A fast method for the quantification of fat fraction and relaxation times: Comparison of five sites of bone marrow

MR-oximetry with fat DESPOT

PURPOSE

The R₁ relaxation rate of fat is a promising marker of tissue oxygenation. Existing techniques to map fat R₁ in MR-oximetry offer limited spatial coverage, require long scan times, or pulse sequences that are not readily available on clinical scanners. This work addresses these limitations with a 3D voxel-wise fat R₁ mapping technique for MR-oximetry based on a variable flip angle (VFA) approach at 3 T.

METHODS

Varying levels of dissolved oxygen (O₂) were generated in a phantom consisting of vials of safflower oil emulsion, used to approximate human fat. Joint voxel-wise mapping of fat and water R₁ was performed with a two-compartment VFA model fitted to multi-echo gradient-echo magnitude data acquired at four flip angles, referred to as Fat DESPOT. Global R₁ was also calculated. Variations of fat, water, and global R₁ were investigated as a function of the partial pressure of O₂ (pO₂). Inversion-prepared stimulated echo magnetic resonance spectroscopy was used as the reference technique for R₁ measurements.

RESULTS

Fat R₁ from Fat DESPOT was more sensitive than water R₁ and global R₁ to variations in pO₂, consistent with previous studies performed with different R₁ mapping techniques. Fat R₁ sensitivity to pO₂ variations with Fat DESPOT (median O₂ relaxivity r_{1, O2} = 1.57× 10^-3 s^-1 mmHg^-1) was comparable to spectroscopy-based measurements for methylene, the main fat resonance (median r_{1, O2}= 1.80 × 10^-3 s^-1 mmHg^-1).

CONCLUSION

Fat and water R₁ can be measured on a voxel-wise basis using a two-component fit to multi-echo 3D VFA magnitude data in a clinically acceptable scan time. Fat and water R₁ measured with Fat DESPOT were sensitive to variations in pO₂. These observations suggest an approach to 3D in vivo MR oximetry.

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.

Voxel-based mapping of five MR biomarkers in the wrist bone marrow

OBJECTIVE

MRI is a reliable and accurate technique to characterize rheumatoid arthritis. The aim of this study was to provide voxel-by-voxel 3D maps of the proton density fat fraction (PDFF), the T₁ of water (T_1W), the T₁ of fat (T_1F), the T₂* of water (T₂*_W), the T₂* of fat (T₂*_F) in the wrist bone marrow.

MATERIALS AND METHODS

The experiments were conducted on 14 healthy volunteers (mean age: 24 ± 4). The data were acquired at 1.5 T using two optimized four-echo 3D 1.2 × 1.2 × 1.2 mm³-isotropic spoiled gradient sequences. A repeatability study was carried out. The measurements were done using a homemade parametric viewer software.

RESULTS

The inter-volunteer results were, on average: PDFF = 86 ± 3%, T_1W = 441 ± 113 ms, T_1F = 245 ± 19 ms, T₂*_W = 6 ± 1 ms and T₂*_F = 16 ± 3 ms. The coefficients of variation were for fat based biomarkers CV_PDFF < 5%, CV_T1F < 15% and CV_T2*F < 10% in the repeatability study.

DISCUSSION

The protocol and quantification tool proposed in this study provide high-resolution voxel-by-voxel 3D maps of five biomarkers in the wrist in less than 4 min of acquisition.

