MR imaging of fat-containing tissues: valuation of two quantitative imaging techniques in comparison with localized proton spectroscopy

Associations of gestational diabetes and proton density fat fraction of vertebral bone marrow and paraspinal musculature in premenopausal women

Background and objective

Fat content in bones and muscles, quantified by magnetic resonance imaging (MRI) as a proton density fat fraction (PDFF) value, is an emerging non-invasive biomarker. PDFF has been proposed to indicate bone and metabolic health among postmenopausal women. Premenopausal women with a history of gestational diabetes (GDM) carry an increased risk of developing type 2 diabetes and an increased risk of fractures. However, no studies have investigated the associations between a history of GDM and PDFF of bone or of paraspinal musculature (PSM), composed of autochthonous muscle (AM) and psoas muscle, which are responsible for moving and stabilizing the spine. This study aims to investigate whether PDFF of vertebral bone marrow and of PSM are associated with a history of GDM in premenopausal women.

Methods

A total of 37 women (mean age 36.3 ± 3.8 years) who were 6 to 15 months postpartum with (n=19) and without (n=18) a history of GDM underwent whole-body 3T MRI, including a chemical shift encoding-based water-fat separation. The PDFF maps were calculated for the vertebral bodies and PSM. The cross-sectional area (CSA) of PSM was obtained. Associations between a history of GDM and PDFF were assessed using multivariable linear and logistic regression models.

Results

The PDFF of the vertebral bodies was significantly higher in women with a history of GDM (GDM group) than in women without (thoracic: median 41.55 (interquartile range 32.21-49.48)% vs. 31.75 (30.03-34.97)%; p=0.02, lumbar: 47.84 (39.19-57.58)% vs. 36.93 (33.36-41.31)%; p=0.02). The results remained significant after adjustment for age and body mass index (BMI) (p=0.01-0.02). The receiver operating characteristic curves showed optimal thoracic and lumbar vertebral PDFF cutoffs at 38.10% and 44.18%, respectively, to differentiate GDM (AUC 0.72 and 0.73, respectively, sensitivity 0.58, specificity 0.89). The PDFF of the AM was significantly higher in the GDM group (12.99 (12.18-15.90)% vs. 10.83 (9.39-14.71)%; p=0.04) without adjustments, while the CSA was similar between the groups (p=0.34).

Conclusion

A history of GDM is significantly associated with a higher PDFF of the vertebral bone marrow, independent of age and BMI. This statistical association between GDM and increased PDFF highlights vertebral bone marrow PDFF as a potential biomarker for the assessment of bone health in premenopausal women at risk of diabetes.

MRI Methods to Visualize and Quantify Adipose Tissue in Health and Disease

MRI is the modality of choice for a vast range of pathologies but also a sensitive probe into human physiology and tissue function. For this reason, several methodologies have been developed and continuously evolve in order to non-invasively monitor underlying phenomena in human adipose tissue that were difficult to assess in the past through visual inspection of standard imaging modalities. To this end, this work describes the imaging methodologies used in medical practice and lists the most important quantitative markers related to adipose tissue physiology and pathology that are currently supporting diagnosis, longitudinal evaluation and patient management decisions. The underlying physical principles and the resulting markers are presented and associated with frequently encountered pathologies in radiology in order to set the frame of the ability of MRI to reveal the complex role of adipose tissue, not as an inert tissue but as an active endocrine organ.

A fat fraction phantom for establishing new convolutional neural network to determine the pancreatic fat deposition

The determination of fat fraction based on Magnetic Resonance Imaging (MRI) requires extremely accurate data reconstruction for the assessment of pancreatic fat accumulation in medical diagnostics and biological research. In this study, the signal model of the oil and water emulsion was created with a 3.0 T field strength. We examined the quantification of the fat fraction from phantom and the intrapancreatic fat fraction using the techniques of magnetic resonance spectroscopy (MRS) and Iterative Decomposition with Echo Asymmetry and Least-Squares estimate (IDEAL) in magnetic resonance imaging (MRI). Additionally, we contrasted expert manual pancreatic fat assessment with MRS and IDEAL pancreatic fat fraction quantification. There was a strong connection between the true fat volume fraction and the fat fraction from IDEAL and MRS (R² = 0.99 and 0.99, respectively). For both phantom and in vivo measurements, Pearson's correlation and linear regression analysis were used. The findings of the in vivo assessment revealed a variable correlation between the pancreatic fat fraction MRI readings. We also used MR-opsy for manual pancreatic fat fraction segmentation since it read pancreatic fat fractions more accurately than IDEAL and MRS, which aided in the development of machine learning's ability to assess pancreatic fat automatically.

Longitudinal relaxation in fat-water mixtures and its dependence on fat content at 3 T

Longitudinal (T₁ ) relaxation of triglyceride molecules and water is of interest for fat-water separation and fat quantification. A better understanding of T₁ relaxation could benefit modeling for applications in fat quantification and relaxation mapping. This work investigated T₁ relaxation of spectral resonances of triglyceride molecules and water in liquid fat-water mixtures and its dependence on the fat fraction. Dairy cream and a safflower oil emulsion were used. These were diluted with distilled water to produce a variety of fat mass fractions (4.4% to 35% in dairy cream and 6.3% to 52.3% in safflower oil emulsion). T₁ was measured at room temperature at 3 T using an inversion recovery STimulated Echo Acquisition Mode (STEAM) MR spectroscopy method with a series of inversion times. T₁ variations as a function of fat fraction were investigated for various resonances. A two-component model was developed to describe the relaxation in a fat-water mixture as a function of the fat fraction. The T₁ of water and of all fat resonances studied in this work decreased as the fat fraction increased. The relative variation in T₁ was different for each fat resonance. The T₁ of the methylene resonance showed the least variation as a function of the fat fraction. The proposed two-component model closely fits the observed T₁ variations. In conclusion, this work clarifies how the T₁ of major and minor fat resonances and of the water resonance varies as a function of the fat fraction in fat-water mixtures. Knowledge of these variations could serve modeling, analysis of MRI measurements in fat-water mixtures, and phantom preparation.

Multiexponential T2 relaxometry of benign and malignant adipocytic tumours

Background

We investigated a recently proposed multiexponential (Mexp) fitting method applied to T2 relaxometry magnetic resonance imaging (MRI) data of benign and malignant adipocytic tumours and healthy subcutaneous fat. We studied the T2 distributions of the different tissue types and calculated statistical metrics to differentiate benign and malignant tumours.

Methods

Twenty-four patients with primary benign and malignant adipocytic tumours prospectively underwent 1.5-T MRI with a single-slice T2 relaxometry (Carr-Purcell-Meiboom-Gill sequence, 25 echoes) prior to surgical excision and histopathological assessment. The proposed method adaptively chooses a monoexponential or biexponential model on a voxel basis based on the adjusted R² goodness of fit criterion. Linear regression was applied on the statistical metrics derived from the T2 distributions for the classification.

