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Clinical imaging of marrow adiposity

https://doi.org/10.1016/j.beem.2021.101511Get rights and content

Research examining bone marrow adipose tissue (BMAT) has rapidly expanded during the last two decades, leading to advances in knowledge on the role of BMAT in the pathogenesis of bone loss and endocrine disorders. Clinical imaging has played a crucial role for the in vivo assessment of BMAT, allowing non-invasive quantification and evaluation of BMAT composition. In the present work, we review different imaging methods for assessing properties of BMAT. Our aim is to review conventional magnetic resonance imaging (MRI), water-fat imaging, and single-voxel proton magnetic resonance spectroscopy (1H-MRS), as well as computed tomography (CT)-based techniques, including single energy and dual energy CT. We will also discuss the clinical applications of these methods in type 2 diabetes mellitus, obesity and anorexia nervosa.

Introduction

Bone marrow adipose tissue (BMAT) research has witnessed considerable advances over the past two decades. The development of ex vivo and in vivo methods to assess BMAT have brought a new perspective into the bone-fat interaction, particularly regarding skeletal metabolism and integrity [1]. BMAT is no longer considered an inert filler of bone marrow space in response to ageing or decrease demand for erythrocyte production [2]. For instance, the accrual of BMAT in response to caloric restriction is a unique behavior of BMAT in contrast to white adipose tissue [3,4]. The increase of BMAT can be seen with ageing and in certain conditions such as immobility, anorexia nervosa, and medication use [∗[4], [5], [6], [7], ∗[8]].

Recent research suggests that BMAT may be a biomarker of osteoporosis, and therefore a possible target for its treatment [[9], [10], [11], [12], [13], ∗[14], [15]]. BMAT may also be involved in the pathophysiology of certain metabolic disorders. For instance, despite preserved bone mineral density (BMD) in type 2 diabetes mellitus or obesity, an increased risk of fracture has been reported [[16], [17], [18]], and the quantity and composition of BMAT may play a role in the pathogenesis of impaired bone health in these disorders [19,20].

Magnetic resonance imaging (MRI) and single-voxel proton magnetic resonance spectroscopy (1H-MRS) allow the non-invasive assessment of BMAT without ionizing radiation [21,22], while computed tomography (CT)-based techniques involve ionizing radiation. However, dual-energy CT (DECT) has the advantage of simultaneous assessment of BMAT and BMD [[23], [24], [25], ∗[26], ∗[27]].

Our aim is to discuss the role of different in vivo imaging techniques in quantifying BMAT with particular focus on MRI- and CT-based techniques. Of note, this review uses the nomenclature and terminology proposed by the International Bone Marrow Adiposity Society (BMAS) [28].

Section snippets

Conventional MRI

Bone marrow composition varies with different physiological and pathological states including ageing, nutrition, or malignancy. Bone marrow signal characteristics on MRI are mainly determined by its fat and water content and are also a function of the pulse sequence used. In adipose tissue and bone marrow, a significant contribution of signal characteristics comes from lipid hydrogens, predominantly those residing in aliphatic (−CH2−) chains of triglycerides, which have short T1 relaxation

Single energy computed tomography

Single Energy Computed tomography (SECT) is based on the principle that the density of a tissue passed by the x-ray can be measured from the calculation of the attenuation coefficient. CT density is not characteristic of a specific chemical composition but rather a function of the atomic number and energy. CT density of tissue is obtained from a linear transformation of attenuation coefficients and is expressed in Hounsfield Units (HU). Air and water densities are arbitrarily defined as −1000HU

Summary

In summary, in vivo imaging methods, especially MR-based techniques have been essential for the advancement of BMAT research. 1H-MRS is the reference method for the in vivo assessment of BMAT, allowing both quantitative and qualitative assessment of BMAT. 1H-MRS only allows single voxel analysis, while WFI techniques provide mapping of BMFF of a large field of view, which is particularly helpful in regions with heterogeneous BMAT distribution. The recent BMAS recommendations aim to standardize

Funding

Miriam A Bredella (NIH K24DK109940).

Declaration of competing interest

The authors have nothing to declare.

Practice points

  • Noninvasive imaging techniques have played an important role for the understanding of BMAT and its relationship to skeletal integrity

  • MRI is the gold standard for the in vivo imaging of BMAT

  • DECT offers the advantage of assessing BMD and BMAT in a single examination but is associated with radiation exposure risk

Research agenda

  • Examine the utility of opportunistic DECT in assessing BMAT and BMD among patients undergoing CT for other clinical

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