Effects of Sex and Age on Fat Fraction, Diffusion-Weighted Image Signal Intensity and Apparent Diffusion Coefficient in the Bone Marrow of Asymptomatic Individuals: A Cross-Sectional Whole-Body MRI Study

We aimed to describe the relationships between the relative fat fraction (%FF), muscle-normalized diffusion-weighted (DW) image signal intensity and water apparent diffusion coefficient (ADC), sex and age for normal bone marrow, in the normal population. Our retrospective cohort consisted of 100 asymptomatic individuals, equally divided by sex and 10-year age groups, who underwent whole-body MRI at 1.5 T for early cancer detection. Semi-automated segmentation of global bone marrow volume was performed using the DW images and the resulting segmentation masks were projected onto the ADC and %FF maps for extraction of parameter values. Differences in the parameter values between sexes at age ranges were assessed using the Mann–Whitney and Kruskal–Wallis tests. The Spearman correlation coefficient r was used to assess the relationship of each imaging parameter with age, and of %FF with ADC and normalized DW signal intensity values. The average %FF of normal bone marrow was 65.6 ± 7.2%, while nSI_b50, nSI_b900 and ADC were 1.7 ± 0.5, 3.2 ± 0.9 and 422 ± 67 μm²/s, respectively. The bone marrow %FF values increased with age in both sexes (r = 0.63 and r = 0.64, respectively, p < 0.001). Values of nSI_b50 and nSI_b900 were higher in younger women compared to men of the same age groups (p < 0.017), but this difference decreased with age. In our cohort of asymptomatic individuals, the values of bone marrow relative %FF, normalized DW image signal intensity and ADC indicate higher cellularity in premenopausal women, with increasing bone marrow fat with aging in both sexes.

Guidelines for Biobanking of Bone Marrow Adipose Tissue and Related Cell Types: Report of the Biobanking Working Group of the International Bone Marrow Adiposity Society

Over the last two decades, increased interest of scientists to study bone marrow adiposity (BMA) in relation to bone and adipose tissue physiology has expanded the number of publications using different sources of bone marrow adipose tissue (BMAT). However, each source of BMAT has its limitations in the number of downstream analyses for which it can be used. Based on this increased scientific demand, the International Bone Marrow Adiposity Society (BMAS) established a Biobanking Working Group to identify the challenges of biobanking for human BMA-related samples and to develop guidelines to advance establishment of biobanks for BMA research. BMA is a young, growing field with increased interest among many diverse scientific communities. These bring new perspectives and important biological questions on how to improve and build an international community with biobank databases that can be used and shared all over the world. However, to create internationally accessible biobanks, several practical and legislative issues must be addressed to create a general ethical protocol used in all institutes, to allow for exchange of biological material internationally. In this position paper, the BMAS Biobanking Working Group describes similarities and differences of patient information (PIF) and consent forms from different institutes and addresses a possibility to create uniform documents for BMA biobanking purposes. Further, based on discussion among Working Group members, we report an overview of the current isolation protocols for human bone marrow adipocytes (BMAds) and bone marrow stromal cells (BMSCs, formerly mesenchymal), highlighting the specific points crucial for effective isolation. Although we remain far from a unified BMAd isolation protocol and PIF, we have summarized all of these important aspects, which are needed to build a BMA biobank. In conclusion, we believe that harmonizing isolation protocols and PIF globally will help to build international collaborations and improve the quality and interpretation of BMA research outcomes.

Variation of the apparent diffusion coefficient of skull bone marrow by age group, pubertal status, and gender in a pediatric population

BACKGROUND

Bone marrow composition varies with stage of development.

PURPOSE

To assess differences in apparent diffusion coefficient (ADC) derived from clivus bone marrow in healthy children by age, pubertal status, and gender as a benchmark when monitoring local and systemic treatment-induced effects.

MATERIAL AND METHODS

Non-oncological pediatric patients (30 pre-pubertal [15 girls, 15 boys] and 30 post-pubertal [15 girls, 15 boys]) with previous normal magnetic resonance imaging (MRI) of the brain including diffusion-weighted magnetic resonance imaging (DW-MRI; 1.5-T Philips Achieva-Ingenia, b-values 0 and 1000s/mm2) were studied. A 4-6 mm diameter region of interest (ROI), drawn within the clivus on two or three DW-MRI slices, yielded mean and centile ADC values. Pubertal status was recognized from imaging appearances of the pituitary gland and from fusion of the spheno-occipital synchondrosis. Correlations between ADC and age were assessed (Pearson's coefficient). Mann-Whitney U tests compared ADC by age, pubertal status, and gender.

