Fat and water magnetic resonance imaging

Water-fat separation from a single spatiotemporally encoded echo based on nominal k-space peaking and joint regularized estimation

PURPOSE

To present a new high-resolution single-point water-fat separation algorithm based on the spatiotemporally encoded chemical shift imaging technique.

THEORY

Identifying water and fat peaks on the ensemble of the nominal k-space profiles of all spatiotemporally encoded lines enables evaluation of the mean off-resonance frequencies of the two components. With utilization of the spatial smoothness and filtering regularizations, the water/fat profiles can be discriminated with twice joint linear least squares estimations line-by-line.

METHODS

The effectiveness of the proposed algorithm was assessed by experiments on oil-water phantoms and in vivo in rats at 7T using a spatiotemporally encoded variant of the multishot spin-echo sequence. The results were compared with those obtained from previously proposed 1-point Dixon, 2-point Dixon, and 3-point IDEAL methods.

RESULTS

The results demonstrate that the new technique can achieve high-quality water-fat separations, comparable in signal-to-noise ratio and contrast to the multipoint methods and is more robust in cases when large areas of low signals or motion artifacts jeopardize the results from the 1-point Dixon method.

CONCLUSIONS

The proposed technique is potentially a new viable alternative for single-point water-fat separation.

Impact of sequential proton density fat fraction for quantification of hepatic steatosis in nonalcoholic fatty liver disease

OBJECTIVE

To determine the utility of sequential MRI-estimated proton density fat fraction (MRI-PDFF) for quantification of the longitudinal changes in liver fat content in individuals with nonalcoholic fatty liver disease (NAFLD).

METHODS

A total of 18 consecutive individuals (M/F: 10/8, mean age: 47.7±9.8 years) diagnosed with NAFLD, who underwent sequential PDFF calculations for the quantification of hepatic steatosis at two different time points, were included in the study. All patients underwent T1-independent volumetric multi-echo gradient-echo imaging with T2* correction and spectral fat modeling.

RESULTS

A close correlation for quantification of hepatic steatosis between the initial MRI-PDFF and liver biopsy was observed (rs=0.758, p<0.001). The median interval between two sequential MRI-PDFF measurements was 184 days. From baseline to the end of the follow-up period, serum GGT level and homeostasis model assessment score were significantly improved (p=0.015, p=0.006, respectively), whereas BMI, serum AST, and ALT levels were slightly decreased. MRI-PDFFs were significantly improved (p=0.004). A good correlation between two sequential MRI-PDFF calculations was observed (rs=0.714, p=0.001). With linear regression analyses, only delta serum ALT levels had a significant effect on delta MRI-PDFF calculations (r2=38.6%, p=0.006). At least 5.9% improvement in MRI-PDFF is needed to achieve a normalized abnormal ALT level. The improvement of MRI-PDFF score was associated with the improvement of biochemical parameters in patients who had improvement in delta MRI-PDFF (p<0.05).

CONCLUSIONS

MRI-PDFF can be used for the quantification of the longitudinal changes of hepatic steatosis. The changes in serum ALT levels significantly reflected changes in MRI-PDFF in patients with NAFLD.

Diagnostic accuracy of dual-echo (in- and opposed-phase) T1-weighted gradient recalled echo for detection and grading of hepatic iron using quantitative and visual assessment

OBJECTIVES

Detection and quantification of hepatic iron with dual-echo gradient recalled echo (GRE) has been proposed as a rapid alternative to other magnetic resonance imaging (MRI) techniques. Co-existing steatosis and T1 weighting are limitations. This study assesses the accuracy of routine dual-echo GRE.

METHODOLOGY

Between 2010 and 2013, 109 consecutive patients underwent multi-echo (ME) MRI and dual-echo GRE for quantification of hepatic iron. Liver iron concentration (LIC) was calculated from ME-MRI. Relative signal intensity (RSI) and fat signal fraction (FSF) were calculated from dual-echo GRE. Four radiologists subjectively evaluated dual-echo GRE (±subtraction). Diagnostic accuracy was compared between techniques and correlated with biopsy using Fisher's exact test, Spearman correlation and regression.

RESULTS

The sensitivity of visual detection of iron ranged from 48 to 55%. Subtraction did not increase sensitivity (p < 0.001). Inter-observer variability was substantial (κ = 0.72). The specificity of visual detection of iron approached 100% with false-positive diagnoses observed using subtraction. LIC showed a higher correlation with histopathological iron grade (r = 0.94, p < 0.001) compared with RSI (r = 0.65, p = 0.02). Univariate regression showed an association between RSI and LIC (B = 0.98, p < 0.001, CI 0.73-1.23); however, the association was not significant with multi-variate regression including FSF (p = 0.28).

CONCLUSIONS

Dual-echo GRE has low sensitivity for hepatic iron. Subtraction imaging can result in false-positive diagnoses.

KEY POINTS

• Routine liver MRI studies cannot effectively screen patients for iron overload. • Concomitant hepatic steatosis and iron limits diagnostic accuracy of routine liver MRI. • Dual-echo GRE subtraction imaging causes false-positive diagnoses of iron overload. • Dedicated MRI techniques should be used to diagnose and quantify iron overload.

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.

Validation of a generic approach to muscle water T2 determination at 3T in fat-infiltrated skeletal muscle

PURPOSE

To introduce a novel method for skeletal muscle water T2 determination in fat-infiltrated tissues, using a tri-exponential fit of the global muscle signal decay.

MATERIALS AND METHODS

In all, 48 patients with various neuromuscular diseases were retrospectively selected and their thigh muscles analyzed. Each patient was imaged using a multispin-echo (MSME) sequence with a 17-echo train. The transmit field (B1+) inhomogeneities were evaluated using the actual flip angle imaging method toward voxel sorting. Muscle water T2 was quantified using a tri-exponential signal decay model. The difference between water T2 of voxels within the same muscle but having different fat ratio was analyzed using nonparametric statistical tests. In addition, we evaluated the correlation between fat ratio and T2 values obtained using both a mono- and tri-exponential approach.

