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

Trajectory correction enables free-running chemical shift encoded imaging for accurate cardiac proton-density fat fraction quantification at 3T

BACKGROUND

Metabolic diseases can negatively alter epicardial fat accumulation and composition, which can be probed using quantitative cardiac chemical shift encoded (CSE) cardiovascular magnetic resonance (CMR) by mapping proton-density fat fraction (PDFF). To obtain motion-resolved high-resolution PDFF maps, we proposed a free-running cardiac CSE-CMR framework at 3T. To employ faster bipolar readout gradients, a correction for gradient imperfections was added using the gradient impulse response function (GIRF) and evaluated on intermediate images and PDFF quantification.

METHODS

Ten minutes free-running cardiac 3D radial CSE-CMR acquisitions were compared in vitro and in vivo at 3T. Monopolar and bipolar readout gradient schemes provided 8 echoes (TE1/ΔTE = 1.16/1.96 ms) and 13 echoes (TE1/ΔTE = 1.12/1.07 ms), respectively. Bipolar-gradient free-running cardiac fat and water images and PDFF maps were reconstructed with or without GIRF correction. PDFF values were evaluated in silico, in vitro on a fat/water phantom, and in vivo in 10 healthy volunteers and 3 diabetic patients.

RESULTS

In monopolar mode, fat-water swaps were demonstrated in silico and confirmed in vitro. Using bipolar readout gradients, PDFF quantification was reliable and accurate with GIRF correction with a mean bias of 0.03% in silico and 0.36% in vitro while it suffered from artifacts without correction, leading to a PDFF bias of 4.9% in vitro and swaps in vivo. Using bipolar readout gradients, in vivo PDFF of epicardial adipose tissue was significantly lower compared to subcutaneous fat (80.4 ± 7.1% vs 92.5 ± 4.3%, P < 0.0001).

CONCLUSIONS

Aiming for an accurate PDFF quantification, high-resolution free-running cardiac CSE-MRI imaging proved to benefit from bipolar echoes with k-space trajectory correction at 3T. This free-breathing acquisition framework enables to investigate epicardial adipose tissue PDFF in metabolic diseases.

Cardio-respiratory motion-corrected 3D cardiac water-fat MRI using model-based image reconstruction

PURPOSE

Myocardial fat infiltrations are associated with a range of cardiomyopathies. The purpose of this study was to perform cardio-respiratory motion-correction for model-based water-fat separation to image fatty infiltrations of the heart in a free-breathing, non-cardiac-triggered high-resolution 3D MRI acquisition.

METHODS

Data were acquired in nine patients using a free-breathing, non-cardiac-triggered high-resolution 3D Dixon gradient-echo sequence and radial phase encoding trajectory. Motion correction was combined with a model-based water-fat reconstruction approach. Respiratory and cardiac motion models were estimated using a dual-mode registration algorithm incorporating both motion-resolved water and fat information. Qualitative comparisons of fat structures were made between 2D clinical routine reference scans and reformatted 3D motion-corrected images. To evaluate the effect of motion correction the local sharpness of epicardial fat structures was analyzed for motion-averaged and motion-corrected fat images.

RESULTS

The reformatted 3D motion-corrected reconstructions yielded qualitatively comparable fat structures and fat structure sharpness in the heart as the standard 2D breath-hold. Respiratory motion correction improved the local sharpness on average by 32% ± 24% with maximum improvements of 81% and cardiac motion correction increased the sharpness further by another 15% ± 11% with maximum increases of 31%. One patient showed a fat infiltration in the myocardium and cardio-respiratory motion correction was able to improve its visualization in 3D.

CONCLUSION

The 3D water-fat separated cardiac images were acquired during free-breathing and in a clinically feasible and predictable scan time. Compared to a motion-averaged reconstruction an increase in sharpness of fat structures by 51% ± 27% using the presented motion correction approach was observed for nine patients.

Real-Time Magnetic Resonance Imaging

Real-time magnetic resonance imaging (RT-MRI) allows for imaging dynamic processes as they occur, without relying on any repetition or synchronization. This is made possible by modern MRI technology such as fast-switching gradients and parallel imaging. It is compatible with many (but not all) MRI sequences, including spoiled gradient echo, balanced steady-state free precession, and single-shot rapid acquisition with relaxation enhancement. RT-MRI has earned an important role in both diagnostic imaging and image guidance of invasive procedures. Its unique diagnostic value is prominent in areas of the body that undergo substantial and often irregular motion, such as the heart, gastrointestinal system, upper airway vocal tract, and joints. Its value in interventional procedure guidance is prominent for procedures that require multiple forms of soft-tissue contrast, as well as flow information. In this review, we discuss the history of RT-MRI, fundamental tradeoffs, enabling technology, established applications, and current trends. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY STAGE: 1.

Improving deuterium metabolic imaging (DMI) signal-to-noise ratio by spectroscopic multi-echo bSSFP: A pancreatic cancer investigation

PURPOSE

Deuterium metabolic imaging (DMI) maps the uptake of deuterated precursors and their conversion into lactate and other markers of tumor metabolism. Even after leveraging ² H's short T₁ s, DMI's signal-to-noise ratio (SNR) is limited. We hypothesize that a multi-echo balanced steady-state free precession (ME-bSSFP) approach would increase SNR compared to chemical shift imaging (CSI), while achieving spectral isolation of the metabolic precursors and products.

METHODS

Suitably tuned ² H ME-bSSFP (five echo times [TEs], ΔTE = 2.2 ms, repetition time [TR]/flip-angle = 12 ms/60°) was implemented at 15.2T and compared to CSI (TR/flip-angle = 95 ms/90°) regarding SNR and spectral isolation, in simulations, in deuterated phantoms and for the in vivo diagnosis of a mouse tumor model of pancreatic adenocarcinoma (N = 10).

RESULTS

Simulations predicted an SNR increase vs. CSI of 3-5, and that the peaks of ² H-water, ² H_6,6' -glucose, and ² H_3,3' -lactate can be well isolated by ME-bSSFP; phantoms confirmed this. In vivo, at equal spatial resolution (1.25 × 1.25 mm² ) and scan time (10 min), ² H_6,6' -glucose's and ² H_3,3' -lactate's SNR were indeed higher for bSSFP than for CSI, three-fold for glucose (57 ± 30 vs. 19 ± 11, P < .001), doubled for water (13 ± 5 vs. 7 ± 3, P = .005). The time courses and overall localization of all metabolites agreed well, comparing CSI against ME-bSSFP. However, a clearer localization of glucose in kidneys and bladder, the detection of glucose-avid rims in certain tumors, and a heterogeneous pattern of intra-tumor lactate production could only be observed using ME-bSSFP's higher resolution.

CONCLUSIONS

ME-bSSFP provides greater SNR per unit time than CSI, providing for higher spatial resolution mapping of glucose uptake and lactate production in tumors.

