Single photon absorptiometry

Single photon absorptiometry is a measuring method for bone density invented by John R. Cameron and James A. Sorenson in 1963. Single photon absorptiometry is a measuring method for bone density invented by John R. Cameron and James A. Sorenson in 1963. Single photon absorptiometry (SPA) was developed in 1963 by Steichen et al. In 1976, it was an important tool for quantifying bone mineralization in infants. The single photon absorption method operates when a certain amount of gamma rays emitted by isotopes, pass through human tissues. There is an exponential function relationship between the number of gamma rays absorbed and the thickness of tissues, and the absorption characteristics of different tissues are different, but the effects of soft tissues and water on gamma rays are the same. Therefore, the influence of soft tissues can be eliminated by a water bath, and the number of gamma rays absorbed by bone tissues can be measured, and then calculated. The bone mineral content (BMC) was calculated. This method is mainly used for bone measurements of limbs and population census with the aid of the water bath. In 1963, the single photon absorption Assay (SPA) invented by Cameron and Sorenson was the first quantitative analysis method applied to the diagnosis of osteoporosis. This method uses the principle that the absorption of radioactive substances by bone tissue is proportional to the bone mineral content. Iodine or Americium gamma photons are used as a light source to penetrate the forearm. After being absorbed by the bone and soft tissue, NaI (Tl) crystal is used to detect the radioactivity counts parallel to the light source. BMC and BMD are obtained by calculating the density of photon energy emitted and emitted. The location of measurement is usually located at the 1/3 junction of the ulna and distal radius, or at the less soft tissue sites such as calcaneus and hand bone, wrapped in a water bag and placed between light source and detector. BMC (g/cm) can be obtained by synthesizing the measured bone gamma photon absorption energy. BMD (g/cm) can be obtained by dividing BMC by bone width. This method can only measure the bone mineral content of limbs. If the isotope source is changed to X-ray source, that is, single energy X-ray absorptiometer (SXA), the principle and determination method is the same as SPA, but the radiation source is different. The basic principle of single-photon bone mineral density measuring instrument is to calculate the attenuation degree of single-energy gamma photon beam through bone tissue. The more attenuation degree is, the more absorbed by bone minerals, the more bone mineral content and the higher bone mineral density are. This method is called gamma-ray absorption method, which is also called single-photon absorption method. This method is the most convenient for measuring radius and ulna, and the object of observation is left. The junction point of the middle and lower 1/3 of the radial and ulnar bones is the measuring point. The height and weight parameters of the observed objects are measured routinely before measurement. Single photon absorption is the earliest method to measure bone mineral density accurately. Its basic principle is that bone mineral density can be obtained by the law of absorption. In this law, the important parameters to be obtained are bone thickness, bone absorption coefficient and radiation intensity (or counting) after bone absorption. The thickness of soft tissue measured by single photon absorption method is the same. Soft tissue does not affect the results of bone tissue measurement. Therefore, the absorption coefficient of a beam of constant energy radiation can be calculated beforehand, and the intensity of radiation (or counting) can be obtained directly in patients' measurement. In the vertical C-frame, the collimated 125I light source (200 mCi or 74 GBq) and the collimated NaI (TI) scintillation detector-photomultiplier tube are mounted in relative geometric shapes to place the measured body parts between the source and the detector. The source and detector assembly are rigidly connected and driven by a motor to cross the longitudinal axis of the bone. An early attempt which used conventional X-rays to measure bone mineral density (BMD) uses stepped wedges made of aluminum or ivory phantom as a calibration tool. The bone mineral density was calculated by visually comparing the bone mineral density and the known density at each step of the phantom. The next improvement in the field of bone mineral density is the single-photon absorption (SPA) method invented by Cameron and Sorenson in 1963. The expensive and potentially dangerous radioactive sources used in SPA and DPA have been replaced by single X-ray absorptiometry (SXA) and dual-energy X-ray absorptiometry (DXA) since the late 1980s. Similar to DPA, the basic principle of DXA is to measure the high and low energy X-ray transmission of stable X-ray sources. The shorter acquisition time, higher accuracy and resolution, and availability exposure can be considered as advantages of using X-rays instead of SPA or DPA. With the increasing popularity of DXA, its application in pediatric research and clinical practice has increased significantly. At the same time, because SPA is only one energy photon, the actual measurement site is limited to limb bones, especially the distal limb bones, while there is a lot of fat and gas around the trunk bone tissue, so the single photon absorption method is 'powerless'. At present, the main improvement is to change the isotope source to X-ray source, which cannot only stabilise the voltage, but also improve the measurement accuracy, resolution and speed. As a result, it has also developed from one-dimensional scanning to two-dimensional scanning, from waveform representation of bone mineral density to matrix arrangement of bone mineral density, which more intuitively reflects bone mineral density.