RESULTS

The results showed that muscle water T2 values obtained using a tri-exponential approach combined with B1+ map-based voxel sorting were independent of the fat infiltration degree inside the muscle (R(2) < 0.03). This was not the case using the mono-exponential model, which measured different T2s between voxels of the same muscle but with various fat ratio (R(2) > 0.67; P < 10e(-4) ).

CONCLUSION

The tri-exponential model is an accurate tool to monitor muscle tissue disease activity devoid of bias introduced by fat infiltration.

Assessing skeletal muscle mass: historical overview and state of the art

BACKGROUND

Even though skeletal muscle (SM) is the largest body compartment in most adults and a key phenotypic marker of sarcopenia and cachexia, SM mass was until recently difficult and often impractical to quantify in vivo. This review traces the historical development of SM mass measurement methods and their evolution to advances that now promise to provide in-depth noninvasive measures of SM composition.

METHODS

Key steps in the advancement of SM measurement methods and their application were obtained from historical records and widely cited publications over the past two centuries. Recent advances were established by collecting information on notable studies presented at scientific meetings and their related publications.

RESULTS

The year 1835 marks the discovery of creatine in meat by Chevreul, a finding that still resonates today in the D3-creatine method of measuring SM mass. Matiegka introduced an anthropometric approach for estimating SM mass in 1921 with the vision of creating a human "capacity" marker. The 1940s saw technological advances eventually leading up to the development of ultrasound and bioimpedance analysis methods of quantifying SM mass in vivo. Continuing to seek an elusive SM mass "reference" method, Burkinshaw and Cohn introduced the whole-body counting-neutron activation analysis method and provided some of the first detailed reports of cancer cachexia in the late 1970s. Three transformative breakthroughs leading to the current SM mass reference methods appeared in the 1970s and early 1980s as follows: the introduction of computed tomography (CT), photon absorptiometry, and magnetic resonance (MR) imaging. Each is advanced as an accurate and/or practical approach to quantifying whole-body and regional SM mass across the lifespan. These advances have led to a new understanding of fundamental body size-SM mass relationships that are now widely applied in the evaluation and monitoring of patients with sarcopenia and cachexia. An intermediate link between SM mass and function is SM composition. Advances in water-fat MR imaging, diffusion tensor imaging, MR elastography, imaging of connective tissue structures by ultra-short echo time MR, and other new MR approaches promise to close the gap that now exists between SM anatomy and function.

CONCLUSIONS

The global efforts of scientists over the past two centuries provides us with highly accurate means by which to measure SM mass across the lifespan with new advances promising to extend these efforts to noninvasive methods for quantifying SM composition.

Putting the fat and water protons to work for you: a demonstration through clinical cases of how fat-water separation techniques can benefit your body MRI practice

OBJECTIVE

Separation methods exploit the different precessional frequencies of fat and water protons. They offer potential benefits to body MRI but are not routinely used in most practices. After a review of the technique, we highlight through cases promising applications of this technology, including using water-only series to obtain more robust fat-suppressed images, shortening MRI scanning protocol time, and achieving perfect coregistering of fat-suppressed and non-fat-suppressed images.

CONCLUSION

As technology has advanced, fat-water separation techniques have shown that they can be valuable tools for a body MRI practice, particularly when the presence or absence of fat, homogeneity of fat saturation, coregistration, or quantitative evaluation is critical.

Chemical shift encoding-based water-fat separation methods

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

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.

Heterogeneity of muscle fat infiltration in children with spina bifida

Children with spina bifida have well recognized functional deficits of muscle, but little is known about the associated changes in muscle anatomy and composition. This study used water-fat magnetic resonance imaging (MRI) to measure fat infiltration in the lower extremity muscles of 11 children with myelomeningocele, the most severe form of spina bifida. MRI measurements of muscle fat fraction (FF) were compared against manual muscle test (MMT) scores for muscle strength. The FF measurements were objective and reliable with mean inter-rater differences of <2% and intraclass correlation coefficients>0.98. There was a significant inverse relationship between muscle FF and MMT scores (P ≤ 0.001). Surprisingly, however, muscles with negligible strength (MMT 0-1) exhibited a bimodal distribution of FF with one group having FF>70% and another group having FF<20%. The MRI also revealed striking heterogeneity amongst individual muscles in the same muscle group (e.g., 4% fat in one participant's lateral gastrocnemius vs. 88% in her medial gastrocnemius), as well as significant asymmetry in FF in one participant with asymmetric strength and sensation. These results suggest that quantitative water-fat MRI may serve as a biomarker for muscle degeneration which may reveal subclinical changes useful for predicting functional potential and prognosis.

Four dimensional spectral-spatial fat saturation pulse design

PURPOSE

The conventional spectrally selective fat saturation pulse may perform poorly with inhomogeneous amplitude of static (polarizing) field (B0 ) and/or amplitude of (excitation) radiofrequency field (B1 ) fields. We propose a four dimensional spectral-spatial fat saturation pulse that is more robust to B0/B1 inhomogeneity and also shorter than the conventional fat saturation pulse.

THEORY

The proposed pulse is tailored for local B0 inhomogeneity, which avoids the need of a sharp transition band in the spectral domain, so it improves both performance and pulse length. Furthermore, it can also compensate for B1 inhomogeneity. The pulse is designed sequentially by small-tip-angle approximation design and an automatic rescaling procedure.

METHODS

The proposed method is compared to the conventional fat saturation in phantom experiments and in vivo knee imaging at 3 T for both single-channel and parallel excitation versions.