Preliminary analysis of interaction of the fat fraction in the sacroiliac joint among sex, age, and body mass index in a normal Chinese population

Objective

Iterative decomposition of water and fat with echo asymmetry and least-squares estimation-iron quantification (IDEAL-IQ) is a noninvasive and objective method used to quantitatively measure fat content. Although this technique has been used in the entire abdomen, IDEAL-IQ findings in the sacroiliac joint (SIJ) have rarely been reported. This preclinical study was performed to quantify the amount of fat in the SIJ in healthy volunteers by IDEAL-IQ.

Methods

From April to November 2017, 60 healthy volunteers with low back pain were included in this retrospective study. The participants were allocated into groups by age (15–30, 31–50, and ≥51 years), sex (male and female), and body mass index (BMI) (<18.5, 18.5–23.9, and ≥24.0 kg/m²). The iliac-side (Fi) and sacral-side (Fs) fat fractions were obtained in all groups. Two- and three-factor multivariate analyses were performed to analyze the effects of sex, age, and BMI on the Fi and Fs.

Results

The interaction among sex, age, and BMI had no statistically significant effect on the dependent variable. Both Fi and Fs were significantly influenced by age. Fs was significantly influenced by sex.

Conclusion

The IDEAL-IQ sequence can be used to quantitatively assess the SIJ fat content in healthy volunteers.

Myocardial T1 and T2 quantification and water-fat separation using cardiac MR fingerprinting with rosette trajectories at 3T and 1.5T

PURPOSE

This work aims to develop an approach for simultaneous water-fat separation and myocardial T₁ and T₂ quantification based on the cardiac MR fingerprinting (cMRF) framework with rosette trajectories at 3T and 1.5T.

METHODS

Two 15-heartbeat cMRF sequences with different rosette trajectories designed for water-fat separation at 3T and 1.5T were implemented. Water T₁ and T₂ maps, water image, and fat image were generated with B₀ inhomogeneity correction using a B₀ map derived from the cMRF data themselves. The proposed water-fat separation rosette cMRF approach was validated in the International Society for Magnetic Resonance in Medicine/National Institute of Standards and Technology MRI system phantom and water/oil phantoms. It was also applied for myocardial tissue mapping of healthy subjects at both 3T and 1.5T.

RESULTS

Water T₁ and T₂ values measured using rosette cMRF in the International Society for Magnetic Resonance in Medicine/National Institute of Standards and Technology phantom agreed well with the reference values. In the water/oil phantom, oil was well suppressed in the water images and vice versa. Rosette cMRF yielded comparable T₁ but 2~3 ms higher T₂ values in the myocardium of healthy subjects than the original spiral cMRF method. Epicardial fat deposition was also clearly shown in the fat images.

CONCLUSION

Rosette cMRF provides fat images along with myocardial T₁ and T₂ maps with significant fat suppression. This technique may improve visualization of the anatomical structure of the heart by separating water and fat and could provide value in diagnosing cardiac diseases associated with fibrofatty infiltration or epicardial fat accumulation. It also paves the way toward comprehensive myocardial tissue characterization in a single scan.

Statistically Segregated k-Space Sampling for Accelerating Multiple-Acquisition MRI

A central limitation of multiple-acquisition magnetic resonance imaging (MRI) is the degradation in scan efficiency as the number of distinct datasets grows. Sparse recovery techniques can alleviate this limitation via randomly undersampled acquisitions. A frequent sampling strategy is to prescribe for each acquisition a different random pattern drawn from a common sampling density. However, naive random patterns often contain gaps or clusters across the acquisition dimension that, in turn, can degrade reconstruction quality or reduce scan efficiency. To address this problem, a statistically segregated sampling method is proposed for multiple-acquisition MRI. This method generates multiple patterns sequentially while adaptively modifying the sampling density to minimize k-space overlap across patterns. As a result, it improves incoherence across acquisitions while still maintaining similar sampling density across the radial dimension of k-space. Comprehensive simulations and in vivo results are presented for phase-cycled balanced steady-state free precession and multi-echo [Formula: see text]-weighted imaging. Segregated sampling achieves significantly improved quality in both Fourier and compressed-sensing reconstructions of multiple-acquisition datasets.

A chemical shift encoding (CSE) approach for spectral selection in fluorine-19 MRI

PURPOSE

To develop a chemical shift encoding (CSE) approach for fluorine-19 MRI of perfluorocarbons in the presence of multiple known fluorinated chemical species.

THEORY AND METHODS

A multi-echo CSE technique is applied for spectral separation of the perfluorocarbon perfluoro-15-crown-5-ether (PFCE) and isoflurane (ISO) based on their chemical shifts at 4.7 T. Cramér-Rao lower bound analysis is used to identify echo combinations with optimal signal-to-noise performance. Signal contributions are fit with a multispectral fluorine signal model using a non-linear least squares estimation reconstruction directly from k-space data. This CSE approach is tested in fluorine-19 phantoms and in a mouse with a 2D and 3D spoiled gradient-echo acquisition using multiple echo times determined from Cramér-Rao lower bound analysis.

RESULTS

Cramér-Rao lower bound analysis for PFCE and ISO separation shows signal-to-noise performance is maximized with a 0.33 ms echo separation. A linear behavior (R²  = 0.987) between PFCE signal and known relative PFCE volume is observed in CSE reconstructed images using a mixed PFCE/ISO phantom. Effective spatial and spectral separation of PFCE and ISO is shown in phantoms and in vivo.

CONCLUSION

Feasibility of a gradient-echo CSE acquisition and image reconstruction approach with optimized noise performance is demonstrated through fluorine-19 MRI of PFCE with effective removal of ISO signal contributions. Magn Reson Med 79:2183-2189, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

3D Cartesian MRI with compressed sensing and variable view sharing using complementary poisson-disc sampling

PURPOSE

To enable robust, high spatio-temporal-resolution three-dimensional Cartesian MRI using a scheme incorporating a novel variable density random k-space sampling trajectory allowing flexible and retrospective selection of the temporal footprint with compressed sensing (CS).

METHODS

A complementary Poisson-disc k-space sampling trajectory was designed to allow view sharing and varying combinations of reduced view sharing with CS from the same prospective acquisition. These schemes were used for two-point Dixon-based dynamic contrast-enhanced MRI (DCE-MRI) of the breast and abdomen. Results were validated in vivo with a novel approach using variable-flip-angle data, which was retrospectively accelerated using the same methods but offered a ground truth.

RESULTS

In breast DCE-MRI, the temporal footprint could be reduced 2.3-fold retrospectively without introducing noticeable artifacts, improving depiction of rapidly enhancing lesions. Further, experiments with variable-flip-angle data showed that reducing view sharing improved accuracy in reconstruction and T₁ mapping. In abdominal MRI, 2.3-fold and 3.6-fold reductions in temporal footprint allowed reduced motion artifacts.

