Today there are radiographic techniques such as DEXA, CT and MRI that allow to visualize and directly measure the variables of body composition (adipose tissue, bone tissue and muscle tissue).
The main limitation of this equipment is that it is not very accessible and very expensive.
DEXA: double energy x-ray absorptiometry
It is based on the principle of differential attenuation of an X-ray beam, at two energy levels, when passing through tissues. This attenuation is recordable and correlated to the body composition of the examined subject.
The device uses a coincident X-ray beam with no dispersion in the environment.
The radiation dose for a single exam is minimal (1 mRem). There is therefore no risk for both the patient and the operator and it is possible to repeat the test a short time later.
Its current use is mainly in the field of bone density determination (osteoporotic pathology), but it has a very high precision even in the evaluation of the athlete's nutritional status.
DEXA allows:
1) an evaluation in weight and percentage of lean mass and fat mass in different body areas. It is therefore possible to determine the areas of fat accumulation and to quantify their weight in grams.
2) a selective evaluation in the various parts of the body of the state of bone mineralization.
The only disadvantage is the high cost of the instrumentation and the execution times (20'-30 ').
DEXA: PRINCIPLE OF OPERATION
When an X-ray or photon source is positioned on one side of an object, the intensity of the ray on the opposite side is related to its thickness, density, chemical composition.
The phenomenon of attenuation depends on the energy of the incident low-energy beam and is regulated by two principles:
photoelectric effect: the photons collide with the electrons of matter and give up all their energy, ceasing to exist
Compton effect: photons collide with the electrons of the absorbent material and give up part of their energy.
The attenuation of the photon energy also depends on:
Initial photon energy
Mass / unit of absorbent material
Mass attenuation coefficient of the absorbent
The result is that from the photon-matter interaction there is an attenuation of the energy which is not linear, and is therefore described by an exponential equation.
DEXA uses a 3-compartment model, body weight is considered as the sum of bone mass + lean mass + mass of soft lean tissue (lean soft tissue mass).
where: Lean soft mass = TOTAL BODY MASS - BONE MASS
