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.     

Continuously moving table 3D MRI with lateral frequency-encoding direction.

A method is presented for 3D MRI in an extended field of view (FOV) based on continuous motion of the patient table and an efficient acquisition scheme. A gradient-echo MR pulse sequence is applied with lateral (left-right (L/R)) frequency-encoding direction and slab selection along the direction of motion. Compensation for the table motion is achieved by a combination of slab tracking and data alignment in hybrid space. The method allows fast k-space coverage to be achieved, especially when a short sampling FOV is chosen along the direction of table motion, as is desirable for good image quality. The method can be incorporated into different acquisitions schemes, including segmented k-space scanning, which allows for contrast variation with the use of magnetization preparation. Head-to-toe images of volunteers were obtained with good quality using 3D spoiled gradient-echo sequences. As an example of magnetization-prepared imaging, fat/water separated images were acquired using chemical shift selective (CHESS) presaturation pulses.     

Principles of whole-body continuously-moving-table MRI

Continuously-moving-table (CMT) imaging is a new and promising approach to virtually extend the field-of-view (FOV) in currently available MRI systems. It shows high potential to improve a number of applications that require a large FOV, such as whole-body contrast-enhanced angiography and contrast-optimized whole-body head-to-toe imaging. In this work, an overview of the different approaches to CMT imaging is given. Basic principles of two- and three-dimensional (2D and 3D) techniques are discussed, with emphasis on performance and image-artifact issues. Potential clinical applications and further desirable improvements are outlined.     

Silicone-specific imaging using an inversion-recovery-prepared fast three-point Dixon technique

PURPOSE: To demonstrate a new hybrid magnetic resonance imaging (MRI) technique capable of simultaneously generating water-specific and silicone-specific images in a single acquisition. MATERIALS AND METHODS: This technique combines short TI inversion-recovery (STIR) technique for robust fat suppression with an efficient fast spin-echo-based three-point Dixon technique for robust separation of remaining water and silicone in the presence of field inhomogeneities. Images demonstrating the feasibility of the technique were acquired with a 1.5-Tesla scanner in a phantom and in a volunteer with both saline and silicone implants in vivo. RESULTS: The new technique provided water-specific and silicone-specific images of diagnostic quality. Separation of the water and silicone chemical species was complete and satisfactory. Compared with a chemical shift-selective technique, the new technique does not rely heavily on field homogeneity and requires the same or even less scan time to acquire images with similar scan parameters, resolution, and signal-to-noise ratio (SNR). CONCLUSION: The feasibility and potential application of the new technique were demonstrated via imaging a phantom and a silicone breast prosthesis in vivo, and it may be used for more consistent imaging of the silicone implants without compromising the image quality or overall scan time.     

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.     

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.     

Fast three-point dixon MR imaging using low-resolution images for phase correction: a comparison with chemical shift selective fat suppression for pediatric musculoskeletal imaging

OBJECTIVE: The purpose of this study is to describe and to implement a new fast three-point Dixon MR imaging sequence with online image reconstruction, and to compare this sequence with conventional chemical shift selective (CHESS) suppression of fat in pediatric musculoskeletal imaging. SUBJECTS AND METHODS: A three-point Dixon technique using a fast spin-echo sequence with a new phase-correction algorithm providing online image reconstruction was implemented on a 1.5-T scanner. Twelve pediatric patients and young adults were imaged with both the new three-point Dixon and conventional CHESS sequences. Three radiologists un-aware of imaging parameters and clinical information independently scored the homogeneity of fat suppression and conspicuity of abnormality using a four-point system. An additional comparison between the two techniques was made using a phantom. RESULTS: The three-point Dixon method showed superior fat suppression and lesion conspicuity (p < 0.001), particularly in the hands and feet, where CHESS is prone to inconsistent fat suppression. The phantom study showed no significant difference in the ratio of suppressed fat signal to background noise and more homogeneous fat suppression using the three-point Dixon method. CONCLUSION: Compared with CHESS, the new fast three-point Dixon sequence with online image reconstruction provides superior fat suppression and lesion conspicuity and can be routinely used in pediatric musculoskeletal imaging.     

Three-dimensional water/fat separation and T2* estimation based on whole-image optimization--application in breathhold liver imaging at 1.5 T

The chemical shift of water and fat resonances in proton MRI allows separation of water and fat signal from chemical shift encoded data. This work describes an automatic method that produces separate water and fat images as well as quantitative maps of fat signal fraction and T2* from complex multiecho gradient-recalled datasets. Accurate water and fat separation is challenging due to signal ambiguity at the voxel level. Whole-image optimization can resolve this ambiguity, but might be computationally demanding, especially for three-dimensional data. In this work, periodicity of the model fit residual as a function of the off-resonance was used to modify a previously proposed formulation of the problem. This gives a smaller solution space and allows rapid optimization. Feasibility and accurate separation of water and fat signal were demonstrated in breathhold three-dimensional liver imaging of 10 volunteer subjects, with both acquisition and reconstruction times below 20 s.     

Interactive continuously moving table (iCMT) large field-of-view real-time MRI.

Continuously moving table (CMT) MRI is a new method that is capable of generating 3D, seamless, large field-of-view (FOV) images by acquiring readouts along the patient superior-inferior axis as the subject is translated through the scanner. For applications that require artifact-free images, such as arterial-phase contrast-enhanced (CE) angiography of the legs, a major challenge is to match the MR data acquisition and patient table motion with the dynamics of blood flow in the region of interest (ROI). Instead of restricting the CMT to predetermined constant table speeds, we adopted a more general approach in which the table motion is decoupled from the phase-encoding order. In our approach the table moves adaptively and in response to operator-provided feedback obtained from viewing real-time preview (or fluoroscopic) images. This interactivity is accomplished by integrating high temporal-spatial resolution encoding of the table position with real-time hybrid-space filling and image reconstruction. Experimental results obtained using our prototype interactive CMT (iCMT) system on a peripheral vascular phantom and five healthy volunteers demonstrate the feasibility of this robust and rapid imaging method for acquiring 3D large-FOV continuous images with patient-specific adaptive table motion profiles.     