CONCLUSION

The complementary-Poisson-disc k-space sampling trajectory allowed a retrospective spatiotemporal resolution tradeoff using CS and view sharing, imparting robustness to motion and contrast enhancement. The technique was also validated using a novel approach of fully acquired variable-flip-angle acquisition. Magn Reson Med 77:1774-1785, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Enhanced phase-sensitive SSFP reconstruction for fat-water separation in phased-array acquisitions

PURPOSE

To propose and assess a method to improve the reliability of phase-sensitive fat-water separation for phased-array balanced steady-state free precession (bSSFP) acquisitions. Phase-sensitive steady-state free precession (PS-SSFP) is an efficient fat-water separation technique that detects the phase difference between neighboring bands in the bSSFP magnetization profile. However, large spatial variations in the sensitivity profiles of phased-array coils can lead to noisy phase estimates away from the coil centers, compromising tissue classification.

MATERIALS AND METHODS

We first perform region-growing phase correction in individual coil images via unsupervised selection of a fat-voxel seed near the peak of each coil's sensitivity profile. We then use an optimal linear combination of phase-corrected images to segregate fat and water signals. The proposed method was demonstrated on noncontrast-enhanced SSFP angiograms of the thigh, lower leg, and foot acquired at 1.5T using an 8-channel coil. Individual coil PS-SSFP with a common seed selection for all coils, individual coil PS-SSFP with coil-wise seed selection, PS-SSFP after coil combination, and IDEAL reconstructions were also performed. Water images reconstructed via PS-SSFP methods were compared in terms of the level of fat suppression and the similarity to reference IDEAL images (signed-rank test).

RESULTS

While tissue misclassification was broadly evident across regular PS-SSFP images, the proposed method achieved significantly higher levels of fat suppression (P < 0.005) and increased similarity to reference IDEAL images (P < 0.005).

CONCLUSION

The proposed method enhances fat-water separation in phased-array acquisitions by producing improved phase estimates across the imaging volume. J. Magn. Reson. Imaging 2016;44:148-157.

Characterization of myocardial T1-mapping bias caused by intramyocardial fat in inversion recovery and saturation recovery techniques

BACKGROUND

Quantitative measurement of T1 in the myocardium may be used to detect both focal and diffuse disease processes such as interstitial fibrosis or edema. A partial volume problem exists when a voxel in the myocardium also contains fat. Partial volume with fat occurs at tissue boundaries or within the myocardium in the case of lipomatous metaplasia of replacement fibrosis, which is commonly seen in chronic myocardial infarction. The presence of fat leads to a bias in T1 measurement. The mechanism for this artifact for widely used T1 mapping protocols using balanced steady state free precession readout and the dependence on off-resonance frequency are described in this paper.

METHODS

Simulations were performed to illustrate the behavior of mono-exponential fitting to bi-exponential mixtures of myocardium and fat with varying fat fractions. Both inversion recovery and saturation recovery imaging protocols using balanced steady state free precession are considered. In-vivo imaging with T1-mapping, water/fat separated imaging, and late enhancement imaging was performed on subjects with chronic myocardial infarction.

RESULTS

In n = 17 subjects with chronic myocardial infarction, lipomatous metaplasia is evident in 8 patients (47%). Fat fractions as low as 5% caused approximately 6% T1 elevation for the out-of-phase condition, and approximately 5% reduction of T1 for the in-phase condition. T1 bias in excess of 1000 ms was observed in lipomatous metaplasia with fat fraction of 38% in close agreement with simulation of the specific imaging protocols.

CONCLUSIONS

Measurement of the myocardial T1 by widely used balanced steady state free precession mapping methods is subject to bias when there is a mixture of water and fat in the myocardium. Intramyocardial fat is frequently present in myocardial scar tissue due lipomatous metaplasia, a process affecting myocardial infarction and some non-ischemic cardiomyopathies. In cases of lipomatous metaplasia, the T1 biases will be additive or subtractive depending on whether the center frequency corresponds to the myocardium and fat being in-phase or out-of-phase, respectively. It is important to understand this mechanism, which may otherwise lead to erroneous interpretation.

Utilization of a balanced steady state free precession signal model for improved fat/water decomposition

PURPOSE

Chemical shift based fat/water decomposition methods such as IDEAL are frequently used in challenging imaging environments with large B0 inhomogeneity. However, they do not account for the signal modulations introduced by a balanced steady state free precession (bSSFP) acquisition. Here we demonstrate improved performance when the bSSFP frequency response is properly incorporated into the multipeak spectral fat model used in the decomposition process.

THEORY AND METHODS

Balanced SSFP allows for rapid imaging but also introduces a characteristic frequency response featuring periodic nulls and pass bands. Fat spectral components in adjacent pass bands will experience bulk phase offsets and magnitude modulations that change the expected constructive and destructive interference between the fat spectral components. A bSSFP signal model was incorporated into the fat/water decomposition process and used to generate images of a fat phantom, and bilateral breast and knee images in four normal volunteers at 1.5 Tesla.

RESULTS

Incorporation of the bSSFP signal model into the decomposition process improved the performance of the fat/water decomposition.

CONCLUSION

Incorporation of this model allows rapid bSSFP imaging sequences to use robust fat/water decomposition methods such as IDEAL. While only one set of imaging parameters were presented, the method is compatible with any field strength or repetition time.

Self-gated fat-suppressed cardiac cine MRI

PURPOSE

To develop a self-gated alternating repetition time balanced steady-state free precession (ATR-SSFP) pulse sequence for fat-suppressed cardiac cine imaging.

METHODS

Cardiac gating is computed retrospectively using acquired magnetic resonance self-gating data, enabling cine imaging without the need for electrocardiogram (ECG) gating. Modification of the slice-select rephasing gradients of an ATR-SSFP sequence enables the acquisition of a one-dimensional self-gating readout during the unused short repetition time (TR). Self-gating readouts are acquired during every TR of segmented, breath-held cardiac scans. A template-matching algorithm is designed to compute cardiac trigger points from the self-gating signals, and these trigger points are used for retrospective cine reconstruction. The proposed approach is compared with ECG-gated ATR-SSFP and balanced steady-state free precession in 10 volunteers and five patients.

RESULTS

The difference of ECG and self-gating trigger times has a variability of 13 ± 11 ms (mean ± SD). Qualitative reviewer scoring and ranking indicate no statistically significant differences (P > 0.05) between self-gated and ECG-gated ATR-SSFP images. Quantitative blood-myocardial border sharpness is not significantly different among self-gated ATR-SSFP ( 0.61±0.15 mm -1), ECG-gated ATR-SSFP ( 0.61±0.15 mm -1), or conventional ECG-gated balanced steady-state free precession cine MRI ( 0.59±0.15 mm -1).

CONCLUSION

The proposed self-gated ATR-SSFP sequence enables fat-suppressed cardiac cine imaging at 1.5 T without the need for ECG gating and without decreasing the imaging efficiency of ATR-SSFP.

Fat water decomposition using globally optimal surface estimation (GOOSE) algorithm

PURPOSE

This article focuses on developing a novel noniterative fat water decomposition algorithm more robust to fat water swaps and related ambiguities.

