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.     

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.     

Fat-suppressed three-dimensional dual echo Dixon technique for contrast agent enhanced MRI

PURPOSE: To develop a fast T1-weighted, fat-suppressed three-dimensional dual echo Dixon technique and to demonstrate its use in contrast agent enhanced MRI. MATERIALS AND METHODS: A product fast three-dimensional gradient echo pulse sequence was modified to acquire dual echoes after each RF excitation with water and fat signals in-phase (IP) and opposed-phase (OP), respectively. An on-line reconstruction algorithm was implemented to automatically generate separate water and fat images. The signal to noise ratio (SNR) of the new technique was compared to that of the product technique in phantom. In vivo abdomen and breast images of cancer patients were acquired at 1.5 Tesla using both techniques before and after intravenous administration of gadolinium contrast agent. RESULTS: In phantom, the new technique yields a close to the theoretically predicted 41% increase in SNR in comparison to the product technique without fat suppression (FS). In vivo images of the new technique show noticeably improved FS and image quality in comparison to the images acquired of the same patients using the product technique with FS. CONCLUSION: The three-dimensional dual echo Dixon technique provides excellent image quality and can be used for T1-weighted, fat-suppressed imaging with contrast agent injection.     

Interleaved water and fat dual-echo spin echo imaging with intrinsic chemical-shift elimination

A new technique was developed to simultaneously acquire water and fat dual-echo spin echo images in a single acquisition period. Chemical shifts between water and fat images are intrinsically eliminated, and the images are combined to form water-plus-fat image. In vivo water-only images show fat suppression superior to that of conventional spin echo images. This technique may be clinically useful for musculoskeletal imaging.     

T2-weighted spine imaging with a fast three-point dixon technique: comparison with chemical shift selective fat suppression

PURPOSE: To develop a phased-array coil-compatible, fast three-point Dixon (TPD) technique, and compare its performance in T2-weighted spine imaging with that of the standard chemical shift selective (CHESS) fat suppression technique. MATERIALS AND METHODS: We acquired T2-weighted spine images of 27 patients using essentially identical scanning parameters with the fast TPD technique and standard fast spin echo (FSE) with CHESS fat suppression. A phased-array coil-compatible image reconstruction algorithm was developed to generate separate water and fat images from the data acquired with the fast TPD technique. Three neuroradiologists independently scored the images from the two different techniques for uniformity of fat suppression and lesion conspicuity using a four-point system (1 = poor, 2 = fair, 3 = good, 4 = best). RESULTS: The reviewers' mean scores were 3.2 and 2.1 for the uniformity of fat suppression, and 3.0 and 2.0 for the lesion conspicuity for the fast TPD and the CHESS fat suppression techniques, respectively. The fast TPD technique was statistically superior to the CHESS technique at P < 0.0005. CONCLUSION: The fast TPD technique provides superior fat suppression and lesion conspicuity, and potentially can be used as an alternative to T2-weighted imaging of the spine.     

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.     

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.     

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.     

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.     

Cardiac CINE imaging with IDEAL water-fat separation and steady-state free precession

PURPOSE: To decompose multicoil CINE steady-state free precession (SSFP) cardiac images acquired at short echo time (TE) increments into separate water and fat images, using an iterative least-squares "Dixon" (IDEAL) method. MATERIALS AND METHODS: Multicoil CINE IDEAL-SSFP cardiac imaging was performed in three volunteers and 15 patients at 1.5 T. RESULTS: Measurements of signal-to-noise ratio (SNR) matched theoretical expectations and were used to optimize acquisition parameters. TE increments of 0.9-1.0 msec permitted the use of repetition times (TRs) of 5 msec or less, and provided good SNR performance of the water-fat decomposition, while maintaining good image quality with a minimum of banding artifacts. Images from all studies were evaluated for fat separation and image quality by two experienced radiologists. Uniform fat separation and diagnostic image quality was achieved in all images from all studies. Examples from volunteers and patients are shown. CONCLUSION: Multicoil IDEAL-SSFP imaging can produce high quality CINE cardiac images with uniform water-fat separation, insensitive to Bo inhomogeneities. This approach provides a new method for reliable fat-suppression in cardiac imaging.     

