Bone Density Measurement

Synonyms

Osteodensitometry engl. : Dual Photon X-Ray = DPX

Definition

In a bone densitometry procedure, a doctor uses a medical-technical procedure to determine the bone density, i.e. ultimately the calcium salt content of the bone and thus its quality. The result of the measurement provides information about how fracture-resistant a bone is and is used primarily to assess the risk of a bone fracture (fracture risk) in the event of existing bone loss (osteoporosis).

Sequence of a bone density measurement

The density or lime salt content of the bones can be determined using various methods. Below is a brief explanation of the procedure for the different methods.

• The DXA ?

  Dual X-ray Absorptiometry: This method measures bone density using X-rays. This requires two X-ray sources. These are slightly different from each other.

  Bone density is measured on two areas of the patient. These are the hip joint and the lumbar spine as standard. The measurement takes 15 to 30 minutes and is not painful or excessively uncomfortable for the patient.

• Quantitative computed tomography ?QCT: This procedure is a specialized computed tomography in which the physical density of the bone is determined very precisely.

  The procedure is similar to conventional computed tomography. During the examination, which usually only takes a few minutes thanks to modern equipment, the patient lies on a height-adjustable table. The images of the bone are also produced here using X-rays.

  No contrast medium is required for imaging. Such images are called native images. Before the images are taken, a detailed plan is made of the area that you would like to view in order to keep the radiation exposure as low as possible.

  Besides quantitative computed tomography, peripheral quantitative computed tomography (pQCT) is also used. These are more compact and less expensive devices that measure the bone density of the periphery, e.g. the arms or legs. The conventional QCT, on the other hand, scans the bone density of the entire body.

There are different methods to measure bone density.

The standard procedure, which is also recognized by the WHO (World Health Organization) and the Umbrella Organization for Osteology as the method of choice, is measurement using X-rays, which is known as Dual-Energy X-Ray Absorptiometry (DXA or DEXA) or two-spectral X-ray absorptiometry. This method is ultimately based on the normal X-ray method, but unlike the latter, it uses not one but two X-ray sources, which differ slightly in energy. The principle of an X-ray image is based on the fact that different materials with different densities (i.e.

also different tissues within a human body) “attenuate”, i.e. absorb, the X-rays that pass through them to different degrees. This is why different gradations of grey can be seen on an X-ray image: Bones appear white because they are usually very dense and slow down the X-rays, while air-filled rooms hardly attenuate the X-rays at all and are therefore black on the image. However, absorption depends not only on the tissue, but also on the energy of the X-rays.

With DEXA, therefore, there are two different values (one for each X-ray tube) for each measuring point in the X-ray image after the measurement has been taken. The combination of these two results can then finally be used to draw conclusions about the density of the bones through the calcium and hydroxyapatite content. However, it is important to note that these values are not real density values in the physical sense (kg/m3), but rather a so-called area-projected mass or area density (kg/m2).

Not all bones are equally well suited for this assessment, so as a rule either the lumbar spine or the thigh bone or hip joint is X-rayed, since the density measurements are most meaningful here. This bone densitometry can be performed either in a hospital or in the practice of an orthopedic surgeon or radiologist. To do this, the patient must lie down on an X-ray table, where he or she is irradiated with X-rays.

The whole procedure takes about 10 minutes. The decisive advantages of this standard measurement are the low radiation exposure, fast execution and a low risk of measurement errors.However, there are other processes that can be used. On the one hand, there is the so-called quantitative computed tomography (QCT) or peripheral quantitative computed tomography (pQCT for peripheral parts of the body such as arms and legs), which are also based on X-ray technology and produce sectional images of the body.

In contrast to DEXA, QCT produces a three-dimensional image, which means that a physical density can actually be calculated for each volume element recorded. In addition, this method allows a more precise differentiation between the outer (corticalis) and inner region of the bones (bone ball or trabecula), which can sometimes play an important role in osteoporosis diagnostics. However, QCT exposes the patient to much higher levels of radiation than DEXA, and pQCT is not necessarily, but studies have shown that it is not as meaningful as the other two.