METHODS

Field map estimation is reformulated as a constrained surface estimation problem to exploit the spatial smoothness of the field, thus minimizing the ambiguities in the recovery. Specifically, the differences in the field map-induced frequency shift between adjacent voxels are constrained to be in a finite range. The discretization of the above problem yields a graph optimization scheme, where each node of the graph is only connected with few other nodes. Thanks to the low graph connectivity, the problem is solved efficiently using a noniterative graph cut algorithm. The global minimum of the constrained optimization problem is guaranteed. The performance of the algorithm is compared with that of state-of-the-art schemes. Quantitative comparisons are also made against reference data.

RESULTS

The proposed algorithm is observed to yield more robust fat water estimates with fewer fat water swaps and better quantitative results than other state-of-the-art algorithms in a range of challenging applications.

CONCLUSION

The proposed algorithm is capable of considerably reducing the swaps in challenging fat water decomposition problems. The experiments demonstrate the benefit of using explicit smoothness constraints in field map estimation and solving the problem using a globally convergent graph-cut optimization algorithm.

Current MR imaging lipid detection techniques for diagnosis of lesions in the abdomen and pelvis

One application of the unique capability of magnetic resonance (MR) imaging for characterizing soft tissues is in the specific detection of lipid. Adipose tissue may be abundant in the body, but its presence in a lesion can greatly limit differential diagnostic considerations. This article reviews MR imaging fat detection techniques and discusses lesions in the abdomen and pelvis that can be readily diagnosed by using these techniques. Traditional fat detection methods include inversion-recovery and chemically selective fat-suppression pulse sequences, with the former being less sensitive to field heterogeneity and less tissue specific than the latter. Chemical shift-based sequences, which exploit the inherent resonance frequency difference between lipid and water to depict intracytoplasmic fat, have great utility for evaluating hepatic steatosis and lesions such as adrenal and hepatic adenomas, hepatocellular carcinoma, focal lipomatosis of the pancreas, and adrenal cortical carcinoma. The signal from large amounts of fat can be suppressed by using a narrow radiofrequency pulse for selective excitation of fat protons (ie, fat saturation imaging), a technique that increases image contrast resolution and highlights lesions such as contrast-enhancing tissue, edema, and blood products. This technique is especially useful for evaluating renal angiomyolipomas, adrenal myelolipomas, ovarian teratomas, and liposarcomas. MR spectroscopy is a promising method for quantifying absolute liver fat concentration and changes in hepatic triglyceride content during treatment. New and evolving techniques include magnetization transfer and modified Dixon sequences. A solid understanding of these techniques will help improve the interpretation of abdominal and pelvic imaging studies.

Variable spatiotemporal resolution three-dimensional Dixon sequence for rapid dynamic contrast-enhanced breast MRI

PURPOSE

To investigate a new variable spatiotemporal resolution dynamic contrast-enhanced (DCE) MRI method termed DIfferential Subsampling with Cartesian Ordering (DISCO), for imaging of breast cancer.

MATERIALS AND METHODS

DISCO combines variable density, pseudorandom k-space segmentation and two-point Dixon fat-water separation for high spatiotemporal resolution breast DCE MRI. During the contrast wash-in phase, view sharing is used to achieve high temporal resolution. Forty patients referred for breast MRI were imaged, 26 using the proposed DISCO sequence and 14 using a conventional low-spatial-resolution dynamic sequence (VIBRANT-FLEX) on a 3 Tesla scanner. DISCO dynamic images from 14 patients were compared with VIBRANT-FLEX images from 14 other patients. The image quality assessed by radiologist image ranking in a blinded manner, and the temporal characteristics of the two sequences were compared.

RESULTS

A spatial resolution of 1.1 × 1.1 × 1.2 mm(3) (160 slices, 28 cm field of view) was achieved with axial bilateral coverage in 120 s. Dynamic images with ∼ 9 s effective temporal resolution were generated during the 2-min contrast wash-in phase. The image quality of DISCO dynamic images ranked significantly higher than low spatial resolution VIBRANT-FLEX images (19.5 versus 9.5, Mann-Whitney U-test P = 0.00914), with no significant differences in the maximum slope of aortic enhancement.

CONCLUSION

DISCO is a promising variable-spatiotemporal-resolution imaging sequence for capturing the dynamics of rapidly enhancing tumors as well as structural features postcontrast. A near 1-mm isotropic spatial resolution was achieved with postcontrast static phase images in 120 s and dynamic phase images acquired in 9 s per phase.

Whole-heart chemical shift encoded water-fat MRI

PURPOSE

To develop and evaluate a free-breathing chemical-shift-encoded (CSE) spoiled gradient-recalled echo (SPGR) technique for whole-heart water-fat imaging at 3 Tesla (T).

METHODS

We developed a three-dimensional (3D) multi-echo SPGR pulse sequence with electrocardiographic gating and navigator echoes and evaluated its performance at 3T in healthy volunteers (N = 6) and patients (N = 20). CSE-SPGR, 3D SPGR, and 3D balanced-SSFP with chemical fat saturation were compared in six healthy subjects with images evaluated for overall image quality, level of residual artifacts, and quality of fat suppression. A similar scoring system was used for the patient datasets.

RESULTS

Images of diagnostic quality were acquired in all but one subject. CSE-SPGR performed similarly to SPGR with fat saturation, although it provided a more uniform fat suppression over the whole field of view. Balanced-SSFP performed worse than SPGR-based methods. In patients, CSE-SPGR produced excellent fat suppression near metal. Overall image quality was either good (7/20) or excellent (12/20) in all but one patient. There were significant artifacts in 5/20 clinical cases.

CONCLUSION

CSE-SPGR is a promising technique for whole-heart water-fat imaging during free-breathing. The robust fat suppression in the water-only image could improve assessment of complex morphology at 3T and in the presence of off-resonance, with additional information contained in the fat-only image.

Water-fat separation imaging of the heart with standard magnetic resonance bSSFP CINE imaging

PURPOSE

To study balanced steady-state free precession CINE phase-sensitive water-fat separation imaging in four cardiac imaging planes to determine the necessary phase correction and image artifacts particular to this technique.

METHODS

Ten healthy volunteers and two subjects with known heart pathologies were studied with standard balanced steady-state free precession CINE imaging. Water-only and fat-only images were calculated using sign detection of the real part of the complex image after phase correction with constant and linear terms. Phase correction values were determined using both manual and automated methods. Differences in phase correction values between imaging planes, cardiac phases, coil elements, automated image reconstruction parameters as well as artifact scores between the automated and manual methods were studied with statistical tests.

RESULTS

Water-fat separation performed well in the heart after constant and linear phase correction. Both constant (p = 0.8) and linear x (p = 1) and y (p = 1) phase correction values did not vary significantly across cardiac phases, but varied significantly among the coils (p < 0.001) and imaging planes (p < 0.001). False water-fat separation artifacts were most frequent in the chest/back and also were present at the mitral and aortic valves.

CONCLUSION

Constant and linear phase correction is necessary to provide consistent results in standard imaging planes using a balanced steady-state free precession water-fat separation postprocessing algorithm applied to standard cardiac CINE imaging.