Results

Healthy subcutaneous fat and benign lipoma were better described by biexponential fitting with a monoexponential and biexponential prevalence of 0.0/100% and 0.2/99.8% respectively. Well-differentiated liposarcomas exhibit 17.6% monoexponential and 82.4% biexponential behaviour, while more aggressive liposarcomas show larger degree of monoexponential behaviour. The monoexponential/biexponential prevalence was 47.6/52.4% for myxoid tumours, 52.8/47.2% for poorly differentiated parts of dedifferentiated liposarcomas, and 24.9/75.1% pleomorphic liposarcomas. The percentage monoexponential or biexponential model prevalence per patient was the best classifier distinguishing between malignant and benign adipocytic tumours with a 0.81 sensitivity and a 1.00 specificity.

Conclusions

Healthy adipose tissue and benign lipomas showed a pure biexponential behaviour with similar T2 distributions, while decreased adipocytic cell differentiation characterising aggressive neoplasms was associated with an increased rate of monoexponential decay curves, opening a perspective adipocytic tumour classification.

Chemical-shift encoding-based water-fat separation with multifrequency fat spectrum modeling in spin-lock MRI

PURPOSE

Chemical exchange saturation transfer is used commonly to generate MRI contrast based on the chemical exchange effect. The spin-lock techniques can also be used to probe the chemical exchange and other molecular motion processes in tissues. The presence of fat can cause errors in spin-lock MRI. Signals from fat are typically suppressed based on spectral selectivity or T₁ nulling approaches in spin-lock imaging. However, these methods cannot be used to suppress fat signals from multiple fat peaks. To address this problem, we report chemical-shift encoding-based water-fat separation approaches with multifrequency fat spectrum modeling.

METHODS

Both the conventional spin-lock and the adiabatic continuous-wave constant-amplitude spin lock (ACCSL) with multi-echo acquisitions are investigated for chemical-shift encoding-based water-fat separation in spin-lock imaging. A comparison is made of reconstructions based on 3 models: a single-peak fat spectrum model, a standard precalibrated proton density 6-peak fat spectrum model, and the self-calibrated relaxation-dependent 3-peak fat spectrum model. Comparisons were performed using Bloch simulations, phantom, and in vivo experiments at 3 T.

RESULTS

Conventional spin-lock acquisitions cannot be used for reliable water-fat separation with a multipeak fat spectrum model. Water-fat separation based on ACCSL acquisitions achieves superior performance compared with the use of conventional spin-lock acquisitions. The best result is achieved from ACCSL acquisition with self-calibrated relaxation-dependent multipeak fat spectrum modeling.

CONCLUSION

The ACCSL acquisition can be used for chemical-shift encoding-based water-fat separation with multipeak fat spectrum modeling. This approach has the potential to improve quantitative analysis using spin-lock MRI for assessing the biochemical properties of tissues.

T2, T2* and spin coupling ratio as biomarkers for the study of lipomatous tumors

BACKGROUND

Subcutaneous fat may have variable signal intensity on T2w images depending on the choice of imaging parameters. However, fatty components within tumors have a different degree of signal dependence on the acquisition scheme. This study examined the use of T2, T2^* relaxometry and spin coupling related signal changes (Spin Coupling ratio, SCr) on two different imaging protocols as clinically relevant descriptors of benign and malignant lipomatous tumors.

MATERIALS AND METHODS

20 patients with benign lipomas or liposarcomas of variable histologic grade were examined at an 1.5 T scanner with Multi Echo Spin Echo (MESE) different echo spacing (ESP) in order to produce bright fat T2w images (ESP: 13.4 ms, 25 equidistant echoes) and dark fat images (ESP: 26.8 ms with 10 equidistant echoes). T2^* relaxometry acquisition comprises 4 sets of in-opposed echoes (2.4-19.2 ms, ESP: 2.4 ms) Multi Echo Gradient Echo (MEGRE) sequence. All parametric maps were calculated on a pixel basis.

RESULTS

Significant differences of SCr were found for five different types of lipomatous tumors (Pairwise t-test with Bonferroni correction): lipomas, well differentiated liposarcomas, myxoid liposarcomas, pleomorphic liposarcomas and poorly differentiated liposarcomas. SCr surpassed the classification performance of T2 and T2^* relaxometry.

DATA CONCLUSION

A novel biomarker based on spin coupling related signal loss, SCr, is indicative of lipomatous tumor histological grading. We concluded that T2, T2^* and SCr can be used for the classification of fat containing tumors, which may be important for biopsy guidance in heterogeneous masses and treatment planning.

Disease progression and outcome measures in spinobulbar muscular atrophy

OBJECTIVE

Spinal and bulbar muscular atrophy (SBMA) is a slowly progressive disease with weakness of bulbar and extremity muscles. There is no curative treatment for the disease, but several clinical trials have been conducted over the past years. The results from these trials have uncovered a great need to develop quantitative, reliable outcome measures. In this study, we prospectively investigated disease progression over 18 months in 29 patients with genetically confirmed SBMA, using quantitative outcome measures, including Dixon magnetic resonance imaging (MRI).

METHODS

We used MRI to assess changes in muscle fat content and stationary dynamometry to assess changes in muscle strength. Disease progression was also investigated with the SBMA functional rating scale, bulbar rating scale, 6-minute walk test, and blood samples, among others.

RESULTS

Mean muscle fat content, muscle strength in knee extensors, handgrip strength, walking distance, and creatinine levels changed significantly. Mean muscle fat content increased by 2 ± 1.25%, and knee extension strength decreased from 83 ± 60 to 76 ± 56Nm, handgrip strength from 31 ± 13 to 29 ± 13kg, walking distance from 362 ± 216 to 336 ± 219m, and creatinine level from 58 ± 21 to 54 ± 20 μmol/l. Functional rating scores did not change.

INTERPRETATION

The present study demonstrates a slow and steady disease progression in SBMA. Dixon MRI detected increases in muscle fat content in all investigated muscles and is therefore a suitable candidate for an outcome measure in natural history or treatment studies in SBMA. The 6-minute walk test and handgrip strength also seem to be reliable outcome measures for SBMA. Ann Neurol 2018;84:762-773.

Comparison of MRI-derived vs. traditional estimations of fatty acid composition from MR spectroscopy signals

INTRODUCTION

The composition of fatty acids in the body is gaining increasing interest, and can be followed up noninvasively by quantitative magnetic resonance spectroscopy (MRS). However, current MRS quantification methods have been shown to provide different quantitative results in terms of lipid signals, with possible varying outcomes for a given biological examination. Quantitative magnetic resonance imaging using multigradient echo sequence (MGE-MRI) has recently been added to MRS approaches. In contrast, these methods fit the undersampled magnetic resonance temporal signal with a simplified model function (expressing the triglyceride [TG] spectrum with only three TG parameters), specific implementations and prior knowledge. In this study, an adaptation of an MGE-MRI method to MRS lipid quantification is proposed.

METHODS

Several versions of the method - with time data fully or undersampled, including or excluding the spectral peak T₂ knowledge in the fitting - were compared theoretically and on Monte Carlo studies with a time-domain, peak-fitting approach. Robustness, repeatability and accuracy were also inspected on in vitro oil acquisitions and test-retest in vivo subcutaneous adipose tissue acquisitions, adding results from the reference LCModel method.

