and finally 5) fat suppression and balanced steady‐state free precession (SSFP) sequences. The basic physical background of these techniques including their specific advantages and disadvantages is given and related to clinical applications. This enables the reader to understand the reasons why some fat suppression methods work better than others in specific clinical settings. J. Magn. Reson. Imaging 2010;31:4–18. © 2009 Wiley‐Liss, Inc.     

Multiecho water‐fat separation and simultaneous R estimation with multifrequency fat spectrum modeling

Multiecho chemical shift–based water‐fat separation methods are seeing increasing clinical use due to their ability to estimate and correct for field inhomogeneities. Previous chemical shift‐based water‐fat separation methods used a relatively simple signal model that assumes both water and fat have a single resonant frequency. However, it is well known that fat has several spectral peaks. This inaccuracy in the signal model results in two undesired effects. First, water and fat are incompletely separated. Second, methods designed to estimate T in the presence of fat incorrectly estimate the T decay in tissues containing fat. In this work, a more accurate multifrequency model of fat is included in the iterative decomposition of water and fat with echo asymmetry and least‐squares estimation (IDEAL) water‐fat separation and simultaneous T estimation techniques. The fat spectrum can be assumed to be constant in all subjects and measured a priori using MR spectroscopy. Alternatively, the fat spectrum can be estimated directly from the data using novel spectrum self‐calibration algorithms. The improvement in water‐fat separation and T estimation is demonstrated in a variety of in vivo applications, including knee, ankle, spine, breast, and abdominal scans. Magn Reson Med 60:1122–1134, 2008. © 2008 Wiley‐Liss, Inc.     

Whole-body 3D water/fat resolved continuously moving table imaging

PURPOSE: To study the feasibility of three-dimensional (3D) whole-body, head-to-toe, water/fat resolved MRI, using continuously moving table imaging technology. MATERIALS AND METHODS: Experiments were performed on nine healthy volunteers, acquiring 3D whole-body head-to-toe data under continuous motion of the patient table. Two different approaches for water/fat separation have been studied. Results of a three-point chemical shift encoding and a spectral presaturation technique were compared with respect to image quality and performance. Furthermore, fast, low-resolution, whole-body water/fat imaging was performed in two minutes total scan time to derive patient-specific parameters such as the total water/fat ratio, the intraperitoneal/extraperitoneal fat ratio, and the body mass index (BMI). RESULTS: Good water/fat separation with decent image quality was obtained in all cases. The three-point chemical shift encoding approach was found to be more efficient with respect to signal-to-noise ratio (SNR) and acquisition time. CONCLUSION: Whole-body water/fat sensitive MRI using continuous table motion is feasible and could be of interest for clinical practice. Some improvements of the method are desirable.     

k‐space water‐fat decomposition with T2* estimation and multifrequency fat spectrum modeling for ultrashort echo time imaging

Purpose:

To demonstrate the feasibility of combining a chemical shift‐based water‐fat separation method (IDEAL) with a 2D ultrashort echo time (UTE) sequence for imaging and quantification of the short T₂ tissues with robust fat suppression.

Materials and Methods:

A 2D multislice UTE data acquisition scheme was combined with IDEAL processing, including T₂* estimation, chemical shift artifacts correction, and multifrequency modeling of the fat spectrum to image short T₂ tissues such as the Achilles tendon and meniscus both in vitro and in vivo. The integration of an advanced field map estimation technique into this combined method, such as region growing (RG), is also investigated.

Results:

The combination of IDEAL with UTE imaging is feasible and excellent water‐fat separation can be achieved for the Achilles tendon and meniscus with simultaneous T₂* estimation and chemical shift artifact correction. Multifrequency modeling of the fat spectrum yields more complete water‐fat separation with more accurate correction for chemical shift artifacts. The RG scheme helps to avoid water‐fat swapping.

Conclusion:

The combination of UTE data acquisition with IDEAL has potential applications in imaging and quantifying short T₂ tissues, eliminating the necessity for fat suppression pulses that may directly suppress the short T₂ signals. J. Magn. Reson. Imaging 2010;31:1027–1034. ©2010 Wiley‐Liss, Inc.     

T1‐corrected fat quantification using chemical shift‐based water/fat separation: Application to skeletal muscle

Chemical shift‐based water/fat separation, like iterative decomposition of water and fat with echo asymmetry and least‐squares estimation, has been proposed for quantifying intermuscular adipose tissue. An important confounding factor in iterative decomposition of water and fat with echo asymmetry and least‐squares estimation‐based intermuscular adipose tissue quantification is the large difference in T₁ between muscle and fat, which can cause significant overestimation in the fat fraction. This T₁ bias effect is usually reduced by using small flip angles. T₁‐correction can be performed by using at least two different flip angles and fitting for T₁ of water and fat. In this work, a novel approach for the water/fat separation problem in a dual flip angle experiment is introduced and a new approach for the selection of the two flip angles, labeled as the unequal small flip angle approach, is developed, aiming to improve the noise efficiency of the T₁‐correction step relative to existing approaches. It is shown that the use of flip angles, selected such the muscle water signal is assumed to be T₁‐independent for the first flip angle and the fat signal is assumed to be T₁‐independent for the second flip angle, has superior noise performance to the use of equal small flip angles (no T₁ estimation required) and the use of large flip angles (T₁ estimation required). Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Noninvasive temperature mapping with MRI using chemical shift water‐fat separation