RESULTS

Age and ADC were significantly negatively correlated (median ADC r=-0.48, mean ADC r=-0.42, P=0.0001 and 0.0008, respectively) which held true when divided by gender. Mean and median ADC differed significantly before and after puberty for the whole population (P=0.0001 and 0.0001, respectively). There was a left shift of the ADC histogram after puberty with significant differences in centile values. ADC differences before and after puberty remained when divided by gender (girls: P=0.04 and 0.009, respectively; boys: P=0.005 and 0.0002, respectively).

CONCLUSION

ADC of clivus bone marrow correlates with age in children. ADC decreases significantly after puberty, likely due to replacement of hypercellular marrow with fat. There are no gender-related differences in clivus bone-marrow ADC before or after puberty.

Confounding factors in multi-parametric q-MRI protocol: A study of bone marrow biomarkers at 1.5 T

OBJECT

The MRI tissue characterization of vertebral bone marrow includes the measurement of proton density fat fraction (PDFF), T₁ and T₂* relaxation times of the water and fat components (T_1W, T_1F, T₂*_W, T₂*_F), IVIM diffusion D, perfusion fraction f and pseudo-diffusion coefficient D*. However, the measurement of these vertebral bone marrow biomarkers (VBMBs) is affected with several confounding factors. In the current study, we investigated these confounding factors including the regional variation taking the example of variation between the anterior and posterior area in lumbar vertebrae, B₁ inhomogeneity and the effect of fat suppression on f.

MATERIALS AND METHODS

A fat suppressed diffusion-weighted sequence and two 3D gradient multi-echo sequences were used for the measurements of the seven VBMBs. A turbo flash B₁ map sequence was used to estimate B₁ inhomogeneities and thus, to correct flip angle for T₁ quantification. We introduced a correction to perfusion fraction f measured with fat suppression, namely f_PDFF.

RESULTS

A significant difference in the values of PDFF, f and f_PDFF, T_1F, T₂*_W and D was observed between the anterior and posterior region. Although, little variations of flip angle were observed in this anterior-posterior direction in one vertebra but larger variations were observed in head-feet direction from L1 to L5 vertebrae.

DISCUSSION

The regional difference in PDFF, f_PDFF and T₂*_W can be ascribed to differences in the trabecular bone density and vascular network within vertebrae. The regional variation of VBMBs shows that care should be taken in reproducing the same region-of-interest location along a longitudinal study. The same attention should be taken while measuring f in fatty environment, and measuring T₁. Furthermore, the MRI-protocol presented here allows for measurements of seven VBMBs in less than 6 min and is of interest for longitudinal studies of bone marrow diseases.

Proximal femur fat fraction variation in healthy subjects using chemical shift-encoding based MRI

The objective of this study was to describe the normal variation of bone marrow fat content in the proximal femur considering the influence of side, age, sex and body mass index using fat fraction MRI. From September 2012 to July 2016, the MRI of 131 patients (258 hips) considered to have a normal MRI appearance were retrospectively evaluated. Patient records were searched to allow calculation of the body mass index (BMI). Water-fat based chemical shift MRI was available for all patients included. Proton density fat fraction maps were calculated, and measurements were performed in the femoral epiphysis, intertrochanteric region, and greater trochanter. The influence of patient age, sex, hip side and BMI on fat fraction values was assessed. Fat fraction was significantly different in the different locations evaluated (P = 0.0001). Patient sex and age significantly influenced fat fraction values in all regions evaluated (P < 0.02) with the exception of the epiphysis for sex (p = 0.07). In all locations, PDFF values were higher in men compared to women (3.3%, 4.4% and 13.1% higher in the epiphysis, greater trochanter and intertrochanteric region respectively). The intertrochanteric region presented the lowest fat fraction values with the highest variation compared to the greater trochanter and the epiphysis. BMI only influenced fat fraction values in the intertrochanteric region of females over 42 years old (P = 0.014). The interobserver variability of the measurements performed was considered to be excellent (ICC = 0.968). In conclusion, patient sex, age, and measurement location significantly influenced fat fraction values indicating that specific standards of reference are needed depending on these factors.