Rapid water and lipid imaging with T2 mapping using a radial IDEAL‐GRASE technique

Three‐point Dixon methods have been investigated as a means to generate water and fat images without the effects of field inhomogeneities. Recently, an iterative algorithm (IDEAL, iterative decomposition of water and fat with echo asymmetry and least squares estimation) was combined with a gradient and spin‐echo acquisition strategy (IDEAL‐GRASE) to provide a time‐efficient method for lipid–water imaging with correction for the effects of field inhomogeneities. The method presented in this work combines IDEAL‐GRASE with radial data acquisition. Radial data sampling offers robustness to motion over Cartesian trajectories as well as the possibility of generating high‐resolution T₂ maps in addition to the water and fat images. The radial IDEAL‐GRASE technique is demonstrated in phantoms and in vivo for various applications including abdominal, pelvic, and cardiac imaging. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Two-point Dixon technique for water-fat signal decomposition with B0 inhomogeneity correction

To separate water and lipid resonance signals by phase-sensitive MRI, a two-point Dixon (2PD) reconstruction is presented in which phase-unwrapping is used to obtain an inhomogeneity map based on only in-phase and out-of-phase image data. Two relaxation-weighted images, a “water image” and a “fat image,” representing a two-resonance peak model of proton density, are output. The method is designed for T₁- or density-weighted spin-echo imaging; a double-echo scheme is more appropriate for T₂-weighted spin-echo imaging. The technique is more time-efficient for clinical fat-water imaging than 3PD schemes, while still correcting for field inhomogeneity.     

3D interleaved water and fat image acquisition with chemical-shift correction.

A new technique, 3D interleaved water and fat image acquisition with chemical-shift correction (3-DIWFAC), was developed to acquire 3D water and fat images in a single acquisition time and to combine the water and fat images to produce chemical-shift-free images. A 3D gradient-recalled-echo (GRE) sequence was implemented with a 1-3-3-1 binomial Shinnar-Le Roux spatial-spectral excitation, and with interleaved phase-encoding lines that alternate between water and fat excitations separated by half TR. Water-only and fat-only images were then realigned to remove chemical shift artifacts. Results from phantoms and human subjects demonstrated that the image contrast was the same as in the regular GRE sequence. With the chemical shift corrected, the shadow artifacts often seen at water and fat boundaries were removed. Since this sequence simultaneously provides water-only images showing cartilage and bone lesions, and water-fat images that depict soft tissue anatomy, it may be clinically useful in musculoskeletal imaging. Magn Reson Med 44:322-330, 2000.     

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.     

Fatty infiltration of the liver: demonstration by proton spectroscopic imaging. Preliminary observations

Two normal volunteers and three patients with CT evidence of fatty infiltration of the liver (two nonuniform, one diffuse) were studied to determine whether magnetic resonance imaging using a pulse sequence designed to differentiate fat and water could be used to detect fatty infiltration of the liver in human beings. The magnetic resonance technique used a modified spin echo technique (simple proton spectroscopic imaging) that was designed specifically to exploit the difference in the rate of precession between the protons in a water molecule and the protons in a fatty acid molecule. Images were obtained using in-phase and opposed techniques and were added or subtracted in order to obtain pure water and pure fat images. Quantitative data showed that fatty liver can be separated from normal liver using the spin echo technique, and that the opposed image of the proton spectroscopic technique is more sensitive to small changes in hepatic fatty content than in-phase images with any echo time.     

T1‐weighted 3D dynamic contrast‐enhanced MRI of the breast using a dual‐echo dixon technique at 3 T

Purpose:

To evaluate a single‐pass fast spoiled gradient echo (FSPGR) two‐point Dixon sequence and a gradient echo sequence with spectral fat suppression in their performance at 3 T for fat suppressed contrast‐enhanced bilateral breast imaging.

Materials and Methods:

Twenty patients were prospectively enrolled in an imaging protocol that included axial Dixon and 3D FSPGR with spectrally selective fat saturation sequences as part of patient care in this study. Qualitative analysis was performed retrospectively by two readers who scored the images for homogeneity and degree of fat saturation, severity of artifacts, and quality of normal anatomical structures. Enhancing lesions were scored according to the confidence with which American College of Radiology (ACR) BI‐RADS magnetic resonance imaging (MRI) features were identified.

Results:

The Dixon sequence showed superior fat saturation homogeneity, quality of posterior anatomical structures, and decreased artifact severity that were statistically significant (P < 0.0001). The degree of fat saturation was scored higher in the Dixon sequence, although the difference did not reach statistical significance. There were no significant differences between the 3D T1‐weighted FSPGR and Dixon groups for assessing lesion features.

Conclusion:

Our findings suggest that the Dixon technique is an effective fat suppression method for contrast‐enhanced breast MRI. The Dixon technique also seemed to provide better anatomical definition of posterior structures and improvement in severity of artifacts. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Susceptibility weighted imaging (SWI).