Tissues containing both water and lipids, e.g., breast, confound standard MR proton reference frequency‐shift methods for mapping temperatures due to the lack of temperature‐induced frequency shift in lipid protons. Generalized Dixon chemical shift–based water‐fat separation methods, such as GE's iterative decomposition of water and fat with echo asymmetry and least‐squares estimation method, can result in complex water and fat images. Once separated, the phase change over time of the water signal can be used to map temperature. Phase change of the lipid signal can be used to correct for non‐temperature‐dependent phase changes, such as amplitude of static field drift. In this work, an image acquisition and postprocessing method, called water and fat thermal MRI, is demonstrated in phantoms containing 30:70, 50:50, and 70:30 water‐to‐fat by volume. Noninvasive heating was applied in an Off1‐On‐Off2 pattern over 50 min, using a miniannular phased radiofrequency array. Temperature changes were referenced to the first image acquisition. Four fiber optic temperature probes were placed inside the phantoms for temperature comparison. Region of interest (ROI) temperature values colocated with the probes showed excellent agreement (global mean ± standard deviation: ?0.09 ± 0.34°C) despite significant amplitude of static field drift during the experiments. Magn Reson Med 63:1238–1246, 2010. © 2010 Wiley‐Liss, Inc.     

Fat‐free MRI based on magnetization exchange

An MRI technique is proposed for complete fat signal elimination. This approach exploits the fact that water rapidly exchanges magnetization with protons in protein and membrane phospholipid of tissue and cells but does not exchange magnetization with triglyceride or fat protons in the tissue. Saturation of the proton signal from protein and membrane phospholipid thus results in partial saturation of the water proton signal, allowing acquisition of an image including a portion of the water signal and the full fat signal. Subtraction of this image from the standard image, containing both water and fat signals, results in an image in which all fat signal is cancelled. This fat‐free image is sensitive to magnetization transfer and to water density and relaxation time, providing the possibility of additional contrast. Unlike most fat suppression techniques, this method is not compromised by the static or radiofrequency field heterogeneity and is equally efficient for all fat resonances independent of their chemical shift frequency. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Multiecho dixon fat and water separation method for detecting fibrofatty infiltration in the myocardium

Conventional approaches for fat and water discrimination based on chemical‐shift fat suppression have reduced ability to characterize fatty infiltration due to poor contrast of microscopic fat. The multiecho Dixon approach to water and fat separation has advantages over chemical‐shift fat suppression: 1) water and fat images can be acquired in a single breathhold, avoiding misregistration; 2) fat has positive contrast; 3) the method is compatible with precontrast and late‐enhancement imaging, 4) less susceptible to partial‐volume effects, and 5) robust in the presence of background field variation; and 6) for the bandwidth implemented, chemical‐shift artifact is decreased. The proposed technique was applied successfully in all 28 patients studied. This included 10 studies with indication of coronary artery disease (CAD), of which four cases with chronic myocardial infarction (MI) exhibited fatty infiltration; 13 studies to rule out arrhythmogenic right ventricular cardiomyopathy (ARVC), of which there were three cases with fibrofatty infiltration and two confirmed with ARVC; and five cases of cardiac masses (two lipomas). The precontrast contrast‐to‐noise ratio (CNR) of intramyocardial fat was greatly improved, by 240% relative to conventional fat suppression. For the parameters implemented, the signal‐to‐noise ratio (SNR) was decreased by 30% relative to conventional late enhancement. The multiecho Dixon method for fat and water separation provides a sensitive means of detecting intramyocardial fat with positive signal contrast. Magn Reson Med 61:215–221, 2009. © 2008 Wiley‐Liss, Inc.     

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.     

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.     

Separation of true fat and water images by correcting magnetic field inhomogeneity in situ

Dixon's method of chemical shift imaging of a two-component system is modified and extended without requiring additional imaging time. The modified method allows one to obtain truly segregated fat and water images of animal tissues. This is accomplished by acquiring additional image data from which information about in situ magnetic field inhomogeneity and bulk magnetic susceptibility can be derived. Applications to various anatomic sections of the normal human body are illustrated. The method is compared with the standard Dixon technique of chemical shift image separation.     

Fat suppression for 1H MRSI at 7T using spectrally selective adiabatic inversion recovery.