RESULTS

On simulations, the proposed method provided TG parameter estimates with the smallest variability, but with a possible bias, which was mitigated by fitting on undersampled data and considering peak T₂ values. For in vitro measurements, estimates for all approaches were correlated with theoretical values and the best concordance was found for the usual MRS method (LCModel and peak fitting). Limited in vivo test-retest variability was found (4.1% for PUFAindx, 0.6% for MUFAindx and 3.6% for SFAindx), as for LCModel (7.6% for PUFAindx, 7.8% for MUFAindx and 3.0% for SFAindx).

CONCLUSION

This study shows that fitting the three TG parameters directly on MRS data is one valuable solution to circumvent the poor conditioning of the MRS quantification problem.

Magnetic resonance spin-spin relaxation time estimation in a rat model of fatty liver disease

PURPOSE

To compare mono- and bi-exponential relaxation model equations to discriminate between normal and fatty liver disease.

MATERIALS AND METHODS

Six rats on a choline deficient amino acid modified (CDAA) diet and six on normal chow were studied. Multiple spin echo images with increasing echo times (TEs) were collected at 9.4T. Pixel-wise T₂ maps were generated using mono-exponential decay function to calculate T_2M , and a bi-exponential to calculate, short T₂ component (T_2S ), long T₂ component (T_2L ), and fractions of these components (ρ_S , ρ_L ), respectively. Statistical F-tests and Akaike's information criterion (AIC) were used to assess the relative performance of the two models.

RESULTS

F-test and AIC showed that in the CDAA group, T₂ bi-exponential model described the signal of T₂ weighted imaging of the liver better than the mono-exponential model. Controls were best described by the mono-exponential model. Mean values for T_2M , T_2L , T_2S , ρ_S , ρ_L were 31.2 ± 0.7 ms, 72.8 ± 3.3 ms, 8.2 ± 0.6 ms,71.2 ± 2.1%, 30.4 ± 1.3%, respectively, in CDAA rats, compared with 18.8 ± 0.5 ms, 32.3 ± 0.7 ms, 9.2 ± 1.8 ms, 79 ± 2%, 21.0 ± 1.1% in controls.

CONCLUSION

In the fatty liver of CDAA rats we have shown that T₂ weighted images fit the bi-exponential model better than mono-exponential decays thus providing a better description of the data.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018;47:468-476.

Non-invasive imaging techniques in assessing non-alcoholic fatty liver disease: a current status of available methods

Non-alcoholic fatty liver disease (NAFLD) is an ailment affecting and increasing a number of people worldwide diagnosed via non-invasive imaging techniques, at a time when a minimum harm caused by medical procedures is rightfully emphasized, more sought after, than ever before. Liver steatosis should not be taken lightly even if its evolution is largely benign as it has the potential to develop into non-alcoholic steatohepatitis (NASH) or even more concerning, hepatic cirrhosis, and hepatocellular carcinoma (HCC). Traditionally, liver biopsy has been the standard for diagnosing this particular liver disease, but nowadays, a consistent number of imagistic methods are available for diagnosing hepatosteatosis and choosing the one appropriate to the clinical context is the key. Although different in sensitivity and specificity when it comes to determining the hepatic fat fraction (FF), these imaging techniques possessing a diverse availability, operating difficulty, cost, and reproducibility are invaluable to any modern physician. Ultrasonography (US), computed tomography (CT), magnetic resonance imaging (MRI), elastography, and spectroscopy will be discussed in order to lay out the advantages and disadvantages of their diagnostic potential and application.

Although imagistics has given physicians a valuable insight into the means of managing NAFLD, the current methods are far from perfect, but given the time, they will surely be improved and the use of liver biopsy will be completely removed.

Fast triple-spin-echo Dixon (FTSED) sequence for water and fat imaging

A number of 'Dixon' techniques based on fast spin echo (FSE) sequence have been proposed and successfully used in many branches of medicine. Some require only one scan, but most of them need multiple scans and long scan times. This article describes a new fast triple-spin-echo Dixon (FTSED) technique suitable for ultra-high field MRI, in which three specific time shifts are introduced in the echo train; thus, three images with defined water-fat phase-differences (0, π, 2π) are encoded in the phase of the acquired images without extreme restrictions upon the echo duration. The water and fat images are then calculated by iterative least-squares estimation method. The sequence was successfully implemented at a 9.4T ultra-high field MRI system and tested on a phantom and a rat.

Short and long time MR signal behavior of randomly distributed water and fat-numerical simulations

The MR time-signal behavior of water has been reported to be different on short and long time scales for systems of randomly distributed perturbers in water in the static dephasing regime. Up to now, the signal of the perturbers in such systems has not been taken into consideration. Water-fat emulsions are macroscopically homogeneous systems and can be considered as microscopically randomly distributed perturbing fat spheres embedded in water. In such water-fat systems, the signal of the perturber, fat, cannot be ignored. Since water and fat are within the same system, the fat signal behavior may show similarities with water, with differences in short and long time scales. This could complicate fat-referenced MR thermometry (MRT) methods such as multi-gradient echo-based (MGE) MRT. Simulations were performed using a numerical phantom comprising spherical fat objects embedded in a spherical water medium. To characterize the fat signal, the theoretical signal description of water was fitted to the simulated fat signal. The simulated signals were sampled as an MGE signal and MGE MRT was used to calculate temperatures. The sampling was done with and without delay, to investigate the effect on the temperature error of the time ranges in which the signal was sampled. It was confirmed that the fat signal behavior was similar to that of water and consisted of two regimes. The separation between the short and long time scales was approximately at 55 ms for fat, as compared with 8.9 ms for water. Without delayed signal sampling, the MGE MRT temperature error was about 2.5°C. With delayed sampling such that both the water and the fat signals were either in the short or in the long time scale the error was reduced to 0.2°C.

The 6-minute walk test, motor function measure and quantitative thigh muscle MRI in Becker muscular dystrophy: A cross-sectional study

Becker muscular dystrophy (BMD) has an incidence of 1 in 16 000 male births. This cross-sectional study investigated the relation between validated functional scores and quantitative MRI (qMRI) of thigh muscles in 20 ambulatory BMD patients, aged 18.3-60 years (mean 31.2; SD 11.1). Clinical assessments included the motor function measure (MFM) and its subscales, as well as timed function tests such as the 6-minute walk test (6MWT) and the timed 10-m run/walk test. Quantitative MRI of the thigh muscles included the mean fat fraction (MFF) using a 2-point Dixon (2-PD) technique, and transverse relaxation time (T2) measurements. The mean MFM value was 80.4%, SD 9.44 and the D1 subscore 54.5%, SD 19.9. The median 6MWT was 195m, IQR 160-330.2. The median 10-m run/walk test was 7.4 seconds, IQR 6.1-9.3. The mean fat fraction of the thigh muscles was 55.6%, SD 17.4%, mean T2 relaxation times of all muscles: 69.9 ms, SD 14.4. The flexors had the highest MFF and T2 relaxation times, followed by the extensors and the adductors. MFF and global T2 relaxation times were highly negatively correlated with the MFM total, D1-subscore and 6MWT, and positively correlated with the 10 m run/walk test time (p < 0.01). Age was not correlated with MFF, global T2 relaxation time or clinical assessments. Both MFF and T2 measures in the thigh muscle were well correlated with clinical function in BMD and may serve as a surrogate outcome measure in clinical trials.

