Modality Review: Introduction to DXA for Preclinical Imaging

When to Consider Using DXA

Dual energy x-ray absorptiometry (DXA/DEXA) is considered the standard of care for diagnosing and following osteoporosis clinically, while the World Health Organization (WHO) considers it the gold standard in measuring bone mineral density, providing the most accurate results. Measurements are considered to be very precise and stable over time, and can allow for evaluation of structural, bone density, and body composition changes over time. Clinically, changes seen on traditional x-ray represent advanced disease which likely will not respond to treatment; changes can be observed much earlier using DXA, when interventions may be effective.

In small animal preclinical research, body composition and bone mineral density measurements are relevant in many areas of research including metabolic bone diseases such as osteoporosis, arthritis, diabetes, metabolic syndrome, obesity, musculoskeletal diseases, drug safety, and toxicology.

iNSiGHT DXA Parameters & Description

Multimodal Imaging

DXA imaging provides a number of measurements from a single scan, looking at both bone and body composition indices, along with providing the x-ray image for additional anatomical reference. These may correlate and be complimentary to additional imaging performed on the same imaging subject, such as MRI, CT, optical etc. to provide further information about a specific disease model or target. Further, if the x-ray image is used to provide anatomical information for a bioluminescence or fluorescence image, for example, this again is a great example of multimodal imaging using the capabilities of a DXA system.

High Throughput Preclinical DXA Imaging

The iNSiGHT system, manufactured by Osteosys and distributed by Scintica, is a fully shielded X-ray cabinet which was specifically designed for preclinical small animal DXA applications, primarily using mice, rats, and similarly sized animals (10 ~ 5000g). Measurements of body composition are taken quickly (~25 seconds), are non-invasive, and use very low-dose radiation; combined this makes the iNSiGHT system ideal for following the same animal over the course of a longitudinal study.

Measurements are made, using dual energy x-rays, to allow each pixel to be assigned to a 3-compartment model – that is either fat mass, non-bone/lean mass, or bone mineral content. DXA does not require any pre-treatment of the animal, contrast, or substrate injection, to acquire the data. It is also non-destructive and provides highly accurate and reproducible measures.

The measurements provided by the iNSiGHT system include the following: Bone mineral density (BMD) in g/cm2, Bone mineral content (BMC) in g, Bone area (BA) in cm2, Tissue area in cm2, Fat tissue as percentage and weight in % and g, Lean tissue as percentage and weight in % and g, and Total weight in g. Additionally, specific bone length measurements can be drawn on the 2D x-ray image. The quantitative results may be tracked over time on the same animal, and/or exported into a comma separated value format for data compilation and comparison.

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Basic Explanation of How Dual Energy X-Ray Absorptiometry Works

Fundamentally, DXA takes advantage of the attenuation of x-ray photons as they pass through the tissues of the body; specifically, that the extent of attenuation varies with the energy of the photons and the density and thickness of the material they are passing through. More specifically, for a given energy level, each tissue will have a unique attenuation characteristic such that the attenuation is a constant – this is called the mass attenuation coefficient. When preforming dual energy x-ray absorptiometry two different x-ray beams, having different energies are generated at the source, passing through the body hitting the x-ray detector.

The acquired images are inherently co-registered as the animal does not move between the acquisitions. Initially, equations are used to determine if a pixel is to be classified as either bone mineral or soft tissue. Bone mineral is a physically dense material made up of phosphorus and calcium, having relatively high atomic numbers, causing significant attenuation of the x-ray beams. Soft tissue is a mixture of muscle, fat, skin, and water; having lower physical density and made up of materials such as hydrogen, carbon, and oxygen having lower atomic numbers, therefore causing lower attenuation of the x-ray beams.

Ultimately the goal is to determine which pixels are considered bone mineral and which are soft tissue. As the equations used presume a two-compartment system, that is bone mineral or soft tissue, any pixel not considered in the bone area measurement is then considered soft tissue. The same set of equations can be used on the soft tissue pixels to determine if the attenuation is a result of lean or fat mass. As soft tissue is inevitably a combination of molecules that make up both lean and fat tissues, each system must be calibrated to appropriate references to ensure appropriate assignment of soft tissue as either lean or fat mass. The values used depend on the lipid and non-lipid biological references used, as well as the energies of the x-rays generated by the system to create the images. As the images generated are two dimensional, and the systems have no way to assess the thickness of the tissue through which the x-rays passed, each pixel can only represent one of the three tissue types in the three-compartment system – bone, fat or lean mass. It is important to understand then that soft tissue located overtop of bone cannot be differentiated, nor can a thick diffuse bone be compared to thin dense bones. Likewise, visceral and subcutaneous adipose tissue located on the trunk which overlay one another cannot be differentiated; however, when located laterally subcutaneous fat will appear within its own pixel and can be differentiated from visceral fat.

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