Interpretation of refraction images in synchrotron based imaging techniques using growth plate injury specimens in an animal model

Abstract

Typical clinical radiography uses the variable absorption of x-radiation of different tissues within the object to produce contrast in the image. The interpretation of this image is based on understanding the anatomy and pathology as well as understanding how the image is produced. One needs to understand what features of the image are representative of the anatomy/pathology, what is inherent to the imaging process and what is artefact. The correlation of contrast in the image with absorption contrast in the object is fairly intuitive. Diffraction Enhanced Imaging (DEI) uses the bending or refraction of radiation as it passes through a tissue interface to produce the contrast in the image in addition to absorption. The ability to interpret the refraction contrast image is not as intuitive as the absorption image. Edges are enhanced and tissues with different densities can look similar. Understanding the image acquisition of refraction based radiography also is not as intuitive as typical absorption radiography. One potential advantage of DEI is the ability to visualize small structures that may not be visible using absorption radiography. The growth plate of long bones and the premature closure or bone bridge/bar formation across the growth plate associated with a fracture was targeted as a study sample. An animal model (juvenile rat) was used for inducing a fracture through the proximal metaphysis of the tibia. The animal was then sacrificed at variable times of healing which is described below. The specimens were then imaged using DEI techniques at Brookhaven National Lab, Upton, New York, at the Biomedical Imaging and Therapy (BMIT) beamline at the Canadian Light Source and using a laboratory based DEI system at Nesch, LLC, Crown Point, Indiana. High resolution absorption images were obtained using a SkyScan micro-CT from Prof. Cooper’s laboratory for comparison of DEI and absorption images. Histological slides were also prepared for correlation of image anatomy. The reason for imaging these specimens using different techniques was to determine potential translation of synchrotron based techniques with lab based or conventional techniques as well as determining what features of imaging could be uniquely done at a synchrotron. Since access to synchrotron biomedical imaging facilities is limited, the potential for some work to be done outside of synchrotron facilities would make research progress more efficient. A detailed analysis of the images was performed. The detail of the bone as well as the fracture was exquisite with the CT data. With the planar images the orientation of the trabeculae of the bone relative to the direction of the analyser crystal (direction of diffraction) changes the appearance and texture of the bone image. It was hoped to visualize the layers of the growth plate (variable calcification) and perhaps the initiation of bone spicules leading to bone bridges across the growth plates at the site of fracture. However, the small size of the object limited observable detail. This work was originally intended to apply a unique imaging technique for the study of growth plate fracture pathophysiology. However, it became clear that the technical image production and interpretation was more important to the project than the individual analysis of each specimen. As a result not all specimens were used, but those selected were used to refine the technique and interpret the synchrotron based images compared to conventional images. The use of DEI for assessing bone bridge formation was promising, but the specimen size limited detail and resolution. This has led to the conclusion that a larger animal model would be more appropriate for this type of study. Further, it was discovered that the bone (growth plate) orientation affected the planar image contrast of the bone / cartilage interface based on long axis orientation relative to the refraction sensitive direction of the DEI system. To more fully exploit this effect, more images at different object orientations would be necessary for interpretation in future work with larger animal models.