Susceptibility differences between tissues can be utilized as a new type of contrast in MRI that is different from spin density, T1-, or T2-weighted imaging. Signals from substances with different magnetic susceptibilities compared to their neighboring tissue will become out of phase with these tissues at sufficiently long echo times (TEs). Thus, phase imaging offers a means of enhancing contrast in MRI. Specifically, the phase images themselves can provide excellent contrast between gray matter (GM) and white matter (WM), iron-laden tissues, venous blood vessels, and other tissues with susceptibilities that are different from the background tissue. Also, for the first time, projection phase images are shown to demonstrate tissue (vessel) continuity. In this work, the best approach for combining magnitude and phase images is discussed. The phase images are high-pass-filtered and then transformed to a special phase mask that varies in amplitude between zero and unity. This mask is multiplied a few times into the original magnitude image to create enhanced contrast between tissues with different susceptibilities. For this reason, this method is referred to as susceptibility-weighted imaging (SWI). Mathematical arguments are presented to determine the number of phase mask multiplications that should take place. Examples are given for enhancing GM/WM contrast and water/fat contrast, identifying brain iron, and visualizing veins in the brain.     

A spin echo chemical shift MR imaging technique

A new method is described that produces images of either the fat or water component in tissues in magnetic resonance imaging. Only a single scan is required, with scan times of a few minutes. Chemical shift selectivity is achieved in the spin echo process by controlling the spectral content of the 180 degree pulse that induces the spin echoes. A theoretical analysis of the selective spin echo process for the case of a radio frequency pulse of constant amplitude shows that spin echoes will be suppressed for certain values of offset frequency that are similar to, but different from, the frequencies at which the Fourier spectrum of the pulse vanishes. The theory was confirmed by experiment on a water phantom. The imaging technique was tested on both a phantom of oil and water and on a human forearm. Excellent suppression of the water signal was found in the fat images, and the small fat component seen in the water images is attributable to components of the triglyceride molecule for which spectral lines overlap those of water. The forearm images also showed blood flow effects in the water image that were not visible in the fat image. The relationship of this method to other proposed methods of chemical shift imaging is discussed.     

Multiecho time‐resolved acquisition (META): A high spatiotemporal resolution dixon imaging sequence for dynamic contrast‐enhanced MRI

Purpose

To evaluate a new dynamic contrast‐enhanced (DCE) imaging technique called multiecho time‐resolved acquisition (META) for abdominal/pelvic imaging. META combines an elliptical centric time‐resolved three‐dimensional (3D) spoiled gradient‐recalled echo (SPGR) imaging scheme with a Dixon‐based fat‐water separation algorithm to generate high spatiotemporal resolution volumes.

Materials and Methods

Twenty‐three patients referred for hepatic metastases or renal masses were imaged using the new META sequence and a conventional fat‐suppressed 3D SPGR sequence on a 3T scanner. In 12 patients, equilibrium‐phase 3D SPGR images acquired immediately after META were used for comparing the degree and homogeneity of fat suppression, artifacts, and overall image quality. In the remaining 11 of 23 patients, DCE 3D SPGR images acquired in a previous or subsequent examination were used for comparing the efficiency of arterial phase capture in addition to the qualitative analysis for the degree and homogeneity of fat suppression, artifacts, and overall image quality.

Results

META images were determined to be significantly better than conventional 3D SPGR images for degree and uniformity of fat suppression and ability to visualize the arterial phase. There were no significant differences in artifact levels or overall image quality.

Conclusion

META is a promising high spatiotemporal resolution imaging sequence for capturing the fast dynamics of hyperenhancing hepatic lesions and provides robust fat suppression even at 3T. J. Magn. Reson. Imaging 2009. © 2009 Wiley‐Liss, Inc.     

Pelvic imaging using a T1W fat-suppressed three-dimensional dual echo Dixon technique at 3T

PURPOSE: To compare two T1-weighted (T1W) fat-suppressed sequences for 3D breath-hold pre- and postcontrast fat-suppressed T1W imaging of the female pelvis at 3T. MATERIALS AND METHODS: Pelvic MRI scans of 16 female patients were retrospectively identified who were scanned with two 3D breath-hold sequences: 1) a fast spoiled gradient echo sequence with spectral inversion at lipids (SPECIAL) (called 3D FSPGR), and 2) a dual-echo two-point Dixon (DE Dixon) sequence. Contrast between soft tissue and fat, soft tissue and fluid, and fat and fluid was measured on pre- and postcontrast images. Additionally, two readers subjectively scored the images for degree and homogeneity of fat suppression plus presence and severity of artifacts. RESULTS: Contrast between muscle and myometrium to fat was improved with the Dixon technique (0.61 vs. 0.09 and 0.7 vs. 0.3, respectively, P < 0.001). Both readers agreed that fat suppression was stronger with the Dixon sequence (P < 0.001 and P = 0.06). Artifacts were equivalent (P = 0.53 and 0.65). CONCLUSION: The 3D DE Dixon sequence achieved stronger fat suppression in the female pelvis when compared to a 3D FSPGR sequence with SPECIAL.