Comparison of Multi-Echo Dixon Methods with Volume Interpolated Breath-Hold Gradient Echo Magnetic Resonance Imaging in Fat-Signal Fraction Quantification of Paravertebral Muscle

Objective

To assess whether multi-echo Dixon magnetic resonance (MR) imaging with simultaneous T2* estimation and correction yields more accurate fat-signal fraction (FF) measurement of the lumbar paravertebral muscles, in comparison with non-T2*-corrected two-echo Dixon or T2*-corrected three-echo Dixon, using the FF measurements from single-voxel MR spectroscopy as the reference standard.

Materials and Methods

Sixty patients with low back pain underwent MR imaging with a 1.5T scanner. FF mapping images automatically obtained using T2*-corrected Dixon technique with two (non-T2*-corrected), three, and six echoes, were compared with images from single-voxel MR spectroscopy at the paravertebral muscles on levels L4 through L5. FFs were measured directly by two radiologists, who independently drew the region of interest on the mapping images from the three sequences.

Results

A total of 117 spectroscopic measurements were performed either bilaterally (57 of 60 subjects) or unilaterally (3 of 60 subjects). The mean spectroscopic FF was 14.3 ± 11.7% (range, 1.9-63.7%). Interobserver agreement was excellent between the two radiologists. Lin's concordance correlation between the spectroscopic findings and all the imaging-based FFs were statistically significant (p < 0.001). FFs obtained from the T2*-corrected six-echo Dixon sequences showed a significantly better concordance with the spectroscopic data, with its concordance correlation coefficient being 0.99 and 0.98 (p < 0.001), as compared with two- or three-echo methods.

Conclusion

T2*-corrected six-echo Dixon sequence would be a better option than two- or three-echo methods for noninvasive quantification of lumbar muscle fat quantification.

Improved method for calculating hepatic steatosis using the hepatorenal index

OBJECTIVES

Marshall et al (AJR Am J Roentgenol 2012; 199:997-1002) initially demonstrated that the hepatorenal index is an effective and noninvasive tool to screen patients for hepatic steatosis. The aim of this study was to determine whether the hepatorenal index can be accurately calculated directly from a picture archiving and communication system (PACS) quickly and efficiently without the need for the multiple steps and specialized software used to calculate hepatorenal index in the study by Marshall et al.

METHODS

We evaluated 99 of the 101 patients included in the study by Marshall et al: patients being followed by hepatologists with plans for liver biopsy. The hepatorenal index was calculated by using Digital Imaging and Communications in Medicine (DICOM) images from a PACS and a markup region-of-interest tool. We compared this value to the value that Marshall et al derived by using specialized software and to standard histologic estimates. We created similar subgroups: patients with steatosis based on histologically estimated intracellular fat exceeding 5% and patients without steatosis.

RESULTS

The mean hepatorenal index ± SD for those with steatosis according to histologic findings was 1.87 ± 0.6, and for those without, it was 1.14 ± 0.2. A hepatorenal index of 1.34 or higher had 92% sensitivity for identifying fat exceeding 5%, 85% specificity, a 94% negative predictive value, and a 79% positive predictive value. Substantial agreement was found between the hepatorenal index calculated from DICOM images and macrovesicular fat categorized at the cut point of 1.34 or higher (κ = 0.76; 95% confidence interval, 0.62-0.88; P < .001).

CONCLUSIONS

The hepatorenal index can be quickly and accurately calculated from DICOM images directly on a PACS without supplementary software.

Measurement of liver fat fraction and iron with MRI and MR spectroscopy techniques

Diffuse liver disease is a widespread global healthcare burden, and the abnormal accumulation of lipid and/or iron is common to important disease processes. Developing the improved methods for detecting and quantifying liver lipid and iron is an important clinical need. The inherent risk, invasiveness, and sampling error of liver biopsy have prompted the development of noninvasive imaging methods for lipid and iron assessment. Ultrasonography and computed tomography have the ability to detect diffuse liver disease, but with limited accuracy. The purpose of this review is to describe the current state-of-the-art methods for quantifying liver lipid and iron using magnetic resonance imaging and spectroscopy, including their implementation, benefits, and potential pitfalls. Imaging- and spectroscopy-based methods are naturally suited for lipid and iron quantification. Lipid can be detected and decomposed from the inherent chemical shift between lipid and water signals, whereas iron imparts significant paramagnetic susceptibility to tissue, which accelerates proton relaxation. However, measurements of these biomarkers are confounded by technical and biological effects. Current methods must address these factors to allow a precise correlation between the lipid fraction and iron concentration. Although this correlation becomes increasingly challenging in the presence of combined lipid and iron accumulation, advanced techniques show promise for delineating these quantities through multi-lipid peak analysis, T2 water mapping, and fast single-voxel water-lipid spectroscopy.

Removal of nuisance signals from limited and sparse 1H MRSI data using a union-of-subspaces model

PURPOSE

To remove nuisance signals (e.g., water and lipid signals) for (1) H MRSI data collected from the brain with limited and/or sparse (k, t)-space coverage.

METHODS

A union-of-subspace model is proposed for removing nuisance signals. The model exploits the partial separability of both the nuisance signals and the metabolite signal, and decomposes an MRSI dataset into several sets of generalized voxels that share the same spectral distributions. This model enables the estimation of the nuisance signals from an MRSI dataset that has limited and/or sparse (k, t)-space coverage.

RESULTS

The proposed method has been evaluated using in vivo MRSI data. For conventional chemical shift imaging data with limited k-space coverage, the proposed method produced "lipid-free" spectra without lipid suppression during data acquisition at 130 ms echo time. For sparse (k, t)-space data acquired with conventional pulses for water and lipid suppression, the proposed method was also able to remove the remaining water and lipid signals with negligible residuals.

CONCLUSION

Nuisance signals in (1) H MRSI data reside in low-dimensional subspaces. This property can be utilized for estimation and removal of nuisance signals from (1) H MRSI data even when they have limited and/or sparse coverage of (k, t)-space. The proposed method should prove useful especially for accelerated high-resolution (1) H MRSI of the brain.

Direct water and fat determination in two-point Dixon imaging with flexible echo times

PURPOSE

Identifying water and fat unambiguously in multipoint Dixon imaging often requires phase correction, which can be challenging and may fail. The purpose of this work is to present a geometric interpretation of the two-point Dixon method with flexible echo times (TEs) and to investigate the conditions under which water and fat can be determined directly without phase correction.

METHODS

Geometrically, the equation for the magnitude of the acquired signal at a given TE represents an ellipse in the water-fat plane centered at the origin. Determining water and fat in two-point Dixon imaging thus amounts to finding the correct intercept between two ellipses from the signals at two TEs. At the right TE combinations, the physicality requirement that water and fat be non-negative can be used to select a unique water and fat solution. Systematic computer simulations were conducted to examine the ranges of the TEs for which this approach is feasible and how different noise levels impact the feasibility. Phantom and in vivo experiments on a 1.5-T whole-body MRI scanner were used to validate the computer simulations.

RESULTS

In simulation and phantom experiments, nearly all pixels of pure water or pure fat were reliably identified based on the physicality requirement alone for a range of practically useful TE combinations (e.g., around 3 ms/6 ms at 1.5 T) and at moderate to high SNR levels (≥ 25). At other TE combinations, finding the correct solution based on the physicality requirement alone was not feasible or became sensitive to noise. In vivo findings were in overall agreement with the simulation and phantom studies, although the percentage of pixels that were correctly determined was lower.