Multipoint Dixon technique for water and fat proton and susceptibility imaging.

Extensions to a previously described three-point Dixon magnetic resonance imaging technique are presented that use alternative water/fat phase-encoding strategies. The technique is generalized to phase encoding of (-theta, O, theta) or (O, theta, 2 theta) radians, and the signal-to-noise ratio (S/N) performance is evaluated. It was found that a theta of 2 pi/3 radians has optimal S/N but that a theta of pi radians is a good compromise and that phase encoding of (O, pi, 2 pi) radians offer an advantage over the previous method, which used (-pi, O, pi) increments, in that a T2' (intravoxel susceptibility dephasing) image may be obtained in addition to the usual water, fat, and Bo images. A new four-point method with phase encoding of (O, pi, 2 pi, 3 pi) radians that can also provide a measure of the spectral width of the fat resonance is suggested. The disadvantages of the method are the extra imaging time and low S/N efficiency.     

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.     

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 .     

The utility of chemical shift imaging and related fat suppression as standalone technique in cryptorchidism using low field MRI

Cryptorchidism is failure of one or both testes to descend completely into the scrotum. The testis can be located anywhere along its journey of descent, between the lower pole of the kidney and the inguinal canal. Objective: The aim of this study was to assess the effectiveness of chemical shift imaging and Dixon based fat suppression using low field MRI in non-palpable undescended testicle. Patients and methods: From July 2017 through February 2018, Twenty eight boys, presented by either unilateral or bilateral cryptorchidism, with total number of forty testicles, underwent MRI study using low field machine in T1-weighted dual gradient-echo in-phase and opposed-phase sequence with Dixon based fat suppression. Results: Based on the laparoscopic/operative data, twenty one testes were located at the inguinal region, whereas fifteen testes were pelvi-abdominal and four were absent. The whole image sets of CSI and Dixon fat suppression had the highest specificity and positive predictive value (100%) and the highest overall accuracy (95%) for detection of undescended testes. Conclusion: chemical shift imaging combined with Dixon based fat suppression is reliable imaging tool as a standalone technique for evaluating cryptorchidism, providing high specificity and diagnostic accuracy.     

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.     

Selective missing pulse steady state free precession (MP-SSFP): inner volume and chemical shift selective imaging in a steady state sequence

PURPOSE: To examine new sequences that restrict acquisition of spins to those excited by both of the RF pulses in missing pulse steady state free precession (MP-SSFP) MRI. MATERIALS AND METHODS: New MP-SSFP sequences were created by replacing one of the slice selective pulses (SSPs) with an orthogonal SSP for inner volume imaging, and with a chemical shift selective (CHESS) pulse for chemical shift imaging. The inner volume sequence was applied to a reduced field of view at the center of a resolution phantom; resulting images were evaluated for differences in the aliased signal. The CHESS sequence was applied to volunteers, as well as to and fat, water, and acetic acid phantoms. Results were evaluated with SNR measurements. RESULTS: The inner volume sequence eliminated the aliased signal, while nonselected fat and water levels were suppressed to that of noise by the CHESS sequence. CONCLUSION: Results suggest a novel steady state technique for rapid inner volume or chemical shift imaging.     

Body MRI using IDEAL

OBJECTIVE: The intrinsic differences of water and fat protons in the MR environment allow selective interrogation of their contribution to the MR signal. Fat-suppression techniques and chemical shift imaging are routinely used in clinical body MRI. Iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) is a novel imaging technique for separating fat and water. CONCLUSION: This article describes the basic principles of IDEAL MRI and illustrates the use of IDEAL imaging as an alternative to fat-suppression techniques and chemical shift imaging for body MRI.     

Continuously moving table SENSE imaging.

A combination of continuously moving table imaging and parallel imaging based on sensitivity encoding (SENSE) is presented. One specific geometry is considered, where the receiver array is fixed to the MR magnet and does not move with the table, which allows for head-to-toe imaging with a small total number of coils. Sensitivity maps are defined for the enlarged virtual field of view and are composed according to the k-space sampling scheme such that established parallel reconstruction techniques are applicable to good approximation. In vivo experiments show the feasibility of this approach, and simulations determine the application range. Three-dimensional head-to-toe imaging of volunteers is performed in 77 s with a SENSE reduction factor of 2 in a virtual field of view of 1800 x 460 x 100 mm(3).     

Whole-body magnetic resonance imaging featuring moving table continuous data acquisition with high-precision position feedback.

A novel setup for whole-body MR imaging with moving table continuous data acquisition has been developed and evaluated. The setup features a manually positioned moving table platform with integrated phased-array surface radiofrequency coils. A high-precision laser position sensor was integrated into the system to provide real-time positional data that were used to compensate for nonlinear manual table translation. This setup enables continuous 2D and 3D whole-body data acquisition during table movement with surface coil image quality. The concept has been successfully evaluated with whole-body steady-state free precession (TrueFISP) anatomic imaging in five healthy volunteers. Seamless coronal and sagittal slices of continually acquired whole-body data during table movement were accurately reconstructed. The proposed strategy is potentially useful for a variety of applications, including whole-body metastasis screening, whole-body MR angiography, large field-of-view imaging in short bore systems, and for moving table applications during MR-guided interventions.     

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.     

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

Purpose

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

Materials and Methods

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

Results

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

Conclusion

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