The CT scanner is typically a large, box-like machine with a central aperture. The participant lies on an examination table that moves into and out of a short tunnel. Rotating around the participant, the X-ray tube and electronic X-ray detectors are located opposite each other in a ring, called a gantry. The X-ray beam is attenuated as it passes through the tissues before reaching the detectors opposite. As the X-ray beam is rotated around the participant, information on the intensity of the attenuated X-ray beam is collected and stored. The X-ray creates cross-sectional slices from 1-10 mm thickness through the participant. Complex algorithms are then applied to build cross-sectional images of the body, which can further be processed into 3D images.
The computer workstation that processes the imaging information is located in a separate control room, where the technologist operates the scanner and monitors the examination in direct visual contact with the ability to hear and talk the subject with the use of a speaker and microphone.
The intensity of the attenuated X-ray beam is expressed as a CT number (also known as the linear attenuation coefficient, or attenuation value). This number is a measure of attenuation relative to air and water expressed in Hounsfield units (HU):
Physical density is the main determinant of attenuation. Lower density tissues such as fat have lower HU values than higher density tissues such as muscle or bone.
The differences in attenuation coefficients among fat-free mass, adipose tissue, and bone make this technique appropriate for quantifying separate adipose tissue compartments and whole-body composition.
Cross-sectional CT images
Cross-sectional CT images are made up of pixels 1 mm by 1 mm square. Each pixel has a CT number or HU, which gives contrast to the image. Each pixel has a HU value on a gray scale that reflects the composition of the tissue. Low HU values correspond to low values for tissue density. Lower densities appear black (e.g. fat) and higher densities white (e.g. bone and muscle).
Single vs multiple slice CT images
To reduce radiation exposure, cost and processing time, a single slice CT image can be used to assess visceral adipose tissue (VAT) and subcutaneous adipose tissue (SCAT). In adults, the single slice is often performed at the level of the L3 vertebral body. However, this method is less accurate than whole abdomen imaging as the inter-individual variation in the distribution of VAT and SCAT across the abdomen is not captured by a single slice alone.
Studies of the whole abdominal region (from T10-T11 to L5-S1 vertebrae), or multiple slices from it, give a more robust estimate of adipose tissue deposition compared to single slice analysis.
Correlations of 0.89 to 0.99 between single scans and the average values for all scans show that a single CT image contains the same information on adiposity as a series of scans.
The following parameters need to be verified with a radiographer prior to image acquisition:
The method has been primarily used in smaller laboratory-based studies. However, it has been applied to population studies, like the Health ABC study, which collected both abdominal and midthigh CT images from approx. 3000 older individuals, and the Framingham Heart Study, which used abdominal multi-detector CTs in over 3500 individuals.
Practical and ethical constraints limit the general use of CT in body composition assessment.
Due to the exposure to ionizing radiation (10 mSv, the equivalent of 100 chest X-rays), the following are avoided:
CT, together with MRI (Magnetic Resonance Imaging) is generally used to validate other methods designed to estimate adipose tissue and skeletal muscle.
Figure 1 Abdominal axial CT scans of a large individual (A) and a thin individual (B). Subcutaneous adipose tissue is divided into superficial and deep subcutaneous adipose tissue by a fascial plane, as indicated by the white arrows.
Liver fat infiltration
Intra muscular fat infiltration
An overview of the characteristics of computed tomography is outlined in Table 1.
Table 1 Characteristics of computed tomography.
|Number of participants||Small|
|Relative cost||High. Often to reduce cost and processing time, a single slice CT image at L3 in adults is used. However, this method is less accurate than the whole abdomen imaging method as the inter-individual variation in the distribution of VAT and SCAT across the abdomen is not capture by a single slice.|
|Researcher burden of data collection||Low|
|Researcher burden of data analysis||Medium. Often to reduce cost and processing time, a single slice CT image at L3 in adults is used. However, this method is less accurate than the whole abdomen imaging method as the inter-individual variation in the distribution of VAT and SCAT across the abdomen is not capture by a single slice.|
|Risk of reactivity bias||No|
|Risk of recall bias||No|
|Risk of social desirability bias||No|
|Risk of observer bias||No|
|Suitability for field use||Not suitable|
|Participant literacy required||No|
Considerations relating to the use of CT in specific populations are described in Table 2.
Table 2 Anthropometry by CT in different populations.
|Infancy and lactation||No*|
|Toddlers and young children||No*|
|Other (obesity)||Yes, however some scanners may not always accommodate large individuals (BMI >40 kg/m2)|
*Because foetus’ and children are more sensitive to radiation, they should have a CT exam only if it is essential for making a diagnosis and not for body composition, unless absolutely necessary and justifiable. Refer to the further considerations section. If CT is used to assess abdominal adipose tissue, a single site is preferable due to concerns with radiation exposure. In children, a single slice higher than L4-L5 level (typically L2-L3 in girls and L1-L2 in boys) has been shown to better represent visceral adipose tissue in the abdomen.
Safety and ethical considerations
What are the radiation risks from CT scanning?
Refer to section: Practical considerations for objective anthropometry