CONCLUSIONS

The problem of direct water and fat determination without phase correction can be understood geometrically. Using the physicality requirement, it is possible to identify the different TE combinations and imaging conditions under which water and fat imaging can be performed either completely without phase correction or by generating a first-pass solution that can be used to improve the processing reliability of a phase-correction based method.

Muscle-fat MRI: 1.5 Tesla and 3.0 Tesla versus histology

INTRODUCTION

We evaluated muscle/fat fraction (MFF) accuracy and reliability measured with an MR imaging technique at 1.5 Tesla (T) and 3.0T scanner strengths, using biopsy as reference.

METHODS

MRI was performed on muscle samples from pig and rabbit species (n = 8) at 1.5T and 3.0T. A chemical shift based 2-point Dixon method was used, collecting in-phase and out-of-phase data for fat/water of muscle samples. Values were compared with MFFs calculated from histology.

RESULTS

No significant difference was found between 1.5T and 3.0T (P values = 0.41-0.96), or between histology and imaging (P = 0.83) for any muscle tested.

CONCLUSIONS

RESULTS suggest that a 2-point Dixon fat/water separation MRI technique may provide reliable quantification of MFFs at varying field strengths across different animal species, and consistency was established with biopsy. The results set a foundation for larger scale investigation of quantifying muscle fat in neuromuscular disorders.

Fat composition changes in bone marrow during chemotherapy and radiation therapy

PURPOSE

To quantify changes in bone marrow fat fraction and determine associations with peripheral blood cell counts.

METHODS AND MATERIALS

In this prospective study, 19 patients received either highly myelotoxic treatment (radiation therapy plus cisplatin, 5-fluorouracil mitomycin C [FU/MMC], or cisplatin/5-FU/cetuximab) or less myelotoxic treatment (capecitabine-radiation therapy or no concurrent chemotherapy). Patients underwent MR imaging and venipuncture at baseline, midtreatment, and posttreatment visits. We performed mixed effects modeling of the mean proton density fat fraction (PDFF[%]) by linear time, treatment, and vertebral column region (lumbar [L]4-sacral [S]2 vs thoracic [T]10-L3 vs cervical[C]3-T9), while controlling for cumulative mean dose and other confounders. Spearman rank correlations were performed by white blood cell (WBC) counts versus the differences in PDFF(%) before and after treatment.

RESULTS

Cumulative mean dose was associated with a 0.43% per Gy (P=.004) increase in PDFF(%). In the highly myelotoxic group, we observed significant changes in PDFF(%) per visit within L4-S2 (10.1%, P<.001) and within T10-L3 (3.93%, P=.01), relative to the reference C3-T9. In the less myelotoxic group, we did not observe significant changes in PDFF(%) per visit according to region. Within L4-S2, we observed a significant difference between treatment groups in the change in PDFF(%) per visit (5.36%, P=.04). Rank correlations of the inverse log differences in WBC versus the differences in PDFF(%) overall and within T10-S2 ranged from 0.69 to 0.78 (P<.05). Rank correlations of the inverse log differences in absolute neutrophil counts versus the differences in PDFF(%) overall and within L4-S2 ranged from 0.79 to 0.81 (P<.05).

CONCLUSIONS

Magnetic resonance imaging fat quantification is sensitive to marrow composition changes that result from chemoradiation therapy. These changes are associated with peripheral blood cell counts. This study supports a rationale for bone marrow-sparing treatment planning to reduce the risk of hematologic toxicity.

Fat and iron quantification in the liver: past, present, and future

Liver fat, iron, and combined overload are common manifestations of diffuse liver disease and may cause lipotoxicity and iron toxicity via oxidative hepatocellular injury, leading to progressive fibrosis, cirrhosis, and eventually, liver failure. Intracellular fat and iron cause characteristic changes in the tissue magnetic properties in predictable dose-dependent manners. Using dedicated magnetic resonance pulse sequences and postprocessing algorithms, fat and iron can be objectively quantified on a continuous scale. In this article, we will describe the basic physical principles of magnetic resonance fat and iron quantification and review the imaging techniques of the "past, present, and future." Standardized radiological metrics of fat and iron are introduced for numerical reporting of overload severity, which can be used toward objective diagnosis, grading, and longitudinal disease monitoring. These noninvasive imaging techniques serve an alternative or complimentary role to invasive liver biopsy. Commercial solutions are increasingly available, and liver fat and iron quantitative imaging is now within reach for routine clinical use and may soon become standard of care.

Long-term risperidone treatment induces visceral adiposity associated with hepatic steatosis in mice: a magnetic resonance approach

Although atypical antipsychotic drugs (APDs) have led to significant advances in the treatment of psychotic disorders, they still induce metabolic disturbances. We aimed at characterizing the metabolic consequences of a risperidone treatment and at establishing a link with noninvasive MR markers, in order to develop a tool for predicting symptoms of the metabolic syndrome. Fat deposition and liver morphometry were assessed by T1-weighted imaging. Fatty acid composition and fat accumulations in tissues were determined using MR spectroscopy with and without water suppression, respectively. Risperidone treatment induced a weight gain accompanied with metabolic disturbances such as hyperglycemic status, an increase in visceral adipose tissue (VAT), and liver fat depositions. Correlations using Methylene-Water Ratio (MWR) and Polyunsaturated Index (PUI) demonstrated a concomitant increase in the weight gain, VAT and liver fat depositions, and a decrease in the quantity of polyunsaturated fatty acids. These results were consistent with a hepatic steatosis state. We evaluated the ability of MR techniques to detect subtle metabolic disorders induced by APDs. Thus, our model and methodology offer the possibility to investigate APDs side effects in order to improve the health conditions of schizophrenic patients.

Femoral bone marrow adiposity and cortical bone cross-sectional areas in men with motor complete spinal cord injury

OBJECTIVES

To (1) quantify yellow and red bone marrow (BM) and cortical bone cross-sectional areas (CSAs) of the femur in persons with motor complete spinal cord injury (SCI) compared with healthy able-bodied control subjects and (2) determine the relationships between yellow and red BM, cortical CSAs, and thigh composition and measurements from dual-energy x-ray absorptiometry in men with complete SCI.

DESIGN

Cross-sectional.

SETTINGS

Clinical hospital and academic settings.

METHODS

Eight persons with motor complete SCI and 6 age-matched healthy control subjects underwent magnetic resonance imaging of both thighs to measure BM adiposity (BMA) and cortical CSA followed by whole-body dual-energy x-ray absorptiometry to measure bone mineral density and body composition for the SCI group.

RESULTS

Cortical bone CSA adjusted to total subperiosteal bone CSA was 1.5-2 times lower in men with SCI compared with able-bodied control subjects across the femoral length (P =.003). Yellow BMA CSA was 2-3 times greater in men with SCI compared with able-bodied control subjects (P < .0001). Opposite relationships were found between the yellow BMA CSA and cortical bone CSAs in men with SCI (negative association) and able-bodied control subjects (positive association). Yellow BMA was negatively associated with bone mineral density and bone mineral content and with skeletal muscle CSA and fat-free mass (P <.05) in men with SCI. Finally, yellow BMA was positively related to thigh subcutaneous adipose tissue.