RESULTS

Compared to the conventional method, the proposed method produces superior fat suppression in the presence of B0 and B1 inhomogeneity and reduces pulse length by up to half of the standard length.

CONCLUSION

The proposed four dimensional spectral-spatial fat saturation suppresses fat more robustly with shorter pulse length than the conventional fat saturation in the presence of B0 and B1 inhomogeneity.

Quantitative proton MR techniques for measuring fat

Accurate, precise and reliable techniques for the quantification of body and organ fat distributions are important tools in physiology research. They are critically needed in studies of obesity and diseases involving excess fat accumulation. Proton MR methods address this need by providing an array of relaxometry-based (T1, T2) and chemical shift-based approaches. These techniques can generate informative visualizations of regional and whole-body fat distributions, yield measurements of fat volumes within specific body depots and quantify fat accumulation in abdominal organs and muscles. MR methods are commonly used to investigate the role of fat in nutrition and metabolism, to measure the efficacy of short- and long-term dietary and exercise interventions, to study the implications of fat in organ steatosis and muscular dystrophies and to elucidate pathophysiological mechanisms in the context of obesity and its comorbidities. The purpose of this review is to provide a summary of mainstream MR strategies for fat quantification. The article succinctly describes the principles that differentiate water and fat proton signals, summarizes the advantages and limitations of various techniques and offers a few illustrative examples. The article also highlights recent efforts in the MR of brown adipose tissue and concludes by briefly discussing some future research directions.

Clinical application of 3D VIBECAIPI-DIXON for non-enhanced imaging of the pancreas compared to a standard 2D fat-saturated FLASH

PURPOSE

To compare a fast 3D VIBE sequence with Dixon fat saturation and CAIPIRINHA acceleration techniques (3D VIBE(CAIPI-DIXON)) to a standard 2D FLASH sequence with spectral fat saturation and conventional GRAPPA acceleration technique (2D Flash(GRAPPA-fs)) for non-enhanced imaging of the pancreas.

METHODS AND MATERIALS

In this retrospective, institutional review board-approved intra-individual comparison study, 29 patients (7 female, 22 male; mean age 60.4 ± 20.9 years) examined on a 48-channel 3.0-T MR system (MAGNETOM Skyra VD 13, Siemens Healthcare Sector, Germany) were included. 3D VIBE(CAIPI-DIXON) (TR/TE-3.95/2.5+1.27 ms; spatial resolution-1.2 × 1.2 × 3.0 mm(3); CAIPIRINHA 2 × 2 [1], acquisition time-0:12 min) and 2D Flash(GRAPPA-fs) (TR/TE-195/3.69 ms; 1.2 × 1.2 × 3.0 mm(3); GRAPPA 2, 3 × 0:21 min) sequences were performed in each subject in random order prior to the administration of an intravenous contrast agent. Two radiologists evaluated the images with regard to diagnostic preference. Semi-quantitative signal ratios were calculated for the pancreas versus the liver, spleen, muscle, and visceral fat. Inter-reader agreement was calculated using unweighted Cohen's kappa. Signal ratio results were analyzed using a univariate analysis of variance. Additional signal-to-noise (SNR) measurements were performed in a phantom.

RESULTS

3D VIBE(CAIPI-DIXON) was preferred in 72.4% (both readers) and 2D Flash(GRAPPA-fs) in 3.4%/6.9% (reader 1/2) of cases with a kappa value of 0.756. The main reasons for this preference were homogenous fat saturation with 3D VIBE(CAIPI-DIXON) and reduced motion artifacts due to a faster acquisition, leading to improved delineation of the pancreas. Signal ratios of pancreatic to fat signal for 3D VIBE(CAIPI-DIXON) (10.08 ± 3.48) and 2D Flash(GRAPPA-fs) (6.53 ± 3.07) were statistically different (P<.001). However, no additional statistically significant differences in signal ratios were identified (range: 0.73 ± 0.18 to 1.37 ± 0.40; .514<P<.961). SNR did not statistically significantly differ between the sequences.

CONCLUSION

3D VIBE(CAIPI-DIXON) enables robust pancreatic imaging with a shorter time and improved fat suppression relative to conventional 2D Flash(GRAPPA-fs). At an acquisition time of 12 seconds, 3D VIBE(CAIPI-DIXON) can be obtained in considerably less time than standard fat-saturated VIBE sequences.

Fat-water interface on susceptibility-weighted imaging and gradient-echo imaging: comparison of phantoms to intracranial lipomas

OBJECTIVE

In a clinical setting, lipoma can sometime show low signal intensity on susceptibility-weighted imaging (SWI) mimicking hemorrhage. The purpose of this study was to evaluate the fat-water interface chemical-shift artifacts between SWI and T2*-weighted imaging with a phantom study and evaluate SWI in lipoma cases.

MATERIALS AND METHODS

SWI, magnitude, high-pass filtered phase, and T2*-weighted imaging of a lard-water phantom were evaluated in the in-phase, out-of phase, and standard partially out-of-phase TE settings used for clinical 3-T SWI (19.7, 20.9, and 20.0 ms, respectively) to identify the most prominent fat-water interface low signal. SWI of five cases of CNS lipoma were retrospectively evaluated by two neuroradiologists.

RESULTS

TE at 19.7 ms (in-phase) showed the minimum fat-water interface low signal in the phase-encoding direction on magnitude, high-pass filtered phase, and SWI. TE at 20.9 ms (out-of-phase) showed the maximum fat-water interface in the phase-encoding direction on magnitude, high-pass filtered phase, and SWI. TE at 20.0 ms (partially out-of-phase) showed more fat-water interface low signal on SWI than on T2*-weighted imaging, especially in the phase-encoding direction. All lipomas in the five patients showed high signal intensity with surrounding peripheral dark rim on SWI.

