**Use of Dual-Energy X-Ray Absorptiometry (DXA) with Non-Human Vertebrates: Application, Challenges, and Practical Considerations for Research and Clinical Practice**

Matthew D. Stone and Alec J. Turner *Kutztown University, USA* 

#### **1. Introduction**

98 A Bird's-Eye View of Veterinary Medicine

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> The applications of Dual-energy X-ray Absorptiometry (DXA) to vertebrate research and veterinary practice are many. DXA has been used successfully to rapidly and non-invasively quantify bone density and body composition in a variety of animals. The use of DXA has been limited, primarily, to basic and applied research, but DXA technology has great promise for clinical practice involving animals. Despite this potential, a number of limitations hinder its use in veterinary practice. These issues must be resolved before DXA can be widely used in traditional veterinary practice and a goal of this chapter is to discuss these limitations. This chapter reviews the past and current uses of DXA in basic and applied research involving non-human vertebrates.

#### **2. DXA theory**

DXA is a non-invasive technique for the determination of body composition. Users of DXA are able to rapidly quantify lean tissue mass, fat mass, total body mass, bone mineral mass, and bone mineral density. A comprehensive review of how DXA quantifies body composition can be found in (Adams, 1997; Jebb, 1997; Peppler & Mazess, 1981; Pietrobelli et al., 1996). The method by which DXA estimates body composition is based on the principle that the intensity of X-rays as they pass through tissues is attenuated in proportion to tissue mass (Figure 1).

The attenuation of a single intensity X-ray beam, as it passes through a single-tissue model (e.g. bone) of unknown mass, can be calculated based on the equation below (modified from Jebb, 1997):

$$R\_- = M\_B(R\_B) \tag{1}$$

where R is the degree of attenuation, MB is the mass of the tissue (e.g. bone), and RB is the tissue-specific attenuation coefficient. In this scenario, bone mass is the unknown of interest

Use of Dual-Energy X-Ray Absorptiomtetry (DXA) with Non-Human Vertebrates:

& fat mass combined) from areas containing bone and soft tissue (Figure 3).

Application, Challenges, and Practical Considerations for Research and Clinical Practice 101

When more than two tissue components exist (e.g. bone, fat, and lean tissue) DXA cannot directly estimate the relative proportion of all three components. DXA indirectly estimates these three components by first distinguishing areas of the scan that contain soft tissue (lean

Fig. 3. X-ray image depicting the method by which DXA estimates body composition from a scan. DXA distinguishes regions (pixels) containing bone (red squares) from regions without

In areas lacking bone, DXA can directly estimate the proportion of fat and lean tissue. For areas that contain bone, DXA determines the proportion of bone and soft tissue. Since DXA cannot distinguish between lean and fat tissue in these areas, DXA applies the proportion of fat and lean tissue, determined at neighboring non-bone areas, to the soft-tissue component

DXA is a non-invasive technique that was originally designed for the purpose of predicting current and future risk of bone fracture in humans by measuring bone mineral density (Grier et al., 1996). DXA can rapidly quantify total body mass, fat mass, lean tissue mass, and bone content and density, so it has potential applications in a variety of research and clinical fields. DXA has been used in and/or has practical applications to the fields of nutrition, sports and exercise science, physical therapy, animal science and nutrition, food science, pharmacology, pathology, metabolism, endocrinology, dentistry, and veterinary medicine. Because DXA is a non-invasive technique it is well suited for longitudinal studies. This advantage allows for reduced sampling error and potentially reduces required sample

To date DXA has been applied to a diversity of small and large animal species, representing most major taxonomic groups. Of the major taxonomic groups, mammals have received the most attention, in part, due to their role as human models of osteoporosis or their role in the food industry. Even though mammals have received the most attention, other taxa are

bone (blue pixels).

of bone containing regions (Pietrobelli et al., 1996).

**3. Basic applications of DXA** 

sizes and reduces those associated costs.

becoming increasingly well represented.

**4. DXA use with animals** 

Fig. 1. Depiction of X-ray intensity attenuation as it passes through a tissue. X-ray attenuation is proportional to tissue mass.

while the degree of attenuation is determined by measuring the difference in intensity of Xrays between the source and detector. The attenuation coefficient is a known constant for a particular tissue that has been derived theoretically and empirically (Jebb, 1997). If more than one tissue type is present, the attenuation of the X-ray is a function of each of the individual tissue components contribution to the total beam attenuation (Figure 2).

Fig. 2. Depiction of X-ray intensity as it passes through two tissue types. The total beam attenuation is a combination of the individual contributions of the two tissues, each at a different rate.

The relationship between the individual tissue contributions to total beam attenuation in a two tissue model (e.g. fat and lean tissue) can be summarized in the following equation, which is an extension of equation 1 (modified from Jebb, 1997).

$$R\_- = M\_F(R\_F) + M\_L(R\_L) \tag{2}$$

Subscripts F and L denote fat and lean tissue contributions to total beam attenuation. In this equation there are two tissues of unknown mass contributing to beam attenuation. Directly solving for both of these unknowns is impossible, so the use of single energy X-ray absorptiometry is limited by its ability to distinguish various tissue components. DXA technology was developed to overcome this limitation. DXA utilizes X-rays of two different peak energies (high and low) and the attenuation of these beams can be used to calculate both unknowns. Each of these beams is attenuated differently when they pass through specific tissues, as indicated in equation 3.

$$R\_{Hlgh} = M\_F(R\_F) + M\_L(R\_L) \tag{3}$$

$$\dots \quad \dots \quad \dots \quad \dots \quad \dots$$

$$R\_{Low} = M\_F(R\_F) + M\_L(R\_L)$$

When more than two tissue components exist (e.g. bone, fat, and lean tissue) DXA cannot directly estimate the relative proportion of all three components. DXA indirectly estimates these three components by first distinguishing areas of the scan that contain soft tissue (lean & fat mass combined) from areas containing bone and soft tissue (Figure 3).

Fig. 3. X-ray image depicting the method by which DXA estimates body composition from a scan. DXA distinguishes regions (pixels) containing bone (red squares) from regions without bone (blue pixels).

In areas lacking bone, DXA can directly estimate the proportion of fat and lean tissue. For areas that contain bone, DXA determines the proportion of bone and soft tissue. Since DXA cannot distinguish between lean and fat tissue in these areas, DXA applies the proportion of fat and lean tissue, determined at neighboring non-bone areas, to the soft-tissue component of bone containing regions (Pietrobelli et al., 1996).