Fat deposition in dilated cardiomyopathy assessed by CMR

OBJECTIVES

The aim of this study was to prospectively investigate the prevalence of fat deposition in idiopathic dilated cardiomyopathy (DCM) by fat-water separation imaging. An auxiliary aim was to determine the relationship between left ventricular (LV) fat deposition and characteristic myocardial fibrosis, as well as cardiac functional parameters.

BACKGROUND

Idiopathic DCM remains the most common cause of heart failure in young people referred for cardiac transplantation; little is known about the clinical value of fat deposition in DCM.

METHODS

A total of 124 patients with DCM were studied after written informed consent was obtained. The magnetic resonance imaging scan protocols included a series of short-axis LV cine imaging for functional analysis, fat-water separation imaging, and late gadolinium enhancement (LGE) imaging. Fat deposition and fibrosis location were compared to the scar regions on LGE images using a 17-segment model. Statistical comparisons of LV global functional parameters, fibrosis volumes, and fat deposition were carried out using the Pearson correlation, Student t test, and multiple regressions.

RESULTS

The patients had a 41.9% (52 of 124) prevalence of positive LGE, and 12.9% (16 of 124) fat deposition prevalence was found in this DCM cohort. The patients with fat deposition had larger LV end-diastolic volume (LVEDV) index (140.8 ± 20.2 ml/m(2) vs. 123.4 ± 15.8 ml/m(2); p < 0.01), larger LV end-systolic volume (LVESV) index (111.3 ± 19.2 ml/m(2) vs. 87.0 ± 20.3 ml/m(2); p < 0.01), and decreased LV ejection fraction (LVEF) (21.1 ± 7.1% vs. 30.0 ± 10.7%; p < 0.01). Higher volumes of LGE were found in the group with myocardial fat deposition (18.39 ± 9.0 ml vs. 13.40 ± 6.54 ml; p = 0.001), as well as a higher percentage of LGE/LV mass (19.11 ± 7.78% vs. 13.60 ± 4.58%; p = 0.000). The volume of fat deposition was correlated with scar volume, LVEF, LVEDV index, and LVESV index.

CONCLUSIONS

Fat deposition is a common phenomenon in DCM, and it is associated with DCM characteristics such as fibrosis volume and LV function.

Fat fraction bias correction using T1 estimates and flip angle mapping

PURPOSE

To develop a new method of reducing T1 bias in proton density fat fraction (PDFF) measured with iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL).

MATERIALS AND METHODS

PDFF maps reconstructed from high flip angle IDEAL measurements were simulated and acquired from phantoms and volunteer L4 vertebrae. T1 bias was corrected using a priori T1 values for water and fat, both with and without flip angle correction. Signal-to-noise ratio (SNR) maps were used to measure precision of the reconstructed PDFF maps. PDFF measurements acquired using small flip angles were then compared to both sets of corrected large flip angle measurements for accuracy and precision.

RESULTS

Simulations show similar results in PDFF error between small flip angle measurements and corrected large flip angle measurements as long as T1 estimates were within one standard deviation from the true value. Compared to low flip angle measurements, phantom and in vivo measurements demonstrate better precision and accuracy in PDFF measurements if images were acquired at a high flip angle, with T1 bias corrected using T1 estimates and flip angle mapping.

CONCLUSION

T1 bias correction of large flip angle acquisitions using estimated T1 values with flip angle mapping yields fat fraction measurements of similar accuracy and superior precision compared to low flip angle acquisitions.

High-resolution 3D radial bSSFP with IDEAL

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

Cardiac imaging techniques for physicians: late enhancement

Late enhancement imaging is used to diagnose and characterize a wide range of ischemic and nonischemic cardiomyopathies, and its use has become ubiquitous in the cardiac MR exam. As the use of late enhancement imaging has matured and the span of applications has widened, the demands on image quality have grown. The characterization of subendocardial MI now includes the accurate quantification of scar size, shape, and characterization of borders which have been shown to have prognostic significance. More diverse patterns of late enhancement including patchy, mid-wall, subepicardial, or diffuse enhancement are of interest in diagnosing nonischemic cardiomyopathies. As clinicians are examining late enhancement images for more subtle indication of fibrosis, the demand for lower artifacts has increased. A range of new techniques have emerged to improve the speed and quality of late enhancement imaging including: methods for acquisition during free breathing, and fat water separated imaging for characterizing fibrofatty infiltration and reduction of artifacts related to the presence of fat. Methods for quantification of T1 and extracellular volume fraction are emerging to tackle the issue of discriminating globally diffuse fibrosis from normal healthy tissue which is challenging using conventional late enhancement methods. The aim of this review will be to describe the current state of the art and to provide a guide to various clinical protocols that are commonly used.

R2* estimation using "in-phase" echoes in the presence of fat: the effects of complex spectrum of fat

PURPOSE

To investigate R2* mapping robustness in the presence of fat using in-phase echoes, without and with spectral modeling of fat (single-peak and multipeak models, respectively), using varying numbers of echoes.

MATERIALS AND METHODS

Data from 88 volunteers (men/women: 52/36, ages: 55.4 ± 12.2) were randomly chosen according to magnetic resonance imaging (MRI) liver fat-fraction (%), and classified into six fat-fraction groups (1: 20 cases, 0%-<10%; 2: 20 cases, 10%-<20%; 3: 20 cases, 20%-<30%; 4: 20 cases, 30%-<40%; 5: 8 cases >40% liver fat; 6: subcutaneous fat from all cases). R2* maps obtained from five in-phase echoes (echo times: 4.8-23.8 msec) were retrospectively reconstructed using single-peak and multipeak fat modeling. R2* maps were also calculated using different numbers (2-5) of echoes.

RESULTS

Multipeak fat corrected R2* mapping is feasible from in-phase echoes, with noise performance comparable to single-peak R2* when using ≥ 4 echoes. Single-peak R2* showed poor robustness to varying echo time combinations in the presence of fat, where using few echoes resulted in large errors. These errors can be reduced using more echoes, or fully corrected using multipeak fat modeling. The mean R2* increased significantly with increasing fat-fraction when using single-peak R2* for any TE combination (P < 0.001), but did not vary when using multipeak R2* for any TE combination (P ≥ 0.158).

CONCLUSION

R2* mapping uncorrected for spectral complexity of fat contains protocol and fat-dependent errors (lack of robustness) in tissues with high fat content. Accounting for complex fat spectrum improves robustness and accuracy of signal fitting, with modest noise performance loss.

Two-dimensional sixteen channel transmit/receive coil array for cardiac MRI at 7.0 T: design, evaluation, and application

PURPOSE

To design, evaluate, and apply a 2D 16-channel transmit/receive (TX/RX) coil array tailored for cardiac magnetic resonance imaging (MRI) at 7.0 T.

MATERIALS AND METHODS

The cardiac coil array consists of two sections each using eight elements arranged in a 2 × 4 array. Radiofrequency (RF) safety was validated by specific absorption rate (SAR) simulations. Cardiac imaging was performed using 2D CINE FLASH imaging, T 2 mapping, and fat-water separation imaging. The characteristics of the coil array were analyzed including parallel imaging performance, left ventricular chamber quantification, and overall image quality.