CONCLUSIONS

After SCI, cortical bone CSA becomes thinner and is associated with greater accumulation of yellow BMA. Yellow BMA is associated with changes in bone CSA and bone mass, as well as increased fat mass, after SCI.

Robust selective signal suppression using binomial off-resonant rectangular (BORR) pulses

PURPOSE

To study the selective signal suppression capability of a binomial off-resonant rectangular (BORR) radiofrequency pulse method.

MATERIALS AND METHODS

The BORR pulse consists of two consecutive rectangular pulses with a phase difference of π. The exact solution of the Bloch equations was used to simulate its frequency response. The BORR pulse was implemented in a gradient echo sequence and tested on phantoms, the knee, and the breast.

RESULTS

The frequency response of the BORR pulse acquired on the phantom confirmed the theory. Broad suppression bands ensured high suppression efficiency and robustness in both in vitro and in vivo scans compared with other saturation pulses.

CONCLUSION

The BORR pulse method provides a simple, efficient, and robust selective signal suppression alternative for three-dimensional short TR (repetition time) imaging.

Quantitative dynamic contrast-enhanced MRI of pelvic and lumbar bone marrow: effect of age and marrow fat content on pharmacokinetic parameter values

OBJECTIVE

The purpose of this study was to determine the effects of age and fat content on quantitative dynamic contrast-enhanced MRI (DCE-MRI) parameters in the bone marrow of the lumbar spine and pelvis. The interreader reproducibility of this technique will also be assessed.

MATERIALS AND METHODS

Forty-three DCE-MRI studies of the female pelvis defined the study group. Quantitative pharmacokinetic perfusion parameters of lumbar and pelvic marrow were analyzed by three readers on a DCE-MRI postprocessing platform. Linear regression analysis was performed to determine the effect of age and marrow fat fraction on the parameters of transfer constant (K(trans)), efflux rate constant (K(ep)), extravascular extracellular space (V(e)), and initial area under the gadolinium curve at 60 seconds (iAUGC(60)). Interreader agreement was assessed by means of intraclass correlation coefficient calculation.

RESULTS

A weak but statistically significant correlation was established between both age and fat fraction and the parameters K(trans) (R(2) = 0.14) and K(ep) (R(2) = 0.09). There was also a weak but statistically significant correlation between fat fraction and V(e) (R(2) = 0.116) and iAUGC(60) (R(2) = 0.108), but no correlation between age and these parameters. Intraclass correlation coefficients of parameter measurements by different readers were all greater than 0.7 at the p < 0.05 level.

CONCLUSION

Age and fat fraction have small measurable effects on quantitative DCE-MRI parameters in bone marrow. However, given the wide interindividual variation of these parameters, these effects are unlikely to confound changes related to malignancy or treatment. Also of note, there was strong interreader reproducibility of parameter measurements among a range of experience levels, suggesting that the reader-reader experience level may not represent a significant source of variability in bone marrow DCE-MRI.

Diagnostic performance of a rapid magnetic resonance imaging method of measuring hepatic steatosis

OBJECTIVES

Hepatic steatosis is associated with an increased risk of developing serious liver disease and other clinical sequelae of the metabolic syndrome. However, visual estimates of steatosis from histological sections of biopsy samples are subjective and reliant on an invasive procedure with associated risks. The aim of this study was to test the ability of a rapid, routinely available, magnetic resonance imaging (MRI) method to diagnose clinically relevant grades of hepatic steatosis in a cohort of patients with diverse liver diseases.

MATERIALS AND METHODS

Fifty-nine patients with a range of liver diseases underwent liver biopsy and MRI. Hepatic steatosis was quantified firstly using an opposed-phase, in-phase gradient echo, single breath-hold MRI methodology and secondly, using liver biopsy with visual estimation by a histopathologist and by computer-assisted morphometric image analysis. The area under the receiver operating characteristic (ROC) curve was used to assess the diagnostic performance of the MRI method against the biopsy observations.

RESULTS

The MRI approach had high sensitivity and specificity at all hepatic steatosis thresholds. Areas under ROC curves were 0.962, 0.993, and 0.972 at thresholds of 5%, 33%, and 66% liver fat, respectively. MRI measurements were strongly associated with visual (r(2) = 0.83) and computer-assisted morphometric (r(2) = 0.84) estimates of hepatic steatosis from histological specimens.

CONCLUSIONS

This MRI approach, using a conventional, rapid, gradient echo method, has high sensitivity and specificity for diagnosing liver fat at all grades of steatosis in a cohort with a range of liver diseases.

Water-fat MRI for assessing changes in bone marrow composition due to radiation and chemotherapy in gynecologic cancer patients

PURPOSE

To assess the feasibility of using fat-fraction imaging for measuring marrow composition changes over large regions in patients undergoing cancer therapy.

MATERIALS AND METHODS

Thirteen women with gynecologic malignancies who were to receive radiation and/or chemotherapy were recruited for this study. Subjects were imaged on a 3T magnetic resonance (MR) scanner at baseline (after surgery but before radiation or chemotherapy), 6 months, and 12 months after treatment. Water-fat imaging was used to generate high-resolution, 3D signal fat fraction (sFF) maps extending from mid-femur to L3. Treatment changes were assessed by measuring marrow sFF in the L4 vertebra, femoral necks, and control tissues.

RESULTS

Pretreatment and 6-month scans were compared in nine women. sFF increased significantly in both the L4 vertebral marrow (P = 0.04) and the femoral necks (P = 0.03), while no significant change was observed in control regions. Qualitatively, chemotherapy changes were more uniform in space, whereas the radiation-induced changes were largest in marrow regions inside and close to the target radiation field.

CONCLUSION

Water-fat MRI is sensitive to changes in red/yellow marrow composition, and can be used for quantitative and qualitative assessment of treatment-induced marrow damage.

ISMRM workshop on fat-water separation: insights, applications and progress in MRI

Approximately 130 attendees convened on February 19-22, 2012 for the first ISMRM-sponsored workshop on water-fat imaging. The motivation to host this meeting was driven by the increasing number of research publications on this topic over the past decade. The scientific program included an historical perspective and a discussion of the clinical relevance of water-fat MRI, a technical description of multiecho pulse sequences, a review of data acquisition and reconstruction algorithms, a summary of the confounding factors that influence quantitative fat measurements and the importance of MRI-based biomarkers, a description of applications in the heart, liver, pancreas, abdomen, spine, pelvis, and muscles, an overview of the implications of fat in diabetes and obesity, a discussion on MR spectroscopy, a review of childhood obesity, the efficacy of lifestyle interventional studies, and the role of brown adipose tissue, and an outlook on federal funding opportunities from the National Institutes of Health.

MRI of spinal bone marrow: part I, techniques and normal age-related appearances

OBJECTIVE

This article reviews MRI protocols, including routine and nonroutine pulse sequences as well as the normal MRI appearance of spinal marrow and expected age-related changes.

CONCLUSION

Routine MRI of the spine provides useful evaluation of the spinal bone marrow, but nonroutine MRI pulse sequences are increasingly being used to evaluate bone marrow pathology. An understanding of MRI pulse sequences and the normal and age-related appearances of bone marrow is important for the practicing radiologist.