CONCLUSION

Fat-water interface is more prominent on the standard TE setting used for clinical SWI (20.0 ms) than that of T2*-weighted imaging and shows a characteristic surrounding peripheral low-signal-intensity rim in lipoma. Knowing the fat-water appearance on SWI is important to avoid misinterpreting intracranial lipomas as hemorrhages.

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.

Usefulness of India ink artifact in steady-state free precession pulse sequences for detection and quantification of intramyocardial fat

PURPOSE

In steady state free precession (SSFP) images acquired with a repetition time/echo time (TR/TE) ≈ 2, fat is surrounded by a black boundary, called "India Ink" artifact. Indian Ink artifact may improve detection of intramyocardial fat. Aims of this study were: (i) to assess the accuracy of SSFP technique for the detection of fat metaplasia in remote myocardial infarction (RMI); (ii) to evaluate the inter- and intraobserver reproducibility for the quantification of intramyocardial fat using SSFP and fast spin echo/short TI inversion recovery (FSE/STIR) techniques.

MATERIALS AND METHODS

A total of 200 patients (age 64 ± 10 years) with RMI (>1000 days) underwent MRI using a 1.5 Tesla (T) scanner. SSFP images (with a TR/TE ≈2), FSE and STIR images were acquired in short and long axis views. Fat was detected in FSE/STIR and SSFP images and its extent manually measured . The inter- and intraobserver agreement for the quantification of fat metaplasia using both the SSFP image and the FSE images was evaluated.

RESULTS

Left ventricle intramyocardial fat was detected in SSFP images of 95 patients (47.5%) and in FSE/STIR images of 84 patients (42%). A very good agreement was found using the SSFP technique between investigators.

CONCLUSION

SSFP sequence with TR/TE=2 is a valuable technique for identifying and quantifying the presence of fat tissue within the left ventricle myocardium in RMI.

MR quantitative biomarkers of non-alcoholic fatty liver disease: technical evolutions and future trends

Non-alcoholic fatty liver disease (NAFLD) is characterized by hepatic steatosis as the earliest manifestation and hallmark, and ranges from benign fatty liver to non-alcoholic steatohepatitis (NASH). Liver biopsy (LB) is considered the reference standard for NAFLD diagnosis, grading and characterization, but it is limited by its invasiveness and observer-dependence. Among imaging surrogates for the assessment of hepatic steatosis, MR is the most accurate. (1)H MR spectroscopy (MRS) provides a quantitative biomarker of liver fat content (LFC) called proton density fat fraction (PDFF), but it is time-consuming, not widely available and limited in sample size. Several MR imaging (MRI) techniques, in particular fat suppression and in-opposed phase techniques, have been used to quantify hepatic steatosis, mainly estimating LFC from water and fat signal intensities rather than proton densities. Several technical measures have been introduced to minimize the effect of confounding factors, in particular a low flip angle, a multiecho acquisition and a spectral modeling of fat with multipeak reconstruction to address respectively T1 effect, T2* effect, and the multifrequency interference effects of fat protons, allowing to use MRI to estimate LFC based on PDFF. Tang et al. evaluated MRI-estimated PDFF, obtained by applying the above-mentioned technical improvements, in the assessment of hepatic steatosis, using histopathology as the reference standard. The identification of PDFF thresholds, even though to be further explored and validated in larger and more diverse cohorts, is useful to identify steatosis categories based on MRI-based steatosis percentages. MRI, with the new refined techniques which provide a robust quantitative biomarker of hepatic steatosis (PDFF) evaluated on the whole liver parenchyma, is a promising non-invasive alternative to LB as the gold standard for steatosis diagnosis and quantification.

Comparing T1-weighted and T2-weighted three-point Dixon technique with conventional T1-weighted fat-saturation and short-tau inversion recovery (STIR) techniques for the study of the lumbar spine in a short-bore MRI machine

AIM

To compare T1-weighted (W) fat-water separation (Dixon's technique) with T1W fat-saturation (FS) and T2W Dixon with short-tau inversion recovery (STIR) images for fat suppression in a short-bore MRI machine.

MATERIALS AND METHODS

Thirteen patients with lumbar disease were studied on using 1.5 T MRI machine. The protocol included T1 FS (with contrast medium administration) and/or STIR and T1W and/or T2W Dixon, for comparison. Three neuroradiologists scored the images from the two-pairs of techniques for fat suppression uniformity and lesion conspicuity. Clinical usefulness of fat-only images was also evaluated.

RESULTS

Regarding uniformity of fat suppression, mean scores were 2.28 (±0.49), 3.06 (±0.49), 2.39 (±0.49), and 3.15 (±0.35) for T1W FS, T1W Dixon, STIR, and T2W Dixon sequences, respectively. For the same pulse sequences, lesion conspicuity scores were 2.78 (±0.50), 2.78 (±0.27), 2.76 (±0.47), and 2.91 (±0.4), respectively. Both T1W and T2W Dixon sequences showed more homogeneous fat-suppression when compared to T1W FS (p = 0.026) and STIR (p = 0.008) techniques, but no significant difference was found for lesion conspicuity. Mean scores for the diagnostic utility of fat-only maps were, respectively, 1.72 (±0.39) and 2.48 (±0.50) for T1W and T2W Dixon.

CONCLUSION

Fat suppression quality was superior with Dixon when compared to the conventional sequences, but not lesion conspicuity, suggesting that both techniques are reliable for diagnosis. Dixon may be advantageous in cases where inhomogeneity artefacts are an issue. Water-only maps appear to be useful in the clinical setting.

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.

