Science to Practice: Good Fat, Bad Fat – Does Location Matter?

A technique that allows rapid noninvasive imaging of abdominal fat without the potentially harmful effects of ionizing radiation would be useful in diagnosing overweight and obese children and in developing effective strategies for weight management.

Siegel et al describe a quick, safe, and reproducible single-section MR method for quantifying subcutaneous and visceral abdominal fat and its distribution in preadolescents and adolescents. Although larger studies are needed, their study results confirm the use of T1-weighted MR imaging at a single level for estimating adipose tissue volumes and distribution.

The Setting

In 2003–2004 in the United States, an alarming 17.1% of children and adolescents were overweight and 32.2% of adults were obese. Despite a national mandate and broad-based public health initiatives aimed at encouraging better nutritional choices and less-sedentary lifestyles, obesity among both children and adults continues to increase.

Our ability to accurately measure abdominal fat may substantially advance strategies for achieving healthy weights in children. There is strong evidence, moreover, that excessive visceral fat may predispose even young children to adult diseases, such as heart disease and diabetes; hence, accurate estimates of this more damaging internal fat are especially critical. Conventional methods of measuring visceral fat, such as anthropometry and dual x-ray absorptiometry (DXA), are imprecise, and DXA subjects the patient to potentially harmful ionizing radiation.

Computed tomography (CT) is far more accurate but also delivers radiation. Magnetic resonance (MR) imaging, however, introduces a noninvasive means of assessing fat, including its pattern of distribution and total volume, but without delivering radiation. In this issue of Radiology, Siegel et al (3) explore the use of T1-weighted MR imaging at a single level as a fast, reproducible means of assessing fat distribution and volume in preadolescents and adolescents.

The Science

The noninvasive measurement of intraabdominal fat with MR imaging has emerged as a reliable predictor of diabetes and other obesity-driven metabolic disorders in adults. Its use in predicting metabolic disorders in children is less clear; although, as with adults, results of MR imaging in children show a correlation among subcutaneous adipose tissue, body mass index, waist circumference, and visceral adipose tissue.

Siegel et al  used a fast MR sequence and semi automated measurements to characterize the total amount of abdominal fat and its distribution by type (subcutaneous vs visceral) in a study cohort of 30 normal-weight, overweight, or overweight diabetic preadolescents and adolescents aged 10 –18 years. This technique allows more rapid sequencing and shorter imaging intervals than does multi-section MR imaging. These features make this method especially attractive for evaluating children. Measurements with both single-section and multi-section methods correlated well with anthropomorphic and DXA measurements as indicators of total body fat.

While subjects of normal weight had the least amount of total adipose tissue and overweight diabetic subjects had the most, the differences in the amount of adipose tissue among the three groups were less striking. Normal-weight subjects and overweight diabetic subjects showed the greatest disparity in visceral fat measurements; overweight non-diabetic subjects and overweight diabetic subjects showed the least. However, Siegel et al did not measure the ratio between visceral and subcutaneous fat—a ratio that is frequently higher in overweight diabetics than in overweight non-diabetic adults.

The authors have established the feasibility of single-section MR imaging for estimating adipose tissue in preadolescents and adolescents by prospectively comparing this technique with multi-section MR imaging and by correlating these results with measurements taken with anthropometry and DXA.

The Practice

Clinical use: Overweight children often become overweight adults (6). As physicians see ever increasing numbers of children with type 2 diabetes and other illnesses associated with metabolic disturbances, the medical community must aggressively seek ways to diagnose this insidious condition in its earliest stages. Although the irreparable systemic damage that accrues from obesity is impossible to evaluate, results of the study by Siegel et al point to a strong relationship between high volumes of visceral adipose tissue and obesity-related illnesses.

The next step in relating intra-adominal fat to diabetes in preadolescents and adolescents is to study a larger cohort. Siegel et al (3) report MR imaging as an accurate, noninvasive, and reproducible means of estimating the volume and distribution of adipose tissue. This study’s major contribution lies in showing that T1-weighted MR imaging at a single level is faster than, and the results are nearly equivalent to, the longer and more labor-intensive multi-section technique.

Furthermore, clinical research protocols aimed at characterizing adipose tissue can be easily incorporated into MR imaging studies that are clinically indicated for a variety of disease states, such as anorexia nervosa, human immunodeficiency virus infection and acquired immunodeficiency syndrome, and Cushing syndrome, in which an understanding of fat distribution
may aid in diagnosis and treatment.

Future opportunities and challenges: The ability to identify childhood predictors of adult disease, especially those that are obesity-driven, has become an increasingly important area of research in pediatrics. The safe, accurate, noninvasive measurement of intra-abdominal fat in children may thus play an important role in the diagnosis of obesity and aid in preventive care. MR imaging offers immediate advantages over non–imaging-based methods, as well as over DXA and CT. The ability to identify and treat obesity in its earliest stages should result in vastly improved patient outcomes, which effectively offsets the relatively high cost of MR imaging units.

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