Fatty infiltration of paraspinal muscles is associated with bone mineral density of the lumbar spine

A total of 88 subjects were enrolled to investigate the relationship between paraspinal muscle fatty infiltration and lumbar bone mineral density (BMD) using chemical shift encoding-based water-fat MRI and quantitative computed tomography (QCT), respectively. A moderate inverse correlation between paraspinal muscle proton density fat fraction and lumbar QCT-BMD was found with age, sex, and BMI controlled.

PURPOSE

To investigate the relationship between paraspinal muscle fatty infiltration and lumbar bone mineral density (BMD).

METHODS

A total of 88 subjects were enrolled in this study (52 females, 36 males; age, 46.6 ± 14.2 years old; BMI, 23.2 ± 3.49 kg/m²). Proton density fat fractions (PDFF) of paraspinal muscles (erector spinae, multifidus, and psoas) were measured at L2/3, L3/4, and L4/5 levels using chemical shift encoding-based water-fat MRI. Quantitative computed tomography (QCT) was used to assess BMD of L1, L2, and L3. The differences in paraspinal muscle PDFF among subjects with normal bone density, osteopenia, and osteoporosis were tested using one-way ANOVA. The relationship between paraspinal muscle PDFF and QCT-BMD was analyzed using linear regression with age, sex, and BMI variables.

RESULTS

PDFF of the erector spinae, multifidus, and psoas of subjects with normal bone density were all significantly less than those with osteopenia and those with osteoporosis (all p < 0.001). There was an inverse correlation between paraspinal muscle PDFF and BMD after controlling for age, sex, and BMI (standardized beta coefficient, - 0.21~- 0.29; all p < 0.05).

CONCLUSIONS

Paraspinal muscle fatty infiltration increased while lumbar BMD decreased after adjusting for age, sex, and BMI. Paraspinal muscles and vertebrae are interacting tissues. Paraspinal muscle fatty infiltration may be a marker of low lumbar BMD. Chemical shift imaging is an efficient and fast quantitative method and can be easily added to the clinical protocol to measure paraspinal muscle PDFF when the patient underwent the routine lumbar MRI with low-back pain.

MR fingerprinting for water T1 and fat fraction quantification in fat infiltrated skeletal muscles

PURPOSE

To develop a fast MR fingerprinting (MRF) sequence for simultaneous estimation of water T1 (T1_H2O ) and fat fraction (FF) in fat infiltrated skeletal muscles.

METHODS

The MRF sequence for T1_H2O and FF quantification (MRF T1-FF) comprises a 1400 radial spokes echo train, following nonselective inversion, with varying echo and repetition time, as well as prescribed flip angle. Undersampled frames were reconstructed at different acquisition time-points by nonuniform Fourier transform, and a bi-component model based on Bloch simulations applied to adjust the signal evolution and extract T1_H2O and FF. The sequence was validated on a multi-vial phantom, in three healthy volunteers and five patients with neuromuscular diseases. We evaluated the agreement between MRF T1-FF parameters and reference values and confounding effects due to B0 and B1 inhomogeneities.

RESULTS

In phantom, T1_H2O and FF were highly correlated with references values measured with multi-inversion time inversion recovery-stimulated echo acquisition mode and Dixon, respectively (R² > 0.99). In vivo, T1_H2O and FF determined by the MRF T1-FF sequence were also correlated with reference values (R² = 0.98 and 0.97, respectively). The precision on T1_H2O was better than 5% for muscles where FF was less than 0.4. Both T1_H2O and FF values were not confounded by B0 nor B1 inhomogeneities.