Utility of STIR MRI in pediatric cervical spine clearance after trauma

OBJECT

Although MRI with short-term T1 inversion recovery (STIR) sequencing has been widely adopted in the clearance of cervical spine in adults who have sustained trauma, its applicability for cervical spine clearance in pediatric trauma patients remains unclear. The authors sought to review a Level 1 trauma center's experience using MRI for posttraumatic evaluation of the cervical spine in pediatric patients.

METHODS

A pediatric trauma database was retrospectively queried for patients who received an injury warranting radiographic imaging of the cervical spine and had a STIR-MRI sequence of the cervical spine performed within 48 hours of injury between 2002 and 2011. Demographic, radiographic, and outcome data were retrospectively collected through medical records.

RESULTS

Seventy-three cases were included in the analysis. The mean duration of follow-up was 10 months (range 4 days-7 years). The mean age of the patients at the time of trauma evaluation was 8.3 ± 5.8 years, and 65% were male. The majority of patients were involved in a motor vehicle accident. In 70 cases, the results of MRI studies were negative, and the patients were cleared prior to discharge with no clinical suggestion of instability on follow-up. In 3 cases, the MRI studies had abnormal findings; 2 of these 3 patients were cleared with dynamic radiographs during the same admission. Only 1 patient had an unstable injury and required surgical stabilization. The sensitivity of STIR MRI to detect cervical instability was 100% with a specificity of 97%. The positive predictive value was 33% and the negative predictive value was 100%.

CONCLUSIONS

Although interpretation of our results are diminished by limitations of the study, in our series, STIR MRI in routine screening for pediatric cervical trauma had a high sensitivity and slightly lower specificity, but may have utility in future practices and should be considered for implementation into protocols.

A historical overview of magnetic resonance imaging, focusing on technological innovations

Magnetic resonance imaging (MRI) has now been used clinically for more than 30 years. Today, MRI serves as the primary diagnostic modality for many clinical problems. In this article, historical developments in the field of MRI will be discussed with a focus on technological innovations. Topics include the initial discoveries in nuclear magnetic resonance that allowed for the advent of MRI as well as the development of whole-body, high field strength, and open MRI systems. Dedicated imaging coils, basic pulse sequences, contrast-enhanced, and functional imaging techniques will also be discussed in a historical context. This article describes important technological innovations in the field of MRI, together with their clinical applicability today, providing critical insights into future developments.

MR Neurography: Advances

High resolution and high field magnetic resonance neurography (MR neurography, MRN) is shown to have excellent anatomic capability. There have been considerable advances in the technology in the last few years leading to various feasibility studies using different structural and functional imaging approaches in both clinical and research settings. This paper is intended to be a useful seminar for readers who want to gain knowledge of the advancements in the MRN pulse sequences currently used in clinical practice as well as learn about the other techniques on the horizon aimed at better depiction of nerve anatomy, pathology, and potential noninvasive evaluation of nerve degeneration or regeneration.

Chemical shift encoded water-fat separation using parallel imaging and compressed sensing

Chemical shift encoded techniques have received considerable attention recently because they can reliably separate water and fat in the presence of off-resonance. The insensitivity to off-resonance requires that data be acquired at multiple echo times, which increases the scan time as compared to a single echo acquisition. The increased scan time often requires that a compromise be made between the spatial resolution, the volume coverage, and the tolerance to artifacts from subject motion. This work describes a combined parallel imaging and compressed sensing approach for accelerated water-fat separation. In addition, the use of multiscale cubic B-splines for B(0) field map estimation is introduced. The water and fat images and the B(0) field map are estimated via an alternating minimization. Coil sensitivity information is derived from a calculated k-space convolution kernel and l(1)-regularization is imposed on the coil-combined water and fat image estimates. Uniform water-fat separation is demonstrated from retrospectively undersampled data in the liver, brachial plexus, ankle, and knee as well as from a prospectively undersampled acquisition of the knee at 8.6x acceleration.

23Na magnetic resonance imaging-determined tissue sodium in healthy subjects and hypertensive patients

High dietary salt intake is associated with hypertension; the prevalence of salt-sensitive hypertension increases with age. We hypothesized that tissue Na(+) might accumulate in hypertensive patients and that aging might be accompanied by Na(+) deposition in tissue. We implemented (23)Na magnetic resonance imaging to measure Na(+) content of soft tissues in vivo earlier, but had not studied essential hypertension. We report on a cohort of 56 healthy control men and women, and 57 men and women with essential hypertension. The ages ranged from 22 to 90 years. (23)Na magnetic resonance imaging measurements were made at the level of the calf. We observed age-dependent increases in Na(+) content in muscle in men, whereas muscle Na(+) content did not change with age in women. We estimated water content with conventional MRI and found no age-related increases in muscle water in men, despite remarkable Na(+) accumulation, indicating water-free Na(+) storage in muscle. With increasing age, there was Na(+) deposition in the skin in both women and men; however, skin Na(+) content remained lower in women. Similarly, this sex difference was found in skin water content, which was lower in women than in men. In contrast to muscle, increasing Na(+) content was paralleled with increasing skin water content. When controlled for age, we found that patients with refractory hypertension had increased tissue Na(+) content, compared with normotensive controls. These observations suggest that (23)Na magnetic resonance imaging could have utility in assessing the role of tissue Na(+) storage for cardiovascular morbidity and mortality in longitudinal studies.

Evaluation of ADC measurements among solid pancreatic masses by respiratory-triggered diffusion-weighted MR imaging with inversion-recovery fat-suppression technique at 3.0T

PURPOSE

The objective of this paper was to investigate the value of apparent diffusion coefficients (ADCs) for differential diagnosis among solid pancreatic masses using respiratory triggered diffusion-weighted MR imaging with inversion-recovery fat-suppression technique (RT-IR-DWI) at 3.0 T.

