Fat fraction mapping using magnetic resonance imaging: insight into pathophysiology

Abstract

Adipose cells have traditionally been viewed as a simple, passive energy storage depot for triglycerides. However, in recent years it has become clear that adipose cells are highly physiologically active and have a multitude of endocrine, metabolic, haematological and immune functions. Changes in the number or size of adipose cells may be directly implicated in disease (e.g. in the metabolic syndrome), but may also be linked to other pathological processes such as inflammation, malignant infiltration or infarction. MRI is ideally suited to the quantification of fat, since most of the acquired signal comes from water and fat protons. Fat fraction (FF, the proportion of the acquired signal derived from fat protons) has, therefore, emerged as an objective, image-based biomarker of disease. Methods for FF quantification are becoming increasingly available in both research and clinical settings, but these methods vary depending on the scanner, manufacturer, imaging sequence and reconstruction software being used. Careful selection of the imaging method-and correct interpretation-can improve the accuracy of FF measurements, minimize potential confounding factors and maximize clinical utility. Here, we review methods for fat quantification and their strengths and weaknesses, before considering how they can be tailored to specific applications, particularly in the gastrointestinal and musculoskeletal systems. FF quantification is becoming established as a clinical and research tool, and understanding the underlying principles will be helpful to both imaging scientists and clinicians.

Magnetic resonance imaging: Proton density fat fraction for assessment of pancreatic fatty infiltration during progression of T2DM bama minipigs

“…Additionally, Bland–Altman plots showed good agreement of PFF measurements between the two interobservers in the present study. Our results are consistent with Bray et al, who show that fat fraction (FF, the proportion of the acquired signal derived from fat protons) has become established as a clinical and research tool …”

“…Our results are consistent with Bray et al, who show that fat fraction (FF, the proportion of the acquired signal derived from fat protons) has become established as a clinical and research tool. 29 Our study shows that there is a strong positive correlation between PFF and HFF; visceral abdominal adipose tissue is the main determinant of both liver and pancreas fat content in T2DM subjects and that liver and pancreas fat content are positively related with feeding time and fasting blood-glucose levels. Rossi et al reported similar results after observations of obese population.…”

Nonalcoholic Fatty Pancreas Disease

“…MR signal arises from protons within different chemical environments (ie, water and fat molecules from body), giving rise to small differences in their resonance frequencies, thereby allowing for quantification. Several MR approaches used to quantify pancreatic fat pancreas include the following techniques. Magnetic resonance spectroscopy (MRS): The noninvasive test that is considered the gold standard to quantify pancreatic fat is MRS. MRS testing requires the user to manually position a voxel to contain as much pancreatic tissue as possible (avoiding blood vessels and the main pancreatic duct) . MRS acquired using point‐resolved spectroscopy or stimulated echo acquisition mode sequences is used to derive a spectrum of the constituent proton signals. Chemical‐shift MRI (CSI): CSI involves acquisition of imaging signal to match the chemical shift between the main fat and water peaks.…”

“…Generated “in‐phase” and “out‐of‐phase” images can then be subtracted to estimate fat content . Errors from background magnetic‐field inhomogeneity may limit quantifying‐tissue fat content, as do difficulties with the quantification of very high/low fat fraction measurements . Qualitative fat assessment is also feasible with CSI. Proton density fat fraction (PDFF) mapping: PDFF mapping addresses limitations of multipoint Dixon quantification, many of which are specifically relevant to pancreatic imaging.…”

“…MR signal arises from protons within different chemical environments (ie, water and fat molecules from body), giving rise to small differences in their resonance frequencies, thereby allowing for quantification. Several MR approaches used to quantify pancreatic fat pancreas include the following techniques.…”