Assessment of abdominal adipose tissue and organ fat content by magnetic resonance imaging

As the prevalence of obesity continues to rise, rapid and accurate tools for assessing abdominal body and organ fat quantity and distribution are critically needed to assist researchers investigating therapeutic and preventive measures against obesity and its comorbidities. Magnetic resonance imaging (MRI) is the most promising modality to address such need. It is non-invasive, utilizes no ionizing radiation, provides unmatched 3-D visualization, is repeatable, and is applicable to subject cohorts of all ages. This article is aimed to provide the reader with an overview of current and state-of-the-art techniques in MRI and associated image analysis methods for fat quantification. The principles underlying traditional approaches such as T(1) -weighted imaging and magnetic resonance spectroscopy as well as more modern chemical-shift imaging techniques are discussed and compared. The benefits of contiguous 3-D acquisitions over 2-D multislice approaches are highlighted. Typical post-processing procedures for extracting adipose tissue depot volumes and percent organ fat content from abdominal MRI data sets are explained. Furthermore, the advantages and disadvantages of each MRI approach with respect to imaging parameters, spatial resolution, subject motion, scan time and appropriate fat quantitative endpoints are also provided. Practical considerations in implementing these methods are also presented.

Frequency response of multipoint chemical shift-based spectral decomposition

PURPOSE

To provide a framework for characterizing the frequency response of multipoint chemical shift based species separation techniques.

MATERIALS AND METHODS

Multipoint chemical shift based species separation techniques acquire complex images at multiple echo times and perform maximum likelihood estimation to decompose signal from different species into separate images. In general, after a nonlinear process of estimating and demodulating the field map, these decomposition methods are linear transforms from the echo-time domain to the chemical-shift-frequency domain, analogous to the discrete Fourier transform (DFT). In this work we describe a technique for finding the magnitude and phase of chemical shift decomposition for input signals over a range of frequencies using numerical and experimental modeling and examine several important cases of species separation.

RESULTS

Simple expressions can be derived to describe the response to a wide variety of input signals. Agreement between numerical modeling and experimental results is very good.

CONCLUSION

Chemical shift-based species separation is linear, and therefore can be fully described by the magnitude and phase curves of the frequency response. The periodic nature of the frequency response has important implications for the robustness of various techniques for resolving ambiguities in field inhomogeneity.

Skin age testing criteria: characterization of human skin structures by 500 MHz MRI multiple contrast and image processing

Ex vivo magnetic resonance microimaging (MRM) image characteristics are reported in human skin samples in different age groups. Human excised skin samples were imaged using a custom coil placed inside a 500 MHz NMR imager for high-resolution microimaging. Skin MRI images were processed for characterization of different skin structures. Contiguous cross-sectional T1-weighted 3D spin echo MRI, T2-weighted 3D spin echo MRI and proton density images were compared with skin histopathology and NMR peaks. In all skin specimens, epidermis and dermis thickening and hair follicle size were measured using MRM. Optimized parameters TE and TR and multicontrast enhancement generated better MRI visibility of different skin components. Within high MR signal regions near to the custom coil, MRI images with short echo time were comparable with digitized histological sections for skin structures of the epidermis, dermis and hair follicles in 6 (67%) of the nine specimens. Skin % tissue composition, measurement of the epidermis, dermis, sebaceous gland and hair follicle size, and skin NMR peaks were signatures of skin type. The image processing determined the dimensionality of skin tissue components and skin typing. The ex vivo MRI images and histopathology of the skin may be used to measure the skin structure and skin NMR peaks with image processing may be a tool for determining skin typing and skin composition.

Assessment of liver fat quantification in the presence of iron

This study assesses the stability of magnetic resonance liver fat measurements against changes in T2* due to the presence of iron, which is a confound for accurate quantification. The liver T2* was experimentally shortened by intravenous infusion of a super paramagnetic iron oxide contrast agent. Low flip angle multiecho gradient echo sequences were performed before, during and after infusion. The liver fat fraction (FF) was calculated in co-localized regions-of-interest using T2* models that assumed no decay, monoexponential decay and biexponential decay. Results show that, when T2* was neglected, there was a strong underestimation of FF and with monoexponential decay there was a weak overestimation of FF. Curve-fitting using the biexponential decay was found to be problematic. The overestimation of FF may be due to remaining deficiencies in the model, although is unlikely to be important for clinical diagnosis of steatosis.

Change in the proton T(1) of fat and water in mixture

This work describes observed changes in the proton T(1) relaxation time of both water and lipid when they are in relatively homogeneous mixtures. Results obtained from vegetable oil-water emulsions, pork kidney and lard mixtures, and excised samples of white and brown adipose tissues are presented to demonstrate this change in T(1) as a function of mixture fat fraction. As an initial proof of concept, a simpler acetone-water experiment was performed to take advantage of complete miscibility between acetone and water and both components' single chemical shift peaks. Single-voxel MR spectroscopy was used to measure the T(1) of predominant methylene spins in fat and the T(1) of water spins in each setup. In the vegetable oil-water emulsions, the T(1) of fat varied by as much as 3-fold when water was the dominant mixture component. The T(1) of pure lard increased by 170 msec (+37%) when it was blended with lean kidney tissue in a 16% fatty mixture. The fat T(1) of lipid-rich white adipose tissue was 312 msec. In contrast, the fat T(1) of leaner brown adipose tissue (fat fraction 53%) was 460 msec. A change in the water T(1) from that of pure water was also observed in the experiments.

Comparison of fat-water MRI and single-voxel MRS in the assessment of hepatic and pancreatic fat fractions in humans

The ability to accurately and noninvasively quantify fatty infiltration in organs such as the liver and the pancreas remains a critical component in understanding the link between obesity and its comorbidities such as type 2 diabetes and fatty liver disease. Single-voxel ((1)H) proton magnetic resonance spectroscopy (MRS) has long been regarded as the gold-standard noninvasive technique for such measurements. Recent advances in three-dimensional fat-water magnetic resonance imaging (MRI) methods have led to the development of rapid, robust, and quantitative approaches that can accurately characterize the proportion of fat and water content in underlying tissues across the full imaging volume, and hence entire organs of interest. One such technique is called IDEAL (Iterative Decomposition with Echo Asymmetry and Least squares estimation). This article prospectively compares three-dimensional (3D) IDEAL-MRI and single-voxel MRS in the assessment of hepatic (HFF) and pancreatic fat fraction (PFF) in 16 healthy subjects. MRS acquisitions took 3-4 min to complete whereas IDEAL acquisitions were completed in 20-s breath-holds. In the liver, there was a strong correlation (slope = 0.90, r(2) = 0.95, P < 0.001) between IDEAL and MRS-based fat fractions. In the pancreas, a poorer agreement between IDEAL and MRS was observed (slope = 0.32, r(2) = 0.51, P < 0.02). The discrepancy of PFF is likely explained by MRS signal contamination from surrounding visceral fat, presumably during respiratory motion. We conclude that IDEAL is equally accurate in characterizing hepatic fat content as MRS, and is potentially better suited for fat quantification in smaller organs such as the pancreas.