Proton magnetic resonance spectroscopic imaging ((1)H MRSI) at 7T offers many advantages, including increased SNR and spectral resolution. However, technical difficulties associated with operating at high fields, such as increased B(1) and B(0) inhomogeneity, severe chemical shift localization error, and converging T(1) values, make the suppression of the broad lipid peaks which can obscure targeted metabolite signals, particularly challenging. Conventional short tau inversion recovery can successfully suppress fat without restricting the selected volume, but only with significant metabolite signal loss. In this work, we have designed two new pulses for frequency-selective inversion recovery that achieve B(1)-insensitive fat suppression without degrading the signal from the major metabolites of interest. The first is a spectrally selective adiabatic pulse to be used in a volumetric (1)H MRSI sequence and the second is a spatial-spectral adiabatic pulse geared toward multi-slice (1)H MRSI. Partial interior volume selection may be used in addition to the pulses, to exclude areas with severe B(0) inhomogeneity. Some differences in the spectral profile as well as degree of suppression make each pulse valuable for different applications. 7T phantom and in vivo data show that both pulses significantly suppress fat, while leaving most of the metabolite signal intact.     

Chemical species separation with simultaneous estimation of field map and T2* using a k-space formulation

Chemical species separation techniques in image space are prone to incorporate several distortions. Some of these are signal accentuation in borders and geometrical warping from field inhomogeneity. These errors come from neglecting intraecho time variations. In this work, we present a new approach for chemical species separation in MRI with simultaneous estimation of field map and T2* decay, formulated entirely in k-space. In this approach, the time map is used to model the phase accrual from off-resonance precession and also the amplitude decay due to T2*. Our technique fits the signal model directly in k-space with the acquired data minimizing the l(2)-norm with an interior-point algorithm. Standard two dimensional gradient echo sequences in the thighs and head were used for demonstrating the technique. With this approach, we were able to obtain excellent estimation for the species, the field inhomogeneity, and T2* decay images. The results do not suffer from geometric distortions derived from the chemical shift or the field inhomogeneity. Importantly, as the T2* map is well positioned, the species signal in borders is correctly estimated. Considering intraecho time variations in a complete signal model in k-space for separating species yields superior estimation of the variables of interest when compared to existing methods.     

Quantification of absolute fat mass using an adipose tissue reference signal model

Purpose

To develop a method for quantifying absolute fat mass, and to demonstrate its feasibility in phantoms and in ex vivo swine specimens at 3 Tesla.

Materials and Methods

Chemical‐shift‐based fat‐water decomposition was used to first reconstruct fat‐only images. Our proposed model used a reference signal from fat in pure adipose tissue to calibrate and normalize the fat signal intensities from the fat‐only images. Fat mass was subsequently computed on a voxel‐by‐voxel basis and summed across each sample. Feasibility of the model was tested in six ex vivo swine samples containing varying mixtures of fat (adipose) and lean tissues. The samples were imaged using 1.5‐mm isotropic voxels and a single‐channel birdcage head coil at 3 Tesla. Lipid assay was independently performed to determine fat mass, and served as the comparison standard.

Results

Absolute fat mass values (in grams) derived by our proposed model were in excellent agreement with lipid assay results, with a 5% to 7% difference (r > 0.99; P < 0.001).

Conclusion

Preliminary results in ex vivo swine samples demonstrated the feasibility of computing absolute fat mass as a quantitative endpoint using chemical‐shift fat‐water MRI with a signal model based on reference fat from pure adipose tissue. J. Magn. Reson. Imaging 2008;28:1483–1491. © 2008 Wiley‐Liss, Inc.     

T1 independent,T2* corrected MRI with accurate spectral modeling for quantification of fat: Validation in a fat‐water‐SPIO phantom

Purpose:

To validate a T₁‐independent, T₂*‐corrected fat quantification technique that uses accurate spectral modeling of fat using a homogeneous fat‐water‐SPIO phantom over physiologically expected ranges of fat percentage and T₂* decay in the presence of iron overload.

Materials and Methods:

A homogeneous gel phantom consisting of vials with known fat‐fractions and iron concentrations is described. Fat‐fraction imaging was performed using a multiecho chemical shift‐based fat‐water separation method (IDEAL), and various reconstructions were performed to determine the impact of T₂* correction and accurate spectral modeling. Conventional two‐point Dixon (in‐phase/out‐of‐phase) imaging and MR spectroscopy were performed for comparison with known fat‐fractions.

Results:

The best agreement with known fat‐fractions over the full range of iron concentrations was found when T₂* correction and accurate spectral modeling were used. Conventional two‐point Dixon imaging grossly underestimated fat‐fraction for all T₂* values, but particularly at higher iron concentrations.

Conclusion:

This work demonstrates the necessity of T₂* correction and accurate spectral modeling of fat to accurately quantify fat using MRI. J. Magn. Reson. Imaging 2009;30:1215–1222. © 2009 Wiley‐Liss, Inc.