Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - ProFatMRI (Magnetic resonance imaging platform for probing fat microstructure)

Teaser

Metabolic syndrome and osteoporosis are the two metabolic diseases with the highest and most rapidly growing prevalence. Both the metabolic syndrome and osteoporosis are being transformed into a major global health concern with severe socioeconomic impact. The clinical...

Summary

Metabolic syndrome and osteoporosis are the two metabolic diseases with the highest and most rapidly growing prevalence. Both the metabolic syndrome and osteoporosis are being transformed into a major global health concern with severe socioeconomic impact. The clinical management of the patients with the two metabolic diseases faces different but severe challenges. On the one hand, metabolic syndrome can be diagnosed with established biomarkers, but the selection of optimal prevention strategies for each individual patient is still problematic. For example, it is unclear why certain lifestyle interventions work in some obese subjects and fail in others. On the other hand, osteoporosis can be treated using known medication, but its current early diagnosis remains insufficient. Specifically, the traditional diagnostic biomarker for osteoporosis, the bone mineral density (BMD), has a compromised precision on predicting fractures. The two metabolic diseases have been linked through the role of fat. Fat is central to their incidence and progression, and the probing of fat cellular properties can provide groundbreaking solutions for overcoming the existing challenges in the diseases early diagnosis and prevention.
In metabolic syndrome, there is evidence supporting a role of brown fat in preventing the disease. Fat can be white or brown. White fat stores excess energy, whereas brown fat converts energy into heat. Excessive white fat in different depots is a characteristic of obesity and insulin resistance. Recent studies on human subjects have shown that brown fat prevalence negatively correlates with BMI and visceral fat volume and that brown fat metabolic activity is lower in diabetics. Therefore, there is evidence supporting a role of brown fat in preventing metabolic syndrome. However, there is no established non-invasive tool to reliably measure brown fat. Brown fat has distinctly different microstructure than white fat: brown fat cells are smaller and enclose more mitochondria than white fat cells.
In osteoporosis, there is evidence supporting a role of marrow fat, in combination with bone mineral density, for monitoring fracture risk. Bone marrow includes both fat and hematopoietic red blood cells and it is not considered any more as a simple filler of the bone cavities. Multiple previous works have shown a negative association between marrow fat content and BMD. It was also recently reported that higher marrow fat content is associated with prevalent vertebral fracture, even after adjustment for BMD. There is therefore evidence supporting a role of bone marrow fat, in combination with BMD, in monitoring fracture risk. Bone marrow fat microstructure is affected by both fat cell-specific microstructure (fat cell size and number) and microenvironment (mineral density). However, there is no non-invasive biomarker to measure marrow fat cellular changes in osteoporosis.
Magnetic resonance imaging (MRI) is the ideal modality for non-invasively measuring fat throughout the body. MRI can also provide unique surrogate biomarkers of tissue microstructure, relying on diffusion and magnetic susceptibility effects. In order to differentiate brown from white fat and characterize the relationship between bone mineral and marrow fat cells, the employed MR methodology needs a technical breakthrough, shifting from the state-of-the-art water-centered paradigm to a fat-centered microstructural MRI paradigm. Namely, methods need to be developed focusing entirely on the protons of the fat molecules. ProFatMRI describes an innovative research program that aims to develop and ex vivo validate diffusion and susceptibility MRI biomarkers of fat microstructure, and in vivo apply them at clinical MRI systems.
The resulting technologies will provide novel ways for selecting optimal individualized prevention strategies in metabolic syndrome and for achieving reliable risk fracture prediction in osteoporosis.

Work performed

Since the beginning of the project, we have been primarily developing and ex vivo validating diffusion and susceptibility MRI biomarkers of fat microstructure. We have already started translating the developed methodologies for in vivo applications at clinical MRI systems.

