In many situations in medicine, structural imaging of tissue does not provide sufficient evidence for effective clinical assessment, and the addition of biochemical information would be beneficial. The ideal situation would be to design hybrid tomography systems which provide good spatial resolution together with high molecular contrast information. Optical spectroscopy can provide inherently high levels of intrinsic molecular contrast, including Raman measurements arising from functional and molecular changes [1
]. Combining optical techniques with anatomical information from other low-contrast, high-resolution imaging modalities has been demonstrated with Magnetic resonance imaging [2
], X ray [3
] and ultrasound [4
]. In these hybrid systems, the spatial structure of tissues is embedded into the optical image reconstruction to provide accurate and high-resolution molecular characterization. Work by Davis et al.
] showed that the use of spatial priors is essential for recovering fluorophore distributions in complex tissue volumes [6
]. The proposed method is extended here to combine Raman spectroscopy with x-ray CT to allow image-guided Raman spectroscopy (IG-RS) using anatomical structures from CT as a priori
knowledge. We present here, to the best of our knowledge, the first known IG-RS estimates using an excised canine limb as a model.
Raman spectroscopy is based on the principle of inelastically scattered light arising from vibrational energy states of molecular bonds, and it provides valuable biochemical markers to study tissue composition. Raman spectroscopy has been studied in various clinical applications [8
]. Our interest is in exploring the use of Raman scattering for studying bone chemical composition and predicting the risk of bone fracture with age and disease, such as osteoporosis. Reports indicate that collagen, the primary organic component of bone extracellular matrix, undergoes changes with osteoporosis [14
]. However, the current gold standard for bone health, dual-energy X-ray absorptiometry (DXA), measures only the mineral component of bone. With Raman spectroscopy both the mineral and organic phases can be measured. Results ex vivo
showed that Raman spectroscopy could distinguish between normal and unhealthy tissue non-invasively [13
] using time-resolved measurements in mouse genotypes. Also, Schulmerich et al.
] have accurately recovered Raman bone spectra through 5 mm of tissue in a canine excised limb using a reflectance-based fiber optic probe. In addition, compositional differences observed in bone excised from women with and without hip fractures illustrated a higher carbonate-phosphate ratio from trabecular bone in women with fracture [19
]. Because of the molecular specificity and quantitative potential of these measurements, deep tissue Raman measurements would provide valuable biomedical information if Raman scatter could be well localized to the region of origin and then used to tomographically recover the contrast (i.e. boundaries) between the desired target and background.
Diffuse optical spectroscopic measurements can be used to overlay molecular information onto imaging systems, but a transmission geometry limits the accuracy of imaging [20
]. Preliminary studies on Raman tomographic imaging using a transmission geometry allowed three-dimensional visualization of spatial changes in biochemical composition [23
], however, the localization of the origin of Raman signal was limited due to the diffuse nature of light propagation. Here we present the use of high-resolution CT images to guide the diffuse modeling of light in order to achieve a high-contrast molecular and structural characterization of tissue with accurate localization of signal origins.