RESULTS

RF characteristics were found to be appropriate for all subjects included in the study. The SAR values derived from the simulations fall well within the limits of legal guidelines. The baseline signal-to-noise ratio (SNR) advantage at 7.0 T was put to use to acquire 2D CINE images of the heart with a very high spatial resolution of (1 × 1 × 4) mm(3) . The proposed coil array supports 1D acceleration factors of up to R = 4 without significantly impairing image quality.

CONCLUSION

The 16-channel TX/RX coil has the capability to acquire high contrast and high spatial resolution images of the heart at 7.0 T.

Recovery of chemical estimates by field inhomogeneity neighborhood error detection (REFINED): fat/water separation at 7 tesla

PURPOSE

To reduce swaps in fat-water separation methods, a particular issue on 7 Tesla (T) small animal scanners due to field inhomogeneity, using image postprocessing innovations that detect and correct errors in the B0 field map.

MATERIALS AND METHODS

Fat-water decompositions and B0 field maps were computed for images of mice acquired on a 7T Bruker BioSpec scanner, using a computationally efficient method for solving the Markov Random Field formulation of the multi-point Dixon model. The B0 field maps were processed with a novel hole-filling method, based on edge strength between regions, and a novel k-means method, based on field-map intensities, which were iteratively applied to automatically detect and reinitialize error regions in the B0 field maps. Errors were manually assessed in the B0 field maps and chemical parameter maps both before and after error correction.

RESULTS

Partial swaps were found in 6% of images when processed with FLAWLESS. After REFINED correction, only 0.7% of images contained partial swaps, resulting in an 88% decrease in error rate. Complete swaps were not problematic.

CONCLUSION

Ex post facto error correction is a viable supplement to a priori techniques for producing globally smooth B0 field maps, without partial swaps. With our processing pipeline, it is possible to process image volumes rapidly, robustly, and almost automatically.

DIfferential Subsampling with Cartesian Ordering (DISCO): a high spatio-temporal resolution Dixon imaging sequence for multiphasic contrast enhanced abdominal imaging

PURPOSE

To develop and evaluate a multiphasic contrast-enhanced MRI method called DIfferential Sub-sampling with Cartesian Ordering (DISCO) for abdominal imaging.

MATERIALS AND METHODS

A three-dimensional, variable density pseudo-random k-space segmentation scheme was developed and combined with a Dixon-based fat-water separation algorithm to generate high temporal resolution images with robust fat suppression and without compromise in spatial resolution or coverage. With institutional review board approval and informed consent, 11 consecutive patients referred for abdominal MRI at 3 Tesla (T) were imaged with both DISCO and a routine clinical three-dimensional SPGR-Dixon (LAVA FLEX) sequence. All images were graded by two radiologists using quality of fat suppression, severity of artifacts, and overall image quality as scoring criteria. For assessment of arterial phase capture efficiency, the number of temporal phases with angiographic phase and hepatic arterial phase was recorded.

RESULTS

There were no significant differences in quality of fat suppression, artifact severity or overall image quality between DISCO and LAVA FLEX images (P > 0.05, Wilcoxon signed rank test). The angiographic and arterial phases were captured in all 11 patients scanned using the DISCO acquisition (mean number of phases were two and three, respectively).

CONCLUSION

DISCO effectively captures the fast dynamics of abdominal pathology such as hyperenhancing hepatic lesions with a high spatio-temporal resolution. Typically, 1.1 × 1.5 × 3 mm spatial resolution over 60 slices was achieved with a temporal resolution of 4-5 s.

MR imaging of the spine at 3.0T with T2-weighted IDEAL fast recovery fast spin-echo technique

Objective

To compare the iterative decomposition of water and fat with echo asymmetry and the least-squares estimation (IDEAL) method with a fat-saturated T2-weighted (T2W) fast recovery fast spin-echo (FRFSE) imaging of the spine.

Materials and Methods

Images acquired at 3.0 Tesla (T) in 35 patients with different spine lesions using fat-saturated T2W FRFSE imaging were compared with T2W IDEAL FRFSE images. Signal-to-noise ratio (SNR)-efficiencies measurements were made in the vertebral bodies and spinal cord in the mid-sagittal plane or nearest to the mid-sagittal plane. Images were scored with the consensus of two experienced radiologists on a four-point grading scale for fat suppression and overall image quality. Statistical analysis of SNR-efficiency, fat suppression and image quality scores was performed with a paired Student's t test and Wilcoxon's signed rank test.

Results

Signal-to-noise ratio-efficiency for both vertebral body and spinal cord was higher with T2W IDEAL FRFSE imaging (p < 0.05) than with T2W FRFSE imaging. T2W IDEAL FRFSE demonstrated superior fat suppression (p < 0.01) and image quality (p < 0.01) compared to fat-saturated T2W FRFSE.

Conclusion

As compared with fat-saturated T2W FRFSE, IDEAL can provide a higher image quality, higher SNR-efficiency, and consistent, robust and uniform fat suppression. T2W IDEAL FRFSE is a promising technique for MR imaging of the spine at 3.0T.

R*(2) mapping in the presence of macroscopic B₀ field variations

R₂ mapping has important applications in MRI, including functional imaging, tracking of super-paramagnetic particles, and measurement of tissue iron levels. However, R₂ measurements can be confounded by several effects, particularly the presence of fat and macroscopic B₀ field variations. Fat introduces additional modulations in the signal. Macroscopic field variations introduce additional dephasing that results in accelerated signal decay. These effects produce systematic errors in the resulting R₂ maps and make the estimated R₂ values dependent on the acquisition parameters. In this study, we develop a complex-reconstruction, confounder-corrected R₂ mapping technique, which addresses the presence of fat and macroscopic field variations for both 2D and 3D acquisitions. This technique extends previous chemical shift-encoded methods for R₂, fat and water mapping by measuring and correcting for the effect of macroscopic field variations in the acquired signal. The proposed method is tested on several 2D and 3D phantom and in vivo liver, cardiac, and brain datasets.

Magnetization-prepared IDEAL bSSFP: a flow-independent technique for noncontrast-enhanced peripheral angiography

PURPOSE

To propose a new noncontrast-enhanced flow-independent angiography sequence based on balanced steady-state free precession (bSSFP) that produces reliable vessel contrast despite the reduced blood flow in the extremities.

MATERIALS AND METHODS

The proposed technique addresses a variety of factors that can compromise the exam success including insufficient background suppression, field inhomogeneity, and large volumetric coverage requirements. A bSSFP sequence yields reduced signal from venous blood when long repetition times are used. Complex-sum bSSFP acquisitions decrease the sensitivity to field inhomogeneity but retain phase information, so that data can be processed with the Iterative Decomposition of Water and Fat with Echo Asymmetry and Least-Squares Estimation (IDEAL) method for robust fat suppression. Meanwhile, frequent magnetization preparation coupled with parallel imaging reduces the muscle and long-T(1) fluid signals without compromising scan efficiency.