Aim A: Fat-specific diffusion MRI
The measurement of restriction in the diffusion properties of fat would enable the non-invasive characterization of adipose tissue microstructure and the non-invasive measurement of adipocyte cell size. However, there are only limited reports in the literature on MR techniques for measuring the self-diffusion properties of fat and no report on measuring lipid droplet size with the gradient hardware of a clinical MR system. We have been developing diffusion-weighted single-voxel magnetic resonance spectroscopy (MRS) and diffusion-weighted (DW) MR imaging techniques targeted for measuring the diffusion properties of fat.

Subaim A1: Fat-specific DW single-voxel MRS
A DW-MRS sequence using STEAM (stimulated echo acquisition model) voxel localization was developed and combined with bipolar gradient readouts to compensate for eddy current effects. An analytical framework of the DW signal dependence on the size of diffusion restriction for spherical lipid droplets was established. To validate the method, oil-in-water emulsions were prepared and examined using DW-MRS, laser deflection and light microscopy. In phantoms, a good correlation between the measured droplet sizes by DW-MRS, laser deflection and microscopy measurements was obtained. The in vivo feasibility of the method was also studied in the tibia bone marrow of volunteers (region minimally affected by motion) to test the method repeatability and characterize microstructural differences at different marrow locations. The coefficient of variation in bone marrow adipocyte size estimation stayed below 15% and the adipocyte diameter measured distally was smaller than the adipocyte diameter measured proximally, in agreement with previous literature [1].
Therefore, our developed approach for fat-specific DW single-voxel MRS showed for the first time that the measurement of large lipid droplet size is possible using the gradient hardware of a clinical 3 T scanner [1]. We have been developing new approaches for specifically translating the methodology into the supraclavicular fat and bone marrow regions.
The translation of the DW-MRS method in the supraclavicular fossa for probing the diffusion properties of water and fat in brown adipose tissue has been challenged by the strong motion effects in the region induced by vessel pulsation and breathing. We have developed a flow-compensated DW-MRS technique and have combined it with cardiac and respiratory triggering to mitigate such effects [2, 3].
The translation of the DW-MRS method in trabecularized bone marrow is challenged by the broad linewidths in trabecularized bone marrow MRS. The broad linewidths of bone marrow spectra complicate the fat peak area extraction and eventually the measurement of the diffusion coefficients of the water and fat peaks. We have been working on improving the characterization of the bone marrow fat MR spectral properties. We have been developing methods using inversion recovery to improve the ability to measure bone marrow fatty acid composition [4]. Based on a meta-analysis of existing data in our group, we have been further characterizing the differences in T2 relaxation times between water and fat [5] and the ability to measure an apparent metric of bone marrow fat unsaturation using short echo time MRS [6].

Subaim A2: Fat-specific DW imaging
Single-voxel DW-MRS enabled the quantification of the diffusion properties of the water and fat peaks, but averaged within a single-voxel. We have been developing methods for spatially resolving the diffusion properties of fat using DW imaging. Given the need to sensitize slow diffusion processes with strong diffusion encoding and long diffusion times, co

Final results

The following progress beyond the state-of-the-art has been also reported:
- Showing the feasibility of DW-MRS measurements in measuring large lipid droplet size using the gradient hardware of a clinical 3 T system
- Showing the sensitivity of quantitative susceptibility mapping for measuring trabecular bone density
- Demonstrating the association of proton density fat fraction in adipose tissue with imaging-based and anthropometric obesity markers in adults

Hypotheses of our ongoing work/results to be expected
- DW-MRI measurements can spatially resolve gluteal fat adipocyte size using the gradient hardware of a clinical 3 T system
- UTE imaging enables the estimation of magnetic susceptibility in cortical bone and improves the estimation of magnetic susceptibility in trabecular bone surrounded by a thick cortical bone layer
- Metrics of supraclavicular fat microstructure improves the differentiation between white and brown adipose tissue and overcome the limitations of fat fraction mapping
- Trabecular bone magnetic susceptibility is a marker of bone density in the lumbar spine