CONCLUSION

The MRF T1-FF sequence derived T1_H2O and FF values in voxels containing a mixture of water and fat protons. This method can be used to comprehend and characterize the effects of tissue water compartmentation and distribution on muscle T1 values in patients affected by chronic fat infiltration.

Marrow adipose tissue imaging in humans

Bone strength is affected not only by bone mineral density (BMD) and bone microarchitecture but also its microenvironment. Recent studies have focused on the role of marrow adipose tissue (MAT) in the pathogenesis of bone loss. Osteoblasts and adipocytes arise from a common mesenchymal stem cell within bone marrow and many osteoporotic states, including aging, medication use, immobility, over - and undernutrition are associated with increased marrow adiposity. Advancements in imaging technology allow the non-invasive quantification of MAT. This article will review magnetic resonance imaging (MRI)- and computed tomography (CT)-based imaging technologies to assess the amount and composition of MAT. The techniques that will be discussed are anatomic T1-weighted MRI, water-fat imaging, proton MR spectroscopy, single energy CT and dual energy CT. Clinical applications of MRI and CT techniques to determine the role of MAT in patients with obesity, anorexia nervosa, and type 2 diabetes will be reviewed.

Quantitative evaluation of T2* relaxation times for the differentiation of acute benign and malignant vertebral body fractures

OBJECTIVES

The aim of this prospective study was to evaluate the diagnostic performance of T2*-weighted magnetic resonance imaging (MRI) to differentiate between acute benign and neoplastic vertebral compression fractures (VCFs).

MATERIALS AND METHODS

Thirty-seven consecutive patients with a total of 52 VCFs were prospectively enrolled in this IRB approved study. All VCFs were categorized as either benign or malignant according to direct bone biopsy and histopathologic confirmation. In addition to routine clinical spine MRI including at least sagittal T1-weighted, T2-weighted and T2 spectral attenuated inversion recovery (SPAIR)-weighted sequences, all patients underwent an additional sagittal six-echo modified Dixon gradient-echo sequence of the spine at 3.0-T. Intravertebral T2* and T2*_ratio (fracture T2*/normal vertebrae T2*) for acute benign and malignant VCFs were calculated using region-of-interest analysis and compared between both groups. Additional receiver operating characteristic analyses were performed. Five healthy subjects were scanned three times to determine the short-term reproducibility of vertebral T2* measurements.

RESULTS

There were 27 acute benign and 25 malignant VCFs. Both T2* and T2*_ratio of malignant VCFs were significantly higher compared to acute benign VCFs (T2*, 30 ± 11 vs. 19 ± 11 ms [p = 0.001]; T2*_ratio, 2.9 ± 1.6 vs. 1.2 ± 0.7 [p < 0.001]). The areas under the curve were 0.77 for T2* and 0.88 for T2*_ratio, yielding an accuracy of 73% and 89% for distinguishing acute benign from malignant VCFs. The root mean square absolute precision error was 0.44 ms as a measure for the T2* short-term reproducibility.

CONCLUSION

Quantitative assessment of vertebral bone marrow T2* relaxation times provides good diagnostic accuracy for the differentiation of acute benign and malignant VCFs.

Normal Values of Magnetic Relaxation Parameters of Spine Components with the Synthetic MRI Sequence

BACKGROUND AND PURPOSE

SyMRI is a technique developed to perform quantitative MR imaging. Our aim was to analyze its potential use for measuring relaxation times of normal components of the spine and to compare them with values found in the literature using relaxometry and other techniques.

MATERIALS AND METHODS

Thirty-two spine MR imaging studies (10 cervical, 5 dorsal, 17 lumbosacral) were included. A modified multiple-dynamic multiple-echo sequence was added and processed to obtain quantitative T1 (millisecond), T2 (millisecond), and proton density (percentage units [pu]) maps for each patient. An ROI was placed on representative areas for CSF, spinal cord, intervertebral discs, and vertebral bodies, to measure their relaxation.