RESULTS

In vivo flow-independent peripheral angiograms with reliable background suppression and high spatial resolution are produced. Comparisons with phase-sensitive bSSFP angiograms (that yield out-of-phase fat and water signals, and exploit this phase difference to suppress fat) demonstrate enhanced vessel depiction with the proposed technique due to reduced partial-volume effects and improved venous suppression.

CONCLUSION

Magnetization-prepared complex-sum bSSFP with IDEAL fat/water separation can create reliable flow-independent angiographic contrast in the lower extremities.

Fat-water separation in dynamic objects using an UNFOLD-like temporal processing

PURPOSE

To separate fat and water signals in dynamic imaging. Because important features may be embedded in fat, and because fat may take part in disease processes, separating fat and water signals may be of great importance in a number of clinical applications. This work aims to achieve such separation at nearly no loss in temporal resolution compared to usual, nonseparated acquisitions. In contrast, the well-known 3-point Dixon method may cause as much as a 3-fold reduction in temporal resolution.

MATERIALS AND METHODS

The proposed approach involves modulating the echo time TE from frame to frame, to force fat signals to behave in a conspicuous manner through time, so they can be readily identified and separated from water signals. The strategy is inspired from the "unaliasing by Fourier encoding the overlaps in the temporal direction" (UNFOLD) method, although UNFOLD involves changes in the sampling function rather than TE, and aims at suppressing aliased material rather than fat.

RESULTS

The method was implemented at 1.5 T and 3 T, on cardiac cine and multiframe steady-state free precession sequences. In addition to phantom results, in vivo results from volunteers are presented.

CONCLUSION

Good separation of fat and water signals was achieved in all cases.

Fat-water selective excitation in balanced steady-state free precession using short spatial-spectral RF pulses

Fat suppression is important but challenging in balanced steady-state free precession (bSSFP) acquisitions, for a number of clinical applications. In the present work, the practicality of performing fat-water selective excitations using spatial-spectral (SPSP) RF pulses in bSSFP sequence is examined. With careful pulse design, the overall duration of these SPSP pulses was kept short to minimize detrimental effects on TR, scan time and banding artifact content. Fat-water selective excitation using SPSP pulses was demonstrated in both phantom and human bSSFP imaging at 3T, and compared to results obtained using a two-point Dixon method. The sequence with SPSP pulses performed better than the two-point Dixon method, in terms of scan time and suppression performance. Overall, it is concluded here that SPSP RF pulses do represent a viable option for fat-suppressed bSSFP imaging.

Chemical shift-based water/fat separation: a comparison of signal models

Quantitative water/fat separation in MRI requires careful modeling of the acquired signal. Multiple signal models have been proposed in recent years, but their relative performance has not yet been established. This article presents a comparative study of 12 signal models for quantitative water/fat separation. These models were selected according to three main criteria: magnitude or complex fitting, use of single-peak or multipeak fat spectrum, and modeling of T(2)(*) decay. The models were compared based on an analysis of the bias and standard deviation of their resulting estimates. Results from theoretical analysis, simulation, phantom experiments, and in vivo data were in good agreement. These results show that (a) complex fitting is uniformly superior to magnitude fitting, (b) multipeak fat modeling is able to remove the bias present in single-peak fat modeling, and (c) a single-T(2)(*) model performs best over a range of clinically relevant signal-to-noise ratios (SNRs) and water/fat ratios.

Magnetic resonance separation imaging using a divided inversion recovery technique (DIRT)

The divided inversion recovery technique is an MRI separation method based on tissue T(1) relaxation differences. When tissue T(1) relaxation times are longer than the time between inversion pulses in a segmented inversion recovery pulse sequence, longitudinal magnetization does not pass through the null point. Prior to additional inversion pulses, longitudinal magnetization may have an opposite polarity. Spatial displacement of tissues in inversion recovery balanced steady-state free-precession imaging has been shown to be due to this magnetization phase change resulting from incomplete magnetization recovery. In this paper, it is shown how this phase change can be used to provide image separation. A pulse sequence parameter, the time between inversion pulses (T180), can be adjusted to provide water-fat or fluid separation. Example water-fat and fluid separation images of the head, heart, and abdomen are presented. The water-fat separation performance was investigated by comparing image intensities in short-axis divided inversion recovery technique images of the heart. Fat, blood, and fluid signal was suppressed to the background noise level. Additionally, the separation performance was not affected by main magnetic field inhomogeneities.

Myocardial fat quantification in humans: Evaluation by two-point water-fat imaging and localized proton spectroscopy

Proton MR spectroscopy ((1)H-MRS) has been used for in vivo quantification of intracellular triglycerides within the sarcolemma. The purpose of this study was to assess whether breath-hold dual-echo in- and out-of-phase MRI at 3.0 T can quantify the fat content of the myocardium. Biases, including T(1), T*(2), and noise, that confound the calculation of the fat fraction were carefully corrected. Thirty-four of 46 participants had both MRI and MRS data. The fat fractions from MRI showed a strong correlation with fat fractions from MRS (r = 0.78; P < 0.05). The mean myocardial fat fraction for all 34 subjects was 0.7 +/- 0.5% (range: 0.11-3%) assessed with MRS and 1.04 +/- 0.4% (range: 0.32-2.44%) assessed with in- and out-of-phase MRI (P < 0.05). Scanning times were less than 15 sec for Dixon imaging, plus an additional minute for the acquisition used for T*(2) calculation, and 15-20 min for MRS. The average postprocessing time for MRS was 3 min and 5 min for MRI including T*(2) measurement. We conclude that the dual echo method provides a rapid means to detect and quantifying myocardial fat content in vivo. Correction/adjustment for field inhomogeneity using three or more echoes seems crucial for the dual echo approach.

Liver fat content determined by magnetic resonance imaging and spectroscopy

Hepatic steatosis as the most prevalent liver disorder can either be related to alcoholic liver disease (ALD) or non-alcoholic fatty liver disease (NAFLD). In both conditions, hepatocytes excessively accumulate fat-containing vacuoles within their cytoplasm, which is the key histological feature. In contrast to ALD, NAFLD is commonly associated with metabolic syndrome, obesity and insulin resistance. To determine increased liver fat content, liver biopsy is currently considered the gold standard. Besides the invasive technique, various other non-invasive techniques have been developed, such as ultrasound, computed tomography (CT), magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) based methods. Among these techniques, ultrasound and CT provide only qualitative information about hepatic steatosis, whereas MRS- or MRI-based methods are able to determine even small amounts of fat accurately. These non-invasive magnetic resonance techniques have already proven their great potential, especially in longitudinal and cross-sectional studies regarding various metabolic conditions and medical treatment regimens. In this review, the most common, non-invasive MRS/MRI techniques for assessment of intrahepatic lipid content are described with their inherent advantages and limitations.