Quantification of hepatic steatosis with MRI: the effects of accurate fat spectral modeling

PURPOSE

To develop a chemical-shift-based imaging method for fat quantification that accounts for the complex spectrum of fat, and to compare this method with MR spectroscopy (MRS). Quantitative noninvasive biomarkers of hepatic steatosis are urgently needed for the diagnosis and management of nonalcoholic fatty liver disease (NAFLD).

MATERIALS AND METHODS

Hepatic steatosis was measured with "fat-fraction" images in 31 patients using a multiecho chemical-shift-based water-fat separation method at 1.5T. Fat-fraction images were reconstructed using a conventional signal model that considers fat as a single peak at -210 Hz relative to water ("single peak" reconstruction). Fat-fraction images were also reconstructed from the same source images using two methods that account for the complex spectrum of fat; precalibrated and self-calibrated "multipeak" reconstruction. Single-voxel MRS that was coregistered with imaging was performed for comparison.

RESULTS

Imaging and MRS demonstrated excellent correlation with single peak reconstruction (r(2) = 0.91), precalibrated multipeak reconstruction (r(2) = 0.94), and self-calibrated multipeak reconstruction (r(2) = 0.91). However, precalibrated multipeak reconstruction demonstrated the best agreement with MRS, with a slope statistically equivalent to 1 (0.96 +/- 0.04; P = 0.4), compared to self-calibrated multipeak reconstruction (0.83 +/- 0.05, P = 0.001) and single-peak reconstruction (0.67 +/- 0.04, P < 0.001).

CONCLUSION

Accurate spectral modeling is necessary for accurate quantification of hepatic steatosis with MRI.

Advanced MRI methods for assessment of chronic liver disease

OBJECTIVE

With recent advances in technology, advanced MRI methods such as diffusion-weighted and perfusion-weighted MRI, MR elastography, chemical shift-based fat-water separation, and MR spectroscopy can now be applied to liver imaging. We will review the respective roles of these techniques for assessment of chronic liver disease.

CONCLUSION

MRI plays an increasingly important role in assessment of patients with chronic liver disease because of the lack of ionizing radiation and the possibility of performing multiparametric imaging.

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

PURPOSE

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

MATERIALS AND METHODS

T(1)-weighted and T(2)-weighted fast spin-echo sequences with IDEAL fat-water separation and two frequency-selective fat-saturation methods (fat-selective saturation and fat-selective partial inversion) were performed on 10 knees of five asymptomatic volunteers. The IDEAL images were reconstructed using a conventional single-peak method and precalibrated and self-calibrated multipeak methods that more accurately model the NMR spectrum of fat. The signal-to-noise ratio (SNR) was measured in various tissues for all sequences. Student t-tests were used to compare SNR values.

RESULTS

Precalibrated and self-calibrated multipeak IDEAL had significantly greater suppression of signal (P < 0.05) within subcutaneous fat and bone marrow than fat-selective saturation, fat-selective partial inversion, and single-peak IDEAL for both T(1)-weighted and T(2)-weighted fast spin-echo sequences. For T(1)-weighted fast spin-echo sequences, the improvement in the suppression of signal within subcutaneous fat and bone marrow for multipeak IDEAL ranged between 65% when compared to fat-selective partial inversion to 86% when compared to fat-selectivesaturation. For T2-weighted fast spin-echo sequences, the improvement for multipeak IDEAL ranged between 21% when compared to fat-selective partial inversion to 81% when compared to fat-selective saturation.

CONCLUSION

Multipeak IDEAL fat-water separation provides improved fat suppression for T(1)-weighted and T(2)-weighted fast spin-echo imaging of the knee when compared to single-peak IDEAL and two widely used frequency-selected fat-saturation methods.

Nonalcoholic fatty liver disease: diagnostic and fat-grading accuracy of low-flip-angle multiecho gradient-recalled-echo MR imaging at 1.5 T

PURPOSE

To assess the accuracy of four fat quantification methods at low-flip-angle multiecho gradient-recalled-echo (GRE) magnetic resonance (MR) imaging in nonalcoholic fatty liver disease (NAFLD) by using MR spectroscopy as the reference standard.

MATERIALS AND METHODS

In this institutional review board-approved, HIPAA-compliant prospective study, 110 subjects (29 with biopsy-confirmed NAFLD, 50 overweight and at risk for NAFLD, and 31 healthy volunteers) (mean age, 32.6 years +/- 15.6 [standard deviation]; range, 8-66 years) gave informed consent and underwent MR spectroscopy and GRE MR imaging of the liver. Spectroscopy involved a long repetition time (to suppress T1 effects) and multiple echo times (to estimate T2 effects); the reference fat fraction (FF) was calculated from T2-corrected fat and water spectral peak areas. Imaging involved a low flip angle (to suppress T1 effects) and multiple echo times (to estimate T2* effects); imaging FF was calculated by using four analysis methods of progressive complexity: dual echo, triple echo, multiecho, and multiinterference. All methods except dual echo corrected for T2* effects. The multiinterference method corrected for multiple spectral interference effects of fat. For each method, the accuracy for diagnosis of fatty liver, as defined with a spectroscopic threshold, was assessed by estimating sensitivity and specificity; fat-grading accuracy was assessed by comparing imaging and spectroscopic FF values by using linear regression.

RESULTS

Dual-echo, triple-echo, multiecho, and multiinterference methods had a sensitivity of 0.817, 0.967, 0.950, and 0.983 and a specificity of 1.000, 0.880, 1.000, and 0.880, respectively. On the basis of regression slope and intercept, the multiinterference (slope, 0.98; intercept, 0.91%) method had high fat-grading accuracy without statistically significant error (P > .05). Dual-echo (slope, 0.98; intercept, -2.90%), triple-echo (slope, 0.94; intercept, 1.42%), and multiecho (slope, 0.85; intercept, -0.15%) methods had statistically significant error (P < .05).

CONCLUSION

Relaxation- and interference-corrected fat quantification at low-flip-angle multiecho GRE MR imaging provides high diagnostic and fat-grading accuracy in NAFLD.

Multiecho water-fat separation and simultaneous R2* estimation with multifrequency fat spectrum modeling

Multiecho chemical shift-based water-fat separation methods are seeing increasing clinical use due to their ability to estimate and correct for field inhomogeneities. Previous chemical shift-based water-fat separation methods used a relatively simple signal model that assumes both water and fat have a single resonant frequency. However, it is well known that fat has several spectral peaks. This inaccuracy in the signal model results in two undesired effects. First, water and fat are incompletely separated. Second, methods designed to estimate T(2) (*) in the presence of fat incorrectly estimate the T(2) (*) decay in tissues containing fat. In this work, a more accurate multifrequency model of fat is included in the iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) water-fat separation and simultaneous T(2) (*) estimation techniques. The fat spectrum can be assumed to be constant in all subjects and measured a priori using MR spectroscopy. Alternatively, the fat spectrum can be estimated directly from the data using novel spectrum self-calibration algorithms. The improvement in water-fat separation and T(2) (*) estimation is demonstrated in a variety of in vivo applications, including knee, ankle, spine, breast, and abdominal scans.

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

PURPOSE

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

Preliminary results in ex vivo swine samples demonstrated the feasibility of computing absolute fat mass as a quantitative endpoint using chemical-shift fat-water MRI with a signal model based on reference fat from pure adipose tissue.