Measurement of relative fat content by proton magnetic resonance spectroscopy using a clinical imager

Postmortem CT and MRI findings of massive fat embolism

OBJECTIVE

To elucidate postmortem computed tomography (PMCT) and postmortem magnetic resonance (PMMR) imaging findings suggesting massive fat embolism.

MATERIALS AND METHODS

Consecutive forensic cases with PMCT and PMMR scans of subjects prior to autopsy were assessed. For PMCT, 16- or 64-row multidetector CT scans were used; for PMMR, a 1.5 T system was used. MRI sequences of the chest area included T2- and T1-weighted fast spin-echo imaging, T2*-weighted imaging, T1-weighted 3-dimensional gradient-echo imaging with or without a fat-suppression pulse, short tau inversion recovery, and in-phase/opposed-phase imaging. At autopsy, forensic pathologists checked for pulmonary fat embolism with fat staining; Falzi's grading system was used for classification.

RESULTS

Of 31 subjects, four were excluded because fat staining for histopathological examination of the lung tissue could not be performed. In three of the remaining 27 subjects, histology revealed massive fat embolism (Falzi grade III) and the cause of death was considered to be associated with fat embolism. CT detected a "fat-fluid level" in the right heart or intraluminal fat in the pulmonary arterial branches in two subjects. MRI detected these findings more clearly in both subjects. In one subject, CT and MRI were both negative. There were no positive findings in the 24 subjects that were fat embolism-negative by histology.

DISCUSSION AND CONCLUSION

In some subjects, a massive fat embolism can be suggested by postmortem imaging with a "fat-fluid level" in the right heart or intraluminal fat in the pulmonary arterial branches. PMMR potentially suggests fat embolism more clearly than PMCT.

FATTY KIDNEY DISEASE: A NEW RENAL AND ENDOCRINE CLINICAL ENTITY? DESCRIBING THE ROLE OF THE KIDNEY IN OBESITY, METABOLIC SYNDROME, AND TYPE 2 DIABETES

Objective: To determine whether fatty kidney disease deserves be designated as a distinct clinical entity similar to fatty liver disease. Methods: Analysis and interpretation of the literature in a novel conceptual framework. Results: The kidney contributes to hyperglycemia, hypertension, inflammatory cytokines, and thus to diabetes and metabolic syndrome. Fat accumulation in and around the kidney drives this process and contributes to progression of chronic kidney disease itself. Weight loss improves these complications of fatty kidney. Diagnosis currently must be inferred from comorbidities but ultimately should be made by imaging once the importance of fatty kidney disease is established, much like fatty liver disease. Conclusion: Fatty kidney disease merits designation as a specific clinical entity similar to fatty liver disease. Greater attention to this may help encourage research into ameliorating the negative consequences of fatty kidney disease and developing new therapies. Abbreviations: BP = blood pressure; CKD = chronic kidney disease; CT = computed tomography; ESRD = end-stage renal disease; FFA = free fatty acid; FKD = fatty kidney disease; GFR = glomerular filtration rate; MetS = metabolic syndrome; MRI = magnetic resonance imaging; NAFLD = nonalcoholic fatty liver disease; RAAS = renin-angiotensin system; SGLT2 = sodium-glucose cotransporter 2; SNS = sympathetic nervous system; T2D = type 2 diabetes; TG = triglyceride.

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.

Assessment of lipids in skeletal muscle by LCModel and AMARES

PURPOSE

To process single voxel spectra of the human skeletal muscle by using an advanced method for accurate, robust, and efficient spectral fitting (AMARES) and by linear combination of model spectra (LCModel). To determine absolute concentrations of extra- (EMCL) and intramyocellular lipids (IMCL).

MATERIALS AND METHODS

Single-voxel proton magnetic resonance spectroscopy (PRESS) was used to obtain the spectra of the calf muscles. Unsuppressed water line was used as a concentration reference. A new prior knowledge for AMARES was proposed to estimate the concentrations of EMCL and IMCL. The prior knowledge was derived from the spectrum of vegetable oil. The results were compared with the values estimated by LCModel. Absolute concentrations of total lipid content in millimoles per kilogram wet weight were used for the comparisons.

RESULTS

Absolute concentrations of total lipid content in skeletal muscle were estimated by AMARES and LCModel. Very good correlation of the total fat (EMCL + IMCL) and IMCL concentrations was achieved between both data processing approaches.