Myocardial Fat Imaging

The presence of intramyocardial fat may form a substrate for arrhythmias, and fibrofatty infiltration of the myocardium has been shown to be associated with sudden death. Therefore, noninvasive detection could have high prognostic value. Fat-water–separated imaging in the heart by MRI is a sensitive means of detecting intramyocardial fat and characterizing fibrofatty infiltration. It is also useful in characterizing fatty tumors and delineating epicardial and/or pericardial fat. Multi-echo methods for fat and water separation provide a sensitive means of detecting small concentrations of fat with positive contrast and have a number of advantages over conventional chemical-shift fat suppression. Furthermore, fat and water–separated imaging is useful in resolving artifacts that may arise due to the presence of fat. Examples of fat-water–separated imaging of the heart are presented for patients with ischemic and nonischemic cardiomyopathies, as well as general tissue classification.

Robust water/fat separation in the presence of large field inhomogeneities using a graph cut algorithm

Water/fat separation is a classical problem for in vivo proton MRI. Although many methods have been proposed to address this problem, robust water/fat separation remains a challenge, especially in the presence of large amplitude of static field inhomogeneities. This problem is challenging because of the nonuniqueness of the solution for an isolated voxel. This paper tackles the problem using a statistically motivated formulation that jointly estimates the complete field map and the entire water/fat images. This formulation results in a difficult optimization problem that is solved effectively using a novel graph cut algorithm, based on an iterative process where all voxels are updated simultaneously. The proposed method has good theoretical properties, as well as an efficient implementation. Simulations and in vivo results are shown to highlight the properties of the proposed method and compare it to previous approaches. Twenty-five cardiac datasets acquired on a short, wide-bore scanner with different slice orientations were used to test the proposed method, which produced robust water/fat separation for these challenging datasets. This paper also shows example applications of the proposed method, such as the characterization of intramyocardial fat.

Whether depositing fat or losing weight, fish maintain a balance

In fish, the relative amount of tissues of different densities changes significantly over short periods throughout the year, depending on the availability of food, nutrition and their developmental status, such as sexual maturation. If a land-living animal accumulates fat it influences not only its general state of health, but also markedly increases its energy expenditure for locomotion owing to the force of gravity. On a body submerged in water, this force, which acts on the centre of gravity (COG), is counterbalanced by a lifting force that is negligible in air and which acts on the centre of buoyancy (COB). Any difference in the longitudinal positions of the two centres will therefore result in pitching moments that must be counteracted by body or fin movements. The displacement of the COG away from the COB is a result of tissues of different density (e.g. bones and fat) not being distributed homogeneously along the body axis. Moreover, the proportions of tissues of different densities change significantly with feeding status. It is still unknown whether these changes produce a displacement of the COG and thus affect the hydrostatic stability of fish. Analysis of computed tomography and magnetic resonance imaging images of Atlantic herring, Atlantic salmon and Atlantic mackerel reveals that the COG is fairly constant in each species, although we recorded major interspecies differences in the relative amount of fat, muscle and bone. We conclude that the distribution of different tissues along the body axis is very closely adjusted to the swimming mode of the fish by keeping the COG constant, independent of the body fat status, and that fish can cope with large variations in energy intake without jeopardizing their COG and thus their swimming performance.

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 T(2) 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.

Turboprop IDEAL: a motion-resistant fat-water separation technique

Suppression of the fat signal in MRI is very important for many clinical applications. Multi-point water-fat separation methods, such as IDEAL (Iterative Decomposition of water and fat with Echo Asymmetry and Least-squares estimation), can robustly separate water and fat signal, but inevitably increase scan time, making separated images more easily affected by patient motions. PROPELLER (Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction) and Turboprop techniques offer an effective approach to correct for motion artifacts. By combining these techniques together, we demonstrate that the new TP-IDEAL method can provide reliable water-fat separation with robust motion correction. The Turboprop sequence was modified to acquire source images, and motion correction algorithms were adjusted to assure the registration between different echo images. Theoretical calculations were performed to predict the optimal shift and spacing of the gradient echoes. Phantom images were acquired, and results were compared with regular FSE-IDEAL. Both T1- and T2-weighted images of the human brain were used to demonstrate the effectiveness of motion correction. TP-IDEAL images were also acquired for pelvis, knee, and foot, showing great potential of this technique for general clinical applications.

Algebraic decomposition of fat and water in MRI

The decomposition of magnetic resonance imaging (MRI) data to generate water and fat images has several applications in medical imaging, including fat suppression and quantification of visceral fat. We introduce a novel algorithm to overcome some of the problems associated with current analytical and iterative decomposition schemes. In contrast to traditional analytical schemes, our approach is general enough to accommodate any uniform echo-shift pattern, any number of metabolites and signal samples. In contrast to region-growing method that use a smooth field-map initialization to resolve the ambiguities with the IDEAL algorithm, we propose to use an explicit smoothness constraint on the final field-map estimate. Towards this end, we estimate the number of feasible solutions at all the voxels, prior to the evaluation of the roots. This approach enables the algorithm to evaluate all the feasible roots, thus avoiding the convergence to the wrong solution. The estimation procedure is based on a modification of the harmonic retrieval (HR) framework to account for the chemical shift dependence in the frequencies. In contrast to the standard linear HR framework, we obtain the frequency shift as the common root of a set of quadratic equations. On most of the pixels with multiple feasible solutions, the correct solution can be identified by a simple sorting of the solutions. We use a region-merging algorithm to resolve the remaining ambiguity and phase-wrapping. Experimental results indicate that the proposed algebraic scheme eliminates most of the difficulties with the current schemes, without compromising the noise performance. Moreover, the proposed algorithm is also computationally more efficient.

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.

Rapid phase-modulated water excitation steady-state free precession for fat suppressed cine cardiovascular MR

BACKGROUND

The purpose of this article is to describe a steady-state free precession (SSFP) sequence for fat suppressed cine cardiovascular magnetic resonance (CMR). A rapid phase-modulated binomial water excitation (WE) pulse is utilized to minimize repetition time and acquisition time.

METHODS

Three different water-excitation pulses were combined with cine-SSFP for evaluation. The frequency response of each sequence was simulated and examined in phantom imaging studies. The ratio of fat to water signal amplitude was measured in phantoms to evaluate the fat suppression capabilities of each method. Six volunteers underwent CMR of the heart at 1.5T to compare retrospectively-gated cine-SSFP with and without water excitation. The ratio of fat to myocardium signal amplitude was measured for conventional cine-SSFP and phase-modulated WE-SSFP. The proposed WE-SSFP method was tested in one patient referred for CMR to characterize a cardiac mass.

RESULTS AND DISCUSSION

The measured frequency response in a phantom corresponded to the numerical Bloch equation simulation demonstrating the widened stop-band around the fat resonant frequency for all water-excitation pulses tested. In vivo measurements demonstrated that a rapid, phase-modulated water excitation pulse significantly reduced the signal amplitude ratio of fat to myocardium from 6.92 +/- 2.9 to 0.8 +/- 0.13 (mean +/- SD) without inducing any perceptible artifacts in SSFP cine CMR.

CONCLUSION

Fat suppression can be achieved in SSFP cine CMR while maintaining steady-state equilibrium using rapid, phase modulated, binomial water-excitation pulses.