X-ray transmission factor in estimating bone density.

Abstract

Bone density, which is affected by the dietary regimen and certain pathologic situations, has become an extremely important measurement in nutrition and biologic research. Although bone ashing technics have been in common use for a long time, measurements of bone density in vivo have been practiced only in recent years. Mack (2, 3), who in 1927 began to use a radiographic photometric method for assessment of the mineral content of bone, has made many valuable contributions in this field. Other investigators have used variations of her technic for measuring the changes in mineral content of bones in man and in experimental animals. The photometric method of Mack has been developed to a high degree of precision' and quantitative bone density values are derived through the use of a recently described semiautomatic integrating microphotometer (4). However, since large amounts of film are consumed in that method, an alternate system which would not require the exposing and processing of film would be advantageous. With this in view, a device which is capable of rapid measurements of the x-ray transmission factor of bone without the use of film has been constructed in this laboratory and will be described herein. Since x-ray transmission is related in a known manner to the density of bone, the transmission factor obviously becomes useful in detecting changes in skeletal mineralization status. The device is similar to that used for measuring linear x-ray absorption coefficients of elements. Figure 1 is a generalized diagram illustrating its principle. A very sharp, collimated and filtered beam of x-radiation is made to pass through the bone for which the transmission factor is to be determined. A short metallic tube on the opposite side of the bone conducts the residual primary rays to an x-radiation detector. The ratio of the initial intensity to the transmitted intensity is determined by comparing the detector readings with the bone in place and, again, not in place. The transmission factor is then determined by means of the equation TF = I/I0 (1) where I is the intensity after passing through the bone and I0 is the initial intensity. The lower the ratio of I to I0, the denser or thicker the bone. There are several complicating factors which affect the apparent x-ray absorption by bone. These will be discussed individually in the following section. Theoretical Aspects The x-ray transmission factor I/I0 is affected by the chemical composition and thickness of the absorber. According to its composition, a material will have a specific linear absorption coefficient which is numerically equal to the fractional reduction of beam intensity per centimeter of absorber.