MATERIALS AND METHODS

20 normal volunteers and 72 patients (Pancreatic ductal adenocarcinoma [PDCA, n=30], mass-forming pancreatitis [MFP, n=15], solid pseudopapillary neoplasm [SPN, n=12], and pancreatic neuroendocrine tumor[PNET, n=15]) underwent RT-IR-DWI (b values: 0 and 600 s/mm(2)) at 3.0 T. Results were correlated with histopathologic data and follow-up imaging. ADC values among different types of pancreatic tissue were statistically analyzed and compared.

RESULTS

Statistical difference was noticed in ADC values among normal pancreas, MFP, PDCA, SPN and PNET by ANOVA (p<.001). Normal pancreas had the highest ADC value, then followed by PNET, PDCA, MFP and SPN. There was noticeable statistical difference in ADC values among PDCA, MFP and normal pancreas by Least Significant Difference (LSD) (p<.001). ADC of SPN was statistically lower than that of PNET (p=0.1800×10(-4)), PDCA (p=0.0300×10(-4)) and normal pancreas (p=0.0007×10(-4)). ADC of PNET was statistically lower than that of normal pancreas (p=0.0360) and higher than that of MFP (p=9.3000×10(-4)).

CONCLUSIONS

ADC measurements using RT-IR-DWI at 3.0T may aid to disclose the histopathological pattern of normal pancreas and solid pancreatic masses, which may be helpful in characterizing solid pancreatic lesions.

Adipose tissue MRI for quantitative measurement of central obesity

PURPOSE

To validate adipose tissue magnetic resonance imaging (atMRI) for rapid, quantitative volumetry of visceral adipose tissue (VAT) and total adipose tissue (TAT).

MATERIALS AND METHODS

Data were acquired on normal adults and clinically overweight girls with Institutional Review Board (IRB) approval/parental consent using sagittal 6-echo 3D-spoiled gradient-echo (SPGR) (26-sec single-breath-hold) at 3T. Fat-fraction images were reconstructed with quantitative corrections, permitting measurement of a physiologically based fat-fraction threshold in normals to identify adipose tissue, for automated measurement of TAT, and semiautomated measurement of VAT. TAT accuracy was validated using oil phantoms and in vivo TAT/VAT measurements validated with manual segmentation. Group comparisons were performed between normals and overweight girls using TAT, VAT, VAT-TAT-ratio (VTR), body-mass-index (BMI), waist circumference, and waist-hip-ratio (WHR).

RESULTS

Oil phantom measurements were highly accurate (<3% error). The measured adipose fat-fraction threshold was 96% ± 2%. VAT and TAT correlated strongly with manual segmentation (normals r(2) ≥ 0.96, overweight girls r(2) ≥ 0.99). VAT segmentation required 30 ± 11 minutes/subject (14 ± 5 sec/slice) using atMRI, versus 216 ± 73 minutes/subject (99 ± 31 sec/slice) manually. Group discrimination was significant using WHR (P < 0.001) and VTR (P = 0.004).

CONCLUSION

The atMRI technique permits rapid, accurate measurements of TAT, VAT, and VTR.

ECG-gated multiecho Dixon fat-water separation in cardiac MRI: advantages over conventional fat-saturated imaging

OBJECTIVE

The purpose of this pictorial essay is to explore the advantages of multiecho Dixon fat-water separation techniques in cardiac MRI. The clinical indications, potential artifacts, and imaging findings with this technique are reviewed.

CONCLUSION

Multiecho Dixon fat-water separation can be used to help characterize cardiac masses, evaluate for myocardial lipomatous infiltration, and diagnose pericarditis. Advantages over conventional fat-saturation techniques include fewer artifacts from background inhomogeneity, improved contrast of microscopic fat, and capability for use in combination with cine and contrast-enhanced imaging.

Spinal fusion-hardware construct: Basic concepts and imaging review

The interpretation of spinal images fixed with metallic hardware forms an increasing bulk of daily practice in a busy imaging department. Radiologists are required to be familiar with the instrumentation and operative options used in spinal fixation and fusion procedures, especially in his or her institute. This is critical in evaluating the position of implants and potential complications associated with the operative approaches and spinal fixation devices used. Thus, the radiologist can play an important role in patient care and outcome. This review outlines the advantages and disadvantages of commonly used imaging methods and reports on the best yield for each modality and how to overcome the problematic issues associated with the presence of metallic hardware during imaging. Baseline radiographs are essential as they are the baseline point for evaluation of future studies should patients develop symptoms suggesting possible complications. They may justify further imaging workup with computed tomography, magnetic resonance and/or nuclear medicine studies as the evaluation of a patient with a spinal implant involves a multi-modality approach. This review describes imaging features of potential complications associated with spinal fusion surgery as well as the instrumentation used. This basic knowledge aims to help radiologists approach everyday practice in clinical imaging.

Development and evaluation of TWIST Dixon for dynamic contrast-enhanced (DCE) MRI with improved acquisition efficiency and fat suppression

PURPOSE

To develop a new pulse sequence called time-resolved angiography with stochastic trajectories (TWIST) Dixon for dynamic contrast enhanced magnetic resonance imaging (DCE-MRI).

MATERIALS AND METHODS

The method combines dual-echo Dixon to generate separated water and fat images with a k-space view-sharing scheme developed for 3D TWIST. The performance of TWIST Dixon was compared with a volume interpolated breathhold examination (VIBE) sequence paired with spectrally selective adiabatic inversion Recovery (SPAIR) and quick fat-sat (QFS) fat-suppression techniques at 3.0T using quantitative measurements of fat-suppression accuracy and signal-to-noise ratio (SNR) efficiency, as well as qualitative breast image evaluations.