RESULTS

Relaxation time means are reported for CSF (T1 = 4273.4 ms; T2 = 1577.6 ms; proton density = 107.5 pu), spinal cord (T1 = 780.2 ms; T2 = 101.6 ms; proton density = 58.7 pu), normal disc (T1 = 1164.9 ms; T2 = 101.9 ms; proton density = 78.9 pu), intermediately hydrated disc (T1 = 723 ms; T2 = 66.8 ms; proton density = 60.8 pu), desiccated disc (T1 = 554.4 ms; T2 = 55.6 ms; proton density = 47.6 ms), and vertebral body (T1 = 515.3 ms; T2 = 100.8 ms; proton density = 91.1 pu). Comparisons among the mean T1, T2, and proton density values showed significant differences between different spinal levels (cervical, dorsal, lumbar, and sacral) for CSF (proton density), spinal cord (T2 and proton density), normal disc (T1, T2, and proton density), and vertebral bodies (T1 and proton density). Significant differences were found among mean T1, T2, and proton density values of normal, intermediately hydrated, and desiccated discs.

CONCLUSIONS

Measurements can be easily obtained on SyMRI and correlated with previously published values obtained using conventional relaxometry techniques.

Surgical and immune reconstitution murine models in bone marrow research: Potential for exploring mechanisms in sepsis, trauma and allergy

Bone marrow, the vital organ which maintains lifelong hemopoiesis, currently receives considerable attention, as a source of multiple cell types which may play important roles in repair at distant sites. This emerging function, distinct from, but closely related to, bone marrow roles in innate immunity and inflammation, has been characterized through a number of strategies. However, the use of surgical models in this endeavour has hitherto been limited. Surgical strategies allow the experimenter to predetermine the site, timing, severity and invasiveness of injury; to add or remove aggravating factors (such as infection and defects in immunity) in controlled ways; and to manipulate the context of repair, including reconstitution with selected immune cell subpopulations. This endows surgical models overall with great potential for exploring bone marrow responses to injury, inflammation and infection, and its roles in repair and regeneration. We review three different murine surgical models, which variously combine trauma with infection, antigenic stimulation, or immune reconstitution, thereby illuminating different aspects of the bone marrow response to systemic injury in sepsis, trauma and allergy. They are: (1) cecal ligation and puncture, a versatile model of polymicrobial sepsis; (2) egg white implant, an intriguing model of eosinophilia induced by a combination of trauma and sensitization to insoluble allergen; and (3) ectopic lung tissue transplantation, which allows us to dissect afferent and efferent mechanisms leading to accumulation of hemopoietic cells in the lungs. These models highlight the gain in analytical power provided by the association of surgical and immunological strategies.

Quantitative MRI and spectroscopy of bone marrow

Bone marrow is one of the largest organs in the human body, enclosing adipocytes, hematopoietic stem cells, which are responsible for blood cell production, and mesenchymal stem cells, which are responsible for the production of adipocytes and bone cells. Magnetic resonance imaging (MRI) is the ideal imaging modality to monitor bone marrow changes in healthy and pathological states, thanks to its inherent rich soft‐tissue contrast. Quantitative bone marrow MRI and magnetic resonance spectroscopy (MRS) techniques have been also developed in order to quantify changes in bone marrow water–fat composition, cellularity and perfusion in different pathologies, and to assist in understanding the role of bone marrow in the pathophysiology of systemic diseases (e.g. osteoporosis). The present review summarizes a large selection of studies published until March 2017 in proton‐based quantitative MRI and MRS of bone marrow. Some basic knowledge about bone marrow anatomy and physiology is first reviewed. The most important technical aspects of quantitative MR methods measuring bone marrow water–fat composition, fatty acid composition, perfusion, and diffusion are then described. Finally, previous MR studies are reviewed on the application of quantitative MR techniques in both healthy aging and diseased bone marrow affected by osteoporosis, fractures, metabolic diseases, multiple myeloma, and bone metastases.

Level of Evidence: 3

Technical Efficacy: Stage 2

J. Magn. Reson. Imaging 2018;47:332–353.