Research updates
Osteoporosis   p 6
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3. How is osteoporosis detected?  Link to the Medical Research Council web site
X-ray of lower spine
Figure 12. X-ray of thoracic spine, showing mild deformation, a classic osteoporotic wedge fracture and a normal spine.
3.1 Fractures
X-ray images are used to establish the presence or absence of fractures. Imaging of the skeleton by plain radiographs has been in medical use for over a hundred years but, although conventional X-ray sources can reveal a fracture, they do not allow useful measurements of bone mineral density (BMD). They can only establish a loss in bone density after at least 30% of the bone has been lost and by this time the disease is already well advanced.
3.2 Measuring bone density: DXA

The measurement of bone mineral density forms the basis for the diagnosis of osteoporosis. The risk of fracture rises as BMD declines. An assessment of bone density provides an estimate of fracture risk in the same way that blood pressure predicts the risk of a stroke. It is possible to choose an arbitrary value of BMD and take this to be the threshold below which fracture risk is considered to be unacceptably high. If an individual has a BMD below this value a diagnosis of osteoporosis is made and treatment considered.

Advances in medical physics have produced accurate and reproducible methods for measuring BMD. The most widely evaluated technique is Dual Energy X-ray Absorptiometry (DXA - pronounced 'dexa'). It has been suggested that a measurement of the hip using this technique should be adopted as a gold standard for the purpose of diagnosis. DXA is also used for monitoring osteoporosis and for research into causes and treatments of the disease.

DXA machine
Figure 13. A DXA instrument in use. The fully dressed subject lies above the dual X-ray source and below the mobile detector.

How does DXA work?

DXA is based on measuring the attenuation of a beam of X-rays when it passes through bone. Click here to find out more about the principles behind the DXA process.

Most instruments occupy an area the size of a single bed and may therefore be used in standard size consulting rooms. DXA machines produce a computer generated image as well as a table of values. Some typical results are shown in Figures 13a and 13b.

Exposure to ionising radiation is minimal (a single scan equates to about a day on holiday in Cornwall and the machine doesn’t require additional protection for staff and patients).

Click here to find out more about DXA.
DXA image of hip
Figure 13a. Image produced by dual energy X-ray absorptiometry (DXA) of hip.

What does it measure?

The bone mineral density at a measurement point corresponds to a particular path of the radiation through the body. It is the mass of bone mineral in the path of the beam divided by the cross sectional area of the beam, expressed as g/cm2. BMD is thus an ‘area density’ rather than a true volumetric density. If two sites have the same true density of bone but site A has a greater depth than site B, the BMD will be higher at site A. It is also possible to scan the bone several times to obtain a series of BMD values which the computer can combine to yield the total mass of bone mineral (in g) in that region. The total bone mineral content is known as BMC.

DXA measurements are highly reproducible (the coefficient of variation is 1-2%). The accuracy is somewhat lower at 5-10%.

Figure 13b. Print out of results of DXA of hip.
Curling accuracy
Figure 14. Accuracy versus precision (roll over the highlighted words).
a) good accuracy but poor precision
b) good precision but poor accuracy
c) accurate and precise.
Question 5
a) Use Figure 14 to describe the difference between accuracy and precision.

b) Bone mineral density is used for:

i. diagnostic purposes to assess and compare with reference values
ii. for monitoring responses to treatment.

Consider each purpose and decide in each case which is more important, accuracy or precision.

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