Facebook Image

How Is Bone Density Measured?

Single- and Dual Photon Absorptiometry.

Single photon absorptiometry (SPA) was initially developed in 1963. This technique involves passing a focused beam of radionuclide radiation across the arm. Because denser tissue (bone) blocks radiation better than soft tissue does, bone density can be deducted from these differences.

Disadvantages of this technique include the requirement for uniform soft tissue around the bone to be measured. In addition it is impossible to use this technique when measuring bone density of the hip and spine, which are sites that are more vulnerable to fractures.

However, SPA has provided important population-based information in epidemiological studies, specifically on the effects of aging of the skeleton.

A more sophisticated version of the same technology used in SPA is dual photon absorptiometry (DPA).

This technique was developed in the early 1970s and the first systems became commercially available in 1980.

DPA uses a radioactive isotope, which emits radiation at two different energy levels, instead of the single energy level used for SPA measurements.

While the body is scanned, the two energy levels are detected and used for mathematical calculations in order to obtain different values for the different amounts of transmissions of the energy through the body. By using this method more accurate bone density values are obtained.

Single Energy X-ray Absorptiometry.

Instead of using a radio nuclide photon source as in SPA, an X-ray source is used in Single Energy X-ray Absorptiometry (SXA). However, this method cannot be used at the spine and hip.

Dual Energy X-ray Absorptiometry.

In 1987 Dual Energy X-ray Absorptiometry (DXA or DEXA) was introduced. Today DXA equipment is widely available and this method is the choice for bone density measurements.

The basic technique of DXA is essentially the same as in DPA except for the radionuclide source, which is replaced with an X-ray source. DXA has several advantages over the older absorptiometry methods. Measurements take less time, exposing the patient to less radiation and most importantly the results of the measurements are more precise. With the development of p-DXA technique it became possible to measure the density of the forearm in a reasonably accurate form, both mid-distal and ultra-distal during the same scan.

Bone Density with OsteoporosisBone Density with Osteoporosis

Radiographic Absorptiometry.

Bone density is determined from X-ray films by reference to a metal calibration wedge placed alongside the hand during an X-ray procedure. Radiographic densitometry is then used to correlate to bone density measurements, but this technique is already outdated.

Quantitative Computed Tomography.

Quantitative Computed Tomography (QCT) represents a modification of conventional CT scanners.

The image produced by CT is generated by computer analysis of numerous X-ray transmission values obtained in different directions. The X-ray source and detector rotate around a patient in a fixed plane. From these CT scans bone density is calculated by reference to the density of calibrated phantoms.

Radiation exposure is substantially higher than with other bone density measuring techniques. This form of measurement is useful in elderly patients who may have age-related osteoarthritis and aortic calcification interfering with absorptiometric measurements.

Quantitative Ultrasound.

During the last few years various new machines have been introduced for the assessment of skeletal status.

By means of Quantitative Ultrasound (QUS), measurements are performed at patella, heel bone, tibia, and ulna.

Mainly the Broadband Ultrasound Attenuation (BUA) and Velocity or Speed of Sound (SOS) are calculated. The systems yield quantitative results averaged over the measurement area, while the Quantitative Ultrasound Index (QUI) and some calcaneal systems also generate an image.

Ultrasound systems have the advantage of obtaining information without the need for ionizing radiation. However, the amount of radiation emitted by modern p-DXA systems is extremely low in comparison. The main reason for purchasing an ultrasound system is the lower price and portability of these devices compared with p-DXA machines.

Long term studies of QUS changes over time are still limited.  Expressing measurement results on a percentage level is misleading because of the different units and calibrations employed. Studies indicate that standardized precision errors of QUS approaches are at least two times higher compared to corresponding DXA results.

Since this technique does not derive absolute data on bone mineral density, QUS can be used only for fracture risk prediction.

Laboratory.

Chemical measurements of blood and urine by biochemical screening assays (bone markers) are used to evaluate bone resorption and formation.

Although biochemical screening assays have improved over the last few years, they still have an accuracy error of up to 30%.

They are only useful for studying bone turnover in large groups of patients where individual errors are statistically compensated.

Chemical analyses such as urinary hydroxyproline and pyridinoline crosslinks or serum alkaline phosphatase cannot be used to diagnose osteoporosis. Usually they cannot be used to evaluate an imbalance between formation and destruction of bone. They are however useful in determining bone turnover, and consequently in identifying patients who are likely to be fast bone losers.