CONCLUSION

Assessment the absolute concentrations of muscular lipids by AMARES and LCModel can be performed with comparable accuracy.

Sonographic subcutaneous and visceral fat indices represent the distribution of body fat volume

BACKGROUND

A reliable method for direct measurement of both subcutaneous and visceral fat volume is the measurement of fat tissue area from tomographic pictures by CT or by MR imaging. However, these are not widely usable because of high cost and/or exposure to radiation.

METHODS

We compared sonographic subcutaneous and visceral fat indices with fat distribution by serial-slice MR imaging in 17 subjects. Sonographic subcutaneous or visceral fat index is standardized thickness of subcutaneous fat tissue or the intra-abdominal depth at the level of umbilicus by height.

RESULTS

Sonographic visceral fat index and intra-abdominal depth were significantly correlated with visceral fat volume by serial-slice MR imaging (r = 0.746, r = 0.726, respectively). Similarly, sonographic subcutaneous fat index and subcutaneous fat thickness were significantly correlated with subcutaneous fat volume by serial-slice MR imaging (r = 0.825, r = 0.816, respectively). The ratio of sonographic visceral fat index and sonographic subcutaneous fat index was closely correlated with the ratio of the visceral fat volume and the subcutaneous fat volume by single-slice MR imaging, which proves to be related to cardiovascular disease risk (r = 0.722).

CONCLUSION

Sonographic subcutaneous or visceral fat index could be an easily measured and inexpensive indicator for the assessment of fat distribution instead of CT or MR imaging.

Evaluation of a quantitative magnetic resonance method for mouse whole body composition analysis

OBJECTIVE

To evaluate applicability, precision, and accuracy of a new quantitative magnetic resonance (QMR) analysis for whole body composition of conscious live mice.

RESEARCH METHODS AND PROCEDURES

Repeated measures of body composition were made by QMR, DXA, and classic chemical analysis of carcass using live and dead mice with different body compositions. Caloric lean and dense diets were used to produce changes in body composition. In addition, different strains of mice representing widely diverse populations were analyzed.

RESULTS

Precision was found to be better for QMR than for DXA. The coefficient of variation for fat ranged from 0.34% to 0.71% compared with 3.06% to 12.60% for DXA. Changes in body composition in response to dietary manipulation were easily detected using QMR. An increase in fat mass of 0.6 gram after 1 week (p < 0.01) was demonstrated in the absence of hyperphagia or a change in mean body weight.

DISCUSSION

QMR and DXA detected similar fat content, but the improved precision afforded by QMR compared with DXA and chemical analysis allowed detection of a significant difference in body fat after 7 days of consuming a diet rich in fat even though average body weight did not significantly change. QMR provides a very precise, accurate, fast, and easy-to-use method for determining fat and lean tissue of mice without the need for anesthesia. Its ability to detect differences with great precision should be of value when characterizing phenotype and studying regulation of body composition brought about by pharmacological and dietary interventions in energy homeostasis.

Proton magnetic resonance spectroscopy for assessment of human body composition

BACKGROUND

The usefulness of magnetic resonance spectroscopy (MRS)-based techniques for assessment of human body composition has not been established.

OBJECTIVE

We compared a proton MRS-based technique with the total body water (TBW) method to determine the usefulness of the former technique for assessment of human body composition.

DESIGN

Proton magnetic resonance spectra of the chest to abdomen, abdomen to pelvis, and pelvis to thigh regions were obtained from 16 volunteers by using single, free induction decay measurement with a clinical magnetic resonance system operating at 1.5 T. The MRS-derived metabolite ratio was determined as the ratio of fat methyl and methylene proton resonance to water proton resonance. The peak areas for the chest to abdomen and the pelvis to thigh regions were normalized to an external reference (approximately 2200 g benzene) and a weighted average of the MRS-derived metabolite ratios for the 2 positions was calculated. TBW for each subject was determined by the deuterium oxide dilution technique.

RESULTS

The MRS-derived metabolite ratios were significantly correlated with the ratio of body fat to lean body mass estimated by TBW. The MRS-derived metabolite ratio for the abdomen to pelvis region correlated best with the ratio of body fat to lean body mass on simple regression analyses (r = 0.918). The MRS-derived metabolite ratio for the abdomen to pelvis region and that for the pelvis to thigh region were selected for a multivariate regression model (R = 0.947, adjusted R(2) = 0.881).

CONCLUSION

This MRS-based technique is sufficiently accurate for assessment of human body composition.