Diffraction-Enhanced Computed Tomographic Imaging of Growing Piglet Joints by Using a Synchrotron Light Source

The objective of this project was to develop and test a new technology for imaging growing joints by means of diffraction-enhanced imaging (DEI) combined with CT and using a synchrotron radiation source. DEI–CT images of an explanted 4-wk-old piglet stifle joint were acquired by using a 40-keV beam. The series of scanned slices was later ‘stitched’ together, forming a 3D dataset. High-resolution DEI-CT images demonstrated fine detail within all joint structures and tissues. Striking detail of vasculature traversing between bone and cartilage, a characteristic of growing but not mature joints, was demonstrated. This report documents for the first time that DEI combined with CT and a synchrotron radiation source can generate more detailed images of intact, growing joints than can currently available conventional imaging modalities.Abbreviations: DEI, diffraction-enhanced imagingDiffraction-enhanced imaging (DEI) is a biomedical imaging technique that, compared with conventional radiography, generates very detailed images with more edge contrast but deposits a lower radiation dose to the object. DEI generates enhanced contrast both from absorption, the process involved in conventional radiography, and from of X-ray refraction, a process that harnesses photons that otherwise typically are imperceptibly diffracted.⁴ The DEI technique collects information from X-rays that are refracted as they pass through tissues that have different refractive indices as it almost completely removes diffracted X-rays. In comparison, conventional radiography produces images from X-rays that are attenuated by the tissues through which they pass, but X-rays that are refracted within those same tissues confound, rather than clarify, image contrast. The creation of contrast from the refraction of X-rays, rather than exclusively from absorption, yields images that display more detail with clearer distinction between tissue interfaces. Refraction-based imaging can reveal tiny structures that are transparent to X-ray attenuation but have sufficient variation in density to produce refraction contrast. Furthermore, refraction-based imaging decreases the required radiation dose.²¹To obviate the superimposing effects in a 2-dimensional DEI refraction image, we considered that combining CT with DEI would yield images with even greater clarity. CT allows a 3D representation of the sample, such that contrast from features at different depths are no longer superimposed on one another but can be separated and viewed as independent structures. Although this advantage is valuable in traditional absorption imaging, the additional features that provide contrast in a refraction-based image enhance the value of CT. Combining DEI technology, which is capable of imaging soft-tissue detail, with CT, which allows segregation of the contrast images at different depths, overcomes limitations of conventional X-ray imaging, namely lack of distinction of soft tissues and 2-dimensionality. As we report here, DEI combined with CT and a synchrotron-generated X-ray source yields 3D images of growing joint tissues at a resolution on the order of micrometers, which is much higher than can be generated using conventional imaging techniques.A synchrotron radiation source was required for the development of DEI because a synchrotron currently is the only source capable of providing an intensely brilliant light (millions of times brighter than sunlight and conventional X-ray sources), is highly collimated (light rays in the beam remain parallel with negligible dispersion over distance), can be made to be monochromatic (having a single wavelength), and can be tuned precisely to an array of energy ranges. The Canadian Light Source (www.lightsource.ca), which began operations in 2005, is one of only 47 synchrotron facilities worldwide and the only such facility in Canada. Although nonsynchrotron sources of X-rays for DEI–CT are conceivable,^16,18 such technology requires considerable image-acquisition time. Regardless, the quality of images generated by using synchrotron technology likely would remain the standard with which any new nonsynchrotron DEI–CT technological innovations would be compared.¹⁴Despite refinements in medical imaging, conventional radiography, CT scanning, and MRI still are insufficient to discern fine details, particularly in growing joints in which soft tissues (including cartilage) predominate and change with physiologic growth. The impetus for the current research was to develop an imaging technique that better demonstrated normal joint characteristics during growth and, in the future, could be applied to pathologic joints for experimental research and eventually clinical applications. In particular, we were motivated by a need to more effectively and reliably image growing joints affected by arthritis, a disease associated with alterations of bone and cartilage growth, tissue morphology and vascularity. Childhood arthritis research likely will benefit from having an improved imaging technique to aid in early diagnosis, monitor disease progression, and assess responses to therapies. The long-term outcomes of childhood arthritis are improved with early diagnosis and prompt and effective response to treatment interventions. Clinical and laboratory-based indicators of inflammation are not always adequate to detect and monitor subclinical intraarticular inflammation which, as with overt disease, can lead to progressive joint damage. Imaging can augment clinical and laboratory assessment of arthritis activity, but even the most sensitive currently available modalities are unable to detect all joint pathology.In juvenile arthritis, joint-imaging outcomes are difficult to evaluate because variations associated with normal growth cannot always be easily discerned from variations induced by the disease. Conventional radiography tends to detect advanced joint damage that has affected bone, but cartilage can be assessed only indirectly, and soft tissue abnormalities cannot be fully evaluated. Consequently, conventional radiography has insufficient sensitivity and specificity to be considered useful for diagnosing or monitoring children with inflammatory joint disease.^6,20 MRI, which evaluates both soft tissues and osteochondral structures, can be used to detect cartilage loss, bone erosions, and synovial hypertrophy in children and adolescents, and contrast-enhanced MRI detects active synovitis.^1,10 However, standardized approaches to acquire and interpret MRI data are not established for children in general and, in particular, for children with arthritis;^12,15 it is not always clear, for example, if observed thinning of cartilage is physiologic or pathologic. Furthermore, although MRI is more sensitive than conventional radiography, MRI too has limited precision in detecting fine structures and pathologic changes; a clinical MRI has less than 50% sensitivity in detecting cartilage damage that subsequently is seen arthroscopically.^8,13CT offers another option for joint visualization, given that it provides high-resolution, 3D images of bone from any angle. Despite its high spatial resolution, however, CT cannot match MRI''s soft-tissue contrast resolution, because CT provides negligible variability of attenuation coefficients of soft tissues so attenuation is nearly the same for cartilage, muscles, and ligaments. Furthermore, CT''s value is offset by the necessity for radiation exposure, a particular concern in the pediatric population. Therefore, for joint research and clinical applications, each of the conventional imaging techniques currently available has limitations. A safe, higher resolution imaging system that generates good contrast for all joint structures is required.Because the DEI technique initially was developed by using a synchrotron light source, we similarly used synchrotron technology in the current experiments. In contrast to conventional X-ray tubes, a synchrotron generates light by using radiofrequency waves and electromagnets to energize and accelerate electrons, thus producing brilliant, highly focused light from the entire wavelength spectrum, including X-rays. For the development and evaluation of DEI–CT imaging of joints, we chose to use healthy commercial piglet stifle joints because porcine stifle joints are anatomically similar to human knees.⁵ In addition, pigs grow quickly, reaching skeletal maturity at the distal femur and proximal tibia in 20 mo,¹⁹ thus allowing for the use of the pig as a model to study growth patterns in normal and disease states in a relatively short time period. The current study aimed to develop and test a new technology for imaging growing joints by using DEI combined with CT and a synchrotron radiation source. This report is the first to document the application of DEI–CT for imaging intact, growing joints.