Bone Density and the DXA Scanner

Pamela Coates

Trabecular & Corticoid

Trabecular

Trabecular more metabolically active

Bone loss leads to thinning and perforation of trabecular plates

Osteoporotic fractures occur at sites with at least 50% trabecular bone

Trabecular Bone

Highlighted areas have

at least 50% trabecular

bone

Peak Bone Density Peak bone density is achieved by late

20s/early 30s

Bone loss after skeletal maturity is about 1% per annum in both sexes

Greater loss in woman for around 3 years following menopause

Bone Structure

Osteoporosis occurs in:

1 in 3 postmenopausal women

1 in 10 men over 50

Classification of Osteoporosis Primary

Type 1 (postmenopausal) Type II (senile) Idiopathic

Secondary Endocrine (hyperthyroidism, Cushing’s) Gastrointestinal (coeliac TPN Crohn’s) Rheumatologocal (RA ankylosing

spondylites) Malignancy Drugs (corticosteroids heparin)

Hologic discovery Scanner

GE Healthcare Lunar Prodigy Scanner

What is DXA

DXA stands for :- Dual Energy X-Ray Absorptiometry

DXA measures, it is NOT an imaging machine

Measurements are taken from the axial skeleton

DXA Scan of Hip

DXA Scan of Lumbar spine

DXA Scan of Forearm

Performed if either of other preferred sites

are unusable or in the case of certain

conditions

Endocrine or gastrointestinal

Total-body scan

Shows body composition

Used for research Occasionally used to monitor weight – fat loss, muscle gain

Collapsed Vertebra

Scan of Lateral spineScan showing collapsed vertebraRecently approved for the detection of aortic calcification

T-score and Z-score

T-score is compared to a population at peak bone density

Z-score is compared to an age, sex and ethnicity matched population

T-score T-score- compared to mean of

population at peak bone mass

T-score >-1 Normal

T-score -1 to -2.4 Osteopaenic

T-score <-2.5 Osteoporotic

X-rays at two energy peaks are passed through the patient

Different tissues adsorb differing amounts of each wave length

Density is calculated by using simultaneous equation on these amounts

How does it work?

There are two manufacturers of DXA Equipment

General Electric Company-GE Healthcare

Lunar Prodigy

Hologic Discovery

Incompatibility

The two scanners should not be mixed

Patient should continue to be scanned on the original machine

They work in different ways

Functionality

GE machinesK-edge filtration with rare earth filter

Hologic machinesAlternates between 2 Kvs

WHERE

Simples!

The Math

Fan Beams

GE narrow fan beamHologic fan beam

They have different edge detection

GE records lower scores than Hologic

Why do we need a DXA in the breast unit

Oestrogen has a beneficial effect on bone density – inhibiting osteoclasts and promoting osteoblasts

Without oestrogen this is reversed leading to loss of bone

Post menopause oestrogen is converted from androgens by the enzyme aromatase

Aromatase Inhibitors

ARIMIDEX (anastrazole)

AROMASIN (exemestane)

FEMARA (letrozole)

Aromatase Inhibitors

As bone density will increase when aromatase inhibitors are prescribed, treatment for loss of bone is given when the T-score is -2 and not the more usual -2.5

Gonadorelin Analogues

Gosrelin (zoladex)

Busrelin

Leuporelin

Prostap

Mortality Rate

Mortality is increased by 20% in the first year after

a hip fracture

UK Annual CostsAnnual cost of treating

osteoporotic fractures in UK is in excess of £1.73 billion

Annual cost of treating coronary heart disease in

the UK is £1.75 billion

The lifetime expectancy of a fracture at age 50 is 40% for women

The likelihood of this will rise for those treated with aromatase

inhibitors!”

References

Fundamentals of Bone Density (NOS 2002

ATC Study

WWW.breastcancer.orgWWW.icmri.comwww.ch.ac.uk

www.osteoporoticdiagnosticcenter.org

ANY QUESTIONS