RESULTS

The water fraction of a uniform phantom was calculated from the following images: 0.66 ± 0.03 for TWIST Dixon; 0.56 ± 0.23 for VIBE-SPAIR, and 0.53 ± 0.14 for VIBE-QFS, while the reference value is 0.70 measured by spectroscopy. For phantoms with contrast (Gd-BOPTA) concentration ranging from 0-6 mM, TWIST Dixon also provides consistently higher SNR efficiency (3.2-18.9) compared with VIBE-SPAIR (2.8-16.8) and VIBE-QFS (2.4-12.5). Breast images acquired with TWIST Dixon at 3.0T show more robust and uniform fat suppression and superior overall image quality compared with VIBE-SPAIR.

CONCLUSION

The results from phantom and volunteer evaluation suggest that TWIST Dixon outperforms conventional methods in almost every aspect and it is a promising method for DCE-MRI and contrast-enhanced perfusion MRI, especially at higher field strength where fat suppression is challenging.

Quantifying hepatic steatosis - more than meets the eye

AIMS

The non-alcoholic fatty liver disease (NAFLD) activity score (NAS) is the histological tool used to assess disease severity based on steatosis, inflammation and hepatocyte ballooning. As steatosis contributes up to three of a potential eight points to NAS, it is important to quantify steatosis accurately. We sought to determine the optimum histological technique for identifying fat in tissue.

METHODS AND RESULTS

Using tissue from a mouse model of NAFLD, with validation in human liver biopsies, the percentage steatosis and fat droplet size were assessed in haematoxylin and eosin (H&E)- and Oil Red-O (ORO)-stained sections by light microscopy and digital image analysis (DIA). Results were compared to biochemical tissue triglyceride content and MRI assessment of hepatic lipid content. H&E steatosis assessment correlated poorly with tissue triglyceride concentration. However, ORO DIA exhibited much higher sensitivity and specificity for steatosis and correlated very well with triglyceride concentration in mouse and human liver (R = 0.706, P = 0.001 and R = 0.894, P =0.041, respectively). MRI-based assessment of steatosis was inaccurate.

CONCLUSIONS

ORO DIA is the most accurate method for detecting and quantifying steatosis. Although H&E-based NAS remains clinically valid in both clinical research and experimental situations, ORO DIA is a more robust technique to assess liver steatosis accurately for NAS scoring.

Proton magnetic resonance imaging with para-hydrogen induced polarization

A major challenge in imaging is the detection of small amounts of molecules of interest. In the case of magnetic resonance imaging (MRI) their signals are typically concealed by the large background signal of e.g. the body. This problem can be tackled by hyperpolarization which increases the NMR signals up to several orders of magnitude. However, this strategy is limited for (1)H, the most widely used nucleus in NMR and MRI, because the enormous number of protons in the body screens the small amount of hyperpolarized ones. Here, we describe a method giving rise to high (1)H MRI contrast for hyperpolarized molecules against a large background signal. The contrast is based on the J-coupling induced rephasing of the NMR signal of molecules hyperpolarized via PHIP and it can easily be implemented in common pulse sequences. We discuss several scenarios with different or equal dephasing times T(2)* for the hyperpolarized and thermally polarized compounds and verify our approach by experiments. This method may open up unprecedented opportunities to use the standard MRI nucleus (1)H for e.g. metabolic imaging in the future.

[Modified DIXON: examination of the different flip angles and echo times for water-fat separation and contrast]

The method known as DIXON is imaging from two different echo times (TE), using the opposite phase of water and fat. Modified DIXON (mDIXON) is the method that is calculated from changing the opposing phases 'in' and 'opposed' phases of the actual measurement, to fit the theoretical value. We reviewed that the effect of the water-fat separation and contrast depended on the different flip angles (FA) and echo times (TE). The diluted gadolinium-ethoxybenzyl-diethylenetriamine pentaacetic acid (Gd-EOB-DTPA) (from 0 to 5 mmol/l) and liver model was stabilized by lard. In order to measure the SNR and %CV of the lard, the various combinations of TE and FA (from 5 to 20 degrees) were changed. The suitable range for the measurement of the SNR and %CV of the lard, and the contrast of gadolinium Gd-EOB-DTPA and the liver model, in the axial view (including 50% fat), was around the 'opposed' phase in the first TE (TE1) and around the 'in' phase in the second TE (TE2) and FA: 15degrees (TE1: 2.0-2.9 ms, TE2: 4.1-5.0 ms). In order to avoid phase dispersion, shorter TE range is more desirable (TE1: 2.0-2.25 ms, TE2: 4.1-4.5 ms). It is our understanding that the water-fat separation is better in these ranges and under these circumstances, good contrast was obtained.

[Comparison of fat suppression techniques of bilateral breast dynamic sequence at 3.0 T: utility of three-point DIXON technique]

The purposes of this study were to determine optimum flip angles (FAs) and to compare the effectiveness of fat suppression and signal homogeneity among three techniques, spectral attenuated with inversion recovery (SPAIR), principle of selective excitation technique (PROSET), and three-point DIXON technique (DIXON), of the bilateral breast dynamic sequence acquired using the optimum FA at 3.0 T. Using a homemade phantom that represented a tumor, fat, and a mammary gland, the optimum FAs were determined from the change of fat signal intensity, signal-to-noise ratio (SNR) of the mammary gland, and contrast ratio (CR) between the tumor and mammary gland. The effectiveness of fat suppression and signal homogeneity were compared in ten breast cancer cases, using the CR between fat and pectoralis muscle signal intensities and the standard deviation (SD) of fat signal intensity, respectively. The optimum FAs for SPAIR, PROSET, and DIXON were 10, 20, and 20 degrees, respectively. The mean CR between fat and pectoralis muscle signal intensities achieved using SPAIR, PROSET, and DIXON were 0.19, 0.30 and 0.40, respectively, and the mean SDs of the fat signal intensities were 90.2, 103.1, and 30.5, respectively. The DIXON technique provided better fat suppression and signal homogeneity than the other two techniques. The results of this study suggest the possible application of the DIXON technique in combination with the optimum FA setting as an effective fat suppression technique for the bilateral breast dynamic sequence at 3.0 T.

