Evaluating CMR
Imaging Techniques
Others
- 1Key points (1 page)
- 2Evaluating CMR: Imaging Techniques (1 page)
- 3Dual-Energy X-Ray Absorptiometry (DEXA) (2 pages)
- 4Ultrasonography (2 pages)
- 5References (1 page)
Ultrasonography
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Figure 2: 
Figure 2: 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 (16). 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 (17). 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 (18) or MRI (17).
Similarly, ultrasonography can be used to estimate intra-abdominal fat by measuring intra-abdominal fat diameter (Figure 2). Intra-abdominal 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 (19, 20). However, it may be hard to measure intra-abdominal 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 intra-abdominal diameter difficult to assess.
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