MRI Methods to Visualize and Quantify Adipose Tissue in Health and Disease

Abstract

MRI is the modality of choice for a vast range of pathologies but also a sensitive probe into human physiology and tissue function. For this reason, several methodologies have been developed and continuously evolve in order to non-invasively monitor underlying phenomena in human adipose tissue that were difficult to assess in the past through visual inspection of standard imaging modalities. To this end, this work describes the imaging methodologies used in medical practice and lists the most important quantitative markers related to adipose tissue physiology and pathology that are currently supporting diagnosis, longitudinal evaluation and patient management decisions. The underlying physical principles and the resulting markers are presented and associated with frequently encountered pathologies in radiology in order to set the frame of the ability of MRI to reveal the complex role of adipose tissue, not as an inert tissue but as an active endocrine organ.

Keywords: MRI; MRS; NAFLD; NASH; adipose tissue; fat fraction; relaxometry; steatosis.

Figure 1

MRI image of lower limb with a brown fat lipoma (hibernoma). Histopathologic examination showed 90% of brown fat composition. The arrow shows the hibernoma mass, while the circle and square denote the bone marrow area subcutaneous fat depot, respectively. (a) T1-weighted with fat suppression; (b) T2- weighted; (c) T2 relaxometry parametric map calculated from multi echo T2 images. Brown fat lipoma exhibits high T2 relaxation rates due to the water component of the brown fat composition; (d) T1 in-phase; (e) T1 out-of-phase; (f) fat fraction map calculated from in- and out-of-phase images. Muscle shows minimal fat fraction, while a higher degree is observed for the bone marrow. Hibernoma exhibits smaller fat fraction, which is in accordance to the histopathologic examination showing BAT dominance over white adipocytes. Subcutaneous fat appears with lower fat fraction than the bone marrow as the technique is unable to quantify percentages of more than 50% fat dominance.