Type:
Educational Exhibit
Keywords:
Not applicable, Osteoporosis, Screening, Absorptiometry / Bone densitometry, Musculoskeletal soft tissue, Musculoskeletal
Authors:
S. Srinivasan, H. Chou, A. Kannivelu; SINGAPORE/SG
DOI:
10.26044/ecr2020/C-13494
Background
Basics of DEXA:
Xrays are not sensitive in detecting bone loss in patients with osteoporosis. Reduction in bone density can be "seen" in radiographs only after 20-30% of bone mass is lost. DEXA is presently the gold standard and widely used technique for assessment of osteoporosis.
Principle:
DEXA principle is based on the fact that attenuation co-efficient of different tissues (such as bone or soft tissues) decrease at different rates with increase in energy. The difference in attenuation is higher with low energy X-ray beams(compared to high energy beams). (Fig. 1).
Fig. 1: Basic principle of DEXA. The attenuation difference between the tissues of different composition is higher for lower energy x-ray beam.
X-ray beams of 2 different energy peak levels (either as pencil/fan beam) are passed through the tissue and the differential attenuation of the tissues namely bone, lean mass/muscle and fat, captured by the detectors are measured by computer algorithms. (The layout of DEXA is shown in Fig. 2 and 3)
Fig. 2: Layout of DEXA machine
Radiation Dose:
The radiation dose is very less. It is less than 1microsievert for bone density studies and less than 10 microsieverts for a whole body study (less than the radiation of a single chest radiograph).
Bone density mesurement:
Usual sites of measurement using DEXA are lumbar spine, femoral neck and rarely distatl radius. The World Health Organization (WHO) defines osteoporosis as bone mineral density (BMD) lower than -2.5 (less than 2.5 SD below young adult mean value) in postmenopausal women and men>50 years of age. BMD with a T-score of −1 to −2.5 is classified as osteopenia (Fig. 4).
FRAX is a newer risk assessment tool, to assess 10-year probability of major osteoporotic fracture based on 8 clinical risk fracture along with the calculated BMD. It was developed and caliberated based on several groups of people all over the world.
Body composition:
DEXA can estimate lean mass and fat mass from the differential absorption of X-rays of two different energies using specific algorithms. The machines also has correction algorithms for the effect of body thickness. DEXA machine automatically assesses and demarcates the body regions(Fig. 5) and minor correction may be needed. The machine calculates the fat mass (g), body lean mass (g) and also BMD (g) from various anatomical regions and gives the whole body values.