The Dual Energy Xray Absortiomety (DEXA)
Authors: Eva Heinen, graduate sports scientist (Univ), Prof. Dr. med. Edgar Heinen
DEXA measuring devices are now available in many clinics and practices; however, they are usually only used to measure bone density, primarily of the lumbar vertebrae and the femoral neck. A decrease in bone density is accompanied by an increase in the likelihood of fractures. As no standardization could be achieved between the various device manufacturers at the beginning of the era of bone density measurement, each manufacturer has its own standard values, which are given as g/cm². Follow-up observations should therefore preferably be made on the same device on which the initial examination was carried out! A certain degree of comparability of the results is achieved by indicating the deviations of the measured values from the average of healthy adults in the form of the standard deviation. A patient who has a bone density of 1 standard deviation below the above-mentioned norm then has a T-score of -1, with 2 standard deviations below the norm a T-score of -2 etc. For further details in connection with osteoporosis, see link.
However, DEXA devices can also be used to determine body composition using special software.
Dual-energy X-ray absorptiometry (DEXA or DXA) is now increasingly regarded as the method of choice in human medicine for determining body composition and body fat in vivo(Henche & Pellico 2005, Heyward 2001).
When measuring body composition using the DEXA method, the different attenuation of X-rays with different radiation energy by the various penetrated media is utilized. An X-ray source emits photons that are filtered in such a way that only two energy peaks penetrate the person being examined. Depending on the masses penetrated, the two beams are attenuated differently and then measured in the detector. For example, the ratio of the attenuation of the beams at 40 keV to the attenuation at 70 keV is 1.21 for fat, 1.37 for soft tissue (lean body mass LBM) and 2.86 for bone. ( For details see Petriobelli et al.: Dual-energy X-ray absorptiometry body composition model: review of physical concepts. Am J Physiol. 1996 Dec; 271 (6 Pt 1): E941-51.) Body composition is calculated from these data.
A DEXA device generally consists of the following components:
an X-ray tube,
a special filter that allows X-rays of two different energy levels to pass through,
a radiation sensor,
a computer with corresponding software and
a positioning table.
The X-ray tube is located under the table and the sensor is located above the table on a C-shaped arm. There are so-called single-beam (or pencil beam) devices, which scan the subject in a meandering pattern with a point-shaped X-ray beam and a single detector, and the newer and faster fan-beam devices, which emit X-rays arranged in a fan shape, which requires several detectors. The advantages of this technology are shorter scanning times (3-4 min), resulting in a higher patient throughput, improved image quality, improved image resolution and reduced motion artifacts. However, this is offset by a higher radiation exposure for the person being examined and for the staff as a disadvantage. Whereas with pencil beam devices the staff can be at a distance of 1 meter from the table and the radiation exposure of the examiner is then well below 1 mSv per year for very many examinations (ICRP recommendation), radiation protection measures are necessary with fan beam devices. A distance of more than 3 meters must be maintained or a radiation protection wall must be erected. The radiation exposure for the person being examined is well below 0.5 µS per examination for pencil beam devices. Considering that the natural radiation exposure > is 1000 µS / year, the very low radiation exposure from such an examination becomes clear.
Unfortunately, the results of the body composition analysis do not match when different device manufacturers are used. For example, the measurements with the Lunar devices give lower values for body fat than measurements with Norland devices. We used the Norland XR 36 device for our measurements and checked the recovery of fat and fat-free mass.
On the one hand, there was no deviation between the measurement of body weight with the scales and the value determined by the device, which would have been more than 200 g for women and men!
In the recovery tests shown, 5.44 kg of fat in the form of oil was recovered at a rate of 101.2 %; 100.3 % of the fat-free mass was recovered. The additional application of 3.11 kg NaCl (0.6%) to the trunk of the test subjects showed a recovery of the fat-free mass of 100.9%; the recovery of the fat amounted to 101.1%; results from which the reliability of the tests with the device can be deduced.
Similar results were obtained by Gottfredsen et al (Body composition analysis by DEXA by using dynamically changing samarium filtration. J Appl Physiol. 1997 Apr; 82 (4): 1200-9 ), who carried out recovery tests in which they applied fat or muscle strips of different weights to the test subjects and then chemically analyzed the overlays.
Unfortunately, it must be pointed out that not all DEXA devices have equally good recoveries. Our own investigations had shown similarly good recovery on a Hologic device. Lunar devices showed considerable differences in fat measurement when compared with Norland devices. I am not aware of any published recovery tests with Lunar devices.
Measurement parameters
The device software determines the following parameters:
Total Bone Mineral Content (BMC) Total bone mineral content [g]
Total Lean Mass (LBM) Total “Fat-free body soft tissue” [g]
Total Fat Mass (FM) Total fat mass [g]
The content of mineral, fat and fat-free soft tissue can be determined separately for each extremity, the thorax, the abdomen and the head, which can provide very interesting aspects, particularly in the area of training.
With other methods, the bone mineral content cannot usually be measured separately. Therefore: BMC + LBM = FFM(fat-freebody mass)
The possibility of defining standard or target values is discussed in the chapters
- FMI – Fat Mass Index
- FFMI – Fat Free Mass Index and
- MMI – Muscle Mass Index
- BMCI – Total Body Mineral Mass Index
received.