Other Imaging Techniques

Dual-energy x-ray absorptiometry (DEXA) was originally designed to measure bone mineral content, but it is now commonly used to assess total and regional fat and fat-free mass (Figure 1) [1,2]. DEXA assesses body composition based on the attenuation of x-rays emitted at two energy levels as they traverse the body [1,2]. DEXA provides less radiation exposure than CT and costs significantly less. A whole body DEXA scan requires 15 to 35 minutes depending on the scanner and is relatively easy to use in most populations as it requires very little participant effort [3]. Once the scan is complete, it can be manually subdivided into truncal and appendicular regions. Measures of total and regional lean mass (coefficient of variation=1 to 7%) [4-6] and fat mass (coefficient of variation=1 to 7%) [5] are highly repeatable, but results may differ depending on the scanner model (e.g., Lunar, Holigic, etc.) [7], the software used (algorithms), and the individual’s sagittal diameter and hydration status [3]. Nevertheless, DEXA measures of total or appendicular fat and lean mass closely match the fat and skeletal muscle values obtained by CT or MRI [4,6,8,9,10]. Accordingly, DEXA is often used to assess total and appendicular percent fat mass with relatively good accuracy.

Nowadays, with the development of advanced softwares, it is possible to estimate visceral fat area using DEXA scan with greater accuracy than initial DEXA-derived values. In this regard, results of several studies that compared visceral adiposity estimated by DEXA to visceral adiposity measured by MRI or CT observed strong correlations between the two measurements [11-14]. However, results of the large multiethnic Dallas Heart Study revealed that visceral adiposity derived from DEXA might be underestimated as compared to MRI in individuals with normal weight while it might be overestimated in individuals with obesity [11]. Moreover, whether DEXA is accurate in specific populations or for long-term use remains to be investigated. Nevertheless, assessing visceral adiposity by DEXA represents a useful alternative to CT or MRI.

Ultrasonography is a clinical tool that is less frequently used to assess body composition [1]. Ultrasonography assesses tissue composition by measuring differences in the reflection off the underlying tissues of high frequency sound waves emitted by a transducer. Soft tissues that readily propagate much of the sound wave and reflect only a small portion back to the transducer will have a smaller “echo” or acoustic impedance, whereas denser tissues such as bone will reflect more of the sound wave and thus produce a greater “echo” or acoustic impedance. Tissues with greater “echo” or acoustic impedance appear brighter. Like other anthropometric measures, ultrasonography is subject to inter- and intra-observer variability [15]. As such, it is important to have standardized protocols and training to ensure the validity of the measures.

Ultrasonography is commonly used to assess subcutaneous fat thickness and is highly repeatable, with a precision similar to skinfold measures [1]. As with skinfolds, total body fat can be estimated using several measures of subcutaneous fat thickness at various body locations, such as the head and neck, forearm, upper arm, trunk, thigh, and lower leg [16]. Ultrasonography has the advantage that measures can be taken in obese subjects at sites not amenable to skinfold measures [1]. Studies report that measures of total body fat by ultrasonography correlate well (r=0.88 to 0.95) with measures by hydrostatic weighing [17] or MRI [16].

Similarly, ultrasonography can be used to estimate visceral fat by measuring visceral fat diameter (Figure 2). Visceral fat diameter is the distance between the abdominal muscle wall and the aorta, generally assessed around the level of the umbilicus or at L4-L5 [18,19]. However, it may be hard to measure visceral fat diameter if there are air-filled structures, such as the lungs or intestines, in the path of the ultrasound beam. Air has extremely low acoustic impedance, which means it reflects very little “echo” or signal back to the transducer. Consequently, if the aorta is located behind an air-filled intestine, ultrasound images may be obscured and the visceral diameter difficult to assess.

Ultrasonography-measured visceral fat thickness is generally a similar or stronger predictor of CT-measured visceral fat compared to anthropometric measures such as body mass index, waist circumference, waist-to-hip ratio, sagittal diameter, or skinfolds [19-23], or other clinical measures such as bioelectrical impedance analysis and DEXA [20]. Furthermore, ultrasonography is a better measure of changes in visceral fat compared to waist-to-hip ratio [24]. It is unclear whether ultrasonography is a better predictor of changes in visceral fat compared to waist circumference.

There is evidence that ultrasonography-measured visceral fat is a stronger predictor of health risk than simple anthropometric measures. Though some have reported that ultrasonography-measured visceral fat is a weaker predictor of health risk compared to waist circumference or DEXA [25], others have suggested that ultrasonography is a better measure of cardiovascular risk compared to waist circumference [18,21] and sagittal diameter [18]. Some have also reported that ultrasonography-measured visceral fat may be comparable to CT-measured visceral fat in predicting alterations in glucose metabolism and fasting lipid levels [26-28]. However, few studies have compared the utility of ultrasonography- and CT-measured visceral fat in predicting health risk, and their findings should be verified with further research.

To date, there is limited research on the usefulness of ultrasonography in assessing health risk compared to CT-measured visceral adiposity or anthropometric tools. Currently, ultrasonography appears to be a potentially useful clinical tool for assessing visceral fat and obesity-related health risk. However, further research is needed to confirm these findings.