Magnetic resonance imaging: the underlying principles

The creation of a magnetic resonance image (MRI) and its inherent contrast are controlled by a variety of anatomical structure- and sequence-dependent parameters. While these may seem confusing to the uninitiated, they provide MRI with great flexibility and make it a powerful clinical tool. This article describes the principles of basic physics behind magnetic resonance spectroscopy (MRS) and imaging, including a basic description of the properties of magnetic resonance compatible nuclei, how a radiofrequency (RF) pulse produces a signal, and how this signal can be spatially encoded to produce an image. The relaxation properties of the MRI signal depend on biological tissue type and can provide information on tissue composition, environment, and pathological changes. The contrast properties within an image can be manipulated based on the relaxation properties of the anatomical sample and the nature of the imaging sequence. The benefits of T1- and T2-weighted images in musculoskeletal imaging and the common sequences used (including turbo spin echo [TSE], fat suppression sequences such as STIR, and rapid breath-hold sequences such as HASTE and FISP) are discussed. The principles behind contrast agents and diffusion-weighted imaging and how they can be applied in the body are considered.

Simultaneous visualization of nerves and vessels of the lower extremities using magnetization-prepared susceptibility weighted magnetic resonance imaging at 3.0 T

BACKGROUND

Identifying the extent of involvement of the vessel and nerve, particularly in regard to preoperative evaluation and precise localization of the tumor and its relation to the structures of the extremities, has important applications for advancing the treatment of lower extremity diseases.

OBJECTIVE

To review the technical feasibility of simultaneous visualization of nerves and vessels of the lower extremities by using magnetization-prepared susceptibility-weighted magnetic resonance (MR) imaging (MP-SWI) at 3.0T.

METHODS

Ten healthy volunteers and 10 patients were studied. Optimized MP-SWI, MR neurography (MRN) based on 3D diffusion-weighted steady-state free precession imaging and contrast-enhanced MR angiography (CE-MRA) sequences were performed for each subject. The means of signal-to-noise ratio (SNR)n, SNRv, SNRm, contrast-to-noise ratio (CNR)n,m and CNRv,m were calculated and the certainty of identifying nerves and vessels was determined. CNRn,m between MP-SWI and MRN, and CNRv,m between MP-SWI and CE-MRA were compared.

RESULTS

MP-SWI provides slightly poorer CNRv,m than CE-MRA, whereas MP-SWI provides a better CNRn,m than MRN. In thin-slice-thickness maximum-intensity projection arbitrary planes, the sciatic nerve and its branches were clearly identified (score 1 or 2 of 2) in 17 subjects (85%); the femoral artery and the main branches were identified (score 1 or 2 of 2) in all 20 subjects (100%). The nerves are isointense to slightly hypointense to muscle, and the vessels show a more obvious hyperintense signal than muscle in MP-SWI.

CONCLUSION

The proposed MP-SWI demonstrates the feasibility of simultaneously visualizing nerves and vessels of the lower extremities without using an exogenous contrast agent. It may enable straightforward localization of a disease process to a specific nerve and vessel.

Volumetric fat-water separated T2-weighted MRI

BACKGROUND

Pediatric body MRI exams often cover multiple body parts, making the development of broadly applicable protocols and obtaining uniform fat suppression a challenge. Volumetric T2 imaging with Dixon-type fat-water separation might address this challenge, but it is a lengthy process.

OBJECTIVE

We develop and evaluate a faster two-echo approach to volumetric T2 imaging with fat-water separation.

MATERIALS AND METHODS

A volumetric spin-echo sequence was modified to include a second shifted echo so two image sets are acquired. A region-growing reconstruction approach was developed to decompose separate water and fat images. Twenty-six children were recruited with IRB approval and informed consent. Fat-suppression quality was graded by two pediatric radiologists and compared against conventional fat-suppressed fast spin-echo T2-W images. Additionally, the value of in- and opposed-phase images was evaluated.

RESULTS

Fat suppression on volumetric images had high quality in 96% of cases (95% confidence interval of 80-100%) and were preferred over or considered equivalent to conventional two-dimensional fat-suppressed FSE T2 imaging in 96% of cases (95% confidence interval of 78-100%). In- and opposed-phase images had definite value in 12% of cases.

CONCLUSION

Volumetric fat-water separated T2-weighted MRI is feasible and is likely to yield improved fat suppression over conventional fat-suppressed T2-weighted imaging.

Robust fat suppression at 3T in high-resolution diffusion-weighted single-shot echo-planar imaging of human brain

Single-shot echo-planar imaging is the most common acquisition technique for whole-brain diffusion tensor imaging (DTI) studies in vivo. Higher field MRI systems are readily available and advantageous for acquiring DTI due to increased signal. One of the practical issues for DTI with single-shot echo-planar imaging at high-field is incomplete fat suppression resulting in a chemically shifted fat artifact within the brain image. Unsuppressed fat is especially detrimental in DTI because the diffusion coefficient of fat is two orders of magnitude lower than that of parenchyma, producing brighter appearing fat artifacts with greater diffusion weighting. In this work, several fat suppression techniques were tested alone and in combination with the goal of finding a method that provides robust fat suppression and can be used in high-resolution single-shot echo-planar imaging DTI studies. Combination of chemical shift saturation with slice-select gradient reversal within a dual-spin-echo diffusion preparation period was found to provide robust fat suppression at 3 T.

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.